The ultrastructure of eukaryotic and prokaryotic cells, the functions of the main organelles, and the events of the cell cycle and mitosis.

What this dot point is asking

Edexcel wants you to describe the ultrastructure of eukaryotic and prokaryotic cells, state the function of the main organelles, and describe the events of the cell cycle and mitosis. In Biology B this links forward to protein synthesis, secretion and the control of cell division (cancer), so examiners often ask you to connect structure to function rather than just list parts.

Eukaryotic cell ultrastructure

Ultrastructure means the detail of cell structure visible only with the electron microscope, which has a much higher resolution than the light microscope because the wavelength of electrons is far shorter than that of light.

Cells are specialised by adjusting the proportions of organelles. A pancreatic acinar cell that secretes digestive enzymes is packed with RER, Golgi and mitochondria; a muscle cell has abundant mitochondria for contraction. Plant cells additionally have a cellulose cell wall, chloroplasts for photosynthesis and a large permanent vacuole surrounded by the tonoplast.

Organelle cooperation: the secretory pathway

Many exam questions test the route a secreted protein takes. The DNA in the nucleus codes for the protein; the mRNA leaves through a nuclear pore; ribosomes on the RER assemble the polypeptide; vesicles carry it to the Golgi for modification; secretory vesicles then carry the finished protein to the cell surface membrane for release by exocytosis. Mitochondria supply the ATP for each step.

Prokaryotic cells

Prokaryotes are typically $1$ to $5$ micrometres across, around ten times smaller in diameter than a typical eukaryotic cell ($10$ to $100$ micrometres). The 70S ribosome difference matters medically because some antibiotics target the bacterial 70S ribosome without harming human 80S ribosomes, giving selective toxicity.

The cell cycle and mitosis

The cell cycle is the sequence a cell goes through from one division to the next.

• Interphase: the longest phase, subdivided into G1 (growth), S (DNA is replicated so each chromosome becomes two identical sister chromatids joined at a centromere) and G2 (further growth and organelle synthesis).
• Mitosis (nuclear division) in four stages:
  • Prophase: chromosomes condense and become visible; the nuclear envelope breaks down; spindle fibres form from the centrioles.
  • Metaphase: chromosomes line up on the equator (metaphase plate), each attached to spindle fibres at its centromere.
  • Anaphase: centromeres divide and sister chromatids are pulled to opposite poles by shortening spindle fibres.
  • Telophase: chromosomes reach the poles, decondense and two new nuclear envelopes form.
• Cytokinesis: the cytoplasm divides, giving two cells.

Mitosis produces two genetically identical diploid daughter cells, used for growth, repair and asexual reproduction. The cell cycle is tightly controlled at checkpoints; loss of control leads to uncontrolled division and tumour formation.

Examples in context

Example 1. Antibiotics and ribosome differences. Tetracycline binds the 30S subunit of the bacterial 70S ribosome and blocks tRNA binding, stopping bacterial protein synthesis. Because human cells use 80S ribosomes with a different structure, the drug does not block human protein synthesis at therapeutic doses. This is a direct exam application of the prokaryote and eukaryote ribosome contrast.

Example 2. Mitotic index in cancer diagnosis. Pathologists count the proportion of cells in mitosis in a tumour biopsy (the mitotic index) to grade how aggressively it is growing. A breast tumour with a mitotic index of $20\%$ is dividing far faster than normal tissue (typically a few percent), which informs treatment urgency. This connects the cell cycle directly to clinical decision-making.

Try this

Q1. State two structures found in a prokaryotic cell but not in a eukaryotic cell. [2 marks]

• Cue. Circular DNA (and/or plasmids), a peptidoglycan cell wall and 70S ribosomes are acceptable.

Q2. Describe what happens to the chromosomes during anaphase. [2 marks]

• Cue. Centromeres divide; sister chromatids are pulled to opposite poles by shortening spindle fibres.

Q3. Explain why a cell that secretes large amounts of enzyme would contain many mitochondria, ribosomes and Golgi bodies. [3 marks]

• Cue. Ribosomes and RER synthesise the protein; Golgi modifies and packages it; mitochondria supply ATP for synthesis and exocytosis.