Cell Structure Function and their Organelles

The human body is composed entirely of cells, the products of cells and various fluids. Cells account for the shape, organization and construction of the body and for carrying on its life processes. In addition, they can reproduce and thus provide the new cells needed for growth and development and for replacement of worn and injured tissues.

Cell Structure

Trillions of cells that make up a human body. Cell size measured in units called microns. 

A microns equals 1/1000th of a millimeter and is symbolized by the greek letter mu (µ).

Human cells also vary in shape and typically their shapes are closely related to their functions. Nerve cells often have long, threadlike extensions that transmit nerve impulses from one part of the body to another.  

The epithelial cells that line the inside the mouth serve to shield underlying cells. These protective cells are thin, flattened and tightly packed, somewhat like the tiles of a floor.

Muscle cells, which function to pull parts closer together, are slender and rodlike, with their ends attached to the parts they move.

 

Composition of cell

Commonly a cell consists of two parts, one within the other and each surrounded by a thin membrane. The inner portion is called the cell nucleus, and it is enclosed by a nucleus membrane. A mass of fluid called cytoplasm surrounds the nucleus and is encircled by a cell membrane or cytoplasmic. 

Cell membrane

The cell membrane is the outermost limit of the cell, but it is more than a simple envelope surrounding the cellular contents.

Plasma Membrane

The membrane serves as “gateway”  through which chemical enter and leave. This gate acts in a special manner, it allows some substances to pass and excludes others. When a membrane functions in this way, it is selectively permeable. 

 

It is known, however, that the mechanism involves the chemical nature of the membrane which is largely protein and lipid (usually phospholipid and cholesterol) with a small amount of carbohydrate.

Since different kinds of molecules are located on the inner and outer surface of the membrane. Various molecules on the outer surface function as receptor sites that can combine with specific chemicals such as hormone, enzyme, neurotransmitter, drugs and other chemical when they occur in the cell’s surroundings. 

Plasma membrane with details

Because the membrane is largely phospholipid, molecules that are soluble in lipids can easily pass through it. On the other hand, molecules of substances like water, which do not dissolve in lipids, cannot penetrate the phospholipid layers. Water and other small molecules, however can pass through the large protein molecules that span the thickness of the membrane and serve as passageways or “pores”.

One type of specialized junction a desmosome, serves to “spot weld” adjacent skin cells so they form a structural unit. The membranes of certain other cells, such as those in heart muscle, are interconnected by gap junctions in the form of tubular channels.

Gap Junction

Cytoplasm

Cytoplasm usually appears as a clear and thick liquid. An electron microscope, reveals cytoplasm to be highly structured and filled with networks of membranes and other organelles. Cytoplasm is site of metabolism reactions, activities of cell occur largely in cytoplasm. 

The following cytoplasmic organelles play specific roles-

Endoplasmic reticulum

The endoplasmic reticulum is a complex network of interconnected membranes. Often a large proportion of its outer membranous surface has numerous tiny, spherical organelles called ribosomes attached to it and for this reason is termed rough endoplasmic reticulum. The remaining surface, which lacks ribosomes, is called smooth endoplasmic reticulum. 

Endoplasmic Reticulum

Functions of endoplasmic reticulum

These membrane form flattened sacs, elongated canals, and fluid-filled vesicles. It seems to function as tubular communication system through which molecules can be transported from one cell part to another.

The endoplasmic reticulum also functions in the synthesis of chemicals. 

Ribosomes function in the synthesis of proteins.

Smooth endoplasmic reticulum contains enzymes involved in manufacturing lipid molecules.

Ribosome

Although many ribosomes are attacked to membranes of the endoplasmic reticulum, other occur as free particles scattered throughout the cytoplasm. Ribosomes are composed of protein and RNA molecules.

Function of ribosomes

Ribosomes function in the synthesis of protein molecules.
 

Protein are used for building cell structure, while other act as enzymes, and still others are transported to the cell membrane and secreted outside the cell. 

Golgi apparatus

The Golgi apparatus is usually centrally located near the nucleus. It is composed of a group of flattened, membranous sacs whose membranes are continuous with those of the endoplasmic reticulum.

Golgi Apparatus

Function of Golgi apparatus

The Golgi apparatus is thought to be involved in the processing of proteins synthesized by the endoplasmic reticulum and in the packaging of these proteins for secretion. 

In some cells, for example the Golgi apparatus seems to manufactured carbohydrate molecules that are combined with protein molecules. The resulting glycoproteins are then packed in bits of the membrane from the Golgi apparatus. Then the package may move to the cell membrane where its contents are released to the outside. 

The Golgi apparatus may also store such packed molecules.

Mitochondria

Mitochondria are relatively large, fluid-filled sacs. They vary in size, can change shape, and often move about in the cytoplasm.

Mitochondria

The membrane surrounding a mitrochondrion has an outer and an inner layer. The inner layer is folded to form partitions called cristae within the saclike structure. Small, stalked particles that contain enzymes are connected to the cristae. 

These enzymes control some of the chemical reactions by which energy is released from glucose and other molecules. The mitochondria also function in transforming this energy into a form that is usable by cell parts. For this reason mitochondria called “power houses” of cells. 

    

Lysosomes

Lysosomes are sometimes difficult to identify because their shapes vary so greatly. However, they commonly appear as tiny, membranous sacs. Inside these sacs are powerful enzymes that are capable of breaking down molecules of protein, carbohydrates and nucleic acids. Such enzymes function to digest various particles that enter cells. Certain white blood cells for example, can engulf foreign substances, such as bacteria.

Lysosomes then fused with the engulfed substances, and Lysosomal enzymes digest the bacteria. Consequently, white blood cells aid in preventing bacterial infections.

Lysosomes also function in the destruction of worn cell parts. In fact, sometimes they are responsible for destroying entire, injured cells that have been engulfed by scavenger cells.

How the lysosomal membrane is able to withstand being digested itself is not understood.

Peoxisomes

Peoxisomes are membranous vesicles that resemble lysosomes in size and shape. They occu in most cells and contain certain enzymes including a group called peroxidases. 

Vacuoles       

Vacuoles are membranous sacs, varying greatly in size. They may be formed by an action of the cell membrane in which a portion of the membrane folds inward and pinches off. As a result, a small, bubblelike vacuole, containing some liquid or solid material that was outside the cell a moment before, appears in the cytoplasm.

Centrosome

A Centrosome (central body) is located in the cytoplasm near the golgi apparatus and the nucleus. It is nonmembranous and consist of two rodlike structures called centrioles, which in turn contain tiny tubelike parts, or microtubules. 

The centrioles lie at right angles to each other and function in cell reproduction. During this process, the centrioles move away from one another and take positions on either side of the nucleus. There they aid in the distribution of the chromosomes, which carry genetic information, to the newly forming daughter cells.

Centrioles also function in initiating the formation of tiny, moving, hairlike projections called cilia that extend outward from the surfaces of some cell membranes and in the formation of the tailike flagella of the sperm cells.

Fibrils and microtubules

Two types of thin, threadlike processes likely to be found in the cytoplasm are fibrils and microtubules. The fibrils are tiny rods, and the microtubules are tubular structures composed of globular proteins. 

Both seem to function in providing support and stability to the cytoplasm and thus help to maintain the shape of the cell.

Fibrils, which are most highly developed in muscle cells (myofibrils), function in the contraction mechanism of these cells.  

   
Microtubules appear in cells during cell reproduction and aid in the distribution of chromosomes to the newly forming cells. They may also function to direct the flow of cytoplasm in cells that move about and may be involved in transporting secretory products to the cellular surfaces. 

  

Cell Nucleus

A cell nucleus is usually located near the center of the cytoplasm. It is relatively large, spherical structure and functions to direct the activities of the cell.

Cell Nucleus

 The nucleus is enclosed in a nuclear membrane, which is porous and allows substances to move between the nucleus and the cytoplasm.

The nucleus contains a fluid (neucleoplasm) in which other structures float.

These structures include the following –   

Nucleolus

A nucleolus or little nucleus, is a small, dense body composed largely of RNA and protein. It has no surrounding membrane and is through to function in the production of ribosomes.

Once the ribosomes are formed, they seem to migrated through the pores in the nuclear membrane and enter the cytoplasm. The nucleus of a cell that synthesizes large amount of proteins may contain several nucleoli.

Chromatin

Chromatin consist of loosely coiled fibers that are present in the nuclear fluid. When the cell begins to undergo the reproductive process, these fibers become more tightly coiled and are transformed into tiny, rodlike chromosomes. 

Chromatin fibers are composed of protein and DNA molecules, which in turn are organized into tiny, beadlike particles called nucleosomes. The DNA molecules contain the information that directs the cell in carrying out its life processes.