Explain how the sub-cellular structures of eukaryotic and prokaryotic cells are related to their functions, including the nucleus, cell membrane, cytoplasm, mitochondria, ribosomes, chloroplasts, cell wall, vacuole, plasmids and flagella.

What this dot point is asking

Edexcel statement 1.1 wants you to compare eukaryotic and prokaryotic cells, identify the sub-cellular structures of animal, plant and bacterial cells, and explain how each structure is related to its function. The command word is explain, so naming a part is not enough; you must say what it does.

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 roughly ten to one hundred times larger by length. Size matters: larger eukaryotic cells need internal organelles and a transport system, while small prokaryotes can rely on diffusion across their whole surface because their surface area to volume ratio is large.

Sub-cellular structures and their functions

Animal cells contain:

• Nucleus: controls the cell's activities and contains the genetic material (DNA), arranged as chromosomes, that codes for proteins. Found only in eukaryotic cells.
• Cytoplasm: the jelly-like fluid where most chemical reactions happen, each controlled by an enzyme.
• Cell membrane: a partially permeable barrier that controls what substances enter and leave the cell.
• Mitochondria: the site of aerobic respiration, releasing 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, stopping it bursting when full of water.
• 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 keeps the cell firm (turgid), which helps support the plant.

Structure related to function

The heart of statement 1.1 is the word "related". Edexcel wants you to connect a structure to the job it does. A muscle cell is packed with mitochondria for the energy to contract; a root hair cell has a large surface area to absorb water and mineral ions; a palisade mesophyll cell is full of chloroplasts to absorb light for photosynthesis. Whenever a question shows a structure, ask what it allows the cell to do.

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 circular loop free in the cytoplasm; in a eukaryote it is enclosed in a nucleus.