(a) Movement of molecules across membranes     

Cell membrane structure

The fluid mosaic model of cell membranes

• Regions of hydrophobic R groups allow strong hydrophobic interactions that hold integral membrane proteins within the phospholipid bilayer
• Some integral membrane proteins are transmembrane proteins
• Integral membrane proteins interact extensively with the hydrophobic region of membrane phospholipids.
• 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

Movement of molecules across the membrane

• Some small molecules, such as oxygen and carbon dioxide, pass through the bilayer by simple diffusion
• Facilitated diffusion is the passive transport of substances across the membrane through specific transmembrane proteins
• To perform specialised functions, different cell types have different channel and transporter proteins
• Most channel proteins in animal and plant cells are highly selective
• Channels are multi-subunit proteins with the subunits arranged to form water-filled pores that extend across the membrane.
• Some channel proteins are gated and change conformation to allow or prevent diffusion
• Ligand-gated channels are controlled by the binding of signal molecules, and voltage-gated channels are controlled by changes in ion concentration
• 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 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 transporter proteins coupled to an energy source.
• 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.

(b) Ion transport pumps and generation of ion gradients

• 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 (an electrical potential difference) is created when there is a difference in electrical charge on the two sides of the membrane.
• Ion pumps, such as the sodium-potassium pump, use energy from the hydrolysis of ATP to establish and maintain ion gradients

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
• 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.

Functions of sodium-potassium pump

• The sodium-potassium pump is found in most animal cells, accounting for a high proportion of the basal metabolic rate in many organisms
• In the small intestine, the sodium gradient created by the sodium-potassium pump drives the active transport of glucose
• The glucose transporter responsible for this glucose symport transports sodium ions and glucose at the same time and in the same direction
• In intestinal epithelial cells the sodium-potassium pump generates a sodium ion gradient across the plasma membrane.
• Sodium ions enter the cell down their concentration gradient; the simultaneous transport of glucose pumps glucose into the cell against its concentration gradient.