AH Biology U1: KA3 Membrane Proteins

Fluid mosaic model

• Describes the structure of the plasma membrane

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

• The position of these proteins is not fixed, rather they can move between the phospholipids

Describe how integral proteins are held within the phospholipid bilayer

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

Location of integral membrane proteins

Within the membrane

Integral membrane proteins interact extensively with what region of membrane phospholipids?

The hydrophobic region

Transmembrane protein

Type of integral protein that spans the entire width of the membrane

Location of peripheral protein

On the surface of the membrane

Describe how peripheral proteins are bound to the surface of membranes

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

The phospholipid bilayer is a barrier to…

Ions and most uncharged polar molecules

What molecules can pass through the bilayer by simple diffusion?

Some small molecules, such as oxygen and carbon dioxide

Facilitated diffusion

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

State what proteins different cell types have which enables them to perform specialised functions

Different channel and transporter proteins

Most channel proteins in animal and plant cells are highly…

Selective

Channels

Multi-subunit proteins with the subunits arranged to form water-filled pores that extend across the membrane

Gated channel proteins

Channel proteins that change conformation to allow or prevent diffusion

Ligand-gated channels are controlled by…

The binding of signal molecules

Voltage-gated channels are controlled by…

changes in ion concentration

Describe how transporter proteins transfer specific substances across the membrane

• 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.

Active transport uses _________ that transfer substances across the membrane _______ their concentration gradient

pump proteins, against

Pumps that mediate active transport are transporter proteins coupled to what? Why?

Pumps that mediate active transport are transporter proteins coupled to an energy source as a source of metabolic energy is required for active transport.

ATPases

Proteins which hydrolyse ATP directly to provide the energy for the conformational change required to move substances across the membrane.

Electrochemical gradient (what is it and what does it determine)

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.

A membrane potential (electrical potential difference) is created when…

There is a difference in electrical charge on the two sides of the membrane

A concentration gradient is created when…

There is a difference in concentration on the two sides of the membrane

Function of ion pumps

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

Function of the sodium-potassium pump

• 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

Describe the process of ion exchange at the sodium-potassium pump

• The pump has high affinity for sodium ions inside the cell

• Binding occurs and the pump is phosphorylated by ATP

• Conformation of the protein changes and affinity for sodium ions decreases, so sodium ions released outside of the cell

• Potassium ions bind outside the cell

• Dephosphorylation occurs and conformation of protein changes, so potassium ions taken into cell. Affinity returns to start

For each ATP hydrolysed, ____ sodium ions are transported _____ of the cell and ____ potassium ions are transported ____ the cell. This establishes:

• 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

The sodium-potassium pump is found in most animal cells, accounting for a high proportion of the…

basal metabolic rate in many organisms

Describe the active transport of glucose in the small intestine (5)

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

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

• 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.