Movement Across Membranes

Question 1

What is the purpose of a membrane?

Answer 1

Maintains the internal environment
Selectively permeable

Question 2

What do impermeable substances need for membrane transport?

Answer 2

Transport proteins
Energy

Question 3

What are channel proteins?

Answer 3

Narrow aqueous pore
Selective to size and charge
Passive
May be gated (voltage or ligand)
Usually ions (e.g. Na+, K+) or water (aquaporins)

Question 4

What are carrier proteins?

Answer 4

Specific binding site
Undergoes a conformational change
Different types of carrier proteins
Active (pumps) or passive

Question 5

What are the different types of carrier proteins?

Answer 5

Uniport – single substance
Symport – two substances in the same direction
Antiport – two substances in the opposite direction

Question 6

What are driving forces?

Answer 6

Forces that drive substances into/out of cells

Question 7

What are 3 main driving forces?

Answer 7

Chemical
Electrical
Electrochemical

Question 8

What are driving forces based on?

Answer 8

Based on the presence of a gradient

Substances either move with the gradient (high to low) or can move against the gradient (low to high) with help

Question 9

What are chemical driving forces?

Answer 9

•Based on concentration differences across the membrane
•All substances have a concentration gradient
•Force directly proportional to the concentration gradient

Question 10

What are electrical driving forces?

Answer 10

•Also known as membrane potential
•Based on the distribution of charges across the membrane
•Only charged substances e.g. Na+, K+
•Force depends on size of membrane potential and charge of the ion

Question 11

What are electrochemical driving forces?

Answer 11

•Combines the chemical and electrical forces
•Net direction is equal to the sum of chemical and electrical forces
•Only charged substances e.g. Na+, K+

Question 12

What are the two main types of membrane transport?

Answer 12

Passive
Active

Question 13

What is passive transport?

Answer 13

•Does not require an input of energy
•Substance moves down its gradient (high to low)
•Two types:
•Simple diffusion e.g. gases
•Facilitated diffusion - mediated by proteins (channel or carrier)

Question 14

Give an example of passive transport

Answer 14

•GLUT4 carrier protein:
•Expressed in skeletal muscle and adipose tissue
•Glucose uptake by facilitated diffusion
•Expression upregulated by insulin

In the absence of insulin, glucose cannot enter the cell
Insulin signals the cell to insert GLUT 4 transporters into the membrane, allowing glucose to enter cell

Question 15

What happens when GLUT4 goes wrong?

Answer 15

•GLUT1 present in many cells, including the brain, where it transports glucose across the blood-brain barrier via facilitative diffusion
•GLUT1 Deficiency Syndrome:
•Very rare disorder
•Mutations in gene that encodes GLUT1
•Less functional GLUT1 -
reduces the amount of glucose available to brain cells
•Symptoms include seizures, microcephaly, developmental delay

Question 16

What is active transport?

Answer 16

•Requires an input of energy
•Substance moves against its gradient (low to high)
•Two types:
•Primary
•Secondary

Question 17

What is primary active transport?

Answer 17

Directly uses a source of energy, commonly ATP
Common example is Na+/K+-ATPase:

Question 18

What happens when Na+/K+-ATPase goes wrong?

Answer 18

•ATP7B protein is a Cu2+-ATPase present in the liver that transports copper into bile
•Wilson’s disease
•Rare disorder
•Mutations in ATP7B gene
•Results in deposition of copper in the liver and other tissues e.g. brain, eyes
•Symptoms include liver disease, tremor,
Kayser-Fleischer rings,

Question 19

What is secondary active transport?

Answer 19

•Transport of a substance against its gradient COUPLED to the transport of an ion (usually Na+ or H+), which moves down its gradient
•Uses energy from the generation of the ions electrochemical gradient (usually by primary active transport)

Question 20

Give an example of secondary active transport?

Answer 20

•Example is the Na+/glucose cotransporter proteins (SGLT):
•Present in intestinal lumen and renal tubules
•Transports glucose from low to high concentration
•Na+/K+-ATPase generates a sodium gradient to enable
co-transport of sodium and
glucose

Question 21

What happens when Na+/glucose cotransporter proteins (SGLT) goes wrong?

Answer 21

•Glucose-Galactose Malabsorption:
•Very rare disorder
•Mutations in SGLT1
•Less functional SGLT1 - inability to transport glucose and galactose, resulting in their malabsorption
•Symptoms include severe, chronic diarrhoea, dehydration, failure to thriv

Question 22

What is cellular signalling?

Answer 22

•Communication between cells takes place via signalling molecules e.g. hormones, neurotransmitters and growth factors
•Signalling molecules bind to receptors:
•Intracellular – e.g. steroid hormones
•Cell-surface – e.g. peptide hormones
•Second messengers e.g. cAMP, IP3, DAG, Ca2+ - amplification
•Affect gene expression in the nucleus either directly or through signalling cascades

Question 23

What happens when cellular signalling goes wrong?

Answer 23

•G proteins integral part of G-protein- coupled receptors on cell membrane surfaces
•Cholera:
•Vibrio cholerae bacteria produce the cholera toxin
•This crosses the cell membrane
•Modifies a subunit of the G protein
•Results in increased second messenger (cAMP) levels
•This stimulates several transporters in the cell membrane of intestinal cells
•Results in massive secretion of ions and water into the gut
•Leads to severe diarrhoea and dehydration that can be fatal

Question 24

What are the other types of membrane transport?

Answer 24

Endocytosis
Exocytosis

Question 25

What is endocytosis?

Answer 25

Transport into a cell

Absorb/engulf molecules into a cell
A portion of the membrane is invaginated to form a membrane bound vesicle called an endosome

Question 26

What is exocytosis?

Answer 26

Transport out of a cell

Vesicle from the Golgi apparatus fuses with the plasma cell membrane, resulting in the expulsion of waste or the secretion of enzymes/hormones

Question 27

What is cystic fibrosis?

Answer 27

•Common hereditary disorder (1 in 25 people are carriers)
•Mutation in CFTR protein
•Chloride channel
•Found in many tissues e.g. pancreas, lungs and skin
•Abnormal function results in sticky, viscous mucus

Question 28

Give examples of drugs that target membrane transporters

Answer 28

•Cardiac glycosides e.g. digoxin
Act on Na+/K+-ATPase in cardiac muscle cells
•Proton pump inhibitors e.g. omeprazole
Act on H+/K+-ATPase in gastric parietal cells
•Loop diuretics e.g. furosemide
Act on NKCC2 cotransporter in thick ascending limb of loop of Henle
•Thiazide diuretics e.g. bendroflumethiazide
Act on NCC cotransporter in distal tubule

Question 29

Where does endocytosis occur?

Answer 29

Occurs in neutrophils & macrophages

They implement phagocytosis whereby they engulf entire cells/macromolecules to form a phagosome

Question 30

What is pinocytosis (drinking)?

Answer 30

Bringing in dissolved solutes

Question 31

What is facilitated diffusion?

Answer 31

The movement of solutes from a region of their high concentration to a region of their low concentration through protein channels (without carrier proteins)

This continues until dynamic equilibrium is reached