Membranes

Question 1

Phospholipid bilayer size

Answer 1

7.5nm

Question 2

Phospholipids

Answer 2

Hydrophilic phosphate head and 2 hydrophobic hydrocarbon tails/chains
Amphipathic
Form bilayers or micelles
Held together by weak hydrophobic interactions and van der Waals forces -> drift laterally -> fluidity

Question 3

Temperature and phospholipids

Answer 3

As temperature decreases -> less active -> closely packed arrangement -> less fluid -> solidifies
Depends on:
1. Ratio of saturated to unsaturated hydrocarbon chains (kinks prevent close packing)
2. Relative amount of cholesterol
3. Length of HC chains

Question 4

Cholesterol structure

Answer 4

Four-ring structure, largely hydrophobic
Amphipathic
Hydrophilic OH group aligns with phosphate heads
Hydrophobic ring structure tucked into hydrophobic core of membrane

Question 5

Cholesterol function

Answer 5

Regulate membrane fluidity -> affects permeability
High temps -> rigid -> stabilises lipid layer through van der Waals interactions -> restricts phospholipids movement -> prevent membrane from being overly fluid

Low temps -> prevent close packing of phospholipids -> hinder solidification/crystallisation -> prevent membrane from being overly firm

Question 6

Proteins

Answer 6

Membrane proteins determine its specific function
Integral/intrinsic proteins (transmembranal/unilateral)
Peripheral/extrinsic proteins

Question 7

Integral/intrinsic proteins

Answer 7

Amphipathic
Hydrophilic regions with amino acids with polar or charged R groups -> exposed to aqueous medium
Hydrophobic region with non-polar R groups -> lie in hydrophobic core

Unilateral vs transmembrane

Question 8

Peripheral/extrinsic proteins

Answer 8

Not embedded but loosely attached to surface of membrane or integral proteins through weak ionic and h-bonds

May be attached to fibres of extracellular matrix on exterior side or filaments of cytoskeleton on cytoplasmic side

Help maintain cell shape and fix location of protein

Question 9

Cytoskeleton

Answer 9

Network of microtubules, microfilaments and intermediate filaments

Question 10

Glycoproteins and glycolipids characteristics

Answer 10

Carbohydrates always projecting out into extracellular matrix
Carbohydrate covalently bonded to HC tail or protein

Question 11

Glycoproteins/glycolipids function (2)

Answer 11

1. Cell-cell recognition
   Carbohydrates -> diversity -> good cell markers
   Important for differentiation and rejection of foreign cells by immune system
2. Cell receptors
   Receptors for hormones in cell-signalling
   Point of attachment for viruses, bacteria and toxins
   Receptors for white blood cells to recognise infected cells to destroy

Question 12

Fluid mosaic model (fluid)

Answer 12

Cell membrane comprises phospholipids and proteins that are free to move laterally
Phospholipids can “flip-flop” but rare occurrence (polar phosphate head will have to pass through hydrophobic core of bilayer -> energetically unfavourable)
Dynamic structure -> continuous lateral motion

Question 13

Fluid mosaic model (mosaic)

Answer 13

Random arrangement of proteins embedded amongst phospholipid molecules -> mosaic pattern

Question 14

Functions of membranes (Internal and surface)

Answer 14

1. Regulate movement of substances across membrane (surface and internal)
   Selectively permeable -> non-polar/uncharged molecules can dissolve and diffuse through hydrophobic core which repels charged and polar molecules

Question 15

Functions of membranes (Internal) (3)

Answer 15

2. Compartmentalisation
   (a) Form unique environment for highly specialised activities
   (b) Accumulation of charged ions and formation of chemical gradients across membranes
   (c) Storage of food source
3. Localisation of proteins of related function
   Allows sequential biochemical processes to be facilitated
4. Increase surface area for chemical reactions

Question 16

Functions of membranes (External) (2)

Answer 16

5. Cell-cell recognition and adhesion
   Glycoproteins and glycolipids for cell-cell recognition -> unique surface topography
   Adhesion -> membrane proteins of adjacent cells can hook together to form junctions (E.g. tight/gap)
6. Signal transduction
   Transmembrane proteins serve as cell surface receptors -> ligands bind to them

Question 17

Reasons for substance movement across membranes (5)

Answer 17

1. Obtain nutrients for energy and raw materials
2. Excrete waste substances
3. Secret useful substances
4. Generate ionic gradients
5. Maintain suitable pH and ionic concentration

Question 18

Passive vs Active processes

Answer 18

Passive -> down concentration gradient -> does not require energy

Active
Polar molecule and ions transported across membrane through protein pumps, against concentration gradient -> requires ATP
Bulk transport -> large molecules via vesicles -> active process not active transport

Question 19

Simple diffusion (Passive)

Answer 19

Movement of non-polar molecules and small polar water molecules directly through membrane, down concentration gradient
No ATP
Occurs until particles evenly distributed -> no net movement -> dynamic equilibrium

Question 20

Factors affective rate of simple diffusion (6)

Answer 20

1. Molecular size
2. Solubility in lipid bilayer
3. Concentration gradient
4. Kinetic energy (temperature) -> more transient pores
5. Surface area of cell membrane
6. Distance

Question 21

Facilitated diffusion (Passive)

Answer 21

Transport proteins (usually transmembrane) needed as ions or polar/charged molecules cannot diffuse through hydrophobic core of plasma membrane

Question 22

Facilitated diffusion transport proteins

Answer 22

Channel proteins
Hydrophilic pore -> only a particular ion or polar molecule can diffuse readily
Some might be gated

Carrier protein
Exists in 2 alternate conformations
Hydrophilic interior contains binding site -> conformation change when solute binds -> exposed to other side
Bidirectional
Depends on chance collision

Question 23

Osmosis (Passive)

Answer 23

Movement of water molecules from region of higher water potential to region of lower water potential through selectively permeable membrane -> simple/facilitated diffusion -> aquaporin proteins

Question 24

Active transport (Active)

Answer 24

Polar/charged molecules and ions against concentration gradient with expenditure of ATP
Specialised carrier proteins called pumps -> one direction only

Question 25

Bulk transport (Active)

Answer 25

Macromolecules or molecules in large quantities

Endocytosis and exocytosis

Question 26

Endocytosis

Answer 26

Uptake of substances into cell/organelle
Infolding/extension of cell surface membrane to form vesicle -> acquire macromolecules and particulate matter

1. Phagocytosis (“Cell eating”) -> solid
   Filaments in cytoskeleton rearranged to help form pseudopodia (uses ATP) -> extends outwards and engulfs particle -> ends fuse to form vesicle/vacuole
2. Pinocytosis (“Cell drinking”) -> liquid
   Small area of plasma membrane invaginate -> form tiny vesicle

Question 27

Exocytosis

Answer 27

Secretion of macromolecules by fusion of vesicles -> release contents of vesicle to extracellular environment
Export manufactured products