A2.2 Cell structure

Cell theory states that living organisms are made of cells, the cell is the basic unit of life, and new cells arise from existing cells.

The theory links observations at different scales: tissues are organized from cells, and cell processes explain organismal functions. Modern evidence adds that cells share chemical continuity and pass genetic information during division.

Use the three core claims:

• all organisms contain one or more cells
• cells are the smallest functional units
• cells come from pre-existing cells

A multicellular muscle works because each cell maintains membranes and ATP production; the tissue is not a separate unit replacing the cells.

Cell theory does not say cells are identical or that non-living particles are cells.

Microscopy skill combines specimen preparation, controlled focusing, calibrated measurement and a calculation that reports actual size.

Prepare a thin temporary mount, add a suitable stain if contrast is needed, place a coverslip, begin with low power and coarse focus, then use fine focus at higher power. Calibrate an eyepiece graticule against a stage micrometer at the selected objective.

If a cell image is 40 mm long at 400x magnification, actual size = 40 mm / 400 = 0.10 mm = 100 micrometres. A scale bar must use the same calibrated relationship.

Convert image and actual size to the same units before calculating. Changing objective magnification requires recalibrating the eyepiece graticule.

Microscopy advances improve resolution, preserve different structures or attach molecular identity; the best method depends on the evidence required.

A fluorescent antibody can locate one membrane protein, whereas freeze fracture can reveal particles embedded in the membrane face without identifying their molecular name.

Electron micrographs and fluorescence images may use false colour. Colour is not automatically part of the specimen.

All cells have a plasma membrane, cytoplasm, ribosomes and genetic material, even though their shapes and internal compartments differ.

The membrane separates the internal reaction space, cytoplasm contains soluble chemistry, ribosomes make polypeptides, and DNA or equivalent genetic material stores instructions. These shared features support the cell-theory definition of a cell.

When identifying a cell, look for:

• boundary and internal fluid
• genetic material
• ribosomes or ribosome-rich regions
• a method of protein synthesis

A prokaryotic cell lacks a nucleus but still has DNA, ribosomes, cytoplasm and a plasma membrane.

A nucleus is not a universal cell feature; it is a eukaryotic compartment.

A typical Gram-positive eubacterium such as Bacillus or Staphylococcus has a cell wall outside a plasma membrane, cytoplasm with 70S ribosomes, and naked circular DNA in a nucleoid region.

A micrograph showing a small walled cell with dispersed ribosomes and no nucleus is consistent with a prokaryote; its DNA occupies a nucleoid rather than a membrane-bound nucleus.

Prokaryotic structure varies, but detailed exceptions such as wall-less bacteria are outside this objective. No nucleus does not mean no DNA.

A eukaryotic cell has a plasma membrane enclosing compartmentalized cytoplasm with 80S ribosomes, a nucleus and membrane-bound organelles.

A secreted protein is synthesized on rough-ER ribosomes, processed through ER and Golgi, then carried in a vesicle to the plasma membrane.

Not every eukaryotic cell displays every organelle equally clearly; identify the cell using a combination of visible structures.

A unicellular organism must perform all essential life processes within one cell: homeostasis, metabolism, nutrition, movement, excretion, growth, response to stimuli and reproduction.

Paramecium uses cilia for movement and feeding, food vacuoles for digestion, and contractile vacuoles for water balance and excretion.

A unicellular organism is a complete living organism, not one specialized cell waiting for a tissue to perform the remaining life processes.

Animal, fungal and plant cells share the eukaryotic core but differ in cell walls, vacuoles, plastids and motile structures.

A cellulose wall, chloroplasts and a large sap vacuole together support a plant-cell identification more strongly than any single feature.

Specialized cells may lack a typical feature; for example, a plant root cell normally lacks chloroplasts. Use combinations and context.

Atypical eukaryotic cells show that one cell does not always contain exactly one nucleus.

A skeletal muscle fibre is one long multinucleate cell, whereas an aseptate fungal hypha contains many nuclei in a cytoplasm not divided by complete cross-walls.

Atypical does not mean non-cellular. Identify the continuous membrane boundary and explain the altered nuclear arrangement.

Identify a cell or organelle in a micrograph by combining visible structures, scale and context; never rely on outline alone.

First read the scale bar, then identify boundaries, then require at least two compatible features. Section angle can hide organelles, so state uncertainty when evidence is incomplete.

A double-membrane organelle with internal cristae is a mitochondrion; a thick wall plus chloroplasts and a sap vacuole supports a plant cell.

Absence from one section is not proof that the whole cell lacks a structure. Use only what the micrograph and scale can support.

A drawing from an electron micrograph records observed structure with clear lines, while an annotation adds a supported function to a labelled feature.

Use a sharp pencil or digital single lines, draw large proportional outlines without shading, include only visible structures, add a title and scale or magnification, and keep label lines ruled and non-crossing.

Annotations must pair structure with function, for example: cristae - membrane surface for aerobic ATP production; rough ER - ribosome-bearing membrane involved in protein synthesis; microvilli - increase exchange surface area.

If an electron micrograph shows a mitochondrion, draw its visible membranes and cristae, label them, and annotate the cristae with their respiratory function.

Do not add textbook structures that are not visible, and do not call a name-only label an annotation when a function is required.