What does it mean to be a cell?. The answer to this question started to unfold more than 300 years ago. The newly invented microscope was used to examine.

What does it mean to be a cell?

What does it mean to be a cell?

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.

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.

Hooke’s Microscope

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

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

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.

How big are cells?

• The units of measurement that we use when talking about the cell are:– the micrometer: m – 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!

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.

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.

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.

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.

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

Prokaryotes

• Prokaryotes are bacteria and cyanobacteria.

• Small (1 to 2m)• Lack membrane bound organelles.• Single circular chromosome.• Plasma membrane surrounded by

outer cell wall of protein and complex carbohydrate (murein)>

Eukaryotes

• Plants, animals and fungi are eukaryotes and are made up of cells 10 to 40m 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.

Prokaryotic vs Eukaryotic Cells

Prokaryotic vs Eukaryotic Cells

Property Prokaryotic Cells Eukaryotic Cells

Size Small (1 – 2m) Large (20 – 30m)

Plasma membrane Yes Yes

Membrane bound nucleus

No Yes

Organelles No Yes

Cytoskeleton No Yes

Exocytosis and endocytosis

No Yes

Mode of cell division Cell fission Mitosis and meiosis

Genetic information Yes: DNA Yes: DNA

Processing of RNA Little Much

Ribosomes Yes: small Yes: large