2.1.5 Transport across membranes

Simple diffusion

Simple diffusion

The movement of small or fat-soluble molecules across a membrane without the need for proteins.

Facilitated diffusion

The movement of ions or polar molecules across a membrane with the help of channel or carrier proteins.

Osmosis

The movement of water across a membrane from an area of higher water potential to an area of lower water potential.

Passive transport

The movement of molecules across a membrane without the input of energy.

Active transport

The movement of molecules across a membrane against the concentration gradient requiring energy, usually ATP.

Endocytosis

The process of bringing materials into a cell by forming vesicles.

Exocytosis

The process of transporting materials out of a cell using vesicles.

Channel protein

Proteins that form channels in the membrane for specific molecules to pass through.

Carrier protein

Proteins that bind to specific molecules, change shape, and transport them across the membrane.

Factors affecting diffusion rate

Variables like temperature, concentration gradient, distance, surface area, and the number of channels influencing the rate of diffusion.

Plasmolysis

Contraction of the protoplast in a plant cell due to water loss, leading to the protoplast pulling away from the cell wall.

Protoplast

Plant cell without its cell wall, comprising all parts of a plant cell except the cell wall.

Crenation

Shrinkage of animal cells causing a notched appearance due to water loss.

Turgid

State of a plant cell where the protoplast pushes against the cell wall.

Cytolysis

Bursting of cells.

Haemolysis

Bursting of red blood cells.

Solute potential (ψS)

Potential due to dissolved solutes in the cytoplasm, always negative.

Pressure potential (ψP)

Pressure exerted by the cell wall on cell contents, opposing water entry.

Water potential (Ψ)

Overall potential of a cell, calculated as the sum of solute potential and pressure potential (Ψ = ψS + ψP).

Osmosis

Movement of water across a semipermeable membrane from a region of low solute concentration to high solute concentration.

Hypotonic solution

Solution with lower solute concentration than the cell, causing water to enter the cell.

Isotonic solution

Solution with the same solute concentration as the cell, resulting in no net flow of water.

Hypertonic solution

Solution with higher solute concentration than the cell, leading to water leaving the cell.

Water potential scale

Summary of water potential values based on solute concentration and pressure in a solution.