Singapore-Cambridge A-Level Biology (H2)

The cell theory

The cell theory has three parts. Living organisms are composed of cells, the cell is the smallest unit of life, and all cells come from pre-existing cells. The second part means that nothing smaller than a cell carries out all the processes of life (metabolism, growth, response to the environment and reproduction) by itself. The third part rules out cells arising from non-living matter under present conditions, and is the reason every cell in a body can be traced back to a single fertilised egg through repeated division.

Eukaryotic cells and their compartments

A eukaryotic cell has a nucleus and a set of membrane-bound compartments. Membranes divide the cell into regions, so enzymes and substrates for one process can be kept together at high concentration and incompatible reactions (for example the breakdown of proteins by lysosomal enzymes) are kept away from the rest of the cytoplasm. Membrane systems also give a large surface area on which enzymes and carriers can be arranged in order.

The cytoplasm is everything inside the cell surface membrane except the nucleus. Its fluid part, the cytosol, is a solution of ions, sugars, amino acids and proteins in which many metabolic reactions, including glycolysis, take place.

Structures and their functions

How the membrane systems work together

The membrane systems form one connected route for making and exporting protein, which is worth following in order because drawings often show only part of it.

1. A protein that is to be secreted is made on a ribosome attached to the rough endoplasmic reticulum, and threaded into the lumen as it is made.
2. In the lumen it folds and may have sugars added.
3. Part of the endoplasmic reticulum membrane pinches off as a transport vesicle carrying the protein.
4. The vesicle fuses with the receiving face of the Golgi body, which modifies the protein further, for example by trimming it or adding more carbohydrate.
5. Vesicles bud from the other face of the Golgi body. A vesicle carrying a secretory protein moves to the cell surface membrane, fuses with it and releases the protein by exocytosis.
6. A vesicle carrying hydrolytic enzymes stays in the cell as a lysosome.

The nuclear envelope is a double membrane perforated by pores, which let mRNA and ribosome subunits out and proteins such as those for transcription in. The outer membrane is continuous with the rough endoplasmic reticulum.

Differences between plant and animal cells

Plant cells have a cellulose wall, chloroplasts (in the green parts) and a large permanent central vacuole surrounded by a membrane called the tonoplast, none of which animal cells have. Animal cells have centrioles, which are absent from the cells of flowering plants. Both kinds contain a nucleus, mitochondria, endoplasmic reticulum, Golgi bodies and 80S ribosomes.

Interpreting micrographs and drawings

In an electron micrograph the structures are recognised by shape and texture. Mitochondria are oval or rod-shaped with internal folds. Rough endoplasmic reticulum is seen as parallel lines of membrane with dots along them, which are ribosomes. A nucleus is a large rounded body with a double outline and often a dark nucleolus. Chloroplasts are lens-shaped with a regular pattern of stacked discs. A Golgi body appears as a few curved, flattened sacs stacked like plates with small vesicles around the edges.

A section through a cell is two-dimensional, so a long tubular mitochondrion cut across looks like a circle and cut along its length looks like a rod. A drawing should show the outline of a structure as a clear continuous line and should not shade.