Eukaryotic and prokaryotic cells

All living things are made of cells — the cell is the basic unit of life. Eduqas splits cells into two great groups:

• Eukaryotic cells have a nucleus that holds the DNA, plus membrane-bound organelles. Animal and plant cells are eukaryotic.
• Prokaryotic cells (e.g. bacteria) are much smaller and have no nucleus — their DNA is a single loop free in the cytoplasm, often with extra rings called plasmids.

Development of the microscope: the light microscope, then the electron microscope, and modern laser imaging each let us see cells in far greater detail — this steadily increased our understanding of the cell as the basic unit of life.

Animal & plant cell organelles

Every animal cell has these shared sub-cellular structures. A plant cell has them all plus three extras:

• Nucleus — controls the cell; contains DNA (the genetic material).
• Cytoplasm — jelly where most chemical reactions happen.
• Cell membrane — controls what enters and leaves the cell.
• Mitochondria — site of aerobic respiration (release energy).
• Cell wall (plant) — made of cellulose; strengthens and supports the cell.
• Chloroplasts (plant) — contain chlorophyll for photosynthesis.
• Permanent vacuole (plant) — filled with cell sap; keeps the cell firm (turgid).
• Plasmids (bacteria) — small extra rings of DNA in prokaryotic cells.

Practical SP1.1: examine plant & animal cells with a light microscope and produce clear, labelled drawings.

Match the function

Bacterial (prokaryotic) cell

A bacterium has a cell wall, cell membrane, cytoplasm and ribosomes, but no nucleus. Its genetic material is a single DNA loop, and it may carry small extra rings of DNA called plasmids.

Watch out: bacteria do have a cell wall and ribosomes, but the wall is not cellulose and they have no mitochondria, chloroplasts or true nucleus.

Whose feature is it?

Tap a feature, then tap the cell type it belongs to. (Some are shared by all.)

Differentiation, specialisation & cancer

As an organism develops, cells become specialised to do a particular job, which makes the organism more efficient. The process of a cell gaining its specialised features is called differentiation.

Cancer: changes to cells can lead to uncontrolled growth and division, forming a tumour. This uncontrolled mitosis is what we call cancer.

Why a long tail?

Microscopy & magnification

Cells are too small to see, so we use microscopes. Electron microscopes and laser imaging reach a far higher magnification and resolution than the light microscope, so they reveal tiny sub-cellular structures.

Misconception: magnification is image ÷ real, not real ÷ image. Always convert both lengths to the same unit first (e.g. mm and mm, or µm and µm).

Your turn — magnification

Your turn — find the real size

Enzymes — lock & key

Enzymes are proteins made by living cells. They are built from a chain of amino acids folded into a specific shape, held by chemical bonds. This shape includes an active site that lets the enzyme work.

• Enzymes catalyse (speed up) reactions without being used up.
• Each has an optimum pH and optimum temperature where it works fastest.
• Boiling denatures most enzymes — heat changes the shape of the active site, so the substrate no longer fits.

Practical SP1.3A: investigate factors (e.g. temperature or pH) affecting the rate of enzyme action.

Your turn — rate of reaction

The cell cycle & mitosis

The nucleus holds DNA as structures called chromosomes. Cells divide by mitosis for growth, repair and replacement. Each division follows the cell cycle:

Key: before a cell divides it must copy its DNA and grow more sub-cellular structures. Mitosis itself is when the chromosomes separate into two new nuclei.

What does mitosis make?

Meiosis vs mitosis & stem cells

Meiosis makes the gametes (sex cells). It halves the chromosome number, and each meiotic division produces four genetically different cells as genes separate and reshuffle.

A stem cell is an undifferentiated cell that can keep dividing and differentiate into many cell types. Some cells never lose the ability to differentiate.

• Embryonic stem cells — from early embryos; can become almost any cell type.
• Adult stem cells — e.g. in bone marrow; form a limited range of cells.
• Plant meristems — in root and shoot tips; can make any plant cell throughout life.

Issues to evaluate: stem cell technology (e.g. embryonic stem cells) has clear benefits but also risks and ethical objections — Eduqas wants you to discuss these, not just list them.

What does meiosis make?

Respiration & digestion

Cellular respiration is exothermic and happens continuously in all living cells, releasing energy as ATP.

• Anaerobic in humans: glucose → lactic acid (+ energy). Less efficient (less ATP), and causes an oxygen debt.
• Anaerobic in yeast (fermentation): glucose → ethanol + carbon dioxide.
• Digestion: fats → fatty acids + glycerol; proteins → amino acids; starch → glucose. These soluble products are absorbed; glucose & fatty acids/glycerol are used in respiration, amino acids build body proteins.

Practical SP1.3B — food tests: starch → iodine goes blue-black; reducing sugar/glucose → Benedict's goes brick-red on heating; protein → biuret goes purple/lilac.

Match each item to its result

Tap a statement on the left, then its matching result on the right.

Measuring respiration rate

The rate of respiration can be found from how much oxygen is used up (or carbon dioxide produced) in a given time. As with any rate:

Respiration is continuous and exothermic, so germinating seeds or small organisms warm up their surroundings slightly and steadily use oxygen — a respirometer measures this.

Your turn — respiration rate

Surface area : volume ratio

The rate at which a cell can exchange substances depends partly on its surface area to volume (SA:V) ratio. As something gets bigger, its volume grows faster than its surface area, so the SA:V ratio falls.

Why it matters: single-celled organisms have a large SA:V and can rely on diffusion. Big animals can't, so they evolve specialised exchange surfaces (lungs, gills, gut villi) with huge surface areas.

Your turn — SA:V ratio

The big ideas to know

You've covered all three parts of Eduqas Topic 1 — prokaryotic & eukaryotic cells, growth & development, and cell metabolism. Press Finish to see your score.