LEARN · PRACTISE · DISCOVER
Living Things, Cells and Microorganisms
EVS (Environmental Studies) in Paper I(A) has a Science half and a Social Studies half. This course covers the science units on the living world and life processes. We begin with the most basic questions: what makes something living, what living things are made of (the cell), and the tiny living things we cannot see (microorganisms). These ideas are taught informally from Class 3 and come up again in many APTET questions.
Living and non-living things
Living things show these characteristics; non-living things do not show all of them.
| Characteristic | Example |
|---|---|
| Made of cells | every plant and animal |
| Need food (nutrition) | plants make food; animals eat food |
| Respire (release energy from food) | all living things respire, day and night |
| Grow | a seedling becomes a tree |
| Respond to stimuli | touch-me-not (Mimosa) leaves fold when touched; we pull our hand from a hot object |
| Excrete wastes | urine, sweat; plants give out excess water |
| Reproduce | seeds, eggs, young ones |
| Move | animals move from place to place; plants show movement of parts (sunflower, Mimosa) |
| Have a life span and die | – |
Movement alone does not prove life: a car moves but is non-living. A seed looks lifeless, but it is living: it is dormant and grows when it gets water, air and warmth.
The cell: basic unit of life
The cell is the basic structural and functional unit of life. Robert Hooke first observed cells in 1665, in a thin slice of cork, using a microscope he built.
- Unicellular organisms have one cell that does all functions: amoeba, paramecium, bacteria, yeast.
- Multicellular organisms have many cells: plants, animals, humans. Similar cells form tissues, tissues form organs, and organs form organ systems.
- Prokaryotic cells have no well-defined nucleus (bacteria); eukaryotic cells have a nucleus with a nuclear membrane (plants, animals, fungi).
Parts of a cell and their functions
| Part | Function |
|---|---|
| Cell membrane | thin outer covering; controls what enters and leaves the cell |
| Cytoplasm | jelly-like substance in which the other parts lie |
| Nucleus | controls all activities of the cell; carries hereditary information (chromosomes) |
| Mitochondria | release energy from food: the “powerhouse of the cell” |
| Chloroplasts (plants) | contain green chlorophyll; make food by photosynthesis |
| Vacuole | stores water and substances; large in plant cells |
| Cell wall (plants) | rigid outer layer of cellulose outside the membrane; gives shape and support |
Plant cell vs animal cell
| Plant cell | Animal cell |
|---|---|
| has a cell wall | no cell wall |
| has chloroplasts | no chloroplasts |
| one large vacuole | small vacuoles, if any |
| usually regular in shape | usually irregular or rounded |
Cells differ in shape and size: nerve cells are long (to carry messages), red blood cells are disc-shaped; the egg of an ostrich is a very large single cell.
Microorganisms: useful and harmful
Microorganisms (microbes) are living things too small to be seen without a microscope.
| Group | Examples |
|---|---|
| Bacteria | Lactobacillus (curd), Rhizobium (root nodules), bacteria causing typhoid, cholera, tuberculosis |
| Fungi | yeast, bread mould, mushrooms (large fungi), ringworm fungus |
| Protozoa | amoeba, paramecium, Plasmodium (malaria) |
| Algae | Spirogyra, Chlamydomonas |
| Viruses | cause common cold, measles, chickenpox, polio, dengue, COVID-19; they reproduce only inside the cells of a host |
Useful microorganisms
- Curd: Lactobacillus bacteria turn milk into curd.
- Bread and idli batter: yeast and other microbes produce carbon dioxide, which makes dough rise (fermentation; Louis Pasteur studied fermentation).
- Medicines: antibiotics such as penicillin, discovered by Alexander Fleming in 1928, come from microorganisms.
- Vaccines: Edward Jenner developed the smallpox vaccine (1796); vaccines protect against diseases.
- Soil fertility: Rhizobium in root nodules of pulses (legumes) fixes nitrogen; decomposers turn dead matter into manure.
Harmful microorganisms
- Diseases in humans: malaria (protozoan, spread by the female Anopheles mosquito), dengue (virus, spread by the Aedes mosquito), cholera and typhoid (bacteria, spread through contaminated water and food), tuberculosis (bacterium, spread through air).
- Diseases in plants and animals: rust of wheat (fungus), citrus canker (bacterium), anthrax in cattle (bacterium).
- Food spoilage: moulds on bread; spoilt milk.
Exam focus
- Robert Hooke (cork, 1665); Fleming (penicillin, 1928); Jenner (smallpox vaccine).
- Cell wall, chloroplasts and a large vacuole: plant cells only.
- Mitochondria = powerhouse; nucleus = control centre.
- Malaria is caused by a protozoan, not a virus; ringworm is caused by a fungus, not a worm.
- Viruses multiply only inside living host cells.
Example
Mrs Sarala sets up a “living or non-living” sort in Class 3 with real objects: a seed, a stone, an ant in a jar, a toy car and a potted plant. Children argue about the seed and the car, then test the seed by sprouting it on wet cotton. The sprout settles the argument.
Key takeaways
- Living things are made of cells, need food, respire, grow, respond, excrete, reproduce and die.
- The cell is the basic unit of life; Robert Hooke first saw cells in cork.
- Plant cells have a cell wall, chloroplasts and a large vacuole; animal cells do not.
- Organisms may be unicellular or multicellular; prokaryotic or eukaryotic.
- Microbes include bacteria, fungi, protozoa, algae and viruses.
- Microbes give curd, bread, medicines, vaccines and fertile soil, but also cause diseases and spoil food.
Practice
- Which characteristic is shown by ALL living things?
- (A) They move from place to place.
- (B) They respire.
- (C) They have legs.
- (D) They make their own food.
- Cells were first observed in a slice of cork by:
- (A) Louis Pasteur
- (B) Alexander Fleming
- (C) Edward Jenner
- (D) Robert Hooke
- Which structure is found in a plant cell but NOT in an animal cell?
- (A) cell wall
- (B) nucleus
- (C) cell membrane
- (D) cytoplasm
- The “powerhouse of the cell” is the:
- (A) nucleus
- (B) chloroplast
- (C) mitochondrion
- (D) vacuole
- The microorganism that turns milk into curd is:
- (A) yeast
- (B) Rhizobium
- (C) Lactobacillus
- (D) Plasmodium
- Malaria is caused by:
- (A) a protozoan (Plasmodium)
- (B) a virus
- (C) a bacterium
- (D) a fungus
- Penicillin was discovered by:
- (A) Robert Hooke
- (B) Louis Pasteur
- (C) Edward Jenner
- (D) Alexander Fleming
- Which of these is a unicellular organism?
- (A) frog
- (B) amoeba
- (C) mango tree
- (D) earthworm
Answers and explanations (tap to show)
- (B) All living things respire; only animals move from place to place, and only green plants make their own food.
- (D) Robert Hooke saw cells in cork in 1665.
- (A) Only plant cells have a cell wall of cellulose.
- (C) Mitochondria release energy from food.
- (C) Lactobacillus bacteria change milk into curd.
- (A) Malaria is caused by the protozoan Plasmodium, spread by the female Anopheles mosquito.
- (D) Alexander Fleming discovered penicillin in 1928.
- (B) An amoeba is a single cell that carries out all life functions.