Membrane transport

Question 1

What are the main roles of a cell membrane?

Answer 1

Selectively permeable barrier

Maintains a constant internal environment both inside the cell and within individual organelles

Question 2

What is a pure lipid bilayer freely permeable to?

Answer 2

Water
Small HYDROPHOBIC molecules
Some small uncharged molecules e.g. CO2, O2

Question 3

Define passive transport and the 4 ways it can be carried out

Answer 3

No input of metabolic energy is required to move the molecule across the membrane 
Simple diffusion
Facilitated diffusion;
Ion channels
Aquaporins

Question 4

Define simple diffusion

Answer 4

A small uncharged molecule e.g. oxygen diffuses in the aqueous environment on one side of the membrane, diffuses into the hydrophobic interior of the bilayer and then into the aqueous environment on the other side, down its concentration gradient
No specificity or selectivity

Question 5

Define facilitated diffusion

Answer 5

Molecule moves down its concentration gradient and no energy is required, but a specific integral membrane protein (carriers, permeates, channels, transporters) are required to facilitate the movement across the bilayer.

Question 6

Are protein channels in facilitated diffusion specific to molecules?

Answer 6

Yes

Question 7

What are protein channels in facilitated diffusion dependent on?

Answer 7

temp, pH, saturation, inhibition

Question 8

How can the affinity for integral membrane transporters for various molecules be measured?

Answer 8

Km - a low Km means a high affinity for the molecule

Question 9

Define ionophore and the two different types

Answer 9

A lipid soluble molecule that is produced to carry ions cross lipid bilayers (often produced by bacteria)
Carrier ionophores are hydrophobic molecules that carry the ion in their core to shield it from the hydrophobic membrane environment
Channel forming ionophores are membrane spanning hydrophobic proteins that form a hydrophilic channel to allow ions to pass freely through

Question 10

Define ions channels

Answer 10

Highly selective proteins that allow rapid and gated passage of anions and cations across the membrane, down their concentration gradient. (A type of facilitated diffusion)

Question 11

Give two things ion channels are essential for

Answer 11

Maintaining osmotic balance
Signal transduction
Nerve impulse

Question 12

Give two examples of ion channels

Answer 12

voltage gated ca2+
Non-selective cation channels
voltage gated K+ channels

Question 13

What type of membrane transport is glucose into erythrocytes? How does this work?

Answer 13

Facilitated diffusion through the glucose transporter ion channel (GLUT1)
Glucose binds to the glucose transporter and the transporter undergoes a conformational change, glucose moves through and diffuses into the cytosol. GLUT1 returns to its original conformation.

Question 14

How and why is glucose changed when it enters the cytosol?

Answer 14

Glucose is always taken up into the erythrocyte and as soon as it enters the cytosol it is acted on by hexokinase and phosphorylated using ATP to ADP, and converted to glucose - 6- phosphate which is no longer a substrate for the glucose transporter. This ensures glucose is not transported back out of the cell.

Question 15

How specific is the glucose transporter?

Answer 15

Very specific to the D-isomer of glucose. Has a low Km for the D isomer and a high Km for the L isomer

Question 16

What are aqauporins? why are the required?

Answer 16

Water channel proteins required for the bulk flow of water across the membrane. Water can diffuse across simply, but when more water needs to be moved the transmembrane protein is used

Question 17

What is the structure of an aquaporin?

Answer 17

4 subunits each with 6 transmembrane alpha helices, with water passing through a pore in each subunit

Question 18

Where are aquaporins abundant?

Answer 18

Erythrocytes and kidney cells

Question 19

Define active transport

Answer 19

Requires the input of energy to move a molecule across the membrane, either form the hydrolysis of ATP or from the movement of an ion down its concentration gradient

Question 20

Explain the Na+/K+ ATP-driven active transport process

Answer 20

All mammalian cells have a high conc of Na+ outside the cell and a low conc inside, with a high conc of K+ inside and a low conc outside.
The Na+/K+ ATPase enzyme maintains this gradient. ATP is hydrolysed, releasing energy (both ions are moved up their concentration gradient) which is used to pump 3 Na+ ions out of the cell and 2 K+ ions into the cell, the polarises the membrane, giving a positive charge on the outside.

Question 21

Why is the Na+/K+ gradient important?

Answer 21

The Na+/K+ gradient is involved in controlling the volume of the cell, driving the active transport of amino acids and sugars, and making nerve and muscle cells electrical excitable.

Question 22

What is a coupled system?

Answer 22

The ATP is not hydrolysed unless the ions are moved and the ions are not moved unless ATP is hydrolysed

Question 23

Define ion driven active transport (co transport)

Answer 23

A form of active transport in which the movement of a molecules across the membrane against its concentration gradient is directly coupled to the movement of an ion down its concentration gradient

Question 24

What is a symport system in co transport? Give an example

Answer 24

Both molecules travel in the same direction
Movement of Na+ into the cell down the concentration gradient can be coupled to the movement of glucose into the cell via the Na+/Glucose transporter

Question 25

What is an antiport system in co transport? Give an example

Answer 25

One molecule goes into the cell while another moves out

E.g. Na+/Ca2+ exchanger maintains a low conc of Ca2+ inside the cell by moving Ca2+ out when Na+ moves in

Question 26

What are cardiotonic steroids used widely for?

Answer 26

Congestive heart failure

Question 27

What is the role of cardiotonic steroids?

Answer 27

They inhibit the Na+/K+ ATPase enzyme to increase the concentration of Na+ inside the cell and decrease the Na+ gradient across the membrane. As the Na+ gradient is required for the Na+/Ca2+ exhancger to move Ca2+ out of the cell (antiport co transport system), this leads to an increased Ca2+ concentration inside the cell, which leads to increased contractions of heart muscles and combats heart failure

Question 28

Where do cardiotonic steroids originate from?

Answer 28

They are naturally occurring plant steroids

Question 29

Where are intestinal epithelial cells found and what is their role? How are they specialised for this?

Answer 29

They line the lumen of the small intestine and are involved in absorbing nutrients from the gut (sugar, amino acids, lipids) and transporting them into the blood stream.
They have a large surface area for absorption

Question 30

What is the structure of intestinal epithelial cells?

Answer 30

They have 2 sides, an apical (brush border) membrane that faces the lumen of the small intestine and the basolateral surface that faces the bloodstream

Question 31

What is the structure/function of the apical membrane on intestinal epithelial cells?

Answer 31

Faces the lumen of the small intestine and has finger-like projections of the membrane that increase the surface area for absorption of amino acids, sugars etc.

Question 32

Where are similar epithelial cells to the small intestine found?

Answer 32

Kidney tubules, to reabsob molecules from urine

Question 33

Where is the Na+/glucose transporter located in intestinal epithelial cells? What is its role?

Answer 33

Na+/glucose transporter is in the apical membrane, and helps the movement of glucose from the gut to the epithelial cells by moving Na+ ions down their concentration gradient through the transporter (SYMPORT SYSTEM)

Question 34

Where is the glucose transporter located in intestinal epithelial cells? What is its role?

Answer 34

In the basolateral membrane

Glucose diffuses by facilitated diffusion through the glucose transporter into the blood stream

Question 35

How is the concentration of Na+ ions in intestinal epithelial cells at low levels?

Answer 35

By the action of the Na+/K+ ATPase in the basolateral membrane

Question 36

When does water move through the intestinal epithelial cells?

Answer 36

Follows glucose as it moves through, by osmosis

Question 37

Why does oral rehydration therapy for cholera need a solution of glucose and Na+ ions?

Answer 37

The uptake of glucose is critically dependent on the presence of Na+ ions in the lumen of the gut. As glucose moves into the body, it alters the osmotic pressure, causing water to follow.

Question 38

Define exocytosis

Answer 38

The secretion of proteins out of the cell across the plasma membrane to the extracellular space

Question 39

Define constitutive and regulated exocytosis

Answer 39

Constitutive is continuous and unregulated, carried out by all cells, secreted proteins and plasma membrane proteins
Regulated only occurs in specialised cells e..g neuronal and in response to a specific signal (e.g. intracellular Ca2+ concentration) and is Ca2+ dependent

Question 40

How does exocytosis occur in the nerve terminal?

Answer 40

Membrane depolarisation activates voltage gated Ca2+ channels that allow Ca2+ to rapidly enter the pre-synaptic nerve terminal, and synaptic vesicles containing neurotransmitters fuse wit the synaptic membrane and release contents to the synaptic cleft

Question 41

Define phagocytosis

Answer 41

cell ingestion of large particles by specialised cells, bacteria are then degraded by enzymes in the lysosome and the remnants are displayed on the cell surface to alert other immune cells (APC)

Question 42

Define pinocytosis

Answer 42

Cell drinking

Question 43

Define receptor mediated endocytosis

Answer 43

The selective uptake of macromolecules form the extra cellular fluid via CLATHRIN COATED PITS and vesicles.
The ligand being taken up binds to a specific cell surface receptor and the receptor molecule complexes accumulate to form a clathrin coated pit, which then forms a clathrin coated vesicle

Question 44

Why does receptor mediated endocytosis occur?

Answer 44

Provides a way of concentrating specific macromolecules that are present at low concs in the extracellular fluid (more efficient than taking up large quantities of extracellular fluid)
It is selective (receptor recognition)

Question 45

Give an example of receptor mediated endocytosis.

What is the downside of this?

Answer 45

Cholesterol uptake using the LDL receptor
This can be exploited by viruses to gain entry to cells; human rhinovirus (common cold) binds to the LDL receptor and is taken up into the cell where it replicates