AQA GCSE Biology: Cell Biology (8461)

This guide covers Topic 1 Cell biology, the first of eight topic/section areas in AQA GCSE Biology (8461), first teaching September 2016. This is a linear, tiered qualification (Foundation and Higher), assessed across two written papers: Paper 1 covers topics 1-4, and Paper 2 covers topics 5-7, with both papers also drawing on Key ideas and Working scientifically.

Where this fits in 8461

Cell biology introduces the structural and functional building blocks – prokaryotic and eukaryotic cells, division, and transport across membranes – that every later topic in Paper 1 depends on, from infection and response through to bioenergetics.

Syllabus coverage

AQA GCSE BIOLOGY (8461) — TOPIC 1 CELL BIOLOGY

• 4.1.1 Cell structure — the differences between eukaryotic cells (plant and animal, with a nucleus) and prokaryotic cells (bacterial, without a nucleus, with a single DNA loop and plasmids); the sub-cellular structures of animal cells (nucleus, cytoplasm, cell membrane, mitochondria, ribosomes) and the additional structures found in plant cells (chloroplasts, a permanent vacuole, and a cellulose cell wall); cell specialisation (for example sperm, nerve and muscle cells in animals, and root hair, xylem and phloem cells in plants) and differentiation; and culturing microorganisms. Required practical activity 1 (using a light microscope to observe, draw and label plant and animal cells, including a magnification scale) sits within this sub-topic
• 4.1.2 Cell division — chromosomes and the role of the nucleus in carrying genetic information; mitosis and the cell cycle; and stem cells
• 4.1.3 Transport in cells — diffusion, osmosis, and active transport across cell membranes

How to approach it

Cell structure (4.1.1) is best learned through comparison – build a side-by-side picture of what animal, plant and bacterial cells have in common and what makes each distinct, since exam questions frequently ask students to identify or explain structural differences. Cell division (4.1.2) hinges on understanding why mitosis exists (growth and repair) and why stem cells matter precisely because they retain the ability to differentiate – know both the process and its biological purpose. Transport in cells (4.1.3) is the most commonly confused sub-topic: keep diffusion, osmosis and active transport clearly distinguished by what is moving (particles vs. water), direction relative to the concentration gradient, and whether energy is required, since exam questions often test this distinction directly.

Official syllabus

AQA GCSE Biology (8461) specification, for first teaching September 2016 — aqa.org.uk.

Eukaryotic and prokaryotic cells

Eukaryotic cells (animals, plants, fungi) have a nucleus enclosing the genetic material and membrane-bound organelles. Prokaryotic cells (bacteria) are much smaller, have no nucleus, and carry a single loop of DNA plus small rings called plasmids.

Structure	Animal	Plant	Bacterial
Nucleus	Yes	Yes	No — DNA loop free in cytoplasm
Cell wall	No	Yes (cellulose)	Yes (not cellulose)
Chloroplasts	No	Yes	No
Permanent vacuole	No	Yes	No
Mitochondria	Yes	Yes	No

Sub-cellular structures worth knowing by function: the nucleus controls activity and holds DNA, mitochondria are the site of aerobic respiration, ribosomes make proteins, chloroplasts absorb light for photosynthesis, and the cell membrane controls what enters and leaves.

Specialisation and differentiation

Cells differentiate to carry out particular jobs. Sperm cells have a tail and many mitochondria for swimming; nerve cells are long with branched ends to carry impulses; muscle cells contain protein fibres that shorten; root hair cells have a large surface area for absorbing water; xylem cells are hollow, dead and strengthened with lignin; phloem cells have sieve plates for transporting dissolved sugars.

In animals most cells differentiate early and lose the ability to change, while many plant cells retain it for life — which is why plants can be cloned from cuttings.

Microscopy

Light microscopes have a maximum magnification of about x2000 and limited resolving power. Electron microscopes have far higher magnification and resolution, which is what revealed sub-cellular structure in detail.

magnification = size of image / size of real object

Always convert to the same units first. 1 mm = 1000 micrometres.

Cell division by mitosis

The cell cycle consists of growth and DNA replication, then mitosis, then cytoplasmic division. Mitosis produces two genetically identical diploid daughter cells, and is used for growth, repair and asexual reproduction.

Stem cells

Stem cells are undifferentiated cells that can divide to produce more cells and then differentiate into one or more specialised cell types. Embryonic stem cells can differentiate into almost any cell type in the body. Adult stem cells, found in tissues such as bone marrow, have a more limited range and can produce only certain cell types, such as the various blood cells. Plants also keep a supply of undifferentiated cells, called meristem cells, in their growing tips (roots and shoots) throughout life. Stem cells have therapeutic uses — bone marrow stem cells are already used to treat some blood disorders, and embryonic stem cells are being researched for treating conditions such as paralysis and diabetes — but their use raises ethical and safety issues, including the destruction of embryos to obtain embryonic stem cells and the risk that transplanted stem cells could transfer a viral infection or trigger an immune reaction.

Transport in cells

Substances move into and out of cells by three processes, distinguished by what is moving, the direction relative to the concentration gradient, and whether energy is needed.

• Diffusion — the net movement of particles (such as gases or dissolved solutes) from an area of higher concentration to an area of lower concentration, down a concentration gradient, until evenly spread. It requires no energy input from the cell. The rate of diffusion increases with a greater concentration gradient, a higher temperature, and a larger surface area.
• Osmosis — the diffusion of water specifically, across a partially permeable membrane, from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution). Like diffusion, it needs no energy input.
• Active transport — the movement of particles from a lower to a higher concentration, against the concentration gradient. Because this is the opposite direction to diffusion, it requires energy released by respiration. Active transport allows a cell to absorb substances it needs even when they are already more concentrated inside it than outside — for example, the absorption of glucose from the gut or from the soil by root hair cells, when the glucose concentration is higher inside the cell already.

Worked example

A cell is 0.05 mm across. Under a microscope its image measures 100 mm. Find the magnification.

Convert: 0.05 mm = 50 micrometres, and 100 mm = 100 000 micrometres

magnification = 100 000 / 50 = x2000

Because the answer is x2000, this is at the limit of a light microscope.

Common mistakes

Saying bacteria “have no DNA” rather than no nucleus — they have DNA, just not enclosed. Writing that plant cells have “a vacuole” without saying permanent, since animal cells can have temporary ones. Mixing up magnification and resolution: magnification makes the image bigger, resolution is the ability to distinguish two close points. Forgetting unit conversion in magnification calculations, which is where most marks are lost. Stating mitosis produces gametes — that is meiosis.

Quick revision checklist

• Compare eukaryotic and prokaryotic cells, with sizes and named structures.
• Give the function of each sub-cellular structure.
• Explain how three specialised cells are adapted to their functions.
• Rearrange and use the magnification equation with correct unit conversion.
• Describe the stages of the cell cycle and state what mitosis produces.
• Explain why electron microscopes changed our understanding of cells.
• Distinguish embryonic and adult stem cells, and state one therapeutic use and one ethical concern.
• Distinguish diffusion, osmosis and active transport by what moves, direction, and energy use.