Molecules combine in very specific ways to form what is called cell organelles (little organs) which are the basic component parts of the cells. Each organelle has a particular functional significance within the cell that allows it to live.
Despite the great variety of cells in the body, they all have the same basic structure. When studying the cell's structure it is helpful to also think of it in three parts:
• the outer part called the cell membrane
• the inner part containing the nucleus
• the middle layer which contains a semi-fluid substance called the cytoplasm, which contains all the cell's organelles.
The cell membrane is a fine membrane that encloses the cell and protects its contents. This membrane is said to be semi-permeable in that it selectively controls the inward and outward movement of molecules into and out of the cell. Oxygen, nutrients, hormones and proteins are taken into the cell as needed, and cellular waste such as carbon dioxide passes out through the membrane. As well as governing the exchange of nutrients and waste materials, its function is to maintain the shape of the cell.
The nucleus is the largest organelle in the cytoplasm and is the control centre of the cell, regulating the cell's functions and directing most metabolic activities. The nucleus governs the specialised work performed by the cell and the cell's own growth, repair and reproduction. All cells have at least one nucleus at some time in their existence. The nucleus is significant in that it contains all the information required for the cell to function and it controls all cellular operations.
The information required by the cell is stored in DNA (deoxyribonucleic acid), which carries the genetic materials for replication of identical molecules. The DNA strands are found in threadlike structures known as chromosomes.
A chromosome is one of the threadlike structures in the cell nucleus that carries the genetic information in the form of genes. The nucleus of a human cell contains 46 chromosomes, 23 of which are maternal and 23 of which are of paternal origin. Each chromosome can duplicate an exact copy of itself between each cell division so that each new cell formed receives a full set of chromosomes.
Chromatin is the substance inside the nucleus that contains the genetic material.
RNA is the molecule that transports the genetic information out of the nucleus and allows translation of the genetic code into proteins.
Inside the nucleus is a dense spherical structure called a nucleolus which contains ribonucleic acid (RNA) structures that form ribosomes.
The nucleus is surrounded by a perforated outer membrane called the nuclear membrane; materials move across it to and from the cytoplasm.
The cytoplasm is the gel-like substance that is enclosed by the cell membrane. The cytoplasm contains the nucleus and the small cellular structures called organelles. Most cellular metabolism occurs within the cytoplasm of the cell.
This is an area of clear cytoplasm found next to the nucleus and contains the centrioles.
Contained within the centrosomes are the small spherical structures called centrioles, which are associated with cell division, or mitosis. During cell division the centriole divides in two and migrates to opposite sides of the nucleus to form the spindle poles.
This is a pair of identical strands that are joined at the centromere and separate during cell division.
The portion of a chromosome where the two chromatids are joined is the centromere.
Ribosomes are tiny organelles made up of RNA and protein. They may be fixed to the walls of the endoplasmic reticulum (known as rough ER) or may float freely in the cytoplasm. Their function is to manufacture proteins for use within the cell and also to produce other proteins that are exported outside the cell.
This is a series of membranes continuous with the cell membrane. It can be thought of as an intracellular transport system, allowing movement of materials from one part of the cell to another. It links the cell membrane with the nuclear membrane and therefore assists movement and materials out of the cell.
It contains enzymes and participates in the synthesis of proteins, carbohydrates and lipids. The endoplasmic reticulum serves to store material, transport substances inside the cell, as well as detoxify harmful agents. Some endoplasmic reticulum appears smooth, while other appears rough due to the presence of ribosomes.
Mitochondria are oval-shaped organelles that lie in varying numbers within the cytoplasm. The mitochondria are said to be the site of the cell's energy production. The mitochondria provide most of a cell's ATP (adenosine triphosphate), which is a compound that stores the energy needed by the cell.
The work of the mitochondria is assisted by enzymes which are proteins that speed up chemical changes. By means of cellular respiration, the mitochondria provide the energy which powers the cell's activities.
These are round sacs present in the cytoplasm. They contain powerful enzymes, which are capable of digesting proteins. Their function is to destroy any part of the cell that is worn out so that it can be eliminated -this is known as lysis.
These are empty spaces within the cytoplasm. They contain waste materials or secretions formed by the cytoplasm and are used for temporary storage, transportation or digestive purposes in different kinds of cells.
This is a collection of flattened sacs within the cytoplasm. The golgi apparatus is typically located near the nucleus and attached to the
Fig 1.2 Structure of a cell
Now complete Activity 1.1 in the resources for this book on Dynamic Learning Online.
endoplasmic reticulum. It is said to be the 'packaging department' of the cell as it stores the protein manufactured in the endoplasmic reticulum and later transports it out of the cell.
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