MYP 3 Sciences 1.1 Microscopes and Cell Observation

MYP 3 Sciences 1.1 Microscopes and Cell Observation - Full notes

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MYP 3 Sciences | 1.1 Microscopes and Cell Observation

MYP 3 SCIENCES · CHAPTER 1: CELLS AND LIVING SYSTEMS

1.1

Microscopes and Cell Observation

Full notes for MYP Year 3 · IB-style

Contents

1	What is a microscope?
2	The parts of a light microscope
3	Using a microscope safely and correctly
4	Preparing slides of cells
5	Drawing and measuring what you see
6	Microscopes in the real world
7	Data-based questions and further practice
8	Common misconceptions and links
9	Exam-style question with markscheme
10	Check your understanding
11	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
name the parts of a light microscope and their jobs
use a microscope safely and focus correctly
calculate total magnification
prepare onion and cheek cell slides
make a scientific drawing
use magnification = image size ÷ real size

Guiding questions

How do microscopes help us see cells? How do we prepare and observe cells safely? How do we record and measure what we see?

How this subtopic is assessed

Criterion A questions test microscope parts, magnification and slide preparation. Criterion B and C questions use cell-counting and measuring investigations. Criterion D questions discuss how microscopes are used in medicine and industry.

Show every step of a calculation and give units.

Criterion	Name	What it rewards
A	Knowing and understanding	recalling, explaining and applying scientific knowledge to familiar and unfamiliar situations
B	Inquiring and designing	questions, hypotheses, variables and methods
C	Processing and evaluating	presenting and interpreting data, evaluating conclusions
D	Reflecting on the impacts of science	applications, implications and limitations of science in society

Command term	What the examiner wants
State / Write down	a brief answer with no working or explanation
Outline	a brief account or summary
Describe	a detailed account without reasons
Explain	a detailed account including causes, reasons or mechanisms
Calculate / Find	a numerical answer with working shown
Show that	obtain the given result, showing every step
Sketch	a graph or diagram showing the general shape and key features
Justify	give valid reasons or evidence for an answer
Evaluate	weigh strengths and limitations and reach a judgement
Suggest / Predict	propose a reasonable answer from the information given

How to use these notes

Each section teaches one topic in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section
Key point	the one idea you must not forget
Common mistake	an error teachers see every year, and how to avoid it
In the real world	where the idea shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 What is a microscope?

Seeing the invisible.

Think about it

Hold up your hand. It is made of millions of cells, but you cannot see a single one. Why not, and what would you need to see them?

Most living things are made of tiny building blocks called cells. A cell is usually far too small to see with your eyes alone: a typical cheek cell is about 60 micrometres (µm) across, and a micrometre is one thousandth of a millimetre. To study cells, scientists use a microscope, an instrument that uses lenses to make small things look bigger.

In this booklet you will learn the parts of a light microscope, how to use one safely, how to prepare your own slides of plant and animal cells, how to work out magnification, and how to draw what you see like a scientist.

1.1 From hand lens to microscope

A hand lens (magnifying glass) is a single curved convex lens. Light from the object bends as it passes through the lens, so your eye sees a bigger image. A hand lens usually magnifies about 5 to 10 times (×5 to ×10). This is great for looking at insects or leaves, but cells need much more.

A light microscope uses two lenses, one after the other: the objective lens near the specimen and the eyepiece lens near your eye. Each lens magnifies the image again, so together they can make a cell look hundreds of times bigger.

Figure 1. A hand lens makes a small object look bigger.

Figure 2. How many times bigger each tool can make things look (log scale).

Key point

Magnify means make something look bigger. A microscope uses lenses to magnify tiny objects like cells.

WORKED EXAMPLE 1 Recall (basic)

Name the type of lens used in a hand lens.

Solution

a convex (magnifying) lensA1

WORKED EXAMPLE 2 Compare (standard)

Give one reason why a light microscope is better than a hand lens for looking at cells.

Solution

it magnifies much more (up to about ×400 in school)A1

cells are too small to see clearly with ×10R1

In the real world

Jewellers and watch makers use a small hand lens called a loupe to check tiny scratches and parts.

1.2 How small is small?

Scientists measure cells in micrometres. There are 1000 micrometres in 1 millimetre (1 mm = 1000 µm). The chart compares the sizes of some tiny things. Notice that the scale goes up in steps of 10 times, because the sizes are so different.

Figure 3. Sizes of some small things (log scale).

WORKED EXAMPLE 3 Convert (basic)

Convert 2 mm to micrometres.

Solution

2 × 1000M1

= 2000 µmA1

WORKED EXAMPLE 4 Convert (standard)

An onion cell is 200 µm long. Write this in mm.

Solution

200 ÷ 1000M1

= 0.2 mmA1

WORKED EXAMPLE 5 Compare (standard)

How many cheek cells (60 µm) would fit side by side across 3 mm?

Solution

3 mm = 3000 µmM1

3000 ÷ 60 = 50 cellsA1

Common mistake

Multiplying when you should divide. Going from mm to µm makes the number BIGGER (×1000); going from µm to mm makes it SMALLER (÷1000).

1.3 A short history

In 1665 Robert Hooke looked at a thin slice of cork through his microscope. He saw tiny empty boxes and called them 'cells' because they reminded him of small rooms. A few years later Antonie van Leeuwenhoek, a Dutch cloth merchant, made tiny powerful lenses and became the first person to see living bacteria and pond organisms. Over the next 200 years, scientists realised that every living thing is made of cells.

Figure 4. What Hooke saw: the walls of dead cork cells.

Figure 5. Some milestones in microscopy.

WORKED EXAMPLE 6 Recall (basic)

Who first used the word 'cell' and what was he looking at?

Solution

Robert HookeA1

a thin slice of corkA1

2 The parts of a light microscope

Know your instrument.

Think about it

Why do you think a microscope needs a light shining up from underneath the slide?

Every part of a light microscope has a job. Light travels up from the lamp (or mirror), through the thin specimen on the slide, through the objective lens, up the body tube and through the eyepiece lens to your eye. If the specimen is too thick, light cannot pass through it and you will only see a dark shadow.

2.1 Labelling the microscope

Learn the names of the parts in the diagram. The nosepiece holds the objective lenses and turns so you can choose a different magnification. The coarse focus knob moves the stage a long way and is used first, at low power. The fine focus knob moves it only a tiny amount and is used to sharpen the image.

Figure 6. The main parts of a school light microscope.

Figure 7. What each part does.

Figure 8. The path of light through a microscope.

WORKED EXAMPLE 7 Function (basic)

State the job of the stage and of the fine focus knob.

Solution

stage: holds the slideA1

fine focus: makes small adjustments to give a sharp imageA1

WORKED EXAMPLE 8 Explain (standard)

Explain why the specimen on a slide must be very thin.

Solution

light has to pass through the specimenR1

a thick specimen blocks the light / cells overlap, so the image is dark or unclearR1

WORKED EXAMPLE 9 Order (standard)

Put these in the order light passes through them: eyepiece, slide, lamp, objective.

Solution

lamp → slide → objective → eyepieceA2

Try this

Cover the labels on the microscope diagram with a sheet of paper and try to name every part from memory.

2.2 Total magnification

Each lens has its magnification printed on it. The total magnification tells you how many times bigger the image is than the real object. You find it by multiplying the two lens magnifications together.

Figure 9. Total magnification with a ×10 eyepiece and different objectives.

WORKED EXAMPLE 10 Calculate (basic)

Eyepiece ×10, objective ×4. Find the total magnification.

Solution

10 × 4M1

= ×40A1

WORKED EXAMPLE 11 Calculate (standard)

Eyepiece ×15, objective ×40. Find the total magnification.

Solution

15 × 40M1

= ×600A1

WORKED EXAMPLE 12 Reverse (standard)

A microscope gives ×400 with a ×10 eyepiece. Which objective is in use?

Solution

400 ÷ 10M1

= ×40 objectiveA1

WORKED EXAMPLE 13 Problem (exam level)

A student wants exactly ×250. She has eyepieces ×5 and ×10 and objectives ×10, ×25 and ×50. Give two combinations that work.

Solution

×10 eyepiece with ×25 objective = ×250A1

×5 eyepiece with ×50 objective = ×250A1

Common mistake

Adding the two numbers. ×10 and ×40 give ×400, not ×50.

2.3 Field of view

The field of view is the circle of light you see when you look into the microscope. When you switch to a higher magnification, everything looks bigger, but you see a smaller part of the slide. So at high power you see fewer cells, but each one is larger and shows more detail.

Figure 10. The same onion skin at low and high power.

Figure 11. Field of view width at three magnifications.

Figure 12. Number of onion cells seen in the field of view.

WORKED EXAMPLE 14 Read graph (standard)

Use the field of view graph to find the width seen at ×100.

Solution

about 1.6 mmA1

WORKED EXAMPLE 15 Explain (standard)

Explain why fewer cells are seen at ×400 than at ×40.

Solution

each cell is magnified more, so looks biggerR1

only a small part of the slide fits in the field of viewR1

WORKED EXAMPLE 16 Estimate (exam level)

At ×100 the field of view is 1.6 mm wide. About 8 cells fit across it in a row. Estimate the length of one cell in µm.

Solution

1.6 mm = 1600 µmM1

1600 ÷ 8 = 200 µmA1

3 Using a microscope safely and correctly

Good habits.

Microscopes are delicate and expensive, and the slides are made of thin glass. Following a set routine protects the lenses, keeps you safe, and helps you find your specimen quickly.

3.1 Handling and safety

Always carry a microscope with two hands. Glass slides and cover slips can break and cut you, so report any breakage to your teacher instead of picking up the pieces. If your microscope has a mirror, never point it at the Sun: focused sunlight can damage your eye.

Figure 13. Six golden rules for using a microscope.

WORKED EXAMPLE 17 Safety (basic)

Give two safety rules for using a microscope.

Solution

carry it with two hands (one under the base)A1

never reflect direct sunlight into the eyepiece / report broken glassA1

3.2 Focusing step by step

Begin with the lowest power objective. Watch from the side as you bring the stage close to the lens, then look through the eyepiece and focus by moving the stage AWAY from the lens. This way the lens never crashes into the slide. Centre what you want to see before switching to a higher objective, then use only the fine focus.

Figure 14. How to focus a light microscope.

WORKED EXAMPLE 18 Method (standard)

Explain why you should start with the lowest power objective.

Solution

the field of view is largest, so the specimen is easier to findR1

the lens is far from the slide, so it is less likely to hit itR1

WORKED EXAMPLE 19 Problem (standard)

A student sees only a bright empty circle at ×400. Suggest two reasons.

Solution

the specimen is not under the lens / not centredR1

the image is out of focus or the slide has no cells in that areaR1

Common mistake

Turning the coarse focus at high power. This can push the objective lens into the slide and break it.

4 Preparing slides of cells

Becoming a cell scientist.

Think about it

Most cells are almost see-through. How could you make them easier to see?

A specimen is the thing you look at. You place it on a glass slide and cover it with a very thin glass square called a cover slip. Many cells are nearly colourless, so we add a stain, a coloured dye that soaks into parts of the cell and makes them show up.

4.1 Onion skin cells (plant cells)

The thin inner skin of an onion layer is only one cell thick, which makes it perfect for a microscope. Iodine solution stains it yellow-brown and makes the nucleus easier to see. The cover slip is lowered at an angle using a mounted needle so that air is pushed out and no bubbles are trapped.

Figure 15. Making an onion skin slide.

Figure 16. A good slide and one with trapped air bubbles.

WORKED EXAMPLE 20 Method (standard)

Explain why the cover slip is lowered at an angle.

Solution

to push the air outR1

so that no air bubbles are trapped and hide the cellsR1

WORKED EXAMPLE 21 Identify (basic)

Name the stain used for onion cells and the colour it gives.

Solution

iodineA1

yellow-brownA1

4.2 Cheek cells (animal cells)

Cells from the inside of your cheek come off easily. They are stained with methylene blue, which colours the nucleus dark blue. Because the cotton bud has been in someone's mouth, it must go straight into disinfectant afterwards.

Feature	Onion skin cell	Cheek cell
shape	regular, box-like	rounded, irregular
cell wall	yes (thick outline)	no
nucleus	yes	yes
stain used	iodine	methylene blue
arrangement	neat rows, touching	separate or in small clumps

Figure 17. Making a cheek cell slide.

Figure 18. What you might see in three different slides.

Figure 19. Comparing onion and cheek cells.

WORKED EXAMPLE 22 Compare (standard)

Give two differences you would see between onion cells and cheek cells.

Solution

onion cells have a regular shape; cheek cells are irregularA1

onion cells have a cell wall (thick outline); cheek cells do notA1

WORKED EXAMPLE 23 Safety (standard)

Explain why the cotton bud is placed in disinfectant.

Solution

it carries saliva and microbesR1

disinfectant kills them so they cannot spreadR1

In the real world

Doctors look at stained cells scraped from the body to check for infections and diseases.

4.3 Living specimens

Not all specimens are dead. A drop of pond water often contains single-celled living things such as Amoeba, Paramecium and Euglena. These can move, so you may need to follow them by gently moving the slide.

Figure 20. Tiny organisms in pond water.

WORKED EXAMPLE 24 Observe (standard)

A student sees an organism moving to the left on the screen. In which direction should she move the slide to follow it, and why?

Solution

to the leftA1

in a school microscope the image is upside down and back to front, so moving the slide the same way the image moves keeps it centredR1

5 Drawing and measuring what you see

Recording like a scientist.

Scientists record observations as clear, simple drawings, not artistic pictures. A good drawing shows only what you actually see, and gives the magnification so other people know how big the real thing is.

5.1 Rules for scientific drawing

Use a sharp pencil and make clear, single, unbroken lines. Do not shade or colour. Draw label lines with a ruler; they should touch the part, never cross each other, and have no arrowheads. Write labels outside the drawing, give it a title, and write the magnification.

Figure 21. A good and a poor scientific drawing.

WORKED EXAMPLE 25 Rules (basic)

Give three rules for a scientific drawing.

Solution

sharp pencil, smooth single lines, no shadingA1

label lines drawn with a ruler that do not crossA1

title and magnificationA1

WORKED EXAMPLE 26 Evaluate (standard)

A student shaded the nucleus and used arrows for labels. Explain what she should change.

Solution

do not shade: draw only the outlineR1

use straight ruled label lines with no arrowheadsR1

5.2 Magnification of an image

You can also work out magnification by comparing the size of an image (for example your drawing or a photo) with the real size of the object. Both sizes must be in the same units.

Figure 22. The magnification triangle: cover the one you want.

Figure 23. A scale bar shows how long the real object is.

WORKED EXAMPLE 27 Calculate (basic)

A drawing of a seed is 40 mm long. The real seed is 4 mm long. Find the magnification.

Solution

40 ÷ 4M1

= ×10A1

WORKED EXAMPLE 28 Calculate (standard)

A photo of a cell at ×500 shows it 30 mm long. Find the real length in mm, then in µm.

Solution

30 ÷ 500 = 0.06 mmM1

= 60 µmA1

WORKED EXAMPLE 29 Calculate (standard)

A real cell is 0.05 mm long. Its image is 20 mm long. Find the magnification.

Solution

20 ÷ 0.05M1

= ×400A1

WORKED EXAMPLE 30 Scale bar (exam level)

In the scale bar figure, 40 mm on the page represents 20 µm. The drawn cell is 48 mm long. Find the real length.

Solution

each 2 mm on the page = 1 µmM1

48 ÷ 2 = 24 µmA1

Common mistake

Using mm for one size and µm for the other. Convert first.

5.3 Measuring with an eyepiece scale

Some microscopes have a tiny ruler inside the eyepiece called an eyepiece graticule. Your teacher can tell you how many micrometres each division is worth at each magnification, and then you can measure cells by counting divisions.

Figure 24. An eyepiece graticule and a stage micrometer used to calibrate it.

WORKED EXAMPLE 31 Measure (standard)

At ×400 one graticule division is 2.5 µm. A cell is 24 divisions long. Find its length.

Solution

24 × 2.5M1

= 60 µmA1

6 Microscopes in the real world

Why it matters.

Microscopes are used every day in hospitals, laboratories, factories and farms. Electron microscopes, which use beams of electrons instead of light, can magnify more than 100 000 times and show the tiny parts inside cells, but they are huge, very expensive and can only look at dead specimens.

6.1 Where microscopes are used

A blood test is often checked under a microscope to count different blood cells or look for parasites such as the one that causes malaria. Food scientists check for harmful bacteria, and forensic scientists compare fibres and hairs from a crime scene.

Feature	Light microscope	Electron microscope
uses	light and glass lenses	a beam of electrons
magnification	up to about ×1500	over ×100 000
specimens	living or dead, in colour	dead only, black and white
cost and size	cheap, fits on a desk	very expensive, fills a room

Figure 25. Some jobs that use microscopes.

WORKED EXAMPLE 32 Compare (standard)

Give one advantage and one disadvantage of an electron microscope compared with a light microscope.

Solution

advantage: much higher magnification and detailA1

disadvantage: very expensive / cannot view living cellsA1

WORKED EXAMPLE 33 Discuss (exam level)

A clinic in a remote village needs to test blood for malaria. Discuss whether it should buy a light microscope or an electron microscope.

Solution

light microscope is cheap, portable and easy to useR1

malaria parasites are big enough to see with a stained blood slideR1

electron microscope is too expensive and needs special training and power: light microscope is the better choiceA1

7 Data-based questions and further practice

Applying the ideas.

7.1 Data: counting cells

A student counted onion cells in the field of view at three magnifications and repeated each count three times.

Magnification	Count 1	Count 2	Count 3	Mean
×40	196	204	200	?
×100	30	34	32	?
×400	2	2	2	?

Figure 26. What happens as magnification increases.

WORKED EXAMPLE 34 Mean (standard)

Calculate the mean number of cells at ×40 and at ×100.

Solution

(196 + 204 + 200) ÷ 3 = 200A1

(30 + 34 + 32) ÷ 3 = 32A1

WORKED EXAMPLE 35 Pattern (standard)

Describe the pattern in the results.

Solution

as magnification increases, the number of cells seen decreasesA1

from 200 at ×40 to only 2 at ×400A1

WORKED EXAMPLE 36 Explain (exam level)

Explain why the counts at ×40 vary more than those at ×400.

Solution

at ×40 there are many cells, some cut by the edge of the circleR1

it is hard to count them all accurately; at ×400 there are only 2 to countR1

7.2 Graded practice

WORKED EXAMPLE 37 Recall (basic)

What does a stain do?

Solution

colours parts of cells so they are easier to seeA1

WORKED EXAMPLE 38 Calculate (basic)

Find the total magnification: eyepiece ×5, objective ×10.

Solution

×50A1

WORKED EXAMPLE 39 Convert (standard)

Convert 0.5 mm to µm.

Solution

0.5 × 1000 = 500 µmA1

WORKED EXAMPLE 40 Calculate (standard)

An image is 24 mm long. The magnification is ×400. Find the real size in µm.

Solution

24 ÷ 400 = 0.06 mmM1

= 60 µmA1

WORKED EXAMPLE 41 Explain (exam level)

A slide shows a dark blurry patch at high power, even after focusing. Suggest two reasons and how to fix them.

Solution

specimen too thick: use a thinner layerR1

lens dirty or air bubble: clean with lens paper / remake slideR1

7.3 Graded practice 2

WORKED EXAMPLE 42 Identify (basic)

Which knob should be used at ×400?

Solution

fine focusA1

WORKED EXAMPLE 43 Calculate (standard)

A hair is 0.08 mm wide. How wide will it look at ×40? Give your answer in mm.

Solution

0.08 × 40M1

= 3.2 mmA1

WORKED EXAMPLE 44 Explain (standard)

Explain why a cell drawing must include the magnification.

Solution

so the reader can work out the real sizeR1

WORKED EXAMPLE 45 Analyse (exam level)

A student says: 'At ×400 my onion cells are bigger than at ×40.' Analyse this statement.

Solution

the real cells are the same sizeR1

only the image is larger because it is magnified more: statement is wrongR1

7.4 Multi-step problem: a pond investigation

WORKED EXAMPLE 46 Pond water (exam level)

A student views pond water with a ×10 eyepiece and ×40 objective. A Paramecium image is 80 mm long. (a) Find the total magnification. (b) Find the real length of the Paramecium in µm. (c) Explain why no stain is used for this slide.

Solution

(a) 10 × 40 = ×400A1

(b) 80 ÷ 400 = 0.2 mmM1

= 200 µmA1

7.5 Exam technique

•Learn the parts of the microscope AND what each part does.
•Total magnification = eyepiece × objective (multiply, never add).
•Convert units before using magnification = image ÷ real.
•Explain method steps by giving the reason (e.g. 'at an angle, so no air bubbles').
•Follow every drawing rule: pencil, no shading, ruled labels, title, magnification.
•In 'evaluate' questions give both sides and finish with a judgement.

7.6 Key terms

Term	Meaning
cell	the basic building block of living things
specimen	the object being looked at
objective lens	lens close to the specimen
eyepiece lens	lens you look through
magnification	how many times bigger the image is than the real object
field of view	the circle of the specimen you can see
stain	dye that colours parts of cells
cover slip	thin glass square placed over the specimen
micrometre (µm)	one thousandth of a millimetre

7.7 Self-test

Try these without notes, then check the answers.

Quick check

1. Which lens do you look through?

2. Total magnification of ×10 and ×40?

3. 1 mm = how many µm?

4. Stain for cheek cells?

5. Which focus knob is used first?

6. Why lower the cover slip at an angle?

Answers: 1. eyepiece lens 2. ×400 3. 1000 µm 4. methylene blue 5. coarse focus (at low power) 6. to avoid air bubbles

7.8 Self-test 2

Students often think…	Actually…
cells get bigger at higher magnification	only the image gets bigger
total magnification = eyepiece + objective	multiply them
a stain kills or harms the cells you want to study	most stains are used on cells that are already dead; living specimens are viewed unstained
shading makes a drawing better	science drawings use clear outlines only
onion cells are green	onion skin has no chloroplasts

Topic	Connection
1.2 Structure and function of cells	organelles
1.3 Plant and animal cells	cell wall, vacuole
Physics: light	lenses and refraction
Maths: ratio and units	magnification

Idea	Remember
Parts	eyepiece, objective, stage, focus
Magnification	eyepiece × objective
Field of view	smaller at high power
Slides	onion (iodine), cheek (methylene blue)
Drawing	rules
Uses	medicine, food, forensics

Part	Answer	Marks
(a)	×400	A1
(b)	80 ÷ 400 = 0.2 mm	M1
	= 200 µm	A1
(c)	iodine	A1
	colours the cells / nucleus so they can be seen	R1
(d)	to push out air so no bubbles form	R1
(e)	cells look bigger / more detail	A1
	fewer cells seen / smaller field of view	A1
(f)	checking blood or tissue samples (e.g. for malaria)	A1