Animal and plant cells: ribosomes.

The idea

Ribosomes are the cell structures where protein synthesis takes place. They are found in the cytoplasm of all cells, including animal, plant and bacterial cells. In an exam, you may be asked to identify ribosomes on a diagram, state their function, or explain why a cell that secretes many proteins, such as a pancreas cell making digestive enzymes, contains many ribosomes. At GCSE, the key idea is simply that they build proteins by joining amino acids together. Because they are very small, they are usually shown as tiny dots on cell diagrams rather than as a labelled organelle with internal detail. Understanding that cells with high rates of protein synthesis require more ribosomes is a common application question.

What earns the marks

• Ribosomes are the site of protein synthesis in the cell.
• Cells that secrete many proteins, such as enzymes, contain large numbers of ribosomes.

Common mistakes and how to put them right

• Confusing ribosomes with mitochondria.

  Put it right: Mitochondria release energy in respiration, whereas ribosomes make proteins.

• Thinking ribosomes are only found in animal cells.

  Put it right: They occur in plant and bacterial cells too.

• Describing ribosomes as membrane-bound.

  Put it right: They have no surrounding membrane, unlike mitochondria and chloroplasts.

Exam tips

• Link ribosomes to protein synthesis whenever you are asked about their function.
• On diagrams, look for small dots in the cytoplasm and label them as ribosomes.