MYP 4 Biology 1.1 Cell Structure and Microscopy

MYP 4 Biology 1.1 Cell Structure and Microscopy - Full notes

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MYP 4 Biology | 1.1 Cell Structure and Microscopy

MYP 4 BIOLOGY · CHAPTER 1: CELLS AND BIOLOGICAL PROCESSES

1.1

Cell Structure and Microscopy

Full notes for MYP Year 4 · IB-style

Contents

1	Cells: the units of life
2	Specialised cells
3	Microscopy
4	Magnification calculations
5	Preparing slides
6	Data-based questions and further practice
7	Common misconceptions and links
8	Exam-style question with markscheme
9	Check your understanding
10	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
describe the functions of organelles
compare animal, plant and bacterial cells
explain adaptations of specialised cells
compare light and electron microscopes
calculate magnification and convert units
prepare and draw slides correctly

Guiding questions

What are cells made of and how do they differ? How are cells specialised for their jobs? How do microscopes reveal the structure of cells?

How this subtopic is assessed

Criterion A questions test organelles, cell types and microscopy. Criterion B and C questions use measurement and magnification data. Criterion D questions evaluate microscope choice and technology.

Always convert units before calculating.

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 Cells: the units of life

Eukaryotic and prokaryotic cells.

Think about it

You are made of about 37 trillion cells. What do they all have in common, and how do they differ?

The cell theory states that all living things are made of one or more cells, that the cell is the basic unit of life, and that all cells come from pre-existing cells. Cells are either eukaryotic (with a nucleus, as in animals, plants, fungi and protists) or prokaryotic (no nucleus, as in bacteria). Most cells are too small to see without a microscope.

1.1 Organelles and their functions

Sub-cellular structures (organelles) each carry out specific jobs. The nucleus contains genetic material and controls the cell. The cell membrane controls what enters and leaves. The cytoplasm is where many chemical reactions happen. Mitochondria carry out aerobic respiration, releasing energy. Ribosomes make proteins.

Figure 1. A typical animal cell.

Figure 2. Main organelles and their functions.

Figure 3. Mitochondria in different cell types (approx.).

Key point

Animal and plant cells are eukaryotic; bacteria are prokaryotic. Magnification = image size ÷ real size.

WORKED EXAMPLE 1 Function (basic)

State the function of mitochondria and ribosomes.

Solution

mitochondria: aerobic respiration, releasing energyA1

ribosomes: protein synthesisA1

WORKED EXAMPLE 2 Explain (standard)

Use the chart to explain why heart muscle cells contain many mitochondria.

Solution

heart muscle contracts continuously and needs much energyR1

mitochondria release energy by aerobic respirationR1

1.2 Plant cells

Plant cells have all the structures of animal cells, plus a cellulose cell wall for support, chloroplasts for photosynthesis, and a large permanent vacuole containing cell sap that keeps the cell firm.

Structure	Animal cell	Plant cell	Bacterial cell
cell membrane	yes	yes	yes
nucleus	yes	yes	no (DNA loop)
cell wall	no	yes (cellulose)	yes (not cellulose)
chloroplasts	no	often	no
permanent vacuole	no	yes	no
plasmids	no	no	often

Figure 4. A typical plant (palisade) cell.

WORKED EXAMPLE 3 Compare (standard)

Give two structures found in plant cells but not animal cells, with functions.

Solution

cell wall: support and strengthA1

chloroplasts: photosynthesis / vacuole: keeps cell firmA1

WORKED EXAMPLE 4 Explain (exam level)

Explain why root cells do not contain chloroplasts.

Solution

roots are underground and receive no lightR1

so photosynthesis cannot occur thereR1

1.3 Bacterial (prokaryotic) cells

Bacteria are much smaller than eukaryotic cells. Their genetic material is a single loop of DNA in the cytoplasm, not in a nucleus, and many have small rings of DNA called plasmids. Some have flagella for movement.

Figure 5. A typical bacterial cell.

Figure 6. Typical sizes of three cell types (log scale).

WORKED EXAMPLE 5 Compare (standard)

Give two differences between bacterial and animal cells.

Solution

bacteria have no nucleus; DNA is a loop in the cytoplasmA1

bacteria have a cell wall and may have plasmidsA1

WORKED EXAMPLE 6 Order (standard)

How many times longer is a typical plant cell than a typical bacterium?

Solution

100 ÷ 2M1

= 50 timesA1

2 Specialised cells

Structure fits function.

2.1 Differentiation

As an organism develops, cells differentiate: they become specialised with structures suited to particular functions. Most animal cells differentiate early; many plant cells keep the ability to differentiate throughout life.

Figure 7. Cell differentiation.

Figure 8. Examples of specialised cells.

WORKED EXAMPLE 7 Adaptation (standard)

Explain how a sperm cell is adapted to its function.

Solution

tail for swimming; many mitochondria release energyR1

acrosome contains enzymes to digest the egg membraneR1

WORKED EXAMPLE 8 Adaptation (standard)

Explain how a red blood cell is adapted to carry oxygen.

Solution

no nucleus, so more room for haemoglobinR1

biconcave shape gives a large surface area for diffusionR1

WORKED EXAMPLE 9 Adaptation (exam level)

Explain how a root hair cell is adapted for absorbing water and mineral ions.

Solution

long extension gives a large surface areaR1

many mitochondria release energy for active transport of ionsR1

2.2 Size and surface area

Cells cannot grow very large because their surface area to volume ratio (SA:V) falls as they grow. A large cell could not exchange substances fast enough through its surface. This is studied further in topic 1.2.

Figure 9. SA:V ratio of cube-shaped cells.

Figure 10. Sizes of biological structures (log scale).

WORKED EXAMPLE 10 SA:V (standard)

Calculate the SA:V of a cube-shaped cell of side 2 µm.

Solution

SA = 6 × 4 = 24 µm²; V = 8 µm³M1

SA:V = 3 : 1A1

3 Microscopy

Seeing cells.

Think about it

Why can't a light microscope show you a ribosome, however much you magnify it?

Microscopes magnify images so we can see cells. Light microscopes use lenses and light; electron microscopes use beams of electrons and have much higher magnification and resolution. Resolution is the ability to distinguish two points close together as separate; magnifying without improving resolution only gives a bigger blurred image.

3.1 The light microscope

Light passes up through the specimen, then through the objective lens and the eyepiece lens. Total magnification = eyepiece magnification × objective magnification. Always start with the lowest-power objective, focus with the coarse focus, then use the fine focus at higher power.

Figure 11. Parts of a light microscope.

Figure 12. Field of view at different magnifications.

WORKED EXAMPLE 11 Total (basic)

Calculate the total magnification with a ×10 eyepiece and ×40 objective.

Solution

10 × 40M1

= ×400A1

WORKED EXAMPLE 12 Field (standard)

Use the graph to find the field of view at ×400 and explain why fewer cells are seen.

Solution

about 0.45 mmA1

a smaller area of the specimen is magnified to fill the viewR1

3.2 Electron microscopes

Transmission electron microscopes (TEM) pass electrons through very thin sections, giving detailed 2D images of organelles such as ribosomes. Scanning electron microscopes (SEM) scan the surface, giving 3D images. Electron microscopes are expensive, large, need a vacuum and cannot view living specimens.

Feature	Light microscope	Electron microscope
uses	light and glass lenses	electron beam and magnets
max magnification	about ×1500	over ×500 000
resolution	about 0.2 µm	down to about 0.1 nm
specimens	living or dead, colour	dead, in vacuum, black and white
cost	cheap, portable	very expensive, large

Figure 13. Resolution of the eye and microscopes (log scale).

Figure 14. Maximum useful magnification (log scale).

Figure 15. Milestones in microscopy.

WORKED EXAMPLE 13 Explain (standard)

Explain why ribosomes cannot be seen with a light microscope.

Solution

ribosomes are about 25 nm acrossR1

smaller than the resolution of a light microscope (about 200 nm)R1

WORKED EXAMPLE 14 Evaluate (exam level)

Evaluate using a light microscope rather than a TEM to study living cells dividing.

Solution

light microscopes can view living cells in colour and are cheapR1

TEM gives far more detail but needs dead, thin sections in a vacuumR1

judgement: light microscope for living processesA1

4 Magnification calculations

Image, real size and units.

4.1 Magnification and units

Magnification = image size ÷ real size. Both sizes must be in the same units. Cells are usually measured in micrometres (µm): 1 mm = 1000 µm and 1 µm = 1000 nm. Very large or small values are written in standard form.

Figure 16. The magnification triangle.

Figure 17. Converting between units.

WORKED EXAMPLE 15 Magnification (standard)

An image of a cell is 40 mm long. The real cell is 50 µm long. Calculate the magnification.

Solution

40 mm = 40 000 µmM1

40 000 ÷ 50 = ×800A1

WORKED EXAMPLE 16 Real size (standard)

A mitochondrion appears 15 mm long in a micrograph at ×5000. Calculate its real length in µm.

Solution

15 mm = 15 000 µmM1

15 000 ÷ 5000 = 3 µmA1

WORKED EXAMPLE 17 Standard form (exam level)

A virus is 0.00012 mm across. Write this in standard form in mm and convert to nm.

Solution

1.2 × 10⁻⁴ mmA1

1.2 × 10⁻⁴ × 10⁶ = 120 nmA1

Common mistake

Forgetting to convert image and real size to the same units before dividing.

4.2 Using a graticule

An eyepiece graticule is a scale in the eyepiece. It is calibrated against a stage micrometer of known size at each magnification, then used to measure cells.

Figure 18. Eyepiece graticule and stage micrometer.

WORKED EXAMPLE 18 Calibrate (exam level)

10 graticule units line up with 0.1 mm on the stage micrometer. A cell measures 24 graticule units. Calculate its length in µm.

Solution

1 unit = 0.01 mm = 10 µmM1

24 × 10 = 240 µmA1

5 Preparing slides

Practical skills.

5.1 Making and staining slides

Specimens must be thin so light can pass through. Stains make structures visible. A cover slip is lowered at an angle to avoid air bubbles.

Figure 19. Preparing an onion epidermis slide.

Figure 20. Common biological stains.

Figure 21. Summary of key relationships.

WORKED EXAMPLE 19 Method (standard)

Explain why a stain is added and why the specimen must be thin.

Solution

stain makes colourless structures such as nuclei visibleR1

a thin layer lets light pass through and avoids overlapping cellsR1

WORKED EXAMPLE 20 Drawing (exam level)

Give three rules for a scientific drawing of cells.

Solution

sharp pencil, clear continuous lines, no shadingA1

label lines with a ruler, not crossingA1

title and magnification or scale barA1

WORKED EXAMPLE 21 Safety (standard)

Give two safety points when preparing cheek cell slides.

Solution

use a sterile swab and dispose of it in disinfectantA1

methylene blue stains skin; wear gloves and eye protectionA1

6 Data-based questions and further practice

Applying the ideas.

6.1 Data: measuring cells

A student measured onion cells at ×400 using a calibrated graticule (1 unit = 2.5 µm).

Cell	1	2	3	4	5
Length / units	84	92	88	80	96

WORKED EXAMPLE 22 Calculate (exam level)

Calculate the mean length in µm and comment on the variation.

Solution

mean = 440 ÷ 5 = 88 unitsM1

88 × 2.5 = 220 µmA1

range 80–96 units: cells vary naturally in sizeR1

6.2 Graded practice

WORKED EXAMPLE 23 Recall (basic)

State the three parts of the cell theory.

Solution

all living things are made of cells; cells are the basic unit of lifeA1

all cells come from pre-existing cellsA1

WORKED EXAMPLE 24 Units (standard)

Convert 0.045 mm to µm and nm.

Solution

45 µmA1

45 000 nmA1

WORKED EXAMPLE 25 Magnification (standard)

A cheek cell is 60 µm wide. How wide will its image be at ×400 (in mm)?

Solution

60 × 400 = 24 000 µmM1

= 24 mmA1

WORKED EXAMPLE 26 Analyse (exam level)

A student says increasing magnification always shows more detail. Analyse this.

Solution

detail depends on resolutionR1

beyond the resolution limit, magnification only makes a larger blurred imageR1

statement incorrectA1

6.3 Graded practice 2

WORKED EXAMPLE 27 Identify (basic)

Name the structure that controls what enters and leaves a cell.

Solution

cell membraneA1

it is partially permeableA1

WORKED EXAMPLE 28 Scale bar (standard)

A scale bar 20 mm long represents 5 µm. Calculate the magnification.

Solution

20 000 ÷ 5M1

= ×4000A1

WORKED EXAMPLE 29 Explain (standard)

Explain why a palisade cell has many chloroplasts and is near the top of the leaf.

Solution

chloroplasts absorb light for photosynthesisR1

near the upper surface it receives the most lightR1

WORKED EXAMPLE 30 Evaluate (exam level)

Evaluate the claim that electron microscopes have made light microscopes obsolete.

Solution

electron microscopes give far higher resolutionR1

light microscopes are cheap, portable and view living, coloured specimensR1

both are still essential: claim incorrectA1

6.4 Graded practice 3

WORKED EXAMPLE 31 Recall (basic)

Give one function each of the cell wall and the vacuole.

Solution

cell wall: strength and supportA1

vacuole: stores cell sap, keeps cell firmA1

WORKED EXAMPLE 32 Calculate (standard)

A bacterium 2 µm long is viewed at ×1000. How long is the image in mm?

Solution

2 × 1000 = 2000 µmM1

= 2 mmA1

WORKED EXAMPLE 33 Explain (standard)

Explain why plasmids are important in genetic engineering.

Solution

they are small rings of DNA that can be removed and changedR1

and inserted into other bacteriaR1

WORKED EXAMPLE 34 Analyse (exam level)

A student draws an onion cell with chloroplasts. Analyse this.

Solution

onion epidermis cells are from a bulb underground / inside the bulbR1

they receive no light, so have no chloroplastsR1

the drawing is incorrectA1

6.5 Multi-step problem: a micrograph

WORKED EXAMPLE 35 Micrograph (exam level)

In a TEM micrograph at ×12 000, a nucleus measures 72 mm. (a) Calculate its real diameter in µm. (b) Could it be seen with a light microscope? (c) Name one structure that could only be seen with the TEM.

Solution

(a) 72 000 µm ÷ 12 000 = 6 µmM1 A1

(b) yes: 6 µm is much larger than 0.2 µm resolutionR1

6.6 Exam technique

•Learn organelles with their functions.
•Compare cell types using a table.
•Convert units before using the magnification formula.
•Distinguish magnification from resolution.
•Link each adaptation of a specialised cell to its function.
•Follow scientific drawing rules.

6.7 Key terms

Term	Meaning
eukaryotic cell	cell with a nucleus
prokaryotic cell	cell without a nucleus (bacteria)
organelle	structure in a cell with a specific function
plasmid	small ring of DNA in bacteria
differentiation	cells becoming specialised
magnification	how many times larger the image is
resolution	ability to distinguish two close points
graticule	measuring scale in the eyepiece

6.8 Self-test

Try these without notes, then check the answers.

Quick check

1. Site of aerobic respiration?

2. Cell wall material in plants?

3. Where is DNA in bacteria?

4. 1 mm in µm?

5. Magnification formula?

6. Resolution of a light microscope?

Answers: 1. mitochondria 2. cellulose 3. loop in the cytoplasm (and plasmids) 4. 1000 µm 5. image size ÷ real size 6. about 0.2 µm

6.9 Self-test 2

Students often think…	Actually…
all plant cells have chloroplasts	only cells exposed to light
bacteria have a nucleus	DNA is free in the cytoplasm
higher magnification always shows more	resolution limits detail
the cell wall controls entry	the membrane does
electron microscopes show colour	images are black and white (coloured later)

Topic	Connection
1.2 Transport	SA:V
1.3 Cell division	mitosis
1.4 Stem cells	differentiation
Physics: light and lenses	magnification

Idea	Remember
Organelles	functions
Cell types	animal, plant, bacterial
Specialised	adaptations
Microscopes	light vs electron
Calculations	magnification, units
Practical	slides, drawing

Part	Answer	Marks
(a)	×400	A1
(b)	80 mm = 80 000 µm	M1
	80 000 ÷ 400 = 200 µm	A1
(c)	nucleus	A1
	vacuole / mitochondria / cellulose cell wall	A1
(d)	ribosomes are about 25 nm	R1
	smaller than the resolution limit (0.2 µm)	R1
(e)	stains nuclei / makes structures visible	R1
(f)	higher resolution / magnification	A1