Movement Across Membranes

Question 1

Membrane permeability

Answer 1

• Maintains the internal environment

•Selectively permeable

•For membrane transport, impermeable substances may need:
•Transport proteins
•Energy

Question 2

Membrane proteins- channels

Answer 2

•Narrow aqueous pore

•Selective:
•Size
•Charge

•Passive

•May be gated (voltage or ligand)

•Usually ions (e.g. Na+, K+) or water (aquaporins)

Question 3

Membrane proteins- Carriers

Answer 3

•Specific binding site

•Carrier undergoes a conformational change

•Different types:
•Uniport – single substance
•Symport – two substances in the same direction
•Antiport – two substances in the opposite direction

•Active (pumps) or passive

Question 4

Driving forces

Answer 4

Three main forces that drive substances into/out of cells:
•Chemical
•Electrical
•Electrochemical

•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 5

Driving forces- Chemical

Answer 5

•Based on concentration differences across the membrane

•All substances have a concentration gradient

•Force directly proportional to the concentration gradient

Question 6

Driving Forces- Electrical

Answer 6

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 7

Driving Forces- Electrochemical

Answer 7

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 8

Types of Membrane Transport

Answer 8

2 main types:

Passive

Active

Question 9

Passive

Answer 9

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 10

Example; Glucose

Answer 10

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

(A) In the absence of insulin, glucose cannot enter the cell

(B) Insulin signals the cell to insert GLUT 4 transporters into the membrane, allowing glucose to enter the cell

Question 11

Example; Too much glucose in pregnancy

Answer 11

Principal fetal nutrient
-Fetal gluconeogenic enzymes inactivated
-low arterial PO2

From maternal circulation

Transports 0.6 mmol/min/g placental tissue (~0.11g glc/min/g)

Carrier system saturates at ~20 mmol/L (Glut-1; ~3.6g/L)
-Fetal glucose levels directly related to mother’s
-No mechanism to limit uptake below saturation point (the point at which no more can be absorbed)
-Excess glucose can cause significant fetal harm

Question 12

When it goes wrong

Answer 12

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 13

Active Transport

Answer 13

Requires an input of energy

Substance moves against its gradient (low to high)

Two types:
-Primary
-Secondary

Question 14

Active Transport- Primary

Answer 14

Directly uses a source of energy, commonly ATP

Common example is Na+/K+-ATPase:
-Pumps 3 Na+ out of the cell, 2 K+ into the cell
-Utilises the hydrolysis of ATP to ADP + Pi

Question 15

When it goes wrong

Answer 15

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 16

Active Transport- Secondary

Answer 16

Transport of a substance against its gradient COUPLED to the transport of an ion (usually Na+ or H+), which moves down its gradient

Secondary as doesn’t use energy directly from ATP but uses energy stored in the, for example Na+ gradient (from high to low), which was created using ATP

Uses energy from the generation of the ions electrochemical gradient (usually by primary active transport)

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 17

When it goes wrong

Answer 17

SGLT1 transports glucose and galactose from the intestinal lumen

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 thrive

Question 18

Cellular signalling

Answer 18

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 19

When it goes wrong

Answer 19

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 Gas subunit (Intra-cellular action)
-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 20

Endocytosis and exocytosis

Answer 20

Large molecules require different methods of transport:

1. Endocytosis:
   Transport into a cell
2. Exocytosis:
   Transport out of a cell

Question 21

Cystic Fibrosis

Answer 21

Common hereditary disorder (1 in 25 people are carriers)

Mutation in CFTR protein
-Chloride channel
-Found in many tissues e.g. Gut, pancreas, lungs and skin
-“secretory epithelium”
-Abnormal function results in sticky, viscous mucus
-No osmotic drag