IGCSE Biology: Cells, Movement of Substances and Enzymes

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IGCSE Biology: Cells, Movement of Substances and Enzymes

TL;DR

Cells are the basic living units with a nucleus (animal) or without (prokaryotic), divided into compartments by membranes that control what enters and leaves. Substances move across membranes by diffusion, osmosis, and active transport—each with different energy costs and uses. Enzymes are biological catalysts that speed up reactions by lowering activation energy, and their activity depends on temperature, pH, and substrate concentration.

1. The Mental Model

Think of a cell like a walled city: the membrane is the border checkpoint that decides what gets in and out. Inside, compartments (organelles) do specific jobs. Substances move across that border in three main ways—some slide through freely (diffusion), some follow water pressure (osmosis), and some need energy to fight their way across (active transport). Enzymes are the city's workers: they grab chemical materials, rearrange them, and spit out products—faster than they'd change on their own.

2. The Core Material

Cell Structure and the Membrane Barrier

A typical animal cell has a nucleus (controls what genes are used), mitochondria (releases energy), ribosomes (build proteins), rough endoplasmic reticulum (makes proteins for export), smooth endoplasmic reticulum (makes lipids and handles calcium), Golgi apparatus (packages proteins), and lysosomes (break things down). The cell membrane is a phospholipid bilayer—two fat layers sandwiched around each other—with proteins studded in and through it. This structure keeps the inside separate from the outside and controls what crosses.

Plant cells add chloroplasts (photosynthesis), a cell wall (rigid support), and a big vacuole (stores water and keeps the plant firm). Prokaryotic cells (bacteria) have no nucleus or membrane-bound organelles—just genetic material floating inside.

How Substances Move: Three Pathways

Diffusion is particles spreading out evenly. If you open a bottle of perfume in a room, the smell spreads because perfume molecules bounce randomly and end up distributed everywhere. No energy needed—it happens naturally. Oxygen and carbon dioxide move in and out of cells this way.

Osmosis is diffusion of water specifically. Water molecules move across a semipermeable membrane (like the cell membrane) from where water is more concentrated (fewer dissolved particles) to where it's less concentrated (more dissolved particles). If a cell sits in salty water, water leaves the cell, and the cell shrinks—this is plasmolysis. Put a cell in pure water, and water enters, and the cell swells. It's still passive—no ATP burned.

Active transport is different: the cell uses energy to pump substances against the concentration gradient—that is, from low concentration to high concentration. Sodium-potassium pumps in nerve cells do this: they use ATP to kick sodium out and potassium in, keeping the gradient sharp. This powers nerve signals and muscle contractions.

graph TD
    A["Substance Needs to Cross Membrane"] --> B{"Is energy available<br/>and necessary?"}
    B -->|No, concentration<br/>gradient favors movement| C{"Is it water?"}
    C -->|Yes| D["Osmosis<br/>water diffuses"]
    C -->|No| E["Simple Diffusion<br/>particles spread out"]
    B -->|Yes, moving against<br/>gradient| F["Active Transport<br/>uses ATP, carrier protein"]
    E --> G["No metabolic cost"]
    D --> G
    F --> H["Cell burns ATP"]

Enzymes: Biological Catalysts

An enzyme is a protein that speeds up a chemical reaction without being used up. It works by lowering the activation energy—the energy barrier the reactants must overcome to turn into products. Without an enzyme, a reaction might take hours; with one, seconds.

How enzymes work:
1. Substrate (reactant) enters the enzyme's active site—a pocket shaped specifically to fit it.
2. Enzyme and substrate form an enzyme-substrate complex.
3. Enzyme lowers the activation energy; the reaction happens.
4. Product leaves; enzyme is unchanged and ready to repeat.

Factors affecting enzyme activity:

Temperature: Each enzyme has an optimal temperature (usually ~37°C for human enzymes). Below it, molecular movement is sluggish and the enzyme works slowly. Above it, the enzyme denatures—its protein structure unravels and the active site is destroyed. This is irreversible.

pH: Enzymes have an optimal pH. Pepsin (in the stomach) works best at pH 2 (very acidic); trypsin (in the small intestine) works best at pH 8 (weakly alkaline). Move too far from optimal pH and the enzyme denatures.

Substrate concentration: At low substrate concentration, adding more substrate speeds up the reaction (more collisions with enzyme). But once the enzyme is saturated (every enzyme molecule is busy), adding more substrate does nothing—you've hit the maximum rate.

Enzyme concentration: More enzyme = more active sites = faster reaction, as long as there's enough substrate.

3. Worked Example

Setup: A cell in a 5% salt solution (hypertonic—more salt outside than inside). We want to predict what happens, then explain using osmosis.

Step 1: Identify the gradient.
The cell's interior is roughly 0.9% salt. The outside solution is 5% salt. Water concentration is higher inside the cell (less dissolved salt means more "pure" water).

Step 2: Apply osmosis.
Water molecules move from high water concentration (inside) to low water concentration (outside). Net water moves out of the cell.

Step 3: Result.
The cell loses water. The cytoplasm shrinks away from the cell membrane. In plant cells, the protoplasm peels away from the cell wall—plasmolysis. The plant cell becomes limp (wilts) because the vacuole has shrunk and no longer pushes outward against the cell wall.

Step 4: Energy involved?
None. Water molecules are bouncing randomly; osmosis is just the statistical outcome of that random motion.

Enzyme Example:
Lactase breaks down lactose (milk sugar) into glucose and galactine. At 20°C, 10 mg of lactase acting on 100 mL of milk for 5 minutes breaks down ~2 g of lactose. At 37°C, the same enzyme and substrate yield ~8 g in 5 minutes. At 65°C, the enzyme denatures and breaks down ~0.5 g (the enzyme is ruined and can't recover).

4. Examiner's Breakdown

4.1 What Examiners Actually Reward

1. "Osmosis is the movement of water molecules across a semipermeable membrane from a region of higher water potential to a region of lower water potential." Use "water potential" language or at minimum say water moves from where water is more concentrated to where water is less concentrated (due to dissolved solutes).

2. "Active transport requires ATP / energy and moves substances against the concentration gradient." Do not say active transport "uses energy to move things faster"—examiners know that's diffusion with a tailwind. Say explicitly: against the gradient.

3. "The enzyme lowers the activation energy of the reaction" OR "the enzyme forms an enzyme-substrate complex with the substrate." Either phrase alone earns credit for mechanism; both earns full marks.

4. "Denaturation is irreversible; the enzyme's active site is permanently altered / destroyed." Don't confuse denaturation (permanent protein unraveling) with temporary slowing at suboptimal conditions.

5. "The cell membrane is selectively permeable / semipermeable because proteins in the phospholipid bilayer control which ions and large molecules can pass." Examiners want you to link permeability to structure.

6. For plasmolysis: "The vacuole shrinks and the protoplasm peels away from the cell wall because the cell loses water by osmosis." Examiners mark cause and effect separately; say both.

4.2 Trapdoor Mistakes

4.3 Score-Boosting Comparisons

5. Now Try It

Your 20-minute task:

You are given a plant cell in a beaker of pure water and a plant cell in a beaker of 10% salt solution. For each:

1. Predict whether the cell will become turgid, remain turgid, or become flaccid.
2. Draw a simple diagram showing the cell membrane and vacuole in each scenario.
3. Explain your prediction using the term "osmosis" and describing the direction water molecules move and why.
4. State whether energy is used in this process and justify your answer.

A top-band answer must include:
- Correct prediction for both cells (pure water → turgid; salt solution → flaccid).
- Diagrams showing vacuole size (large in pure water, shrunken in salt water).
- Explicit statement: "Water molecules move from where water concentration is higher to where it is lower" (or "from lower to higher solute concentration").
- Explicit statement: "No ATP is used because osmosis is a passive process."

Success looks like: You've written a full explanation for both cells, named osmosis correctly, described the direction of water movement, and explained why no energy is involved—all in under 200 words.

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