The tiny organisms that make curd, bread and medicines, and the ones that cause disease and spoilage.
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Bacteria are microscopic, single-celled prokaryotes with no true nucleus. By shape they are cocci (spherical), bacilli (rod-shaped), spirilla (spiral) and vibrios (comma-shaped). They multiply by binary fission, which can be very fast. Useful bacteria: Lactobacillus turns milk into curd; Rhizobium in root nodules of legumes fixes atmospheric nitrogen; decomposer bacteria recycle dead matter; Acetobacter makes vinegar; Streptomyces gives the antibiotic streptomycin.
A bacterium divides every 20 minutes. How many are there after 2 hours, starting from one?
2 hours = 120 min = 6 divisions, so 2^6 = 64 bacteria.
How many after 4 hours at the same rate?
4 hours gives 12 divisions: 2^12 = 4,096 bacteria.
Why do legume crops need less nitrogen fertiliser?
Rhizobium in their root nodules fixes nitrogen from the air into forms the plant can use.
Disease-causing bacteria cause cholera (Vibrio cholerae), typhoid (Salmonella typhi), tuberculosis (Mycobacterium tuberculosis), tetanus and diphtheria in humans, and citrus canker in plants. Some bacteria spoil food and make toxins. Food can be protected by refrigeration, boiling, salting, drying and pasteurisation.
Name two bacterial diseases of humans.
Any two: cholera, typhoid, tuberculosis, tetanus, diphtheria.
Why is milk pasteurised?
Heating to about 63 °C for 30 minutes (or 72 °C for 15 seconds) kills disease-causing bacteria without changing the milk much.
Why does food keep longer in a refrigerator?
Low temperature slows the growth and division of bacteria, though it does not kill them.
Fungi are eukaryotes with no chlorophyll. Yeast (a single-celled fungus) ferments sugar to make alcohol and makes bread dough rise. Penicillium gives penicillin, the first antibiotic discovered by Alexander Fleming (1928). Mushrooms such as the button mushroom are edible and rich in protein. Harmful fungi: Rhizopus (bread mould), moulds on food and leather, ringworm and athlete's foot in humans, and rust and smut of crops. Some mushrooms are poisonous.
Name the fungus used in (a) baking, (b) making an antibiotic.
(a) Yeast. (b) Penicillium.
Name two diseases caused by fungi.
Any two: ringworm, athlete's foot, rust of wheat, smut of crops.
Why should wild mushrooms never be eaten unless identified by an expert?
Some species are highly poisonous and look like edible ones.
1. Name the four shapes of bacteria.
2. Which bacterium converts milk into curd?
3. What is nitrogen fixation? Name a bacterium that does it.
4. Name an antibiotic got from bacteria and one got from fungi.
5. Why are decomposers called nature's recyclers?
6. One cell divides every 20 minutes. How many cells after 1 hour?
7. Name one use of yeast.
8. Who discovered penicillin, and in which year?
9. Name two ways of preserving food from bacteria.
10. Name a harmful bread mould.