Key Area 3 - Membrane Proteins

Question 1

Movement of Molecules Across Membranes

The Fluid Mosaic Model of Cell
Membranes

Question 2

What does the plasma membrane consist of?

Answer 2

A fluid mosaic bilayer of phospholipids and proteins.

Hydrophilic head align to the cytoplasm and the hydrophobic tails point inwards.

Question 3

What holds integral membrane proteins within the phospholipid bilayer?

Answer 3

Regions of hydrophobic R groups allow
strong hydrophobic interactions that hold
integral membrane proteins within the
phospholipid bilayer.

Question 4

Integral membrane proteins interact with what?

Answer 4

Integral membrane proteins interact
extensively with the hydrophobic region of
membrane phospholipids.

Question 5

Some integral membrane proteins can be what?

Answer 5

Some integral membrane proteins are
transmembrane proteins.

Question 6

Explain peripheral membrane proteins

Answer 6

Peripheral membrane proteins have
hydrophilic R groups on their surface and are bound to the surface of membranes, mainly by ionic and hydrogen bond interactions.

Question 7

What do peripheral membrane proteins interact with?

Answer 7

Many peripheral membrane proteins interact with the surfaces of integral membrane proteins.

Question 8

What is the phospholipid bilayer?

Answer 8

The phospholipid bilayer is a barrier to ions
and most uncharged polar molecules.

Question 9

What molecules can pass through the bilayer by simple diffusion?

Answer 9

Some small molecules, such as oxygen and
carbon dioxide, pass through the bilayer by
simple diffusion.

Question 10

What is facilitated diffusion?

Answer 10

Facilitated diffusion is the passive transport
of substances across the membrane through specific transmembrane proteins.

Question 11

To perform specialised functions, different
cell types have different what?

Answer 11

To perform specialised functions, different
cell types have different channel and
transporter proteins.

Question 12

What are channels?

Answer 12

Channels are multi-subunit proteins with the subunits arranged to form water-filled pores that extend across the membrane.

Most channel proteins in animal and plant
cells are highly selective.

Question 13

List the other names in transport:

Answer 13

Uniport - one substance moved.
Contrasport - two substances moved.
Symport - both in same direction across membrane.
Antiport - move in opposite directions.

Question 14

How do some channel proteins allow or prevent diffusion?

Answer 14

Some channel proteins are gated and
change conformation to allow or prevent
diffusion

Question 15

Explain the two gated channel proteins

Answer 15

Transport is always passive. Solute passage can be gated or ungated.

Ligand Gated Channel Proteins - allow passage of solutes by altering the conformation.

Volted Gated Channel Proteins - allow solute passage by changing ion concentration.

Question 16

What are ligand-gated and voltage-gated channels controlled by?

Answer 16

Ligand-gated channels are controlled by the binding of signal molecules, and voltage-gated channels are controlled by changes in ion concentration

Question 17

What do transporter proteins bind to and undergo?

Answer 17

Transporter proteins bind to the specific
substance to be transported and undergo a
conformational change to transfer the solute across the membrane.

Transporters alternate between two
conformations so that the binding site for a
solute is sequentially exposed on one side of the bilayer, then the other.

Passive Transport - with the concentration or electrochemical gradient (facilitated diffusion).

Active Transport - against the concentration or electrochemical gradient.

Question 18

What does active transport use?

Answer 18

Active transport uses pump proteins that
transfer substances across the membrane
against their concentration gradient.

Pumps that mediate active transport are
transporter proteins coupled to an energy
source.

Question 19

What is required for active transport?

Answer 19

A source of metabolic energy is required for active transport.

Question 20

Why do some active transport proteins hydrolyse ATP directly?

Answer 20

Some active transport proteins hydrolyse
ATP directly to provide the energy for the
conformational change required to move
substances across the membrane.

ATPases hydrolyse ATP.

Question 21

Ion Transport Pumps and Generation of Ion Gradients

Question 22

What combines to form the
electrochemical gradient?

Answer 22

For a solute carrying a net charge, the
concentration gradient and the electrical
potential difference combine to form the
electrochemical gradient that determines the transport of the solute.

Question 23

How is a membrane potential created?

Answer 23

A membrane potential (an electrical potential difference) is created when there is a difference in electrical charge on the two sides of the membrane.

Question 24

What uses energy from the hydrolysis of ATP and why?

Answer 24

Ion pumps, such as the sodium-potassium
pump, use energy from the hydrolysis of ATP to establish and maintain ion gradients.

Question 25

Explain how the sodium-potassium pump transports ions

Answer 25

The sodium-potassium pump transports ions against a steep concentration gradient using energy directly from ATP hydrolysis.

It actively transports sodium ions out of the
cell and potassium ions into the cell.

The pump has high affinity for sodium ions
inside the cell; binding occurs;
phosphorylation by ATP; conformation
changes; affinity for sodium ions decreases;
sodium ions released outside of the cell;
potassium ions bind outside the cell;
dephosphorylation; conformation changes;
potassium ions taken into cell; affinity returns to start.

For each ATP hydrolysed, three sodium ions are transported out of the cell and two
potassium ions are transported into the cell. This establishes both concentration gradients and an electrical gradient.

Question 26

Where is the sodium-potassium pump found and what does it account for?

Answer 26

The sodium-potassium pump is found in most animal cells, accounting for a high proportion of the basal metabolic rate in many organisms.

Question 27

In the small intestine, the sodium gradient
created by the sodium-potassium pump
drives the active transport of glucose. Explain

Answer 27

In intestinal epithelial cells the sodium-potassium pump generates a sodium ion gradient across the plasma membrane.

Question 28

What does the glucose transporter responsible for glucose symport transport?

Answer 28

The glucose transporter responsible for this glucose symport transports sodium ions and glucose at the same time and in the same direction.

Sodium ions enter the cell down their
concentration gradient; the simultaneous transport of glucose pumps glucose into the cell against its concentration gradient.