A2.2 Cell structure

A cell is the smallest membrane-bounded unit that can carry out the processes of life. A tissue is different: it is a group of cells cooperating in a shared function.

Cell theory states that:

• living organisms consist of one or more cells
• cells are the basic units of structure and function
• new cells arise from pre-existing cells

The theory is a powerful biological model, not a claim that every cell has the same appearance. Later examples such as multinucleate fibres and mature cells without nuclei test its boundaries without removing the central idea that living organization is cellular.

At one objective, 10 graticule divisions correspond to 20 µm, so each division represents 2 µm. A cell spanning 18 divisions is therefore 36 µm long. If its printed image is 54 mm long, convert 54 mm to 54,000 µm before calculating: $M = 54{,}000/36 = 1{,}500\times$.

Calibration belongs to a particular objective lens. Changing the objective changes the real distance represented by one graticule division, so the calibration must be repeated.

Magnification makes an image larger; resolution allows two close points to be seen as separate. Enlarging beyond the resolution limit only produces a larger blur.

Preparation can introduce artefacts. Confidence increases when different preparation methods reveal a consistent structure, rather than when one impressive image is accepted alone.

None of these components alone is a cell. Together they create a bounded chemical system that can maintain itself, use information and make the proteins needed for its activities.

Plasmids, capsules, pili, flagella and specialized membrane regions occur in some bacteria, not all. E. coli, Bacillus and Staphylococcus also differ in shape and wall organization, so no optional feature should be treated as the definition of a prokaryote.

For a secreted protein, information and material move through a coordinated route: nuclear gene → transcription → rough-ER ribosome → ER lumen → transport vesicle → Golgi modification and sorting → secretory vesicle → plasma membrane.

The cytoskeleton gives internal organization and shape. Microtubules guide vesicles and move chromosomes; actin microfilaments resist tension and contribute to cell movement and shape change.

A unicellular organism cannot delegate essential work to tissues. Nutrition, metabolism, homeostasis, excretion, response, movement, growth and reproduction must all be achieved within one cell.

The structures differ, but the same functional problem remains: matter, information and energy must be coordinated inside one bounded cell.

Classification should use a combination of structures. A wall alone does not prove that a cell is plant, and the absence of a chloroplast does not exclude plant tissue because many plant cells are non-photosynthetic.

These cases do not show that cell theory is useless. They show that specialization can redistribute cellular functions across shared cytoplasm or cooperating cells, so “one small compartment with one nucleus” is not the theory itself.

Absence is weaker than presence: a thin section may miss an organelle that exists elsewhere in the cell. Identification should therefore rest on visible positive evidence whenever possible.

Before adding an annotation, ask whether the feature can be pointed to in the drawing and whether the claimed function follows from that observed structure. Biological knowledge may guide interpretation, but it must not fabricate visual evidence.

The engulfing host was not already a modern eukaryotic cell. Endosymbiosis is part of the explanation for how eukaryotic complexity arose.

No single observation proves the full history. The theory is strong because structural, biochemical and sequence evidence converge on the same ancestry and make successful predictions about organelle genomes.

The evidence strongly supports bacterial origins of mitochondria and chloroplasts; it does not imply that every eukaryotic organelle arose by endosymbiosis.

Most cells in a multicellular organism descend from one original cell and therefore contain essentially the same genome. They differ because developmental signals and position cause different genes to be expressed or suppressed.

different gene expression → different RNAs and proteins → different proteomes → different structures and reactions → specialized cell functions

A muscle fibre produces abundant contractile proteins and organizes them into myofibrils. A protein-secreting cell develops extensive rough ER and Golgi apparatus. Their specialization reflects different use of shared genetic information, not a different complete genome.

Division of labour increases efficiency but also dependence: a highly specialized cell often cannot perform every process needed for independent life.

Multicellularity evolved independently in animals, plants, fungi and several algal lineages. Its repeated origin shows that there is no single compulsory route, but stable multicellular organisms must solve a common set of coordination problems.

Larger size can reduce predation and broaden resource use; specialization can make tissues more efficient. These gains are selected only when they outweigh the energy cost of coordination and the loss of cellular independence.