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| Structure of the cell |
INTRODUCTION
Cells are basic units of life. All living organisms are com- posed of many blocks of cells. Each single has all the characteristics of life. Cell is defined as structural and functional unit of the living body. The space outside the cell is called extracellular space.
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 such as bacteria or toxic substances into the body.
6. Reproduces by division. There are some exceptions such as 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.
Primary Tissues
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.
Types of Organs
Organs are of two types, namely tubular or hollow or- gans 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. Repro- ductive system is involved in the reproduction of species. Endocrine system is concerned with growth of the body and regulation and maintenance of normal life. Musculo- skeletal 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 sur- rounding the nucleus.
Thus, the structures of cell are of three categories:
l. Cell membrane.
II. Cytoplasm.
III. Nucleus.
CELL MEMBRANE
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| Cell membrane |
Cell membrane is a protective sheath, enveloping the cell body. It is also known as plasma membrane or plasma- lemma. 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 111A.
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. Electron microscopic study reveals three layers of cell Major 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 sub- stances. The other two layers are protein layers formed by proteins. Cell membrane contains some carbohydrate molecules also. 2. "Danielli-Davson model' was the first proposed basic accepted by scientists for many years. This model was basically a 'sandwich of lipids' covered by proteins
Structural Model of the Cell Membrane
1. Danielli-Davson model
Danielli-Davson 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 on both sides.
2. Unit membrane model
In 1957, JD Robertson replaced 'Danielli-Davson model' by 'Unit membrane model' on the basis of electron mi- croscopic 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 pro- teins are found to float in the lipid layer instead of forming the layers of the sandwich type model.
Lipid Layer of the Cell Membrane
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| Lipids of the cell membrane |
The central lipid layer is a bilayered structure. This is formed by a thin film of lipids. The characteristic feature of lipid layer is that, it is fluid in nature and not a solid structure. So, the portions of the membrane move from one point to another point along the surface of the cell.The materials dissolved in lipid layer also move to all areas of the cell membrane.
Major lipids:
1.Phospholipids
2.Cholesterol
1. Phospholipids
Phospholipids are the lipid substances containing phosphorus and fatty acids. Phospholipid molecules are arranged in two layers. Each phospholipid molecule resembles the headed pin in shape. The outer part of the phospholipid molecule is called the head portion and the inner portion is called the tail portion. repelled by water (hydrophobic). Head portion is the polar end and it is soluble in water and has strong affinity for water (hydrophilic). Tail portion is the non-polar end. It is insoluble in water and Two layers of phospholipids are arranged in such a way that the hydrophobic tail portions meet in the center of the membrane. Hydrophilic head portions of outer layer face the ECF and those of the inner layer face ICF mi- (cytoplasm).
2. Cholesterol
Cholesterol molecules are arranged in between the phospholipid molecules. Phospholipids are soft and oily lipids in the membrane. So, cholesterol is responsible for structures and cholesterol helps to 'pack' the phospho- the structural integrity of lipid layer of the cell membrane.
Functions of Lipid Layer in Cell Membrane
Lipid layer of the coll mombrane is a semipermeable membrane and allows only the fat-soluble substances to pass through it. Thus, the fat-soluble substances like oxygen, carbon dioxido and alcohol can pass through this lipid layer. Tho water-soluble substances liko glucose, urea and electrolytes cannot pass through this layer.
Protein Layers of the Cell Membrane
Protein layers of the cell membrane are electron-dense layers. These layers cover the two surfaces of the contral lipid layer. Protein layers give protection to the central lipid layer. The protein substances present in those layers are mostly glycoproteins.
Protein molecules are classified into two categories:
1. Integral proteins or transmembrane proteins.
2. Peripheral proteins or peripheral membrano proteins.
1. Integral proteins
Integral or transmembrane proteins or intrinsic proteins are the proteins that pass through entire thickness of cell membrane from one side to the other side. These proteins are tightly bound with the cell membrane.
2.Peripheral proteins
Peripheral proteins or peripheral membrane proteins or extrinsic proteins are the proteins, which aro partially embedded in the outer and inner surfaces of the cell mem- brane and do not penetrate the cell membrane. Peripheral proteins are loosely bound with integral proteins or lipid layer of cell membrane. So, these protein molecules dis- sociate readily from the cell membrane.
Types of peripheral proteins
Peripheral proteins are of two types:
i. Intracellular peripheral proteins, which are located on inner surface of cell membrane. Examples are receptors, transport proteins and some enzymes.
ii. Extracellular peripheral proteins that are situated on outer surface of cell membrane. Examples are anti- gens, adhesion proteins and some enzymes.
Functions of Proteins in Cell Membrane
1. Integral proteins provide the structural integrity of the cell membrane.
2. Channel proteins help in the diffusion of water-solu- ble substances such as glucose and electrolytes.
3. Carrier or transport proteins help in the transport of substances across the cell membrane by means of active or passive transport.
4. Some carrier proteins act as pumps, by which ions are transported actively across the cell membrane.
5. Receptor proteins serve as the receptor sites for hor- mones and neurotransmitters.
6. Some of the protein molecules form the enzymes and control chemical (metabolic) reactions within the cell membrane.
7. Some proteins act as antigens and induce the pro- cess of antibody formation.
8. Some proteins called cell adhesion molecules are responsible for attachment of cells to their neighbors or to basal lamina.
Carbohydrates of the Cell Membrane
Some of the carbohydrate molecules present in cell mem- brane are attached to proteins and form glycoproteins (proteoglycans). Some carbohydrate molecules are attached to lipids and form glycolipids. Carbohydrate molecules form a thin and loose cover- ing over the entire surface of the cell membrane called
Functions of Carbohydrates in Cell Membrane
1. Carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell.
2. Glycocalyx from the neighboring cells helps in the tight fixation of cells with one another.
3. Some carbohydrate molecules function as the recep- tors for some hormones.
FUNCTIONS OF CELL MEMBRANE
1. Protective Function- Cell membrane protects the cytoplasm and organelles present in the cytoplasm.
2.Selective Permeability- Cell membrane acts as a semipermeable membrane, which allows only some substances to pass through it and acts as a barrier to other substances.
3.Absorptive Function- Nutrients are absorbed into the cell through the cell membrane.
4.Excretory Function- Metabolism and other waste products from th cell are excreted out through the cell membrane.
5.Exchange of gases-Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane.
6. Maintenance of shape and size of cell- Cell membrane is responsible for the maintenance of shape and size of the cell.
Read more about cell - Organelles Of Cell And Gene
Source K Sembulingam's physiology
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