The ultrastructure of eukaryotic and prokaryotic cells, the functions of organelles, the differences between plant and animal cells, and the levels of organisation from cells to organisms.

What this dot point is asking

WJEC wants you to describe the ultrastructure of eukaryotic cells, state the function of each organelle, compare prokaryotic and eukaryotic cells, distinguish plant and animal cells, use magnification calculations, and understand the levels of organisation from cell to organism.

Eukaryotic ultrastructure

A protein destined for secretion shows how the organelles cooperate. It is made on ribosomes on the rough ER, transported through the ER, pinched off in a vesicle, modified and packaged by the Golgi, then carried in another vesicle to the cell-surface membrane for release by exocytosis. Cells that secrete a lot, such as pancreatic cells making digestive enzymes, have abundant rough ER, Golgi and mitochondria.

Plant versus animal cells

Plant cells have three structures animal cells lack: a cellulose cell wall (support and shape, freely permeable), chloroplasts (the site of photosynthesis, containing chlorophyll on stacked thylakoid membranes) and a permanent vacuole filled with cell sap (maintains turgor and support). Animal cells may have centrioles, involved in spindle formation during cell division, which most plant cells lack.

Prokaryotic cells

Microscopy and magnification

The light microscope magnifies up to about $1500$ times and resolves to about $200$ nm, limited by the wavelength of light. The electron microscope uses electrons (much shorter wavelength) and resolves to about $0.2$ nm, revealing ultrastructure such as cristae and ribosomes.

Levels of organisation

Specialised cells group into tissues (similar cells with a shared function, such as muscle), tissues into organs (such as the heart, which contains muscle, nervous and connective tissue), organs into organ systems (such as the circulatory system), and systems together form the organism. This hierarchy lets a large multicellular body divide labour efficiently.

Examples in context

Example 1. Pancreatic acinar cells. These cells secrete digestive enzymes such as amylase and trypsinogen. Under the electron microscope they are packed with rough endoplasmic reticulum, a large Golgi body and many mitochondria, a structure that directly reflects their function of synthesising, packaging and exporting large amounts of protein, an examiner-favourite link between ultrastructure and role.

Example 2. Antibiotics targeting the prokaryotic cell. Penicillin works because it disrupts the synthesis of peptidoglycan in bacterial cell walls, weakening the wall so the bacterium bursts. Human cells have no peptidoglycan wall, so the drug harms bacteria but not us, a clean illustration of why the structural differences between prokaryotic and eukaryotic cells matter in medicine.

Try this

Q1. State the function of the Golgi body. [1 mark]

• Cue. Modifies and packages proteins into vesicles for secretion.

Q2. A cell appears $50$ mm long under a magnification of $1000$ times. Calculate its actual length in micrometres. [2 marks]

• Cue. Actual $= 50 \text{ mm} \div 1000 = 0.05$ mm $= 50$ micrometres.

Q3. Explain why a cell that secretes large amounts of protein has many mitochondria. [2 marks]

• Cue. Mitochondria make ATP by aerobic respiration; protein synthesis, packaging and exocytosis all require ATP, so a high secretion rate needs a high ATP supply.