Prokaryotic and eukaryotic cellsEdexcel A-Level Biology B: Revision notes

Cell theory states that cells are the fundamental unit of structure, function and organisation in all living organisms, and that new cells arise from existing cells. In complex organisms, cells are organised: similar specialised cells form a tissue, tissues combine to form an organ, and organs working together form an organ system. For example, in the digestive system the stomach is an organ made of epithelial, muscle and glandular tissue.

There are two types of cell. Prokaryotic cells (bacteria) are small and have no nucleus or membrane-bound organelles. Eukaryotic cells (animals, plants, fungi, protoctists) have a nucleus and membrane-bound organelles.

A bacterial cell, about 1 to 5 µm long, has:

• A cell wall of peptidoglycan, which supports the cell and prevents bursting
• A cell surface membrane inside the wall
• The nucleoid, a region of the cytoplasm containing a single circular DNA molecule, not enclosed by an envelope
• Plasmids, small circular loops of DNA carrying extra genes, such as antibiotic resistance
• 70S ribosomes, smaller than eukaryotic ribosomes, which make proteins
• No mitochondria, endoplasmic reticulum or other membrane-bound organelles

The Gram stain separates bacteria by wall structure. Cells are stained with crystal violet, treated with iodine and washed with alcohol, then counterstained with safranin.

• Gram-positive: a thick peptidoglycan wall and no outer membrane. It retains the crystal violet and appears purple.
• Gram-negative: a thin peptidoglycan layer with an outer membrane outside it. It loses the crystal violet and takes up the counterstain, so it appears pink/red.

The difference affects antibiotics. Penicillin stops peptidoglycan cross-links forming as walls are built, so cells burst by osmosis. It reaches the peptidoglycan easily in Gram-positive bacteria. In Gram-negative bacteria the outer membrane reduces the entry of many antibiotics, so they are often less susceptible. Human cells have no peptidoglycan wall, so these antibiotics have little effect on them.

• Nucleus: contains DNA, surrounded by a nuclear envelope with pores; controls the cell. The nucleolus makes ribosomal RNA and assembles ribosomes.
• 80S ribosomes: make protein, free or on the RER.
• Rough endoplasmic reticulum (RER): ribosome-covered membranes; folds and transports proteins.
• Smooth ER (SER): no ribosomes; makes lipids and steroids.
• Golgi apparatus: modifies and packages proteins into vesicles; makes lysosomes.
• Lysosomes: vesicles of digestive enzymes that break down worn-out organelles and engulfed material.
• Mitochondria: site of aerobic respiration, making ATP.
• Centrioles: form spindle fibres during cell division (animal cells).
• Cell wall: cellulose in plants; supports and protects.
• Chloroplasts: site of photosynthesis in plants.
• Vacuole and tonoplast: the vacuole stores sap and keeps cells turgid; the tonoplast is its membrane.

Magnification is how many times larger the image is than the object: magnification = image size ÷ actual size. Resolution is the ability to distinguish two close points as separate. Higher magnification without higher resolution only gives a bigger, blurry image.

• Light microscope: magnification up to about ×1500, resolution about 200 nm. Specimens can be living, and colours can be seen.
• Transmission electron microscope (TEM): electrons pass through a very thin section; resolution about 0.1 nm to 1 nm because electrons have a much shorter wavelength than light. Specimens are dead and in a vacuum; the image is 2D.
• Scanning electron microscope (SEM): electrons bounce off the surface, giving a 3D image of the surface, with a lower resolution than the TEM.

Staining increases contrast because most cell structures are transparent. Methylene blue or iodine are used for light microscopy and heavy metal salts for electron microscopy.

Worked example: a TEM image of a mitochondrion is 45 mm long at ×15 000, so actual length = 45 ÷ 15 000 = 0.003 mm = 3 µm (1 mm = 1000 µm).

An eyepiece graticule is a scale in the eyepiece with arbitrary divisions. It is calibrated with a stage micrometer, a slide with a scale of known length, usually 1 mm divided into 100 divisions of 10 µm. Line the two scales up and compare them.

Worked example: if 5 stage divisions (50 µm) equal 20 eyepiece divisions, one eyepiece division = 50 ÷ 20 = 2.5 µm. A cell that spans 12 divisions is 12 × 2.5 = 30 µm long.

The graticule must be recalibrated for each objective lens. To draw a specialised tissue, use a sharp pencil, clear continuous lines and no shading, draw a few cells in proportion, label and give the magnification or a scale bar.