CDS Biology · Cell Biology
Cell Wall, Membrane and Transport
Which living things have a cell wall and what it is made of, how the cell membrane is built, and how substances cross it: diffusion, osmosis and active transport.
Why this matters
Eleven CDS questions from 2017 (II) to 2026 (II). Three ask which cell has no cell wall (always the animal: 2019 (II), 2021 (I), 2023 (II)), two ask how the membrane is built, and five are about crossing it, from osmosis to active transport.
Concept 1 of 3: The cell wall: who has one and what it is made of
A cell wall is a stiff layer outside the cell membrane. It gives the cell a fixed shape and stops it bursting when water rushes in. Plants, fungi and bacteria all have one, each made of a different material. Animal cells have no wall at all, only the soft membrane, which is why an animal cell can change its shape.
Definition
Cell walls:
- Plants: cellulose.
- Fungi (mushrooms, yeast): chitin.
- Bacteria: peptidoglycan. The antibiotic penicillin stops bacteria building this wall, so it kills them without harming human cells.
- Diatoms: a glassy wall of silica.
- Animals (including humans and worms such as the tapeworm): no cell wall, only the cell membrane.
| Living thing | Cell wall made of |
|---|---|
| Plants | Cellulose |
| Fungi, such as mushrooms | Chitin |
| Bacteria | PeptidoglycanQ CDS 2026 (I): penicillin spares human cells because it targets the bacterial cell wall. |
| Diatoms | Silica |
| Animals, including humans | No cell wallQ Asked three times: CDS 2019 (II), 2021 (I) and 2023 (II). The organism without a wall is always the animal. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2023 · CDS (II) 2023 — General Knowledge · Q45Moderate
Example 1 · Cell Biology · Cell Wall, Membrane and Transport
Cell wall is absent in which one of the following organisms ?
Fungi and bacteria do have walls
A mushroom (a fungus) has a chitin wall and a bacterium a peptidoglycan wall. Only animal cells, human or tapeworm, have none.
Not every wall is cellulose
Cellulose is the plant wall. Fungi use chitin and bacteria peptidoglycan, which is why penicillin harms bacteria and not plants or people.
Concept 2 of 3: The cell membrane
Every cell, plant or animal, is wrapped in a thin, soft membrane. It is made of two layers of fat-like phospholipid molecules, with proteins floating in it like tiles in a mosaic. It lets some things through and keeps others out. Because it is flexible, a cell like an amoeba can fold it around food and swallow it.
Definition
The plasma membrane:
- A bilayer of phospholipids with proteins embedded in it, and cholesterol among the lipids in animal cells. This picture is the fluid mosaic model.
- It is selectively permeable (semi-permeable): water and small gases such as oxygen and carbon dioxide pass easily; many other substances need carriers.
- It is flexible, so a cell can engulf food or particles by folding it inwards (endocytosis), as an amoeba does.
- It is not made of cellulose: that is the cell wall, outside it in plants.
| Statement about the cell membrane | True or false |
|---|---|
| It is a phospholipid bilayer with proteins and cholesterol in it | TrueQ CDS 2022 (II): the bilayer is of phospholipid, not of protein or neutral lipid. |
| It is flexible and lets the cell engulf material | TrueQ CDS 2022 (I): this is the correct statement; the other three are false. |
| It is made of cellulose | False: that is the plant cell wall |
| It lets every substance through | False: it is selectively permeable |
| It stops oxygen and carbon dioxide | False: these gases diffuse through it |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2022 · CDS (II) 2022 — General Knowledge · Q70Moderate
Example 2 · Cell Biology · Cell Wall, Membrane and Transport
Which one of the following is a feature of Membrane ?
A bilayer of lipid, not of protein
The two layers are phospholipid. Proteins sit in the bilayer; they do not form it. An option calling it a protein bilayer, or a neutral-lipid bilayer without proteins, is wrong.
Membrane and wall are different layers
The membrane is in every cell and is made of lipid and protein. The cell wall is outside it, only in plants, fungi and bacteria, and is the part made of cellulose (in plants).
Concept 3 of 3: Diffusion, osmosis and active transport
Things on their own spread from where there is more of them to where there is less: that is diffusion, and it costs nothing. Osmosis is the same thing for water, through a membrane that lets water pass but holds back the dissolved substance. To push something the other way, from less to more, the cell has to spend energy from respiration: that is active transport.
Definition
Moving across the membrane:
- Diffusion: from higher to lower concentration. No energy needed.
- Osmosis: water moves from its higher concentration (a dilute solution) to its lower concentration (a strong solution) through a semi-permeable membrane.
- Facilitated diffusion: down the gradient through a protein channel or carrier, still with no energy. Na⁺ and Cl⁻ ions crossing through protein channels is an example.
- Active transport: against the gradient, using energy (ATP) from respiration. Examples: reabsorbing glucose and amino acids in the kidney, loading sugar into phloem, absorbing glucose from the gut.
- Tonicity: in an isotonic solution (same strength as the cell, like normal saline for red cells) nothing changes; in a hypotonic one (weaker, like pure water) the cell swells and may burst; in a hypertonic one (stronger) it shrinks.
- Reverse osmosis: a pressure greater than the osmotic pressure is put on salt water, forcing pure water out through the membrane. This is how sea water is made drinkable.
| Process | Direction | Energy used | Example |
|---|---|---|---|
| Diffusion | High to low concentration | None | Oxygen entering a cell |
| Osmosis | Water, through a semi-permeable membrane, from dilute to strong solution | None | Root hairs taking in waterQ CDS 2021 (I): the membrane is semi-permeable, not fully permeable and not the cell wall. |
| Facilitated diffusion | High to low, through a protein channel | None | Na⁺ and Cl⁻ through protein channelsQ CDS 2026 (II): this is the one process in the list that is not active transport. |
| Active transport | Low to high concentration | ATP from respiration | Glucose reabsorbed in the kidney |
| Reverse osmosis | Water forced out of a strong solution | Pressure above the osmotic pressure | Desalination of sea waterQ CDS 2017 (II): the pressure applied is larger than the osmotic pressure. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2019 · CDS (II) 2019 — General Knowledge · Q78Moderate
Example 3 · Cell Biology · Cell Wall, Membrane and Transport
Which one of the following is active transport ?
Active transport runs on respiration
Active transport moves a substance against the gradient using ATP from respiration. 'Without energy', 'energy from photosynthesis' or 'along the gradient' are the wrong versions.
A protein channel is not active transport
Ions flowing down their gradient through a protein channel is facilitated diffusion and uses no energy. Kidney reabsorption, loading phloem and taking glucose from the gut all pump against the gradient.
Isotonic means no change
In an isotonic solution such as normal saline the water moving in equals the water moving out, so red cells neither swell nor shrink.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
The cell wall: who has one and what it is made of5 rows
| Living thing | Cell wall made of |
|---|---|
| Plants | Cellulose |
| Fungi, such as mushrooms | Chitin |
| Bacteria | PeptidoglycanQ CDS 2026 (I): penicillin spares human cells because it targets the bacterial cell wall. |
| Diatoms | Silica |
| Animals, including humans | No cell wallQ Asked three times: CDS 2019 (II), 2021 (I) and 2023 (II). The organism without a wall is always the animal. |
The cell membrane5 rows
| Statement about the cell membrane | True or false |
|---|---|
| It is a phospholipid bilayer with proteins and cholesterol in it | TrueQ CDS 2022 (II): the bilayer is of phospholipid, not of protein or neutral lipid. |
| It is flexible and lets the cell engulf material | TrueQ CDS 2022 (I): this is the correct statement; the other three are false. |
| It is made of cellulose | False: that is the plant cell wall |
| It lets every substance through | False: it is selectively permeable |
| It stops oxygen and carbon dioxide | False: these gases diffuse through it |
Diffusion, osmosis and active transport5 rows
| Process | Direction | Energy used | Example |
|---|---|---|---|
| Diffusion | High to low concentration | None | Oxygen entering a cell |
| Osmosis | Water, through a semi-permeable membrane, from dilute to strong solution | None | Root hairs taking in waterQ CDS 2021 (I): the membrane is semi-permeable, not fully permeable and not the cell wall. |
| Facilitated diffusion | High to low, through a protein channel | None | Na⁺ and Cl⁻ through protein channelsQ CDS 2026 (II): this is the one process in the list that is not active transport. |
| Active transport | Low to high concentration | ATP from respiration | Glucose reabsorbed in the kidney |
| Reverse osmosis | Water forced out of a strong solution | Pressure above the osmotic pressure | Desalination of sea waterQ CDS 2017 (II): the pressure applied is larger than the osmotic pressure. |
Watch out for (7)
- Fungi and bacteria do have walls→ The cell wall: who has one and what it is made of
- Not every wall is cellulose→ The cell wall: who has one and what it is made of
- A bilayer of lipid, not of protein→ The cell membrane
- Membrane and wall are different layers→ The cell membrane
- Active transport runs on respiration→ Diffusion, osmosis and active transport
- A protein channel is not active transport→ Diffusion, osmosis and active transport
- Isotonic means no change→ Diffusion, osmosis and active transport
Test yourself on Cell Biology
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.