Cell

 We have learnt that cellular organization is the primary characteristic of all living organisms and that the cell is the fundamental structural and functional unit of living organisms.

We see the structural organization of a book in the above flow chart. Similarly, there are organizational levels in living organisms, too. These are : cells, tissues, Let’s recall. organs, organ systems, organism. Cells form the basis of the structure and function of all living organisms. It is only with the help of cells that living organisms carry out all their different life processes.

Measurement and observation of cells

 In 1673, Anton van Leeuwenhoek assembled various lenses to construct a microscope. He was the first to observe live bacterial and protozoan cells under the microscope. Cells are extremely minute in size. We cannot see cells with the naked eye. Micrometre and nanometre are the units used for measuring their sizes. The compound microscope is used for observing cells. An object on a glass slide magnified many times by the lenses of this microscope.

Take a piece/segment of onion and carefully separate the thin skin from its concave surface with the help of forceps. Place the membrane on a glass slide and put a drop of water on it. (Take care that the membrane does not get folded while placing on the slide). Put a drop of a dilute solution of iodine or eosin over it and observe under the 10X objective of the compound microscope. Don’t forget to put a cover-slip over the onion skin on the slide before placing it under the microscope.

In the same way, observe the cells from various parts of plants like leaves, bark, root tips, etc. Do you remember that last year you had observed the amoeba and paramoecium that are found in water ?

Cell size

 There is great variation in the shapes of cells. Their shapes are mainly related to their function. Observe the cells of different shapes shown below.

Cells are of many different shapes e.g. circular, rod-shaped, columnar, spiral, oval, rectangular, etc. Each cell contains various components for carrying out the life-processes of the living organism. These components are called organelles. These organelles are studied in detail with the help of the electron microscope which can magnify images up to (2 x 109 ) two billion times their actual size. There are two main types of cells – animal cells and plant cells. These cells consist of various types of membrane-bound cell organelles. Plant cells have a definite shape due to the presence of the cell wall around them. Besides, unlike animal cells, plant cells contain single large vacuole. All these cells are known as eukaryotic cells.

Which are the common components of plant and animal cells ? Which are the different ones ? The nucleus is the most important organelle of the cell. There is a porous double membrane around it. The nucleus controls all functions of the cell. The endoplasmic reticulum is a sprawling net-like organelle. Its function is to make necessary changes in the proteins produced by ribosomes and send them to the Golgi bodies. Golgi bodies are made up of several flat sacs. Their function is the proper distribution of proteins. Mitochondria and plastids are organelles with double outer coverings. As mitochondria produce energy, they are called the powerhouses of the cell. The chloroplasts in plant cells carry out the function of photosysthesis. Vacuoles help to throw out waste products of the cell. Vacuoles in animal cells are small whereas there is only one large vacuole in a plant cell.

Micro-organisms

1. What is meant by micro-organisms ?
2. Categorise the following organisms into two groups, according to size – amoeba, paramoecium, euglena, snail, elephant, pigeon, worms.

 We have learnt that there are countless living organisms on the earth. Of these, the organisms which cannot be seen with our eyes but can only be observed under a microscope are called micro-organisms.

Occurence of micro-organisms

Micro-organisms are present all around us in the air, water, soil, food, sewage, garbage as well as in the bodies of plants and animals, including humans. Some micro-organisms are solitary, that is, they live singly, e.g. amoeba, paramoecium, whereas some live in colonies. Some micro-organisms live on the remains of dead plants and animals.

Observation and measurement of micro-organisms

1. Keep a moistened piece of bread or bhakari in a closed box. Observe it after 3-4 days using a magnifying lens.
2. Observe a drop of muddy water or water from a stagnant puddle, under the compound microscope.
3. Take a drop of yoghurt or buttermilk on a glass slide and observe it under a compound microscope. In your notebook, draw sketches of the microbes you observe.

Nature of micro-organisms

Among the sketches you have drawn , do you find any of the micro-organisms you see below? What inferences can you draw about their sizes ?

Some micro-organisms like the fungus that grows on bread or strands of algae in ponds are multi-cellular. However, most micro-organisms such as bacteria and viruses are unicellular. They have a somewhat different cellular structure. They do not have the membrane-bound organelles found in eukaryotic cells. The plasma membrane, cytoplasm and nucleoid are their only components. Such cells are called prokaryotic cells.

Growth of micro-organisms

Each micro-organism needs a specific environment for growth and reproduction. Many microbes need oxygen for their growth whereas some microbes can grow without oxygen. Some micro-organisms survive even in extreme and adverse conditions like the ocean floor, ice in polar regions, hot water springs, etc. During adverse conditions, micro[1]organisms form a thick covering around themselves and stop their life-processes. On return of favourable conditions, they come out of the protective covering and continue their life processes.

Useful micro-organisms

Take two earthen pots half filled with soil and mark them ‘A’ and ‘B’. Mix some waste material like garden waste, dung, fruit peel, vegetable stalks, paper scrap, etc. with the soil in pot ‘A’.

Mix things like pieces of glass, scrap metal, plastic bags, etc. with the soil in pot ‘B’. Keep both pots at the same spot in the garden and observe them after 3 to 4 weeks.

We have seen that some micro-organisms present in the soil and those in the root nodules of leguminous plants convert atmospheric nitrogen into its compounds. These nitrogenous compounds help to increase soil fertility and thereby the protein content of the pulses grown in that soil. Project : Visit the garbage depot near your village/city. Find out the reason for burying the garbage in the large pits there ?

If microbes are allowed to grow in batter, dough, fruit juices, etc., they break down these substances producing new compounds as they grow and multiply in them. This microbial process is used in the production of various common foodstuffs.

Sometimes, when you are ill, the doctor prescribes capsules or injections of medicines like penicillin. These types of medicines destroy the pathogens and retard their growth. They are called antibiotics. Antibiotics are produced with the help of specific microbes. Diseases like tuberculosis, typhoid, cholera, etc. which were previously considered incurable, have now come under control because of antibiotics. Domestic animals too can be protected from various diseases by mixing antibiotics with their food. Plant diseases can also be controlled with the help of antibiotics.

A vaccine is produced in a laboratory with the help of microbes, that gives immunity against a particular disease. If we have been vaccinated against any disease, our immunity i.e. resistance to that disease, increases, so that the possibility of contracting that disease is greatly reduced.

Microbes are also used in processes like tanning of skin, production of ropes and strings from agave. Some microbes use oil for their growth. Such microbes are used to clear a layer of oil floating on the surface of an ocean or lake formed due to a leak or a spill i.e. to clear an oil slick. Farm waste, human urine and faeces, wet garbage, etc. is collected and used in a biogas plant to produce biogas and fertilizer.

Harmful micro-organisms

If jars of pickles, jams (murabba), etc. are opened after a long time, a round layer of white scum or black particles may appear to have formed on the surface. In summer, milk and meat get spoilt quickly. Fungus grows quickly on moist and stale food. What do we do with such spoilt food? Why ?

Pathogens : Disease-producing micro-organisms

Pathogens may be present in water bodies contaminated with sewage and dirt from the surroundings, in food left uncovered in unhygienic conditions with houseflies sitting on it, etc. If such contaminated food or water is consumed, we may fall ill with diseases of the alimentary canal, like amoebiasis, typhoid, cholera, hepatitis, gastro, etc. Pathogens are released in the air when a person having an infection of the respiratory tract sneezes or coughs. A healthy person may get infected with such pathogens on breathing in the same air and contract diseases like common cold, cough, diphtheria, pneumonia, tuberculosis, etc.

Mosquitoes reproduce in places like heaps of garbage, drains, stagnant water, etc. Microbes that cause diseases like malaria, dengue, elephantiasis, yellow fever, chikungunya, Zika fever, etc. gain entry into the human body through the bite of a female mosquito.