Biology · Unit 2

The Cell

Cells and their organelles

The cell is the unit biology is built on: every living thing is either one cell or a colony of them, and almost everything in later units is really a story about what cells do. Getting the parts straight here pays off for years.

The unit covers the organelles and their jobs, what separates a plant cell from an animal one, and how cell theory was arrived at in the first place. That last part matters more than it looks - cells were invisible until the microscope, so the history of the idea is also a lesson in how instruments change what can be known.

This unit breaks down into 20 short steps and 129 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 129
Difficulty: 1-3

What this unit covers

Plant vs Animal Cells
From Cells to Bodies & Microscopes
Cell Theory & the Unit of Life
Organelles and Their Jobs
Cell Types & Numbers

Where this fits

Do this before DNA and Genetics, Cell Transport and Energy, or any of the body-system units. All of them assume you can picture a cell.

Where people slip

Diagrams make cells look like flat, tidy cross-sections with everything spaced out. They are three-dimensional, crowded and in constant motion, and the diagram is a convenience rather than a picture of the thing.

How the unit is structured

The Cell runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 129 in the unit altogether.

Cell Theory & the Unit of Life

True or falseLevel 1
1. All living things are made of one or more cells.
Answer: True
One of the main ideas of cell theory is that every living thing is built from at least one cell.
Build the sentenceLevel 2
2. Build the sentence about who first named cells.
Answer: Robert Hooke first named the cell
Robert Hooke first used the word 'cell' in 1665 when he looked at a slice of cork through a microscope.
Multiple choiceLevel 2
3. According to cell theory, where do brand-new cells come from?
- From other living cellscorrect
- From the air
- From plain water
- They appear on their own
Cell theory states that all cells come from cells that already exist, usually when one cell divides in two.
Fill the blankLevel 3
4. During metaphase the chromosomes line up across the middle of the cell, at a position called the metaphase ____.
- platecorrect
- pole
- furrow
- spindle
At metaphase the chromosomes align on the metaphase plate, midway between the two spindle poles.
Picture questionLevel 3
5. 🧬 This is a replicated chromosome. What are the two identical halves joined at the centromere called?
- Sister chromatidscorrect
- Daughter cells
- Homologous partners
- Centrioles
After S phase each chromosome consists of two identical sister chromatids held together at the centromere until they separate in anaphase.
Sequence recallLevel 3
6. Remember and repeat the order of these discoveries about cells.
Answer: Hooke sees and names cells in cork -> Leeuwenhoek observes living microbes -> Schleiden and Schwann state all living things are made of cells -> Virchow states cells come from existing cells
The story of cell theory runs from Hooke naming cells, to Leeuwenhoek seeing living microbes, to Schleiden and Schwann's cell theory, to Virchow showing cells come from cells.

Cell Types & Numbers

Fact or fibLevel 1
7. A sperm cell has a long tail that helps it swim toward the egg.
Answer: True
The sperm cell's tail whips back and forth so it can swim to reach an egg cell.
Multiple choiceLevel 1
8. A nerve cell is very long and thin. What does this shape help it do?
- Carry messages quickly over long distancescorrect
- Store lots of water
- Soak up sunlight for food
- Float easily in the blood
Nerve cells are long and thin so they can carry electrical messages across your body.
Choose all that applyLevel 2
9. Which of these cells have NO nucleus? Select all that apply.
- Mature red blood cellcorrect
- Bacteriumcorrect
- Human muscle cell
- Plant leaf cell
Bacteria never have a nucleus, and mature red blood cells push theirs out to make more room for oxygen.
Fill the blankLevel 2
10. Many bacteria carry small extra rings of DNA, separate from their main chromosome, called ____.
- plasmidscorrect
- grana
- cristae
- lysosomes
Plasmids are small circular DNA molecules in bacteria that often carry genes such as those for antibiotic resistance.
Guess the numberLevel 2
11. A unicellular organism, such as a bacterium, is made of how many cells?
Answer: 1 cells
Unicellular means 'one cell,' so the whole organism is made of just a single cell.
Odd one outLevel 3
12. Three of these are materials that form a cell wall. Which one is the odd one out?
- Keratincorrect
- Cellulose
- Peptidoglycan
- Chitin
Cellulose (plants), peptidoglycan (bacteria) and chitin (fungi) form cell walls, but keratin is a structural protein in hair and skin, not a wall.

From Cells to Bodies & Microscopes

Fill the blankLevel 1
13. Magnification tells you how many times ____ an object looks under a microscope.
- biggercorrect
- colder
- heavier
- faster
Magnification is how many times larger something appears compared with its real size.
Multiple choiceLevel 1
14. Robert Hooke looked at a thin slice of cork under a microscope. What did he name the little boxes he saw?
- Cellscorrect
- Atoms
- Germs
- Crystals
The tiny boxes reminded Hooke of small rooms called cells, and the name stuck.
Put in orderLevel 1
15. Put these in order from smallest to largest in the body.
Answer: Cell -> Tissue -> Organ -> Organ system
Cells join to make tissues, tissues form organs, and organs work together as organ systems.
Guess the numberLevel 2
16. About how many cells make up the human body? (in trillions)
Answer: 37 trillion cells
Scientists estimate the human body is built from around 37 trillion cells.
Choose all that applyLevel 3
17. Why is the solution used to break open cells kept cold, isotonic, and buffered? Select all that apply.
- Cold, to slow enzymes that could digest the organelles
- Isotonic, to stop organelles bursting or shrinking by osmosis
- Buffered, to keep the pH steady so proteins are not denatured
- Warm, to speed up respiration in the organelles
A cold, isotonic, buffered solution protects organelles from enzyme damage, osmotic bursting, and pH changes.
Tap the pairsLevel 3
18. Tap the matching pairs about cell signalling.
Answer: Ligand = Signalling molecule that binds a receptor; Receptor = Protein that detects and responds to a signal; Transduction = Relaying the signal onward inside the cell; Response = The cell's final reaction to the signal
A ligand binds a receptor (reception); transduction relays the signal inside the cell; the response is the cell's final reaction.

Organelles and Their Jobs

Fill the blankLevel 1
19. The ____ is the control center that holds the cell's DNA.
- nucleuscorrect
- membrane
- cytoplasm
- ribosome
The nucleus stores the cell's DNA and directs what the cell does, acting like a control center.
Match the pairsLevel 1
20. Match each organelle to the job it does.
Answer: Nucleus = Control center; Mitochondria = Releases energy; Ribosome = Makes proteins; Cell membrane = Gatekeeper
Each organelle has its own job: the nucleus controls, mitochondria power, ribosomes build proteins, and the membrane guards the cell.
Build the sentenceLevel 3
21. Build the sentence describing the fluid mosaic model of the cell membrane.
Answer: proteins float in a phospholipid bilayer
The fluid mosaic model pictures proteins floating within a fluid double layer (bilayer) of phospholipids.
Choose all that applyLevel 3
22. Which of these are parts of the endomembrane system? Select all that apply.
- Endoplasmic reticulumcorrect
- Golgi apparatuscorrect
- Lysosomescorrect
- Mitochondria
The endomembrane system includes the nuclear envelope, ER, Golgi, lysosomes and vesicles; mitochondria have their own separate membranes and are not part of it.
Fact or fibLevel 3
23. The microtubules inside cilia and flagella are arranged in a 9+2 pattern.
Answer: True
Cilia and flagella contain a ring of nine microtubule pairs around two central microtubules, the classic 9+2 arrangement.
Guess the numberLevel 3
24. For every ATP it uses, how many sodium ions does the sodium-potassium pump move out of the cell?
Answer: 3 sodium ions
Each cycle of the Na-K pump exports 3 sodium ions and imports 2 potassium ions, powered by one molecule of ATP.

Plant vs Animal Cells

Fact or fibLevel 1
25. Plant cells have a stiff cell wall around them.
Answer: True
Plant cells are surrounded by a strong cell wall that gives them shape and support, while animal cells do not have one.
Spell itLevel 1
26. Spell the green organelle that lets plant cells capture sunlight to make food.
Answer: chloroplast
Chloroplasts are the green organelles that let plant cells use sunlight to make their own food.
Choose all that applyLevel 2
27. Which of these are found in plant cells but NOT in animal cells? Select all that apply.
- Cell wallcorrect
- Chloroplastcorrect
- Nucleus
- Mitochondria
Cell walls and chloroplasts are special to plant cells, while a nucleus and mitochondria are found in both plant and animal cells.
Multiple choiceLevel 2
28. A root hair cell has a long thin bump sticking out. Why is this useful?
- It adds surface area to soak up more water and minerals from the soilcorrect
- It helps the plant stand up straight
- It catches sunlight to make food
- It scares away hungry insects
The root hair's long shape increases surface area so the plant can absorb more water and minerals.
Fill the blankLevel 3
29. The thick fluid that surrounds the thylakoids inside a chloroplast is called the ____.
- stromacorrect
- matrix
- cytoplasm
- cell sap
The stroma is the fluid of the chloroplast surrounding the grana, and it is where the light-independent reactions occur.
True or falseLevel 3
30. Plant cells finish cytokinesis by building a new cell plate across the middle, instead of pinching in with a cleavage furrow.
Answer: True
A rigid cell wall stops plant cells pinching, so they form a cell plate from Golgi vesicles that grows into a new dividing wall.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise The Cell

129 questions across 20 steps. Start with step one and crawl at your own pace.

Play this unit

Read about The Cell

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

More units in Biology

See all 18 units in Biology →