this him 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.

Functions

Cells that transport substances across their cell membranes have tight junctions. For example, the cells that line the intestine absorb nutrients through pores in the exterior surface of the cell membrane. The nutrients then move through the cell and out another pore to the extracellular matrix on the other side of the cell, and then into a blood vessel. The entry pores must be kept separate from those that allow passage out of the cell for this process to work efficiently. The presence of tight junctions near the outer surface is thought to maintain this separation.

Without the tight junctions, the entry pores could migrate into the region of the exit pores.

Anchoring junctions

At an anchoring junction, on the insides of the neighboring cells, are rivet-like thickenings called plaques. Each plaque is made of protein. Keratin filaments attach the plaques to the insides of the cell membranes. These filaments are part of the cell's cytoskeleton. Thinner proteins called transmembrane linker proteins cross the space between the cells.

Locations

There are two types of anchoring junction: desmosomes and hemidesmosomes. Desmosomes have larger transmembrane linker proteins than the other junctions. These linkers are called cadherins. Anchoring junctions are used to connect epithelia (lining cells) to a basement membrane.

Together with gap junctions, desmosomes form intercalated discs (complex junctions) between cardiac muscle cells.

Functions

• Anchoring junctions bind cells together in sheets or masses that form strong structural units.

• The networks that these junctions form in tissues distribute tension, helping to prevent tearing. This is why anchoring junctions are prevalent in areas that are subjected to the mechanical stress of pulling and stretching.

• Within intercalated disks, for example, desmosomes prevent adjacent cells from separating during heart contractions.

Cilia (Singular: cilium)

These appear in large numbers as tiny, hairlike fronds on the cell's exposed surface. Each cilium is made of bundles of microtubules (tiny tubes) covered by the cell membrane.

Function

Cilia's wave-like movement enables them to carry matter in one direction over the cell's surface. Ciliated cells that line the airways move mucus (thick, slimy fluid) toward the pharynx (throat) to be removed by swallowing. This gets rid of the airborne dust and bacteria trapped in the mucus and helps clean and protect the lining.

Flagella (Singular: flagellum)

These are structurally the same as cilia but are longer and occur singly in humans.

Function

Flagella are used to move the cell itself. The only example of human cells with flagella are sperm (male sex cells). The sperm uses its flagellum to propel itself toward the female egg cell (ovum).