Cellular Processes

Question 1

Structure of Plasma Membrane

Answer 1

Phospholipid bilayer of thickness 8nm. Contains proteins which diffuse laterally through the membrane.
Glycolipids form a sugar coating called a glycocalyx on the membrane.
Phospholipids tend to remain on one face.

Question 2

Factors affecting membrane fluidity

Answer 2

Length of tails: Length increases dispersion forces between tails and lipids pack together more tightly.
Saturation of tails: Reduced saturation means more kinks in the tail. This prevents tight packing and allows increased movement.

Question 3

Integral Protein Function

Answer 3

Receptors, cell identity markers, linkers (joins cell to extracellular fibres), enzymes, ion channels and carriers.

Question 4

Peripheral Protein Function

Answer 4

Moving cytoskeleton and maintaining cell shape.

Question 5

Extracellular Na, Cl and K concentration

Answer 5

Na: 150mmol
K: 5mmol
Cl: 150mmol

Question 6

Intracellular Na, Cl and K concentration

Answer 6

Na: 15mmol
K: 150mmol
Cl: 10mmol

Question 7

Gated ion channels

Answer 7

Water filled protein pore with hydrophobic amino acids lining the regions facing into the hydrophobic core.
Does not require ions to bind so does not exhibit saturation kinetics.
Gating regulated by conformational change upon stimulus (phosphorylation, ligand, potential).
Movement of ions through this is electrogenic.

Question 8

Carrier-Mediated Facilitated Diffusion

Answer 8

Passive process- but proteins required to undergo conformational change.
Protein is ‘opened’ to one side to allow ions to bind and trigger a conformational change.
Vmax is applicable as ions must bind- saturation kinetics apply.

Question 9

Primary Active Transport

Answer 9

Movement of molecules against their concentration using energy from ATP hydrolysis- 30% of cellular ATP consumption.

Question 10

Mechanism of NaKATPase

Answer 10

3Na and ATP bind to protein and ATP is hydrolysed to release energy. Conformational change occurs and protein opens up towards extracellular space for Na to diffuse out and 2K bind. ADP and Pi dissociates and conformational change is reversed.

Question 11

Secondary Active Transport

Answer 11

Uses energy stored in ion gradients created by primary active transport ions. Can be symporters or antiporters. Involved in absorption of glucose, secretion of Cl- and removal of H+.

Question 12

Paracellular Transport

Answer 12

Governed by the permeability of the tight junction. Leaky tight junctions have lower resistance to ion movement and allows for bulk movement of material. This means of transport dominates if tight junction resistance is low.
Tight junction resistance increases towards the distal side of tracts, as absorption becomes more controlled.

Question 13

Transcellular Transport

Answer 13

Movement of solutes through the cell via transport proteins. Protein activity is controlled so this pathway is used in the movement of specific molecules in controlled amounts.
Requires polarity to be maintained by tight junctions.
This means of transport dominates if tight junction resistance is high.

Question 14

Process of Glucose Absorption

Answer 14

NaKATPase actively transports Na+ out of the cell through the apical membrane to set up Na+ gradient.
Na+ diffuses down SGLT along with glucose to accumulate it above its concentration gradients.
Glucose can only diffuse out of the cell via GLUT proteins in the basolateral membrane.
Movement of Cl- and H2O in the paracellular pathway follows- isotonic fluid absorption.

Question 15

Oral Rehydration Therapy

Answer 15

Using a NaCl solution with glucose. By stimulating glucose reabsorption, isotonic fluid absorption occurs and reduces loss of water and ions.

Question 16

Reasons why glucose isn’t absorbed

Answer 16

Glucose/galactose malabsorption syndrome: Impairment of SGLT due to autosomal recessive mutation prevents glucose absorption . Leads to loss of water by osmosis. Therapy is to use fructose as source of carbohydrate as it uses GLUT5.
Glycosuria: When there is excess glucose in the blood, SGLTs in the renal tubules will be saturated and some glucose will not be reabsorbed and end up in the urine.

Question 17

Mechanism of Chloride Secretion

Answer 17

Active transport of 3 Na+ out by NaKATPase to accumulate an inward electrochemical gradient. Cl- is actively transported in along with Na and K by secondary transport via NKC (NaK2Cl) channels.
K+ diffuses out passively to hyperpolarise the cell and make the electrochemical gradient directed outwards more steep.
Cl- diffuses out through the CFTR, which is highly regulated and is thus the rate limiting step of chloride secretion.
Na+ follows in the paracellular pathway to maintain charge neutrality and water follows to maintain osmolarity- isotonic fluid secretion.

Question 18

Mechanism of Secretory Diarrhea

Answer 18

Overstimulation of CFTRs of crypt cells in intestine and colon by excess endogenous secretagogues due to tumours/inflammation, or by enterotoxins.
Irreversible binding and activation of adenylate cyclase results in constant phosphorylation (and opening) of the CFTR. Leads to excessive rate of chloride secretion (as it’s dependent of CFTR activity).
Extensive loss of water and electrolytes due to excess isotonic fluid secretion. Can be treated by oral rehydration therapy while epithelium is replaced naturally.

Question 19

CFTR Structure

A little late in the stack yea

Answer 19

Channel protein with binding sites for ATP and an R-Domain which can be phosphorylated to regulate activity.
ATP can only bind when r-domain is phosphorylated.

Question 20

Cystic Fibrosis

Answer 20

Caused by an autosomal recessive mutation. Prevents the diffusion of Cl- into the lumen so there is no paracellular isotonic fluid secretion. Results in very thick mucus which can block tracts such as GE tract. Mucus is not removed so pathogens are accumulated within it. Leads to increased likelihood of infection and inflammation.

Question 21

Sweat Formation

Answer 21

Produced by isotonic fluid secretion initiated by secretion of Cl- through either CFTRs or CLCAs activated by Ca2+. Excess Cl- is reabsorbed by depolarised duct cells by secondary reabsorption when Na diffuses down the ENaC while Cl- follows down the concentration. Water is not reabsorbed due to tight junctions and absence of aquaporins.

Question 22

Why does Cystic Fibrosis lead to Salty Sweat?

Answer 22

CFTR cannot secrete Cl- but the CLCA can. However the Cl- is not reabsorbed as only CFTRs are present on the duct cells. This means Na+ is also not reabsorbed, leading to NaCl present in the sweat.