The cell as a working system: organelles and membrane transport

Compartmentalisation makes cells efficient

A eukaryotic cell is organised into compartments whose membranes create different chemical conditions. This separation prevents incompatible reactions from interfering with one another and concentrates enzymes and substrates where they are needed.

The nucleus stores most genetic information; ribosomes translate mRNA; rough endoplasmic reticulum handles many proteins for secretion or membranes; smooth endoplasmic reticulum participates in lipid synthesis and detoxification; the Golgi apparatus modifies, sorts and packages products; lysosomes contain hydrolytic enzymes; and mitochondria support aerobic ATP production.

The plasma membrane is a dynamic boundary

The fluid-mosaic model describes a phospholipid bilayer containing proteins, cholesterol and carbohydrate-bearing molecules. Hydrophilic heads face watery environments, while hydrophobic tails form an interior barrier to ions and most polar molecules.

Transport proteins give the membrane selective permeability. Channel proteins form hydrophilic routes; carrier proteins bind specific solutes and change shape. Receptors receive signals, enzymes catalyse reactions, and glycoproteins contribute to cell recognition.

Passive transport follows gradients

Simple diffusion is the net movement of particles from higher to lower concentration because random molecular motion produces more movement away from the crowded region than toward it. Small non-polar molecules such as oxygen and carbon dioxide can diffuse through the bilayer.

Facilitated diffusion also moves substances down an electrochemical gradient, but it requires channels or carriers. No metabolic energy is used to drive the movement. Because the number of transport proteins is limited, carrier-mediated transport can reach a maximum rate.

• A steeper gradient generally increases net diffusion rate.
• A larger surface area increases the number of particles that can cross per unit time.
• A shorter diffusion distance increases exchange efficiency.
• Higher temperature increases molecular kinetic energy.

Osmosis is the movement of water

Osmosis is the net movement of water through a selectively permeable membrane from higher water potential to lower water potential. Adding a non-penetrating solute lowers water potential because fewer water molecules are free to move and because water associates with solute particles.

An animal cell in a strongly hypotonic solution may swell and lyse because it lacks a cell wall. A plant cell becomes turgid as the wall resists further expansion. In a hypertonic solution, an animal cell shrinks while a plant cell may plasmolyse as the protoplast pulls away from the wall.

Active and bulk transport require cellular energy

Active transport moves a solute against its concentration or electrochemical gradient using energy. Primary active transport uses ATP directly, while secondary active transport uses the energy stored in an ion gradient created by another pump.

Endocytosis brings large material into the cell by vesicle formation; exocytosis releases material when a vesicle fuses with the plasma membrane. These bulk processes alter membrane shape and are distinct from movement through a channel or carrier.