The cell theory
Three ideas underpin biology: all living things are made of cells, the cell is the basic unit of life, and all cells come from existing cells.
Two cell types
- Prokaryotic cells (bacteria and archaea) have no membrane-bound nucleus. DNA floats in a region called the nucleoid. They are small and simple.
- Eukaryotic cells (animals, plants, fungi, protists) have a nucleus and other membrane-bound organelles.
All cells have a plasma membrane, cytoplasm, ribosomes and DNA.
Key organelles
- Nucleus: holds DNA.
- Ribosomes: build proteins. They float free or sit on the rough ER.
- Rough ER: studded with ribosomes; makes proteins destined for export or membranes.
- Smooth ER: makes lipids and detoxifies.
- Golgi apparatus: modifies, sorts and packages proteins into vesicles.
- Mitochondria: make most of the cell's ATP by respiration.
- Lysosomes: digest waste and worn-out parts.
- Chloroplasts (plants and algae): carry out photosynthesis.
- Cell wall (plants, fungi, bacteria): rigid outer layer. Animal cells do not have one.
- Large central vacuole (plants): stores water and maintains pressure.
The plasma membrane
The membrane is a phospholipid bilayer. Each phospholipid has a hydrophilic (water-loving) head and two hydrophobic tails. In water, the heads face outward toward the watery surroundings and the tails tuck inward, away from water. Proteins embedded in it act as channels, pumps and receptors. The membrane is selectively permeable.
Moving across the membrane
Passive transport needs no cell energy; substances move down a concentration gradient (from high to low).
- Diffusion: movement of small, nonpolar molecules such as oxygen.
- Facilitated diffusion: through channel or carrier proteins.
- Osmosis: diffusion of water across a membrane toward the side with more dissolved solute.
Active transport uses ATP to move substances against the gradient (low to high), for example the sodium-potassium pump. Large particles move by endocytosis (in) and exocytosis (out).
Tonicity
- Hypotonic surroundings (less solute outside): water moves in, and an animal cell swells and may burst, while a plant cell becomes firm.
- Hypertonic surroundings (more solute outside): water leaves and the cell shrinks.
- Isotonic: no net movement.
Worked example
A red blood cell is placed in pure water.
- Water has no solutes, but the cell's interior has many. So the water is hypotonic to the cell.
- Water moves in by osmosis from high water concentration to low.
- Animal cells have no wall, so the cell swells and can burst (lysis).
In contrast a plant cell in the same water swells until its wall pushes back, creating turgor pressure. That is why plants stay upright and wilt without water.