Cell membrane

Question 1

Explain the fluid mosaic model.

Answer 1

• mosaic: random arrangement of proteins which are embedded in the phospholipid bilayer
• fluid: phospholipids and proteins can move laterally due to weak hydrophobic interactions

Question 2

Functions of phospholipid bilayer

Answer 2

• regulates movement of substances moving in and out of the cell by acting as a barrier to charged ions, polar and large molecules
• act as a boundary between intracellular and extracellular aqueous env
• allows compartmentalisation within a cell

Question 3

Functions and structure of cholesterol

Answer 3

Structure:
- slightly amphipatic, it has hydrophilic, polar OH group, and a hydrophobic 4 ring structure
- the OH group of cholesterol aligns with the charged phosphate heads of the phospholipids while the rest of it embedded in the hydrophobic core

Functions:
- regulates membrane fluidity
- prevents close packing of phospholipids, prevents membrane from being overly firm at low temperatures
- prevents membrane from being overly fluid at high temperatures as cholesterol restricts phospholipid movement
- anchor proteins in the membrane

Question 4

Functions and structure of proteins

Answer 4

• function as channels/carriers for facilitated diffusion and active transport
  1. channel proteins provide a hydrophilic pore for small and polar ions to diffuse across the membrane from a high to low solute conc
  2. carrier proteins
  –> bind the solute on one side, protein undergoes conformational change that allows access of solute to opposite side of the membrane
  –> pumps that use ATP. to move solutes against a conc gradient
  3. functions as enzymes
  4. functions as receptor proteins
  5. stabilise membrane structure, as they are non-covalently bonded to cytoskeleton and extracellular matrix

Question 5

glycoproteins and glycolipids

Answer 5

• function as markers/recognition sites in cell-cell recognition and adhesion
• functions as receptors

Question 6

function of membranes

Answer 6

1. regulate movement of substances as membranes are selectively permeable and act as a boundary
2. membranes allow for compartmentalisation which allow
   - unique environments to be formed for highly specialised activities
   - spatial separation of biochemical processes and thus their sequential operation within a cell
   - accumulation of ions to higher concentrations
3. act as surface for chemical reactions to occur in a. sequential manner
4. increase surface area for chemical reactions
5. enable communication between a cell and its surroundings
   –> cell-cell recognition and. adhesion
   –> signal transduction

Question 7

simple diffusion

Answer 7

net movement of small, non-polar molecules from a region of high conc to a region of low conc down a conc gradient

Question 8

facilitated diffusion

Answer 8

net movement of molecules down. a conc gradient with the help of channel of carrier proteins, no ATP is required

Question 9

active transport

Answer 9

ATP is required for the movement of molecules against a conc gradient with the help of pumps.

Question 10

bulk transport

Answer 10

• an active process that requires ATP
• exocytosis: secretion of macromolecules to the exterior by the fusion of the vesicle membrane with plasma membrane
• phagocytosis: outward extension of the cell surface surface membrane known as pseudopia which engulfs, wraps around solid particles. Ends of pseudopia fuses and pinches off to form a phagocytotic vesicle
• pinocytosis: a small area of the plasma membrane invaginates and liquids taken into the cell via vesicles

Question 11

osmosis

Answer 11

net movement of water molecules down a wpt gradient through a selectively permeable membrane