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CELL - THE BASIC UNIT OF LIFE

Introduction

A cell is a microscopic, structural and functional unit of living organisms capable of independent existence. (e.g. Ameba). All living things are composed of cells. Some functioning cells come together to form a tissue and tissues collectively form organs. In more complex living organisms, organs work together for the purpose of survival as system. However, in all living organisms, the cell is a functional unit and all of biology revolves around the activity of the cell.

An Animal Cell

The study of cell is impossible without the microscope. The first simple microscope was prepared by Anton Van Leeuwenhoek (1632-1723) who studied the structure of bacteria, protozoa, spermatozoa, red blood cells etc. The word ‘cell’ was first coined by Robert Hooke in 1665 to designate the empty honey-comb like structures viewed in a thin section of bottle cork, which he examined. He was impressed by the microscopic compartments in the cork as they reminded him of rooms in a monastery, which are known as cells. He therefore referred to the units as cells. In 1838, the German botanist Matthias Schleiden proposed that all the plants are made up of plant cells. Then in 1839, his colleague, the anatomist Theodore Schwann studied and concluded that all animals are also composed of animal cells. Schwann and Schleiden studied a wide variety of plant and animal tissues and proposed the "cell theory" in 1839. It stated that "all organisms are composed of cells." But still the real nature of a cell was in doubt. Cell theory was again rewritten by Rudolf Virchow in 1858.

In his theory he said that all living things are made up of cells and that all cells arise from pre-existing cells. It was German biologist Schulze who found in 1861 that the cells are not empty as were seen by Hooke but contain a ‘stuff’ of life called protoplasm.

During the 1950s scientists developed the concept that all organisms may be classified as prokaryotes or eukaryotes. For example, in prokaryotic cells, there is no nucleus; eukaryotic cells have a nucleus. Another important difference between prokaryotes and eukaryotes is that the prokaryotic cell does not have any intracellular components. Bacteria and blue- green algae come under the prokaryotic group, and protozoa, fungi, animals, and plants come under the eukaryotic group.

Modern cell theory

Modern biologists have made certain additions to the original cell theory, which now states that:

1. All organisms are made up of cells.

2. New cells are always produced from pre-existing cells.

3. The cell is a structural and functional unit of all living things.

4.The cell contains hereditary information which is passed on from cell to cell during cell division.

5. All cells are basically the same in chemical composition and metabolic activities.

 A Plant Cell

Structure of cell

Both prokaryotic and eukaryotic cells possess the basic features of a plasma membrane and cytoplasm. The plasma membrane is the outermost surface of the cell, which separates the cell from the environment. The cytoplasm is the aqueous content within the plasma membrane.

Plasma membrane : It is like any other membrane in the cell but it plays a very important function. It forms the border of a cell, so it is also called the cell membrane. It is primarily composed of proteins and phospholipid. The phospholipids occur in two layers referred to as a bilayer. Protein is embedded within the lipid layer, or attached to the surface of it. The plasma membrane is elastic and very fluid because of protein and lipid. Normally the function of the plasma membrane is that of a gate-keeper. It allows certain important substances to enter and exit the cell.

Cytoplasm and organelles : The cytoplasm is a semi-solid substance, which is present in the cell and which gives structure, size, shape and foundation to the cell. It is enclosed by the plasma membrane. Within the cytoplasm are a number of microscopic bodies called organelles that perform various functions essential for the survival of the cell.

Endoplasmic reticulum with the nucleus and the Golgi complex

Endoplasmic reticulum (ER) : is one of the important organelles present in the cytoplasm. Endoplasmic reticulum is a series of membranes, which extend throughout the cytoplasm in eukaryotic cells. In certain cases in ER there are submicroscopic bodies called ribosomes, which are involved in production of proteins.

Rough ER : In this kind of ER the ribosomes are present on the surface. The endoplasmic reticulum is responsible for protein synthesis in a cell. Ribosomes are sub-organelles in which the amino acids are actually bound together to form proteins. There are spaces within the folds of ER membrane are known as cisternae.

Smooth ER : This type of ER does not have ribosomes.

Another organelle is the Golgi body or Golgi apparatus (G.A.). The Golgi body is a series of flattened sacs usually curled at the edges. Proteins, which have formed in ER, are processed in G.A. After processing, the final product is discharged form the G.A. At this time the G.A. bulges and breaks away to form a dropline vesicle known as secretory vesicles. The vesicles move outward to the cell membrane and either insert their protein contents in the membrane, or release their contents outside the cell.

There is another organelle, which is related to the Golgi apparatus called the lysosome. The lysosome is derived from the Golgi body. It is a sac of enzymes in the cytoplasm, used for digestion within the cell. These enzymes break down particles of food taken into the cell and make the food product available for use. There are also cytoplasmic organelles called peroxisomes in the cell, which produces the enzymes to degrade fat molecules.

Mitochondria : is another organelle of the cell. It is called the "power house of the cell" because it stores and releases the energy of the cell. The energy released is used to form ATP (adenosine triphosphate).

Nucleus : Prokaryotic cells don’t have a nucleus but eukaryotic cells have a nucleus situated in the cytoplasm. The nucleus mainly contains DNA ( i.e., Deoxyribonucleic acid). DNA is organized into linear units called chromatin. Genes are the functional segments within the chromosome. Each chromosome consists of approximately 1,000,000 genes. The chromatin is coiled around nuclear proteins called histones. When chromatin is coiled, it forms

chromosomes. Genes contain the coding for all the proteins in a cell of an animal or plant. The nucleus of the cell is surrounded by an outer membrane called the nuclear envelope. The nuclear membrane resembles the plasma membrane in its function. It is also a double layer membrane consisting of two lipid layers similar to those in the plasma membrane. Pores in the membrane allow the internal nuclear part to communicate with the cytoplasm of the cell.

Prokaryotic cells don’t have a nucleus but they do possess DNA, which exists freely in the cytoplasm. In bacteria a single looped chromosome consists of 4,000 genes. In plant cells, organelles called chloroplasts exist. Due to chloroplasts, plants look green in color. The chloroplasts function in the process of photosynthesis.

During this process, chloroplasts convert the energy in the sunlight into energy of carbohydrate molecules. Energy from the sun comes in the form of photons i.e. a package of energy that gets converted into carbohydrate energy. Chloroplasts consist of a green pigment called chlorophyll. Because chlorophyll molecules absorb most wavelengths of light except green, they reflect green light and appear green to our eyes. chlorophyll is normally present in that area of a plant where sunlight can reach easily. For example, the leaves and stem of a plant are green. On the contrary, the roots don’t have chloroplast so they are not green.

Cytoskeleton : is an interconnecting system of fibers and threads and interwoven molecules that give structure to the cell. The main component of the cytoskeleton are microtubules, microfilaments and intermediate filaments. They are all made up of proteins.

Centriole : is another organelle present in the cell. It is cylindrical in shape and always occurs in pairs. Centrioles are involved in cell division. Vacuole: Another organelle seen in the plant cell is the vacuole. The vacuole forms about 75% of the plant cell. In the vacuole, the plant stores nutrients as well as toxic wastes. If pressure increases within the vacuole it can increase the size of the cell. In this case the cell will become swollen. If the pressure increases further the cell will get destroyed.

Many cells have structures attached to them called Flagella or Cilia. Flagella are seen in the single-celled plant and protozoans, and cilia are commonly seen in animal cells. Flagella are long hairlike extensions that extend from the cell and help in locomotion. Animal sperm has flagella, which permit locomotion. Cilia are shorter and more numerous than flagella. Rows of cilia move in waves to move the cell (prokaryotes like paramecia), or to move fluids around the cell (e.g., respiratory epithelial cells). These cells help in the removal of particles from the tract.

Cell Wall : Plant cells possess a cell wall. It is a structure, which is present outside the cell membrane. It is not very thick. In bacteria the cell wall is very thick and rigid: this gives shape to the bacteria. In an eukaryote cell, the cell wall is not identical in different animals. In fungi, the cell wall is made up of chitin, which is a polysaccharide. In plant cell there is no chitin. Cell walls are composed of another polysaccharide called cellulose.

The cell wall provides support to the structure to the cell. It also saves the cell from mechanical pressure. it is not a selective (semipermeable) membrane like the plasma membrane. When bacteria enter the human body the cell wall this is recognized as a foreign substance in the body; this is how our immune system recognizes and destroys bacteria.

Movement through the plasma membrane

The cell membrane separates the cell from the external environment. In order to communicate with the external environment for the purpose of survival (e.g., for the consumption of food, minerals etc.), there is movement in the cytoplasm and plasma membrane. This movement occurs through several mechanisms, which are listed below:

Osmosis : One method of movement through the membrane is osmosis. Osmosis is the movement of water. Osmosis often occurs across a membrane that is semipermeable. A semipermeable membrane allows only certain molecules to pass through while keeping other molecules out. Osmosis is really a type of diffusion involving only water molecules.

Diffusion : Another method of movement through the membrane is diffusion. Diffusion is the movement of molecules from a region of higher concentration to one of lower concentration. This movement occurs due to molecules, which constantly collide with each other. The total effectual momentum of the molecules is away from the region of high concentration to the region of low concentration.

Diffusion is the random movement of molecules. The exchange of molecules (taking place from a higher concentration region to a lower concentration region) leads to the formation of concentration gradient. Diffusion phenomena can be seen by letting a drop of dye into water. The color of the dye gets diffused throughout the water.

Facilitated diffusion : A third method is facilitated diffusion, which occurs across the plasma membrane. This type of diffusion is very specialized. This occurs only in cases where specific proteins in the membrane permit only certain molecules across the membrane. These membrane proteins allow movement in the direction that diffusion would normally take from a region with a higher concentration of molecules to a region with a lower concentration of molecules. No energy use is required for facilitated diffusion.

Active transport : A fourth method for movement across the membrane is active transport. When active transport takes place, a protein moves a certain substance across the membrane, usually from a region of lower concentration to a region of higher concentration. As you know this movement is against the concentration gradient, hence energy is required for this movement. Normally the cell gets its energy from ATP (adenosine triphosphate). For example, in cardiac muscles, active transport takes place. In these cells, sodium ions are constantly transported out of the cell. The cellular compartment is a region of high concentration of sodium ions. Buildup of electrically charged ions allows changes in voltage over the cell membrane, which affects contraction of muscle cells.

Endocytosis : This another mechanism of movement across the plasma membrane. In this type, a small patch of plasma membrane encloses particles or tiny volumes of fluid, which are at or near the cell surface. The membrane enclosure then sinks into the cytoplasm and breaks off from the membrane, forming a vesicle that moves into the cytoplasm. When the vesicle contains particulate matter, the process is called phagocytosis. When the vesicle contains liquids or droplets of fluids the process is called pinocytosis.