Animal and plant cells: Students should be able to explain how the main sub-cellular structures, including the nucleus, cell membranes, mitochondria, chloroplasts in plant cells and plasmids in bacterial cells are related to their functions.

The idea

Every sub-cellular structure is matched to a job, so link each part to what it does. The nucleus holds DNA and controls the cell's activities, including protein synthesis. The cell membrane is a partially permeable barrier controlling what enters and leaves. Mitochondria are the site of aerobic respiration, releasing energy for the cell. Chloroplasts, found in plant cells, contain chlorophyll and absorb light for photosynthesis. Plasmids are small circular DNA loops in bacterial cells, carrying extra genes such as antibiotic resistance.

For example: A muscle cell has many mitochondria because it respires rapidly, while a root hair cell has no chloroplasts because it is underground.

What earns the marks

• The nucleus contains the genetic material (DNA) and controls the cell's activities, including the synthesis of proteins.
• The cell membrane controls the movement of substances into and out of the cell and is partially permeable.
• Mitochondria are the site of aerobic respiration, where energy is released for the cell's processes.
• Chloroplasts contain chlorophyll and absorb light energy for photosynthesis; they occur in plant cells, not animal cells.
• Plasmids are small circular loops of DNA in bacterial cells that carry additional genes, such as those for antibiotic resistance.
• A correct explanation links each structure to its function rather than naming the structure alone.

Common mistakes and how to put them right

• Saying mitochondria 'make energy' rather than releasing energy during respiration.

  Put it right by writing that respiration releases energy from glucose.

• Confusing the cell membrane with the cell wall.

  Put it right by stating the membrane controls entry and exit while the wall provides support.

• Stating that plasmids are the main bacterial chromosome.

  Put it right by describing plasmids as small additional DNA loops.

• Claiming all plant cells contain chloroplasts.

  Put it right by noting that only photosynthesising plant cells do, so root cells lack them.

Exam tips

• Use the command word: 'explain' needs a because or so link between structure and function.
• Name the structure and its function in the same sentence to keep the link clear.
• For a comparison question, state clearly which structures are found in plant cells but not animal cells.
• Check that each structure you name is matched to a job, not just listed.