UPSC Biology Cell Cycle and Cell Division Cell

Cell

 

 

Components of Cell?

In the living organism there are two types of cellular organizations. If we look at very simple of cellular like bacteria and blue-green algae, we will discover cells that have no defined nucleus, these are prokaryotes cells. The cells which have definite nucleus are known as eukaryote. But the things which both have in common is that there are compartments surrounded by some type of membranes. These are called cell membranes.

 

Cell membranes: It is like a plastic bag with some tiny holes that bag holds all of the cell pieces foreign particles outside the cell. The holes are there to let some things move in and out of the cell. Compounds called proteins and phospholipids make the basic bag. The proteins are found around the holes and help move molecules in and out of the cell. Substances like  and  can move across the cell membranes by a process called diffusion. Diffusion is a process of movements of substance from a region of high concentration to a region where its concentration is low. Water also obeys the law of diffusion. The movement of water molecules is called osmosis.

 

Cytoplasm: It is the fluid that fills a cell. Scientists used to call the fluid proto plasm, cytoplasm contain many specialized cell called organ cells. Each of these organ cells performs a specific function for the cell.

 

Cell organelles: Organelles are living part of the cell have definite shape, structure and functions. To keep their function different from each other these organelles use membranes bound little structure with in themselves. Some of the important organelles are:

 

(a) Endoplasmic reticulum: It is a network of tulsular membranes connected at one plasma membranes. Endoplasmic reticular (ER) are two types:-rough endoplasmic reticular (RER) and Smooth endoplasmic reticulum (SER).

 

Functions of ER:

• It forms the supporting skeleton frame work of the cell.
• It provides a pathway for distribution of nuclear material.
• It provides surface for various enzymatic reactions.

 

(b) Ribosomes: It synthesis protein, and ER sent these protein in various part of the cell. Where as SER helps in the manufacture of fats.

 

Functions of these proteins and fats:

• Protein and fat (lipid) help in building the cell membranes. This process is known as membranes biogenesis.
• Some other protein and fat functions as enzymes and hormones.
• SER plays a crucial role in detoxifying many poisons and drugs.

 

(c) Golgi apparatus: It is found in most cell. It is another packaging organelle like the molecules and combines them to make molecules and combines them to make molecules that are more complex. It then takes those big molecules, packages them in vesicles and either stores them for faster use or sends them out of the cell.

 

Other functions:

• Its functions include the storage modifications and packaging of products in vesicles.
• It is also the organ Elle that builds lysosomes (cells digestion machines).

 

(d) Lysosomes: It is a kind of waste disposal system of the cell.

 

It helps to keep the cell clean by digesting any foreign material.

• Old organs cell end up in the lysosomes.

When the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore lysosomes are also known as the “suicide ways” of the cell.

 

(e) Mitochondria: It is known as the power house of the cell. The energy required for various chemical activities headed for life is released by mitochondria in form of ATP (adenosine-tri-phosphate) molecules.

 

• ATP is known as the energy currency of the cell. The body uses energy are stored in ATP for making new chemical compounds and for mechanical work.
• Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes, therefore mitochondria are able to make their own protein.
• Mitochondria is absent in bacteria and the red blood cells of mammals and higher animals.

 

(f) Centrioles: It is a micro-tubular structure; centrioles are concerned with cell-division. It initiates cell division.

 

(g) Plastids: These are present only in plant cells. There are two types of plastids:- chromoplastes (colour plastids) and leucoplast (white or colourless plastids).

 

• Chloroplast impact colour to flower and fruits.
• Plastids are self-replicating. i.e. they have the power to divide, as they contain DNA, RNA and ribosomes.
• Plastids contains the pigment chlorophyll that is known as chloroplast. It is the site for photo synthesis.

The above mentioned cell organelles are the living part of the cell but there are some non - living parts with in the cell like vacuoles and granules.

 

Vacuoles: It is a fluid filled spaces enclosed by membranes. It is a storage sacs for solid or liquid contents. It stores excess water, minerals, food substance, pigments and waste products. Its size in animal is small and in plant it is big. Many substances of importance in the life of the plant cell are stored in vacuoles. These are amino acids sugars. It also Contain Various organic acid and some proteins.

 

Granules: It is not bounded by any membranes. It stores fats, proteins and carbohydrates.

 

Cell nucleus: The cell nucleus acts like the brain of the cell. It helps control eating, movement and reproduction. Not all cells have a nucleus. The nucleus contain, the following components:

 

(a) Nuclear envelope: It surrounds the nucleus and all of its contents nuclear envelope is a membrane similar to the cell membranes around the whole cell.

 

(b) Chromatin: When the cell is in resting state there is something called chromatin in the nucleus. Chromatin is made up of DNA, RNA and nucleus protein. DNA and RNA are the nucleus acids inside the cell. When the cell is going to divide, the chromatin become very compact. It condenses when the chromatin comes together we can see the chromosomes.

 

(c) Chromosomes: Chromosomes make organism what they are. They carry all the information used to help a cell grow, thrive and reproduce.

 

• Chromosomes are made up of DNA.
• Segments of DNA in specific patterns are called genes.
• In prokaryotes, DNA floats in the cytoplasm in an area called the nucleoid.
• Chromosomes are not always visible. They usually sit around uncalled and as loose shards called chromation.
• When it is time for all cells to reproduce, they condense and wrap up very tightly. The tightly round DNA in the chromosome.
• Chromosomes are usually found in pairs.
• Human Beings probably have 46 chromosomes (23 pairs).
• Pear only have 12, a dog has 78 chromosomes.
• The number of chromosomes is not related to the intelligence or complexity of the creature.

 

(d) Nucleolus: It is a dense spherical granule contained within the nucleus, its size is related to the synthetic activity of the cell. Neurons cell have a comparatively larger nucleate than those cell have no synthetic activity. The nucleus stores proteins.

 

Cell Division

Organisms grow and reduce through cell division. Plants continue to grow by cell division all their lives. But in most animals cells divide more slowly once the body taken shape. There are two methods of replication mitosis and meiosis.

 

(a) Mitosis: The main theme of this replication is that mitosis is the simple duplication of a cell and all of its parts. It duplicates its DNA and the two new cells (daughter cells) have the same pieces and generic code. Beyond the idea that two identical cells are created, these are five steps in this process. You should remember the term PMATI. It breaks down to :

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
5. Interphase

 

The Phases:

(i) Prophase: a cell gets the idea that it is time to divide. First it has to get everything ready. Cell need to duplicate DNA, get certain places in the right position (centrioles) and generally prepare the cell for the process of mitotic division.

(ii) Metaphase: The DNA lines up along a central axis and then DNA condensed into chromosomes.

(iii) Anaphase: Here the separation begins. Half of the chromosomes are pulled to one side of the cell half to go the other way.

(iv) Telophase: Now the division is finishing up. We have now two separate cells each with half of the original DNA.

(v) Interphase: This is the normal state of the cell.

 

(b) Meiosis: It’s for sexual reproduction. The main theme of meiosis is that there are two cell division. Mitosis has one division and meiosis has two divisions in this process four cells are created where there was originally one.

 

 

Meiosis happens when its time to reproduce an organisms. The steps of meiosis are very simple. When we break it down its just two PMATI’s in a row. The interphase that happens between the two processes is very short and the DNA is not duplicated. Meiosis is the great process that shuffles the cell’s gene-sis around. Instead of creating two new cells with equal number of chromosomes (like mitosis). The cell does a second division soon after the first. This second division divides the number of chromo-somes in half. Scien-tists call this process meiosis I and II, but its just two PMATI’s.

 

• Meiosis I: This is basically live PMATI of a regular mitosis. Pairs of chromosomes are lined up at the centre of the cell and then pulled up the centre of the cell and then pulled to each side. Meiosis is a bit different because there are something called crossing-over happens with the DNA. This crossing over is an exchange of genes. The genes are mixed up not resulting in a perfect duplicate like mitosis. The cell divides, having two new cells with a pair of chromosomes each. Since this is meiosis. There begins again.

 

• Meiosis II: In this division the DNA that remains in the cell begins to condense and form short chromosomes and the centre of the cell and the centrioles are in position for the duplication. Each one splits into two pieces. They don’t divide up the DNA between the cells. They split the DNA that exits. Each daughter cell will get one half of the DNA needed to make a functioning cell. When it’s all over we left with four haploid cells (means half the regular number) that are called gametes. The eventual purpose of the gametes. The eventual purpose of the gametes will be to find other gametes with which they can combine.

 

Some important facts regarding cells:

• Nerve cells in animals are the longest cells.

 

• Smallest human cell is red blood cell.

 

• Largest human cell is female ovum.

 

• The single largest cell in the world is of an ostrich.

 

• The smallest cells are those of the mycoplasma.

 

• Every minute about 3 millions cells in our body die.

 

• Sieve tube in plants and the mature mammalian red blood cells do not have a nucleus.

 

• The red blood cell carries respiratory gases.

 

• Sieve cells in plants transport nutrients in plants.

 

• The lysosomal enzymes of the sperm cells digest the limiting membranes of the ovum (egg). Thus the sperm is able to enter the ovum.

 

• During the transformation of tadpole into frog. The embryonic tissues like gills and tail are digested by the lysosome.

 

• Mitochondria contain DNA, hence capable of replication.

 

• Matrix is a transparent, homogenous semifluid substance. In its active state. It remains saturated with water.

 

Comparisons between Plant Cell and Animal Cell

 

 

Part of cell	Plant	Animal
Cell membranes	present	present
Endoplasmic reticulum	present	Present but absent in RBC and embryonic cell.
Ribosome	present	Present
Mitochondria	present	Present but absent in RBC and bacteria
Golgi complex	present	Present but absent in mature RBC
Nucleus	present	present
Granules.	present	present

 

 

• Nucleus is absent in mature mammalian red blood cells and sieve tubes in the phloem tissue of vascular tube.

 

 

Plant Cell		Animal Cell
1.	Nucleus elliptical in shape.	1.	Nucleus rounded in shape
2.	Mitochondria fewer.	2.	Mitochondria numerous.
3.	Plant cells do not burst if placed in hypotonic solution.	3.	Animal cells usually burst if placed in hypotonic solution
4.	Centrioles absent except in lower plants.	4.	Centrioles present.
5.	Spindle formed during cell division is an astral type.	5.	Spindle formed during cell division is of amphiastral type.
6.	Golgi body has distyosomes.	6.	Golgi apparatus consists of a single complex.
7.	Lysosomes rare.	7.	Lysosomes present in animal cells.
8.	Glyoxysomes present.	8.	Glyxosomes absent.
9.	Crystals of inorganic substances occur inside.	9.	Crystals do not occur.
10.	Adjacent cells connected through	10.	Adjacent cells connected by a number of junctions.
11.	Cytokinesis by middle lamella.	11.	Cytokinesis by cleavage.

 

 

Dissimilarities:

Cell part	Plant	Animals
Cell wall	present	absent
Lysosomes	absent	present
Centrioles	absent	present
Plastids	present	absent
Vacuoles	present	absent