Topic B: MEMBRANE AND TRANSP

Question 1

What is the fluid mosaic model?

Answer 1

Fluid:
Phospholipids and proteins are able to move freely within the phospholipid bilayer.
Phospholipids move laterally within a layer freq. but rarely flip-flop across the membrane as it is unlikely that the hydrophilic heads will pass through the hydrophobic core of the membrane.
Embedded proteins move slowly as they are larger, some have restricted movement as they are attached to the cytoskeleton.
Mosaic:
Protein molecules are embedded and scattered among the phospholipids.

Question 2

What is the structure and function of the phospholipids?

Answer 2

Structure:
Phospholipids are amphipathic. It consists of a hydrophilic phosphate group and 2 hydrophobic FA chains/tails.
They are arranged as a bilayer in the cell membrane. The hydrophobic phosphate heads face aq medium and vice versa.
Most phospholipids have a choline group attached to the phosphate head.
Function:
Arrangement of the phospholipids in the form of a bilayers allows it to form a barrier in an aq environment.

Question 3

What is the structure of extrinsic proteins?

Answer 3

Extrinsic proteins are loosely attached to membrane surface and are exposed to the aq medium of extracellular fluid and cytoplasm.
They are globular proteins found on either sides of the bilayers and are soluble in aq medium.
External extrinsic proteins are attached to fibers of extracellular matrix to keep the cell in place.
Internal extrinsic proteins are attached to cytoskeleton, they are involved in maintaining cell shake and motility.

Question 4

What is the structure of intrinsic proteins?

Answer 4

Intrinsic proteins are embedded in hydrophobic core of phospholipid bilayer. They may be unilateral or transmembrane.
They contain both hydrophob and hydrophil regions.
The hydrophob regions form hydrophob interacxns with the hydrophob FA tails of phospholip bilayer.
The hydrophil regions form H bonds with aq medium and hydrophil phosphate heads of the phospholip molqs

Question 5

What are the functions of membrane proteins?

Answer 5

1. Channel proteins have hydrophil pores which allow movement of hydrophil moleqs such as charged ions and polar molecules.
2. Carrier proteins have specific binding sites which binds to specific solutes and transports them across the cell membrane down/against their conc gradient using ATP.
3. Many enzymes are embedded within membranes for specific functions. In certain cases, several enzymes can function as a team that carries out sequential steps of a metabolic pathway.
4. Receptor proteins have sites of attachments for signal molecules to bind, allowing for cell signalling.

Question 6

What is the structure of glycoproteins?

Answer 6

GP: Proteins with addition of short hydrocarbon chains
GL: Phospho/Lipids with addition of short hydrocarbon chains.
Both are found on the outer surface of the membrane of all eukaryotic cells.

Question 7

What is the function of glycoproteins and glycolipids?

Answer 7

The main function of glycoproteins and glycolipids is for cell-cell recognition where cells are able to recognise and bind to one another.
Another function is for cell-cell adhesion. The carbohydrate chains of 2 adj. cells may bind with each other, causing adhesion btw cells.
They also act as receptor sites for signal molecules.

Question 8

What is the structure of cholesterol?

Answer 8

Cholesterol is a class of sterols found only in the membranes of animal cells.
Cholesterol consists of a hydrophil region and a hydrophob region. HL region interacts with the HL heads of PPL moleq.
The HP region of the cholesterol interacts with the HP FA tails of PPL bilayer.
Cholesterols are interspersed among the bilayer hindering the close packing of PPL.

Question 9

What is the function of cholesterol?

Answer 9

Helps to regulate fluidity of cell membrane in animal cells.
Maintains mechanical stability of the membrane.
Prevents leakage of small polar molecules.

Question 10

What are the factors affecting membrane fluidity?

Answer 10

1. Temperature: At low temps moleqs have low KE and thus the membrane is less fluid. At high temps moleqs have high KE, vibrating faster and membrane is more fluid. PPL may move far apart freq, creating gaps in the membrane, increasing its permeability.
2. Lipid composition: Unsat FA chains have kinks at C=C, preventing FA chains from packing closely tgt. Generally, the higher the prop. of unsat FA chains, the more fluid the membrane.
3. Presence of cholesterol. At high temps Cholesterol prevents PPL from moving too far apart by forming hydrophobic interactions with FA tails. At low temps cholesterol prevents PPL from packing too close together, preventing cell freeze.

Question 11

What are the functions of cell membranes?

Answer 11

1. Boundary: CSM forms a boundary that separates the cell contents from external environment, allowing the internal env of the cell to be kept relatively constant in order to function optimally.
2. Compartmentalisation: Internal cell membranes enables compartmentalisation of cell contents and prevents indiscriminate mixing. This allows for specialisation of cell function by concentrating specific substances.
3. Partailly selective barrier: Cell membranes are partially permeable and prevent free interchange of materials. HP tails repel HL moleqs such as ions and polar subst. Close packing of PPL prevents large moleqs such as glucose from passing through. It allows for the passage of HP moleqs such as non-polar moleqs and small polar moleqs.

There is more but then this would be too long.

Question 12

What are the 2 types of molecules?

Answer 12

Hydrophobic moleqs: Non-polar moleqs. Small hydrophobic non-polar molecules such as oxygen are able to dissolve on HP FA tails of PPL bilayer and pass through the membrane.
HP moleqs: Small polar substances such as water are able to pass through the small temp gaps btw PPL and move across the membrane. Large polar substances such as glucose are not able to pass thru the membrane easily and require help from transport proteins. Charged ions are repelled by HP core of the phospholipid bilayer, cannot pass through the membrane easily.

Question 13

What is passive transport?

Answer 13

Passive transport is the movement of substances down a concentration gradient without the use of ATP Concentration gradient of substance determines the direction of net movement. Energy for the process comes from the kinetic energy of the molecules alone.
There are 3 types of passive transport: diffusion, facilitated diffusion and osmosis

Question 14

What is diffusion and what diffuses across the PPL bilayer?

Answer 14

Diffusion refers to the net movement of molecules down concentration gradient i.e. from region of higher concentration to region of lower Concentration until dynamic equilibrium reached (no net movement of molecules).
Substances that diffuse across the phospholipid bilayer includes:
Lipid-soluble molecules such as steroid hormones
Small hydrophobic non-polar molecules (O2)
Small polar molecules such as water molecules which pass through small temporary gaps between phospholipids.

Question 15

What are the factors affecting the rate of diffusion?

Answer 15

Size of molecule: smaller molecules diffuse at a faster rate
Solubility of molecule: more soluble molecules difuse at faster rate
Concentration gradient: greater the concentration gradient, faster the diffusion rate
Distance shorter the distance, faster the difusion rate
Surface Area: greater the surface area, faster the difusion rate

Question 16

What is facilitated diffusion?

Answer 16

Facilitated diffusion is the net movement of molecules down a concentration gradient ie from a region of higher concentration to a region of lower concentration with the help of specific transport proteins until a dynamic equilibrium is reached.
Substances that diffuse by facilitated diffusion include Charged ions e g. Na, K. Small polar molecules e.g. water large polar molecules e.g. glucose

Question 17

How do transport and channel proteins function?

Answer 17

**Channel proteins **which form *hydrophilic pores *in the membrane to allow hydrophilic molecules such as charged ions and polar molecules to pass through the hydrophobic fatty acid tails of the phospholipid bilayer. They are usually gated to control the entry and exit of molecules. For example, voltage-gated channel proteins transport ions such as sodium and potassium ions across membrane. Another example of a channel protein is aquaporin which transports water molecules.
Carrier proteins which have specific binding sites for a specifc solute. Carrier proteins undergo change in conformatíon once solute binds to it and constantly flip between two states so that thay alternatively open to opposite sides of the membrane.

Question 18

What is osmosis?

Answer 18

Osmosis is the net movement of water molecules down a water potential gradient ie. from a region of less negative water potential to a region more negative water potential across a partially permeable membrane until a dynamic equilibrium is reached Water molecules can pass through small temporary gaps in the membrane or via channel proteins.

Question 19

What is water potential?

Answer 19

Water potential ψw (Greek letter psi, pronounced ‘sy), is the net tendency of water to diffuse out of a solution. It is measured in pressure units (kPa, kilopascals) Pure water has the highest water potential of ψw = 0. Addition of solutes to pure water results in water potential ψw to be lowered, becoming negative (<0). This is because solute particles attract water molecules to them, restricting the movement of water molecules, therefore the more negative the water potential ψw becomes.
Water potential ψw of a solution is always negative.

Question 20

What is active transport?

Answer 20

Active transport is the movement of substances against a concentration gradient, using energy in the form of ATP, i.e. from a region of lower concentration on one side of a partially permeable membrane to a region of higher concentration on the other side.
The movement is highly selective and usually unidirectional. It involves specific transmembrane carrier proteins called ‘pumps which use ATP to change its conformation. The conformation change in a manner which translocates the solute attached to the protein across the membrane.

Question 21

What is bulk transport?

Answer 21

It is a mode of transport is used to transport large molecules such as protein polysaccharides. Energy in the form of ATP is required for the process.
They are endo and exocytosis.

Question 22

What are the types endocytosis?

Answer 22

Endocytosis is the process of intake of substances by invagination of cell surface membrane or extension of pseudopodia. A small area of cell surface membrane therefore pinches to form a vesicle containing the substance.
Pinocytosis involves the uptake of droplets of extracellular fluid via tiny vesicles. This process is unspecific as all solutes dissolved in the droplets are taken in. This is carried out via the invagination of the cell surface membrane This is useful in the uptake of dissolved digested nutrients.

Phagocytosis is explained on another card

Question 23

What is the process of phagocytosis?

Answer 23

Phagocytosis involves uptake of large, solid materials such as when white blood cell ingests a bacterium. Cells specializing in this process are called phagocytes. This process is specific as phagocytes recognize the molecules before engulfing them. During phagocytosis, the cell extends pseudopodia to engulf the bacteria. A phagocytic vesicle (phagosome) is formed with the bacterium enclosed.
The lysosome membrane fuses with membrane of the phagocytic vesicle. Hydrolytic enzymes in lysosomes digest the contents into soluble products. These soluble products diffuse into the cytoplasm for cell use. Membrane of vesicle fuses with the cell surface membrane, releasing any undigested materials out of the cell by exocytosis.

Question 24

What is receptor mediated endocytosis?

Answer 24

Receptor mediated endocytosis involves the uptake of bulk quantities of specific substances even though those substances may not be very concentrated in the extracelular fluid. This process is specific as specific substances (ligands) bind to the proteins with specific receptor sites exposed to the extracellular fluid, whch are embedded in the cell surface membrane.
The receptor proteins then cluster in regions of the membrane called coated pits which are lined on their cytoplasmic side by a layer of coat oroteins An example of coat protein is clathrin.
Next each coated pit forms a vesicle containing the ligand molecules by invagination of the cell surface membrane. After the ingested material is liberated from the vesicle, the emptied receptors are recycled to the cell surface membrane by vesicle.

Question 25

What is exocytosis?

Answer 25

Exocytosis involves transport of materials out of cell. Materials to be transported out are enclosed in a vesicle, usually from Golgi apparatus.
Secretory vesicle move along the cytoskeleton towards the cell surface membrane. Membrane of secretory vesicle fuses with the cell surface membrane. Contents are then discharged from the cell by exocytosis This process is used in secretion of hormones and digestive enzymes by secretory cells of endocrine glands and stomach/intestines, respectively.

Question 26

Describe osmosis in animal cells.

Answer 26

Animal cells do not have cell wall. Water potential of the cell is dependent on the solute potential which is determined by the amount of dissolved solutes in cytoplasm. (\w = ys). Solute potential (ws) is the measure of decrease in water potential in the system due to the presence of solutes.