Cell biology

The stem cellstem cell
Cells which can divide repeatedly without becoming differentiated and have the capacity to develop into a diverse range of specialised cell types. s of an early embryoembryo
The name for a group of cells that are developing into a fetus. In humans this is from implantation to the 8th week of development.  are all very similar and all have the potential to become any other cell in the body – they are pluripotentpluripotent
(of an immature cell or stem cell) capable of differentiating to form almost any type of cell in the body except those of the placenta and amniotic membranes. . But by the time a baby is born, many of the cells have differentiated and become specialised for particular functions in the body. Examples include muscle cells that contract in response to a stimulus, neurones that carry electrical impulses around the body, and secretory cells that produce substances such as mucus, acid, enzymes or hormones for the body to use.

Different cells are adapted for different functions in an organism, such as the sensory cells (green) highlighted in this confocal micrograph of a Drosophila melanogaster fruit fly (Chun Han, CC BY-NC-ND 3.0)

It is very useful to consider some of the major adaptations for different functions seen in cells. Looking at differences in the ultrastructureultrastructure
The structure of a cell as seen using an electron microscope. of a cell can give you a clear insight into the functions of that cell in the body.

Example 1: Transport

There are many situations where it is important to move substances into or out of cells. For example the products of digestion moving into the cells of the ileum, the hormones produced by glandular cells moving into the bloodstream or the Na⁺, K⁺ or Cl^- ions involved in the electrochemical events of an action potentialaction potential
The wave of positive charge which passes along an axon in response to a stimulus.  moving into or out of a neurone.

Cells may be adapted to allow rapid transport across their internal or external membranemembrane
A thin, flexible sheet-like structure that acts as a lining or a boundary in an organism. s in a number of ways:

• by an increase in the surface area of the internal membranes (such as many stacks of Golgi apparatus), or of the cell surface membrane (such as the presence of microvilli).
• by an increase in the number of protein channels and carrier molecules in their membranes.
• by mechanisms to set up concentrationconcentration
  The amount of a substance (solute) in a solution. , water potentialwater potential
  A measure of the relative tendency of water to move from one area to another. Pure water has the highest water potential as all the water molecules are free to move. Every solution has a lower water potential than pure water as some of the molecules are associated with the solute particles. or electrochemical gradients across membranes.

Many fat cells have few mitochondriamitochondria
Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse , but special brown fat cells found in newborn babies and hibernating animals are full of mitochondria which undergo a special form of aerobic respiration designed to produce heat to keep the body warm (Image credits: mitochondria Keith Porter/ CC BY-NC-ND 3.0 hedgehog Jamain/ CC-BY-SA-3.0,2.5,2.0,1.0)