Eukaryotic and prokaryotic cell structureEdexcel A-Level Biology A: Revision notes

All living organisms are made of cells, and all cells share some common features: a cell-surface membrane, cytoplasm, DNA as genetic material, and ribosomes for protein synthesis. Cells are either eukaryotic (with a nucleus and membrane-bound organelles) or prokaryotic (no nucleus and no membrane-bound organelles).

The ultrastructure of a cell is the detail seen only with an electron microscope.

The nucleus is surrounded by a double membrane, the nuclear envelope, with nuclear pores. It contains chromatin (DNA and protein). DNA codes for proteins, so the nucleus controls the cell's activities, and mRNA leaves through the pores.

The nucleolus, a dark-staining region inside the nucleus, makes ribosomal RNA (rRNA) and assembles ribosomes.

Ribosomes are the site of protein synthesis. They are made of rRNA and protein and are not membrane-bound. Eukaryotic ribosomes are 80S (larger); prokaryotic ribosomes are 70S (smaller).

Rough endoplasmic reticulum (rER) is a network of flattened membrane sacs covered in ribosomes. It makes and transports proteins. Smooth endoplasmic reticulum (sER) has no ribosomes and makes and stores lipids.

The Golgi apparatus is a stack of flattened sacs that modifies and packages proteins into vesicles.

Lysosomes are small, round, single-membrane vesicles containing hydrolytic enzymes. They digest worn-out organelles and engulfed bacteria. The membrane keeps the enzymes from damaging the rest of the cell.

Mitochondria have a double membrane. The inner membrane is folded into cristae, giving a large surface area for the electron transport chain and ATP synthase. The matrix contains enzymes for the Krebs cycle. Mitochondria make most of the cell's ATP by aerobic respiration, so cells with a high energy demand have many.

Centrioles are pairs of hollow cylinders of microtubules, found in animal cells. They organise the spindle during cell division.

Prokaryotic cells such as bacteria are much smaller than eukaryotic cells and have:

• a cell wall (made of peptidoglycan) that supports the cell
• a capsule that protects the cell, for example from drying out or phagocytosis
• circular DNA lying free in the cytoplasm (no nucleus)
• plasmids: small loops of extra DNA, often carrying genes such as antibiotic resistance
• 70S ribosomes
• a flagellum for movement and pili for attachment
• mesosomes: infoldings of the cell-surface membrane that were once thought to carry out respiration, now often considered artefacts of preparation.

Resolution is the ability to distinguish two points as separate. Electrons have a shorter wavelength than light, so an electron microscope has a much higher resolution (about 0.2 nm) than a light microscope (about 200 nm).

To recognise organelles: the nucleus is large and round with pores and a dark nucleolus; mitochondria are oval with a folded inner membrane; rER is parallel membranes studded with dots; the Golgi is a stack of curved sacs with vesicles; lysosomes are small, dark, round and single-membraned.

Magnification = image size ÷ actual size. Worked example: a 45 mm image at ×15 000 magnification is 45 ÷ 15 000 = 0.003 mm = 3 µm.