1. What does it mean to be a cell?

2. The answer to this question started to unfold more than 300 years ago.
The newly invented microscope was used to examine a variety of living material from bark to sperm.

3. Robert Hooke (1665) built himself a crude microscope (x30 magnification) and examined slices of cork cut with a penknife.
He saw a network of tiny boxlike compartments that he called cellulae – meaning little rooms.
Of course Hooke did not observe cells, but rather the empty cell walls of dead plant tissue.

4. Hooke’s Microscope

5. Leeuwenhoek developed a better microscope (x 270 magnification) around the same time.
He was the first to observe living cells, specifically bacteria and yeast.
By the 1830’s improved lenses lead to higher magnifications and better resolution, allowing Schwann to postulate the Cell Theory in 1839

6. Cell Theory Cell Theory has three basic tenents:
All organisms consist of one or more cells
The cell is the basic structural unit of all organisms
All cells arise from pre-existing cells

7. How else do we study cells?
Our present day knowledge of the cell has resulted from three intertwined scientific disciplines:
Cytology
Biochemisty
Genetics
Modern biology is very much focused on the dynamic and molecular nature of the cell.

8. How big are cells?
The units of measurement that we use when talking about the cell are:
the micrometer: mm – the size of cells and larger organelles
The nanometer: nm – smaller organelles and macromolecules
The angstrom: Å – small molecules and atoms
Cells come in many shapes and sizes!

9. What is a cell?
A cell is no more or less than a chemical production house.
It must produce energy and a variety of complex substances so that it can continue to function, grow, reproduce and respond to stimuli in its environment.
It is the requirements and nature of the chemical reactions within the cell that governs cell characteristics such as cell size and complexity.

10. Factors influencing cell size
The main constraint on cell size is the need to maintain an adequate surface area/volume ratio.
Problem: as a cell increases in size its surface area does not keep pace with its volume, making passage of materials in and out of the cell more difficult.

11. Factors influencing cell size
Cell size is limited by the rates at which molecules can move around the cell.
In general, molecules move by diffusion from areas of high concentration to areas of low concentration.
The rate of diffusion varies inversely with the size of the molecule – bigger molecules diffuse more slowly.
The cell is limited by the diffusion rates of the molecules it contains.

12. Factors influencing cell size
Cell size is limited by the need to maintain adequate concentrations of essential molecules for the processes the cell must carry out.
For a chemical reaction to occur reactants must collide with or find to other molecules.
The frequency of such random collisions will be greatly increased by having larger concentrations of the reactants.
Every time the 3 dimensions of a cell doubles a particular molecule would need to increase eight-fold to maintain the same concentration.

13. Cell design
There are two fundamentally different plans of cellular organisation or design.
It is a size thing!!!
Based upon their cellular organisation, all organisms can be divided into two broad groups:
prokaryotes
eukaryotes

14. Prokaryotes Prokaryotes are bacteria and cyanobacteria.
Small (1 to 2mm)
Lack membrane bound organelles.
Single circular chromosome.
Plasma membrane surrounded by outer cell wall of protein and complex carbohydrate (murein)>

15. Eukaryotes
Plants, animals and fungi are eukaryotes and are made up of cells 10 to 40mm in size.
Have an internal membrane system which forms the nuclear membrane and many other organelles.
Organelles are subcellular structures involved in specific functions of the cell.
It is easier for the cell to maintain appropriately high concentrations of particular reagents within the volume of a particular organelle, rather than the volume of the whole cell.

16. Prokaryotic vs Eukaryotic Cells

17. Prokaryotic vs Eukaryotic Cells
Property
Prokaryotic Cells
Eukaryotic Cells
Size
Small (1 – 2mm)
Large (20 – 30mm)
Plasma membrane
Yes
Membrane bound nucleus No
Organelles
Cytoskeleton
Exocytosis and endocytosis
Mode of cell division
Cell fission
Mitosis and meiosis
Genetic information
Yes: DNA
Processing of RNA
Little
Much
Ribosomes
Yes: small
Yes: large