AQA GCSE Biology 8461: Cell structure – Study Guide

This study guide teaches section 4.1.1 Cell structure (4.1.1.1 to 4.1.1.6) of the AQA GCSE Biology (8461) specification, for first teaching 2016 with exams from 2018 (version 1.0). Cell biology is assessed on Paper 1, which covers topics 1 to 4 and is set at Foundation and Higher Tier. One statement in 4.1.1.6 is marked (HT only) and is labelled Higher tier only below. Required practical activities 1 and 2 both sit in this section.

The Cell biology study guide summarises the whole topic; this page goes deeper on 4.1.1 only. For extra questions on division and transport, use the Cell biology practice questions. Revise with the Cell structure revision notes and test yourself with the Cell structure practice questions. The course hub is AQA GCSE Biology, and the printable checklist lists every topic. To find your gaps, try a free diagnostic.

What this section covers

Spec	What you must be able to do	Tier
4.1.1.1 Eukaryotes and prokaryotes	Compare eukaryotic and prokaryotic cells; work with cell sizes, prefixes and standard form; make order of magnitude calculations	Both
4.1.1.2 Animal and plant cells	Relate the nucleus, membranes, mitochondria, chloroplasts and plasmids to their functions; list the parts of animal and plant cells; use estimation to judge relative size or area	Both
Required practical 1	Use a light microscope to observe, draw and label plant and animal cells, with a magnification scale	Both
4.1.1.3 Cell specialisation	Explain how sperm, nerve, muscle, root hair, xylem and phloem cells are adapted to their function	Both
4.1.1.4 Cell differentiation	Explain the importance of differentiation; compare animals and plants	Both
4.1.1.5 Microscopy	Compare light and electron microscopes (magnification and resolution); use magnification = size of image / size of real object	Both
4.1.1.6 Culturing microorganisms	Binary fission; broth and agar; aseptic technique; πr²; bacterial numbers from a mean division time	Both, except standard form for bacterial numbers (Higher tier only)
Required practical 2	Investigate the effect of antiseptics or antibiotics on bacterial growth by measuring zones of inhibition	Both

4.1.1.1 Eukaryotes and prokaryotes

Eukaryotic cells are plant and animal cells. They have a cell membrane, cytoplasm, and genetic material enclosed in a nucleus.

Prokaryotic cells are bacterial cells. They are much smaller. They have cytoplasm and a cell membrane, surrounded by a cell wall. The genetic material is not in a nucleus. It is a single DNA loop, and there may be one or more small rings of DNA called plasmids.

Scale, prefixes and standard form

You need the prefixes centi, milli, micro and nano.

Unit	Symbol	In metres
centimetre	cm	10⁻² m
millimetre	mm	10⁻³ m
micrometre	μm	10⁻⁶ m
nanometre	nm	10⁻⁹ m

Each step from mm to μm to nm is a factor of 1000: multiply going to the smaller unit, divide going to the larger.

Most bacteria are a few micrometres across or less; a plant or animal cell can be up to about 100 times wider than a typical bacterium.

An order of magnitude is a factor of 10. Two sizes that differ by a factor of 100 (10²) differ by two orders of magnitude.

Worked example 1. A plant cell is 0.1 mm long. A bacterium is 1 μm long. Write both lengths in metres in standard form and find how many orders of magnitude apart they are.

plant cell: 0.1 mm = 0.1 × 1000 = 100 μm = 1 × 10⁻⁴ m
bacterium:  1 μm = 1 × 10⁻⁶ m
ratio = (1 × 10⁻⁴) / (1 × 10⁻⁶) = 10²

The plant cell is 100 times longer: two orders of magnitude. Subtracting the powers (−4 − (−6) = 2) is quicker.

4.1.1.2 Animal and plant cells

Most animal cells have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant cells often have these plus chloroplasts and a permanent vacuole filled with cell sap. Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

Structure related to function

Structure	Function, and how its features fit it
Nucleus	Holds the genetic material, which controls the activities of the cell
Cytoplasm	Where most chemical reactions happen
Cell membrane	Controls which substances pass into and out of the cell
Mitochondria	Site of aerobic respiration, releasing energy; cells that need a lot of energy have many
Ribosomes	Where protein synthesis happens
Chloroplasts (plants)	Contain chlorophyll, which absorbs light for photosynthesis
Permanent vacuole (plants)	Filled with cell sap; helps keep the cell firm
Cellulose cell wall (plants, algae)	Strengthens the cell
Plasmids (bacteria)	Small rings of extra DNA, separate from the main DNA loop

For a 2-mark “explain”, name the structure, then say what it does: “many mitochondria [1] release energy for contraction [1]”.

Using estimation

Estimate when an exact measurement is not possible or not needed: a structure has unclear edges or an irregular shape, or you only need to compare sizes. Count how many times one structure fits into another, or treat a shape as a rectangle or circle.

Worked example 2. In a micrograph, a leaf cell is 50 μm long and about 20 μm wide. About 10 chloroplasts fit end to end along its length. The vacuole fills about three-quarters of the cell.

chloroplast length ≈ 50 / 10 = 5 μm
cell area ≈ 50 × 20 = 1000 μm²   (treat the cell as a rectangle)
vacuole area ≈ 0.75 × 1000 = 750 μm²

Give these as “about 5 μm” and “about 750 μm²”.

Required practical 1: observing cells with a light microscope

1. Take a thin layer of cells, such as onion epidermis, so light passes through.
2. Place it on a slide in a drop of stain (iodine solution for plant cells) to make the nucleus visible.
3. Lower a coverslip at an angle to avoid trapping air bubbles.
4. Clip the slide on the stage. Start with the lowest-power objective lens.
5. Focus with the coarse, then fine, focus. Switch to a higher-power objective and refocus with the fine focus.
6. Draw in pencil with clear, unbroken lines and no shading. Label with straight lines that do not cross.
7. Include a magnification scale, such as a scale bar.

Total magnification = eyepiece lens magnification × objective lens magnification. A ×10 eyepiece with a ×40 objective gives ×400.

4.1.1.3 Cell specialisation

Specialised cells have structures that fit them to a job in a tissue, organ, organ system or whole organism. For an unfamiliar cell, give each feature, then what it allows.

Cell	Key features	How they help
Sperm cell	Tail; many mitochondria; enzymes in the head	Swims to the egg; mitochondria release energy for swimming; enzymes digest a way into the egg
Nerve cell	Long; branched endings	Carries impulses over long distances; connects with many other cells
Muscle cell	Protein fibres that can contract; many mitochondria	Shortens to move bones or organs; energy for contraction
Root hair cell	Long, thin projection	Large surface area for absorbing water and mineral ions from soil
Xylem cell	Dead cells forming hollow tubes; walls strengthened with lignin	Water and mineral ions flow through freely; tubes do not collapse and support the plant
Phloem cell	Cells joined end to end; end walls with pores	Dissolved sugars move from one cell to the next

4.1.1.4 Cell differentiation

As an organism develops, cells differentiate to form different types of cells. As a cell differentiates, it gains the sub-cellular structures it needs for a certain function. It has then become a specialised cell.

• Most types of animal cell differentiate at an early stage.
• Many types of plant cell keep the ability to differentiate throughout life.
• In mature animals, cell division is mainly restricted to repair and replacement.

Why differentiation matters: a multicellular organism needs many cell types for different jobs. Differentiation produces them, and they then form tissues and organs.

4.1.1.5 Microscopy

Light microscopes came first; the electron microscope came later. It has much higher magnification and much higher resolving power than a light microscope.

• Magnification is how many times bigger the image is than the real object.
• Resolution is the ability to tell apart two points that are close together.

A light microscope cannot resolve detail finer than about 200 nm, however much the image is enlarged. The electron microscope resolves far finer detail, so biologists could see and understand many more sub-cellular structures, such as ribosomes.

The magnification equation

magnification = size of image / size of real object

Rearranged: real size = image size / magnification. Both sizes must be in the same unit. Magnification has no unit.

Worked example 3. In an electron micrograph a mitochondrion is 36 mm long. The magnification is ×18 000. Find its real length in micrometres and in metres in standard form.

real size = 36 / 18 000 = 0.002 mm
0.002 mm × 1000 = 2 μm
2 μm = 2 × 10⁻⁶ m

Worked example 4 (scale bar). A scale bar is 10 mm long and labelled 10 μm. A cell in the same image is 45 mm long.

magnification = 10 mm / 10 μm = 10 000 μm / 10 μm = ×1000
real length = 45 mm / 1000 = 0.045 mm = 45 μm

4.1.1.6 Culturing microorganisms

Bacteria multiply by simple cell division called binary fission. This can happen as often as once every 20 minutes if there are enough nutrients and a suitable temperature. Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate.

Uncontaminated cultures are needed to investigate the action of disinfectants and antibiotics.

Aseptic technique, and why each step matters

Step	Reason
Sterilise Petri dishes and culture media before use	Kills any microorganisms already present, so only the chosen bacteria grow
Pass the inoculating loop through a flame	Kills microorganisms on the loop before it is used to transfer bacteria
Secure the lid with adhesive tape	Stops microorganisms from the air getting in, and stops the lid coming off
Store the dish upside down	Condensation does not drip onto the agar
Incubate at 25°C in school labs	Lower temperatures make the growth of pathogens that could harm humans less likely

Calculating bacterial numbers

number of divisions = time available / mean division time
bacteria at end = bacteria at start × 2^(number of divisions)

Worked example 5. A culture starts with 150 bacteria. The mean division time is 30 minutes. How many bacteria are there after 3 hours?

3 hours = 180 minutes
divisions = 180 / 30 = 6
number = 150 × 2⁶ = 150 × 64 = 9600

Higher tier only: give the answer in standard form: 9.6 × 10³ bacteria.

Area of a colony or clear zone

Use πr², where r is the radius (half the diameter).

Worked example 6. A clear zone around an antibiotic disc has a diameter of 18 mm.

r = 18 / 2 = 9 mm
area = π × 9² = 254.469... ≈ 254 mm²

Required practical 2: antiseptics and zones of inhibition

Spread bacteria evenly over an agar plate using aseptic technique. Add paper discs soaked in different antiseptics (or antibiotics), plus a control disc soaked in sterile water. Tape and incubate the plate. A clear zone of inhibition forms where bacteria are killed or stopped from growing. Measure each zone’s diameter and find its area with πr². A larger area means a more effective antiseptic. Control disc size, soaking time, bacterium, temperature and incubation time.

Common errors

• Writing that bacteria have “no DNA”. They have DNA: a single loop, plus plasmids, but no nucleus.
• Giving “vacuole” for plant cells without “permanent”.
• Giving bacteria a cellulose cell wall. Cellulose walls are in plant and algal cells only.
• Multiplying when you should divide in a unit conversion. mm → μm is ×1000; μm → mm is ÷1000.
• Using the diameter instead of the radius in πr².
• Treating magnification and resolution as the same.
• Counting divisions wrongly: 2 hours at 20 minutes per division is 6 divisions.
• Saying cultures are incubated at 25°C “because bacteria grow best there”. The reason is safety.

Official syllabus

AQA GCSE Biology (8461) specification, for first teaching 2016, exams from 2018, version 1.0, published by AQA – section 4.1.1 Cell structure (4.1.1.1 to 4.1.1.6), with required practical activities 1 and 2.