Pearson Edexcel IGCSE Biology: Cell Structure and Biological Molecules — Revision Notes (4BI1)

Related: Cell structure and biological molecules study guide

Condensed, exam-focused notes for sub-topics (b) and (c) of Topic 2, Pearson Edexcel International GCSE Biology (4BI1), Issue 3.

2.2-2.3 Cell structures and functions

Structure	Function
Nucleus	Contains genetic material, controls cell activities
Cytoplasm	Site of chemical reactions
Cell membrane	Controls substances entering/leaving the cell
Cell wall (plant only)	Provides structural support
Mitochondria	Site of respiration, releases energy
Chloroplasts (plant only)	Site of photosynthesis
Ribosomes	Site of protein synthesis
Vacuole (large, permanent, plant)	Stores cell sap, maintains turgor

• Always be ready to state both what a structure IS and what it DOES — questions ask for either given the other.

2.4 Plant vs animal cells

• Shared: nucleus, cytoplasm, cell membrane, mitochondria, ribosomes.
• Plant-only: cell wall, chloroplasts, large permanent vacuole.
• Never state that animal cells possess a cell wall or large vacuole.

2.5B-2.6B Cell differentiation and stem cells (Paper 2 only, not in the Science Double Award)

• Cell differentiation: importance in developing specialised cells.
• Stem cells: advantages and disadvantages of medical use — weigh both sides for a given context.

2.7-2.8 Biological molecules

• Elements present: know which elements make up carbohydrates, proteins, lipids.
• The building-block pattern (the single most useful idea in this section): a large molecule is built from smaller repeating units.
  • Starch/glycogen ← simple sugar units.
  • Protein ← amino acid units.
  • Lipid ← fatty acid + glycerol units.
• This pattern transfers directly to later topics on digestion, where these molecules are broken back down into their component units.

2.9 Practical: testing food samples

• Tests for glucose, starch, protein, fat in a food sample (linked to the biochemical test methods — know reagent and expected positive result for each).

2.10-2.14 Enzymes

• Enzymes act as biological catalysts in metabolic reactions.
• Temperature: activity increases toward the optimum, then denatures (active site changes shape, substrate no longer fits) at high temperature — permanent, unlike low-temperature reduced activity.
• pH: extreme pH also denatures the enzyme by altering the active site shape.

Exam technique for this topic

The recurring exam trap is describing an enzyme as “killed” by heat rather than explaining denaturation as a specific change in active site shape that prevents substrate binding — always use the term denaturation and explain the mechanism, not just the outcome. When predicting an enzyme’s activity at a given temperature, structure the answer in three parts: below optimum (lower kinetic energy, fewer successful collisions, reduced but not permanently damaged activity), at optimum (maximum rate), and well above optimum (denatured, activity drops sharply and does not recover). The key distinguishing point examiners look for is that denaturation at high temperature is permanent, while reduced activity at low temperature is reversible.

Worked example: identifying an unlabelled cell

An exam question shows a micrograph or diagram of a cell without a label, containing a cell wall, large central vacuole, and chloroplasts, and asks candidates to identify the cell type and justify the answer. A strong answer identifies it as a plant cell and justifies this specifically: the cell wall (providing structural support, not found in animal cells), the large permanent vacuole (storing cell sap and maintaining turgor, not found in animal cells) and chloroplasts (the site of photosynthesis, only present in photosynthetic plant cells) are each named individually as the evidence, rather than a vague “it looks like a plant cell.” This structured, evidence-citing approach — naming the specific structures that justify the identification, not just stating the conclusion — is what separates a full-mark answer from a partially-correct one on this kind of question.

Worked example: applying the building-block pattern to digestion

Later topics on digestion ask candidates to explain how large food molecules are broken down for absorption, and the building-block pattern from this topic is the direct foundation for that explanation. Starch, a large molecule built from many simple sugar units joined together, is broken down by digestive enzymes (amylase) into individual simple sugar units small enough to be absorbed into the bloodstream; similarly, proteins are broken down into individual amino acids, and lipids into fatty acids and glycerol. Recognising that digestion is essentially the building-block pattern running in reverse — breaking a large molecule back down into the smaller units it was built from — is a transferable insight worth carrying forward well beyond this specific topic.

Self-test

1. Name two functions found only in plant cells, with the structure responsible for each.
2. What is the building-block pattern, and how does it apply to starch?
3. Why does an enzyme’s activity fall at very high temperatures?
4. Is reduced activity at low temperature permanent or reversible?
5. Name the four biological molecules tested for in the food sample practical.

Answers: 1. Photosynthesis (chloroplasts) and structural support (cell wall) — or storage of cell sap/turgor (large permanent vacuole). 2. A large molecule is built from smaller repeating units; starch is built from simple sugar units. 3. The active site denatures (changes shape), so the substrate no longer fits and the reaction can no longer be catalysed. 4. Reversible — activity increases again if warmed back toward the optimum. 5. Glucose, starch, protein, fat (each with its own specific positive-result test and reagent to recall precisely).