Cell Structure and Organisation — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

Until microscopes became powerful enough to view individual cells, nobody knew what living organisms were made of.

All cells are surrounded by a cell surface membrane that controls the exchange of materials between the internal cell environment and the external environment. It is partially permeable: some substances can pass through while others cannot.

The nucleus is present in all eukaryotic cells and is relatively large.

Mitochondria (singular mitochondrion) are the site of aerobic respiration and are just visible with a light microscope. They have a double membrane, with the inner membrane folded into cristae to increase the surface area for the enzymes of respiration, and a fluid matrix.

Ribosomes may be free in the cytoplasm of all cells or part of the rough endoplasmic reticulum in eukaryotic cells. They are not surrounded by a membrane; they are formed in the nucleolus and are made of almost equal amounts of RNA and protein. They are the site of translation.

There are two types of endoplasmic reticulum (ER). Rough ER (RER) is formed from folds of membrane continuous with the nuclear envelope and is covered in ribosomes. Smooth ER (SER) has no ribosomes and is involved in lipid synthesis. Under the electron microscope the presence or absence of ribosomes distinguishes them.

The Golgi apparatus (also called the Golgi body) is a stack of flattened membrane sacs, similar in appearance to the smooth ER. The cis face lies near the RER and the trans face lies near the cell surface membrane. It modifies and packages proteins and lipids into vesicles.

Lysosomes are specialised vesicles containing hydrolytic enzymes. They break down waste materials such as worn-out organelles, and in white blood cells they digest pathogens that have been taken in by phagocytosis.

Centrioles are made of hollow fibres called microtubules, which are protein filaments that can move substances around inside a cell and support the shape of the cell from the inside. Two centrioles at right angles form a centrosome, which organises the spindle fibres during nuclear division in animal cells.

Many organelles work together to produce and secrete proteins.

The RER and Golgi are thus involved in producing, packaging and transporting proteins in a cell.

Animal and plant cells are eukaryotic; bacteria are prokaryotes. Prokaryotic cells are much smaller than eukaryotic cells and differ in several structural and physiological ways that affect their metabolism and reproduction.

Not all structures are present in all prokaryotes (for example, only some have a capsule or flagella). Remember that size is not a structural feature, so do not use size as a structural difference.

Cells can become specialised for specific functions. For example, epithelial cells in the small intestine are specialised to absorb food efficiently, having microvilli to give a large surface area. Specialised cells are grouped into tissues (similar cells working together), tissues into organs, and organs into organ systems.

An epithelial cell of the small intestine shows how structure suits function: microvilli give a large surface area for absorption, many mitochondria supply ATP for active transport, and tight junctions between neighbouring cells stop substances leaking between them. Similarly, a cell that secretes enzymes has extensive RER and Golgi. Use the pattern 'feature, effect, function' when linking structure to role.

Many biological structures are too small to be seen by the naked eye.

For a liquid specimen, add a few drops of the sample to the slide with a pipette.

Specimens are often stained because cytoplasm and other structures may be transparent or hard to distinguish. Most colours seen in light micrographs result from added stains. Examples include toluidine blue and phloroglucinol (used on a leaf section), iodine solution for starch, and methylene blue for nuclei.

Observations are recorded as labelled biological drawings: line drawings showing the features observed.

Magnification is how many times bigger the image is than the real specimen: magnification = image size / actual size.

Worked calibration

• the smallest stage micrometer divisions are 0.01 mm.
• If 40 graticule divisions line up with one micrometer division, then 40 graticule divisions = 0.01 mm = 10 µm, so one graticule division = 10 / 40 = 0.25 µm.
• A cell measuring 20 graticule divisions is 20 x 0.25 = 5 µm long.

Electron microscopes can produce highly detailed images of animal and plant cells because they have a higher resolution, or resolving power (the ability to distinguish two points as separate), than a light microscope.

Generally, if you can see internal structures the image was taken with a TEM, and if the image appears 3D, an SEM was used.

A stained TEM of the nucleus is 2D and high resolution, showing a double envelope with pores and dark chromatin.

Typical limits

• a light microscope resolves objects about 200 nm apart and magnifies up to about x1500;
  • a transmission electron microscope resolves about 0.2 nm in theory and magnifies to over x500 000.
• Because the nucleus (about 10 µm), mitochondria (about 1 to 2 µm) and ribosomes (about 25 nm) differ greatly in size, only the electron microscope can show ribosomes.
• Magnification calculations: actual size = image size divided by magnification, with all lengths converted to the same unit first.