The cell which literally means "small chamber" is the basic unit of a living organism. The human body consists of trillions of cells, each carrying out its own functional and structural role. The cells are then organized into tissues, and tissues are in turn organized into organs.
Every human cell is enclosed by the cell membrane. Inside the cell, we can find two components namely: 1) the nucleus which is a spherical structure which is usually placed at the centre of the cell. 2) Cytoplasm, which is the protoplasmic fluid found outside the nucleus. The Cytoplasm has two components: (1) the cell organelles and (2) Cytosol which is a gelatinous solution. It is a fluid that surrounds the organelles and contains microtubules which provide the cell with its cytoskeleton.
The cell or Plasma membrane is primarily made of two things: 60% of protein and 40% of fat. And the major membrane lipids are the Phospolipids. These lipids are arranged in a bio-molecular layer. These lipids are arranged in a bio-molecular layer. This bilayer structure has two ends, each having different characteristics.
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<image 2: cell membrane>
The cell membrane helps in supporting and retaining the cytoplasm. And it plays the role of acting as a selective barrier to the passage of molecules, allowing some molecules to pass while not allowing others. And the membrane of the organelle allows for a selective passage of molecules between organelles and the cytosol. The advantage of such selective movement is that the cell can confine certain products of chemical reactions to the required specific organelle.
Besides acting as a selective barrier, the plasma membrane also carries out the role of detecting chemical signals that arise in the neighbouring cells.
2aiii. Cell organelles
The cell organelles are found in the cytoplasm. The following are the various cell organelles.
The Nucleus is the largest organelle found in the cell structure. It is usually placed at the centre of the cell and it contains two components a) Nucleolus b) Chromatin. Usually most cells contain a single nucleus but there are some special cells E.g. Skeletal muscle cells that contain multiple nuclei whereas mature red blood cells have no nucleus.
The primary function of the nucleolus is to store and transmit genetic information to the next offspring generation of cells. This information is also used to make proteins which determine the structure and function of the cell.
<image 3: cell nucleus>
The Nucleolus is a densely stained filamentous body found inside the nucleus. It is not covered by a membrane. This structure contains protein granules that synthesize RNA (Ribonucleic acid). And these RNA units move out to the cytoplasm through some circular openings which are known as nuclear pores.
This is found within the nucleus. The DNA in association with proteins forms a fine thready network called the Chromatin. These threads contain the genetic information that determines the process of heredity. The units for heredity determination are called genes. These genes are organized linearly on rod like bodies called chromosomes. A human being has 23 pairs with 46 chromosomes. The Chromatin condenses to form chromosomes during cell division.
Ribosomes are the organelles responsible for the manufacture of proteins. They are known as the protein houses of the cell. They contain rRNA (ribosomal RNA) which provides the genetic information to synthesize amino acids into proteins.
<image 4: Ribosome>
These Ribosomes are either found freely in the cytoplasm or they are attached to the endoplasmic reticulum. (Described in next point).
These Ribosomes can also be seen as attached to each other forming chains which are known as polyribosomes or polysomes.
3. Endoplasmic reticulum
The most extensive organelle of the cell is the endoplasmic reticulum. We can distinguish the endoplasmic reticulum. It serves two purposes. It acts as a channel for the passage of chemical products in a cell. And it also serves as a store of the synthesised products. We can also distinguish the endoplasmic reticulum into two: rough and smooth.
a) The rough Endoplasmic Reticulum is granular in structure and contains the Ribosomes, and this it is involved in the processing of proteins.
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b) The smooth Endoplasmic Reticulum does not contain Ribosomes. It has a tubular structure. In the smooth Endoplasmic Reticulum, the lipid molecules (fatty acid) are synthesised, and it also stores and releases calcium ions.
<image 5: Endoplasmic Reticulum >
4. Golgi apparatus
It consists of a series of flattened sacs forming a cup-shaped structure. The Golgi apparatus receives the proteins that arrive from the Endoplasmic reticulum. It modifies the proteins as they move from one compartment level of the Golgi apparatus to the next. And finally, it is responsible for the packaging of materials and guiding it to the transport vesicles.
<image 6: Golgi apparatus >
The Endosomes are found between the cell membrane and the Golgi apparatus. They are responsible for the sorting, modification and directing of vesicular transport in the cell.
The Mitochondria is the power-house of the cell and are primarily involved with the production of energy in the form of Adenosine triphosphate (ATP). It carries out the process of cellular respiration, where oxygen is consumed and CO2, heat and water are released.
<image 7: Mitochondria>
The Mitochondria is a rod like structure with a double membrane. The outer membrane is smooth but the inner membrane has many folds or shelf-like structures called cristae, which contains nearly 60 enzymes which are responsible for carrying out the process of ATP production.
The Lysosomes are known as suicide bags of the cell. They are spherical or oval in shape with a single membrane; which encloses a fluid that contains powerful digestive enzymes which are responsible for the destruction of damaged cells and organelles. It is also responsible for the digestion of phagocyted materials like bacteria. It plays a significant role in the make up of defense systems of the body.
Besides the organelles, the cell cytoplasm also contains various protein filaments. This filamentous network of the cell is known as cytoskeleton of the body. It is responsible for the maintenance and growth of the cell shape and to activate cell movements.
There are three kinds of cytoskeleton.
a) Microfilaments - which are made of the protein actin, and make the main part of the cytoskeleton.
b) Intermediate filaments - these are developed In the regions which are subject to mechanical stress.
c) Microtubules - These are hollow tubes which are from the protein tubules. This is the most rigid filament. They maintain the cell shape.
This is present near to the nucleus. It is spherical in shape and contains two small granules called centrioles. The centrosome is responsible for the formation of spindle fibres and astral rays during cell division.
These are hairlike extension found in the surface of some epithelial cells e.g. Those that are lining the trachea.
<image 8: Cillia>
These are cell membrane projections that serve the purpose of increasing the surface area of a cell. E.g. the lining of the intestine.
<image 9: Villi>
This is relatively longer and is typically found in single number e.g. the sperm has a flagellum.
<image 10: Flagella>
2aiv. Components of the Cellular environment
Water is useful as a solvent and takes part in many metabolic processes. Generally comprises 60-90% of a living organism.
These are atoms or molecule with an imbalances number of electrons and protons. The ions can be found in both the intra and extra cellular fluid. Important ions include: Sodium, Potassium, Calcium and Chloride.
This is an important source of energy for cells. It comprises 3% of the biomass of the typical cell, it is composed of carbon, hydrogen and oxygen atoms. The different types of carbohydrates include:
This is mainly involved in long term energy storage. It comprises 40% of the dry mass of a cell. It is composed largely of carbon and hydrogen. It is generally insoluble in water. Different lipids include triglycerides, phospholipids and steroids.
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It comprises 50-60% of the dry mass of the cell and is made from amino acids and is linked by peptide bonds. They contain enzymes that act as catalysts which lower the amount the activation energy and speed up the reaction.
Deoxyribonucleic acid (DNA) is responsible for controlling the cell functions via transcription and translation; these processes are in charge of the protein synthesis of a cell. The DNA is a complex molecule that has the capacity to duplicate itself exactly
Transcription: DNA produces mRNA (Messenger RNA).
Translation: mRNA moves to the nucleus and is used for protein production. This process requires the mRNA, tRNA (Transfer RNA), amino acids and a ribosome.
Ribonucleic acid plays an active role as a catalyst in protein synthesis and it controls gene expression.
mRNA - Messenger RNA guides the proteins to the Ribosomes.
tRNA - Transfer RNA delivers amino acids to the Ribosomes.
rRNA - Ribosomal RNA binds the amino acids to form proteins.
2av. Movement of molecules across all membranes
As we have studied, that the plasma membrane which is a thin layer of lipids separates the inner contents of a cell from the surrounding extracellular fluid. In addition to this membrane, the organelles have membranes too.
These membranes play an important role in the movement of molecules and ions between the various organelles and cytosol as well as between the cytosol and the extracellular fluid. This is carried out by several mechanisms which can be classified into two types.
A) Passive process
B) Active process
A) Passive process
These processes do not consume energy from the cell. They include:
1) Simple diffusion
3) Facilitated diffusion
1) Simple diffusion
The movement of a substance from a region of high concentration to a region of low concentration is known as diffusion.
This is the diffusion of substance through a semi-permeable membrane like plasma or cell membrane from a region of low concentration to a region of high concentration.
3) Facilitated diffusion
This is the diffusion of a substance through a cell membrane from a region of high concentration to a region of low concentration. This process is assisted by the use of carriers.
The substance or solvent is pressurized from a high concentration region to a low concentration region through the cell membrane by the action of hydrostatic pressure.
B) Active processes
These processes consume energy from the cell. It includes the process of Active transport, which is the movement of a substance across a cell membrane from a region of low concentration to a region of high concentration using a carrier molecule that utilizes the cell energy.
In this active process, the particles of a very large size are made to enter a cell by the process of diffusion or by active transport through a vesicle formed by a part of the cell membrane.
2avi. Cell metabolism
This is the sequence of metabolic processes that take place in a cell to breakdown energy into ATP molecules. These processes occur in the mitochondria which is the power house of the cell. The reactions include catabolic reactions which include the oxidation of one molecule of glucose and the breaking down of another through glycolsis into two molecules of pyruvic acid to give out ATP molecules.
C6H12O6 + 6O2 â†’ 6CO2 + 6H2O
A total of 38 ATP produced per glucose molecule in the whole process.
Besides Glucose, Fats (Lipids) and protein are also metabolized to produce energy.
2avii. Cell differentiation
This is the mechanism by which a generic cell becomes a specialized type of cell meant for a particular function. E.g. when a zygote develops into a multi-cellular system of tissue and cells.
2b. Cell division
2bi. Introduction to cell division
Each cell has its own lifespan and characteristic size of growth. When it reaches a certain level of maturity, then the cell divides into two daughter cells. These daughter cells replace the worn out cells or diseased cells. This type of cell division is known as mitosis. The process of cell division is always continuous.
The cell life is classified into two phases:
2. Mitotic phase
This is the preparation stage of the cell before the cell division. It is the longest phase. In this phase, the cell is very active and it includes cell grwth and DNA duplication. The chromatin reticulum becomes distinct and the centrioles divides into two.
B. Mitotic phase
This is a continuous phase and it is comprised of two parts.
1. Mitosis 2. Cytokinesis
This is the division of the nucleus and it is done in four sub phases.
In this phase, the chromosomes change from long their threads into short thick structures by the process of repeated coiling and condensation. Then the chromosomes divide into two halves known as chromatids. These chromatids are held together by the centromere. The centrioles move to opposite sides of the nucleus and each centrioles is then covered by fibres that radiate out to form aster. The fibres connect the two centrioles and are known as mitotic spindle, and these elongate accordingly.
<image 11: Prophase>
The chromosomes place themselves in an orderly manner at the equatorial plane, i.e. the mid way between the two centrioles. This is a very slow stage. The centromere that holds the chromosome pair attaches to a spindle fibre between the centrioles.
<image 12: Metaphase>
The centromere that holds the chromosome pair together detaches. The chromosomes move towards the opposite directions by the help of the spindle fibre towards the centrioles.
<image 12: Anaphase>
The movement of chromosomes towards the centrioles gets completed. The division of cytoplasm gets initiated slowly. The Chromosomes expand and lengthen forming the chromatin reticulum. Nuclear membranes start appearing around each set of chromosomes. Spindle fibres disappear. And a cleavage furrow begins to appear in the midway.
<image 13: Telephase>
This is the division of the cytoplasm. A nucleus is formed within each set of chromosomes and thereby forming two daughter cells.
<image 14: Cytokinesis>
2c. Heredity and its utility in Physical education
2ci. Introduction to heredity
Heredity is the transmission of genetic or biological traits from one generation to its offspring. There are million of traits that are passed through this process, E.g. Hair colour, facial features, hair structure, skin colour. These traits help the body on how to develop its organs and tissues.
Heredity functions by the work of structures that are found in the nucleus of the cell, these structures are known as chromosomes. They are found in pairs, a human being has 23 pairs, that is 46 chromosomes in all. And each Chromosome contains Deoxyribonucleic acid (DNA). The Chromosome also contains a special type of protein known as histone proteins.
<image 15: Chromosomes>
The DNA as we have discussed earlier is a molecule that has the capacity to duplicate itself exactly. The DNA molecules have information coded into their sequence of nucleotides which help in the synthesis of proteins. The cell's DNA is found inside the nucleus. And the protein synthesis is carried out in the cytoplasm.
<image 16: DNA>
Even though the DNA contains the information for specifying the sequences of amino acids in proteins, it does no participate directly in the making of protein molecules. The transmission of information from the DNA inside the nucleus to the cytoplasmic site of protein synthesis is done by the RNA molecules. So that is, Genetic information moves from DNA to RNA and then to the proteins. And this is known as transcription. And the process that uses the information for the assembly of protein is known as translation.
DNA â†’ RNA â†’ Protein
<image 14: RNA>
Each chromosome has thousands of units made of DNA, and these units are known as Genes. A Gene is a sequence of DNA nucleotides that enclose the information that specify the amino acid sequence of a single polypeptide chain which is known as a gene. Therefore, we can say that the gene is a unit of hereditary information.
The complete set of genetic information that is coded in the DNA of a cell in an organism is known as its genome. The human being genome contains around 30,000 to 40,000 genes.
2cii. Role of Chromosomes
When the normal human body cell divides, the chromosomes make duplicates of themselves. And this way, the new generation of cells still has the same DNA which contains 23 pairs of chromosomes with the same gene assemblage. Whereas in the case of sex cells, the male cell which is known as the sperm and the female cell which is known as the ovum, have just 23 single chromosomes in each, instead of being in pairs.
During the process of fertilization, the sperm cell and ovum fuse to form a zygote which has the total number of chromosomes. And from this point onwards, the zygote begins to reproduce and develop itself by normal cell division which is known as mitosis (discussed earlier).
The genetic material of this new cell formed from the zygote will contain the traits of both the parents. It will grow to become another unique individual, but carrying some traits from both the parents. And the expression of how these traits influence the development process is based on how the parents' genes interact with each other.
2ciii. Role of genes
Some genes are of dominant type and other genes are of Recessive type. The presence of one or two dominant genes causes the expression of that particular trait.; the common example is that of the gene of brown eyes in humans which is dominant over the gene for blue eyes. And to display a recessive trait, such as to have blue eyes, both the genes from each parent should be recessive.
There many characteristic traits that need more than one gene, e.g. skin colour is controlled by several genes. And some other traits are influenced by an external influence. E.g. Intelligence may be genetically constructed by it is also determined by the environmental factors. And sex-linked traits are the traits that are related to the X-Chromosome, e.g. Colour blindness.
Genetic defects are also passed on from one generation to another generation by the process of heredity. Many genes that result in diseases have been found out and studied, and this allows for the parents to have the option of genetic counseling and testing during the pregnancy period.
2civ. Role in Physical education
Each cell of the human body contains 46 chromosomes except for the sex cells, the male sperm cell and the female ovum cell fuse together to form the zygote which contains the 46 chromosomes. And then these particular 46 chromosomes are copied into each and every cell that forms the new embryo. And so in this way a child inherits the characteristics of each parent.
So the physical traits of an individual greatly depends on the genetic material that he/she receives form the parent cells. By understanding and studying the genetic background of an individual, one can be able to assess to a great extent what a particular type of human body is capable of doing.
The study of heredity in Physical education can also help in understanding and preventing various gene related diseases or defects. Some defects that are normally detected in a Physical test are E.g. Knock Knee, Colour blindness.
By studying the various genes related to the defects, one can provide genetic counseling to the parents of an unborn child regarding the genetic constitution.
Genetic counseling is the education of individuals about the various inherited disorders.
An exciting international project of the recent years is the Human Genome project which has been involved in mapping all the genes of the human body. And with the advances in DNA mapping, it is possible to get rid of many gene-related diseases.
The use of modern techniques of genetic engineering in the process of heredity can enable use to remove individual genes from a cell. These processes are leading to the treatment known as gene therapy, which allows defective genes to be replaced by normal healthy functioning genes.