THE FUNDAMENTAL UNIT OF LIFE: CELL · THE FUNDAMENTAL UNIT OF LIFE: CELL CLEAR VISION COACHING, Ph 98269-46131 Page 1 Cell: • The bodies of living organisms are made up of microscopic

THE FUNDAMENTAL UNIT OF LIFE: CELL

CLEAR VISION COACHING, Ph 98269-46131 Page 1

Cell:

• The bodies of living organisms are made up of microscopic units called cells.

• The cell is the basic structural and functional unit of living organisms.

• The number of cells may vary from organisms to organisms.

• A single cell may constitute whole organism. Such organisms are called unicellular organisms and include bacteria, amoeba, paramecium and yeast.

• The bodies of fungi, plants and animals are made up of many cells packed together. These organisms are called multicellular organisms.

Cell structure: The size, shape, number and volume of the cell vary greatly among unicellular and multicellular organisms. 1. Cell shape: The basic shape of cell is spherical but is mostly determined by the specific

function of the cell. Thus, the shape of the cell may be variable or fixed.

2. Cell size: The size of different cell ranges between broad limits. Some plant and animal cells are visible to the naked eye and few are visible only through microscope.

3. Cell volume: The volume of a cell is fairly constant for a particular cell type and is independent of the size of the organism. For example, kidney or liver cells are about the same size in the bull, horse and mouse.

4. Cell number: The number of cells in most multicellular organisms is indefinite, but the number of cell may be fixed in some multicellular organisms such as rotifers and nematodes.

Structurally the cell is formed of three parts: 1. Plasma membrane 2. Cytoplasm 3. Nucleus

1. Plasma membrane or cell membrane:

• Each cell is bounded by extremely delicate, thin, elastic, living membrane called plasma membrane.

• In plant cell, it is present on the inner side of the cell wall.

• It is a selectively permeable membrane which allows only some substances into the cell and out of the cell.

• The plasma membrane is made up to two layers of lipid (fat) molecules with protein molecules. Functions:

• It gives a definite shape to the cell.

• It provides protection to the internal contents of the cell.

• It regulates the entry and exit of the substances in and out of the cell.



The movement of substance take place into the cell through different methods:

Diffusion:

• Substance like carbon dioxide, oxygen, water, etc. Can move across the cell membrane by a process called diffusion.

• Diffusion is a spontaneous movement of molecules from a region of high concentration to one of lower concentration. It is faster in the gaseous phase than in liquid and solids.

• When gases like carbon di oxide and oxygen accumulate inside the cell, their concentration is greater inside compared to the external environment is to the outside by diffusion, i.e., from their region of higher concentration to lower concentration. Similarly, when the concentration is greater outside, oxygen and carbon di oxide move inside the cell from the external environment.

Osmosis:

• The spontaneous movement of water molecules through a selective permeable membrane is called osmosis.

• Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to water from a region of low water concentrations.

Transport of water between cells and their environment: Osmosis will take place and the movement of water molecules will depend on the concentration of the surrounding solution. 1. In a hypotonic solution, the cell will gain water by osmosis. This is because there is a

higher water concentration outside the cell. The cell sap, or cytoplasm, has less water molecules than the outer solution. Water molecules pass in both directions across the cell membrane. But more water enters the cell then goes out of it. So the net movement of water is into the cell, due to which the cell increases in volume, or swells up. This is called endosmosis.

2. When the cell is placed in an isotonic solution, which has exactly same water concentration as the cell, there will be no net movement of water molecules across the cell membrane. Although water passes the cell membrane in both directions, the amount coming in is the same as the amount going out. So there is no net movement of water and the cell remains the same size.

3. When a cell is immersed in a hypertonic solution, water will diffuse out of the cell by osmosis. This is because there is a lower concentration of water outside the cell. Although water molecules pass the cell membrane in both directions, more water goes out of the cell than enters it. As a result, the cell will shrink. This process is celled exosmosis.



Cell wall:

• Plant cells have an additional protective wall outside the plasma membrane, called the cell wall.

• It is relatively thick, nonliving and rigid envelope. It is permeable and is made up of cellulose. Cellulose is a complex substance and provides structural strength to plants.

• Plant cell on loosing water through osmosis, there is a shrinkage or concentration of the content of the cell away from the cell wall. This phenomenon is known as plasmolysis. Functions:

• It determines the shape of the plant cell.

• It prevents desiccation (drying up) of cells.

• It protects the plasma membrane and internal structure of the cell.

• It helps in the transport of various substances in and out of the cell. Nucleus:

• The nucleus is a major, centrally located spherical cellular component.

• The nucleus is surrounded by a double membrane called the nuclear envelope, which separates it from the surrounding cytoplasm.

• The nuclear envelope contains many pores (the nuclear pores) and closes the liquid ground substance, the nucleoplasm.

• Inside nucleoplasm is a tangled mass of threadlike structure called chromatin. Chromatin materials are composed of DNA (deoxyribonucleic acid) and proteins. DNA stores all the necessary information for the cell to function, to grow and to reproduce further cells of the next generation.

• When a cell starts to divide, the tangled mass of chromatin condense into long threads and finally, rod like bodies called chromosomes. The chromosomes contain stretches of DNA which carry information for protein synthesis. These stretches of DNA are called genes.

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:

• The part of the cell which occurs between the plasma membrane and nuclear membrane is called the cytoplasm.

• Many small organelles are found inside the cytoplasm. These are known as cytoplasmic organelles. Examples of such organelles are nucleus, mitochondria, chloroplast, endoplasmic reticulum, golgi apparatus, plastids, lysosome, ribosomes and vacuoles.



1. Endoplasmic reticulum:

• The endoplasmic reticulum is a network of tube like structure running through the cytoplasm.

• Some endoplasmic reticulums are connected to the nuclear membrane, while others are connected to the cell membrane.

• Endoplasmic reticulum is of two types: rough and smooth. a) Rough endoplasmic reticulum: Those reticulum in which ribosomes are attached

are called rough endoplasmic reticulum (ERE). b) Smooth endoplasmic reticulum: Those reticulum in which no ribosomes are

attached are called smooth endoplasmic reticulum (SER).

Function:

• It is associated with the synthesis, storage and transport of metabolic products.

• Rough endoplasmic reticulum is associated with the synthesis of proteins.

• Smooth endoplasmic reticulum secretes lipids which along with proteins constitute cell membrane by a process called membrane biogenesis.

• It helps in the formation of the cell plate and nuclear membrane during cell divisions.

Ribosomes:

• Ribosomes are extremely small, round bodies found either in the free state in the cytoplasm or attached to the surface of the ER. Function:

• It acts as a platform or work place for the synthesis of proteins.

Mitochondria:

• Mitochondria are small, rod-shaped organelles found in large numbers. Each mitochondria is bounded by two membranes – outer and inner.

• The outer membrane is smooth and inner membrane is pushed inwards at intervals forming crests called cristae. The cristae lie in a ground substance called matrix.

• Mitochondria possess enzyme for the oxidation of carbohydrates. This process releases energy in the form of ATP (adenosine triphosphate) which is energy rich compound and hence, are called power house of the cell. Function:

• Mitochondria provide energy for the vital activities of living cells.

• They are able to make some of their own proteins; so they are regarded as semiantonomous organelles.



Golgi apparatus:

• Golgi bodies, or golgi complex are formed by stacks of flattened (saucer-shaped) membranes or flattened sacs called cisternae.

• It consists of a set of membrane bounded, fluid-filled vesicles and vacuole.

• In plant cells Golgi apparatus are called dictyosomes. Functions:

• They store, modify, package and condense the proteins synthesized in the ribosomes.

• They form the cell plate during cell division.

• They set aside digestive enzymes in tiny membrane bound vesicles which become lysosomes.

Plastids:

• Plastids are double-membraned organelles which are found only in plant cells.

• Plastids are self-replicating organelles like the mitochondria i.e. they have the power to divide.

• Plastid shows two distinct regions – grana and stroma. Grana are stacks of membrane-bound, flattened, discoid sacs containing chlorophyll molecules. These molecules are responsible for the production of food by the process of photosynthesis and hence, are called the kitchen of cell. The homogenous matrix in which grana are embedded is known as stroma.

• Plastids are of following three types: a) Leucoplasts:

leucoplasts are colourless plastids. They store food in the form of starch, protein and lipids.

b) Chloroplasts: chloroplasts are green plastids which contain chlorophyll. Green leaves have chloroplasts.

c) Chromoplasts: Chromoplasts are variously coloured plastids present in flower and fruits.

Functions:

• By trapping solar energy, green plastids manufacture through photosynthesis.

• Chromoplasts provide colour to various following parts.

• Leucoplasts helps in the storage of proteins, starch and lipids (oil). Lysosomes:

• Lysosomes are saclike, small, spherical, single membrane-bound vesicles which contain enzymes. These enzymes are synthesized in the rough endoplasmic reticulum, which are brought to the golgi complex.

• They occur in animal cells and in the merismatic cells of a few plants.



Functions:

• They help in breaking down (digesting) large molecules of the cell hence, called digestive bags.

• They work in defence against bacteria and viruses.

• During breakdown of cell structure, when the cell gets damaged, lysosomes may burst and the enzymes eat up their own cell. Therefore, lysosomes are also known as suicide bag of a cell.

• Lysosomes also remove the worn out and poorly working cellular organelles by digesting them to make way for their new replacements. Thus, lysosomes form a kind of garbage disposal system of the cell.

Vacuoles:

• Vacuoles are fluid-filled or solid-filled and membrane bounded spaces.

• In animal cell, the vacuoles are smaller in size and lesser in number. They store water, glycogen and proteins.

• In plant cell, vacuoles are larger and permanent and occupy up to 90% of the cell volume.

• The vacuole is filled with cell sap which is a watery solution rich in sugar, amino acids, proteins, minerals and metabolic wastes. Functions:

• In animals, vacuoles are often associated with the maintenance of water balance.

• They store various substance including waste products. Difference between plant cell and animal cell:

S.No. Plant cell Animal cell

1 Plant cell are usually larger than animal cell.

Animal cell are generally smaller in size.

2 The plasma membrane of a plant cell is surrounded by a rigid wall.

Cell wall is absent.

3 Plastids (leucoplasts, chloroplasts, chromoplasts) are present in plant cell.

Plastids are absent.

4 Vacuoles are present in abundance. They are larger in size.

Vacuoles are less in number and smaller in size.

5 Plant cells have many simple units of golgi complex, called dictyosomes.

Animal cells have single highly elaborate golgi complex.





Difference between prokaryotic cell and eukaryotic cell:

S.No. Prokaryotic cell Eukaryotic cell

1. Size of the cell is generally small (1-10 µm).

Size of cell is generally large (5-100 µm).

2. Nucleus is absent (nuclear region or nucleoid is not surrounded by a nuclear membrane).

Nucleus is present (nuclear membrane is surrounded by a nuclear membrane).

3. It contains single chromosomes. It contain more than more one chromosomes.

4. Nucleolus is absent. Nucleus is present. 5. Membrane bound organelles are

absent. Cell organelles such as mitochondria, plastids, endoplasmic reticulum, golgi apparatus.

6. Examples: bacteria and blue green algae.

Examples: all other organisms.



NOTES

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THE FUNDAMENTAL UNIT OF LIFE: CELL · THE FUNDAMENTAL UNIT OF LIFE: CELL CLEAR VISION COACHING, Ph 98269-46131 Page 1 Cell: • The bodies of living organisms are made up of microscopic