1.1 The molecules of life
Most of the dry mass of living things is made of four groups of organic molecules. Carbohydrates (glucose, starch, glycogen, cellulose) are a quick energy source and building material. Lipids (fats, phospholipids, steroids) store energy long-term and form the main material of membranes. Proteins are chains of 20 kinds of amino acids joined by peptide bonds, working as enzymes, carriers, hormones and structural proteins. Nucleic acids (DNA and RNA) store and pass on hereditary information.
Most of these molecules are built by joining small units (monomers) with the loss of water — dehydration synthesis; breaking them down by adding water is called hydrolysis. A protein's function depends on its three-dimensional shape, and its amino acid sequence (primary structure) decides how it folds. That is why changing a single amino acid can ruin a protein's function.
1.2 Enzymes: biological catalysts
Enzymes speed up reactions millions of times by lowering the activation energy needed to start them, and they are not used up. The substrate binds to the enzyme's active site, which changes shape slightly to fit it (the induced-fit model). This is why each enzyme is specific to a particular substrate.
Enzyme rate is affected by temperature, pH, substrate concentration and inhibitors. Most human enzymes work fastest around 37 °C; high temperatures break hydrogen bonds and permanently change the enzyme's shape (denaturation). The stomach enzyme pepsin works best around pH 2, while trypsin in the small intestine works best around pH 8. Rate rises with substrate concentration until every active site is busy and the rate reaches a maximum (saturation).
1.3 Crossing the membrane
The cell membrane is a fluid mosaic: a phospholipid bilayer with proteins embedded in it. Diffusion is the movement of substances from where they are more concentrated to where they are less concentrated, with no energy used. Oxygen and carbon dioxide cross the membrane directly; large molecules such as glucose and charged ions pass through channel or carrier proteins (facilitated diffusion). Osmosis is the diffusion of water across a partially permeable membrane, from the side with less solute to the side with more.
A cell placed in a hypertonic (more concentrated) solution loses water: a red blood cell shrinks, and in a plant cell the cytoplasm pulls away from the wall (plasmolysis). In a hypotonic solution it gains water: a red blood cell may burst, but a plant cell is protected by its wall and becomes firm (turgor). Active transport moves substances against a concentration gradient, using ATP. Each cycle of the sodium–potassium pump moves 3 Na⁺ ions out of the cell and 2 K⁺ ions in.
FormulaSurface-area-to-volume ratio = surface area ÷ volume · Magnification = image size ÷ actual size
Key terms
- Activation energy
- The minimum energy needed to start a reaction.
- Denaturation
- A protein losing its three-dimensional shape and its function.
- Osmosis
- Water moving across a partially permeable membrane towards the side with more solute.
- Active transport
- Moving substances against a concentration gradient using ATP.
End-of-unit problems
Solve each one yourself first, then open the solution.
Calculate the surface-area-to-volume ratios of three cube-shaped cell models with sides of 1 cm, 2 cm and 3 cm. In which is exchange by diffusion most efficient?
Show the solution
Surface = 6a², volume = a³, ratio = 6/a. 1 cm: 6/1 = 6; 2 cm: 24/8 = 3; 3 cm: 54/27 = 2. The 1 cm cube has the most surface per unit volume, so diffusion is most efficient there.
In a micrograph, a cell 50 µm across appears 5 mm wide. What is the magnification?
Show the solution
First make the units match: 5 mm = 5000 µm. Magnification = image ÷ actual = 5000 ÷ 50 = 100. The micrograph is magnified 100 times.
Red blood cells and cells from a plant leaf are both put in distilled water. Explain the difference you will see.
Show the solution
Distilled water is hypotonic to both cells, so water enters them by osmosis. The red blood cell has no wall, so it swells and bursts (haemolysis). The plant cell also takes in water, but its strong cellulose wall limits expansion; it doesn't burst but becomes turgid (turgor pressure).
In an enzyme experiment, the rate rises when the temperature goes from 20 °C to 37 °C; at 60 °C the reaction stops and doesn't restart when cooled back to 37 °C. Explain these results.
Show the solution
20→37 °C: molecules gain kinetic energy, enzyme–substrate collisions become more frequent and the rate rises. 60 °C: the hydrogen bonds holding the active site's shape break, the enzyme is denatured and the substrate can no longer bind. Denaturation is permanent, so cooling doesn't restore the function.
🦉 An easier explanation: Professor Owl's lesson: Cells: the building blocks of life