Archive for category Biology
Basic Structural Cell Components in Brief
Cell membrane structure
The cell membrane is a thin sheet of fats, or lipids, interspersed with large protein molecules. A lipid molecule has two halves: a water-soluble end and a water-repelling one. A membrane is a double layer of phospholipid molecules.
The water-repelling ends form the outer surfaces and the water-soluble ends mingle in between.
The cell membrane controls the flow of materials in and out of the cells, and maintains the cell’s integrity.
Lipid bilayer
This fatty double layer contains:
• Phospholipids -fat molecules containing phosphorus;
• cholesterol -fat molecules that stabilize the lipid layer;
• glycoproteins, which are made up of protein and carbohydrate (sugar) molecules on the extracellular (outer) surface of the protein.
Cell membrane
Microvilli
These are tiny, fingerlike projections, or folds, of the cell membrane itself.
The function of microvilli is to increase the surface area of the cell. This allows the cell to absorb more substances from their surroundings and also to secrete molecules more efficiently.
Types of cell junction
Tight, or impermeable, junction
This is formed by protein molecules of neighboring cell membranes fusing together. Tight junctions are found between the epithelial cells that line the digestive tract.
Desmosome, or anchoring, junction
At these junctions, on the insides of the neighboring cells, are rivetlike thickenings called plaques. These are attached to the opposite side of the cell membrane by keratin filaments (flexible protein strands also used in hair).
Linker proteins extend from the plaques and cross the space between the cells.
Gap, or communicating, junction
Proteins pass through both the membranes of two adjacent cells.
The proteins are arranged into groups (connexons) which form a hollow channel through the cell membranes.
These junctions are found in heart muscles and in the muscles of the gut.
Selective barrier
The cell membrane is a protective barrier that controls what substances travel in and out of the cell. Although oxygen and carbon dioxide pass through freely, other substances have difficulty getting through the membrane.
The cytoplasm
The cytoplasm (semifluid mixture) is the cellular material outside of the nucleus (control center) and inside the cell (plasma) membrane. In humans, as well as all plants and other animals, cytoplasm consists of cytosol (a gellike fluid), cytoplasmic organelles (miniorgans), and inclusions (chemical substances). Prokaryotic cells (those of bacteria and primitive algae) have a cytoplasm and inclusions, but no organelles.
As a whole, the cytoplasm assists in the movements of organelles and the transport of substances within the cell; provides an environment in which biochemical reactions can occur; and helps to support and shape the cell.
Cytosol
This is a gel-like, semitransparent fluid mostly comprised of water. It contains dissolved sugars, salts, and other solutes.
Larger molecules, such as proteins, form colloids. The cytosol holds the other elements of the cytoplasm in suspension.
Many vital substances, such as starch, are stored in the cytosol in this way until they are needed by the organelles in the cell.
The cytosol is able to change from a semifluid to a more solid state (in conjunction with the cytoskeleton). This is important for many cell functions. Read the rest of this entry »
Structural Organelles in Human Cells
Centrioles
Centrioles are a type of organelle. They occur in pairs and lie at right angles to each other near the nucleus (control center) of the cell. They are bundles of microtubules (tiny tubes) like those used in the cytoskeleton. The microtubules are arranged in nine groups of three, forming a tube.
Cilia and flagella formation
Centrioles form the bases of cilia and flagella (movable projections from the cell membrane).
1The centrioles multiply.
2They migrate to the cell membrane.
3 Each centriole sprouts microtubules that push the cell membrane outward to form cilia.
4 A flagellum results when microtubules form a single, longer projection.
Peroxisomes
Peroxisomes are a type of organelle found in human body cells, and those of all plants and animals. They are single-membrane sacs suspended in the cell’s cytoplasm. Peroxisomes are very similar to lysosomes, but tend to be smaller.
They contain powerful enzymes (biological catalysts). The number of peroxisomes in a cell varies from one to several hundred. They are especially prevalent in the liver and kidney cells.
Peroxisomes are formed by growth and binary fission (simple division into two) of other peroxisomes.
Functions
• Peroxisomes detoxify harmful and poisonous substances such as alcohol, hydrogen peroxide, and formaldehyde.
• Peroxisomes disarm dangerous free radicals.
These are energetic chemicals with unpaired electrons. They can scramble the structure of vital compounds such as DNA (deoxyribonucleic acid), proteins, and lipids (fats).
• Peroxisomes also break down fatty acids. Although free radicals and hydrogen peroxide are natural byproducts of cell activities, if they accumulate in tissues to certain levels they can become very harmful.
Tight intercellular junctions
Tight, or impermeable, junctions are formed by protein molecules of neighboring cells fusing together like a zipper. There is no intercellular space between cells at a tight junction.
Tight junctions are found in epithelial tissues. These tissues:
• cover the body surface as skin;
• line internal cavities;
• form glands.
A special type of epithelial tissue called endothelium lines the walls of the heart, blood, and lymph vessels. In the brain, the endothelial cells of capillaries (the smallest blood vessels) have tight junctions. Tight junctions between epithelial cells are formed only between those parts of the cell junctions near the free surface. Read the rest of this entry »