Cell Structure and Function

cholesterol

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diffusion

1. The passive movement of molecules or particles along a concentration gradient, or from regions of higher to regions of lower concentration. 2. The spontaneous net movement of particles down their concentration gradient (i.e. difference in the concentrations of substances or molecules between two areas). 3. (Cell biology): a type of passive transport, therefore, it is a net movement of molecules in and out of the cell across the cell membrane along a concentration gradient. Unlike active transport, diffusion does not involve chemical energy. When molecules move (diffuse) via special transport proteins found within the cell membrane, it is called facilitated diffusion, otherwise it is only simple diffusion. An example of diffusion in biological system is diffusion of oxygen and carbon dioxide across the alveolar-capillary membrane in mammalian lungs.

hypotonic

1. having a lesser degree of tone or tension, as in a 'hypotonic muscle' 2. having a lesser osmotic pressure in a fluid compared to another fluid, as in a 'hypotonic solution' - compare hypertonic, isotonic 3. refers to a solution with a comparatively lower concentration of solutes compared to another

isotonic

2. Pertains to a muscular contraction in which the muscle remains to be in a relatively constant tension while its length changes, as in isotonic muscle. 3. Isoosmotic, i.e. having the same (or equal) osmotic pressure and same water potential since the two solutions have an equal concentration of water molecules. Mahesh. 4. Pertaining to a solution that has the same tonicity as some other solution with which it is compared. For example, blood serum is isotonic to a physiologic salt solution. Solutions that have same tonicity will result in no net flow of water across the cell membrane.

transport proteins

A transport protein (variously referred to as a transmembrane pump, transporter protein, escort protein, fatty acid transport protein, cation transport protein, or anion transport protein) is a protein which serves the function of moving other materials within an organism. Transport proteins are vital to the growth and life of all living things. There are several different kinds of transport proteins. Carrier proteins are proteins involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane.[1] Carrier proteins are integral membrane proteins; that is they exist within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport. These mechanisms of movement are known as carrier mediated transport.[2] Each carrier protein is designed to recognize only one substance or one group of very similar substances. Research has correlated defects in specific carrier proteins with specific diseases.[3] A membrane transport protein (or simply transporter) is a membrane protein[4] which acts as such a carrier.

hypertonic

Having a higher osmotic pressure in a fluid relative to another fluid. Of or pertaining to a solution (e.g. extracelllular fluid) with higher solute concentration compared with another. (see: hypotonic, isotonic). For example, if the extracellular fluid has greater amounts of solutes than the cytoplasm, the extracellular fluid is said to be hypertonic. A cell placed in a highly concentrated solution will result in the water molecules diffusing out of the cell. Eventually, the cell will shrink. It should be noted, however, that not all cells in a hypertonic solution will shrink. The cells have ways to circumvent hypertonicity (see osmoregulation).

prokaryote

No nucleus - DNA found in the nucleoid Circular DNA No membrane bound organelle Smaller (1-10 micrometers) Earlier in evolution Cell wall Cell membrane Cytoplasm Ribosomes Cilia All unicellular Examples: bacteria, archae

eukaryote

Nucleus - DNA is in the nucleus Stranded DNA Membrane bound organelles - mitochondria, chloroplasts, golgi body, lysosomes, ER Larger (10-100 micrometers) Plants and fungi have a cell wall All other things are similar in eukaryotes Uni and multicellular Examples: animals, plants, fungi, protest

phospholipid bilayer definition

The two layers of phospholipids arranged in such a way that their hydrophobic tails are projecting inwards while their polar head groups are projecting on the outside surfaces This arrangement of phospholipids in bilayer is used to describe the cell membranes of many animal and plant cells. The two layers of phospholipids are arranged in a way that their hydrophobic tails are projecting to the interior whereas their hydrophilic heads are projecting the exterior. This organization of phospholipids in the cell membranes makes the latter selectively permeable to ions and molecules.

passive transport

a movement of biochemicals and other atomic or molecular substances across membranes. Unlike active transport, it does not require an input of chemical energy, being driven by the growth of entropy of the system. The rate of passive transport depends on the (semi-)permeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids and proteins. The four main kinds of passive transport are diffusion, facilitated diffusion, filtration and osmosis.

endocytosis

a process by which cells absorb molecules (such as proteins) by engulfing them. It is used by all cells of the body because most substances important to them are large polar molecules that cannot pass through the hydrophobic plasma or cell membrane. The process which is the opposite to endocytosis is exocytosis.

selectively permeable membrane

a semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion"

vesicle

a small bubble within a cell, and thus a type of organelle. Enclosed by lipid bilayer, vesicles can form naturally, for example, during endocytosis. Alternatively, they may be prepared artificially, when they are called liposomes. If there is only one phospholipid bilayer, they are called unilamellar vesicles; otherwise they are called multilamellar. The membrane enclosing the vesicle is similar to that of the plasma membrane, and vesicles can fuse with the plasma membrane to release their contents outside of the cell. Vesicles can also fuse with other organelles within the cell

cell membrane

all cells contain cell membranes, which almost always are made up of a double-layered sheet called a lipid bilayer. the cell membrane regulates what enters and leaves the cell and also protects and supports the cell

cilia

build from microtubules from the cell surface that enable cells to swim rapidly through liquid. microtubules in cilia and flagella (plural) are arranged in a 9+2 pattern

lysosome

clean-up crew! lysosomes are small organelles filled with enzymes. lysosomes break down lipids, carbohydrates, and proteins into small molecules that can be used by the rest of the cell. they are also involved in breaking down organelles that have outlived their usefulness. lysosomes perform the vital function of removing "junk" that might other wise accumulate and clutter up the cell. a number of serious human diseases can be traced to lysosomes that fail to function properly. lysosomes are found in animal cells but also in a few types of plant cells

nucleus

control center of the cell. contains nearly all of the cell's DNA and with it, the coded instructions for making proteins and other important molecules. surrounded by a nuclear envelope composed of two membranes. nuclear envelope is dotted with thousands of nuclear pores, which allow materials to move in and out of the nucleus. chromosomes are also found in the nucleus. chromosomes are spread throughout the nucleus in the form of chromatin-a complex of DNA bound to proteins

cytoskeleton

eukaryotic cells are given their shape and internal organization by a network of protein filaments known as the cytoskeleton. certain parts of the cytoskeleton also help transport materials between different parts of the cell, much like the conveyor belts that carry materials from one part of a factory to another. cytoskeletal components may also be involved in moving the entire cell as in cell flagella and cilia. the cytoskeleton helps the cell maintain its shape and is also involved in movement

endoplasmic reticulum

eukaryotic cells contain an internal membrane system known as the endoplasmic reticulum. the ER is where lipid components of the cell membrane are assembled, along with proteins and other materials that are exported from the cell

hydrophobic

fatty acid portion of this kind of lipid (lipid bilayer) are hydrophobic, or "water-hating"

centriole

in animal cells, organelles called centrioles are also formed from tubulins. centrioles are located near the nucleus and help organize cell division. centrioles are not found in plant cells.

Golgi apparatus

in eukaryotic cells, proteins produced in the rough ER move next into an organelle called the Golgi apparatus, which appears as a stack of flattened membranes. as proteins leave the rough ER, molecular "address tags" get them to the right destinations. as these tags are "read" by the cell, the proteins are bundled into tiny vesicles that bud from the ER and carry them to the Golgi apparatus. the Golgi apparatus modifies, sorts, and packages proteins and other materials from the ER for storage in the cell of release outside the cell.

properties of lipids

layered structure of cell membranes reflects the chemical properties of the lipids that make them up. lipids have oily fatty acid chains attached to chemical groups that interact strongly with water. when these lipids, including the phospholipids that are common in animal cell membranes, are mixed with water, their hydrophobic fatty acid "tails" cluster together while their hydrophilic "heads" are attracted to water. a lipid bilayer is the result.

organelle

little organs inside the cell

cell wall

many cells, including most prokaryotes, also produce a strong supporting layer around the membrane known as a cell wall. main function of the cell wall is to support, shape, and protect the cell. most prokaryotes and many eukaryotes have cell walls. animal cells do not have cell walls. cell walls lie outside the cell membrane. most cell walls are porous enough to allow water, oxygen, carbon dioxide.. etc through. cell walls provide much of the strength needed for plants to stand against the force of gravity

microfilament

microfilaments are threadlike structures made up of a protein called actin. they form extensive networks in some cels and produce a tough flexible framework that supports the cell. microfilaments also help cells move. microfilament assembly and disassembly are responsible for the cytoplasmic movements that allow amoebas and other cells to crawl along surfaces

microtubule

microtubules are hollow structures made up of proteins known as tubulins. in many cells, they play critical roles in maintaining cell shape. microtubules are also important in cell division, where they form a structure known as the mitotic spindle, which helps to separate chromosomes.

mitochondrion

nearly all eukaryotic cells, including plants, contain mitochondria. mitochondria are the power plants of the cell. mitochondria convert the chemical energy stored in food into compounds that are more convenient for the cell to use

vacuole

storage room! many cells contain large, saclike, membrane-encolsed structures called vacuoles. vacuoles store materials like water, salts, proteins, and carbohydrates. in many plant cells, there is a single, large central vacuole filled with liquid. vacuoles are also found in some unicellular organisms and in some animals.

exocytosis

the durable process by which a cell directs the contents of secretory vesicles out of the cell membrane and into the extracellular space. These membrane-bound vesicles contain soluble proteins to be secreted to the extracellular environment, as well as membrane proteins and lipids that are sent to become components of the cell membrane. However, the mechanism of the secretion of intra-vesicular contents out of the cell is very different from the incorporation of ion channels, signaling molecules, or receptors at the cell membrane. While for membrane recycling and the incorporation of ion channels, signaling molecules, or receptors at the cell membrane complete membrane merger is required, for cell secretion there is transient vesicle fusion with the cell membrane in a process called exocytosis, dumping its contents out of the cell's environment. Examination of cells following secretion using electron microscopy, demonstrate increased presence of partially empty vesicles following secretion. This suggested that during the secretory process, only a portion of the vesicular content is able to exit the cell. This could only be possible if the vesicle were to temporarily establish continuity with the cell plasma membrane, expel a portion of its contents, then detach, reseal, and withdraw into the cytosol (endocytose). In this way, the secretory vesicle could be reused for subsequent rounds of exo-endocytosis, until completely empty of its contents.

lipid bilayer

the lipid bilayer gives cell membranes a flexible structure that forms a strong barrier between the cell and its surroundings

main components of cell membrane

the main components of cell membranes are lipids and proteins. cell membranes are composed primarily of phospholipids arranged in a bilayer, as well as integral and peripheral membrane proteins. This is known as the fluid mosaic model. Components of cell membrane 1. Phospholipids 2. Glycoproteins 3. Carbohydrates 4. Cholesterol

active transport

the movement of a substance across a cell membrane against its concentration gradient (from low to high concentration). In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose and amino acids. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. Active transport uses cellular energy, unlike passive transport, which does not use cellular energy. Active transport is a good example of a process for which cells require energy. Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.

nucleolus

where assembly of the ribosome begins, contained inside the nucleus

ribosome

proteins are assembled on the ribosomes. ribosomes are small particles of RNA and protein found throughout the cytoplasm in all cells. ribosomes produce proteins by following coded instructions that come from DNA.

transport vesicle

transport vesicles can move molecules between locations inside the cell, e.g., proteins from the rough endoplasmic reticulum to the Golgi apparatus,

chloroplast

plants and some other organisms contain chloroplasts. chloroplasts are the biological equivalents of solar power plants. chloroplasts capture the energy from sunlight and convert it into food that contains chemical energy in a process called photosynthesis

flagella

plural of the cilia

cytoplasm

portion of cell outside the nucleus. nucleus and cytoplasm work together in the business of life. eukaryotic cells have both nucleus and cytoplasm whereas prokaryotic cells only have cytoplasm

facilitated diffusion

process of passive transport (as opposed to active transport), with this passive transport aided by integral membrane proteins. Facilitated diffusion is the spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane integral proteins. The facilitated diffusion may occur either across biological membranes or through aqueous compartments of an organism.[1] Polar molecules and charged ions are dissolved in water but they cannot diffuse freely across the plasma membrane due to the hydrophobic nature of the fatty acid tails of phospholipids that make up the lipid bilayers. Only small nonpolar molecules, such as oxygen can diffuse easily across the membrane. All polar molecules are transported across membranes by proteins that form transmembrane channels. These channels are gated so they can open and close, thus regulating the flow of ions or small polar molecules. Larger molecules are transported by transmembrane carrier proteins, such as permeases that change their conformation as the molecules are carried through, for example glucose or amino acids. Polar molecules, such as retinol or lipids are poorly soluble in water. They are transported through aqueous compartments of cells or through extracellular space by water-soluble carriers as retinol binding protein. The metabolites are not changed because no energy is required for facilitated diffusion. Only permease changes its shape in order to transport the metabolites. The form of transport through cell membrane which modifies its metabolites is the group translocation transportation. Glucose, sodium ions and chloride ions are just a few examples of molecules and ions that must efficiently get across the plasma membrane but to which the lipid bilayer of the membrane is virtually impermeable. Their transport must therefore be "facilitated" by proteins that span the membrane and provide an alternative route or bypass. Various attempts have been made by engineers to mimic the process of facilitated transport in synthetic (i.e., non-biological) membranes for use in industrial-scale gas and liquid separations, but these have met with limited success to date, most often for reasons related to poor carrier stability and/or loss of carrier from the membrane.

osmosis

the net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in order to equalize the solute concentrations on the two sides.[1][2][3] It may also be used to describe a physical process in which any solvent moves, without input of energy,[4] across a semipermeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations.[5] Although osmosis does not require input of energy, it does use kinetic energy [6] and can be made to do work.[7] One frame of a computer simulation of osmosis Net movement of solvent is from the less concentrated (hypotonic) to the more concentrated (hypertonic) solution, which tends to reduce the difference in concentrations. This effect can be countered by increasing the pressure of the hypertonic solution, with respect to the hypotonic. The osmotic pressure is defined to be the pressure required to maintain an equilibrium, with no net movement of solvent. Osmotic pressure is a colligative property, meaning that the osmotic pressure depends on the molar concentration of the solute but not on its identity.

hydrophilic

the opposite end of the lipid bilayer is hydrophilic, or "water-loving"

smooth ER

the other portion of the ER is known as the smooth ER because ribosomes are not found on its surface. in many cells, the smooth ER contains collections of enzymes that perform specialized tasks, including the synthesis of membrane lipids and the detoxification of drugs

fluid mosaic model

the plasma membrane is described to be fluid because of its hydrophobic integral components such as lipids and membrane proteins that move laterally or sideways throughout the membrane. That means the membrane is not solid, but more like a 'fluid'. the membrane is depicted as mosaic because like a mosaic that is made up of many different parts the plasma membrane is composed of different kinds of macromolecules, such as integral proteins, peripheral proteins, glycoproteins, phospholipids, glycolipids, and in some cases cholesterol, lipoproteins. according to the model, the plasma membrane is a lipid bilayer (interspersed with proteins). It is so because of its phospholipid component that can fold in itself creating a double layer - or bilayer - when placed in a polar surrounding, like water. This structural feature of the membrane is essential to its functions, such as cellular transport and cell recognition

rough ER

the portion of the ER involved in the synthesis of proteins is called the rough ER. given its name because of the ribosomes found on its surface. newly made proteins leave these ribosomes and are inserted into the rough ER, where they may be chemically modified. proteins made on the rough ER include those that will be release, or secreted, from the cell as well as many membrane proteins and proteins destined for lysosomes and other specialized locations within the cell.