Chapter 4 - Cell structure and membranes

Pinocytosis (endocytosis by a cell of liquid containing dissolved substances)

("cellular drinking"), vesicles bring fluids and dissolved substances, including proteins, into the cell. Sometimes called fluid endocytosis, pinocytosis is relatively nonspecific regarding what it brings into the cell. For example, pinocytosis goes on constantly in the endothelium—the single layer of cells that separates a blood capillary from the surrounding tissue. Pinocytosis allows cells of the endothelium to rapidly acquire fluids and dissolved solutes from the blood.

phagocytosis (endocytosis by a cell of another cell or large particles)

("cellular eating"), a very large vesicle called the phagosome is formed when a cell engulfs and ingests a large particle or even an entire cell. Phagocytosis is initiated by specific cell membrane proteins called receptors that recognize a specific ligand on the surface of the particle or cell. The binding of the ligand to the receptor causes the phagocytic cell to engulf the particle or other cell. Phagocytosis is restricted to specialized cells; for example, unicellular protists use phagocytosis for feeding, and some white blood cells use phagocytosis to engulf foreign cells and substances. The phagosome that forms usually fuses with another vesicle in the cell called the lysosome, where the vesicle's contents are digested

Vacuoles (occur in many eukaryotic cells, but particularly those of plants and fungi)

(A membrane-enclosed organelle in plant cells that can function for storage, water concentration for turgor, or hydrolysis of stored macromolecules.)

phospholipid bilayer

A biological membrane's characteristics are determined by the chemical properties of its lipid and protein constituents. In essence, a membrane consists of a ________________ with numerous associated proteins that are embedded in it or attached to it on the outside or inside

Peripheral membrane proteins

A cell membrane is associated with many types of proteins. Which type of protein would most easily be removed from the cell membrane using aqueous conditions in a laboratory technique?

energy

A cell requires __________ to make the molecules it needs for activities such as growth, reproduction, responsiveness, and movement. Mitochondria (found in all eukaryotic cells) transfer chemical-bond energy from glucose to ATP, which is then used to fuel a variety of endergonic processes. Chloroplasts (found in plants and other photosynthetic cells) capture light energy from sunlight and convert it into chemical-bond energy.

thylakoids

A flattened sac within a chloroplast. Thylakoid membranes contain all of the chlorophyll in a plant, in addition to the electron carriers of photophosphorylation. Thylakoids stack to form grana.

Cytoplasmic streaming

A form of amoeboid movement in which a fluid cytoplasm moves and stretches the organism's body in new directions.

cilia

A hairlike organelle used for locomotion or signaling. Generally shorter than a flagellum.

Unsaturated fatty acids would replace saturated fatty acids.

A hibernating animal must make changes at the cellular level to prevent damage in colder temperatures. How might cell membranes of these animals be affected?

flagella

A long, whiplike appendage that propels cells. Prokaryotic flagella differ sharply from those found in eukaryotes.

Tonicity

A measure of the relative concentration of membrane-impermeable solutes on either side of a membrane. Determines the direction and extent of net water movement across a membrane.

Secondary lysosome

A membrane-enclosed organelle formed by the fusion of a primary lysosome with a phagosome, in which macromolecules taken up by phagocytosis are hydrolyzed into their monomers.

phagosome

A membrane-enclosed vesicle inside a cell that results from infolding of the cell membrane and enclosing a particle to be taken into the cell.

fluid mosaic model

A molecular model for the structure of biological membranes consisting of a fluid phospholipid bilayer in which suspended proteins are free to move in the plane of the bilayer.

cell wall

A relatively rigid structure that encloses cells of plants, fungi, many protists, and most prokaryotes, and which gives these cells their shape and limits their expansion in hypotonic media. (Cell walls are tough, flexible structures that differ in their chemical makeup depending on the organism and cell type. Some are composed primarily of carbohydrates (cellulose in plants, chitin in fungi), others primarily of protein (in some archaea), and still others a mix (peptidoglycans in bacteria). The cell wall serves two primary functions. The first is structural support, and the second is protection from physical damage and pathogens. Cells with walls can be hypertonic with respect to their environment because the wall limits their volume and keeps them from bursting)

Efficiency of transport of materials within a cell decreases with size, thus limiting the maximum cell size.

A researcher growing cells in culture observed that the cells always divided once they reached a certain size. When she added more nutrients to the culture medium or adjusted the temperature of the growing chamber, the cells grew at different rates but always divided once they reached a certain size. Which statement explains these observations?

bacterial chromosome

A single, circular DNA molecule containing the vast majority of the genes present in a bacterial cell.

nucleolus

A small, generally spherical body found within the nucleus of eukaryotic cells. The site of synthesis of ribosomal RNA.

contractile vacuoles

A specialized vacuole that collects excess water taken in by osmosis, then contracts to expel the water from the cell.

use active transport

A species of fish that lives in the ocean has cells with a lower solute concentration than the surrounding ocean waters. To maintain cell homeostasis under these conditions, these fish

Golgi Apparatus

A system of concentrically folded membranes found in the cytoplasm of eukaryotic cells; functions in secretion from the cell by exocytosis.

endomembrane system

A system of intracellular membranes that exchange material with one another, consisting of the nuclear envelope, Golgi apparatus, endoplasmic reticulum, and lysosomes when present.

endoplasmic reticulum (ER)

A system of membranous tubes and flattened sacs found in the cytoplasm of eukaryotes. Exists in two forms: rough ER, studded with ribosomes; and smooth ER, lacking ribosomes.

in a direction opposite to the one in which diffusion would move them.

Active transport moves molecules

1; 25

All biological membranes contain proteins. Typically, cell membranes have about ___ protein molecule for every ____ phospholipid molecules. This ratio varies depending on membrane function.

ATP, which is used as an energy source.

All the following may be involved in facilitated diffusion except

fatty acid chain length and degree of unsaturation and in the kinds of polar (phosphate-containing) groups present

Although all biological membranes share a similar structure, there are many different kinds of phospholipids, and membranes from different cells and other membrane-enclosed compartments may vary greatly in their lipid composition. For example, phospholipids can differ in their _____________(number of carbon atoms) _____________(number of double bonds in their fatty acids), ______________. The most common fatty acids in membranes have chains with 16-18 carbon atoms and 0-2 double bonds. Saturated fatty acid chains (those with no double bonds) allow close packing of phospholipids in the bilayer, whereas the "kinks" caused by double bonds in unsaturated fatty acids (see Figure 3.1A) make for less dense packing.

phagocytosis involves engulfment of larger substances, including whole cells.

An important difference between phagocytosis and receptor-mediated endocytosis is that

sodium-potassium(Na+-K+)pump

An integral membrane protein (anti-porter) that carries out primary active transport of ions; it uses energy from ATP to pump sodium ions out of a cell and potassium ions into the cell. Also called a sodium-potassium ATPase.

channel proteins

An integral protein in a cell membrane that forms an aqueous passageway across the membrane in which it is inserted and through which specific solutes (e.g., specific ions) may pass by diffusion in the direction dictated by their electrochemical gradient.

carrier proteins

An integral protein in a cell membrane that noncovalently binds specific molecules and transports them through the membrane. Many are capable of moving molecules against their electrochemical gradients by direct or indirect use of ATP-bond energy.

chloroplast

An organelle bounded by a double membrane containing the enzymes and pigments that perform photosynthesis. Chloroplasts occur only in eukaryotes.

lysosome

An organelle containing digestive enzymes

They hold cells together in tissues. They contribute to the physical properties of cartilage, skin, and other tissues. For example, the mineral component of bone is laid down on an organized extracellular matrix. They help filter materials passing between different tissues. This is especially important in the kidney. They help orient cell movements during embryonic development and during tissue repair.

Animal cells lack cell walls, but many are surrounded by, or in contact with, an extracellular matrix consisting of the fibrous protein collagen and gel-like proteoglycans (proteins with many attached carbohydrate residues). A third group of proteins links the collagen and the proteoglycan matrix together. The extracellular matrices of animal cells have several roles:

Carbohydrates

Cell-to-cell adhesion often occurs through the interactions of

Exocytosis

Cells secrete materials to the exterior environment through the process of

eukaryotes

Cells that contain nuclei. (which include plants, fungi, protists and animals) are much larger (10−100 μm), have a nucleus, and have numerous membrane-enclosed organelles, each of which has a characteristic structure and functions

prokaryotes

Cells that do not contain nuclei (archaea and bacteria) which are generally small (0.1−5 μm in diameter), single-celled, lacking a nucleus, and usually without membrane-enclosed internal compartments

temperature

Changes in ________________ impact the physical state (fluidity) of membrane lipids This affects protein function & the cell membrane's effectiveness as a barrier

temperature effects on membranes

Changes in temperature impact the physical state (fluidity) of membrane lipids This affects protein function & the cell membrane's effectiveness as a barrier

can allow movement of substances either into or out of the cell.

Channel proteins

eukaryotes have a nucleus; prokaryotes do not

DNA is organized differently... whether the genetic material is housed in a nucleus distinguishes eukaryotes and prokaryotes (greater compartmentalization in general in eukaryotes)

against their concentration gradients

Diffusion and osmosis tend to equalize the concentrations of solutes on either side of a membrane. However, one hallmark of cells is that their internal solute concentrations are quite different from their external environment. To achieve this, a cell must sometimes move substances ___________________. This process requires the input of energy and is known as active transport. Often the energy source is the hydrolysis of the nucleotide ATP.

• Cell membrane & nuclear membrane are parts of a larger collection of functionally distinct phospholipid bilayers • Called the endomembrane system

Endomembrane System

division of labor

Eukaryotic cells have extensive membrane compartmentalization and thus are able to effectively separate certain molecules and biochemical reactions from the rest of the cell. This impressive "____________" provides possibilities for regulation and efficiency that were important in the evolution of plants and animals.

Glucose Transporters (A carrier protein used to facilitate diffusion of the monosaccharide glucose.)

Facilitated diffusion of the monosaccharide glucose is accomplished with carrier proteins called

surface area-to-volume ratio

For any cell, organism, or geometrical solid, the ratio of surface area to volume; this is an important factor in setting an upper limit on the size a cell or organism can attain.

small cells

For the average prokaryote, diffusion is an efficient way of moving molecules across the cell membrane à mm / ms

Hypertonic

Having a higher concentration of membrane-impermeable solutes; said of one solution compared with another.

hypotonic

Having a lower concentration of membrane-impermeable solutes; said of one solution compared with another.

both undergo dynamic shifts between assembly and disassembly

How are microfilaments and microtubules similar?

Analysis of their membrane lipids provides insights on the fundamental principles of cell membrane structure

How do cell membranes vary across the Tree of Life?

complex solution

How do the first three factors apply to a cell surrounded by a membrane? The cytoplasm is largely a water-based (aqueous) solution, and so is the surrounding environment. In a _______________ (one with many different solutes), the diffusion of each solute depends only on its own concentration, not on the concentrations of other solutes. So one might expect a substance with a higher concentration inside the cell to diffuse out, and one with a higher concentration outside the cell to diffuse in.

Channel proteins form a pore through the membrane to allow substances to pass, whereas carrier proteins bind to and change conformation as they transport substances through the membrane.

How does facilitated diffusion by channel proteins differ from facilitated diffusion by carrier proteins?

diameter (of the molecules or ions: smaller molecules diffuse faster.) temperature (of the solution: higher temperatures lead to faster diffusion because particles have higher thermal energy and thus move faster.) concentration gradient (in the system—that is, the change in solute concentration with distance in a given direction. The greater the concentration gradient, the more rapidly a substance diffuses.) area across which a substance diffuses and the distance it moves (A larger surface area permits more rapid diffusion -this explains why in many cells the cell membranes are folded- keeping the distance a solute must diffuse as short as possible maximizes the diffusion rate.)

How fast a substance diffuses, measured as the net movement of particles per unit of time, depends on several factors:

active transport against the concentration gradient of Ca2+

If a cell is bathed in an extracellular solution high in calcium ions, it would use _______ to maintain a normal, low intracellular concentration of calcium ions.

active transport

If an animal cell needs to acquire more of a particular amino acid for a physiological process, it can employ _______ when the amino acid is more concentrated inside the cell than in the extracellular fluid.

Hexagonal phase aggregates

If temperature is too high, ____________________________will form, disrupting the bilayer and destroying the boundary between cell & environment

Gel Phase

If temperature is too low, membrane lipids will pack closely into a _______________, which impairs protein movement & function

the blood cells will swell and possibly burst

In animals, the solute balance of the plasma surrounding blood cells must be carefully maintained so that the blood cells are not affected by changes in fluid or solute balance. If the plasma becomes hypotonic compared with the blood cells, what will be the effect?

fatty acid tails point toward each other

In biological membranes the lipid bilayer contains phospholipids, each with a head and two fatty acid tails. They are arranged so that the

Cytosol

In eukaryotic cells, the breakdown of energy-rich molecules such as the monosaccharide glucose begins in the __________. The molecules that result from this partial degradation enter the mitochondria, the primary function of which is to harvest the chemical-bond energy of those molecules in a form the cell can use, namely the energy-rich nucleotide ATP.

A protein's function may depend on its presence in a specific location.

In the fluid mosaic model, some proteins are able to move freely within the membrane, while others, such as proteins in a muscle cell membrane, are anchored or restricted in their movement. What is the best explanation for a cell's restriction of some proteins to a specific region of the cell membrane?

Homeoviscous adaptation in caribou

In winter, caribou do not keep their outer extremities as warm as their body core - this cuts down on energy costs To maintain proper membrane fluidity at cold temperature, caribou increase the proportion of unsaturated fatty acids at their extremities

digestive enzymes

Lysosomes are important to eukaryotic cells because they contain

secrete

Many macromolecules are simply too large and too charged or polar to pass through biological membranes. In one sense, this is a fortunate property—cellular integrity depends on containing these macromolecules in specific locations. However, some cells must sometimes take up or ________ (release to the external environment) intact large molecules. Even if a membrane were permeable to such molecules, they would diffuse extremely slowly because of their large size. It takes a completely different mechanism than diffusion to move intact large molecules across membranes.

homeoviscous adaptation

Many organisms adjust the lipid composition of their membranes to maintain appropriate fluidity

membrane fluidity (pp)

Membrane lipid composition is well-matched to physiological temperature to maintain proper ________________

internal; external

Membranes separate an ________ environment from the __________ environment, giving cells or cell compartments the ability to regulate their internal composition.

outer; inner

Mitochondria have two membranes. The ___________ membrane has large pores and is thus permeable to most substances. The ________ membrane separates the biochemical processes of the mitochondrion from the surrounding cytosol. The inner membrane is extensively folded into structures called cristae, and the fluid-filled region inside the inner membrane is referred to as the mitochondrial matrix. The mitochondrion contains many enzymes for energy metabolism, as well as DNA and ribosomes for the synthesis of a small proportion of the mitochondrial proteins.

gated

Most ion channels are "________," meaning they can be opened or closed to ion passage. A gated channel opens when a stimulus causes a change in the three-dimensional shape of the channel. In some cases this stimulus is the binding of a chemical signal, or ligand. Channels controlled in this way are called ligand-gated channels. In contrast, a voltage-gated channel is stimulated to open or close by a change in the voltage (electric charge) across the membrane.

channels called plasmodesmata

Movement of materials among plant cells is made possible by

Facilitated diffusion

Osmosis, the movement of water though specialized membrane channels, is an example of

deficient receptor-mediated endocytosis of LDLs in the liver.

Persons with the inherited disease familial hypercholesterolemia have very high levels of cholesterol in their blood because of

8 nanometers (0.008 micrometer)

Phospholipids are the major lipid component of biological membranes. Phospholipids form thin bilayers, about _____________ thick, with their charged, hydrophilic phospholipid "heads" on the external surfaces interacting with the aqueous environments, and their nonpolar hydrophobic fatty acid "tails" interacting with each other on the inside of the membrane. The hydrophobic interior acts as a barrier and prevents the diffusion of polar molecules and ions through the membrane.

Turgor Pressure

Plants depend on their cell walls to limit the amount of water entering their cells and maintain _______, which helps keep plants upright.

channel proteins and carrier proteins

Polar molecules, especially large ones, and charged substances do not readily diffuse across phospholipid bilayers. However, they can cross the membrane passively (that is, without the input of energy) with the help of two kinds of integral transmembrane proteins,

interactions with cytoskeletal proteins on the membrane's inner face.

Protein movement within a membrane may be restricted by

one on the interior of the cell, usually to microfilaments or to intermediate filaments in the cytoplasm just below the cell surface, and the other to collagen in the extracellular matrix.

Proteins connect the cell membrane to the extracellular matrix. These proteins (for example, integrin) span the cell membrane and have two binding sites:

it allows more opportunities for regulating gene expression

Scientists have observed that in prokaryotes, translation of messenger RNAs begins before transcription has completed. In contrast, translation is completely separate from transcription in eukaryotes because DNA is isolated in the nucleus away from ribosomes, which carry out protein synthesis in the cytoplasm. What advantage does this compartmentalization provide eukaryotes?

Their membrane lipids will contain a higher proportion of saturated fatty acids than lipids of related bacteria living at lower temperatures.

Some bacteria can live at temperatures near the boiling point of water in Grand Prismatic hot spring in Yellowstone National Park. Make a prediction about the membrane lipid composition of these bacteria.

cell junctions

Specialized structures associated with the cell membrane that join cells in multicellular organisms. Some contribute to cell adhesion, others to intercellular communication.

Cell Theory

States that cells are the basic structural and physiological units of all living organisms, and that all cells come from preexisting cells.

replacing saturated fatty acids with unsaturated fatty acids.

Temperature affects the fluidity of plasma membranes. Some organisms maintain their membrane fluidity when it is extremely cold by

volume; surface area

The __________ of a cell determines the amount of metabolic activity, and thus the cell's need for resources and its rate of waste production per unit of time. Since substances enter and exit the cell through the cell membrane, cell _______________ determines the rate at which resources can enter and wastes can leave a cell.

carbohydrates

The _____________ associated with membranes are attached either to lipids, to form glycolipids, or to proteins, to form glycoproteins (when there are a few carbohydrate residues) or proteoglycans (when there are many carbohydrate residues). They are generally located on the outside of the cell, where they interact with substances in the external environment. While glycolipids can have important effects on membrane fluidity, in general carbohydrates associated with membranes function in cell recognition. Chains of monosaccharides can generate a large diversity of linear or branched structures that play roles in cell-to-cell communication and cell adhesion (the ways in which cells attach to one another, or to a substrate).

stroma

The aqueous fluid inside of the chloroplast surrounding the thylakoid membranes. The location of carbohydrate synthesis during photosynthesis.

helps maintain the internal environment of the cell.

The cell membrane

Lipids and proteins

The cell membrane contains carbohydrates that can serve as recognition sites for other molecules. The carbohydrates may be covalently bonded to

invaginate

The cell membrane is said to _______ as it folds inward to form a pocket around materials in the environment during phagocytosis, pinocytosis, and receptor-mediated endocytosis.

water flow

The concentration of membrane-impermeable solutes outside the cell determines the direction of _________ ________ for all animal cells.

cytoplasm

The contents of the cell, excluding the nucleus. (substances determine the cell's structure and function)

active transport

The energy-dependent transport of a substance across a biological membrane against a concentration gradient—that is, from a region of low concentration (of that substance) to one of high concentration.

provide strength and stability

The function of the clathrin molecules that coat vesicles in receptor-mediated endocytosis is to

on the exterior surface of the membrane.

The hydrophilic regions of a membrane protein are most likely to be found

It supports the cell and maintains its shape. It maintains the position of internal cell compartments called organelles and other particles within the cell. It moves organelles and other particles around within the cell. It interacts with extracellular structures, helping anchor the cell in place.

The interior of a cell is supported by a meshwork of protein filaments called the cytoskeleton, literally "cell skeleton." Each type of filament is a polymer, made up of protein monomers. The cytoskeleton has several important roles:

Cytoskeleton

The network of microtubules, intermediate filaments, and microfilaments that gives a eukaryotic cell its shape and its capacity to arrange its organelles and to move.

Rough Endoplasmic Reticulum (RER)

The portion of the endoplasmic reticulum in which the outer surface has attached ribosomes.

achieving equilibrium of solute and solvent molecules in a solution.

The process of diffusion leads to

osmotic pressure

The property of aqueous solutions that determines the direction of osmotic water movement when water can move from one solution to another. When water moves by osmosis between two solutions, it always moves from the lower to the higher osmotic pressure. Each individual dissolved entity (regardless of chemical nature) contributes approximately equally to the osmotic pressure of a solution.

nucleoid

The region that harbors the chromosomes of a prokaryotic cell. Unlike the eukaryotic nucleus, it is not enclosed by a membrane.

•Complexes of rRNA & and ribosomal proteins • Where proteins are synthesized in a process called translation, using mRNA instructions derived from the DNA

The ribosome: the cell's protein-making factory

lipid composition (Phospholipids with longer fatty acid chains have more bonds with one another than those with shorter chains and thus have lower fluidity. Saturated phospholipids pack more tightly together than those with kinked, unsaturated fatty acids, resulting in less fluid membranes. Cholesterol is able to alter interactions among the fatty acid side chains and alter membrane fluidity. At low temperatures cholesterol increases membrane fluidity, and at high temperatures it decreases fluidity. In plants, phytosterols are similar in structure to cholesterol and modify fluidity in plant cell membranes in a similar manner.) and Temperature (Membrane fluidity declines under cold conditions because molecules move more slowly at lower temperatures. Changes in fluidity can alter the function of a membrane. A familiar example is when your fingers get numb after contact with ice, which causes a reduction in membrane fluidity in the nerve cells, which decreases nerve activity. To overcome the potentially deleterious effects of reduced fluidity at low temperature, organisms can alter the lipid composition of their membranes in cold areas of their body, or when their environment gets colder. Specifically, in cells that experience colder temperatures, phospholipids that have saturated, long fatty acid chains can be replaced with those that have unsaturated, short chains, and the amount of cholesterol can be altered appropriately. Such changes increase fluidity and are critical for the survival of plants, bacteria, and hibernating animals during winter.)

The two most important factors influencing membrane fluidity are:

50

There are at least ___________ different kinds of intermediate filaments that are grouped into six different molecular classes based on their amino acid sequence. Many occur in just a few cell types in animals. Intermediate filaments are tough, ropelike protein assemblages 8-12 nm in diameter (see Figure 4.12B). They are more permanent than the other two types of filaments and do not show dynamic instability.

phagocytosis, pinocytosis, and receptor-mediated endocytosis:

There are three types of endocytosis:

a concentration gradient established by a previous reaction to drive transport.

There are two basic types of active transport. A main difference between them is that only one uses

Storage (Like all cells, plant cells produce a variety of toxic by-products and waste products. Plants store many of these in vacuoles. Because they are poisonous or distasteful, these stored materials deter some animals from eating the plants, and may thus contribute to the plants' defenses and survival.) Structure (In many plant cells, enormous vacuoles take up more than 90 percent of the cell volume and grow as the cell grows. The presence of dissolved substances in the vacuole causes water to enter it from the cytoplasm -which in turn takes up water from outside the cell-, making the vacuole swell like a water-filled balloon. The plant cell wall resists the swelling, causing the cell to stiffen from the increase in water pressure. This turgor pressure helps support the plant.) Reproduction (Some pigments in the petals and fruits of flowering plants are contained in vacuoles. These pigments—the red, purple, and blue anthocyanins—are visual cues that help attract animals, which assist in pollination and seed dispersal.) Catabolism (In the seeds of some plants, the vacuoles contain enzymes that hydrolyze stored seed proteins into monomers. The developing plant seedling uses these monomers as building blocks and sources of energy.)

There can be one large vacuole or many small ones in a cell. Plant vacuoles have several functions:

movement by diffusion of O2 in H2O

Time to move a net distance by diffusion: 1 mm = 10^-3 seconds 1 mm = 1 hour 10 m = 1000 years! (Diffusion is efficient mode of transport only over a short distance)

Membrane lipids in the phosphorus-poor ocean

Typically, 20-30% of phosphorus in a cell is used for membrane lipids Much of the ocean is very low in phosphorus What can you do if you live in a P-poor environment? Membrane lipids are amphipathic molecules, but not all are phospholipids Examples of different non-phosphorus polar head groups (shaded) illustrates how membrane lipid structure may vary

Several water molecules move one after the other through the opening provided by a channel protein, while glucose is transported across the membrane via a carrier protein one molecule at a time.

Water moves across a cell membrane through a channel protein, whereas glucose moves across a cell membrane via a carrier protein. Which statement provides a valid comparison of these two membrane transport systems?

the hereditary material that directs cell functions

What does the nucleoid region of a prokaryotic cell contain?

Our current understanding is that Eukarya originated from within a group of Archaea called the Asgard Archaea

What was the host cell that engulfed the ancestral mitochondrion?

concentrations

When determining whether osmosis will occur between two solutions separated by a membrane, one must know the ______________ of each solute in the two solutions, and the permeability of the membrane to the solutes.

Pancreatic digestive enzymes by exocytosis

Which is a correct pairing of a substance and the process by which it is transported across cell membranes?

forming the receptors that bind a specific molecule

Which is a function of the proteins lining the extracellular surfaces of coated pits involved in receptor-mediated endocytosis?

receptor-mediated endocytosis

Which is the most likely method for a cell to acquire molecules of one particular compound from the environment?

The cell membrane undergoes compositional change.

Which occurs during both endocytosis and exocytosis?

all of the above

Which of the following are examples of passive transport? A.Diffusion. B.Facilitated diffusion with a channel protein. C.Facilitated diffusion with a carrier protein. D.All of the above. E.None of the above.

DNA and Ribosomes

Which of the following features are shared by all cells?

small, hydrophobic molecules like O2

Which of the following kinds of molecules can freely cross the cell membrane?

D. all of the above

Which of the following statements about members of the Eukarya are correct? A.Their cells exhibit greater compartmentalization than those of prokaryotes. B.Most are single-celled microorganisms. C.They are more closely related to Archaea than they are to Bacteria. D.All of the above. E.None of the above.

Intermediate filaments

Which of the following structures is (are) involved with anchoring cell structures within a cell?

Phenylalanine, leucine, valine

Which sequence of amino acids in an integral membrane protein would most likely be found in the internal region of the cell membrane?

The external environment is hypotonic with respect to the trout's cells, so the trout must protect itself from water moving into cells from the outside.

Which statement correctly describes a trout that lives in fresh water?

fast; slower; slow

While diffusion is quite _______ in gases, it is substantially _________ in liquids such as water. Diffusion is generally a very ________ process in living tissues except over very short distances.

flip over to the other side

While phospholipid molecules can easily move laterally within a membrane, it is rare for a phospholipid in one half of the bilayer to spontaneously ________________. For that to happen, the polar part of the molecule would have to move through the hydrophobic interior of the membrane. Since spontaneous flip-flops are rare, the inner and outer halves of the bilayer may be quite different in the kinds of phospholipids they contain.

Microfilaments

______ determine cell shape and function in cell movement. Microfilaments are actin polymers, each about 7 nanometers (nm) in diameter, that are assembled from actin monomers that usually add to the filament at one end (the "plus end") and are removed at the other end (the "minus end")

plastids

__________ are present in the cells of plants and algae, and like mitochondria, contain circular DNA molecules and can divide autonomously. There are several types of plastids, each specialized for the storage of polysaccharides or pigments. An important type of these is the chloroplast

microtubules

____________ are long, hollow, unbranched cylinders about 25 nm in diameter. Microtubules are polymers of the protein tubulin. Tubulin dimers consist of one molecule each of α-tubulin and β-tubulin. Each microtubule cylinder consists of 13 chains of tubulin dimers surrounding the hollow center. Like microfilaments, microtubules show dynamic instability, with plus and minus ends and various associated proteins. However, unlike actin microfilaments that are stable in length when rates of assembly and disassembly are equal, microtubules have a growth phase, followed by a phase where no assembly or disassembly occurs, and then a rapid disassembly phase. Both assembly and disassembly occur from the same (plus) end.

membrane fluidity

____________ is important for function. Because phospholipids bond with one another through van der Waals interactions, they move laterally in the membrane quite easily, as these weak bonds break and re-form. A membrane composed only of phospholipids would be about as fluid as olive oil.

intermediate filaments

_____________ anchor cell structures in place. In some cells, they help maintain the positions of the nucleus and other organelles in the cell. Intermediate filaments also resist tension.

all cells need

_______________ to take in nutrients, to remove waste products (molecules to find each other in the cell) (e.g., for chemical reactions)

facilitated diffusion; channel proteins

_________________ is protein-assisted passive transport of molecules across the membrane from areas of higher to lower concentration •__________________like aquaporin provide hydrophilic passages for molecules to traverse membrane

Carrier proteins

________________facilitate diffusion of polar molecules by binding its substrate on one side of the membrane and releasing it on the other •Example: glucose transporters

small

________cells have a high surface area-to-volume ratio

Glucose

________is the major energy source for cells in most organisms, and so many cell membranes contain glucose transporters. Since glucose is usually broken down as soon as it enters the cell, there is almost always a concentration gradient favoring glucose entry (that is, a higher concentration outside the cell than inside).

biofilm

a community of microorganisms embedded in a polysaccharide matrix, forming a highly resistant coating on almost any moist surface

plasmodesmata

a cytoplasmic strand connecting two adjacent plant cells

nuclear envelope

a double membrane that encloses the cell nucleus

collagen

a fibrous protein found extensively in bone and connective tissue

proteoglycans

a glycoprotein containing a protein core with attached long, linear carbohydrates sides chains

motile cilia

a hairlike organelle used for locomotion by many unicellular organisms and for moving water and mucus by many multicellular organisms.

cell membrane

a lipid bilayer also containing proteins and other molecules that enclose the cytoplasm of the cell and separates it from the surrounding environment (performs the vital role of keeping the internal chemicals of a cell, which carry out life's functions, different from the chemical composition outside the cell)

glycolipids

a lipid to which sugars are attached

Primary lysosomes

a lysosome before fusion with an endosome

extracellular matrix

a material of heterogeneous composition surrounding cells and performing many functions, including adhesion of cells

Receptor-mediated endocytosis (endocytosis initiated by macromolecular binding to a specific membrane receptor)

a mechanism for bringing specific large molecules, recognized by specific receptors, into the cell. The receptors bind their specific ligand and then migrate laterally in the membrane to pits on the surface of the cell membrane. These pits are slight depressions that have their cytoplasmic surfaces coated by a protein, which is often clathrin. Clathrin (or other protein) molecules strengthen and stabilize the pit and the vesicle after it forms. The uptake process is similar to that in phagocytosis. However, in receptor-mediated endocytosis, the receptor is specific for a given macromolecule or aggregate, making it possible to control the abundance in the cell of each type of ingested molecule.

primary cilium

a nonmotile hairlike organelle used for signaling in vertebrate cells

phytosterols

a plant compound with structural similarity to cholesterol. Affects membrane fluidity

exocytosis

a process by which a vesicle within a cell fuses with the cell membrane and releases its contents to the outside (moves materials out of the cell)

endocytosis

a processes by which liquids or solid particles are taken up by a cell through invagination of the cell membrane (a group of processes that bring small molecules, macromolecules, large particles, and even other cells into eukaryotic cells)

tubulin

a protein that polymerizes to form microtubules

glycoproteins

a protein to which sugars are attached

ribosomes

a structure in the cell that is the site of protein synthesis

ion channels

a transmembrane protein that allows specific ions to diffuse across the membrane in which it is embedded

aquaporins

a transport protein in plant and animal cell membranes through which water passes by osmosis (increase water permeability)

transmembrane protein

an integral membrane protein that spans the phospholipid bilayer (Proteins are asymmetrically distributed on the inner and outer surfaces of membranes, giving the two surfaces different properties. An integral membrane protein that extends all the way through the phospholipid bilayer and protrudes on both sides of a bilayer)

mitochondrion

an organelle in eukaryotic cells that contains the enzymes of the citric acid cycle, the respiratory chain, and oxidative phosphorylation

organelles

any of the membrane-enclosed structures within a eukaryotic cell. examples include the nucleus, endoplasmic reticulum, and mitochondria

Gap junctions (A 2.7-nanometer gap between cell membranes of two animal cells, spanned by protein channels. Gap junctions allow chemical substances or electric signals to pass from cell to cell.)

are channels that run between membrane pores in adjacent cells. Gap junctions allow substances such as ions, small molecules, and electric signals to pass between cells. In the heart, for example, gap junctions allow the rapid spread of electric current mediated by ions so the heart muscle cells can beat in unison.

glyoxysomes (An organelle found in plants, in which stored lipids are converted to carbohydrates.)

are organelles that carry out the conversion of stored lipids to carbohydrates. This pathway is unique to plants, so these organelles are found only in plant cells, particularly germinating seeds.

peroxisomes (An organelle that houses reactions in which toxic peroxides are formed and then converted to water)

are small (0.2−1 μm diameter) organelles that accumulate and break down toxic peroxides, such as hydrogen peroxide (H2O2), which occur as the by-products of some biochemical reactions in many eukaryotes.

archaea, bacteria, and eukarya

biologists classify all living things into three domains:

sugars and amino acids

carrier proteins transport polar molecules such as:

eukaryotic cells

cells that contain (or used to contain) a nucleus

passive transport

diffusion across a membrane; may or may not require an integral membrane protein

simple diffusion

diffusion that doesn't involve a direct input of energy or assistance by integral membrane proteins

Hypotonic

freshwater environments are _______________

Isotonic

having the same concentration of membrane-impermeable solutes; said of two solutions

cells are the fundamental units of life, all living organisms are composed of cells, and all cells come from preexisting cells

he cell theory, developed in the nineteenth century, was the first unifying principle of biology and has three critical components:

desmosomes (an adhering junction between animal cells.)

hold adjacent cells together with stable protein connections. This provides mechanical stability for tissues such as skin that are under stress due to movement. Desmosomes do, however, permit some materials to move around in the extracellular matrix between cells.

tight junctions, desmosomes, and gap junctions

in multicellular organisms, cells are joined via specialized cell junctions. what are three types of junctions in animals:

selective permeability

in reference to membranes, the characteristic of allowing certain substances to pass through while other substances are excluded

hydrophobic

lipids form the _________ core of the membrane

receptor-mediated endocytosis

mammalian cells acquire specific macromolecules from their environments by the process of:

charged (ions-H+, Na+), large uncharged polar (sugar) (impeded by the hydrophobic interior of the membrane)

many important molecules cannot freely cross the membrane

tubulin, and they are essential in chromosome movement during mitosis.

microtubules are made of

osmosis

movement of water across a selectively permeable membrane, from a hypotonic solution to a hypertonic solution

the Endosymbiont Theory II

origin of the chloroplast

the endosymbiont theory I

origin of the mitochondrion

facilitated diffusion

passive movement through a membrane involving a specific integral membrane protein; does not proceed against a concentration gradient

nuclear pores

perforations in the nuclear envelope through which the regulated passage of large molecules occurs

tight junctions (a junction between epithelia cells in which there is no gap between adjacent cells)

prevent substances from moving through spaces between cells. For example, the epithelium of the urinary bladder contains tight junctions to prevent urine from leaking into the adjacent abdomen.

peripheral membrane proteins

proteins associated with but not embedded within the cell membrane

anchored membrane proteins

proteins that are associated with the membrane by covalent attachments to lipids.

integral membrane proteins

proteins that are at least partially embedded in the cell membrane

diffusion

random movement of molecules or other particles, resulting in even distribution of the particles when no barriers are present (a net movement from regions of higher concentration to regions of lower concentration.)

composed of protein and nucleic acid components

ribosomes are:

Surface area-to-volume ratio

small cell size is a practical necessity arising from the decrease in the ______________ for an object that increases in size but remains the same shape. For example, the surface area and volume of a sphere are proportional to the radius squared and cubed, respectively. Thus doubling the radius of a sphere increases the surface area by fourfold and increases the volume by eightfold, resulting in the surface area-to-volume ratio declining by twofold

integral membrane proteins (are at least partly embedded in the phospholipid bilayer and thus have both exposed hydrophobic regions that interact with the interior of the membrane, and exposed hydrophilic regions that interact with the aqueous environment external to the membrane.), anchored membrane proteins (are covalently attached to fatty acids or other lipid groups but otherwise have no exposed hydrophobic regions. The hydrophobic lipid groups insert into the phospholipid bilayer and hold these proteins in association with the membrane.), and peripheral membrane proteins (lack exposed hydrophobic groups and are not embedded in the bilayer. Instead, they have polar or charged regions that interact with exposed parts of integral membrane proteins, or with the charged heads of phospholipid molecules.)

some amino acids contain nonpolar hydrophobic R groups, whereas others contain polar or charged hydrophilic R groups. The arrangement of these amino acids in a membrane protein determines whether the membrane protein will insert into the nonpolar lipid bilayer and how it will be positioned. Membrane proteins and lipids generally interact noncovalently. The polar or charged regions of proteins interact with the charged heads of phospholipids, and the nonpolar hydrophobic regions of both molecules interact. There are three general types of membrane proteins:

Hydrophobic (CO2, O2, N2) and small uncharged polar (H2O-slowly)

some molecules can freely cross the membrane barrier

catabolism

the breakdown of complex molecules into simpler ones; releases energy

helps maintain the internal environment of the cell

the cell membrane

dynamic instability

the co-occurrence of addition (assembly) and removal (disassembly) of subunits from microfilament and microtubules

cytosol

the fluid portion of the cytoplasm, excluding organelles and other solids (intracellular fluid of the cell)

turgor pressure

the force exerted by a plant cell against the cell wall when it has taken up water by osmosis

DNA

the nucleus contains most of the cell's ________, and is usually the largest organelle.

anabolism

the synthesis of complex molecules from simpler ones; requires an input of energy and captures it in the chemical bonds that are formed

osmolarity

the total number of solute particles per liter of water

Primary active transport (Active transport in which ATPATP is hydrolyzed, yielding the energy required to transport an ion or molecule against its concentration gradient.)

uses chemical-bond energy released during the hydrolysis of ATP or another chemical reaction, or light energy, to transport a solute against its concentration gradient. For example, the concentration of potassium ions (K+) inside a cell is often much higher than the concentration in the fluid bathing the cell. In contrast, the concentration of sodium ions (Na+) is often much higher outside the cell. These gradients are maintained by an integral protein in the cell membrane that pumps Na+ out of the cell and K+ into the cell. Appropriately called the sodium-potassium(Na+−K+)pump, this integral membrane-spanning protein catalyzes the hydrolysis of a molecule of ATP to ADP and a free phosphate ion (Pi) and uses the transferred energy to bring two K+ ions into the cell and to export three Na+ ions

Secondary active transport (A form of active transport that does not use ATPATP as an energy source; rather, transport is coupled to ion diffusion down a concentration gradient established by primary active transport. Also called coupled transport or co-transport.)

uses the energy stored in an ion concentration gradient or an electrical gradient, previously generated by primary active transport, to transport the solute against its concentration gradient. Secondary transport is often termed coupled transport or co-transport because two substances are simultaneously transported across a membrane by one protein. For example, once the Na+-K+ pump establishes a concentration gradient of sodium ions by primary active transport, the passive diffusion of some Na+ back into a cell can provide energy for the secondary active transport of glucose into the cell. The coupling of Na+ movement down its concentration gradient with glucose transport up its concentration gradient is achieved by the sodium-glucose co-transporter. This co-transporter is used when glucose is absorbed from the digestive tract, where it is at low concentration, into the bloodstream, where it is at high concentration. In some cases of secondary active transport, the ion and the transported molecule move in opposite directions, whereas in other cases they move in the same direction (as for glucose and Na+ in the cells lining the digestive tract). Secondary active transport aids in the uptake of amino acids and sugars, which are essential raw materials for cell maintenance and growth.

primary active transport and secondary active transport

what are the two basic types of active transport?

(Hämmerling's experiments with Acetabularia on the role of the nucleus (1940s)) Nucleus is located in the base of this single-celled organism

what is the role of the nucleus?

the cell

what is the simplest structural unit of a living organism?

they are the sites where autophagy occurs

what is true about lysosomes?

vesicles

within the cytoplasm, a membrane-enclosed compartment that is associated with other organelles; the Golgi apparatus is one example

equation for osmotic pressure (where c is the osmolarity, R is the gas constant, and T is the absolute temperature -R and T may be familiar to you from your study of chemistry-. Note that osmotic pressure is proportional to the osmolarity 'c'—the more dissolved solutes there are, the higher the osmotic pressure.)

Π = cRT

Mitochondria and Chloroplast DNA

• Both have DNA • DNA is in the form of a closed circle, like bacteria • These DNA circles are much smaller than the chromosomes of most bacteria ... many genes have been transferred to the nucleus or simply lost over time

the nucleus

• Compartment surrounded by a double membrane (nuclear envelope) • Membrane-enclosed compartments = organelles • Houses most of the cell's genetic information (DNA) and is where transcription occurs in eukaryotes

Functions of smooth ER

• Compartment with enzymes for the synthesis of lipids, as well as diverse functions that vary with cell type • Example: detoxification enzymes in liver cells

Endoplasmic Reticulum (ER)

• Extensive membrane network continuous with the outer membrane of the nuclear envelope • Rough ER coated with ribosomes on the surface of the membrane; Smooth ER lacks ribosomes

membrane proteins and transport

• How do cells get other dissolved molecules (solutes) across this hydrophobic barrier? • Membrane also contains proteins à fluid-mosaic model • Many proteins involved in solute transport

Nucleolus & ribosomal RNA

• Nucleolus is the site of transcription of RNA & protein components of ribosomes, as well as assembly of ribosome subunits • Ribosome subunits exported through pores to cytoplasm

membrane adaptation in Archaea

• Ratio of tetraether to diether lipids increases with increasing temperature • Tetraether lipids form monolayers that are highly stable due to restricted motility of the hydrocarbon chains

Energy

•Active transporters are proteins that move substances against their concentration gradient, which requires ________ •Example: the sodium-potassium pump, which creates a charge difference (voltage) across the cell membrane (membrane potential) •

cellular life

•All cellular organisms belong to one of three groups: Bacteria, Eukarya, or Archaea

prokaryotes versus eukaryotes

•All cellular organisms belong to one of three groups: Bacteria, Eukarya, or Archaea •Bacteria & Archaea are prokaryotes •Eukarya are eukaryotes

key points

•All membrane lipids are amphipathic molecules that self-assemble into a stable boundary between cell & environment •Phospholipids are the most abundant lipids of most but not all membranes •On the Tree of Life, there is variation in the nature of the hydrophobic region of phospholipids & how it is linked to glycerol •Some organisms primarily use membrane lipids other than phospholipids

about cells

•All organisms are composed of cells, and all cells come from preexisting cells • All cells share certain basic features

fatty acid saturation and fluidity

•At a given temperature, membranes with a higher proportion of unsaturated fatty acids are more fluid

evidence that they were "eaten"

•Both mitochondria and chloroplasts have double membranes • This suggests that they had been engulfed by a eukaryotic host (phagocytosis) but retained rather than digested

Epulopiscium and the wrinkled membrane

•Cell membrane not smooth ... wrinkles SA (& SA/V) • Molecules don't have far to diffuse • Not violating any fundamental biological principle!

organisms in isotonic environments

•Cells of most terrestrial animals are bathed in isotonic fluid •Seawater is isotonic to marine invertebrates like this larvacean •This achieves water balance: no net water movement into or out of the cell, so no need to osmoregulate

DNA packaging in the nucleus

•DNA packaged in units called chromosomes • DNA wrapped around histone proteins to form nucleosomes • Nucleosomes aggregate to form dense chromatin

diffusion across the cell membrane

•Diffusion is the process by which molecules move from areas of higher to lower concentration as a consequence of their random movements as they collide with water molecules •Diffusion directly across the cell membrane by a small nonpolar molecule requires no energy input from the cell: an example of passive transport

cell membrane diversity

•Domains Eukarya & Bacteria both have: à Phospholipid bilayer à Hydrophobic part made of fatty acids à Fatty acids ester-linked to glycerol à (This is the phospholipid structure we discussed earlier in class) •Membranes of Archaea: à Phospholipid bilayer OR monolayer à Hydrophobic part made of repeated isoprene units à Hydrophobic side chains ether-linked to glycerol

tour of cell organization

•Eukaryotic cells organized in compartments • Different local environments for specialized cell functions • Creates a smaller space for molecules to find each other

direction

•Facilitated diffusion speeds up passive transport across the membrane... •But does not alter the ____________of transport - it is always down the concentration gradient •Many solutes in a cell are required at concentrations that are either much higher or lower than the environment: How is this accomplished?

rapidly

•For the average prokaryote, this will happen ___________ by diffusion (mm / ms) • Larger cells (e.g., eukaryotes) tend to compartmentalize molecules to increase their odds of finding each other

•For example, Paramecium controls solute concentration and water balance (osmoregulation) by expelling excess water from its contractile vacuole

•How do organisms deal with hypotonicity?

membrane fluidity and function at temperature extremes

•If temperature is too low, membrane lipids will pack closely into a gel phase, which impairs protein movement & function • If temperature is too high, hexagonal phase aggregates will form, disrupting the bilayer and destroying the boundary between cell & environment

hypertonic

•In _________________ environments (low water availability), solute concentration is higher outside of cell • H2O will tend to flow out of cell

SA:V issues set limits on cell size and shape

•Larger (multicellular) organisms don't generally have larger cells than smaller organisms... just more cells • Cells that exchange a lot of material with their surroundings often increase SA without a large increase in V (e.g., microvilli of intestinal cells)

Energy

•Many solutes need to be at a higher concentration in the cell than in the environment, which isn't possible by diffusion •Active transporters are proteins that move substances against their concentration gradient, which requires _________

high temperature

•Membrane lipids of bacteria living at ________________ are rich in saturated fatty acids... have a high melting temperature, maintain membrane integrity at higher temperatures

membrane fluidity and function

•Membranes must be fluid to work properly •Membrane fluidity affects both its permeability & membrane protein function •The proportion of saturated vs unsaturated fatty acids is a major determinant of membrane fluidity in bacterial & eukaryotic membranes

summary of key points

•Membranes must be fluid to work properly •Membrane fluidity is influenced by membrane lipid composition & temperature •Membrane lipid composition is well-matched to an organism's optimal physiological temperature •Many organisms can adjust membrane lipid composition in response to a change in the temperature of the environment

Mitochondria and Chloroplasts (where did they come from? - •1905: Konstantin Mereschkowsky proposes that chloroplasts were derived from photosynthetic bacteria that had been engulfed by a eukaryotic cell based on similarities between how they divide • 1960s: Lynn Margulis independently has same idea, the Endosymbiont Theory)

•Mitochondrion: Organelle found in most eukaryotes responsible for much of the ATP production in the cell • Chloroplast: Organelle in plants and algae responsible for photosynthesis More on these organelles in "Cellular respiration & photosynthesis" (•Mitochondria & chloroplast DNAs encode their own ribosomes • Same size as bacterial ribosomes • Related to bacterial ribosomes)

navigating the tree of cellular life

•Modern organisms at the tips of the tree •Can follow each tip backwards in time, ultimately to the universal ancestor at the root of the tree •Relatedness is measured by recency of common ancestry (common ancestors are nodes where 1 branch splits into 2)

osmosis and its impact on cell water balance

•Osmosis: diffusion of water molecules across a membrane ... the direction of net water movement depends on the nature of the concentration gradient between cell and environment • •Here, the solute concentration inside of cells is higher than in the environment (free water concentration is lower in the cell), so net water movement is into the cell • •We call such environments hypotonic (tonicity measures the ability of a solution to cause a cell to gain or lose water) (water flows into the cell) •Freshwater environments are hypotonic • •How do organisms deal with hypotonicity? • •For example, Paramecium controls solute concentration and water balance (osmoregulation) by expelling excess water from its contractile vacuole

phospholipids can self-assemble into membranes

•Partly hydrophilic (water-soluble) • • Partly hydrophobic (water-insoluble) • • Can self-assemble in water into a stable bilayer that shields the hydrophobic region, exposes the hydrophilic heads to water

Golgi Apparatus

•Receives transport vesicles from ER on its cis face for modification, storage and transport to other destinations from its trans face • Transport vesicles have molecules that recognize docking sites of destination

Functions of Rough ER

•Rough ER ribosomes produce proteins destined for export (secretion) from cell; contrast with proteins made by free ribosomes in the cytoplasm, which remain in cell • These proteins exit ER via transport vesicles

Glycolipids & aminolipids in SAR11 bacteria

•SAR11 bacteria are the most abundant organisms in the ocean (~1028 cells) • • When P is limited, these bacteria produce a mix of glycolipids & aminolipids instead of phospholipids

cell membrane

•Separates inside (of cell) from outside (environment) • Fluid-mosaic model: the membrane is a mosaic of proteins embedded in a fluid matrix of membrane lipids • Selective permeability: some molecules cross it more easily than others

surface area (SA) to volume (V) effects

•Smaller cells also have more SA per unit volume V, since volume increases faster than SA • Can more efficiently exchange molecules with environment • Tend to grow faster

Halophilic Archaea: life at high salinity

•Some Archaea can live at salt concentrations 10x greater than seawater! • These organisms raise solute concentration inside of the cell by pumping in potassium (K+) •This prevents the cell from losing water to the environment • Osmoregulatory molecules like this are called compatible solutes

Sulfolipids in Prochlorococcus

•The cyanobacterium Prochlorococcus is responsible for much of the photosynthesis in the (sub)tropical ocean • • When P is limited, Prochlorococcus uses sulfolipids instead of phospholipids! • • Less than 1% of phosphorus taken up from the environment is incorporated into membranes - reducing the demand for P •

increases

•The net displacement of a molecule from its starting point does not increase linearly with time by diffusion... • • It ___________ with the square root of time (√t)

summary of diffusion in small cells

•We have just seen how diffusion works well for small organisms like an average-sized bacterium • Microorganisms also have high SA:V for efficient diffusion

A. mediterranea

•What will the cap of this chimeric cell look like? A.A. mediterranea B.A. crenulata