Cell Wall, Vacuole and Specialised Cells
The cell-surface membrane, plant cell wall and vacuole, and how specialised cells, tissues, organs and systems are organised, with an adaptation example. Includes the 4-mark exam question, a worked model answer and the examiner trap on not confusing the cell wall with the membrane.
Chapters (8)
Exam question [4 marks]
A plant cell has a cell wall and a large vacuole. Explain how each helps the plant cell. Then explain how microvilli adapt a cell in the lining of the small intestine.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Cell wall (made of cellulose) is rigid / strong, so it supports the cell OR stops it bursting when water enters by osmosis (either one suffices) | "strong wall resists bursting", "gives shape / support" | "protects the cell" alone; "the cell wall controls what enters and leaves"; "cell wall is the cell membrane" |
| 2 | Vacuole contains cell sap and, when full of water, makes the cell turgid, which supports the plant | "turgid", "pushes against the wall", "keeps cell firm", "stores sugars / ions" with turgor link | "stores stuff" alone; "stores water" with no consequence stated |
| 3 | Microvilli are folds / projections of the cell-surface membrane that increase its surface area | "finger-like folds", "larger membrane area" | "microvilli increase the surface area of the cell wall"; "make the cell bigger" |
| 4 | So more digested food molecules can be absorbed / absorption is faster | "greater rate of absorption", "more space for transport proteins" | "helps absorption" with no reference to surface area |
Marks 1 and 2 are for the plant cell, 3 and 4 for the intestinal cell. Each mark links a feature to a function; a function stated with no feature, or a feature with no function, scores nothing. Tonoplast (the vacuole membrane) is creditable but adds no mark beyond 4.
Model answer (4/4)
The cell wall is made of cellulose and is rigid and strong, so it supports the cell and stops it bursting when water enters by osmosis. The vacuole contains cell sap, and when it is full of water the cell becomes turgid, which supports the plant. Microvilli are folds of the cell-surface membrane that increase its surface area, so more digested food molecules can be absorbed and absorption is faster.
Examiner trap
- Saying the cell wall is the 'cell membrane'
- Describing an adaptation without linking the feature to its function
Key terms
- cell-surface membrane
- cell wall
- vacuole
- tonoplast
- specialised cell
- tissue
- organ
- organ system
- adaptation
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:28Cell-surface membrane
Start at the edge of every cell. The cell-surface membrane. It is a thin layer that surrounds the cell and controls what enters and leaves. It is partially permeable, so it lets some substances through and keeps others out. Animal cells, plant cells, fungi, every cell has one.
0:47The plant cell wall
Now plants. Outside the membrane, plant cells have a cell wall. This is a separate layer. It is not the cell membrane. In plants it is made mainly of cellulose, which you met in the carbohydrates lessons. Algae and fungi have walls too, made of different chemicals. The wall is rigid and strong. It gives the cell its shape, supports the plant, and stops the cell bursting when water enters by osmosis.
1:16The vacuole and tonoplast
Inside a plant cell sits a large vacuole. It is surrounded by its own membrane, called the tonoplast. The vacuole contains cell sap, a solution of sugars, mineral ions and sometimes pigments. When it is full of water, it pushes the cell against the wall. The cell is firm, which we call turgid, and that helps support the plant.
Lose water, and the cells go floppy, or flaccid, and the plant wilts.
1:45Specialised cells and tissues
In a complex multicellular organism, cells become specialised. They are adapted for one job. Similar specialised cells group together into a tissue. Different tissues form an organ. And several organs working together make an organ system.
Cell, tissue, organ, system. That order is worth learning.
2:07Adaptation: intestine cell
Here is a classic adaptation. The cell lining the small intestine. Its surface membrane is folded into tiny finger-like projections called microvilli. Microvilli increase the surface area of the membrane. A bigger surface area means more space for digested food molecules to be absorbed, so absorption is faster.
That is the pattern for every adaptation. Feature, then function, then the reason it works.
3:08Examiner trap and recap
The trap. The cell wall is not the cell membrane. The wall is outside, the membrane is inside. And never stop at the feature. Always add the result: a bigger surface area, so more absorption.
Recap. Membrane controls entry, wall supports, vacuole keeps the cell firm, and microvilli add surface area. Next time we compare prokaryotes with eukaryotes, and see what bacteria do differently.
Watch first
This lesson builds on:
- Nucleus, Mitochondria and Chloroplasts · AQA 7402 · 3.2.1.1
Keep going
More from 3.2 Cells
- Nucleus, Mitochondria and Chloroplasts · AQA 7402 · 3.2.1.1
- Rough ER, Golgi and Lysosomes Explained · AQA 7402 · 3.2.1.1
- Prokaryotic vs Eukaryotic Cells · AQA 7402 · 3.2.1.2
- Viruses: Structure and Non-Living Status · AQA 7402 · 3.2.1.2
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-024.