Eukaryotic and prokaryotic cells, the sub-cellular structures of animal, plant and bacterial cells, specialised cells, and using a light microscope with the magnification equation.

What this dot point is asking

Edexcel wants you to describe the differences between eukaryotic and prokaryotic cells, identify the sub-cellular structures of animal, plant and bacterial cells and state the function of each, recognise specialised cells, and use a light microscope including the magnification equation with correct units.

Eukaryotic and prokaryotic cells

Prokaryotic cells are typically about $1$ to $5$ micrometres ($\mu m$) across, while eukaryotic cells are usually $10$ to $100\,\mu m$, so they are around ten to one hundred times larger by length. This size difference matters because larger cells need internal organelles to organise their reactions, while small prokaryotes can rely on diffusion across their whole surface.

Sub-cellular structures and their functions

Animal cells contain:

• Nucleus: controls the cell and contains the genetic material (DNA) that codes for proteins. Found in eukaryotic cells only.
• Cytoplasm: the jelly-like substance where most of the cell's chemical reactions happen, controlled by enzymes.
• Cell membrane: controls what substances enter and leave the cell (it is partially permeable).
• Mitochondria: the site of aerobic respiration, which releases the energy the cell needs. Active cells such as muscle and liver cells have many mitochondria.
• Ribosomes: the site of protein synthesis, where amino acids are joined to make proteins.

Plant cells contain all of the above, plus:

• Cell wall: made of cellulose; it strengthens the cell and gives it a fixed shape.
• Chloroplasts: contain the green pigment chlorophyll and are the site of photosynthesis. They are absent from cells that do not photosynthesise, such as root cells.
• Permanent vacuole: a large central space filled with cell sap; it helps keep the cell firm (turgid), which supports the plant.

Specialised cells

A central idea is that a cell's structure suits its job. A sperm cell has many mitochondria for the energy to swim and a tail (flagellum); an egg cell has nutrients in its cytoplasm and a haploid nucleus; a ciliated epithelial cell has hair-like cilia to sweep mucus; a root hair cell has a large surface area for absorbing water and ions. Edexcel often asks you to link a structure you can see to the function it carries out.

Microscopy and magnification

A light microscope magnifies up to about $\times 1500$ to $\times 2000$ with limited resolution; an electron microscope uses electrons for much higher magnification and resolution, revealing fine detail of organelles.

Try this

Q1. Name two structures found in a plant cell but not in an animal cell. [2 marks]

• Cue. Chloroplasts and a cellulose cell wall (a permanent vacuole is also acceptable).

Q2. State one difference between the genetic material of a prokaryotic and a eukaryotic cell. [1 mark]

• Cue. In a prokaryote the DNA is a single loop free in the cytoplasm; in a eukaryote it is enclosed in a nucleus.