Cell structure
Levels of Organisation
Before examining individual cells, it is essential to understand how they fit into the larger organism. The syllabus defines a hierarchy of complexity:
| Syllabus Example |
|---|
| Palisade mesophyll cell, neurone, bacterium. |
| Epithelial tissue (trachea), root tissue. |
| Leaf, stomach, brain. |
| Shoot system (leaves, stems), Root system (roots). |
| Human, tree, bacterium. |
Why this matters: Examiners often ask you to identify the level of organisation in a diagram. Remember that an organ system is defined by the organs it contains (e.g., leaves and stems), not just the process (photosynthesis).
Plant vs. Animal Cells
Both plant and animal cells are eukaryotic, meaning they contain a true nucleus and membrane-bound organelles. However, plants have specific structures for support and photosynthesis that animals lack.
| Function |
|---|
| Provides structural support and prevents bursting. |
| Controls what enters and leaves the cell. |
| Contains genetic material; controls cell activities. |
| Gel-like substance where metabolic reactions occur. |
| Site of photosynthesis; contains chlorophyll. |
| Site of aerobic respiration (releases energy). |
| Site of protein synthesis. |
| Contains cell sap; maintains turgor pressure. |
Key Distinction: Not all plant cells have chloroplasts. Only cells exposed to light (like those in leaves) contain them. Root cells, for example, do not.
Bacterial Cells (Prokaryotes)
Bacteria are prokaryotic cells. They are generally much smaller than plant or animal cells and lack a true nucleus and membrane-bound organelles.
| Function |
|---|
| Prevents the cell from bursting in hypotonic solutions. |
| Controls entry and exit of substances. |
| Site of metabolic reactions. |
| Protein synthesis. |
| Contains genetic information controlling cell activities. |
| Carry non-essential genes (e.g., antibiotic resistance). |
Comparison: Bacteria have a cell wall, but it is chemically different from plant cell walls. Bacteria lack mitochondria and chloroplasts; they perform respiration and photosynthesis (if photosynthetic) across their cell membrane.
New cells are produced by the division of existing cells. This is a fundamental principle of cell theory. Cells do not arise spontaneously.
Specialised Cells
Specialised cells have specific structures adapted to their function. This is known as structure-function relationship.
| Specialisation & Function |
|---|
| Has hair-like projections (cilia) that beat to move mucus and trapped particles out of the airways. |
| Has a large projection (hair) to increase surface area for water absorption. Contains many mitochondria for active transport. |
| Contains many chloroplasts near the top of the cell to maximise light absorption for photosynthesis. |
| Long axon to conduct electrical impulses over long distances; dendrites receive signals. |
| Biconcave shape increases surface area for oxygen uptake. No nucleus to make more space for haemoglobin. |
| Sperm: Tail for movement, many mitochondria for energy. Egg: Large cytoplasm/nutrients for embryo growth. |
Note: Root hair cells do not contain chloroplasts because they are underground and receive no light.
⚠︎ Confusing Cell Wall and Cell Membrane Functions
The Error: Students often state that the cell wall controls what enters or leaves the cell.
The Correction: The cell membrane is the selectively permeable barrier that controls substance movement. The cell wall is fully permeable and provides only structural support.
⚠︎ Misidentifying Mitochondria Function
The Error: Stating that mitochondria 'produce' energy.
The Correction: Energy cannot be created or destroyed. Mitochondria release energy from glucose via aerobic respiration. Use the phrase: 'Site of aerobic respiration' or 'Releases energy for metabolic processes.'
⚠︎ Assuming All Plant Cells Have Chloroplasts
The Error: Identifying chloroplasts in a diagram of a root cell or stating that all plant cells photosynthesise.
The Correction: Chloroplasts are only found in parts of the plant exposed to light (e.g., leaves, stems). Root hair cells and epidermal cells typically lack them.
Describing Functions Accurately
When to use: When asked to describe the function of the nucleus or mitochondria.
Why examiners accept this: Examiners look for specific keywords. For the nucleus, 'contains genetic material' and 'controls cell activities' are the accepted phrases. For mitochondria, 'aerobic respiration' is the key term. Avoid vague terms like 'powerhouse' without explanation.
Example:
Question: State the function of the nucleus.
Correct Answer: Contains DNA/genetic material and controls the activities of the cell.
Question: State the function of the nucleus.
Correct Answer: Contains DNA/genetic material and controls the activities of the cell.
Identifying Plant vs. Animal Cells in Diagrams
When to use: When asked to identify whether a cell is plant or animal based on a diagram.
Why examiners accept this: You must cite two distinct features. Citing only one may not be sufficient for full marks in structured questions. The presence of a cell wall and chloroplasts (or a large permanent vacuole) confirms a plant cell.
Example:
Question: Give two features that identify this as a plant cell.
Correct Answer: 1. Cell wall. 2. Chloroplasts.
Question: Give two features that identify this as a plant cell.
Correct Answer: 1. Cell wall. 2. Chloroplasts.
Identifying Structures in Diagrams
Q:
Identify the structure labelled A in the diagram of a plant cell. [1]
A:
Cell wall
Q:
Which structure controls what enters and leaves the cell? [1]
A:
Cell membrane
Comparing Cell Types
Q:
State two structures found in plant cells but not in animal cells. [2]
A:
- Cell wall. 2. Chloroplasts.
Q:
State one structure found in bacterial cells that is not found in animal cells. [1]
A:
Cell wall (or circular DNA / plasmids).
Specialised Cells
Q:
State the function of a root hair cell. [1]
A:
Absorption of water (and mineral ions).
Q:
Explain how a red blood cell is adapted for its function. [2]
A:
- Biconcave shape increases surface area for oxygen uptake. 2. No nucleus allows more space for haemoglobin.
Levels of Organisation
Q:
Describe the term 'organ system' using an example from a plant. [2]
A:
An organ system is a group of organs working together. Example: The shoot system (consisting of leaves and stems) works together for photosynthesis and support.