What makes something alive?
Living things need food and air, make energy, grow, sense the world, and make more of their kind. These are life processes. In humans several body parts work together to do them.
Breathing and the heart
Breathing: air goes into the lungs. Oxygen passes into the blood. Carbon dioxide leaves the blood and we breathe it out.
Heart: the heart is a pump. Each beat pushes blood around the body, carrying oxygen and food to every cell. At rest an adult heart beats about 70 times a minute and we breathe about 14 times a minute.
When we exercise muscles need more oxygen and release more energy, so heartbeat and breathing both speed up. Resting makes them slow down again.
Food, energy and growth
Food is broken into tiny pieces in the stomach and intestines. These pieces enter the blood. Cells use them with oxygen to release energy, which we use to move, think, keep warm and grow. Waste goes out as breath, sweat and urine.
Microbes: tiny living things
Microbes (micro-organisms) are living things too small to see without a microscope. They include bacteria, yeast and other fungi, algae and viruses (viruses live only inside other living cells).
Many microbes multiply fast. If one bacterium splits every hour: 1, 2, 4, 8, 16, 32, 64 after six hours.
Uses of microbes
- Food: curd (milk bacteria), bread and idli (yeast), cheese, pickles.
- Medicine: some medicines such as antibiotics come from microbes; vaccines protect from disease.
- Farms: microbes in soil break down dead plants and make compost.
- Cleaning: microbes break down sewage and waste.
Some microbes cause disease (cholera, malaria, flu). Washing hands, boiled water and vaccines keep us safe.
Try it
Sit quietly and count your pulse on your wrist for 15 seconds. Multiply by four. Then jog on the spot for one minute and count again. Next, mix a spoon of sugar, a little dry yeast and warm water in a bottle and fix a balloon on top. Watch it fill with gas.
Key formulas and definitions
- Pulse per minute = beats in 15 s x 4
- Resting: heart about 70 per minute, breathing about 14 per minute
- Microbes doubling every hour: after n hours = 2^n
- Microbes help: food, medicine, compost, cleaning
Worked examples
1. Ravi counts 20 pulse beats in 15 seconds. What is his pulse per minute?
20 x 4 = 80 beats per minute.
2. One microbe doubles every hour. How many are there after 5 hours?
2^5 = 32 microbes.
3. Why does the dough rise after yeast is added?
Yeast feeds on sugar and makes carbon dioxide gas. The gas makes bubbles in the dough and pushes it up.
Common mistakes
- Thinking all microbes are harmful. Many are useful.
- Thinking breathing and respiration are the same. Breathing is moving air; respiration is releasing energy in cells.
- Thinking the heart makes blood. The heart pumps blood.
- Counting a 15-second pulse without multiplying by four.