Module 1: Principles of Cell Function

Question 1

What are the 2 main functions of Phospholipid movement?

Answer 1

1. Lateral movement (10^7 times per second)

2. Flip-flop (once per month)

Question 2

Describe 2 properties of membrane fluidity?

Answer 2

1. Unsaturated hydrocarbon tails with kinks = fluid

2. Saturated hydrocarbon tails = viscous

Question 3

Cholesterol increases fluidity, describe with reference to different temperatures:

Answer 3

1. High temperatures stabilises and raises melting point (more viscous)
2. Low temperatures prevents stiffening (more fluid)

Question 4

List the 7 things that Phospholipid bilayers contain:

Answer 4

1. Fibres of extracellular matrix (ECM)
2. Glycoproteins with carbohydrates attached (surface/peripheral)
3. Cholesterol (internal)
4. Microfilaments of cytoskeleton (inside)
5. Peripheral proteins
6. Integral Proteins: (Hydrophilic regions on the outside, hydrophobic regions in the middle, N-terminus on the outside, C-terminus on the inside)
7. Glycolipids

Question 5

Describe the Fluid Mosaic Model

Answer 5

• Mouse cell and human cell combined as hybrid cell

- Mixed surface proteins within an hour due to frequent lateral movement

Question 6

List the 6 major functions of membrane proteins:

Answer 6

1. Transport (active/facilitated)
2. Enzymatic activity
3. Signal transduction ( receives signalling molecule, performs signal transduction)
4. Cell to cell recognition (via glycoproteins/peripheral proteins)
5. Intercellular joining
6. Attachment to the cytoskeleton and ECM (extracellular matrix)

Question 7

Describe selective permeability and transport

Answer 7

• lipid bilayers are impermeable to most essential molecules and ions
• only allows some water molecules and small, uncharged molecules
• not permeable to ions, hydrophilic molecules and macromolecules

Question 8

Describe Passive transport

Answer 8

With time, due to random motion, molecules become equally distributed to eliminate concentration gradients via diffusion

Question 9

Describe Osmosis

Answer 9

• diffusion of water through a selectively permeable membrane into another aqueous compartment containing solute at a higher concentration
• equalises concentration

Question 10

Define ‘Osmotica’ and give examples

Answer 10

• things that are osmotically active

e. g. Ions, sugars, proteins

Question 11

Define tonicity, list the 3 different types of tonicity and the differences in plant and animal cells

Answer 11

• Tonicity is the ability of a solution to cause a cell to gain or lose water
• Isotonic: solute conc. is the same, no net movement
• Hypertonic: solute conc. is GREATER than that inside the cell, cell loses water: cell shrivels
• Hypotonic: solute conc. is LESS than that inside the cell, cell gains water: cell bursts (or lyses)
• Animal cells can lyse (burst) or shrivel
• Plant cells can be turgid, flaccid or plasmolysed

Question 12

Outline Facilitated Diffusion and list the 3 types of proteins used in this process

Answer 12

• transport proteins facilitate passive movement of molecules across the plasma membrane for specific proteins
  Channel proteins: allow a specific molecule or ion to cross the membrane (e.g. Aquaporins - for water, Ion-gated channels)
  Transport proteins: allow passage of hydrophilic substances (facilitated or active)
  Carrier proteins: bind to molecules and change their shape to shuttle them across the membrane

Question 13

Outline Active Transport

Answer 13

• against concentration gradient (requires ATP, e.g. sodium-potassium Na+/K- ATPase)
• an electrogenic pump is a transport protein that generates voltage across a membrane (Sodium-potassium pump is a major electrogenic pump of animal cells, proton pump is main electrogenic pump of plants, fungi and bacteria)

Question 14

Outline Co-transport

Answer 14

• coupled transport by a membrane protein
• when active transport of a solute indirectly drives transport of another solute
• e.g. plants use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell (proton pump pumps H+ out of cell, sucrose-H+ co-transporter transports sucrose into the cell, puling H+ in)
• sodium-potassium pump works with Na+ glucose co-transporter to drive glucose uptake

Question 15

Describe bulk transport across the plasma membrane occurs by exocytosis and endocytosis

Answer 15

Endocytosis: pinches off cell membrane to transport product from outside
Exocytosis: cell membrane of vesicle fuses with cell membrane to deliver product to outside
Phagocytosis: engulfing particles
Pinocytosis: drinking particles
Receptor-mediated endocytosis: coated vesicles form from receptors receiving signals, which are carried inside vesicle (may be alongside other particles)

Question 16

Outline local signalling

Answer 16

Paracrine signalling: secreting cells discharge molecules of a local regulator into the extracellular fluid of a target cell
Synaptic signalling: a nerve cell releases neurotransmitter to the post-synaptic neutron, triggering an action potential

Question 17

Outline long-distance signalling

Answer 17

Hormonal signalling: specialised endocrine cells secrete hormones into body fluids, e.g. blood, that can reach virtually any cell

Question 18

Describe the 3 stages of cell signalling

Answer 18

1. Reception: a receptor protein in the plasma membrane receives a signalling molecule
2. Transduction: relay molecules translate the signal in a signal transduction pathway
3. Response: a cellular response is activated

Question 19

Give 2 types of Reception

Answer 19

1. Hydrophilic signalling molecule and cell-surface receptor
2. Carrier protein: small hydrophobic signalling molecule and intracellular receptor

Question 20

List 3 types of responses

Answer 20

1. Gene activation
2. Enzyme stimulation
3. To rearrange the cytoskeleton

Question 21

List and describe the 3 types of Ion Channel Receptors in order of speed

Answer 21

1. Ion Channel Receptors: Na+ channel opened by ligand: fast neurotransmitter
2. G Protein-Coupled receptors: 7 transmembrane-spanning regions: applies to all aspects of physiology and pharmacology
3. Tyrosine Kinase Linked Receptors (e.g. Insulin receptors): metabolism, cell growth, cell reproduction

Question 22

List and describe the 1 type of Intracellular receptors

Answer 22

1. Steroid Receptors (e.g. testosterone): long-distance signalling that can persist in blood stream from hours to months

Question 23

Outline 3 points of G-Protein Coupled Receptors

Answer 23

• largest family of receptors
• activated by: light, ions, odourants, neurotransmitters, hormones, peptides, proteins, etc.
• interact with G-proteins to control cellular response

Question 24

Outline the 7 steps of how G-Protein coupled receptors work:

Answer 24

1. Signal molecule (adrenaline) binds to G-protein coupled receptor
2. It facilitates a G-protein to gain energy from a GTP molecule, converting it to GDP (di instead of tri)
3. The G-protein uses this energy to send a signal to adenyl cyclase
4. Adenyl cyclase converts ATP into cyclic AMP (cAMP)
5. Phosphodiesterase regales the duration of activity of cAMP by converting it to its inactive form, AMP
6. One of cAMP’s various activities is that it can activate protein kinase A (PKA)
7. PKA can in turn produce cellular responses specific to the signal molecule (adrenaline in this case)

Question 25

Outline 2 points with regards to Tyrosine kinase linked receptors:

Answer 25

• phosphorylation cascades mediated by enzymes called kinases transfer phosphate and activate or inactivate target proteins
• Phosphorylation results in: conformational change, protein-protein interactions, change in cellular location

Question 26

Outline 1 point with regards to Insulin receptors (part of Tyrosine kinase linked receptors):

Answer 26

• insulin’s role is to increase glycogen synthesis. One pathway is via PI- 3 kinase

Question 27

Outline the 7 steps of how Tyrosine kinase linked receptors work (in particular: the inactivation of glycogen synthase kinase 3 to indirectly increase glycogen synthesis):

Answer 27

1. Insulin binds to insulin receptor
2. Insulin receptor substrate I (IRS1) activates PI- 3 kinase
3. PI-3 kinase converts PIP2 to PIP3
4. PIP3 indirectly activates protein kinase B via phosphorylation, which can itself perform phosphorylation on a range of target proteins
5. One such protein is glycogen synthase kinase 3 (GSK -3), the role of which is to inactive glycogen synthase (GS)
6. Phosphorylation inactivates GSK-3, so that it cannot phosphorylates GS and make it inactive
7. This is an inhibitory pathway that inhibits another inhibitory pathway. Indirectly, insulin increases glycogen synthesis

Question 28

Outline 5 points with regards to Steroid hormones:

Answer 28

1. Steroid hormones are hydrophobic, and must therefore be transported inside the cell membrane by HORMONE RECEPTOR COMPLEXES
2. They require specific carrier proteins in the blood, unlike hydrophilic proteins
3. They travel freely in the extracellular fluid and through the plasma membrane
4. Within the cytoplasm, they require being carried by hormone receptor complexes
5. They are carried to the nucleus where they are received by a receptor and promote the synthesis of a specific protein