Adv Biology - 1.3 - Membrane Proteins

What is the fluid mosaic model?

A model that describes the structure of the plasma membrane.

What are membranes composed of?

A phospholipid bilayer with a patchwork of proteins.

Do the heads of the phospholipids have charge?

Yes the heads are charged and this makes them polar and hydrophilic.

Do the tails of the phospholipids have charge?

No the tails don’t have charge and this makes them non-polar and hydrophobic.

Why is the mosaic model fluid?

It is fluid because the phospholipids are constantly moving.

How are integral membrane proteins held firmly within the phosphate bilayer?

Regions of hydrophobic R groups (on the protein) allow strong hydrophobic interactions to take place with the phospholipid tails.

Some integral proteins are transmembrane proteins, what does this mean?

Transmembrane proteins span the entire width of the membrane.

What are some example of transmembrane proteins?

Channels and transporters.

How are peripheral membrane proteins bound to the surface of membranes?

They have hydrophilic R groups on their surface and bind to the surface of membranes by ionic and hydrogen bond interactions.

What do many peripheral membrane proteins do?

Many interact with the surfaces of integral membrane proteins.

The phospholipid bilayer is a barrier to what?

It is a barrier to ions and most uncharged polar molecules

How do small molecules, like oxygen and carbon dioxide, pass through the bilayer?

Small molecules pass through the bilayer by simple diffusion.

What is facilitated diffusion?

The passive transport of substances across the membrane through specific transmembrane proteins.

What does passive mean?

It means it does not require energy.

How else does water pass through the membrane?

As well as simple diffusion, water passes across the membrane in water channels called aquaphorins.

What do different cell types have so that they can perform specialised functions?

They have different channel and transporter proteins.

Most channel proteins in animal and plant cells are highly …

Selective

How do some channel proteins allow or prevent diffusion?

The channels are gated and change conformation to allow or prevent diffusion

How are the gates controlled?

The gates respond to a stimulus which causes them to open or close?

How are ligand-gated channels controlled?

The stimulus is chemical, they are controlled by the binding of signal molecules.

How are voltage-gated channels controlled?

The stimulus is electrical, they are controlled by changes in ion concentration.

How do transporter proteins function?

They bind to the specific substance that needs transported and undergo a conformational change to transfer the solute across the membrane.

What is active transport?

Transport that requires energy.

How does active transport move substances across the membrane?

It uses pump proteins to transfer substances against their concentration gradient.

What is required for active transport?

A source of metabolic energy.

What do some active transport proteins do to provide the energy for the conformation change needed to move substances?

Hydrolyse the ATP directly.

What happens to a solute carrying a net charge? (ion gradient)

The concentration gradient and electrical potential difference combine to form the electrochemical gradient that determines the transport of the solute.

What does the electrochemical gradient determine?

The transport of the solute.

What energy do ion pumps use?

Energy from the hydrolysis of ATP.

What do ion pumps do?

Use energy from ATP hydrolysis to establish and maintain ion gradients.

What is an example of an ion pump?

The sodium-potassium pump.

What does the sodium-potassium pump do?

It transports ions against a steep concentration gradient using energy directly from ATP hydrolysis.

What type of transport is the sodium potassium pump?

Active transport.

Which directions do the ions go in the sodium potassium pump? (in or out of the cell)

The sodium is actively transported out of the cell and the potassium is actively transported into the cell.

The sodium potassium pump is found in …

Most animal cells.

What does the sodium-potassium pump account for in many organisms?

A high proportion of the basal metabolic rate.

What drives the active transport of glucose in the small intestine?

The sodium gradient created by the sodium-potassium pump.

What is a symport?

An integral membrane protein that transports two substances across the membrane simultaneously and in the same direction.

What does the glucose transporter responsible for this glucose symport do?

Transports glucose and sodium at the same time and in the same direction.