HomeWork Due: Friday, Sept. 13, 2024

Question 1

Describe the features of the cell membrane in detail.

Answer 1

• The features of the cell membrane are Fluidity & Asymmetry.
• Fluidity is determined by the lipid bilayer and membrane proteins
• the fluidity of the the lipid bilayer involves the mobility patterns (lateral diffusion, flip-flop, rotation, flexion) which require different amounts of energy
• lateral diffusion requires the least amount of energy and occurs most frequently in the lipid bilayer, while flip-flop rarely occurs due to the large amounts of energy required to carry out this mobility pattern
• there are many factors affecting the fluidity of membrane lipids including: saturation degree and length of hydrocarbon tail, dual regulatory role of cholesterol, ratio of lecithin vs sphingomyelin and the influence of membrane protein
• the fluidity of membrane proteins involves using mobility patterns that require overcoming a small energy barrier: lateral diffusion and rotation only
• proteins cannot flip-flop because the molecules are larger in size and polar
• lateral diffusion is demonstrated in fluorescence recovery after photobleaching (FRAP)
• asymmetric distribution of lipids in the membrane involves different distribution between: the two layers of lipids, different kinds of cells and different kinds of organelle
• proteins differing with each other on location of the membrane and the transmembrane proteins having polarity, contribute to the assymetric distribution of proteins in the cell membrane
• saccharides distribute only on the outer membrane face to contribute to cell membrane asymmetry

Question 2

Main points of fluid mosaic model and lipid raft model.

Answer 2

Fluid Mosaic Model
- the fluid mosaic model is dynamic because it is involved in transport of molecules in and out of the cell
- the mobility patterns of the lipid bilayer and membrane proteins provide membrane fluidity
- various arrangements of lipids, proteins and saccharides in and around the membrane contribute to its asymmetry

Lipid Raft Model
- dynamic assemblies of proteins and lipids floating freely in the bilayer of cell membranes
- having weak protein-protein, protein-lipid and lipid-lipid interactions that reinforce one another and partition the interacting components of the bilayer into raft domains
- the main components of lipid raft domains are cholesterol, sphingolipids (sphingomyelin), glycolipids, some transmembrane proteins, etc.
- raft domains have increased membrane thickness due to their composition
- functions of the lipid raft model include: signal transduction, transport of proteins and cholesterol, metabolic reactions, etc.

Question 3

Define passive transport

Answer 3

Passive Transport
- also called Simple diffusion or passive diffusion
- membrane transport that does not require energy to move substances across the cell membrane
- the concentration differs between both sides of the membrane
- the molecules transported have low molecular weight and are fat-soluble
- does not require membrane transport proteins

Question 4

Define active transport

Answer 4

Active Transport
- is defined as carrier protein mediated movement up the gradient
- solutes always move up a concentration or electrochemical gradient
- active transport always requires the input of energy

Question 5

Define Cotransport

Answer 5

• Cotransport is coupled or paired transport of molecules across the cell membrane involving Symport and antiport

Question 6

Sodium/potassium pump:

Answer 6

Concept
- the Na+/K+ ATPase requires K+ outside, Nà+ and ATP inside and is inhibited by ouabain
- the ratio of Nà+:K+ pumped is 3:2 for each ATP hydrolyzed
- the Nà+/K+ ATPase is a P-type pump in which ATPase sequentially phosphorylates and dephosphorylates itself during the pumping cycle
- this pump is found only in animals

Mechanism
- 3 Nà+ are taken from the inside
- ATP phosphorylates alpha subunits
- a conformational change following phosphorylation expels 3 Nà+ to outside
- the pump opens to outside, ready to start the second half of the cycle
- two K+ accepted from outside
- de phosphorylation triggers conformational change
- 2 K+ expelled to inside and the pump returns to initial state

Question 7

Two classes of membrane transport proteins and the transport they mediate

Answer 7

Carrier protein
- can carry out both passive and active transport
- bind one or more solute molecules on one side of the membrane and then undergo a conformational change that transfers the solute to the other side of the membrane
- mediates facilitated diffusion and active transport

• facilitated diffusion occurs using carrier proteins by binding with the solute specifically and changing the conformation.
• the diffusion rate is influenced by the concentration gradient and the number of carrier proteins
• occurs downhill and does not need ATP
• active transport occurs uphill and requires ATP
• the carrier has the specificity and variability
• three types of carrier-mediated transport: uniport, symport and antiport
• uniport: transporting a single molecule in a single direction
• symport: transporting two molecules in the same direction
• antiport: transporting two molecules in opposite directions

Channel protein
- can carry out only passive transport (downhill) and does not interact with the solute
- also known as ion channels or gated ion channels
- highly selective
- high speed
- has specific gated regulator: ligand-gated, voltage-gated, stress activated

Question 8

Types of endocytosis and exocytosis

Answer 8

Endocytosis
- invaginate and then pinch off to form an intercellular vesicle which end up in lysosome
- forms of endocytosis: pinocytosis, phagocytosis and receptor-mediated
- pinocytosis: imports extracellular molecules dissolved or suspended in fluid by forming vesicles from the plasma membrane
- phagocytosis: the uptake of large molecules
- receptor-mediated: molecules that will be internalized bind to the specific receptors on the surface of the plasma membrane forming an internalized coat vesicle

Exocytosis
- secretory vesicles fuse with plasma membrane
- types of exocytosis: constitutive exocytosis pathway (constitutive secretion) and regulated exocytosis pathway (regulated secretion)