Biology - The structure of cell

The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life.[1] Some organisms, such as most bacteria, are unicellular (consist of a single cell). Other organisms, such as humans, are multicellular.
In form 1, we learned that the living component of a cell is called protoplasm, which is the combination of the cytoplasm and nucleus. The protoplasm is being surrounded by the plasm membrane. And now, in form 4, we have learned that the cytoplasm contains structures called organelles. Every organelles perform specific functions with enable the cell to function as a unit of life.
Cell membrane
The cell membrane is a general term that refers to all membranes that found in the cell. The membrane at the surface that enclose the contents of the cell is called plasma membrane. The plasma membrane is the combination of the protein and phospholipids. It form a boundary that separates the content of the cell from the outer environmen. The plasma membrane is semi- permeable which act as a selective barrier that can regulates the movement of these substances into and out the cytoplasm.
Cell wall
The cell wall is the rigid outer layer that surrounds the plasma membrane of plant cells. It is composed of the cellulose, a tough and fibrous carbonhydrates. The cell wall is permeable to allow substances move freely into and out of the cell. It gives a shape to the cell and provide mechanical support to the cell. It also protects the plant cell from rupturing due to excessice intake water.
Cytoplasm
The cytoplasm is a region between the nucleus and the plasma membrane. It consists of a jelly-like matrix in which the organelles of the cell is suspended. Cytoplasm contains organic and inorganic substances and act as a medium for biochemical reactions in the cell. It also provides the external environment to the organelles.
Nucleus
The nucleus appears a large, dense, spherical organelle enclosed by a nuclear membrane. It contain the nucleoplasmand a dark spehrical structure called the nucleolus. Chromatin, the form of choromosomes take when the cell is not dividing, is contained withing the nucleoplasm. Chromosomes carry DNA which determines the characteristic and functions of the cell. The nucleus control all the activities which takes place in cell.
Vacuole
A vacuole is fluid-filled sac which is surrounded by tonoplast, a semi-permeable membrane. The fluid contain a vacuole is called cell sap. A typical plany cell has a large central vacuole while a young plant cell has a numerous smaller vacuoles. Freshwater microorganism such as Paramecium sp. has contractile vacuole and food vacuole. The vacuole act as a storage place in a cell. The cell sap contains water, organic acids, sugars, amino acids, mineral salts, waste substances, pigments and metabolic by- products.The cell sap supports herbaceuos plants to prevent them wilt.
Mitochondria
Mitochondria is a small spherical or cylindrical-shaped organelles. Mitochondria are involved in cellular respiration. It has double layer. The process of releasing energy through the oxidation of food substance such as glucose. Energy is generated in the form of ATP which can be used readily be cell.
Ribosomes
It is compact, spherical organelles. Each ribosomes is composed of two-sub units, each consisting of RNA and protein. It is found either suspended freely in the cytoplasm or attached to the surface of the rough endoplasmic reticulum Ribosomes are the sites of protein synthesis. The instruction required for the synthesis of these proteins is stored in the chromatin.
Endoplasmic reticulum (ER)
Consist of an extensive network of folded membranes which form interconnected tubes and sacs in the cytoplasm. The membranes of the ER are continuous with the nuclear envelope.There are two types of ER: smooth ER which does not has ribosomes and also rough ER that has ribosomes. The rough ER transport protein made by the ribosomes while the smooth ER synthesis lipids and carries out detoxification of drugs and metabolic products.
Golgi apparatus
It consists of stack of flattened membrane-bound sacs. New membrane is continuously added to one end of the Golgi apparatus and buds off as vesicles at the other end. Functions as a processing packaging and transport centre of carbohydrates, proteins and gylcoproteins.
Lysosomes
It is a small, spherical sacs surrounded by a single membrane. It also contains hydrolytic enzymes which is a digestive enzymes. It function as digestive compartments in a cell. The hydrolytic enzymes digest or break down complex organic molecule. In certain unicellular organism, lysosomes fuse with food vacuoles and release their enzymes into these vacuoles to digest the contents of the vacuole. The breakdown products are absorbed into the cytoplasm of the cell.
Centrioles
Centrioles is a small cylindrical structures that occurs in pairs, next to the nucleus in animal cells. It is composed of a complex arrangement of microtubules. Centrioles does not present in plant cells. Centrioles form spindle fibres during cell division in animal cells.
Chloroplast
Choroplast are lens-shaped organelles. They also have double membrane. The internal chloroplast membranes contain the green pigment, chlorophyll. Chlorophyll captures the sunlight energy and converts light energy into chemical energy during photosynthesis. The green pigment of chlorophyll gives plants their green colour.

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