Animal and Plant Cell Structure
This topic covers the essential components, or organelles, inside eukaryotic cells, which form the basis of all animal and plant life. Understanding the specific role of each part is key to answering questions on how cells live, get energy, and are structured.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Eukaryotic cells, found in plants and animals, have a true nucleus containing their genetic material.
- Plant cells have three structures not found in animal cells: a cellulose cell wall for support, chloroplasts for photosynthesis, and a large permanent vacuole to maintain turgor pressure.
- The nucleus acts as the control centre, containing DNA which codes for proteins and directs cell activities.
- Mitochondria are the sites of aerobic respiration in ALL eukaryotic cells (both plant and animal), breaking down glucose to release energy (ATP).
- The cell membrane is a selectively permeable barrier that controls the passage of substances into and out of the cell.
- Chloroplasts are the site of photosynthesis in green plant cells, using light energy to convert carbon dioxide and water into glucose.
Definitions
- Eukaryotic Cell
- A cell that possesses a membrane-bound nucleus and other membrane-bound organelles. Animals, plants, fungi, and protists are eukaryotes.
- Organelle
- A specialised sub-unit within a cell that has a specific function, e.g., the mitochondrion.
- Cytoplasm
- The jelly-like material filling the cell, enclosed by the cell membrane. It is where most metabolic reactions occur and where organelles are suspended.
Worked example
A scientist observes a eukaryotic cell and notes that it contains a cell wall, mitochondria, and a nucleus, but no chloroplasts. Which of the following statements is a valid conclusion? A) The cell is from a plant leaf. B) The cell cannot carry out aerobic respiration. C) The cell is an animal cell. D) The cell could be from a plant root.
- 1
Analyse the features given:
The cell has a cell wall and nucleus (making it eukaryotic, not a typical animal cell) and mitochondria.
The absence of chloroplasts is a key detail.
- 2
Evaluate option A:
A plant leaf cell is specialised for photosynthesis and would contain many chloroplasts.
This is incorrect.
- 3
Evaluate option B:
The cell contains mitochondria, which are the site of aerobic respiration.
Therefore, it can carry out this process.
This is incorrect.
- 4
Evaluate option C:
Animal cells do not have a cell wall.
This is incorrect.
- 5
Evaluate option D:
A plant root cell provides anchorage and absorbs water/minerals.
It is not exposed to light and does not photosynthesise, so it lacks chloroplasts.
It still needs to respire for energy, so it has mitochondria.
It is a plant cell, so it has a cell wall and nucleus.
This conclusion is consistent with all the observations.
Answer: D) The cell could be from a plant root.
Common mistakes
- ×Assuming plant cells don't respire. A very common mistake is to think that because plants photosynthesise, they don't need to respire. All living cells, including plant cells, respire 24/7 to release energy from the food they make.
- ×Misidentifying organelles on diagrams. Chloroplasts and mitochondria can look similar in simple diagrams. Remember, chloroplasts are generally larger and disc-shaped, while mitochondria are often shown with internal folds (cristae).
- ×Forgetting the function of the vacuole. In plant cells, the large central vacuole is not just for storage; it is filled with cell sap and pushes the cytoplasm against the cell wall, creating turgor pressure that helps support the plant.
No-calculator tips
- ✓Handle unit conversions for cell sizes mentally. Remember the key steps: 1 mm = 1000 micrometres (µm), and 1 µm = 1000 nanometres (nm). Converting from a larger unit to a smaller one means multiplying by 1000 (shifting the decimal point 3 places right).
- ✓Use scale bars for estimation. If a diagram has a 10 µm scale bar, you can quickly estimate an organelle's size by seeing how many times it fits along the bar. If a nucleus is half the length of the bar, it's roughly 5 µm wide.
- ✓Know relative organelle sizes to aid identification. In a typical cell, the nucleus is the largest organelle. In a mature plant cell, the vacuole can be even larger. Chloroplasts are usually significantly larger than mitochondria.