Cambridge International AS & A Level Biology 9700: Cells as the basic units of living organisms – Revision Notes

These notes condense section 1.2, Cells as the basic units of living organisms, from Topic 1 Cell structure of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. They cover learning outcomes 1 to 7 of 1.2. It is AS Level content, examined on Papers 1 and 2, and Paper 4 can draw on it. For full explanations and worked examples, use the 1.2 study guide.

Course hub: Cambridge A Level Biology · Checklist: 9700 checklist · Practise: 1.2 practice questions · Whole topic: Cell structure revision notes

1.2.1 Organelles by number of membranes

Sorting organelles by their membranes answers a surprising number of Paper 1 questions.

Membranes	Structures
Two (an envelope)	Nucleus (nuclear envelope with pores), mitochondria, chloroplasts
One	Cell surface membrane, RER, SER, Golgi body, lysosomes, vesicles, tonoplast
None	Ribosomes, nucleolus, centrioles, microtubules, cell wall

Cilia and microvilli are extensions of the cell, so they are covered by the cell surface membrane rather than having their own.

Structure and function in one line each

Structure	Key structure	Function
Cell surface membrane	Phospholipid bilayer with proteins	Partially permeable; controls entry and exit
Nuclear envelope	Two membranes, nuclear pores; outer one joins RER	Separates DNA from cytoplasm; pores let substances through
Nucleolus	Dense region, no membrane	Makes rRNA; assembles ribosome subunits
RER	Flattened sacs with ribosomes	Transports proteins made on its ribosomes
SER	Tubules, no ribosomes	Makes lipids, including steroids
Golgi body	Curved stack of sacs plus vesicles	Modifies and packages proteins; makes lysosomes
Mitochondria	Double membrane, cristae, matrix, circular DNA, 70S ribosomes	Aerobic respiration; makes ATP
Ribosomes	Two subunits, RNA + protein	Protein synthesis
Lysosomes	Single membrane, hydrolytic enzymes	Digest worn organelles and engulfed material
Microtubules	Hollow tubes of tubulin	Cytoskeleton; move organelles; spindle
Centrioles	Pair of cylinders, 9 triplets of microtubules	Organise the spindle (animal cells)
Cilia	9 + 2 microtubules, basal body	Beat to move fluid or mucus
Microvilli	Folds of cell surface membrane	Increase surface area for absorption
Chloroplasts	Double membrane, grana, stroma, circular DNA, 70S ribosomes	Photosynthesis
Cell wall	Cellulose; freely permeable	Shape; resists bursting
Plasmodesmata	Channels through walls, lined by membrane	Link cytoplasm of neighbouring cells
Vacuole + tonoplast	Cell sap inside a partially permeable membrane	Turgor; storage

Ribosome sizes

80S  -> cytoplasm (free or on RER) of eukaryotic cells
70S  -> mitochondria and chloroplasts; all of a bacterium's ribosomes
S    -> measures sedimentation rate: larger S = larger particle

1.2.2 Micrograph method in steps

1. Identify the image type. Colour or a stain and low detail means a light micrograph. Black and white with membranes visible means TEM. A 3-D surface means SEM.
2. Look for a nucleus. No nucleus and no double-membrane organelles means a prokaryote.
3. Look for a wall, chloroplasts and a large vacuole. Any of these means a plant cell.
4. Count the membranes around each body to name it (see the table above).
5. Use the scale bar if you must give a size. Measure the bar and the object in mm, then actual size = (object ÷ bar) × bar value.

Light microscope resolution is about 200 nm, so ribosomes, ER and Golgi detail need an electron micrograph.

Small worked reminders

Scale bar. A bar labelled 5 µm measures 20 mm on a micrograph. A chloroplast in the same image measures 28 mm.

actual length = (28 / 20) x 5 um = 7 um
magnification = 20 mm / 5 um = 20 000 um / 5 um = x 4000

Magnification has no units. Give the size in µm, the unit that suits a cell or organelle.

“Explain why this cell is a prokaryote.” [2] Write: there is no nucleus (no nuclear envelope), so the DNA lies free in the cytoplasm [1]; there are no organelles surrounded by a double membrane, such as mitochondria [1]. Do not write “it has no organelles”: the ribosomes are visible.

“Suggest why this cell has many mitochondria.” [2] Write: mitochondria are the site of aerobic respiration, which makes ATP [1]; the cell carries out an energy-requiring process, such as protein synthesis or active transport, which uses that ATP [1]. Name a process that fits the cell in the question.

Reading a secretory cell. Lots of RER, a large Golgi body, many vesicles near the cell surface and many mitochondria together point to a cell that makes and releases protein. The whole route is set out in the Cell structure study guide.

1.2.3 Plant versus animal: the short list

Only in plant cells: cellulose cell wall, plasmodesmata, chloroplasts, large permanent vacuole with tonoplast. Only in animal cells: centrioles, and in some cells cilia and microvilli. In both: nucleus, ER, Golgi body, mitochondria, ribosomes, lysosomes, microtubules, cell surface membrane.

1.2.4 ATP

State it plainly: cells use ATP from respiration for energy-requiring processes, such as protein synthesis, active transport, cell division and muscle contraction. More mitochondria means a higher energy demand.

1.2.5 and 1.2.6 Typical bacterium

Feature	Bacterium	Eukaryotic cell
Cells per organism	Unicellular	Often many
Diameter	Generally 1–5 µm	Usually larger
Wall	Peptidoglycan	Cellulose (plant) or none (animal)
DNA	Circular, free in cytoplasm	Linear, inside a nucleus
Ribosomes	70S only	80S in cytoplasm, 70S in mitochondria and chloroplasts
Double-membrane organelles	Absent	Nucleus, mitochondria, chloroplasts (plants)

1.2.7 Viruses

Non-cellular. Nucleic acid core (either DNA or RNA) + protein capsid. Some have an outer envelope of phospholipids.

Must-know distinctions

• Cilia vs microvilli. Cilia: long, 9 + 2 microtubules, beat. Microvilli: short folds, no microtubule core, absorb.
• Centriole vs basal body. Same structure; a centriole organises the spindle, a basal body anchors a cilium.
• Nucleus vs nucleolus. The nucleus has an envelope; the nucleolus is a region inside it with no membrane.
• Tonoplast vs cell surface membrane. Both partially permeable; the tonoplast surrounds the vacuole.
• Capsid vs envelope. Every virus has a protein capsid; only some have a phospholipid envelope.
• 70S in bacteria vs 70S in eukaryotes. Same size; in eukaryotes they are only inside mitochondria and chloroplasts.

Quick self-test

1. Name the two organelles in a plant cell that contain small circular DNA.
2. Which structure makes ribosomal RNA, and is it membrane-bound?
3. State two differences between cilia and microvilli.
4. Name the membrane around the large permanent vacuole.
5. A cell has a peptidoglycan wall and 70S ribosomes only. What kind of cell is it?
6. Name three structures surrounded by a double membrane in a plant cell.
7. State what cells use ATP from respiration for.
8. Give the three possible components of a virus and say which one only some viruses have.
9. A virus 100 nm across has an image 25 mm across. Calculate the magnification.
10. A mitochondrion 1.2 µm long is photographed at × 25 000. How long is its image in mm?
11. Why can’t ribosomes be seen with a light microscope?
12. Name two structures that let you tell a TEM of a liver cell from one of a bacterium.

Answers

1. Mitochondria and chloroplasts.
2. The nucleolus; no, it has no membrane.
3. Any two: cilia contain a 9 + 2 arrangement of microtubules, microvilli do not; cilia beat to move fluid, microvilli increase surface area for absorption; cilia are much longer.
4. The tonoplast.
5. A prokaryotic cell (a bacterium).
6. Nucleus (nuclear envelope), mitochondria, chloroplasts.
7. As the energy source for energy-requiring processes, for example protein synthesis or active transport.
8. Nucleic acid core (DNA or RNA), protein capsid, phospholipid envelope; only some have the envelope.
9. 25 mm = 25 000 000 nm; 25 000 000 ÷ 100 = × 250 000.
10. 1.2 µm × 25 000 = 30 000 µm = 30 mm.
11. They are smaller than the resolution of a light microscope (about 200 nm), so they cannot be distinguished.
12. Any two: nucleus with nuclear envelope; mitochondria; RER or Golgi body (any double-membrane organelle or membrane system).

Where marks are usually lost

• Writing “mitochondria contain DNA” without “circular” when the question asks how it differs from nuclear DNA.
• Saying 70S ribosomes are found only in prokaryotes.
• Describing the nuclear envelope as a single membrane, or forgetting the pores.
• Calling the nucleolus an organelle with a membrane.
• Giving “cell membrane” when the syllabus term is cell surface membrane, or “vacuole membrane” when tonoplast is asked for.
• Listing plant features in a “compare” answer without saying what the animal cell has instead.
• Stating that a bacterium has “no organelles”. It has no double-membrane organelles, but it has ribosomes.
• Saying viruses are made of cells, or have a cell wall.
• Confusing the basal body of a cilium with a microvillus.
• Forgetting unit conversion (mm to µm or nm) before using a scale bar or magnification.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, Cambridge University Press & Assessment – Topic 1, Cell structure (section 1.2 Cells as the basic units of living organisms).