Class 9 BiologyChapter 2NCERT 2.1
How do we study cells?
Your eye stops at a tenth of a millimetre. Here is how biologists go further.
Think it over
- Draw two dots on paper and slowly push them together. At what point do they become one blob, and is that the paper or your eye?
- A grain of sand is easy to see. An onion cell is hundreds of times smaller across. What tool would you build to see it?
- If you can only see a cell made bigger, how could you ever know its real size?
In one minute
- From about 25 cm away, the human eye can tell two points apart only if they are at least 0.1 mm apart. This is its limit of resolution.
- Most cells are far smaller than that, so we need lenses. A light microscope uses an objective lens and an eyepiece; their powers multiply.
- Robert Hooke first saw cells in 1665, in a thin slice of cork, with a microscope he built himself.
- You can estimate a cell's real size: field of view in µm divided by the number of cells across it.
- Electron microscopes use a beam of electrons instead of light and show details down to nanometres.
Where your eye gives up
Hold a page at a comfortable reading distance, about 25 cm, which is the near point of the eye. Two dots on it look separate only while there is at least 0.1 mm of gap between them. Bring them any closer and your eye merges them into one point.
That smallest gap is called the limit of resolution of the human eye. Anything smaller than about 0.1 mm across is simply invisible to the unaided eye, however hard you stare.
Most plant and animal cells are only 10 to 100 micrometres across: from a tenth of that limit up to about the limit itself. So most are too small to see clearly, and to study cells, biologists had to find ways to see past the eye. Scroll the zoom below from your own height all the way down to an atom, and watch which tool you need at each step.
1 mA child, a Great Indian Bustard
Everyday sizes. Your eyes manage these easily.
- 1 m: A child, a Great Indian BustardUnaided eye. Everyday sizes. Your eyes manage these easily.
- 10 cm: A chicken eggUnaided eye. One egg yolk is one cell, a giant one, before the chick starts to grow.
- 1 mm: A fish eggUnaided eye. Still visible. The eye tells two points apart down to about 0.1 mm.
- 100 µm: An amoebaLight microscope. A single cell at the very edge of what eyes can see. A light microscope shows it clearly.
- 10 µm: Most plant and animal cellsLight microscope. Most cells are 10 to 100 µm across, from a tenth of the eye's limit up to about the limit itself, so they are too small to see clearly. A school microscope shows them.
- 1 µm: Most bacteria, a mitochondrionLight microscope. Bacteria are about 1 to 10 µm. Organelles this small show little detail under light.
- 100 nm: VirusesElectron microscope. Too small for any light microscope. Here you need a beam of electrons.
- 10 nm: Ribosomes, proteins, the cell membraneElectron microscope. A ribosome is a few tens of nm; the membrane is only 7 to 10 nm thick.
- 1 nm: Small moleculesElectron microscope. Glucose, water and salt live down here, a millionth of a millimetre across.
- 0.1 nm: AtomsElectron microscope. The bottom of the ladder: a ten-billionth of a metre.
- Limit of resolution
- The smallest gap at which two points still look separate. For the eye, 0.1 mm.
- Micrometre (µm)
- One thousandth of a millimetre. 1 mm = 1000 µm.
- Nanometre (nm)
- One billionth of a metre, or one thousandth of a micrometre.
Quick check
Two tiny dots are 0.05 mm apart on a page held 25 cm from your eye. What do you see?
Show the answer
B. One dot, because they are closer than the eye can resolve 0.05 mm is less than the eye's limit of resolution of 0.1 mm, so the two points blur into one.
Lenses that make small things look big
A convex lens bends light so that an object looks larger than it is. A microscope puts two lenses to work one after the other: the objective lens near the object makes a bigger image, and the eyepiece near your eye enlarges that image again.
In 1665, Robert Hooke looked at a very thin slice of cork through a microscope he had designed himself, which magnified about 200 to 300 times. He saw tiny box-like compartments packed together and called them cells. The name stuck for every living thing since.
Your school light microscope works with ordinary light and usually has objectives of 10X and 40X. Tap the parts below to see what each one does.
The lens you look through. It magnifies the image made by the objective again, often 10X.
Holds the eyepiece and the objective lenses the right distance apart, and light travels up it.
The lenses closest to the slide. They make the first enlarged image; school microscopes often have 10X and 40X.
The flat platform where the slide sits, over a hole that lets light through.
Reflects light up through the hole in the stage and through the slide.
Moves the lenses a long way up or down to bring the slide roughly into focus.
Makes tiny movements for a sharp focus, especially at high magnification.
The arm you hold to carry the microscope, with one hand under the base.
The heavy foot that keeps the microscope steady.
Quick check
Which pair of lenses does the actual magnifying in a light microscope?
Show the answer
B. Objective lens and eyepiece The objective makes the first enlarged image and the eyepiece enlarges it again. The mirror only lights the slide and the knobs only focus.
Estimate the size of a cell
Under a microscope you see a cell made bigger, so how do you get its real size? Use something you already know the size of: the circle of light you see, called the field of view.
First put a transparent ruler on the stage and read how many millimetres fit across the circle. Then swap the ruler for an onion peel slide and count how many cells sit in a straight line across the same circle. One division gives the size of one cell.
Field of view
Read where the edge of the circle meets the ruler.
Cells across the line
Switch to the onion peel and tap each cell on the dashed line.
Size of one cell
Fill in both readings
How much bigger does it look?
This field of view is 5 mm wide, which is 5000 µm, and 25 onion cells fit across it.
Size of one cell = 5000 ÷ 25 = 200 µm.
Size of one cell = diameter of the field of view in µm ÷ number of cells across it. If the field is 5 mm wide, that is 5 × 1000 = 5000 µm; with 25 cells across, each cell is about 5000 ÷ 25 = 200 µm.
How much bigger does the cell look? Multiply the powers of the two lenses. A 10X eyepiece with a 10X objective gives 100X, so a 200 µm cell looks as big as a 20 mm object would to the naked eye.
Quick check
The field of view is 4 mm wide and 20 cells fit across it. About how big is one cell?
Show the answer
C. 200 µm 4 mm = 4000 µm, and 4000 ÷ 20 = 200 µm.
Beyond light: the electron microscope
A light microscope cannot show the tiny parts inside a cell clearly. Electron microscopes use a beam of electrons in place of light, and give images detailed enough to show structures only a few nanometres across. Most organelles were first seen properly this way.
Over the years, microscopes have been improved in three ways: resolution (how clearly close details stay separate), contrast (how much the parts differ in brightness) and magnification (how much bigger the image is). A scanning electron microscope can even show the pores called stomata on the underside of a Colocasia leaf.
- Resolution
- Clarity: how well two close details stay separate.
- Contrast
- The difference in brightness between parts of the object.
- Magnification
- How many times bigger the image is than the object.
Quick check
Why can an electron microscope show ribosomes when a light microscope cannot?
Show the answer
B. It uses a beam of electrons and resolves far smaller details Ribosomes are only tens of nanometres across. An electron beam resolves details down to the nanometre scale; visible light cannot.