1.3 Membrane Proteins

Question 1

What are the two parts of the cell membrane?

Answer 1

phospholipid bilayer and the membrane proteins

Question 2

Describe the chemistry of the phospholipid bilayer and the direction of lipids

Answer 2

Chemically it is a ‘hybrid molecule’ with one end that is strongly hydrophobic (the tails) and one that is hydrophilic (the heads).

The bilayer is created by phospholipids organising themselves so that the tails face into the bilayer and the heads face out into the aqueous medium (this is either the cytoplasm or the external fluid)

Question 3

Describe the movement of molecules through membranes in regards to chemistry

Answer 3

The hydrophobic centre allows small non-polar molecules such as oxygen and carbon dioxide to pass directly through the cell membrane. Larger signalling molecules can also pass straight through as long as they are hydrophobic.

The centre of the membrane acts as a barrier to charged ions (H+, Na+, amino acids) and most polar molecules (water, glucose).

Question 4

What phrase can be used to describe the cell membrane structure

Answer 4

Fluid Mosaic

(The cell membrane is embedded with proteins which form a patchy mosaic)

Question 5

What are the two classes of membrane proteins?

Answer 5

Pheripheral and Integral

Question 6

Describe integral proteins

Answer 6

Integral proteins are held embedded within the membrane, this occurs due to strong hydrophobic interactions with the phospholipid tails

Some integral proteins only extend part way into the bilayer while others are transmembrane and span the entire width of the membrane, for example channel proteins, transporters and many receptors.

Question 7

Describe Peripheral proteins

Answer 7

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

Peripheral proteins on the inside of the membrane give mechanical support and shape to the cell.

Question 8

What types of molecules can pass through the phospholipid bilayer?

Answer 8

hydrophobic molecules

Question 9

How do Hydrophilic molecules pass through the phospholipid bilayer?

Answer 9

With the assistance of a membrane protein.

Question 10

What do membrane proteins do?

Answer 10

control the concentration of ions and other molecules within the cell

Question 11

Talk about transmembrane proteins in terms of specificity

Answer 11

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

Question 12

What is the role of Channel Proteins?

Answer 12

These proteins allow molecules to move from one side of the membrane to the other without using energy (passive transport)

Question 13

What is facillitated diffusion?

Answer 13

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

Question 14

Some channel proteins are gated and allow or preven diffusion by undergoing

Answer 14

Confirmational Change

Question 15

What Types of gated channel proteins are there and what types of stimuli do they respond to?

Answer 15

ligand-gated - Chemical stimuli
voltage-gated - Electrical stimuli

Question 16

How do ligand-gated channels work?

Answer 16

A signal molecule binds to the protein and changes it confirmation. This change will either open or close the channel.

Question 17

Describe voltage-gated channels

Answer 17

If there is a large enough change in the ion concentrations across the membrane, this can change the confirmation of the protein.

Again, this change will either open or close the channel.

Question 18

Talk about transporter proteins in terms of the movement of substances

Answer 18

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

Question 19

Describe active transport and transfer of Molecules

Answer 19

Active transport uses pump proteins that transfer substances across the membrane against their concentration gradient, these pumps are coupled to an energy source. Due to this a source of metabolic energy is required for active transport.

Question 20

Where does the energy for active transport come from?

Answer 20

requires the energy from the direct hydrolysis of ATP or more accurately; the protein requires the binding of a phosphate from ATP.

Question 21

What is the electrochemical gradient and what does it do?

Answer 21

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 22

When is a membrane potential created and what maintains it?

Answer 22

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

Ion pumps use energy from the hydrolysis of ATP to establish and maintain ion gradients.

Question 23

One example of an ion pump

Answer 23

sodium-potassium pump

Question 24

What is the purpose of the sodium-potassium pump

Answer 24

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

Question 25

What is the direction of active transport in the sodium-potassium pump

Answer 25

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

Which goes against their concentration gradients by using ATP to generate new ion gradients.

Question 26

What number of sodium ions and potassium ions are involved in the sodium poatssium pump

Answer 26

Each time 3 sodium ions are pumped out and 2 potassium ions are pumped in

Question 27

Describe the stages of the sodium potassium pump

Answer 27

1. The transporter protein has ion-binding sites exposed to the cytoplasm (inside the cell). In this state, the protein has a high affinity for 3 sodium ions therefore binding occurs.
2. Phosphorylation by ATP causes a conformational change in the protein
3. Now that the protein’s shape has changed, the affinity for sodium decreases and the ions are released outside the cell.
4. This new confirmation has a high affinity for potassium and as such, 2 ions will bind to the newly exposed binding sites on the part of the protein that is on the outside of the cell.
5. Dephosphorylation occurs causing the protein to revert to its original confirmation.
6. This confirmation has a low affinity for potassium ions and so they are released inside the cell. Since it is back in its original confirmation, the protein now has a high affinity for sodium and the cycle can start again.

Question 28

What types of cells is the sodium potassium pump most commonly found in and what does that mean for the cell?

Answer 28

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

Question 29

How does The sodium-potassium pump affect the small intestine?

Answer 29

In the small intestine, the sodium gradient created by the sodium-potassium pump drives
the active transport of glucose. This allows glucose to be absorbed from the small intestine into the bloodstream.

Question 30

Describe the motion of the glucose transported involved in the transport of glucose in the small intestine.

Answer 30

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.