ICSE Class 7 Biology Chapter 09 Life Under A Microscope

Life Under a Microscope - The Micro-Organisms

Syllabus

1. What are microorganisms?

2. Unicellular plants and animals - Chlorella, Chlamydomonas, desmids, diatoms, yeasts, Various types of bacteria. Amoeba, Paramoecium, Euglena.

3. Multicellular plants and animals: Filamentous algae - Spirogyra; Fungi - bread mould Colonial organisms - Volvox

4. Conditions for the growth of microorganisms

5. Useful microorganisms - e.g. fermentation, setting of curds tanning of leather, retting of fibres, formation of compost and manure, biogas, gobar gas etc. Harmful microorganisms - disease causing viruses, bacteria, fungi, protozoa - only as examples - symptoms, vectors and spread need not be done. How to avoid infections and spoilage of food.

- Studying a drop of pond water under a microscope (D/E).

- Study of permanent slides of microorganisms - drawing the same (E).

- A simple experiment to determine conditions needed for the growth of mould on bread - need for a control to be discussed (E).

- Experiment to determine the conditions needed for the setting of curds (E).

What Are Microorganisms?

Invention of the microscope opened up a vast world of tiny microscopic organisms that man was not able to see so far. The tiniest animals known to us up to that time were the insects like ants, lice or the fleas. One could at best distinguish their head and legs but, hardly anything inside their bodies. Similarly, the smallest plants, the humans knew in the earlier days were the green layer of mosses growing on the damp walls or on the stones, or the green thread-like algae floating on the pond, but they knew nothing of their internal structure. Today, we know about thousands and thousands of different kinds of living organisms visible only through a microscope (Fig. 9.1). For this reason, such organisms are categorised as micro-organisms (micro: small/minute, organism: living being). The branch of science which deals with the study of micro-organisms is called microbiology.

Teacher's Note

When we leave food uncovered or observe green growth on damp walls, we're witnessing microorganisms at work - organisms too small to see without a microscope but crucial to understanding life around us.

Micro-Organisms Occur Everywhere

Micro-organisms occur everywhere in the world - from the Arctic to Antarctic and from the deep ocean bottoms to the high mountain tops. They are found in the air, in the soil, in water and even inside our own bodies. They may be found on all kinds of food stuffs, and more so in the decomposing food wastes. They are found on the surface, inside our mouth and in the intestines, and far more numerous even in our faeces (stools). Certain micro-organisms are useful to us such as the ones used in making curd, vinegar, alcohol, bread, etc. Some micro-organisms are sources of medicines like the antibiotics. Some are natural inhabitants of our intestines and they are beneficial in producing some necessary vitamins. Many others are harmful, causing diseases like dysentery, malaria, tuberculosis, and so on. But there are as well quite many that are harmless living freely in waters or in soil contributing to the natural cycles of various elements.

Teacher's Note

Microorganisms are everywhere - in the air we breathe, the food we eat, and even in our digestive system - making them one of the most important yet invisible parts of our environment.

Activity 1

Students can request the teacher to help them to prepare a fresh slide of a drop of pond water so that they can observe some microorganisms.

Materials required: Slide, coverslip and a microscope.

Procedure:

1. Collect some water in a small bottle from a near by pond.

2. Put one drop of this water on a slide and place a cover slip over it.

3. Wipe off excess water with a blotting paper, and examine the slide under the low and high powers of microscope.

4. You may find a variety of micro-organisms in it. It will show some actively swimming forms, some fixed to sand particles and some others just creeping slowly.

5. Try to identify them with the help of diagrams given above and draw these organisms in your record book.

Teacher's Note

Observing microorganisms from pond water under a microscope helps us understand that life thrives in places we rarely think about, like the water in a nearby pond.

Micro-Organisms Can Be Categorised Into Five Major Groups

1. Bacteria, 2. Algae 3. Fungi, 4. Protozoa, and 5. Viruses.

1. Bacteria (Fig. 9.2)

Bacteria (sing. bacterium) are the smallest and structurally the simplest organisms. They are typically one-celled, rarely exceeding 3 to 5 microns in length.

There are four common forms of bacteria (coccus, bacillus, spirillum and vibrio).

(A) Coccus form (Gk. kokkos: a berry). These bacteria are spherical or ovoid in shape and they occur in four types:

(a) Single bodies: monoccus type

(b) In doubles: diplococcus type

(c) In grape-like clusters: staphylococcus type

(d) In strings: streptococcus type.

(B) Bacillus form (bacillus: rod). These are rod-shaped. These may also occur singly or in two's or three's, joined end to end in long chains.

(C) Spirillum form. These are spiral-shaped.

(D) Vibrio form. These are short, curved, appearing comma-shaped. Cholera bacteria (Vibrio cholerae) are of vibrio type.

Structure of a Bacterium Cell

A bacterium cell has most primitive nucleus not bound by nuclear membrane and has only chromatin material in the centre. Outermost cell wall is made up of largely protein-like material. The cell wall is surrounded by a gelatinous or proteinaceous capsule. Nucleolus, mitochondria and plastids are absent. Reserve food is glycogen (Fig. 9.3).

Many bacteria possess one or more flagella that help them to move in liquid medium. Non-flagellated bacteria gets carried by wind, water and through contact.

Nutrition

Mostly, the bacteria lack chlorophyll and depend on other sources for their food.

Bacteria obtain their nutrition in three ways:

(i) Most bacteria draw nourishment from decaying dead animals and plants, these are categorized as saprophytes (sapro: rotten).

(ii) Many bacteria draw nourishment from the body of their hosts. These are called parasites.

(iii) Some bacteria live in association with other organisms obtaining nourishment from them without causing any harm, and may even provide some benefit. Such bacteria are termed symbionts.

Respiration

Some bacteria respire by absorbing oxygen (aerobic respiration), while others without oxygen (anaerobic respiration).

Reproduction

Bacteria reproduce very fast by asexual method or binary fission (dividing into two). A full grown bacterium, when conditions are favourable, divides into two daughter cells; on maturity after growth they divide and redivide further into a number of cells. The curd bacteria divide every 20 minutes or so, and can potentially produce billions of bacteria in 24 hours.

Harmful Bacteria

1. Spoilage of food. Bacteria ferment food products and leave poisonous substance which causes food poisoning, e.g., botulism is a condition caused due to eating up EXPIRED tinned food, inadequately STERILISED, canned or bottled food. Bacterium Clostridium botulinum multiplies and produces a powerful poison which affects the nervous system and causes botulism.

2. Diseases. Bacteria cause a number of diseases in humans, e.g., typhoid, tetanus, tuberculosis, pneumonia, cholera, diphtheria etc. Canker disease in orange and lemon, black root in cabbage, potato, apple, etc.

Teacher's Note

Bacteria are responsible for food spoilage and many diseases, which is why we refrigerate food and practice good hygiene - simple actions that directly impact our daily health.

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