Cell structure and organisation, at a glance

A cell has different structures that work together. Identify a structure by its position and appearance, then explain the job it performs.

• The membrane controls exchange; the cell wall supports a plant cell.
• The nucleus contains genetic information; many chemical reactions happen in the cytoplasm.
• Chloroplasts absorb light for photosynthesis. A large sap-filled vacuole helps a plant cell stay firm.

Keep in mind: The wall provides support and is freely permeable. The cell membrane inside it controls the movement of substances into and out of the living cell.

Typical plant and animal cells share a membrane, cytoplasm and nucleus. A cellulose wall and a large permanent vacuole distinguish a typical mature plant cell.

• Plant cells also contain mitochondria and ribosomes; these are not exclusive to animal cells.
• Chloroplasts are evidence of a photosynthetic cell, but their absence alone does not identify an animal cell.

Keep in mind: Plant cells respire and contain mitochondria. Photosynthetic plant cells contain chloroplasts as well.

A stain improves contrast; it does not add a new organelle. Identify what the image actually resolves and distinguish an observation from an inference.

• A thin specimen lets light through. A suitable stain makes some structures easier to distinguish.
• Begin with low power to locate the specimen, then use fine focus at higher power.
• A large clear region is not evidence that a plant cell is empty: its vacuole contains cell sap.

Keep in mind: A cell membrane may be too thin or too close to the wall to distinguish separately in this view. Limited resolution or contrast is not proof of absence.

Draw the features you observe, preserve their proportions, and compare image size with actual size in the same units.

• Use clear single lines, appropriate proportions and labels that end at the named structure.
• Magnification = image size / actual size, with both lengths in the same units.
• 1 mm = 1000 micrometres. Magnification has no unit.

Keep in mind: Resizing changes image magnification. Use a scale bar that resizes with the image, or the dimensions and magnification explicitly given for the version in the question.

Mitochondria are sites of aerobic respiration; ribosomes synthesise proteins. A cell needs both processes to function.

• Mitochondria: aerobic respiration and energy release.
• Ribosomes: protein synthesis.
• Use internal folds or small dense dots as evidence, with the image context.

Keep in mind: Ribosomes synthesise proteins. Mitochondria are sites of aerobic respiration, releasing energy for cell activities.

An adaptation answer needs a chain: a named structure, the advantage it provides and the function that benefits.

• Root hair: a long extension gives a larger membrane surface for absorption.
• Muscle cell: many mitochondria release energy for contraction.
• Mature mammalian red blood cell: no nucleus leaves more room for oxygen-carrying haemoglobin.

Keep in mind: Its long projection increases the membrane surface area in contact with the soil. The relevant advantage is more surface for absorption, not simply a larger volume.