cell membrane and transport

Question 1

What are the five functions of biological membranes?

Answer 1

1. Definition of cell’s boundaries: keeps interior separated from environment, selectively permeable to keep substances inside/outside of cell
2. Organisation & localisation of function: Molecules or structures with specific functions are embedded in membranes or localised within organelles.
3. Regulation of cell’s contents
4. Signal transduction: detection of specific signals, triggering specific responses
5. Cell-to-cell communication

Question 2

What are the characteristics of the fluid mosaic model?

Answer 2

1. The fluid layer is asymmetrical.
   * The two lipid layers (bilayer) may differ in composition and arrangement of proteins and
   lipids.
2. The phospholipid bilayer is fluid or mobile, i.e. lateral movement of phospholipids is possible.
3. The unit membrane is a dynamic structure, where the embedded proteins can float, some moving
   freely while others are fixed in positions by microfilaments on the cytoplasmic face.
4. Membranes are amphipathic.

Question 3

Describe the role of phospholipids in the membrane.

Answer 3

They are responsible for the formation of bilayers in an aqueous environment. They are amphipathic due to their hydrophobic tail and hydrophilic head. This forms an effective hydrophobic barrier against polar and charged solutes.

Question 4

Describe membrane fluidity.

Answer 4

As hydrophobic interactions are weak, phospholipid molecules are free to move about laterally. It is however quite rare for the molecules to flip transversely across
the membrane. This is because the hydrophilic part of the molecule must cross the hydrophobic core of the membrane in order to do so.

Question 5

How does temperature affect membrane fluidity?

Answer 5

Low temperature - KE of hydrocarbon chains decreases, mow tightly packed and increased hydrophobic interactions restricts their motion, bilayer exists in a semisolid state/less fluid.
High temperature - KE increases, increased lateral movements, flexing of chains and transverse flipping, overcoming hydrophobic interactions, increased space between molecules and bilayer is more fluid.

Question 6

How do the nature of fatty acid chains affect membrane fluidity?

Answer 6

As length of fatty acid chains increases, membrane fluidity decreases. The longer the hydrocarbon chains, the higher the melting point (phase transition temperature) due to increased hydrophobic interactions
between hydrocarbon chains.
As degree of saturation increases, membrane fluidity decreases. Saturated lipids — long straight chains, close packing and membrane solidification
Unsaturated lipids — kinks, prevent close packing and membrane fluidity

Question 7

What are cholesterols?

Answer 7

Cholesterols are steroids commonly found wedged between phospholipid molecules in the
cell membranes of animal cells. They have no glycerol backbone, fatty acid tail and are lipids that are not soluble in aqueous environment.

Question 8

What are the effects of cholesterol on cell membranes?

Answer 8

Membrane stability — rigid steroid ring interferes with motions of hydrocarbon chains, enhances stability
Membrane fluidity — high temp, restrain movement and interfere with motions, decreasing fluidity. low temp — prevents close packing, increased fluidity. I.e. dual effects on fluidity, ‘temperature buffer’
Membrane permeability — decreases permeability by filling in spaces and plugging transient gaps

Question 9

Describe integral proteins.

Answer 9

Deeply embedded in bilayer, unilateral or transmembrane. Amphipathic, held in place by extensive hydrocarbon interactions, insoluble and released only through use of detergents or non-polar solvents

Question 10

Describe peripheral proteins.

Answer 10

Loosely bound, found on both sides of membrane: held by network proteins on cytoplasmic side, held by fibres of extracellular matrix on exterior side. Rich in hydrophilic amino acid, soluble and easy release by mild treatment

Question 11

Describe role of proteins in anchorage.

Answer 11

Anchoring proteins attach the cell membrane to other substances,
stabilise the position of the cell membrane and can help maintain cell shape. can coordinate
extracellular and intracellular changes.
- On the cytoplasmic side, they are bound to microfilaments of the
cytoskeleton.
- On the exterior side, they may attach the cell to fibres of the
extracellular matrix.

Question 12

Describe the two types of proteins in their role of transport.

Answer 12

Carrier proteins: bind solutes and transport them across membrane, conformational change when binding occurs and return to original form when released. Can be via facilitated diffusion or active transport.
Channel proteins: water-filled central pore/hydrophillic channel to permit movement of solutes. Leak channels permit movement at all times (eg aquaporins, Na+ or K+ leak channels). Gated channels can open or close to regulate ion passage.

Question 13

Describe role of proteins in enzymatic activity.

Answer 13

• These enzymes catalyse reactions in the extracellular fluid or within the cytosol, depending on the location of the active site.
• In some instances, several enzymes can be grouped together to carry out sequential steps in a metabolic pathway.

Question 14

Describe role of proteins in signal transduction.

Answer 14

• These proteins have very specific 3D conformations, receptor molecules for chemical signalling between cells.
• Chemical signalling works by the binding of a ligand to the receptor protein which triggers changes in the cell.
• Cell membranes differ in the type and number of receptor proteins they contain. It is these differences that account for the differing sensitivities to hormones and neurotransmitters.

Question 15

Describe role of proteins in cell-to-cell recognition.

Answer 15

• Recognition proteins (identifiers or cell identity markers) are usually
  glycoproteins.
• There is a wide array of possible shapes to the carbohydrate side
  chains; hence each cell type has its own specific markers.
• This enables cells to recognise other cells, and provides a means for foreign markers to be recognised and attacked by the immune system.

Question 16

Describe role of proteins in intercellular joining.

Answer 16

Membrane proteins of adjacent cells may adhere together in various kinds of intercellular junctions, such as gap junctions and tight junctions.

Question 17

Compare simple and facilitated diffusion.

Answer 17

Simple diffusion — small/hydrophobic molecules that can cross the bilayer directly. Facilitated — larger, hydrophilic substances.
Simple — directly across plasma membrane. Facilitated — transport protein is used to enhance/increase rate of transport.
Simple — either direction until dynamic equilibrium. Facilitated — usually from exterior to interior.

Question 18

What factors affect the rate of diffusion?

Answer 18

1. Concentration gradient
2. Distance over which diffusion occurs
3. Area across which diffusion occurs.
4. Structure (presence of transient gaps, type and number of transport proteins)
5. Size and type of diffusing molecule
6. Temperature

Question 19

Define osmosis.

Answer 19

Osmosis is the net movement of freely moving water molecules from a region of less negative
water potential to a region of more negative water potential through a selectively permeable
membrane.

Question 20

Water potential equation

Answer 20

Water potential of cell = Solute potential + pressure potential

Question 21

Describe solute potential.

Answer 21

Solute potential is the measure of the ability of a solute to make the water potential more negative. Solute potential is always negative. The more solute molecules present, the more negative the solute potential.

Question 22

Describe pressure potential.

Answer 22

Pressure potential is the measure of the pressure exerted by the cell wall on its contents. It is not applicable for animal cells as they lack cell walls.
If pressure is applied to pure water or a solution, its water potential becomes less negative. This is
due to pressure applied which forces water to move from 1 place to another. Pressure potential increases as the cell absorbs water and increases in volume. It is a
positive value, since it tends to move water out of the cell as opposed to solute potential which tends to move water into the cell.

Question 23

Describe how a plant cell would change in a solution of more negative water potential.

Answer 23

Water potential in cell is less negative than that of solution. Water leaves the cell by osmosis. Water is lost first from the cytoplasm through the cell
membrane, and then from the vacuole through the tonoplast.
The protoplast (the living contents of the cell surrounded by the cell wall) shrinks and eventually pulls away from the cell wall. This process is known as plasmolysis, and the cell is said to be plasmolysed.
The point at which plasmolysis begins is known as incipient plasmolysis — the protoplast has just ceased to exert any pressure against the cell wall as the cell
membrane starts to pull away from the cell wall. The cell is said to be in a flaccid state. Water continues to leave until the contents in the protoplast are of the same water potential as the external environment. No further shrinkage occurs. For a population of plant cells, incipient plasmolysis is defined as the point where 50% of
the plant cells are plasmolysed.

Question 24

Describe active transport in cells, utilising carrier proteins.

Answer 24

As movement occurs against a concentration gradient, conformational change in the transport protein is necessary to ensure that there is no “leakage” of solutes back across the membrane down the concentration gradient.
1. The process begins when solutes on the cytoplasmic side of the plasma membrane bind to a
specific binding sites on the transport protein.
2. ATP then transfers a phosphate group to the transport protein.
3. This causes the protein to change its conformation in such a way that the solute is released
on the other side of the membrane.
4. This phosphate group detaches and the transport protein returns to its original conformation.

Question 25

What is primary active transport and what active transport carriers are involved?

Answer 25

The direct use of ATP by a carrier protein to perform transport. 1. Uniport – Only one substance is transported across the membrane at a time 2. Symport – Two substances are transported across the membrane together in the same direction 3. Antiport – Two substances are transported across the membrane together in opposite directions (sodium-potassium pump, transports 3 Na+ out for every 2 K+ ions)

Question 26

Describe the 3 types of endocytosis.

Answer 26

1. Phagocytosis Large solid particles such as food particles, bacteria etc, are taken into the cell. Pseudopodia extend outwards from the cell to enclose the solid particle. The vesicles formed usually fuse with lysosomes which contain hydrolytic enzymes to digest these macromolecules. 2. Pinocytosis Droplets of extracellular fluid are incorporated into small vesicles. The process is non-specific, and the cell takes in all solutes dissolved in the droplet. 3. Receptor-mediated endocytosis coated pits form vesicles when specific molecules bind to receptor proteins on the cell surface. Coated pits are reinforced on their cytoplasmic side by the protein clathrin. The vesicles formed are known as clathrin-coated vesicles. This is a selective form of endocytosis which allows the cell to take up only specific substances from the extracellular fluid

Question 27

Describe exocytosis.

Answer 27

Exocytosis is a process where the cell secretes macromolecules by fusion of vesicles with the plasma membrane. The vesicles usually bud off from the ER or GA, migrate to and fuse with the plasma membrane. This process is used by secretory cells to export products, or for the removal of waste materials.