SQA National 5 Biology

Cell ultrastructure

The parts inside a cell are called organelles when they do a particular job, and their detailed structure (seen with an electron microscope) is the cell's ultrastructure. Four kinds of cell are met in the course: animal, plant, fungal (yeast is the usual example) and bacterial.

A fungal cell looks like a plant cell in having a wall, a vacuole and a nucleus, but it has no chloroplasts, so a fungus cannot photosynthesise and has to take in food that is already made. A bacterial cell is much simpler. It has a cell wall, a membrane, cytoplasm and ribosomes, but no nucleus and no other organelles such as mitochondria. Its main DNA lies free in the cytoplasm as a single chromosome, and it often has one or more plasmids as well.

Functions in more detail

The cell membrane is a thin layer around the cytoplasm. It is selectively permeable, so it lets some substances through and holds others back, which is how the cell keeps its contents different from its surroundings.

The nucleus holds the cell's DNA, arranged in chromosomes. The genes on the DNA carry the instructions for making proteins, and through the proteins they control how the cell works and what it becomes.

Mitochondria are the site of aerobic respiration, the stage that releases most of the energy from glucose. A cell that needs a lot of energy has more of them, which is why muscle cells and the cells that line the small intestine are packed with mitochondria compared with a skin cell.

Ribosomes are where amino acids are joined together to make protein, following a copy of the instructions in a gene.

A chloroplast contains the green pigment chlorophyll, which traps light energy for photosynthesis. Chloroplasts are found in the cells of the green parts of a plant, such as the leaf, and not in root cells that never see light.

The permanent vacuole of a plant cell is filled with a solution of sugars, salts and other substances called cell sap. When it is full it pushes the cytoplasm against the wall and keeps the cell firm, and the whole plant is supported in part by the firmness of its cells.

A plasmid is a small ring of DNA, separate from the main chromosome. It carries a few genes, such as ones that give resistance to an antibiotic, and bacteria can pass plasmids to one another. Plasmids are also the tools used in genetic engineering.

Cell walls

The cell wall lies outside the cell membrane. It is strong and fairly rigid, so it supports the cell and stops it swelling until it bursts when it takes in water. In plants the wall is made of cellulose. Fungi and bacteria also have a wall, but it is made of different materials and not cellulose. Animal cells have no wall at all, only a membrane, which is why they can change shape and why they burst in pure water.

Comparing the four cell types

A useful way to sort a cell from a description is to ask three questions. Has it a nucleus? If not, it is bacterial. Has it chloroplasts? If so, it is a plant cell. Has it a wall but no chloroplasts? Then it is fungal (and a bacterial cell would already have been picked out by the first question). A cell with a nucleus but no wall is an animal cell.