Cells

Introduction

The cell is the basic structural and functional unit of the living human (functional means they can reproduce). Each cell is a highly organised structure, and different cells are important in the different body functions. The external wall of the cell is called the cell membrane (or plasma membrane) and it is made up of lipid and protein molecules. Cell membranes come in a variety of sizes and shapes. Microvilli are wavey plasma projections that add surface area and save space. If the cell is designed to absorb something then the more surface area it has the more it can absorb.



The lipid layer is biomolecular (2 symmetrical rows of lipid molecules with their tails pointing to one another), we call this a phospolipid bilayer. The heads are water loving (hydrophilic) and the tails are water hating (hydrophobic). Protein molecules are Asymmetrical, integral molecules are imbedded through the lipid layers and peripheral molecules are attached at an extracellular level.



External to the plasma membrane, all animal cells have a fuzzy coat called the glycocalyx (Gly-co-cal-ix). Each cell type has a different pattern of sugars acting as specific biological markers, recognised by approaching cells and this gives the ability of cell-cell recognition.

__**Cytoskeleton **__ The cytoskeleton is a collection of protein filaments and cylinders that determine the shape of a cell, lend it structural support, organise its contents, move substances through the cell, and contribute to movements of the cell as a whole. Basically the cytoskeleton is made up of rods running through the cytosol they shape the cell and generate various movements.



Microfilaments are protein strands that form a network on the cytoplasmic side of the plasma membrane.

Intermediate filaments resist stresses placed on a cell and participate in junctions that attach some cells to their neighbours.

Microtubules are hollow, they hold organelles in place, form bundles that maintain cell shape and rigidity, and act somewhat like railroad tracks to guide organelles and molecules to specific destinations in a cell.

__** Cell Junctions **__ In many **animal** tissues (e.g., connective tissue), each cell is separated from the next by an extracellular coating or matrix. However, in some tissues (e.g., epithelia), the plasma membranes of adjacent cells are pressed together. We are looking at 3 kinds of junctions that occur in the human body:
 * **Tight junctions**
 * **Gap junctions**
 * **Desmosomes**

Epithelia are sheets of cells that provide the interface between masses of cells and a cavity or space (a lumen).
 * //Tight Junctions//**
 * The portion of the cell exposed to the lumen is called its **apical** surface.
 * The rest of the cell (i.e., its sides and base) make up the **basolateral** surface.

Tight junctions seal adjacent epithelial cells in a narrow band just beneath their apical surface.

Tight junctions perform two vital functions:


 * They prevent the passage of molecules and ions through the space between cells. So materials must actually enter the cells (by diffusion or active transport) in order to pass through the tissue. This pathway provides control over what substances are allowed through.
 * They block the movement of integral membrane proteins (red and green ovals) between the apical and basolateral surfaces of the cell. Thus the special functions of each surface, for example
 * receptor-mediated endocytosisat the apical surface
 * exocytosis at the basolateral surface can be preserved.
 * The epithelia of lung tissue is an example of where you would find tight junctions

Gap junctions are intercellular channels some 1.5�2 nm in diameter. These permit the free passage between the cells of ions and small molecules. They are cylinders constructed from 6 copies of transmembrane proteins called **connexins**. Because ions can flow through them, gap junctions permit changes in membrane potential to pass from cell to cell.
 * //Gap Junctions//**


 * Examples:**
 * The action potential in heart (cardiac) muscle flows from cell to cell through the heart providing the rhythmic contraction of the heartbeat.
 * At some so-called electrical synapses in the brain, gap junctions permit the arrival of an action potential at the synaptic terminals to be transmitted across to the postsynaptic cell without the delay needed for release of a neurotransmitter.
 * As the time of birth approaches, gap junctions between the smooth muscle cells of the uterus enable coordinated, powerful contractions to begin.

**//Desmosomes//** Desmosomes are localised patches that hold two cells tightly together. They are common in epithelia (e.g., the skin). Desmosomes are attached to intermediate filaments of keratin in the cytoplasm.

Carcinomas are cancers of epithelia. However, the cells of carcinomas no longer have desmosomes. This may partially account for their ability to metastasize.

The Cell: Contents
The Cell: Objectives Cellular Organelles