Transport across the Plasma Membrane

Transport across the Plasma Membrane

The plasma membrane is the boundary of a cell, controlling the movement of substances in and out of the cell. The concentration of ions on either side of the membrane is vastly different:

• Na+ and Ca2+ concentrations are higher outside the cell, while K+ concentration is higher inside the cell.

The lipid bilayer of the membrane prevents the free movement of solutes and ions:

• Small, nonpolar molecules (O2, CO2, N2, steroid hormones) can pass completely through the membrane.

• Polar but electrically neutral molecules of small size (H2O, ethanol) can cross the membrane, but large molecules (like glycerol) cannot.

• Charged molecules, including inorganic ions, are blocked by the membrane, even if they are small.

To help transport substances across the membrane, cells use transport proteins.

• Transport proteins are divided into two main types, differing in how they recognize and transport solutes:

• Passive transport: Does not require energy, transport follows the concentration gradient.

• Active transport: Requires energy, transport goes against the concentration gradient.

Passive transport includes:

• Simple diffusion:

• Does not require energy.

• Substances move down the concentration gradient, from high to low concentration.

• Example: O2 and H2O diffuse into the cell, CO2 diffuses out of the cell.

• Facilitated diffusion:

• Does not require energy.

• Uses transport proteins to move substances down the concentration gradient.

• Example: Glucose and amino acids are transported from the blood into the cell.

• Osmosis:

• The diffusion of water across the membrane, from high water potential (low solute concentration) to low water potential (high solute concentration).

• Water transport channels (aquaporins) are used in osmosis.

• Creates isotonic, hypertonic (solute concentration greater than intracellular), and hypotonic (solute concentration less than intracellular) environments.

• The solute concentration inside the cell is often greater than the extracellular solution, so water usually enters the cell, causing the cell to swell.

• Cells use several ways to avoid lysis due to osmotic pressure:

• Protozoa expel water from the cell periodically and store water in vacuoles.

• Plants have cell walls for protection.

Active transport includes:

• Characteristics:

• Requires energy.

• Transports against the concentration gradient.

• Uses transport proteins, controlled by the concentration gradient and membrane potential.

• Three forms of transport:

• Uniport: Transporting one substance down or against its concentration gradient.

• Symport: Transporting two substances in the same direction, one down its gradient and one against its gradient.

• Antiport: Transporting two substances in opposite directions, one down its gradient and one against its gradient.

Transport proteins:

• Used to transport most organic substances across the membrane.

• Highly selective, often each protein only transports one type of molecule.

• Each membrane contains a characteristic set of membrane proteins.

• Transport proteins carry uncharged substances down their concentration gradient (passive) like glucose.

• Transport proteins carry charged substances (ions) based on the electrochemical gradient (concentration and potential) (active).

• Active transport goes against the electrochemical gradient.

• There are three ways of active transport:

• ATP-powered transport: Uses ATP to transport substances against their gradient.

• Coupled transport: Uses the electrochemical gradient of one substance to transport another substance against its gradient.

• Light-driven transport: Uses light energy to transport substances.

The Na+-K+ pump:

• Animal cells use energy from ATP hydrolysis to pump Na+ out of the cell and K+ into the cell.

• Steps of operation:

1. 3 intracellular Na+ ions bind to the transport protein.

2. ATP binds to the protein and phosphorylates it, releasing ADP.

3. The protein changes conformation and releases the 3 Na+ ions into the extracellular space.

4. 2 extracellular K+ ions bind to the protein.

5. The protein changes conformation and releases the Pi.

6. The protein returns to its original conformation, the affinity for K+ decreases and K+ is released into the cell.

• Result: Creates a Na+ concentration gradient across the plasma membrane, with intracellular Na+ concentration 10-30 times lower than extracellular.

• This Na+ concentration gradient works together with the membrane potential gradient.

Coupled transport:

• Animal cells use the Na+ electrochemical gradient to actively transport glucose into the cell (symport).

• Due to the transmembrane Na+ electrochemical gradient, Na+ and glucose enter the specific channel together.

• Glucose is transported against its concentration gradient.

• Glucose can be transported actively or passively, depending on the state of the cell.

• Plant cells also perform this, but replace the Na+ gradient with the H+ gradient.

Ion channels and membrane potential:

• Ion transport channels are characterized by:

• Ion selectivity.

• Channel opening signals.

• Ion-selective channels are based on:

• Size.

• Shape.

• Charge.

• Membrane potential is regulated by the transport of ions across the membrane.

• Ion channels are classified based on channel opening signals.

• Ion channels switch between open and closed states randomly, not continuously maintaining one state.

• More than a million ions can pass through one ion channel per second, 1000 times faster than the rate of transport proteins.

Other forms of transport across the membrane:

• Exocytosis: Cell releases substances to the outside.

• Endocytosis: Cell absorbs substances from the outside.

• Pinocytosis: Cell absorbs liquids.

• Phagocytosis: Cell absorbs solids.