Chapter 4 The Cell Membrane Separates the Interior of the Cell from Its Environment

myelin

which encloses portions of some neurons (nerve cells) and acts as an electrical insulator, have only one protein for every 70 lipids.

actin

A protein that makes up the cytoskeletal microfilaments in eukaryotic cells and is one of the two contractile proteins in muscle. (See also myosin.)

Biologists classify all living things into three domains:

Archaea, Bacteria, and Eukarya

eukaryotes

(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

1. phagocytosis

-("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

The cytoskeleton has several important roles:

-It supports the cell and maintains its shape. -It maintains the position of internal cell compartments called organelles (described in Key Concept 4.5) 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.

Info:

-It takes a completely different mechanism than diffusion to move intact large molecules across membranes. -Both processes are possible because membranes can fuse with and bud off vesicles quite easily. (exocytosis and endocytosis) -Cholesterol and triglycerides, which have low solubility in water, are packaged by liver cells into lipoprotein particles which are released into the bloodstream. -When a particular cell requires cholesterol, it produces low-density lipoprotein (LDL) receptors, which are inserted into the cell membrane and allow LDL binding and subsequent endocytosis. In most people, the liver takes up unused LDLs for recycling.

Microfilaments (more info)

-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") -In a filament of constant length, assembly and disassembly are in equilibrium, such that the two processes occur at the same rate. The relative rates of assembly and disassembly are not the same, however, in filaments that shorten or lengthen. Assembly and disassembly may be altered by the binding of other, non-actin proteins at the two ends. The combined effects of adding monomers (assembly) and removing them (disassembly) and the roles of extra proteins that can regulate how much these processes occur make the lengths of microtubules quite variable, a situation termed dynamic instability. -Microfilaments are important in maintaining overall cell shape and causing localized shape changes in regions of the cell. -For example, microfilaments are involved in the flowing movement of the cytoplasm called cytoplasmic streaming, -Microfilaments are also involved in the formation of cellular extensions called pseudopodia (pseudo, "false," + podia, "feet") that enable some cells, such as Amoeba, to move

Microtubules (More Info)

-Microtubules are long, hollow, unbranched cylinders about 25 nm in diameter. Microtubules are polymers of the protein tubulin.

Flagella and Cilia

-Motile cilia and flagella are involved in movement either of the entire cell (as in protists, a group of unicellular eukaryotic organisms) or, in some ciliated cells, of fluid across the cell (as in the human respiratory system). -The microtubules that are found in motile cilia and flagella not only support these structures but also cause them to bend, which moves them. -Movement of cilia and flagella is driven by a motor protein called dynein that is present on the outer nine pairs of microtubules.

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

-The diameter of the molecules or ions: smaller molecules diffuse faster. -The temperature of the solution: higher temperatures lead to faster diffusion because particles have higher thermal energy and thus move faster. -The 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. -The 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). As you will see in Chapter 30, keeping the distance a solute must diffuse as short as possible maximizes the diffusion rate.

Info:

-The volume 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 surface area determines the rate at which resources can enter and wastes can leave a cell. -As the surface area-to-volume ratio declines, it becomes more difficult for a cell to obtain sufficient resources and eliminate waste per of unit time, which favors small cell size. In addition, substances must move from one location to another within the cell; the smaller the cell, the more easily this is accomplished.

intermediate filaments (more Info)

-There are at least 50 different kinds of intermediate filaments that are grouped into six different molecular classes based on their amino acid sequence. -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. -Intermediate filaments anchor cell structures in place -Intermediate filaments also resist tension.

The extracellular matrices of animal cells have several roles:

-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.

There are three general types of membrane proteins:

1. Integral membrane proteins 2. Anchored membrane proteins 3. Peripheral membrane proteins

The two most important factors influencing membrane fluidity are:

1.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 2.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. -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.

endocytosis

A process by which liquids or solid particles are taken up by a cell through invagination of the cell membrane. (Contrast with exocytosis.) -Endocytosis is a general term for a group of processes that bring small molecules, macromolecules, large particles, and even other cells into eukaryotic cells

glucose transporters

A carrier protein used to facilitate diffusion of the monosaccharide glucose.

biofilm

A community of microorganisms embedded in a polysaccharide matrix, forming a highly resistant coating on almost any moist surface. RE: Some bacterial cells also produce an extracellular matrix under some conditions. This matrix is especially important when bacteria form large aggregations of cells fixed to a surface in a so-called biofilm. -The bacterial extracellular matrix facilitates communication among cells and the exchange of metabolites and genetic information.

collagen

A fibrous protein found extensively in bone and connective tissue.

Secondary active transport

A form of active transport that does not use ATP 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. (Contrast with primary active transport.) -Secondary active transport aids in the uptake of amino acids and sugars, which are essential raw materials for cell maintenance and growth.

cytoplasmic streaming

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

proteoglycans

A glycoprotein containing a protein core with attached long, linear carbohydrate side chains. -when there are many carbohydrate residues

Sub: motile cilia

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

cilia

A hairlike organelle used for locomotion or signaling. Generally shorter than a flagellum. -Cilia are short, only 0.25 micrometer in length, and can occur singly, as a primary cilium, or in tens to hundreds of motile cilia.

cell membrane

A lipid bilayer also containing proteins and other molecules that encloses the cytoplasm of the cell and separates it from the surrounding environment. -Typically, cell membranes have about 1 protein molecule for every 25 phospholipid molecules. This ratio varies depending on membrane function.

glycolipids

A lipid to which sugars are attached. -While glycolipids can have important effects on membrane fluidity, in general carbohydrates associated with membranes function in cell recognition.

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.

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. -describes membrane structure in terms of random movement ("fluid") and separateness of the proteins and lipids ("mosaic").

Sub: primary cilium

A nonmotile hairlike organelle used for signaling in vertebrate cells. -In the primary cilium, the central pair of unfused microtubules is missing and the cilium is unable to move. -The primary cilium is important in signaling rather than in movement. Primary cilia have been shown to detect a variety of stimuli, including various chemical signals, light, gravity, and changes in osmolarity and temperature. -They are also involved in setting up the left-right axis during vertebrate development; abnormal primary cilia can result in organs being on the wrong side of the body.

phytosterols

A plant compound with structural similarity to cholesterol. Affects membrane fluidity. -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.

tubulin

A protein that polymerizes to form microtubules. -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. -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. -Microtubules form a rigid internal skeleton for some cells or cell regions, and they act as a framework along which motor proteins such as kinesins can move various organelles and other items within the cell.

glycoproteins

A protein to which sugars are attached.

cell wall

A relatively rigid structure exterior to the cell membrane that encloses the cell. -RE: 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).

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. -Although the phospholipid bilayer is somewhat permeable to water molecules, many types of cells have specialized protein channels, called aquaporins, that increase water permeability.

3. microtubules

A tubular structure composed of tubulin dimers found in the centrioles, spindle apparatus, cilia, flagella, and cytoskeleton of eukaryotic cells. Microtubules play roles in the motion and maintenance of shape of eukaryotic cells. -which are the largest in diameter and made of tubulin

Primary active transport

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

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. -uses the transferred energy to bring two K+ ions into the cell and to export three Na+ions

transmembrane protein

An integral membrane protein that extends all the way through the phospholipid bilayer and protrudes on both sides of a bilayer.

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. -As the name indicates, channel proteins form channels in the membrane through which certain substances can pass.

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. -Carrier proteins bind to specific substances and speed up their diffusion across the membrane.

mitochondrion

An organelle in eukaryotic cells that contains the enzymes of the citric acid cycle, the respiratory chain, and oxidative phosphorylation. -a compartment inside the cell specialized for energy production, there is 1 protein for every 5 lipids.

extracellular matrix

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. -A material of heterogeneous composition surrounding cells and performing many functions, including adhesion of cells.

organelles

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

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.

eukaryotic cells

Cells that contain (or used to contain) a nucleus -In eukaryotic cells (those with a nucleus; see Key Concept 4.5), including plants, fungi, protists and animals, large molecule transport involves packaging the molecules and a small volume of fluid into small, membrane-enclosed structures termed vesicles.

prokaryotic cells

Cells that do not contain (and have never contained) a nucleus, such as bacteria.

2. intermediate filaments

Cytoskeleton components with a diameter that falls between those of the larger microtubules and those of the smaller microfilaments.

osmosis

Movement of water across a selectively permeable membrane, from a hypotonic solution to a hypertonic solution. -The movement of water from low to high solute concentration disperses the energy in the solute molecules, by making the solute concentrations on the two sides of the membrane more similar.

2. Desmosomes

Desmosomes 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. -RE: An adhering junction between animal cells.

Passive transport

Diffusion across a membrane; may or may not require an integral membrane protein. (Contrast with active transport.) -Passive transport does not require an input of energy. For example, if a membrane is permeable to a particular substance, that substance can simply diffuse (as we describe below) across the membrane from a region of higher concentration to a region of lower concentration.

active transport

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 against their concentration gradients. This process requires the input of energy and is known as active transport.

simple diffusion

Diffusion that doesn't involve a direct input of energy or assistance by integral membrane proteins. -In general, membranes are more permeable to molecules that are small, nonpolar, uncharged, and hydrophobic; the more lipid-soluble the molecule is, the more rapidly it diffuses through the lipid bilayer.

Dynein

Dyneins cause the microtubule doublets to slide past one another in localized regions on one side of the cilium or flagellum, which causes the entire cilium or flagellum to bend in that location. -The movement of dynein contractions along the cilium or flagellum results in a powerful beat that is able to propel fluid relative to the cell.

4.4 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.

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. ex:

For example, phospholipids can differ in their fatty acid chain length (number of carbon atoms) and degree of unsaturation (number of double bonds in their fatty acids), and in the kinds of polar (phosphate-containing) groups present. 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.

3. Gap junctions

Gap junctions 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. -RE: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.

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.

isotonic

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

1. microfilaments

In eukaryotic cells, a fibrous structure made up of actin monomers. Microfilaments play roles in the cytoskeleton, in cell movement, and in muscle contraction.

cell junctions

In multicellular organisms, cells are joined via specialized cell junctions. -Specialized structures associated with the cell membrane that join cells in multicellular organisms. Some contribute to cell adhesion, others to intercellular communication.

2. pinocytosis

In pinocytosis ("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.

4.2 selective permeability

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

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.

Interaction between membrane proteins and lipids:

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.

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

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. These binding sites are noncovalent and reversible.

facilitated diffusion

Passive movement through a membrane involving a specific integral membrane protein; does not proceed against a concentration gradient. (Contrast with active transport, diffusion.)

Diffusion

Random movement of molecules or other particles, resulting in even distribution of the particles when no barriers are present.

Reminder of amino acids:

Recall from Figure 3.14 that some amino acids contain nonpolar hydrophobic R groups, whereas others contain polar or charged hydrophilic R groups.

3. Receptor-mediated endocytosis

Receptor-mediated endocytosis is 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 (FIGURE 4.10). 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.

coupled transport

Secondary transport is often termed coupled transport or co-transport because two substances are simultaneously transported across a membrane by one protein.

flagella

Some cells have projections from their cell membrane termed flagella (singular flagellum) and cilia (singular cilium) (FIGURE 4.15) that include a microtubule component. -A long, whiplike appendage that propels cells. Prokaryotic flagella differ sharply from those found in eukaryotes. -Flagella are long, 100-200 micrometers, and occur singly, in pairs, or up to several per cell.

cell theory

States that cells are the basic structural and physiological units of all living organisms, and that all cells come from preexisting cells. -Cells are the fundamental units of life. -All living organisms are composed of cells. -All cells come from preexisting cells.

4.5 catabolism

The breakdown of complex molecules into simpler ones; releases energy. (Contrast with anabolism.)

dynamic instability

The co-occurrence of addition (assembly) and removal (disassembly) of subunits from microfilaments and microtubules. Re: The combined effects of adding monomers (assembly) and removing them (disassembly) and the roles of extra proteins that can regulate how much these processes occur make the lengths of microtubules quite variable.

cytoplasm

The contents of the cell, excluding the nucleus.

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. -Active transport, by contrast, requires an input of energy and the assistance of specialized membrane proteins.

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.

cytosol

The fluid portion of the cytoplasm, excluding organelles and other solids.

turgor pressure

The force exerted by a plant cell against the cell wall when it has taken up water by osmosis. -it keeps the green parts of plants upright and is the driving force for enlargement of plant cells

cytoskeleton

The interior of a cell is supported by a meshwork of protein filaments called the cytoskeleton, literally "cell skeleton." -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. -each type of filament is a polymer, made up of protein monomers.

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. -is defined as the pressure that needs to be applied to a solution to prevent the flow of water across a membrane by osmosis.

anabolism

The synthesis of complex molecules from simpler ones; requires an input of energy and captures it in the chemical bonds that are formed. (Contrast with catabolism.)

1. Tight junctions

Tight junctions 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. -RE: A junction between epithelial cells in which there is no gap between adjacent cells.

Three types of junctions in animals:

Tight junctions, Desmosomes, and Gap junctions

4.3 secrete

To discharge a substance from a cell or gland.

exocytosis

Transport of large molecules or particles out of the cells via vesicles is called exocytosis, and import of such molecules is called endocytosis. -Exocytosis is the process by which materials packaged in vesicles are secreted from the cell (see Figure 4.9A)

prokaryotes

Unicellular organisms that do not have nuclei. Not a monophyletic group; as commonly used, includes the bacteria and prokaryotic archaeans. A term of convenience encompassing all cellular organisms that are not eukaryotes. (Contrast with eukaryotes.) -RE: which are generally small (0.1−5 μm in diameter), single-celled, lacking a nucleus, and usually without membrane-enclosed internal compartments -Prokaryotes also possess tiny encapsulin nanocompartments, 25-30 nanometers in diameter. -Like microcompartments, these structures are not enclosed inside a lipid-based membrane, but instead are enclosed by a protein (encapsulin in the case of microcompartments). -Another compartment, which has a different kind of function, is the gas vesicle (45 to over 100 nm in diameter), which is found in some bacteria and archaea and give cells buoyancy in the aquatic environments in which they float. Gas vesicles are used to position the cell at an optimal depth in the water column for nutrient acquisition.

osmolarity

We quantify osmotic pressure of a solution by measuring its osmolarity, which is the total number of solute particles per liter of water (the concentration of solute particles). The equation for osmotic pressure is Π = cRT where c is the osmolarity, R is the gas constant, and T is the absolute temperature

plasmodesmata

While plant cells are joined together by their cell walls (see Figure 4.16B), communication between adjacent cells is made possible by numerous cell membrane-lined channels called plasmodesmata. -A cytoplasmic strand connecting two adjacent plant cells. -Plasmodesmata are similar to gap junctions between animal cells, but they allow passage of larger substances. Plasmodesmata allow water, ions, small molecules, hormones, and even some RNA and protein molecules to move between connected cells. -In this way, energy-rich molecules such as sugars can be shared among plant cells, and plant hormones can affect growth at sites far from where the hormones were synthesized.

vesicles

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

The cytoskeletal protein filaments are classified into three groups:

microfilaments, intermediate filaments, and microtubules

There are three types of endocytosis:

phagocytosis, pinocytosis, and receptor-mediated endocytosis

There are two basic types of active transport:

primary and secondary

The organisms in Archaea and Bacteria are collectively called ->

prokaryotes