Notes on Cells. Ib Topic 2

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

Cells are the smallest unit of life
The cell is the smallest unit of organisation that can show all the characteristics of living processes
Organelles often require the cooperation of other organelles for their successful function
Endosymbiont theory

Cells come from only from other cells

Where do cells come from?

Cells carry out a form of cell division to form new cells. This process of cell replication in eukaryotes is called mitosis and in prokaryotes is called binary fission The parental cell divides to produce identical daughter cells.

This aspect of cell theory suggests that all cells therefore have a common ancestor, the original ancestral cell form which all other cells have arisen by descent. (origin of cellular life).

This relationship of common ancestor suggest thereof re that all organisms are related.

2.1.3 State that unicellular organisms carry out all the functions of life
State:means to give a specific name, value or other brief answer without explanation or calculation.

1. metabolism which includes respiration the synthesis of ATP.
2. response to a change in the environment
3. homeostasis the maintenance and regulation of internal cell conditions. growth which for a unicellular organism means an increase in cell size and volume. reproduction which for the unicellular organism is largely asexual through cell division to form a clone. nutrition which means either the synthesis of organic molecules or the absorption of organic matter.

2.1.4 Compare the relative sizes of molecules, cell membrane thickness, viruses bacteria, organelles and cells, using the appropriate SI unit
Compare: Give an account of similarities and differences between two or more items, referring to both all of them account.

We depend on the microscope for our observation of cellular structures. Observations of this type are for the most part dependable but we must consider the introduction of 'artifacts' by those processes that prepare the material for microscopy. These artifacts are a consequence of specimen dehydration, contrast enhancement (staining), radiation and microscope function. These artifacts can lead to image or data distortions and misinterpretation.

Relative sizes
1. molecules (1nm).
2. cell membrane thickness (10nm).
3. virus (100nm).
4. bacteria (1um).
5. organelles (less 10um).
6. cells (