THE FUNDAMENTAL UNIT OF LIFE

PRESENTED BY

ANIL KUMAR

TGT ( Sc. )

INSIDE

• CELL AS A BASIC UNIT OF LIFE
• PROKARYOTIC & EUKARYOTIC CELLS
• STRUCTURE OF CELL
• PLASMA MEMBRANE
• NUCLEUS
• CYTOPLASM
• DIFFERENCES BETWEEN PLANTS & ANIMAL CELLS

CELL AS A BASIC UNIT OF LIFE

• Our earth is inhabited by different kinds of living organisms, which look very different from each other. These living organisms are bacteria, protista (Amoeba, Chlamydomonas), fungi, plants and animals. The bodies of living organisms are made up of microscopic units called cells. The cell has same central position in biology as the atom in the physical sciences. The cell is the basic structural and functional unit of living organisms.

• On the basis of number of cells an individual is made up of organisms, are divided into two categories : - 1.UNICELLULAR ORGANISMS :- Organisms made up of one cell only

2.MULTI-CELLULAR ORGANISMS : - Organisms made up of more than one cell.

PROKARYOTIC & EUKARYOTIC CELLS

• According to above criteria, all living organisms present on earth can be classified into following two types :

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NON-CELLULAR ORGANISMS

• Non cellular organisms which do not contain any cell in their body organisation e.g., viruses. Viruses lack any membrane and hence do not show characteristics of life until they enter a living body (i.e. prokaryotic cell or eukaryotic cell) to use its cell machinery to reproduce.

CELLULAR ORGANISMS

• Cellular organisms which contains either one or many cells in their bodies, e.g., bacteria, plants and animals.
• Cellular organisms are again divided into following two main types :

1. Prokaryotes

2. Eukaryotes

STRUCTURE OF CELL

• The size, shape, number and volume of the cell vary greatly among unicellular and multicellular organisms.

CELL SHAPE

• The basic shape of eukaryotic cell is spherical but the shape of cell is ultimately determined by the specified function of the cell. Thus, the shape of the cell may be variable (i.e. frequently changing its shape) or fixed. Variable or irregular shape occurs in amoeba and white blood cells or leucocytes. In fact, leucocytes are spherical in the circulating blood, but in other conditions they may produce pseudopodia and become irregular in shape.

CELL SIZE

• The size of different cells ranges between broad limits. Some plant and animal cells are visible to the naked eye. Most cells, however, are visible only with a microscope, since they are only a few micrometres in diameter. A micrometre (µm) is one thousand of a millimetre. The size of cells varies from the very small cells of bacteria (0.2 to 5.0 µm) to the very large eggs of the ostrich (18 cm) (Among the multicellular plants, the large cell is the ovule of Cycas).

CELL VOLUME

• The volume of a cell is fairly constant for a particular cell type & is independent of the size of the organism. For example, kideny or liver cells are about the same size in the bull, horse and mouse. The difference in the total mass of the organ or organism depends on the number, not on the volume of the cells. Thus, the cells of an elephant are not necessarily large than those of other tiny animals or plants. The large size of the elephant is due to the large number of cells present in its body.

CELL NUMBER

• The number of cells in most multicellular organisms is indefinite, but the number of cells may be fixed in some multicellular organisms such as rotifers and nematodes. The number of cells in multi-cellular organisms usually is correlated with size of the organism. Thus, small sized organism has less number of cell in comparison to large sized organisms.

DETAILED STRUCTURE OF CELL

• Though their shape, size and activities vary, all cells have three major functional regions :
• The plasma membrane or cell membrane.
• The nucleus, and
• The cytoplasm.

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PLASMA MEMBRANE

NATURE & OCCURRENCE

• Most cellular organelles such as mitochondria, chloroplasts, lysosomes, peroxisomes, Golgi apparatus, nucleus and endoplasmic reticulum are all enclosed by the unit membrane. The cell surface membrane or plasma membrane is the outer covering of each cell. It is present in cells of plants, animals and micro organisms.

STRUCTURE

• Plasma membrane is a living, thin, delicate, elastic, selectively permeable membrane. It is about 7 µm (70Å) thick. Under the light microscope this merely appears as a single line. However, the development of electron microscope has made it possible to investigate the detailed structure of biological membranes.

FUNCTIONS

• Plasma membrane permits the entry and exit of some materials in the cells. It also prevents movement of some other materials. Therefore, the plasma membrane is called a selectively permeable membrane.
• Entry or exit of materials through the plasma membrane take place by two processes.
• Diffusion
• Osmosis

NUCLEUS

NATURE & OCCURRENCE

• The nucleus is a major, centrally located spherical cellular component. It is bounded by two nuclear membranes, both forming a nuclear envelope. Nuclear envelope encloses a space between two nuclear membranes and is connected to a system of membrances called the ER (endoplasmic reticulums).

FUNCTIONS

• The nucleus controls all metabolic activities of the cell. If the nucleus is removed from a cell, the protoplasm ultimately dries up and dies.
• It regulates the cell cycle.
• It is concerned with the transmission of hereditary traits from the parent to offspring.

CYTOPLASM

NATURE & OCCURRENCE

• The part of the cell which occurs between the plasma membrane and nuclear envelope is called the cytoplasm. The inner granular mass of the cytoplasm is often called endoplasm, while the outer, clearer (glassy) layer is called cell cortex or ectoplasm.
• Cytoplasm consists of an aqueous ground substance, the cytosol, containing a variety of cell organelles and other inclusions such as insoluble waste and storage products (starch, glycogen, lipid, etc.)

CYTOSOL

• It is the soluble part of cytoplasm. It forms the ground substance or “background material” of the cytoplasm and is located between the cell organelles. Cytosol is about 90 per cent water and forms a solution which contains all biochemicals of life.
• Cytosol (cytoplasm) acts as a store of vital chemicals such as amino acids, glucose, vitamins, ions, etc. It is the site of certain metabolic pathways, such as glycolysis. Synthesis of fatty acids, nucleotides, and some amino acids also take place in the cytosol.

CELL ORGANELLES

• A cell has to perform different functions with the help of its various membrane bound organelles :
• It has to synthesize substances, eg. Protein synthesis by ribosomes, photosynthesis of food by chloroplasts.
• It has to secrete cell products, e.g. enzymes, ormones, mucus, etc.
• It has to digest those substances which are taken up by the cell during endocytosis.
• It has to generate energy by mitochondria.

SOME CELL ORGANELLES

ENDOPLASMIC RETICULUM �(ER)

• Inside the cell there exists a membranous network enclosing a fluid-filled lumen which almost fills up the intracellular cavity. It is called endoplasmic reticulum (ER). On the one end ER is connected to the outer membrane of the nucleus and on the other end to the plasma mebrane.
• Endoplasmic Reticulum is of two types :
• Rough endoplasmic reticulum (RER).
• Smooth endoplasmic reticulum (SRE).

RIBOSOMES

• Ribosomes are dense, spherical and grannual particles which occur freely in the matrix (cytosol) of remain attached to the endoplasmic reticulum (RER). Chemically, the major constituents of ribosomes are the ribonucleic acid (RNA) and proteins. Lipids are virtually absent in ribosomes.
• Ribosomes play an impartant part in the synthesis of proteins.

RIBOSOMES

GOLGI APPARATUS

• The Golgi apparatus, first described by Camillo Golgi, consists of a system of membrane-bound vesicles arranged aprpoximately parallel to each other in stacks called cisterns.
• It is used for storage, modification and packaging of products in vesicles. It helps in the formation of lysosomes.

GOLGI APPARATUS

LYSOSOMES

• Lysosomes are a kind of waste disposal system of the cell. Each lysosome is a small vesicle srrounded by a single membrane and contains powerful enzymes. These enzymes are capable of digesting or breaking down all organic material.
• During breakdown of cell structure, when the cell gets damaged, lysosomes may burst and the enzymes eat up their own cells.

MITOCHONDRIA

• Mitochondira are known as the power houses of the cell. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphoshate) molecules. ATP is known as the energy currency of the cell.
• Mitochondira are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochnodira are able to make some of their own proteins.

MITOCHONDRIA

PLASTIDS

• Plastids occur in most plant cells and are absent in animal cells. Plastids are of following three types :
• Chromoplasts : Coloured plastids
• Chloroplasts : Green coloured plastids
• Leucoplasts : The colourless plastids
• Chloropasts trap solar energy & utilise it to manufacture food for the plant.
• Chromoplasts impart various colours to flowers to attract insects for pollination.
• Leucoplasts store food in the form of carbohydrates, fats and protein.

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VACUOLES

• Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90%of the cell volume.
• Vacuoles help to maintain the osmotic pressure in a cell (osmoregulation). They store toxic metabolic by-products or end products of plant cells. They provide turgidity and rigidity to the plant cells.

DIFFERENCES BETWEEN �PLANT & ANIMAL CELLS

ANIMAL CELL

PLANT CELL

THANKS