How to study cells: the limit of the eye and the discovery of the cell
Every living thing you know, the neem tree outside your school, the pigeon on the balcony, the mould on old bread and you yourself, is built from cells.
A cell is the smallest unit that is alive: it takes in materials, uses energy, gets rid of waste and makes more of itself.
Yet you have never seen one of your own cells with your bare eyes.
This first section explains why, and how scientists first got round the problem.
You need to know what limits the human eye, how the cell was discovered, and how to work with the tiny units in which cells are measured.
A quick exploration: take a sheet of paper and, with a very sharp pencil, make two dots as close together as you can.
Hold the paper at a comfortable reading distance, about 25 cm from your eye.
If the dots are far enough apart you see two dots.
Now try a pair that almost touch.
At some point the two dots merge and you see only one.
Your eye has reached its limit.
The limit of resolution of the human eye
The ability to see two very close points as separate is called resolution.
Your eye is good, but not perfect.
When you look at something from about 25 cm (the near point, the closest distance at which a normal eye sees clearly without strain), two points must be at least about 0.1 mm apart to be seen as two.
If they are closer, the eye blends them into one point.
This smallest separation, 0.1 mm, is the limit of resolution of the human eye.
Why it works (not examined): the eye forms an image on the retina, which is lined with light-sensitive cells packed side by side.
Two points are seen as two only if their images fall on different sensing cells with an unstimulated one between them.
Below about 0.1 mm at 25 cm, both images land on neighbouring cells and the brain reports one blur.
Most plant and animal cells are between about 10 µm and 100 µm across, that is 0.01 mm to 0.1 mm.
So almost all cells are at or below the limit of the eye, and their insides (a nucleus, the parts around it) are far smaller still.
That is why a cell biologist cannot work without a microscope.
Units for very small things
You will meet three units again and again in this chapter.
Learn the steps between them; each is a factor of 1000.
Unit
Symbol
In metres
Useful link
millimetre
mm
one-thousandth of a metre
1 mm = 1000 µm
micrometre
µm
one-millionth of a metre
1 µm = 1000 nm
nanometre
nm
one-billionth of a metre
1 nm = 0.001 µm = 0.000001 mm
To go to a smaller unit, multiply by 1000; to go to a bigger unit, divide by 1000.
A quick check: the number should get bigger when the unit gets smaller, because more small units fit into the same length.
The ladder below places some cells and cell parts against these units.
Each step down is ten times smaller.
Interpret it: everything below the 0.1 mm mark needs help to be seen.
A light microscope takes you down to cells, nuclei and bacteria; an electron microscope (later in this chapter) takes you down to ribosomes and the thickness of the cell membrane.
Robert Hooke and the first "cells"
The story of the cell begins with a lens.
A convex lens, or two lenses used together, makes a small object look larger.
In 1665 the English scientist Robert Hooke built his own microscope, which could magnify about 200 to 300 times.
He cut a very thin slice of cork (the light, spongy bark of the cork oak, used to stopper bottles) and looked at it.
He saw that the cork was made of many tiny box-like compartments, packed side by side like the cells of a honeycomb.
He called them cells, from a Latin word for a small room.
Hooke described his observations in his book Micrographia.
A thin slice of cork looks like this under a light microscope.
Notice that the compartments are empty: only the walls are left.
Two things are worth understanding about this discovery.
- Hooke saw walls, not life. Cork is made of dead cells. What Hooke saw were the empty walls left behind, with nothing living inside. The living contents of cells (the nucleus and the jelly around it) were studied later, with better microscopes.
- The idea came before the full picture. Hooke's word "cell" stuck, and over the next two centuries scientists found that every plant and animal they examined was made of cells. That work led to the cell theory, which you will meet at the end of this chapter.
In each Key terms list, a definition printed in italics is the wording to learn and write in your answers.
- Limit of resolution of the human eye: the smallest gap at which two points still look separate. When viewed from about 25 cm (the near point of human eye), two points separated by about 0.1 mm can be seen as distinct; otherwise, they appear as a single point. This is called the limit of resolution of the human eye, which is 0.1 mm.
- Cell (Hooke's discovery): Robert Hooke was the first person to observe a cell in 1665 using a self-designed microscope (capable of about 200–300X magnification). While examining a thin slice of cork, he observed small box-like compartments and named them ‘cells’.
- Unit conversions: 1 millimetre (mm) = 1000 micrometre (µm); 1 µm = 1000 nanometres (nm); a nanometre is one-billionth of a metre.
- Resolution: the ability to show two very close points as separate and distinct.
A human cheek cell is about 60 µm across.
Explain, with a calculation, why you cannot see a single cheek cell with your unaided eye. [2]
1. Convert the limit of the eye to the same unit: 0.1 mm = 0.1 × 1000 = 100 µm (1).
2. Compare: 60 µm is smaller than 100 µm, the limit of resolution of the eye, so the cell (and any detail inside it) cannot be seen as a separate object from 25 cm (1).
Answer: the cheek cell is below the eye's limit of resolution of 100 µm (0.1 mm), so a microscope is needed.
Four pairs of dots are drawn on a card held 25 cm from a student's eye.
The gaps are 0.02 mm, 0.05 mm, 0.08 mm and 0.3 mm.
Which pair will the student see as two separate dots?
(A) 0.02 mm (B) 0.05 mm (C) 0.08 mm (D) 0.3 mm
1. Only a gap of about 0.1 mm or more is resolved by the eye.
2. Of the four gaps, only 0.3 mm is larger than 0.1 mm.
Answer: (D) / 0.3 mm (1)
The cell membrane is about 8 nm thick.
(a) Write this thickness in micrometres and in millimetres.
(b) How many times larger is the eye's limit of resolution than this thickness? [3]
1. Nanometres to micrometres: divide by 1000. 8 ÷ 1000 = 0.008 µm (½).
2. Micrometres to millimetres: divide by 1000 again. 0.008 ÷ 1000 = 0.000008 mm (½).
3. Put both lengths in the same unit. 0.1 mm = 0.1 × 1000 × 1000 = 100 000 nm (1).
4. Divide: 100 000 nm ÷ 8 nm = 12 500 (1).
Answer: 0.008 µm = 0.000008 mm; the eye's limit is 12 500 times the membrane's thickness.
This is why the membrane's structure can be studied only with an electron microscope.
Class IX has no board examination: your school sets the annual examination, usually in the same question types as the Class X board paper.
In any "define" item on resolution, write the whole idea, distance and value: "from about 25 cm, two points closer than about 0.1 mm appear as a single point".
"The eye cannot see small things" earns nothing.
- Writing that Hooke saw living cells with nuclei. Cork cells are dead; he saw only their box-like walls.
- Multiplying when you should divide in a conversion. Going from µm to mm (a bigger unit) the number must get smaller: 250 µm = 0.25 mm, not 250 000 mm.
- Giving the limit of resolution without the viewing distance. The 0.1 mm value holds when the object is about 25 cm from the eye.
- Confusing magnification with resolution. Making an image bigger does not by itself make two close points look separate; resolution is about clarity.
Check your understanding
1. The limit of resolution of the normal human eye, for an object held about 25 cm away, is about
(A) 0.01 mm
(B) 0.1 mm
(C) 1 mm
(D) 0.1 µm [1]
2. Assertion (A): Robert Hooke saw nuclei inside the cells of the cork slice he examined in 1665.
Reason (R): Cork is made of dead cells, so only their box-like walls remain.
Choose the correct option from (A), (B), (C) and (D).
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
(B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
(C) Assertion (A) is true, but Reason (R) is false.
(D) Assertion (A) is false, but Reason (R) is true. [1]
3. Define the limit of resolution of the human eye, and state why most cells cannot be seen without a microscope. [2]
4. A bacterium is about 2 µm long.
(a) Write its length in millimetres. (1)
(b) Calculate how many such bacteria, placed end to end, would make a line 0.1 mm long. (1)
(c) State whether a single bacterium can be seen with the unaided eye, giving a reason. (1) [3]
5. A craftsman in Moradabad engraves two tiny marks on a brass plate. The marks are 0.04 mm apart. A customer looking at the plate from a normal reading distance says she can see only one mark. The craftsman hands her a magnifying glass, and she now sees two.
(a) Why did the customer see only one mark at first? (1)
(b) How did the magnifying glass help her see two marks? (1) [2]
6. Consider the following statements about the discovery of the cell.
(i) Hooke used a microscope that he designed himself.
(ii) Hooke examined a thin slice of cork.
(iii) Hooke saw small box-like compartments and named them cells.
(iv) Hooke described the nucleus as the control centre of the cell.
Which statements are correct?
(A) (i) and (iv) only
(B) (ii) and (iii) only
(C) (i), (ii) and (iii)
(D) (i), (ii), (iii) and (iv) [1]
Answers
1. (B) / 0.1 mm (1)
2. (D) / Assertion (A) is false, but Reason (R) is true. (1) The assertion is false: cork cells are dead, so Hooke saw only empty compartments. The reason is true.
3. When viewed from about 25 cm (the near point of human eye), two points separated by about 0.1 mm can be seen as distinct; otherwise, they appear as a single point; this is the limit of resolution of the human eye, 0.1 mm (1). Most cells are about 10 µm to 100 µm across, at or below 0.1 mm, so they and their parts cannot be seen as separate objects without a microscope (1).
4. (a) 2 ÷ 1000 = 0.002 mm (1)
(b) 0.1 mm = 100 µm; 100 ÷ 2 = 50 bacteria (1)
(c) No: 2 µm (0.002 mm) is far below the eye's limit of resolution of 0.1 mm (1).
5. (a) The gap of 0.04 mm is less than the eye's limit of resolution of 0.1 mm, so the two marks appeared as one point (1).
(b) The lens magnified the plate, so the image of the gap became larger than 0.1 mm and the eye could resolve the two marks (1).
6. (C) / (i), (ii) and (iii) (1). Statement (iv) is wrong: Hooke saw only walls of dead cells; he did not describe a nucleus.