Animal and Plant Cells

This lesson covers the detailed structure and function of animal and plant cells as specified in the AQA GCSE Combined Science Trilogy specification (8464). You need to be able to identify subcellular structures, explain their functions and compare animal and plant cells.

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Animal Cells

Animal cells are eukaryotic cells. They contain several key subcellular structures, each with a specific function.

Subcellular Structures of an Animal Cell

Subcellular Structure	Function
Nucleus	Contains genetic material (DNA) in the form of chromosomes. Controls cell activities and cell division.
Cell membrane	A thin, flexible barrier that controls the movement of substances into and out of the cell. It is selectively permeable.
Cytoplasm	A jelly-like substance filling the cell. Most chemical reactions take place here, many controlled by enzymes.
Mitochondria	The site of aerobic respiration. Glucose and oxygen react here to release energy for the cell's processes.
Ribosomes	Tiny structures (not visible with a light microscope) where protein synthesis occurs. Found free in the cytoplasm or attached to the endoplasmic reticulum.

Exam Tip: When asked to label a diagram of an animal cell, always include: nucleus, cell membrane, cytoplasm, mitochondria and ribosomes. These are the five subcellular structures you must know for AQA Combined Science.

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Plant Cells

Plant cells are also eukaryotic. They have all the structures found in animal cells, plus three additional structures.

Additional Subcellular Structures in Plant Cells

Subcellular Structure	Function
Cell wall	A rigid outer layer made of cellulose. It provides structural support and prevents the cell from bursting when water enters by osmosis.
Permanent vacuole	A large central vacuole filled with cell sap (a dilute solution of sugars, salts and sometimes pigments). It maintains turgor pressure, keeping the cell firm.
Chloroplasts	Contain the green pigment chlorophyll, which absorbs light energy for photosynthesis. Found mainly in leaf cells and cells near the surface of the plant.

Exam Tip: Not all plant cells have chloroplasts. Root cells, for example, are underground and do not receive light, so they have no chloroplasts. Only cells exposed to light contain chloroplasts.

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Comparing Animal and Plant Cells

Feature	Animal Cell	Plant Cell
Nucleus	Yes	Yes
Cell membrane	Yes	Yes
Cytoplasm	Yes	Yes
Mitochondria	Yes	Yes
Ribosomes	Yes	Yes
Cell wall	No	Yes (cellulose)
Permanent vacuole	No (may have small temporary vacuoles)	Yes (large, central)
Chloroplasts	No	Yes (in green parts)

graph TD
    subgraph "Shared Structures"
        A["Nucleus"]
        B["Cell membrane"]
        C["Cytoplasm"]
        D["Mitochondria"]
        E["Ribosomes"]
    end
    subgraph "Plant Cell Only"
        F["Cell wall<br/>(cellulose)"]
        G["Permanent vacuole<br/>(cell sap)"]
        H["Chloroplasts<br/>(photosynthesis)"]
    end

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Functions of Subcellular Structures — In Detail

The Nucleus

• Contains DNA arranged in long, thread-like structures called chromosomes.
• DNA carries the genetic code — the instructions for making proteins.
• The nucleus controls the cell's activities by determining which proteins are made.
• Surrounded by a nuclear membrane with small pores that allow substances (such as mRNA) to pass in and out.

Mitochondria

• Small, sausage-shaped organelles with a folded inner membrane (cristae) that increases the surface area for respiration reactions.
• The site of aerobic respiration:

$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$glucose+oxygen→carbon dioxide+water

• Cells that require lots of energy (e.g. muscle cells, sperm cells) have many mitochondria.

Ribosomes

• Very small structures — approximately 20 nm in diameter.
• Cannot be seen with a light microscope; only visible with an electron microscope.
• They read the instructions from the DNA (via messenger RNA) and assemble amino acids into proteins.

Cell Membrane

• Made of a phospholipid bilayer with embedded proteins.
• Controls which substances pass into and out of the cell.
• Small molecules (e.g. oxygen, carbon dioxide, water) can pass through; larger molecules may need special transport proteins.

Cell Wall (Plants Only)

• Made of cellulose — a tough, fibrous carbohydrate.
• Fully permeable — it does not control what passes through (that is the job of the cell membrane).
• Provides structural support and prevents the cell from bursting when it absorbs water.

Chloroplasts (Plants Only)

• Contain chlorophyll, a green pigment that absorbs light energy (mainly from the red and blue parts of the spectrum).
• The site of photosynthesis:

$\text{carbon dioxide} + \text{water} \xrightarrow{\text{light energy}} \text{glucose} + \text{oxygen}$carbon dioxide+waterlight energy​glucose+oxygen

• Found in large numbers in palisade mesophyll cells (the main photosynthesising cells in a leaf).

Permanent Vacuole (Plants Only)

• A large, central, fluid-filled sac surrounded by a membrane called the tonoplast.
• Contains cell sap — a solution of dissolved sugars, mineral salts and sometimes pigments.
• Maintains turgor pressure: when the vacuole is full of water, it pushes the cytoplasm against the cell wall, making the cell turgid (firm). This helps the plant stay upright.

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Worked Example

Question: A student is looking at two cells under a microscope. Cell A has a nucleus, mitochondria, ribosomes and a cell membrane. Cell B has all of these structures plus a cell wall, chloroplasts and a permanent vacuole. Identify each cell type.

Solution:

• Cell A is an animal cell — it contains the five basic eukaryotic structures but lacks a cell wall, chloroplasts and a permanent vacuole.
• Cell B is a plant cell — it contains all the structures of an animal cell plus a cellulose cell wall, chloroplasts and a permanent vacuole.

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Worked Example — Explaining Function

Question: Explain why muscle cells contain large numbers of mitochondria.

Solution:

Muscle cells require a large amount of energy to contract and produce movement. Energy is released by aerobic respiration, which takes place in the mitochondria. Having a large number of mitochondria means more aerobic respiration can take place, transferring more energy to meet the cell's high energy demands.

Exam Tip: When explaining why a cell has many mitochondria, always link it to the function of the cell (e.g. contraction, active transport) and state that mitochondria are the site of aerobic respiration, which releases energy.

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Common Mistakes