membrane transport

Passive transport refers to the movement of molecules across a membrane without the need for energy input.
- Energy Requirement: No energy is required because transport occurs down a concentration gradient.
- Gradient Movement: Molecules move from areas of high concentration to areas of low concentration.

Diffusion: The process by which molecules spread from areas of high concentration to areas of low concentration.
- Example: Movement of oxygen from the lungs into the bloodstream.
Osmosis: A specific type of diffusion involving the movement of water through a semipermeable membrane.
- Water molecules move from areas of low solute concentration to areas of high solute concentration.

Gradient: Refers to the difference in concentration across a space.
- High Concentration: Area with a greater number of molecules.
- Low Concentration: Area with fewer molecules.
- Movement is described as occurring "along the gradient" or "down the gradient."

Facilitated Diffusion: Involves transport proteins to help move substances across membranes.
- Example: Aquaporins are channel proteins that allow water to move rapidly across cell membranes.
- Function: Aquaporins can increase water transport rate, especially when the body is in need of water (e.g., dehydration situation).
- Biological Purpose: Facilitated transport allows for rapid movement of essential substances in and out of cells without energy expenditure.

Mechanically Gated Channels: Open or close in response to mechanical forces.
- Example: Cochlear hair cells detect sound vibrations; mechanical action opens ion channels, allowing ions to flow and generate electrical signals.
- Energy Requirement: No energy is expended during this process.

Membrane Potential: The difference in charge across a membrane.
- Described as net positive charge outside the cell and net negative charge inside the cell.
- Biological Importance: Net negative charge is crucial for many cellular functions and the overall electrical activity of cells (e.g., neuron activity).
Electrochemical Gradient: Combination of the concentration gradient and the electrical gradient across the membrane, affecting ion movement.
- Example: Sodium-Ion Concentrations: 6 sodium ions found outside the cell (extracellular) and 2 chloride ions resulting in a net negative charge within the cell.
- Key Concept: Ions move based on both their concentration and electrical gradients.

Potassium ions (K+) demonstrate selective permeability and gradient movement.
- Potassium Channels: Allow potassium ions to move down their concentration gradient across the cell membrane, influenced by the electrical gradient as well.
- Voltage-Gated Potassium Channels: Open in response to changes in membrane potential, allowing K+ to flow and contribute to the membrane potential.

Active Transport: Requires energy input to move substances against their concentration gradients.
- Energy Source: Often ATP is used in these processes.
- Types of Active Transport:
- Primary Active Transport: Direct use of energy from ATP.
- Secondary Active Transport: Utilizes the electrochemical gradient created by primary active transport to drive the movement of another molecule.

Function: Pumps sodium ions out of the cell and potassium ions into the cell against their concentration gradients.
- Mechanism:
1. Sodium ions bind to the pump.
2. ATP phosphorylates the pump, causing a conformational change that releases sodium ions outside the cell.
3. Potassium ions then bind to the pump.
4. The pump returns to its original shape, releasing potassium ions into the cell.
- Importance: Maintains essential concentration gradients for cell function, particularly in neurons.

Coupled Transport: Moves one molecule against its concentration gradient while simultaneously moving another molecule down its gradient.
- Examples include symporters and antiporters that facilitate the movement of sodium with another substance.
- Symport: Both molecules move in the same direction, one with the gradient and one against.
- Antiport: Opposing directions, where one substance moves in while the other moves out.
- Energy Consideration: These processes utilize the electrochemical gradient as the energy source, not ATP directly.