Cell structure and organisation: the ultrastructure and functions of eukaryotic organelles; the differences between prokaryotic and eukaryotic cells; microscopy and magnification; and the organisation of cells into tissues, organs and systems.

What this dot point is asking

Eduqas wants you to identify eukaryotic organelles and state their functions, contrast prokaryotic and eukaryotic cells, use microscopy and the magnification equation, and explain how cells are organised into tissues, organs and systems. This is a Core Concepts statement, so it is sampled in all three papers.

Eukaryotic organelles

A eukaryotic cell is defined by a true membrane-bound nucleus and membrane-bound organelles. You must identify each from an electron micrograph and give its function:

• Nucleus: holds the DNA (as chromatin); the envelope has pores for mRNA export; the nucleolus makes ribosomes.
• Mitochondria: the site of aerobic respiration (the Krebs cycle in the matrix, oxidative phosphorylation on the inner membrane folded into cristae).
• Chloroplasts (plants): the site of photosynthesis (light-dependent reactions on the thylakoids, the Calvin cycle in the stroma).
• Ribosomes (80S): the site of translation; found free or on the rough endoplasmic reticulum.
• Rough endoplasmic reticulum: studded with ribosomes, processes and folds proteins.
• Smooth endoplasmic reticulum: makes lipids and steroids.
• Golgi body: modifies, sorts and packages proteins into vesicles.
• Lysosomes: contain hydrolytic enzymes for breaking down material.
• Cell wall (plants, made of cellulose) and permanent vacuole maintain shape and turgor.

Prokaryotic cells

Prokaryotic cells (bacteria) are the contrast: typically $1$ to $5$ micrometres across, with no nucleus and no membrane-bound organelles. Their DNA is a single circular molecule free in the cytoplasm, often with smaller plasmids; their ribosomes are smaller (70S); and their wall is made of peptidoglycan (murein), not cellulose. Some have a capsule, flagella or pili.

Microscopy and magnification

Distinguish magnification (how many times larger the image is) from resolution (the smallest distance at which two points can still be told apart). Resolution is limited by the wavelength of the radiation used, which is why a light microscope resolves only to about $200$ nanometres while an electron microscope, using electrons of much shorter wavelength, resolves ultrastructure. The transmission electron microscope (TEM) shows internal detail; the scanning electron microscope (SEM) shows 3D surfaces.

The magnification equation is:

Always convert to the same units first ($1 \text{ mm} = 1000$ micrometres; $1$ micrometre $= 1000$ nanometres). To measure real sizes under a light microscope, calibrate an eyepiece graticule against a stage micrometer of known scale.

Cell organisation

Differentiated cells express only part of their genome, giving them specialised structures. They are organised into tissues (groups of similar cells, for example epithelium), organs (several tissues working together, for example the stomach) and organ systems (several organs, for example the digestive system).

Examples in context

Example 1. Why secretory cells are packed with rough endoplasmic reticulum. Cells that export a lot of protein, such as the pancreatic cells that make digestive enzymes, have abundant rough endoplasmic reticulum, Golgi bodies and mitochondria, a direct structure-to-function link examiners reward.

Example 2. Resolution sets the limit, not magnification. Early microscopists could magnify endlessly but saw nothing new because the light microscope's resolution is capped by the wavelength of light. The electron microscope's far shorter wavelength is what revealed organelle ultrastructure.

Try this

Q1. State two structural differences between a prokaryotic and a eukaryotic cell. [2 marks]

• Cue. Any two: prokaryotes have no nucleus (circular DNA free in the cytoplasm) versus a membrane-bound nucleus; no membrane-bound organelles; 70S versus 80S ribosomes; a peptidoglycan versus cellulose wall.

Q2. Explain the difference between magnification and resolution. [2 marks]

• Cue. Magnification is how many times larger the image is; resolution is the smallest distance at which two points can be distinguished, limited by wavelength.

Q3. An image of a cell is 60 mm long at a magnification of times 1500. Calculate the actual length in micrometres. [2 marks]

• Cue. $60 \text{ mm} = 60\,000$ micrometres; $\dfrac{60\,000}{1500} = 40$ micrometres.