Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.1Cell Structure

Cell Structure — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 12 parts Lesson 5 of 47 Unit quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

Eukaryotic Cell Structure and Specialisation — AQA A LEVEL BIOLOGY 7402 notes: Describe the cell theory, eukaryotic organelles and how cells are specialised and organised into tissues.The cell theory states that all living organisms are composed of one or more cells and that all …

  • Cell surface membrane
  • Mitochondria
  • Chloroplasts
  • Golgi apparatus
  • Muscle cells
  • Sperm cell

Section 1 of 12Cell theory and eukaryotic organelles

The cell theory states that all living organisms are composed of one or more cells and that all cells arise from pre-existing cells. Eukaryotic cells contain these structures:

Cell theory and eukaryotic organelles
Cell surface membrane
Cell surface membrane a phospholipid bilayer with embedded proteins.
Nucleus
Nucleus contains linear DNA bound to proteins as chromosomes.
Mitochondria
Mitochondria site of aerobic respiration and ATP production.
Chloroplasts
Chloroplasts site of photosynthesis in plants and algae.
Golgi apparatus
Golgi apparatus modifies and packages proteins and lipids.
Lysosomes
Lysosomes vesicles containing hydrolytic enzymes that digest worn-out organelles or pathogens.
Rough ER
Rough ER covered in ribosomes; makes and transports proteins. Smooth ER: synthesises and stores lipids and carbohydrates.
Ribosomes
80S in eukaryotes (70S in prokaryotes).
Vacuole
(plants): helps maintain cell pressure and structure.

Section 2 of 12Specialised cells

In multicellular organisms eukaryotic cells become specialised for specific functions. Their structure is adapted to their role, which is why different cell types can look very different. Specialisation involves the presence or absence of particular organelles.

Muscle cells
(skeletal, smooth, cardiac): protein filaments (actin and myosin) slide over each other to cause contraction; a high density of mitochondria supplies ATP.
Sperm cell
Sperm cell fertilises the egg and delivers the father's DNA. The head contains a haploid nucleus; the acrosome contains digestive enzymes to penetrate the egg; the mid-piece is packed with mitochondria to provide energy for movement.
Xylem vessels
Xylem vessels transport water and dissolved mineral ions. There are no end walls between cells, forming a continuous tube; cells are dead and lack organelles so water flows unimpeded.
Phloem
Phloem transports sugars and amino acids (translocation). Living cells are joined end to end by sieve plates for solute flow and have few subcellular structures to aid transport.

Section 3 of 12Cell organisation

Specialised cells of the same type form a tissue, a group of similar cells working together.

Cell organisation
Tissues form organs, and organs form organ systems.
For example, epithelial cells in the small intestine absorb nutrients.
Neurones (nerve cells) form nervous tissue, which makes up the brain and the central nervous system; rod and cone cells form the retina (nervous tissue) of the eye, which is part of the visual system.

Section 4 of 12Answering exam questions on adaptation

Think about what the cell needs to do its job and which organelles would help. For example, a cell that photosynthesises needs many chloroplasts; a cell that carries out active transport needs many mitochondria.

Section 5 of 12Worked examples

  1. Explain how the structure of a sperm cell adapts it for its function. [4]4 marks

  2. Explain why xylem vessels are well adapted to transport water. [3]3 marks

  3. Define the term tissue and give an example. [2]2 marks

Section 6 of 12Prokaryotic cells

Animal and plant cells are eukaryotic, whereas bacteria are prokaryotes. Prokaryotic cells are much smaller than eukaryotic cells. They differ in having no nucleus or other membrane-bound organelles. Learn the typical structures found in prokaryotic cells and their functions:

Cell wall
Cell wall made of murein (a glycoprotein), which supports the cell and prevents bursting.
Plasmids
Plasmids small circular DNA molecules, often carrying genes such as antibiotic resistance.
Circular DNA
Circular DNA free in the cytoplasm and not associated with proteins.
70S ribosomes
70S ribosomes for protein synthesis.
Capsule
Capsule and flagellum in some bacteria, for protection and movement.

Section 7 of 12Comparing prokaryotes and eukaryotes

FeatureProkaryotesEukaryotes
SizeMuch smaller, about 0.5 to 5 µmLarger, about 10 to 100 µm
NucleusNone; DNA free in cytoplasmPresent, enclosed by a nuclear envelope
DNACircular, not bound to proteinsLinear, bound to histone proteins
Ribosomes70S80S
OrganellesNo membrane-bound organellesMitochondria, ER, Golgi and others
Cell wallMureinCellulose in plants, chitin in fungi, none in animals
DivisionBinary fissionMitosis

Section 8 of 12Viruses

Viruses are acellular infectious particles. They are considered non-living because they cannot reproduce independently (they require a host cell) and they do not carry out protein synthesis (they use the protein-building machinery in host cells). They also do not respire or grow.

Section 9 of 12Why size and structure matter

Bacteria are typically about 1 to 5 µm long, while animal and plant cells are about 10 to 100 µm.

Why size and structure matter
Because prokaryotes are small, their surface area to volume ratio is large, so diffusion supplies the cell quickly.
Their lack of membrane-bound compartments means ribosomes, enzymes and DNA are all in the cytoplasm, which allows rapid gene expression.
Make sure you can label a labelled diagram of a bacterium and state the function of each part, for example the murein wall resists osmotic bursting and the plasmid carries extra genes.

Section 10 of 12Worked examples

  1. State three differences between prokaryotic and eukaryotic cells. [3]3 marks

  2. Explain why viruses are classed as non-living. [2]2 marks

  3. Suggest why prokaryotes are not limited by the lack of organelles. [2]2 marks

Section 11 of 12Key terms

TermMeaning
OrganelleSpecialised structure inside a cell with a specific function
TissueGroup of similar cells working together
AcrosomeRegion of a sperm head containing digestive enzymes
TranslocationTransport of sugars and amino acids in phloem
Rough EREndoplasmic reticulum with ribosomes; makes proteins
LysosomeVesicle containing hydrolytic enzymes
TermMeaning
ProkaryoteCell lacking a nucleus and membrane-bound organelles
PlasmidSmall circular DNA molecule in bacteria
MureinGlycoprotein that forms the bacterial cell wall
VirusAcellular infectious particle that needs a host cell
70SSize of ribosomes in prokaryotes

Section 12 of 12Exam technique

1
Describing mitochondria as "producing energy" - they transfer energy to ATP.
2
Forgetting that xylem cells are dead and phloem cells are living.
3
Listing organelles without linking them to the function of the specialised cell.
4
Confusing a tissue with an organ.
Avoid these mistakes
Saying bacteria have "no DNA" - they have DNA, but it is free in the cytoplasm and not in a nucleus.
Stating that viruses are cells.
Writing that prokaryotic ribosomes are the same as eukaryotic - they are 70S and 80S respectively.

Last updated: