SPI Biology · Cell Biology
Cell Structure
Every living thing is made of cells, and each cell has a membrane around it, cytoplasm inside it and small parts called organelles that each do one job.
Why this matters
Six questions in the bank, five of them EASY: how a large animal begins life, what a potato peel is made of, the feature that tells a plant cell from an animal cell, what the cell membrane is made of and what it does, and the suicidal bag of the cell. Each is one fact about a cell part.
Concept 1 of 3: The cell as the unit of life, plant and animal cells
The cell is the smallest unit that is alive. An elephant and a bacterium both start as one cell; the elephant's single cell, the zygote, divides again and again. Plant and animal cells share most parts. The parts only one of them has are the ones that tell them apart.
Definition
- Robert Hooke first saw cells in 1665, in a thin slice of cork.
- Every organism, however large, begins life as a single cell, the zygote.
- Unicellular organisms (Amoeba, Paramecium, bacteria) are one cell; multicellular ones (plants, animals) are many.
- Both plant and animal cells have a cell membrane, cytoplasm, a nucleus and mitochondria.
- Only plant cells have a cell wall (made of cellulose) and chloroplasts, and they have one large vacuole.
- Animal cells have centrosomes and only small vacuoles, if any.
| Part | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent The cell wall is the surest feature that tells a plant cell from an animal cell. |
| Chloroplasts | Present in green parts | Absent |
| Vacuole | One large central vacuole | Small or none |
| Centrosome | Absent | Present |
| Cell membrane, nucleus, mitochondria | Present | Present |
A part that both cells have cannot tell them apart.
Practice this conceptself-check · 5 quick reps
The same idea in a practice question:
Practice questionEasy
Example 1 · Cell Biology · Cell Structure
In a large animal like elephant life begins with :
Picking a part both cells share
Mitochondria, a nucleus and a cell membrane are in plant and animal cells alike, so none of them can tell the two apart. Look for the cell wall, chloroplasts or a large vacuole.
Thinking a big animal starts as many cells
Every organism, an elephant or a whale included, begins as one cell, the zygote. The many cells come later, by division.
Concept 2 of 3: The cell membrane
The cell membrane is a thin, flexible skin around the cell. It decides what may come in and what may go out, like a gatekeeper who checks both doors.
Definition
- Also called the plasma membrane.
- Made mainly of lipids and proteins (a double layer of lipid with proteins in it).
- It is selectively permeable: it lets some substances pass and stops others.
- It controls both the entry and the exit of materials.
- In a plant cell it lies just inside the cell wall. The cell wall is freely permeable; the membrane is not.
| Feature | Cell membrane | Cell wall |
|---|---|---|
| Found in | All cells | Plant cells (also fungi and bacteria) |
| Made of | Lipids and proteins | Cellulose (in plants) |
| Lets substances through | Selectively: it controls entry and exit | Freely The membrane controls traffic both ways, not just one way. |
| Nature | Thin, flexible, living | Rigid, gives shape and support |
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The same idea in a practice question:
Practice questionEasy
Example 2 · Cell Biology · Cell Structure
What is the main composition of the cell membrane?
Only entry, or only exit
The cell membrane controls materials going in and coming out. An option that says it controls only one direction is wrong.
Carbohydrates as the main material
The membrane has a few short sugar chains on its surface, but it is made mainly of lipids and proteins. Nucleic acids (DNA, RNA) are in the nucleus, not the membrane.
Concept 3 of 3: Cell organelles and their jobs
Organelles are the small working parts inside a cell, each with one job. The questions name a job or a nickname and ask for the organelle.
Definition
- Nucleus: holds the chromosomes (DNA) and controls the cell.
- Mitochondria: release energy from food: the powerhouse of the cell.
- Ribosomes: make proteins.
- Lysosomes: bags of digestive enzymes. When the cell is damaged they burst and digest it: the suicidal bags.
- Golgi apparatus: packs and sends out materials.
- Endoplasmic reticulum: a network of channels that carries materials.
- Plastids (chloroplasts): photosynthesis, in plant cells.
- Vacuoles: store water, food and waste.
| Organelle | Job | Nickname |
|---|---|---|
| Nucleus | Controls the cell, holds DNA | Control centre |
| Mitochondria | Release energy | Powerhouse of the cell |
| Lysosome | Digests worn out parts and germs | Suicidal bag Lysosomes, not vacuoles, are the suicidal bags. |
| Ribosome | Makes proteins | Protein factory |
| Chloroplast | Photosynthesis | Kitchen of the cell |
Practice this conceptself-check · 4 quick reps
The same idea in a practice question:
Practice questionEasy
Example 3 · Cell Biology · Cell Structure
Which of the following cell organ is called the siucidal bag?
A vacuole as the suicidal bag
A vacuole is a store of water and waste. The suicidal bag is the lysosome, because its enzymes digest the cell itself when the cell is damaged.
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 as the unit of life, plant and animal cells5 rows
| Part | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent The cell wall is the surest feature that tells a plant cell from an animal cell. |
| Chloroplasts | Present in green parts | Absent |
| Vacuole | One large central vacuole | Small or none |
| Centrosome | Absent | Present |
| Cell membrane, nucleus, mitochondria | Present | Present |
A part that both cells have cannot tell them apart.
The cell membrane4 rows
| Feature | Cell membrane | Cell wall |
|---|---|---|
| Found in | All cells | Plant cells (also fungi and bacteria) |
| Made of | Lipids and proteins | Cellulose (in plants) |
| Lets substances through | Selectively: it controls entry and exit | Freely The membrane controls traffic both ways, not just one way. |
| Nature | Thin, flexible, living | Rigid, gives shape and support |
Cell organelles and their jobs5 rows
| Organelle | Job | Nickname |
|---|---|---|
| Nucleus | Controls the cell, holds DNA | Control centre |
| Mitochondria | Release energy | Powerhouse of the cell |
| Lysosome | Digests worn out parts and germs | Suicidal bag Lysosomes, not vacuoles, are the suicidal bags. |
| Ribosome | Makes proteins | Protein factory |
| Chloroplast | Photosynthesis | Kitchen of the cell |
Watch out for (5)
- Picking a part both cells share→ The cell as the unit of life, plant and animal cells
- Thinking a big animal starts as many cells→ The cell as the unit of life, plant and animal cells
- Only entry, or only exit→ The cell membrane
- Carbohydrates as the main material→ The cell membrane
- A vacuole as the suicidal bag→ Cell organelles and their jobs
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