Exploring Eukaryotic Cells: Nucleus, Membranes, and Energy Revision Notes | A Level AQA Biology

1. Why Eukaryotic Cell Structure Matters

Every function your body performs — from contracting a muscle to reading this sentence — depends on what is happening inside individual cells. The structures within those cells are not random; each one is precisely built to do a specific job.

AQA examiners regularly ask you to link structure to function, and to explain how a cell is adapted for its role. That means you need to know not just what each organelle looks like, but why it looks that way. That is the thread running through this entire lesson.

2. The Cell-Surface Membrane

Picture the cell-surface membrane as a highly selective border control. It does not keep things in — it decides, moment by moment, what enters and what leaves. This selectivity is what keeps the internal environment of the cell stable.

The phospholipid bilayer is the core of the membrane. Each phospholipid molecule has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) fatty acid tails. In an aqueous environment, the molecules spontaneously arrange into a bilayer — heads facing outward into the watery surroundings, tails tucked inward away from water.

Proteins embedded in this bilayer carry out most of the membrane's active work: some act as channels or carriers for specific molecules, others function as receptors for signalling molecules such as hormones. The fluid mosaic model describes this arrangement — 'fluid' because the phospholipids can move laterally, and 'mosaic' because of the varied protein pattern.

• Controls entry and exit of substances — maintains a stable internal environment
• Acts as a site for cell signalling via receptor proteins
• Provides a partially permeable barrier — small, non-polar molecules (e.g. oxygen, carbon dioxide) cross freely; larger or charged molecules require protein channels or carriers
• Involved in cell recognition through glycoproteins and glycolipids on the outer surface

The cell-surface membrane is not unique to the outer boundary of the cell — the same phospholipid bilayer structure forms the membranes of every organelle inside the cell. Keeping that in mind will help you see the pattern as we move through the nucleus, mitochondria, and chloroplasts.

3. The Nucleus: Control Centre of the Cell

If the cell-surface membrane is border control, the nucleus is the government. It holds the instructions — in the form of DNA — that direct every activity the cell carries out. Understanding its structure tells you exactly how those instructions are stored and accessed.

The nuclear envelope is a double membrane — two phospholipid bilayers — perforated by nuclear pores. These pores are not holes; they are large protein complexes that regulate the passage of molecules such as messenger RNA (mRNA) out of the nucleus and proteins into it. This controlled traffic is essential for gene expression.

Inside the nucleus, DNA is wound around histone proteins to form chromatin. When a cell is not dividing, chromatin is loosely arranged, allowing transcription enzymes to access genes. During cell division, chromatin condenses into the distinct, visible structures we call chromosomes. In eukaryotes, these chromosomes are linear — a key distinction from the circular DNA of prokaryotes.

The nucleolus is a dense, non-membrane-bound region within the nucleus. Its job is to transcribe ribosomal RNA and assemble the subunits of ribosomes. Cells that produce large quantities of protein — such as secretory cells — often have a particularly prominent nucleolus, because they need to manufacture ribosomes rapidly to keep up with protein synthesis demand.

A cell with one or more nucleoli is the norm — the number can vary depending on how active the cell is in protein synthesis. Cells that are highly active in secretion or growth may contain multiple nucleoli to meet the demand for ribosome production.

4. Mitochondria: Powering the Cell

Now that we have established how the nucleus directs the cell, we can turn to the organelles that provide the energy to carry out those instructions. Mitochondria are where the majority of ATP — the cell's usable energy currency — is produced through aerobic respiration.

The structure of a mitochondrion is a masterclass in surface area maximisation. The outer membrane forms a smooth boundary, but the inner membrane is folded into structures called cristae. These folds dramatically increase the surface area available for the electron transport chain — the final, most productive stage of aerobic respiration — allowing far more ATP to be generated per mitochondrion.

Between the two membranes lies the intermembrane space, and inside the inner membrane is the matrix. The matrix contains enzymes for the Krebs cycle, mitochondrial DNA (circular, like that of prokaryotes), and mitochondrial ribosomes (70S, smaller than eukaryotic cytoplasmic ribosomes which are 80S). The presence of their own DNA and ribosomes is strong evidence for the endosymbiotic theory — the idea that mitochondria were once free-living prokaryotes engulfed by a host cell.

The number of mitochondria in a cell is directly related to its energy demands. Cardiac muscle cells, for example, contain hundreds of mitochondria packed tightly together, because the heart must contract continuously without fatigue. This is a classic example of the structure-to-function relationship that AQA examiners test repeatedly.

5. Chloroplasts: Capturing Light Energy

Mitochondria release energy stored in organic molecules. Chloroplasts do the opposite — they capture energy from sunlight and use it to build organic molecules from carbon dioxide and water. Both organelles are energy transducers, but they work in opposite directions.

The internal architecture of a chloroplast is organised around two distinct regions. The grana are stacks of flattened membrane sacs called thylakoids. Chlorophyll and other photosynthetic pigments are embedded in the thylakoid membranes, and this is where the light-dependent reactions of photosynthesis occur — capturing light energy and using it to produce ATP and reduced NADP.

Surrounding the grana is the stroma — a protein-rich fluid containing the enzymes needed for the light-independent reactions (the Calvin cycle). Carbon dioxide is fixed here into organic molecules using the ATP and reduced NADP produced in the grana. Like mitochondria, chloroplasts contain their own circular DNA and 70S ribosomes, again pointing to their prokaryotic ancestry.

Chloroplasts are found only in plant cells and algae — animal cells cannot photosynthesise and therefore have no chloroplasts. This is a straightforward distinction, but it feeds directly into questions about cell adaptations, which we will address in the next section.

6. Applying Structure to Cell Adaptations

The real test of your understanding is not recalling what organelles look like — it is using that knowledge to explain why a particular cell is built the way it is. AQA 'suggest' and 'explain' questions frequently present an unfamiliar cell type and ask you to reason from first principles.

The logic is always the same: identify the cell's function, identify which organelles support that function, and then explain which structural features of those organelles make them particularly effective. Here is a worked example to show you how that reasoning chain works in practice.

Worked example: Applying Cell Structure Knowledge to an Exam Adaptation Question

A palisade mesophyll cell in a leaf is adapted for photosynthesis. Explain how the structure of its organelles supports this function. [4 marks]

1. Palisade cells are the primary site of photosynthesis in leaves, so the key organelle is the chloroplast. — Always start by naming the organelle linked to the function stated in the question. This earns the first mark and frames the rest of your answer.
2. Palisade cells contain a large number of chloroplasts, positioned near the upper surface of the leaf to maximise light absorption. — Quantity and position are both structural adaptations. Examiners award marks for specific detail — 'many chloroplasts' is better than 'chloroplasts are present'.
3. Each chloroplast contains thylakoid membranes stacked into grana. The large surface area of the thylakoid membranes provides more sites for photosynthetic pigments and the light-dependent reactions. — This is the structure-to-function link the mark scheme is looking for. Naming 'grana' and 'thylakoid membranes' with a reason scores higher than a vague reference to 'membranes inside the chloroplast'.
4. Palisade cells also contain many mitochondria to supply ATP for active transport of ions and for biosynthesis reactions within the cell. — A well-adapted cell often relies on more than one organelle. Including mitochondria here shows higher-order reasoning and can secure an additional mark.

Answer: Palisade cells contain numerous chloroplasts with extensive thylakoid membranes (grana) that maximise the surface area for light absorption and the light-dependent reactions. The large number of chloroplasts positioned near the leaf surface increases the rate of photosynthesis. Mitochondria provide ATP for other cellular processes, supporting the high metabolic activity of the cell.

Notice how each point in that answer names a structure, describes a feature of that structure, and then explains the functional consequence. That three-part pattern — structure, feature, function — is the template for any adaptation question at A Level.

7. Exam Technique and Common Mistakes

You now have the content knowledge. The final step is making sure you translate that knowledge into marks under exam conditions. The mistakes below are the ones that cost students marks most frequently on this topic.

Bringing this lesson together: the cell-surface membrane controls what enters and leaves the cell; the nucleus directs all cellular activity through linear, protein-bound DNA and produces ribosomes via the nucleolus; mitochondria release energy as ATP through aerobic respiration, with cristae maximising the surface area for this process; and chloroplasts capture light energy for photosynthesis, with thylakoid membranes and stroma serving distinct but complementary roles. Every structural feature you have studied exists because it makes the cell better at its job — and that is precisely the argument AQA wants you to make in the exam.