Membrane Proteins

Question 1

Fluid mosaic model of cell membranes

Answer 1

The membranes of cells are made of a phospholipid bilayer with globular proteins penetrating the bilayer or attached to its surface.

This is called the Fluid mosaic model of cell membranes

The phospholipid bilayer can move so it is fluid, and the seperate proteins are like the small pieces of a mosaic

Question 2

Integral membrane proteins

Answer 2

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

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

Some integral membrane proteins are transmembrane proteins

Question 3

Peripheral membrane proteins

Answer 3

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

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

Question 4

Phospholipid bilayer

Answer 4

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

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

Question 5

Facilitated diffusion

Answer 5

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

Requires no conformational change to the protein channel as the proteins make it easier for the ion or molecule to move passively across the membrane.

Question 6

Channel Proteins

Answer 6

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

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

Most channel proteins in animal and plant cells are highly selective

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

Question 7

Gated channels

Answer 7

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

There are 2 types -

Ligand-gated channels -
Ligand-gated channels are controlled by the
binding of signal molecules

Voltage-gated channels -
voltage gated channels are controlled by changes in
ion concentration

Question 8

Transporter proteins

Answer 8

Transporter proteins bind to the specific substance to be transported and undergo a conformation 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

Question 9

Active Transport

Answer 9

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

A source of metabolic energy is required for active transport

Pumps that mediate active transport are coupled to an energy source

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

Question 10

ATPases

Answer 10

ATPases hydrolyse ATP