Plant cell is a unit or independent tiny or independent tiny or microscopic mass of protoplasm enclosing in it a denser spherical or oval body called the nucleus and bounded by a distinct wall the cell wall. Protoplasm and nucleus are living, while the cell wall is non living; the latter has been formed by the protoplasm to maintain its shape and firmness and to afford necessary protection. Cells vary widely in shape and size. They may be spherical, oval, polygonal, cubical or narrow and elongated.
All cells are enclosed in plasma membrane that serves as their outer boundary, separating the cytoplasm from the external environment. This is called plasma membrane. It allows the cell to take up and retain certain substances while excluding others. All membrances have the same basic molecular organisation. They have double layer of phospholipids interspersed with proteins.
The phospholipids molecules in the plasma membrane are arranged in two parallel layers. Their non polar hydrophobic ends face each other, where as their polar hydrophilic ends are associated with carbohydrates, protein etc.
Plasma membrane also contains several types of lipids like cholesterol. In certain animal cells cholesterol may constitute upto 50 percent of the lipid molecules in plasma membrane. It is absent plasma membrane of most plant and bacteria cells.
In 1972 singer and Nicholson proposed a working model of plasma membrane known as fluid mosaic model. It is the lipid bilayer is retained as the core of the membrane. These lipid molecules are present in a fluid state. Capable of rotating and moving laterally within the membrane. The structure and arrangement of membrane proteins in the fluid mosaic model are like ice bergs in the sea. The proteins occur as a mosaic of discontinuous particles that penetrate deeply into and even completely through the lipid sheet. The components of plasma membrane are mobile and capable of coming together to engage in various types of transient or semi permanent interaction.
A class of proteins that are directly incorporated within the lipid bilayer are intrinsic proteins some of these proteins are believed to provide a channel through which water soluble substances such as ions can pass back and forth between the extra cellular and intra cellular compartment. Extrinsic proteins are a class of proteins located entirely outside the lipid bilayer on extra cellular or cytoplasmic surface. It exhibit loose association with membrane surface. Those proteins which possess lipid or carbohydrate side chains are arranged as mosaics within the cell membrane.
Plasma membrane functions as protection of cell cytoplasm, and to regulate the flow of solutions and material in and out of the cell.
Protoplasm outside nucleus is called cytoplasm. Cytoplasm appears as a semi fluid colloid that fills the cell. The cytoplasm exhibits active streaming movements around the inner surface of the cell. This movement is known as cyclosis. Cytoplasm is composed of several types of organelles occupying half the volume of cell and a fluid of matrix, the cytosol (cell division) in which the organelles reside. The cytosol is a watery solution of slats, sugar, amino acids, proteins, fatty acids, nucleotides and other material. Giving shape and organization to the cytoplasm is a network of protein fibres, the cytoskeleton many organelles and individual molecules of the cytoplasm are thought to be attached to the cytoskeleton.
Observations under electron microscope reveal that the cytoplasm is not a simple colloid since it contains many different kinds of minute organelles and also a mesh of tiny filaments the micro fibrils that form a sort of skeleton giving rigidity to cell and helping unicellular organisms in movement. A variety of them are membrane bound. These organelles are grouped on the basis of membrane. Membrane bound organelles are endoplasmic reticulum, Mitochondria, Golgi apparatus lysogomes, plastids and microbodies. Non membrane organlles are Ribosome, Centriole and vacuole.