General Characteristics of Cell

CELL

All the living things are composed of cells. A single cell is the smallest unit that has all the characteristics of life. Cell is defined as the structural and functional unit of the living body.

General Characteristics of Cell

Each cell in the body:

1. Needs nutrition and oxygen

2. Produces its own energy necessary for its growth, repair and other activities

3. Eliminates carbon dioxide and other metabolic wastes

4. Maintains the medium, i.e. the environment for its survival

5. Shows immediate response to the entry of invaders like bacteria or toxic substances into the body

6. Reproduces by division. There are some exceptions like neuron, which do not reproduce.

TISSUE

Tissue is defined as the group of cells having similar function. There are many types of tissues in the body. All the tissues are classified into four major types which are called the primary tissues. The primary tissues include:

1. Muscle tissue (skeletal muscle, smooth muscle and cardiac muscle)

2. Nervous tissue (neurons and supporting cells)

3. Epithelial tissue (squamous, columnar and cuboidal epithelial cells)

4. Connective tissue (connective tissue proper, cartilage, bone and blood).

ORGAN

An organ is defined as the structure that is formed by two or more primary types of tissues, which execute the functions of the organ. Some organs are composed of all the four types of primary tissues. The organs are of two types, namely tubular or hollow organs and compact or parenchymal organs. Some of the organs in the body are brain, heart, lungs, stomach, intestine, liver, gallbladder, pancreas, kidneys, endocrine glands, etc.

SYSTEM

The organ system is defined as group of organs that work together to carry out specific functions of the body. Each system performs a specific function. Digestive system is concerned with digestion of food particles. Excretory system eliminates unwanted substances. Cardiovascular system is responsible for transport of substances between the organs. Respiratory system is concerned with the supply of oxygen and removal of carbon dioxide. Reproductive system is involved in the reproduction of species. Endocrine system is concerned with growth of the body and regulation and maintenance of normal life.Musculoskeletal system is responsible for stability and movements of the body. Nervous system

controls the locomotion and other activities including the intellectual functions.

STRUCTURE OF THE CELL

Each cell is formed by a cell body and a membrane covering the cell body called the cell membrane. Cell

body has two parts, namely nucleus and cytoplasm surrounding the nucleus. Thus, the structure

of the cell is studied under three headings:

1. Cell membrane

2. Cytoplasm

3. Nucleus.

CELL MEMBRANE

Cell membrane is a protective sheath, enveloping the cell body. It is also known as plasma membrane or

plasmalemma. This membrane separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF). The cell membrane is a semipermeable membrane. So, there is free exchange of certain substances between ECF and ICF. Thickness of the cell membrane varies from 75 to 111Å .

COMPOSITION OF CELL MEMBRANE

Cell membrane is composed of three types of substances:

1. Proteins (55%)

2. Lipids (40%)

3. Carbohydrates (5%).

STRUCTURE OF CELL MEMBRANE

On the basis of structure, cell membrane is called a unit membrane or a three-layered membrane. The electron microscopic study reveals three layers of cell membrane, namely, one central electron-lucent layer and two electron- dense layers. The two electron-dense layers are placed one on either side of the central layer. The central layer is a lipid layer formed by lipid substances. The other two layers are protein layers formed by proteins. Cell membrane contains some carbohydrate molecules

also.

Structural Model of the Cell Membrane

1. Danielli-Davson model

‘DanielliDavson model’ was the first proposed basic model of membrane structure. It was proposed by

James F Danielli and Hugh Davson in 1935. And it was accepted by scientists for many years. This model was basically a ‘sandwich of lipids’ covered by proteins on both sides.

2. Unit membrane model

In 1957, JD Robertson replaced ‘DanielliDavson model’ by ‘Unit membrane model’ on the basis of electron microscopic studies.

3. Fluid mosaic model

Later in 1972, SJ Singer and GL Nicholson proposed ‘The fluid mosaic model’. According to them, the membrane is a fluid with mosaic of proteins (mosaic means pattern formed by arrangement of different colored pieces of stone, tile, glass or other such materials). This model is accepted by the scientists till now. In this model, the proteins are found to float in the lipid layer instead of forming the layers of the sandwich-type model.