Principles of Cell Function (1)

Question 1

Cellular membranes

Answer 1

Cellular membranes are fluid mosaics of proteins and a phospholipid bilayer.

Question 2

Phospholipids

Answer 2

Phospholipids are abundant amphipathic molecules and components of cellular membranes. They consist of hydrophilic heads (phosphate group) and hydrophobic tails (two fatty acids).

Question 3

Movement of phospholipids

Answer 3

Lateral movement - often

Flip-flop - once per month

Question 4

Describe how phospholipids affect membrane fluidity

Answer 4

Unsaturated carbon tails of the phospholipids have kinks which make it difficult for the phospholipids to layer, making the membrane more fluid.

Saturated fatty acid tails - viscous

Question 5

Cholesterol

Answer 5

Prevents movement of the phospholipids as a result of mechanical force or temperature change.

Question 6

Six major functions of membrane proteins

Answer 6

1. Transport
2. Enzymatic activity
3. Signal transduction
4. Cell-cell recognition (glycol-protein)
5. Intercellular joining
6. Attachment to the cytoskeleton and extracellular matrix (ECM)

Question 7

What are lipid bilayers impermeable to?

Answer 7

Ions
Small hydrophilic molecules (glucose
Macromolecules like proteins and RNA

Question 8

What are lipid bilayers permeable to?

Answer 8

Some permeability to water molecules and other SMALL, UNCHARGED (lipophilic), molecules like oxygen and carbon dioxide.

Question 9

What is passive transport?

Answer 9

Diffusion of a substance across a membrane with no energy investment.

Diffusion: with time - due to random motion molecules become equally distributed. i.e. to eliminate concentration gradients - provided molecules can cross the membrane.

Question 10

Describe osmosis

Answer 10

Diffusion of water through a selectively permeable membrane into another aqueous compartment containing solute at a higher concentration. (water wants to be at equilibrium)

Plasma membrane is semi-permeable!

Question 11

What is tonicity?

Answer 11

The ability of a solution to cause a cell to gain or lose water

Question 12

Describe isotonic, hypertonic and hypotonic solutions

Answer 12

Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane.

Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water.

Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water.

Question 13

What are the effects of different solutions on erythrocytes and plant cells?

Answer 13

Hypo: RBC lysed, plant cell turgid
Hyper: RBC shrivelled, plant cell plasmolysed
Iso: RBC normal (300osmol/L), plant cell flaccid

Question 14

Channel proteins

Answer 14

Channel proteins provide facilitated diffusion. They have a hydrophilic channel that certain molecules or ions can use as a tunnel

• Aquaporins
• Ion channels (open or close in response to stimulus)

Question 15

Transport proteins

Answer 15

Allow passage of hydrophilic substances across a membrane.
A transport protein is specific for the substance it moves

Facilitated diffusion or active transport

Question 16

Carrier proteins

Answer 16

Bind to molecules and and change shape to shuttle them across the membrane

Question 17

Passive transport

Answer 17

Down a concentration gradient

Question 18

Active transport

Answer 18

Against a concentration gradient

Requires work - from energy of ATP

e.g. Na+/K+-ATPase = sodium-potassium pump

Question 19

Electrogenic pump

Answer 19

transport protein that generates a voltage across a membrane

Question 20

Sodium-potassium pump

Answer 20

major electrogenic pump of animal cells

EP - transport protein that generates a voltage across a membrane

Question 21

Proton pump

Answer 21

main electrogenic pump of plants, fungi, and bacteria

EP - transport protein that generates a voltage across a membrane

Question 22

Cotransport

Answer 22

occurs when active transport of a solute drives transport of another solute

e.g. plants commonly use the gradient of protons generated by proton pumps to drive the active transport of other nutrients into the cell

Question 23

Bulk transport

Answer 23

Endocytosis and exocytosis

Types of endocytosis:
Phagocytosis -engulfing particles
Pinocytosis - cells drinking
Receptor-mediated endocytosis

Example of exocytosis:
Neurotransmitter release from neurons

Question 24

Cell to cell communication stages

Answer 24

1. Reception
2. Transduction
3. Response

Question 25

Ion channel receptors

Answer 25

Plasma membrane receptor

Na+ channels open by ligand - fast neurotransmission

Question 26

G protein-coupled receptors

Answer 26

Plasma membrane receptor
7 transmembrane-spanning regions - all aspects of physiology and pharmacology

First messenger —> receptor —> g-protein —> Second messengers—> Kinase —> cellular response

Adrenaline (first messenger) binds to a GP linked receptor. Interacts with heterotrimeric G proteins which binds to a target enzyme to make a second messenger (like cAMP). This causes protein kinase A to induce a cellular response.

Question 27

Tyrosine kinase linked receptors

Answer 27

Plasma membrane receptor
e.g. insulin receptors - metabolism, cell growth, cell reproduction

Insulin binds to tyrosine kinase linked receptor. Kinases transfer phosphates and activate or inactivate target proteins.

Question 28

Steroid receptors

Answer 28

Intracellular receptor

Question 29

Protein phosphorylation (in TKLRs)

Answer 29

Causes conformational change

Protein-protein interactions

Change in cellular location

These changes lead to activation or inactivation of the target proteins