Notes on Cells. Ib Topic 2

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Topic 2: Cells

2.1.1 Outline the cell theory
Outline: To give a brief account or summary

All living things are made of cells
Cells are the smallest unit of life
Cells come from pre-existing cells

2.1.2 Discuss the evidence for the cell theory
Discuss: Give an account including, where possible, a range of arguments for and against the relative importance of various factors, or comparisons of alternative hypotheses.

1: All living things are made of cells
When living things are observed under the microscope they consistently appear to be composed of cells. This is a argument of induction.

P: All living things seen so far are made of cells
C: Therefore, all living things are made of cells

However, there are some exceptions to the cell theory which test this rule of cell theory:

Muscle cells: challenges the idea that a cell has one nucleus.

Muscle cells have more than one nucleus per cell

Muscle Cells called fibres can be very long (300mm).

They are surrounded by a single plasma membrane but they are multi-nucleated.(many nuclei). This does not conform to the standard view of a small single nuclei within a cell

Fungal Cells: challenges the idea that a cell is a single unit.

Fungal Hyphae: again very large with many nuclei and a continuous cytoplasm

The tubular system of hyphae form dense networks called mycelium.

Like muscle cells they are multi-nucleated

They have cell walls composed of chitin

The cytoplasm is continuous along the hyphae with no end cell wall or membrane

Protoctista: Challenges the idea that a cell is specialised

A cell capable of all necessary functions Amoeba Single celled organisms have one region of cytoplasm surrounded by a cell membrane.

The protoctista cell is unusual in that it performs all functions. Such cells are usually much larger than other cells such that some biologist consider them 'acellular', that is, non-cellular. You can almost see amoeba visually...
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