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

This study guide teaches 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. It works through all seven learning outcomes in 1.2. This is AS Level content: it is examined on Paper 1 (Multiple Choice) and Paper 2 (AS Level Structured Questions), and Paper 4 can draw on it because A Level questions need knowledge of the AS content. Microscope skills from section 1.1 are covered in the topic-level pages linked below.

Course hub: Cambridge A Level Biology · Printable checklist: 9700 checklist · Condensed version: 1.2 revision notes · Test yourself: 1.2 practice questions

For the whole of Topic 1, including magnification and the graticule, see the Cell structure study guide. The secreted-protein pathway and the free versus bound ribosome comparison are explained there, so this guide does not repeat them.

What this section covers

Outcome	What you must be able to do
1.2.1	Recognise the listed eukaryotic organelles and outline their structure and function
1.2.2	Describe and interpret photomicrographs, electron micrographs and drawings of typical plant and animal cells
1.2.3	Compare the structure of typical plant and animal cells
1.2.4	State that cells use ATP from respiration for energy-requiring processes
1.2.5	Outline the key structural features of a typical bacterium
1.2.6	Compare a typical bacterium with typical plant and animal cells
1.2.7	State the structure of viruses and that they are non-cellular

The organelle list in 1.2.1 is “limited to” the items named, so learn every one.

1.2.1 Eukaryotic organelles: structure and function

For each organelle learn its appearance, its number of membranes and its function.

Membranes and the nucleus

• Cell surface membrane. A phospholipid bilayer with proteins. In a transmission electron micrograph it shows as a thin dark-light-dark line. It is partially permeable and controls what enters and leaves the cell. Its detailed structure belongs to Topic 4, Cell membranes and transport.
• Nucleus. The largest organelle in most cells. It contains the DNA as chromatin. It is bounded by the nuclear envelope, which is two membranes with nuclear pores through it. The outer membrane is continuous with the rough endoplasmic reticulum.
• Nucleolus. A dense, darker region inside the nucleus with no membrane of its own. It makes ribosomal RNA and assembles ribosome subunits.

The internal membrane system

• Rough endoplasmic reticulum (RER). A network of flattened membrane sacs (cisternae) with ribosomes on the outer surface. Proteins made on these ribosomes enter the cisternae and are transported through the cell.
• Smooth endoplasmic reticulum (SER). Membrane tubules and sacs with no ribosomes. It makes lipids, including steroids.
• Golgi body (Golgi apparatus or Golgi complex). A stack of curved, flattened sacs, with vesicles at the edges. Vesicles from the RER fuse with it. It modifies proteins (for example, adding carbohydrate to make glycoproteins), packages them into vesicles and makes lysosomes.
• Lysosomes. Small spherical vesicles surrounded by a single membrane. They contain hydrolytic (digestive) enzymes, kept apart from the rest of the cytoplasm. They break down worn-out organelles and material taken into the cell.

Energy organelles

• Mitochondria. Surrounded by an envelope of two membranes. The inner membrane is folded into cristae, and the fluid inside is the matrix. They are the site of aerobic respiration, which makes ATP. The matrix contains small circular DNA and 70S ribosomes.
• Chloroplasts (plant cells that photosynthesise). Also surrounded by a double-membrane envelope. Inside, the stroma contains stacks of thylakoid membranes called grana, and often starch grains. They are the site of photosynthesis. Like mitochondria, they contain small circular DNA and 70S ribosomes.

Ribosomes

Ribosomes are not surrounded by a membrane. Each is made of two subunits of RNA and protein, and they are the site of protein synthesis. The S number measures how fast a particle sediments in a centrifuge, so a larger S means a larger particle.

• 80S ribosomes are in the cytoplasm, free or attached to the RER.
• 70S ribosomes are in mitochondria and chloroplasts.

Microtubules, centrioles, cilia and microvilli

• Microtubules. Long, hollow tubes made of the protein tubulin. They help form the cytoskeleton, which supports the cell and moves vesicles and organelles. They also form the spindle in cell division.
• Centrioles. A pair of short hollow cylinders, usually at right angles to each other, near the nucleus in animal cells. Each cylinder is a ring of nine groups of three (triplets of) microtubules. They are involved in organising the spindle. Plant cells of flowering plants have no centrioles.
• Cilia. Hair-like extensions of the cell surface, covered by the cell surface membrane. Inside is a ring of nine pairs of microtubules around two central microtubules (the 9 + 2 arrangement). Each cilium grows from a basal body, which has the same structure as a centriole. Cilia beat to move fluid or mucus over a surface, for example in the airways.
• Microvilli. Small finger-like folds of the cell surface membrane, much smaller than cilia and with no 9 + 2 core. They increase the surface area for absorption, for example on cells lining the small intestine.

Plant cell structures

• Cell wall. Made mainly of cellulose, outside the cell surface membrane. It is freely permeable. It gives the cell its shape and stops it bursting when it takes in water.
• Plasmodesmata. Narrow channels through the walls of neighbouring cells, lined by cell surface membrane. They connect the cytoplasm of one cell with the next, so substances can pass between cells.
• Large permanent vacuole and tonoplast. A large central space filled with cell sap. It is bounded by a partially permeable membrane called the tonoplast. Water taken into the vacuole pushes the cytoplasm against the wall and keeps the cell turgid.

1.2.2 Interpreting micrographs and drawings

Decide first what kind of image you have.

• Light micrograph (photomicrograph). Resolution is limited to about 200 nm. You can see the nucleus, cell wall, chloroplasts and large vacuole. Mitochondria may appear only as tiny dots. Ribosomes, ER and Golgi detail are not visible.
• Transmission electron micrograph (TEM). A thin section, in black and white. Internal membranes, cristae, ribosomes and nuclear pores are visible.
• Scanning electron micrograph (SEM). Shows surfaces in three dimensions, such as cilia or microvilli on the outside of a cell.

Recognition clues:

You see	It is
Large round body, double membrane with gaps, dark patch inside	Nucleus with pores and nucleolus
Oval body, double membrane, inner one folded into shelves	Mitochondrion
Larger body, double membrane, internal stacks of membranes	Chloroplast (grana)
Parallel flattened sacs covered in dots	RER
Curved stack of sacs with vesicles budding off, no dots	Golgi body
Small round vesicle with uniform dark contents	Lysosome
Hollow cylinders in a pair, near the nucleus	Centrioles

Worked example 1: identifying a cell from a micrograph

A TEM of a cell shows: a thick layer outside the cell surface membrane; no nucleus; scattered small dark dots in the cytoplasm; no structure with a double membrane. A 2 µm scale bar measures 25 mm and the cell measures 45 mm long. Identify the type of cell and calculate its length and the magnification.

Step 1: Use the scale bar. 25 mm represents 2 µm, so 45 mm represents (45 ÷ 25) × 2 = 3.6 µm.

Step 2: Magnification = image size ÷ actual size. Convert the bar to the same unit: 25 mm = 25 000 µm. 25 000 ÷ 2 = × 12 500.

Step 3: Interpret. No nucleus and no double-membrane organelles means a prokaryote. A wall outside the membrane and a length within 1–5 µm fit a typical bacterium. The dark dots are 70S ribosomes.

Answer: a bacterium (prokaryotic cell), 3.6 µm long, magnification × 12 500.

1.2.3 Comparing plant and animal cells

Both are eukaryotic, so both have a nucleus, RER, SER, Golgi body, mitochondria, 80S ribosomes, lysosomes, microtubules and a cell surface membrane.

Feature	Typical plant cell	Typical animal cell
Cell wall	Present, cellulose	Absent
Plasmodesmata	Present	Absent
Chloroplasts	In photosynthesising cells	Absent
Vacuole	Large, permanent, with tonoplast	Small and temporary, if present
Centrioles	Absent (flowering plants)	Present
Cilia, microvilli	Absent	Present in some cells
Shape	Regular, fixed by wall	Variable

Worked example 2: a “compare” answer

Compare the structure of a palisade mesophyll cell with a liver cell. [4]

Give matched pairs and at least one similarity:

1. Both have a nucleus, mitochondria and 80S ribosomes (similarity).
2. The palisade cell has a cellulose cell wall; the liver cell has none.
3. The palisade cell has chloroplasts; the liver cell has none.
4. The palisade cell has a large permanent vacuole with a tonoplast; the liver cell does not.
5. The liver cell has centrioles; the palisade cell does not.

Each point names both cells. “Plant cells have a wall” on its own does not compare.

1.2.4 ATP from respiration

You only need to state this: cells use ATP made in respiration as the immediate energy source for energy-requiring processes. Examples include protein synthesis, active transport, cell division, muscle contraction and the movement of vesicles. This is why cells with a high energy demand, such as secretory cells, have many mitochondria. The detail of how ATP is made comes later in Topic 12, Energy and respiration.

1.2.5 The typical bacterium

The syllabus lists six features of a typical prokaryotic cell:

• unicellular: each bacterium is one cell
• generally 1–5 µm in diameter, much smaller than most eukaryotic cells
• a cell wall made of peptidoglycan, not cellulose
• circular DNA in the cytoplasm, not enclosed in a nucleus
• 70S ribosomes only
• no organelles surrounded by double membranes: no nucleus, mitochondria or chloroplasts

1.2.6 Bacterium versus plant and animal cells

Feature	Typical bacterium	Plant cell	Animal cell
Nucleus with envelope	No	Yes	Yes
Main DNA	Circular, in cytoplasm	Linear, in nucleus	Linear, in nucleus
Small circular DNA	It is the main DNA	In mitochondria and chloroplasts	In mitochondria
Ribosomes	70S only	80S in cytoplasm, 70S in organelles	80S in cytoplasm, 70S in mitochondria
Cell wall	Peptidoglycan	Cellulose	None
Double-membrane organelles	None	Nucleus, mitochondria, chloroplasts	Nucleus, mitochondria

Notice that 70S ribosomes and circular DNA are not unique to bacteria. They are also found inside mitochondria and chloroplasts. A question that asks which feature “only prokaryotes” have needs care.

1.2.7 Viruses

All viruses are non-cellular. A virus has:

• a nucleic acid core, either DNA or RNA
• a capsid made of protein around it
• in some viruses, an outer envelope made of phospholipids.

A virus has no cytoplasm, no ribosomes and no cell surface membrane of its own. This is why section 1.2 asks whether the cell is the basic unit of life. Viruses are needed later in Topic 10, Infectious diseases.

Common errors

• Calling the nuclear envelope a single membrane. It is two membranes.
• Saying the nucleolus is an organelle with a membrane. It has none.
• Writing that only bacteria have 70S ribosomes. Mitochondria and chloroplasts have them too.
• Mixing up cilia and microvilli. Only cilia have the 9 + 2 microtubule core and beat.
• Stating that plant cells have no mitochondria because they have chloroplasts. They have both.
• Writing “the vacuole membrane” instead of “tonoplast” when the name is asked for.
• Saying a virus has a cell wall or a cell membrane. The envelope is phospholipid, but the virus is not a cell.
• Labelling a lysosome as a ribosome. Ribosomes are much smaller and have no membrane.

Next steps

• 1.2 revision notes for quick recall
• 1.2 practice questions with mark-by-mark answers
• Free 9700 AS diagnostic

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).