Animal and plant cells — AQA GCSE Combined Science

Most animal cells share a common set of parts, each with a job. The nucleus contains DNA and controls the cell. The cytoplasm is where most chemical reactions happen. The cell membrane controls what enters and leaves. Mitochondria are the site of aerobic respiration, releasing energy. Ribosomes are the site of protein synthesis. Animal cells do not have a cell wall or chloroplasts, and they store carbohydrate as glycogen. For example, a liver cell has many mitochondria and ribosomes because it is metabolically active. When you describe an animal cell, name each part and give its function, and be ready to compare it with a plant cell.

The nucleus is the membrane-bound organelle found in eukaryotic cells such as animal and plant cells. It contains the cell's genetic material, mainly DNA arranged into chromosomes, and controls the cell's activities by regulating which proteins are made. In an animal cell the nucleus is usually a rounded structure near the centre, while in a plant cell it is often pushed to one side by the large permanent vacuole. A useful method is to picture a cell as a factory: the nucleus is the control room holding the master instructions. Students should be able to identify the nucleus on a light-microscope image or diagram, state that it contains genetic material, and explain that it coordinates cell activities including growth and division.

Cytoplasm is the jelly-like substance that fills the cell and surrounds the organelles. It is mainly water with dissolved substances, and it is where many of the cell's chemical reactions take place. In animal and plant cells the cytoplasm contains the nucleus, mitochondria, ribosomes and other structures, holding them in position. In prokaryotic cells such as bacteria, the cytoplasm also holds the genetic material because there is no nucleus. A helpful method is to imagine the cytoplasm as the fluid in which the cell's machinery floats and works. Students should identify cytoplasm on diagrams, describe it as the site of chemical reactions, and explain that it supports and separates the cell's structures.

The cell membrane is the thin boundary that surrounds every cell, whether prokaryotic or eukaryotic. It is a partially permeable barrier built from a phospholipid bilayer with embedded proteins. It holds the cytoplasm and cell contents together, separates the cell from its surroundings, and controls which substances enter and leave. Small molecules such as oxygen and carbon dioxide diffuse across it, water moves by osmosis, and dissolved substances may be moved by active transport. In animal cells it is the outermost layer; in plant cells it lies just inside the cell wall. A useful method is to compare it with a selectively permeable gate: glucose and ions can be admitted or expelled, while large molecules cannot pass freely.

Mitochondria are organelles found in the cytoplasm of eukaryotic cells, including animal, plant, algal and fungal cells. They are the site of aerobic respiration, where glucose and oxygen are used to release energy that the cell needs for processes such as growth, movement and active transport. Cells that demand a lot of energy, such as muscle cells, sperm cells and liver cells, contain many mitochondria, whereas less active cells contain fewer. A useful method in exams is to observe the number of mitochondria on an electron micrograph and relate this number to the cell's energy demand. For example, a sperm cell has many mitochondria to release energy for swimming.

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.

Animal cells contain a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant cells contain all of these parts as well, but they often also have three additional structures: a cell wall made of cellulose, chloroplasts containing chlorophyll, and a permanent vacuole filled with cell sap. The cell wall gives the cell strength and support and keeps its shape. Chloroplasts absorb light energy for photosynthesis, so they are green and are found mainly in leaf cells. The permanent vacuole stores cell sap and helps keep the cell firm. Not every plant cell has chloroplasts, for example root hair cells do not, because they are underground and receive no light. When comparing cell types, list the shared parts first and then the extra plant-cell parts.

Chloroplasts are the organelles in plant cells where photosynthesis occurs. They contain the green pigment chlorophyll, which absorbs light energy so that the plant can make glucose from carbon dioxide and water. Chloroplasts are found in the green parts of plants, such as the leaves, and are numerous in cells like palisade cells. They are absent from animal cells and from plant cells that do not photosynthesise, such as root hair cells. When plant cells are viewed under a light microscope, chloroplasts appear as small green, oval structures in the cytoplasm. Their presence is a key feature that distinguishes plant cells from animal cells. For example, a palisade cell contains many chloroplasts near the top of the leaf, which increases the amount of light absorbed for photosynthesis.

The permanent vacuole is a large, fluid-filled organelle found in plant cells. It is filled with cell sap, a solution of sugars, salts and other substances. The vacuole helps maintain turgor pressure, keeping the cell firm and supporting the plant. It also stores nutrients and waste products. When a plant cell is placed in pure water, water enters by osmosis, the vacuole expands, and the cell becomes turgid. In a concentrated sugar solution, water leaves the vacuole, and the cell becomes plasmolysed. Animal cells do not have a permanent vacuole; they may have small temporary vacuoles. The presence of a permanent vacuole is a key feature that distinguishes plant cells from animal cells. For example, in a wilted plant, the vacuoles have lost water, so the cells are no longer turgid and the plant droops.

Plant and algal cells are eukaryotic cells that, unlike animal cells, possess a cell wall. This wall is made of cellulose, a carbohydrate polymer that forms strong fibres. It lies outside the cell-surface membrane and provides structural support, helping the cell keep its shape and resist bursting when water enters by osmosis. For example, a root hair cell in a plant has a cellulose wall that prevents the cell from bursting in dilute soil water, while still allowing water and mineral ions to pass to the membrane. The wall is fully permeable, so it does not control what enters or leaves; that is the role of the cell-surface membrane. Algal cells also have cell walls, although the wall material can vary between algal species. Recognising the presence of a cell wall helps distinguish plant and algal cells from animal cells in microscopy and in cell models.

Estimation means making a sensible approximate judgement of size or area without measuring every detail. It is useful when comparing sub-cellular structures because actual dimensions are often too small to measure directly with a school microscope. For example, you can estimate that a mitochondrion is about 1–2 µm long while a chloroplast is about 3–10 µm long, so the chloroplast is larger. To estimate from a microscope image, use the magnification or a scale bar: measure the image size, then divide by magnification to find actual size. Estimation is also used when a structure is irregular, when only part of it is visible, or when you need a quick comparison rather than an exact value. It is not a guess; it uses known reference sizes and simple calculations to judge relative size or area.

This practical develops skill in using a light microscope safely and correctly to view cells. You prepare a slide, for example onion epidermis for a plant cell and human cheek cells for an animal cell, then focus using the coarse and fine adjustment knobs, starting on the lowest power objective. You observe the cells, then produce clear biological drawings with sharp pencil lines, no shading, and labels with straight lines that touch the structure. Every drawing must include a magnification scale, such as a line labelled with its actual length and the calculated magnification, so the size of the specimen is communicated. Magnification is calculated as image size divided by actual size, with units converted consistently. Accurate observation, correct technique and clear, labelled drawings with a scale are all assessed.