OCR GCSE Biology A: Microscopy, eukaryotic and prokaryotic cells and electron microscopy (J247) – Revision Notes

These notes condense statements B1.1a to B1.1c of OCR GCSE (9-1) Gateway Science Suite Biology A (J247), Version 4.0, first assessment in 2018. No statement in B1.1 is Higher Tier only. For full explanations and longer worked examples, read the study guide, then try the practice questions. Course pages: OCR GCSE Biology hub, checklist and the Cell level systems overview. To find your weak spots first, try a free 10-minute diagnostic.

B1.1a: Light microscope and staining

Parts and jobs

Part	Job
Lamp	Lights the specimen from below
Stage	Holds the slide
Objective lens	First magnification; rotating turret, several powers
Eyepiece lens	Second magnification; the lens you look through
Coarse and fine focus	Sharpen the image
Slide and cover slip	Hold a thin specimen flat; cover slip keeps it flat, stops it drying and protects the lens

Total magnification = eyepiece × objective. For example ×10 × ×40 = ×400.

Stains

• Cells are mostly colourless, so little contrast shows without a stain.
• A stain either makes a colourless specimen visible, or colours different structures or tissues differently so they can be told apart.
• Typical pairings: methylene blue with cheek cells, iodine solution with onion epidermis.

Method in steps: preparing and viewing a slide

1. Place a thin layer of cells in a drop of liquid on a slide.
2. Add a drop of stain.
3. Lower a cover slip at an angle with a needle so no air bubbles are trapped.
4. Clip on the stage; start with the lowest-power objective.
5. Focus with coarse then fine focus; change to a higher-power objective only when sharp.
6. Draw with a pencil: clear lines, no shading, label lines not crossing, title with the magnification.

Magnification formula triangle

Want	Method
Magnification	divide image by actual
Actual size	divide image by magnification
Image size	multiply actual by magnification

Unit conversions: ×1000 going from mm to µm, and again from µm to nm (so ×1 000 000 from mm to nm); divide to go back.

Standard form: a number between 1 and 10 times a power of 10. 0.0000032 m = 3.2 × 10⁻⁶ m. 4500 nm = 4.5 × 10³ nm = 4.5 × 10⁻⁶ m.

Worked reminders

• Image 24 mm, ×400: actual = 24 ÷ 400 = 0.06 mm = 60 µm.
• Scale bar 25 mm labelled 10 µm: magnification = 25 000 µm ÷ 10 µm = ×2500.
• Actual 1.5 µm, ×8000: image = 12 000 µm = 12 mm.
• Drawing 50 mm, real length 0.25 mm: magnification = 50 ÷ 0.25 = ×200.

Estimation: a rough value from a quick count or round numbers, used to check a calculated answer. If 5 cells fit across a 2.0 mm field, each is about 0.4 mm (400 µm). A calculated answer of 4 mm for one cell must be wrong.

B1.1b: Sub-cellular structures

Structure	Key fact to state	Cells
Nucleus	Contains the genetic material (chromosomes); controls the cell	Animal, plant
Genetic material	DNA; genes carry instructions for proteins	All
Chromosomes	Long DNA molecules carrying many genes	Linear in nucleus; one circular chromosome in bacteria
Plasmids	Small rings of extra DNA; genes such as antibiotic resistance	Bacteria
Mitochondria	Contain enzymes for cellular respiration	Animal, plant
Chloroplasts	Contain chlorophyll for photosynthesis	Green plant cells
Cell membrane	Selective barrier; receptor molecules	All
Ribosomes	Site of protein synthesis	All

Other structures that appear on diagrams: cytoplasm (where many reactions occur), plant cell wall (cellulose), permanent vacuole (plants), bacterial cell wall (not cellulose).

Eukaryote against prokaryote

	Eukaryotic	Prokaryotic
Nucleus	Yes	No; DNA free in cytoplasm
DNA	Several linear chromosomes	One main circular chromosome plus plasmids
Mitochondria	Yes	No
Chloroplasts	In some plant cells	No
Ribosomes	Yes	Yes
Size	Larger	Much smaller (around a few µm)

Must-know distinctions

• Genetic material or chromosome? Genetic material is the DNA; a chromosome is one long coiled DNA molecule.
• Chromosome or plasmid? Plasmids are small extra rings, not the main chromosome.
• Mitochondria or chloroplasts? Mitochondria are for respiration (release energy) and are in nearly all eukaryotic cells; chloroplasts are for photosynthesis (use light) and only in some plant cells.
• Cell wall or cell membrane? The membrane controls what crosses; the wall supports.

Structure and function pairings

• Many mitochondria: cell uses lots of energy (muscle).
• Many chloroplasts: cell photosynthesises (palisade layer of a leaf).
• Many ribosomes: cell makes lots of protein.
• Receptors on the membrane: only cells with the matching receptor respond to a given chemical signal.

B1.1c: Electron microscopy

• TEM sends electrons through a very thin specimen.
• Higher resolution than a light microscope: closer points are seen as separate.
• Light microscope resolution is limited to roughly 200 nm.
• Revealed fine detail: internal structure of mitochondria and chloroplasts, nuclear envelope, ribosomes (about 25 nm, invisible by light microscopy).
• Increased understanding: we could link detailed structure to function, and see differences between prokaryotes and eukaryotes.
• Limits: dead, treated specimens (possible artefacts), monochrome image, expensive equipment, vacuum.
• Light microscope: living cells possible, colour, cheap, quick.

Magnification and resolution are different. Magnification enlarges the image; resolution decides how much fine detail is visible.

Worked reminder (TEM)

Image 36 mm, ×30 000: actual = 36 ÷ 30 000 = 0.0012 mm = 1.2 µm = 1200 nm = 1.2 × 10⁻⁶ m.

Light microscope and TEM compared

	Light microscope	TEM
Radiation	Visible light	Beam of electrons
Resolution	Lower (about 200 nm)	Much higher
Specimen	Can be living	Dead, very thin, treated
Image	Colour (often stained)	Black, white and grey
Cost and size	Cheap, portable	Expensive, large
Shows	Nucleus, cells, large organelles	Internal detail of organelles, ribosomes

Method in steps: any magnification question

1. Write the formula: magnification = image size ÷ actual size.
2. Underline what you are asked for and what is given.
3. Put image and actual size into the same unit (usually µm). Convert mm to µm by multiplying by 1000.
4. Rearrange before substituting if the unknown is not the magnification.
5. Calculate, then give the unit.
6. Check by estimation: is the size sensible for this kind of cell or organelle? Animal cells are tens of µm across, bacteria a few µm, mitochondria around 1 µm.

Second worked reminder (scale bar)

A scale bar is 20 mm long and labelled 2 µm. A structure measures 30 mm on the same image.

1. Magnification = 20 000 µm ÷ 2 µm = ×10 000.
2. Actual length = 30 000 µm ÷ 10 000 = 3 µm.
3. In standard form: 3 × 10⁻⁶ m.

Notice that you can skip the magnification: 30 ÷ 20 × 2 = 3 µm.

Command words in this topic

• Describe (a microscope method): give the steps in order, in words.
• Explain (a structure and its function): state what the structure contains or is, then link it to what the cell does.
• Calculate: show the formula, the conversion and the unit.
• Suggest (why a stain is used, or why a TEM image shows more): apply the idea of contrast or resolution to the situation given.

Quick self-test

1. Name the part of the microscope that holds the slide.
2. A ×10 eyepiece and ×10 objective are used. What is the total magnification?
3. A cell is 0.08 mm long. Give this in µm.
4. An image is 45 mm and the real length is 15 µm. Calculate the magnification.
5. Write 0.0000048 m in standard form.
6. Name the molecules in the cell membrane that detect chemical signals.
7. State the function of ribosomes.
8. Where is the genetic material of a bacterium?
9. State one reason a TEM shows more detail than a light microscope.
10. A TEM image has magnification ×20 000. A structure is 40 mm long on the image. Find its real length in µm.

Answers

1. The stage.
2. 10 × 10 = ×100.
3. 0.08 × 1000 = 80 µm.
4. 45 mm = 45 000 µm; 45 000 ÷ 15 = ×3000.
5. 4.8 × 10⁻⁶ m.
6. Receptor molecules.
7. Protein synthesis.
8. Free in the cytoplasm, as a circular chromosome, with plasmids; there is no nucleus.
9. It has higher resolution, so structures closer together are seen as separate.
10. 40 ÷ 20 000 = 0.002 mm = 2 µm.

Where marks are usually lost

• Dividing actual by image instead of image by actual.
• Forgetting to convert mm to µm before dividing.
• A positive power on a tiny length, such as 3.2 × 10⁶ m when 3.2 × 10⁻⁶ m was meant.
• Writing “magnifies more” instead of “has higher resolution”.
• Saying a bacterium has a nucleus, or that plasmids are its only DNA.
• Describing mitochondria as “making energy” rather than releasing it in respiration.
• Giving the function of the membrane as “keeps everything in” rather than a selective barrier.
• Putting chloroplasts in root cells or animal cells.
• Using a stain without saying why: state that it makes structures visible or distinguishes them.

Official syllabus

Aligned to OCR GCSE (9-1) Gateway Science Suite Biology A (J247), Version 4.0, published by OCR, first assessment in 2018, statements B1.1a to B1.1c.