Cell Biology

Cell theory

All living organisms are made of cells. Cells can thus be considered “units of life”.

All cells consist of a cytoplasm enclosed in a plasma membrane but we can easily recognize prokaryotic cells from eukaryotic cells:

• Prokaroytic cells, Bacteria and Archaea, are not compartmentalized
• Eucaryotic cells contain separate compartments

In eukaryotic cells, some compartments are surrounded by two membranes (nucleus, mitochondria, chloroplasts) whereas others are delimited by one membrane (reticulum, Golgi apparatus, vesicles…).

Animal and plant cells, both eukaryotic, differ by the presence of chloroplasts, vacuoles and cell walls (only present in plant cells).

Unicellular and multicellular organisms

Unicellular organisms, like Paramecium, consist of only one cell, whereas multicellular organisms, like Humans, are usually composed of many cells.

In unicellular organisms, all functions are carried out by one single cell.

In multicellular organisms, cells can have distinct structures and functions within the organism. They’ve undergone differentiation. They still have the same genetic information as the rest of the cells in the organism but they only express a particular set of genes which gives them their specificities.
We can also identify stem cells, like early embryo cells, that still have the capacity of dividing and giving diverse cell lines.

Cell cycles

All cells undertake a precise cycle where they divide into two daughter cells. This cell cycle can be split in two: interphase and mitosis.

Interphase consists of G1, S and G2 phases. Cells that do not divide anymore are stopped during the G1 phase. In the S phase (synthesis), genetic information is duplicated and chromosomes are doubled (two identical chromatids).

Mitosis, or cell division, consists of prophase, metaphase, anaphase and telophase.

• Prophase: Chromosomes supercoil and the nucleus membrane disappears
• Metaphase: Chromosomes align on the metaphasic plate
• Anaphase: The chromatid of every chromosome is pulled towards one pole of the cell
• Telophase: Chromosomes uncoil and the cell divides to form two identical cells (they have the same genetic information)

This sequence of events is controlled by proteins which are expressed specifically at precise moments of the cycle, and enzymes which bind proteins. If this regulation fails, cell division can grow out of control and may lead to cancer.

The membrane structure

Different membrane structure models have preceded the current fluidic mosaic model.

Based on chemical analysis and before electron micrography, the Davson and Danielly model suggested a phospholipidic bilayer packed between two proteins layers. Electron micrographs, proteins analysis and cell fusion experiments leaded Singer and Nicolson to propose a new model, also known as the fluidic mosaic model, which is still used nowadays.

1. Phospholipids are amphipathic, meaning that a part of the molecule is hydrophilic (phosphate head) whereas the other is hydrophobic (hydrocarbon chains). This property orientates the molecules and gives to the plasma membrane its basic architecture.
2. Cholesterol, another lipid, is a key component of plasma membranes. It decreases both membrane fluidity and permeability.
3. Membrane proteins can be peripheral (attached on one side only) or integral (embedded in the phospholipid bilayer). Sugar units can bound some of those proteins. They’re called glycoproteins. Membrane proteins can have many important functions, such as docking, communication, nutrition, etc. 
   

Membrane permeability and movements

Diffusion is a passive movement of particles from a compartment where they’re highly concentrated towards a compartment where they’re less concentrated, i.e. following the concentration gradient.
Osmosis is a passive movement of water from a compartment diluted in solute particles towards a compartment that is more concentrated.

Membranes permeability depends on the nature, size and polarity of particles. As they’re made of lipids, small uncharged particles tend to diffuse easily through. Larger particles or charged ones require specific pathways. We can thus distinguish passive diffusion of water, dioxygen and carbon dioxide and facilitated diffusion through specific proteins (channels) of glucose or chloride, potassium and sodium ions.

Some particle transports require energy to occur. Those transports can happen against the concentration gradient. In this case, we speak of « active transport ». It uses ATP as an energy source. The sodium-potassium pump that insures unbalance of both ions across neuron membrane is a typical example of such active transport.