Cell is the basic living structural and functional unit of the body.
Cytology: – It is a branch of science concerned with a study of cells.
Cell Theory explains about
- All living organisms are composed of cell and cell products.
- Cell is the basic unit of structure & function of all living organisms.
- All cells come from the division of pre existing cell.
- An organism as a whole can be understood through the collective activities & interactions of its cells.
We can divide the cell in to four principal parts: –
- Plasma (cell) membrane: it is the outer lining, limiting membrane separating the cell internal parts from extra cellular materials & external environment.
- Cytoplasm: cytoplasm is the substance that surrounds organelles and is located between the nucleus and plasma membrane
- Organelles: these are permanent structures with characteristic morphology that are highly specialized in specific cellular activity.
- Inclusions: they are the secretions and storage products of cells.
Figure 1- Cell
1.1 Cell membrane (Plasma membrane)
- Each cell has a limiting boundary, the cell membrane, plasma membrane or plasmalemma.
- It is a living membrane, outermost in animal cells.
- Plasma membrane maintains the integrity of the cell.
- It keeps the cell and its contents separate and distinct from the surrounding.
- It is a double layered measuring about 4.5 nm and made of phospholipids, cholesterol, glyco-lipid, & carbohydrate (oligosaccharides).
- The bi-layer is self-sealing. If a needle is injected and pulled out, it automatically seals.
- The plasma membrane is made of proteins and lipids and several models were proposed regarding the arrangement of proteins and lipids. The fluid mosaic model proposed by Singer and Nicholson (1972) is widely accepted.
According to the fluid mosaic model,
- The plasma membrane is composed of a lipid bilayer of phospholipid molecules into which a variety of globular proteins are embedded.
- Each phospholipid molecule has two ends, an outer head hydrophilic i.e. water attracting, and the inner tail pointing centrally hydrophobic, i.e. water repelling.
- The protein molecules are arranged in two different ways:
- Peripheral proteins or extrinsic proteins: these proteins are present on the outer and inner surfaces of lipid bilayer.
- Integral proteins or intrinsic proteins: These proteins penetrate lipid bilayer partially or wholly.
Figure 2- Plasma Membrane
- The plasma membrane encloses the cell contents.
- It provides cell shape (in animal cells) e.g. the characteristic shape of red blood cells, nerve cells, bone cells, etc
- It allows transport of certain substances into and out of the cell but not all substance, so it is termed selectively permeable.
- Separate cell from one another.
- Provide an abundant surface on which chemical reaction can occur.