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2.1.2Cell Structure

Structure of Eukaryotic Cells — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 7 of 47 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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2.1.2 Structure of Eukaryotic Cells — AQA A Level Biology 7402 notes: Describe the structure and function of the organelles of animal and plant cells as seen with light and electron microscopes.Eukaryotic cells (animals, plants, fungi and protoctists) have a nucleus and other mem…

  • membrane-bound organelles
  • double membrane (nuclear envelope)
  • nuclear pores
  • Rough endoplasmic reticulum (RER)
  • Smooth endoplasmic reticulum (SER)
  • Golgi apparatus

Section 1 of 9Eukaryotic cells

Eukaryotic cells (animals, plants, fungi and protoctists) have a nucleus and other membrane-bound organelles. Organelles allow compartments in which particular reactions occur in the best conditions (compartmentalisation), and increase efficiency. Cell structure is studied with microscopes; the electron microscope reveals the ultrastructure of organelles.

Section 2 of 9Organelles of the nucleus and protein synthesis

Organelles of the nucleus and protein synthesis
Nucleus
Nucleus surrounded by a double membrane (nuclear envelope) with nuclear pores that allow mRNA, ribosome subunits and other molecules to pass.
  • Contains chromatin (DNA wound around histones) and one or more nucleoli, where rRNA is made and ribosomes assembled.
  • It controls cell activity by storing genes and making mRNA.
Ribosomes
80S, made of rRNA and protein, site of translation; free in cytoplasm or on the RER.
Rough endoplasmic reticulum (RER)
Rough endoplasmic reticulum (RER) system of flattened sacs (cisternae) continuous with the nuclear envelope and coated with ribosomes; folds and transports proteins made by the attached ribosomes.
Smooth endoplasmic reticulum (SER)
Smooth endoplasmic reticulum (SER) no ribosomes; makes and stores lipids and steroids and detoxifies drugs.
Golgi apparatus
Golgi apparatus stack of flattened, curved sacs; modifies proteins (e.g. adds carbohydrates to make glycoproteins), packages them into vesicles, and makes lysosomes.

Section 3 of 9Energy and breakdown organelles

Mitochondria
Mitochondria double membrane; the inner membrane is folded into cristae to give a large surface area for electron transport chains and ATP synthase; the fluid matrix holds enzymes of the Krebs cycle, small circular DNA and 70S ribosomes. Site of aerobic respiration.
Chloroplasts
Chloroplasts in plants and algae; double membrane; contain stacks of thylakoids (grana) with chlorophyll for the light-dependent reaction, and a fluid stroma for the Calvin cycle, with DNA, ribosomes and starch grains.
Lysosomes
Lysosomes membrane-bound vesicles of hydrolytic (digestive) enzymes; break down worn-out organelles, and bacteria taken in by phagocytosis.
Vacuole
Vacuole in plant cells a large sac of cell sap surrounded by a membrane (the tonoplast), keeps the cell turgid and stores sugars, ions and pigments.

Section 4 of 9Cell surface and support structures

Cell surface and support structures
Cell surface membrane
Cell surface membrane phospholipid bilayer with proteins; controls what enters and leaves, and is involved in cell signalling.
Cell wall
(plants, algae, fungi) - plants: cellulose microfibrils giving strength and support and preventing bursting; fungal walls contain chitin. The wall is fully permeable.
Cytoskeleton
Cytoskeleton protein fibres and microtubules that maintain shape, move organelles, and make up centrioles, cilia and flagella.
Centrioles
Centrioles hollow cylinders of microtubules in animal cells that organise the spindle in cell division.

Section 5 of 9Animal and plant cells compared

FeatureAnimal cellPlant cell
Cell wallAbsentCellulose wall
ChloroplastsAbsentPresent in photosynthetic cells
VacuoleSmall or absentLarge permanent vacuole
CentriolesPresentAbsent in higher plants
StoreGlycogenStarch
ShapeVariableFixed, regular

Section 6 of 9Using the light and electron microscope

A light microscope can show nucleus, cell wall, chloroplasts, large vacuole and cell membrane (as a boundary).

Using the light and electron microscope
Ultrastructure
The electron microscope shows ultrastructure: ribosomes, cristae, nuclear pores, ER and Golgi.
Remember that the cell surface membrane, ribosomes and the internal structure of mitochondria cannot be resolved in a light microscope.
When identifying organelles in micrographs, look for the shape and the membranes: two membranes with folded inner membrane (mitochondrion); stacked discs in a green structure (chloroplast); flattened sacs with dots (RER); curved stack with vesicles budding off (Golgi).
Calculate real sizes using magnification (see 2.2).

A practical pointto see organelles in the light microscope, tissue is often stained.

1
Methylene blue stains nuclei in animal cells, iodine solution stains starch grains and nuclei in plant cells, and eosin stains cytoplasm.
2
In an electron microscope, heavy-metal stains such as osmium and lead add contrast to membranes.
3
Transmission electron microscopes (TEM) show a thin slice of internal structure, so the same organelle can look quite different depending on the angle of the section: a mitochondrion cut lengthways looks elongated, but cut across looks circular.
4
Be careful not to identify a structure only by one section.

Finally, link the organelles together

  • nucleus (information), ribosomes and RER (protein synthesis), Golgi (modification and packaging), vesicles (transport), mitochondria and chloroplasts (energy), lysosomes (digestion).
  • Learn to give one function and one structural feature for each.

In exam answers, avoid vague phrases such as 'makes energy' or 'controls the cell'; say 'produces ATP by aerobic respiration' or 'contains DNA that codes for proteins'.

Section 7 of 9Exam-style questions

  1. Describe the function of the Golgi apparatus in a secretory cell. [3]3 marks

  2. Explain how the structure of a mitochondrion is related to its function. [4]4 marks

  3. A cell is 80 µm long in an image magnified x400. Calculate the length of the image. [2]2 marks

Section 8 of 9Key terms

TermMeaning
OrganelleA specialised structure within a cell that carries out a particular function.
NucleolusRegion in the nucleus where rRNA is made and ribosomes assembled.
CristaeFolds of the inner mitochondrial membrane.
Golgi apparatusOrganelle that modifies, packages and sorts proteins.
LysosomeVesicle containing hydrolytic enzymes.
TonoplastMembrane surrounding the plant vacuole.
UltrastructureFine detail of cell structure visible with an electron microscope.
CytoskeletonNetwork of protein fibres giving shape and movement.

Section 9 of 9Exam technique

1
Saying the nucleus has a single membrane. It has a double membrane (envelope) with pores.
2
Forgetting that the RER's ribosomes make protein, not the RER itself.
3
Writing that plant cells have no mitochondria. They have both mitochondria and chloroplasts.
4
Saying the cell wall is the selectively permeable barrier. It is fully permeable; the membrane controls entry.
5
Confusing cristae (inner membrane folds) with grana (stacks of thylakoid).

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