Lenses and microscopes
A grandfather reads the newspaper through reading glasses. If you place a round glass flask full of water over a page, the letters look bigger too, because the curved water acts like a lens.
A lens is a curved piece of glass that is thick in the middle and thin at the edge, like a lentil seed (that is where the name comes from). A microscope uses lenses to show us things too small for the naked eye.
Key idea 1: From lens to microscope — Lenses were improved again and again, until the microscope opened up a hidden world of tiny living things.
- Easy: A lens is thick in the middle and thin at the edges.
- Medium: In 1665 Robert Hooke published Micrographia. In a thin slice of cork he saw many small empty boxes like a honeycomb and called each a "cell".
- Exam-type: Antonie van Leeuwenhoek made better lenses and clearly saw bacteria and blood cells, so he is called the Father of Microbiology.
In the figure: The curved glass is fat in the centre and thin at the rim, just like a lentil seed.
Key idea 2: How powerful are the tools? — Hooke's microscope showed things about 200 to 300 times bigger; a school microscope shows 100 to 400 times; an electron microscope about 10,00,000 times.
- Easy: A school microscope magnifies 100 to 400 times.
- Medium: An electron microscope can show parts inside a cell.
- Exam-type: A low-cost foldable paper microscope shows less detail, but it brings the microscopic world to many people.
The human eye can see only objects above a certain size, so tools were needed to discover microbes.
Thinking Hooke saw living cells in cork. He saw the empty walls of the compartments, and he gave the word "cell" its scientific meaning.
Pair each name with one fact. Memory clue: Hooke = Honeycomb = cork cells; Leeuwenhoek = bacteria.
Spot the difference
| Point | Robert Hooke | Antonie van Leeuwenhoek |
|---|---|---|
| Time | 1665 | 1660s |
| Saw | cork cells | bacteria, blood cells |
| Known for | naming the "cell" | Father of Microbiology |
Practice questions
- 1What is the shape of a lens?
- 2Which instrument opened up the world of tiny living things?
- 3Who named the "cell"?
- 4Why do letters look bigger through a water-filled round flask?
- 5About how many times does a school microscope magnify?
- 6What did Hooke see in a slice of cork?
- 7Arrange in order of power: electron microscope, magnifying glass, school microscope.
- 8Why is a paper microscope useful even though it shows less detail?
Show answers (try them yourself first)
- 1Thick in the middle and thin at the edges, like a lentil seed.
- 2The microscope.
- 3Robert Hooke.
- 4The curved, water-filled flask works like a magnifying lens.
- 5100 to 400 times.
- 6Many small empty compartments that reminded him of a honeycomb.
- 7Magnifying glass, school microscope, electron microscope.
- 8It is low-cost, so many more people can observe microbes.
Structure of a cell
A wall is built from bricks; a living body is built from cells. Peel a thin skin from an onion and look at it under a microscope: you will see closely packed rectangular boxes. Each box is a cell.
The cell is the basic unit of life. Every typical cell has a cell membrane, cytoplasm and a nucleus. Plant cells also have a cell wall.
Key idea 1: Jobs of the parts — Each part of the cell has its own work.
- Easy: The cell membrane encloses the cell and lets needed materials in and waste out.
- Medium: Most life processes take place in the cytoplasm; the nucleus regulates all the activities and growth.
- Exam-type: The cell wall of a plant cell gives rigidity and strength, so plant cells are packed firmly together.
In the figure: Both cells have a membrane, cytoplasm and nucleus. Only the plant cell has a cell wall, green chloroplasts and a large vacuole.
Key idea 2: Plant cell and animal cell — Plastids such as chloroplasts contain chlorophyll for photosynthesis; the large vacuole stores substances, removes waste and keeps the cell's shape. Animal cells have none or only tiny vacuoles.
- Easy: A cheek cell has no cell wall; an onion peel cell has one.
- Medium: Safranin (red) stains onion peel, methylene blue stains cheek cells; glycerin stops them from drying.
- Exam-type: A stain increases contrast, so cells are seen clearly; a folded peel or an air bubble spoils the view.
A cell is not a simple bag of liquid; it is a complex structure whose parts each do a special job.
Saying that animal cells have a cell wall. Only plant (and fungal and bacterial) cells have one.
Think of a city. Memory clue: Membrane = gate, Cytoplasm = workshop, Nucleus = boss (office).
Spot the difference
| Part | Animal cell | Plant cell |
|---|---|---|
| Cell wall | absent | present |
| Chloroplast | absent | present in green parts |
| Large vacuole | none or very small | large |
Practice questions
- 1What is the basic unit of life?
- 2Name the three basic parts of a cell.
- 3Which part controls all the activities of a cell?
- 4What is the use of the cell wall in plants?
- 5Why is glycerin put on the slide?
- 6Which stain is used for cheek cells and which for onion peel?
- 7Name two parts found in a plant cell but not in an animal cell.
- 8A student folds the onion peel and traps air under the coverslip. What goes wrong?
Show answers (try them yourself first)
- 1The cell.
- 2Cell membrane, cytoplasm and nucleus.
- 3The nucleus.
- 4It gives rigidity and strength to the plant.
- 5It keeps the cells from drying and gives a clearer view.
- 6Methylene blue for cheek cells; safranin for onion peel.
- 7Cell wall and chloroplasts (also a large vacuole).
- 8The folds and bubbles hide the cells, so they cannot be seen clearly.
Structure of a cell
In the app you can move this figure with your finger.
Open in the appShapes of cells and levels of organisation
Bricks make a wall, walls make a room, rooms make a house. In the same way, cells make tissues, tissues make organs, and so on.
The body of an organism is organised in levels: cell → tissue → organ → organ system → organism. The shape of a cell suits the job it does.
Key idea 1: Levels of organisation — A group of similar cells is a tissue; different tissues form an organ; organs working together form an organ system.
- Easy: A group of similar muscle cells is muscle tissue.
- Medium: The stomach is an organ; the stomach, food pipe and intestine together are part of the digestive system.
- Exam-type: A multicellular organism begins as a single cell (the egg) that divides again and again; a unicellular organism does all its work in one cell.
In the figure: Read left to right and down: each level is built from the one before it.
Key idea 2: Shape suits function — A muscle cell is spindle-shaped and can contract; a nerve cell is long and branched to carry messages quickly; a cheek cell is thin and flat to form a protective lining.
- Easy: A muscle cell is spindle-shaped.
- Medium: Nerve cells are long and branched so that they reach all parts of the body.
- Exam-type: The yolk of an ostrich egg is a single cell, the largest known, about 130 to 170 mm across; the shell and white are not part of that cell.
The cell is to the body as a brick is to a wall.
Mixing up tissue and organ. A tissue has similar cells; an organ has different tissues.
Learn the order as one word. Memory clue: C-T-O-S-O = "Can The Old Sage Observe?" (Cell, Tissue, Organ, organ System, Organism).
Spot the difference
| Level | What it is | Example |
|---|---|---|
| Cell | basic unit of life | a nerve cell |
| Tissue | group of similar cells | muscle tissue |
| Organ | different tissues together | the stomach |
| Organ system | organs working together | the digestive system |
Practice questions
- 1What does a group of similar cells form?
- 2What shape is a muscle cell?
- 3What do nerve cells carry?
- 4Write the levels of organisation in order.
- 5Why are nerve cells long and branched?
- 6How do food-pipe muscle cells push food down?
- 7A student says an ostrich egg yolk cannot be one cell because it is big. Reply.
- 8Give one example each of a unicellular and a multicellular organism.
Show answers (try them yourself first)
- 1A tissue.
- 2Spindle-shaped.
- 3Messages.
- 4Cell, tissue, organ, organ system, organism.
- 5To reach different parts of the body and pass messages quickly.
- 6By contracting and relaxing in a wave-like manner.
- 7The yolk is a single cell; the shell and white are extra non-cellular material.
- 8Unicellular: Amoeba or bacteria; multicellular: a pea plant or a human.
Microorganisms: kinds and places
A tomato left outside gets a powdery, cotton-like growth, but pickles in a jar stay fine for months. Microbes cause the first; salt and oil stop them in the second.
Microorganisms (microbes) are living things too small to see with the naked eye. Some are made of one cell (unicellular), others of many cells (multicellular), and they live almost everywhere.
Key idea 1: The main groups — Bacteria and protozoa are unicellular; fungi and algae may be unicellular or multicellular. Viruses are even smaller and are not made of cells.
- Easy: Amoeba and Paramecium are protozoa.
- Medium: Yeast is a unicellular fungus; mould is a multicellular fungus.
- Exam-type: Fungi have a cell wall but no chloroplasts, so they cannot make food by photosynthesis; bacteria have a nucleoid, not a well-defined nucleus.
In the figure: Each card gives the main facts about one group.
Key idea 2: Where they live and how we meet them — Microbes live in water, soil, air, food, hot springs, snowy places and even inside our gut. Salt or sugar in high amounts keeps them from growing on pickles and murabbas. Viruses multiply only inside a living host cell.
- Easy: Microbes are found everywhere.
- Medium: High salt or sugar acts as a preservative.
- Exam-type: Use a spoon or gloves, not bare hands, when you collect moist soil.
A microscope showing 100 to 400 times is enough to see most pond-water microbes.
Thinking all microbes are harmful or all are helpful. Some help us and some cause disease.
List the groups in one go. Memory clue: B-P-F-A = "Be Positive, Find Answers" (Bacteria, Protozoa, Fungi, Algae), and V for virus.
Spot the difference
| Group | Cells | Example |
|---|---|---|
| Bacteria | one cell, nucleoid | Lactobacillus |
| Protozoa | one cell | Amoeba, Paramecium |
| Fungi | one or many | yeast (one), mould (many) |
| Algae | one or many, green | microalgae such as Spirulina |
Practice questions
- 1What are microorganisms?
- 2Name one unicellular microorganism.
- 3Where can microbes be found?
- 4How is yeast different from mould?
- 5What do bacteria have instead of a nucleus?
- 6Why do pickles not get spoiled by microbes?
- 7Why can fungi not make their own food?
- 8How are viruses different from other microorganisms?
Show answers (try them yourself first)
- 1Living things too small to be seen with the naked eye.
- 2Amoeba (or bacteria, Paramecium).
- 3In water, soil, air, food and even inside our body.
- 4Yeast is unicellular; mould is multicellular.
- 5A nucleoid.
- 6The high salt or sugar does not let the microbes grow.
- 7They have no chloroplasts, so they cannot photosynthesise.
- 8Viruses are not made of cells and multiply only inside a living host cell.
Microbes and our daily life
The curd at home, the soft bread from the bakery and the compost pit in the school garden all depend on microbes. Many microbes are our silent helpers.
Helpful microbes decompose waste into manure, make foods such as bread and curd, add nitrogen to the soil and give us oxygen.
Key idea 1: Cleaning and recycling — Fungi and bacteria break dead plants and animals into simple substances rich in nutrients; this is called decomposition.
- Easy: Fruit and vegetable peels in a soil pit turn into manure.
- Medium: Some microbes that live without oxygen break down waste and release biogas, a mixture of carbon dioxide and a lot of methane, used as fuel.
- Exam-type: Ananda Mohan Chakrabarty developed a bacterium that breaks down oil spills.
In the figure: Yeast, Lactobacillus and Rhizobium each do a different useful job.
Key idea 2: Food and farming — Yeast respires and releases carbon dioxide bubbles that make dough fluffy. Lactobacillus turns milk into curd with lactic acid. Rhizobium in the root nodules of legumes traps nitrogen from air, so farmers grow legumes in rotation. Microalgae release more than half of Earth's oxygen.
- Easy: Yeast makes bread and cakes soft.
- Medium: Milk curdles in a warm place because the bacteria grow well in warmth.
- Exam-type: Dough A with yeast rises and dough B without yeast does not, so yeast is the cause; B is the comparison.
Without microbes, dead matter would pile up and nutrients would not return to the soil.
Saying yeast gives out oxygen. Yeast releases carbon dioxide.
One helper, one job. Memory clue: Y-L-R = Yeast raises, Lactobacillus sours, Rhizobium feeds (nitrogen).
Spot the difference
| Microbe | Where it works | Its job |
|---|---|---|
| Yeast | dough | releases carbon dioxide |
| Lactobacillus | milk, idli batter | makes lactic acid |
| Rhizobium | legume roots | traps nitrogen |
Practice questions
- 1What is the breaking down of dead plants and animals by microbes called?
- 2Which microbe makes dough fluffy?
- 3Which bacterium helps form curd?
- 4Why does milk turn into curd in a warm place but not in a cold one?
- 5What is biogas mainly made of?
- 6Where do Rhizobium bacteria live?
- 7Why do bean farmers need less nitrogen fertiliser?
- 8Snehal mixes dry leaves with peels in pit A but not in pit B. What is she testing?
Show answers (try them yourself first)
- 1Decomposition.
- 2Yeast.
- 3Lactobacillus.
- 4The curd bacteria grow well in warm conditions.
- 5Carbon dioxide and a high proportion of methane.
- 6In the root nodules of legumes such as peas, beans and lentils.
- 7Rhizobium in the nodules traps nitrogen from the air for the plants.
- 8Whether mixing dry leaves changes how manure forms.