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

This guide covers statements B1.1a, B1.1b and B1.1c of Topic B1 (Cell level systems) in the OCR GCSE (9-1) Gateway Science Suite Biology A specification (J247), Version 4.0, for first assessment in 2018. None of these three statements is marked as Higher Tier only, so everything here applies to both Foundation and Higher Tier. Revise it with the revision notes, then test yourself with the practice questions. The wider topic is on the Cell level systems overview, and the course hub is at OCR GCSE Biology with a printable checklist. If you are unsure where to start, take a 10-minute diagnostic.

What this unit covers

Statement	What you must be able to do
B1.1a	Describe how light microscopes and stains are used to view cells: lenses, stage, lamp, slides and cover slips, stains for colourless specimens or to highlight structures, and calculation of magnification
B1.1b	Explain how the main sub-cellular structures of eukaryotic (plant and animal) and prokaryotic cells relate to their functions: nucleus, genetic material, chromosomes, plasmids, mitochondria, chloroplasts, cell membranes, ribosomes
B1.1c	Explain how electron microscopy has increased our understanding of sub-cellular structures, through increased resolution in a transmission electron microscope (TEM)

The specification also lists mathematical outcomes for this topic: number, size and scale and the relationship between units; estimation and when to use it; and calculating with numbers in standard form. All three appear below.

B1.1a: Light microscopes and staining

The parts and what they do

Part	Job
Lamp (or mirror)	Shines light up through the specimen
Stage	Flat platform that holds the slide; the specimen sits over the hole in it
Objective lenses	Several lenses on a rotating turret, commonly ×4, ×10 and ×40; they make the first magnified image
Eyepiece lens	The lens you look through; commonly ×10; it magnifies the image again
Coarse and fine focus wheels	Move the stage or lens to bring the image into focus

Light has to get through the specimen, so it must be a thin layer, mounted on a slide (a glass rectangle) and covered with a cover slip (a thin square of glass or plastic). The cover slip flattens the specimen, keeps it from drying and protects the lens. Lower it at an angle, using a mounted needle, so that no air bubbles are trapped.

Total magnification = eyepiece magnification × objective magnification. Pair a ×10 eyepiece with a ×40 objective and the result is ×400.

Why stain?

Most cells are almost colourless, so their parts hardly differ in how much light they absorb. A stain is a coloured chemical that soaks into a specimen. It does one of two jobs:

• it makes a colourless specimen visible against the background; or
• it colours different structures or tissues differently, so they stand out from one another.

Methylene blue is commonly used on cheek cells, and iodine solution on onion epidermis. Each stains the nucleus and the cytoplasm to different depths, so the nucleus shows up as a darker spot. The slide in each practical is made by wiping or peeling a thin layer of cells, placing it in a drop of liquid on the slide, adding stain, and lowering the cover slip.

Using the microscope

1. Start with the lowest-power objective and the stage as low as possible.
2. Use the coarse focus until the cells are roughly sharp, then the fine focus.
3. Move to a higher-power objective only once the image is sharp; refocus with the fine focus only.
4. Draw what you see with a sharp pencil, a clear outline, no shading, and a title stating the total magnification.

Magnification calculations

The relationship is:

magnification = image size ÷ actual size

To find the real size, divide the image size by the magnification. To find the image size, multiply the real size by the magnification. You measure the image size on the picture itself. Use the same units for image and actual size before dividing, or convert at the end.

Units: there are 1000 µm in 1 mm and 1000 nm in 1 µm. Going from mm to µm, multiply by 1000.

Worked example 1: finding actual size. A cheek cell is 24 mm across on a light micrograph with magnification ×400.

1. actual size = image size ÷ magnification = 24 ÷ 400 = 0.06 mm
2. Convert to µm: 0.06 × 1000 = 60 µm

Worked example 2: from a printed scale bar. A scale bar printed below an image is 25 mm long and is labelled 10 µm. A cell on the same image measures 62 mm.

1. Magnification = 25 mm ÷ 10 µm. Convert 25 mm to 25 000 µm: 25 000 ÷ 10 = ×2500.
2. Actual cell length = 62 ÷ 2500 = 0.0248 mm = 24.8 µm. (A shortcut gives the same answer: 62 ÷ 25 × 10 = 24.8 µm.)

Worked example 3: finding image size. A bacterium is 1.5 µm long. How long will it be on an image of magnification ×8000?

1. image size = actual size × magnification = 1.5 × 8000 = 12 000 µm
2. Convert to mm: 12 000 ÷ 1000 = 12 mm

Worked example 4: finding magnification of a drawing. You draw an onion cell 50 mm long. A measurement gives its real length as 0.25 mm. Magnification = 50 ÷ 0.25 = ×200.

Estimation

Estimation is a sensible approximate answer, useful when you cannot measure exactly or want to check a result. For instance, a field of view is 2.0 mm across and about 5 cells fit across it in a line. Each cell is about 2.0 ÷ 5 = 0.4 mm, which is 400 µm. If your calculated cell size is 4 mm, an estimate like this tells you something is wrong.

Standard form

Very small sizes are tidier in standard form, a number between 1 and 10 multiplied by a power of 10. A length of 0.0000032 m is 3.2 × 10⁻⁶ m. In the same way, 1.2 µm = 1.2 × 10⁻⁶ m, and 60 µm = 6.0 × 10⁻⁵ m.

Magnification is not resolution

Magnification makes an image larger. Resolution is the smallest gap at which two neighbouring points still look like two. A light microscope can only separate points that are more than about 200 nm (0.2 µm) apart. Enlarging an image beyond this only makes a bigger, blurrier picture. This is why a better instrument, not simply more magnification, is needed to see small structures (B1.1c).

B1.1b: Sub-cellular structures and their functions

Eukaryotic cells (animal and plant cells) have genetic material enclosed in a nucleus. Prokaryotic cells (bacteria) have no nucleus. They are much smaller, typically a few micrometres long.

Structure	What it contains or is	Function	Where
Nucleus	Genetic material, in the form of chromosomes	Controls the activities of the cell	Animal and plant cells
Genetic material	DNA	Carries the instructions (genes) for making proteins	All cells
Chromosomes	Long, coiled DNA molecules	Each carries many genes	Linear in the nucleus of eukaryotes; a single circular chromosome in bacteria
Plasmids	Small rings of DNA, separate from the main chromosome	Carry extra genes, such as ones for antibiotic resistance	Bacteria
Mitochondria	Contain enzymes for cellular respiration	Site where energy is released from glucose	Animal and plant cells
Chloroplasts	Contain chlorophyll	Absorb light for photosynthesis	Plant cells in green parts
Cell membrane	Contains receptor molecules	Selective barrier controlling what enters and leaves; receptors detect chemical signals	All cells
Ribosomes	Tiny structures	Site of protein synthesis	All cells

The cytoplasm is the jelly-like fluid where many of the cell’s chemical reactions happen. Plant cells also have a cellulose cell wall and a permanent vacuole, and bacteria have a cell wall that is not made of cellulose. These do not appear in the specification’s list of structures to include for this statement, but you will meet them in diagrams.

Linking structure to function

• Mitochondria hold the enzymes for cellular respiration, so a cell that uses a lot of energy, such as a muscle cell, has many.
• Chloroplasts contain chlorophyll, so only cells that photosynthesise have them. Root cells have none.
• The cell membrane is selective: it lets some molecules cross and blocks others. Receptor molecules in it bind particular chemicals, such as a hormone, so only cells with the matching receptor respond.
• Ribosomes build proteins. Cells that make a lot of protein have many.
• Plasmids can be copied and passed between bacteria, which is one way a trait like antibiotic resistance spreads.

Eukaryote or prokaryote?

Feature	Eukaryotic cell	Prokaryotic cell
Nucleus	Present	Absent; DNA lies free in the cytoplasm
Chromosomes	Several, linear	One main, circular
Plasmids	Not found in the animal and plant cells you study	Often present
Mitochondria and chloroplasts	Mitochondria present; chloroplasts in some plant cells	Absent
Ribosomes	Present	Present
Typical size	Larger	Much smaller

B1.1c: Electron microscopy

A transmission electron microscope (TEM) passes a beam of electrons through a very thin slice of specimen. The image appears in black, white and grey, and the specimen is in a vacuum. Electrons behave as waves of much shorter wavelength than visible light. That gives far higher resolution, so detail lying much closer together still looks separate. The detail shown has been the key to our modern understanding of cell structure.

With a light microscope you can see a nucleus, and often the cell membrane as a boundary. A TEM shows what is inside them: the folded inner membrane of a mitochondrion, the internal membranes of a chloroplast, the double layer of the nuclear envelope. Ribosomes, about 25 nm across, are too small to see with a light microscope but appear as tiny dots in a TEM image. Bacterial structures, including the lack of a nucleus, could also be examined in far more detail.

Worked example 5: a TEM calculation. A mitochondrion appears 36 mm long on a TEM image of magnification ×30 000.

1. actual length = 36 ÷ 30 000 = 0.0012 mm
2. Convert to µm: 0.0012 × 1000 = 1.2 µm
3. In nm: 1.2 × 1000 = 1200 nm. In metres: 1.2 × 10⁻⁶ m.

Limits of the TEM

Specimens must be dead, thin and treated before viewing, so living processes cannot be watched and the preparation may produce features that are not in the living cell (artefacts). The equipment is large and expensive. A light microscope can be used to look at living cells, in colour, cheaply and quickly. The two instruments therefore complement each other.

Common errors

• Turning the formula upside down: magnification is the image divided by the real size, never the other way round.
• Mixing units, such as dividing millimetres by micrometres without converting first.
• Writing “mitochondria produce energy”. Say they release energy from glucose in respiration.
• Saying the electron microscope “magnifies more” without mentioning resolution.
• Saying bacterial DNA is “in the nucleus”. Bacteria have no nucleus.
• Putting chloroplasts in every plant cell.

Official syllabus

This guide is aligned to OCR GCSE (9-1) Gateway Science Suite Biology A (J247), Version 4.0, published by OCR (Oxford Cambridge and RSA), first assessment in 2018, Topic B1, statements B1.1a to B1.1c.