Write down the structure of an animal cell.

STRUCTURE OF AN ANIMAL CELL: 

Cell Membrane: This is the outermost boundary of an animal cell. It is also called the plasma membrane. In fact, a cell membrane surrounds the cytoplasm and nucleus in all types of cells. However, in plants and bacteria, cell membrane itself is surrounded by a cell wall. Cell membranes are composed of lipids and proteins (lipoproteins). A cell membrane is selectively or semi-permeable.

Fluid Mosaic Model of Cell Membrane:

Singer and Nicholson proposed a model of plasma membrane in 1972 According to this model:
(1) Lipids are in the form of a fluid bilayer.
(ii) Lipids are either phospholipids or glycolipids suspended in water.
(iii)• Protein molecules float like icebergs in the sea in the lipid bilayer.
(iv) Proteins are completely or partially embedded in the lipid bilayer forming a mosaic arrangement

Functions: 

(1) Cell membrane regulates selective movement of molecules into and out of the cell. It allows certain molecules to pass through it while others can not pass through it.

(2) Cell membrane takes in material by infolding in the form of vacuoles. It is called endocytosis.

Cytoplasm:

It is a viscous opaque (translucent) fluid portion of the cell which is present between the cell membrane and nuclear membrane. It consists of an aqueous ground substance called cytosol and granular portion called cytoplasmic organelles Its chemical analysis shows that it is composed of organic compounds such as proteins. carbohydrates, lipids (fats), enzymes and some inorganic compounds, e.g. water and salts. Some important cytoplasmic organelles found in eukaryotic cells are as under:
1. endoplasmic reliculum.
2. Golgi complex.
3. mitochondria.
4. plastids (in case of plant cells).
5. ceritrioles.
6. ribosomes.
7 vacuoles.

Functions: 

 
(1) It serves as a storehouse of vital chemicals.
(2) It is a site of metabolic reactions like protein synthesis, glycolysis, etc.
(3) Many reactions can occur at the same time in different regions of the cytoplasm.

Mitochondria (singular mitochondrion):

They are an important organelle of a eukaryotic cell because they provide energy to the cell and are  called the powerhouse of the cell. 

They are generally rod like or bean shaped bounded by double membrane. Their outer membrane is smooth while the inner membrane is folded. These infoldings are called cristae while the fluid present inside is called matrix. The number of mitochondria in a cells relates to its activities. In different animal cells, number of mitochondria differ. The cells which are more active e.g. liver cells have more than one thousand mitochondria while ear lobes have very small numbers only 50.

Function:

(1) Mitochondria play an important role in respiration.
(2) They contain enzymes which break down the food for the production of energy.

Endoplasmic Reticulum:

It is a network of channels extending throughout cytoplasm from plasma membrane to nuclear membrane. They are tube like. They are of two types.
(1) smooth endoplasmic reticulum (SER): It is nongranular because ribosomes are not attached to it.
(2) rough endoplasmic reticulum (RER): It is granular as ribosomes in the form of small granules are attached to it.

Function:

(1) Endoplasmic reticulum plays an important role in the transport of material from one part of the cell to the other.
(2) It is involved in the synthesis of proteins.
(3) It provides mechanical support to the cell so that its shape is maintained.
(4) It detoxifies the harmful drugs.

Nucleus (Karyon):

It was discovered by Robert Brown in 1831. It may be spherical or irregular in shape. In animal cells, it is usually present in the centre but in plant cells due to the presence of a large vacuole, it is pushed towards the cell membrane. Nucleus is surrounded by a double membrane called the nuclear membrane. This membrane has a large number of pores. Nucleus is filled with a gel like substance called nucleoplasm. The nucleoplasm contains nucleoli and a network of thread like structures called chromatin network. The threads of chromatin become prominent during cell division. Each thread is called chromosome. Chromosomes are made up of deoxyribonucleic acid (DNA) and proteins. DNA plays an important role in the inheritance of characters as well as in controlling and regulating the activities of cells. The number of chromosomes in the cells of a particular species always remains constant. It is the characteristic of that species.

Function:

Nucleus controls all the activities of cells.

GOIgi Complex:

They were discovered by Camillo Golgi and thus called Golgi complex or Golgi bodies or Golgi apparatus. They are a set of smooth membranes that are stacked into flattened, fluid filled sacs or vesicles containing carbohydrates, glyco-proteins and enzymes. They are especially prominent in glandular cells. In animal cells, usually a Golgi apparatus is found in each cell. In plant cells, there may be more.

Function:

(1) They are mainly concerned with cell secretions.
(2) In plants they synthesize cell walls.

Centrioles or Centrosome:

A rounded structure, the centrosome is present in the cytoplasm near the nucleus in animals cells. A centrosome contains two centrioles. Higher plants lack centrioles. Each centriole consists of a cylindrical. array of nine rows of microtubules. Both the centrioles lie perpendicular to one another.

Function:

They form fibrous protein spindles, which help in movement of chromosomes towards poles during animal cell division.

Ribosomes:

They are granules, which are rich in ribonucleic acid (RNA). They are found either freely suspended in the cytoplasm or attached to the surface of rough endoplasmic reticulum. They are the only organelle found in prokaryotic cells. They are the only organelle, which are not bound by a membrane.

Function:

They serve as sites for protein synthesis and hence called the protein factories of the cell.

Nucleolus:

It is found inside the nucleus. It has got no membrane. There may be one or more nucleoli in the nucleus. It disappears during the cell division and reappears later.

Functions:

1. The ribosomal RNA is synthesized and stored in the nucleolus.
2. It is the nucleolus where ribosomes are assembled and exported to cytoplasm via nuclear pores.

Vacuoles:

They are liquid filled cavities in the cytoplasm. They are surrounded by a membrane called tonoplast. In plant cells, there is usually one large permanent vacuole. In animal cells, there may be many small temporary vacuoles.

FUNCTIONS:

(1). They are connected with  the storage of cell sap.

(2) .In small organisms , water and waste materials are excreted through special vacoules called contractile vacoules and food is digested in food vacoules.