1.1Eukaryotes and prokaryotes
Every living thing is made of cells, and every cell is one of two types. The difference that matters is where the genetic material is kept.
In a eukaryotic cell — animal, plant or fungal — the DNA is enclosed inside a nucleus. In a prokaryotic cell — a bacterium — there is no nucleus. The genetic material is a single loop of DNA sitting free in the cytoplasm, and there may also be one or more small rings of DNA called plasmids. A prokaryotic cell is much smaller, and it has a cell wall surrounding its cell membrane.
- Eukaryotic cell
- A cell whose genetic material is enclosed in a nucleus. Plant, animal and fungal cells are eukaryotic.
- Prokaryotic cell
- A much smaller cell with no nucleus. Its genetic material is a single DNA loop in the cytoplasm. Bacteria are prokaryotic.
- Plasmid
- A small ring of DNA, separate from the main DNA loop, found in prokaryotic cells.
You are also expected to handle the scale of cells, and to make order of magnitude calculations — comparisons rounded to the nearest power of ten. A typical animal cell is about 10–30 μm across; a bacterium is about 1–5 μm, so an animal cell is one order of magnitude — ten times — wider.
Sizes this small are written in standard form, and you are expected to use the prefixes centi, milli, micro and nano: 1 m = 100 centimetres (cm), 1 cm = 10 millimetres (mm), 1 mm = 1000 micrometres (μm) and 1 μm = 1000 nanometres (nm).
Worked example
A cheek cell is 0.00003 m wide. A bacterium is 2 × 10⁻⁶ m wide. How many times wider is the cheek cell?
- Put both into standard form: 0.00003 m = 3 × 10⁻⁵ m.
- Divide: (3 × 10⁻⁵) ÷ (2 × 10⁻⁶).
- Divide the numbers and subtract the powers: 3 ÷ 2 = 1.5, and 10⁻⁵ ÷ 10⁻⁶ = 10¹.
Answer: 1.5 × 10¹ = 15 times wider — one order of magnitude.
Common mistake
Prokaryotes are not cells without DNA. They have DNA — it simply is not enclosed in a nucleus. And a plasmid is a small extra ring, not the main DNA loop.
1.1Animal and plant cells
Learn each sub-cellular structure together with its job. Questions here nearly always ask what a structure does, or why a particular cell has a lot of one of them.
Most animal cells have a nucleus, cytoplasm, a cell membrane, mitochondria and ribosomes. Plant and algal cells have all of those, and in addition often have chloroplasts and a permanent vacuole filled with cell sap. Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.
The specification also asks for the function of plasmids in bacterial cells. A plasmid is a small ring of DNA separate from the main loop, carrying extra genes — genes for antibiotic resistance, for example — which bacteria can pass to one another.
You are not always given exact measurements, and some questions ask you to judge a size or an area rather than calculate one. Estimate when no measurement is available, when the numbers are awkward and a rough answer settles the comparison anyway, or when all you need is the order of magnitude. Round to one significant figure, do the arithmetic, and say that the answer is an estimate.
Watch the difference between length and area: if one structure is a fifth as wide as another, it covers about a twenty-fifth of the area, because area scales with the square of the width.
Worked example
A cheek cell is about 60 μm across. Its nucleus looks roughly one fifth as wide. Estimate what fraction of the cell's area the nucleus takes up.
- Estimate the width of the nucleus: 60 ÷ 5 = about 12 μm.
- Area scales with the square of the width, so compare (1/5)².
- (1/5)² = 1/25.
Answer: About 1/25, or roughly 4%, of the cell's area — an estimate, not a measurement.
Required practical 1
Use a light microscope to observe, draw and label plant and animal cells
- Prepare a slide: a thin piece of onion epidermis for plant cells, or cheek cells smeared onto the slide for animal cells.
- Add a drop of stain — iodine for plant cells, methylene blue for animal cells — so the structures show up.
- Lower the coverslip at an angle with a mounting needle, to avoid trapping air bubbles.
- Start with the lowest power objective lens, focus, then move to a higher power.
- Draw with a sharp pencil, using clear unbroken lines and no shading, and label the structures you can see.
Watch out: The drawing must include a magnification scale. Marks are lost for shading, sketchy lines and missing labels more often than for the biology itself. Afterwards, compare it with a photograph or micrograph of the same tissue — images of cells from videos, bioviewers, photographs and micrographs are meant to be used alongside your own drawings, and they show up what you have missed.
Common mistake
Plant cells do have mitochondria — they respire as well as photosynthesise. And not every plant cell has chloroplasts: root cells have none, because there is no light underground.
1.2Cell specialisation
A specialised cell has a structure that suits one particular job in a tissue, an organ, an organ system or the whole organism. Marks come from linking the two: name the feature, then say what it allows the cell to do.
Common mistake
"It has a tail" scores nothing on its own. "It has a tail so it can swim to the egg" is the answer. Always finish the sentence with the function.
- Specialised cell
- a cell with a structure that suits one particular job in a tissue, organ, organ system or the whole organism
1.3–1.6Microscopy
Two things separate one microscope from another: magnification and resolution. An electron microscope has much higher magnification and much higher resolving power than a light microscope, so it can be used to study cells in far finer detail. That is how biologists came to see and understand many more sub-cellular structures — a light microscope simply cannot resolve them.
Microscopy techniques have developed over time, and what biologists could see developed with them. Early light microscopes in the seventeenth century were good enough to show that tissue is built from cells at all; better lenses and stains through the nineteenth century sharpened that picture. But a light microscope is limited by the wavelength of light — it cannot resolve anything closer together than about 0.2 μm — so sub-cellular detail stayed out of reach. The electron microscope, developed in the 1930s, uses a beam of electrons instead, and its far higher magnification and resolving power increased understanding of sub-cellular structures enormously: ribosomes and the internal structure of mitochondria were seen for the first time. You are not asked for dates — only for why the newer technique shows more, which comes down to magnification and resolution.
- Magnification
- How many times larger the image is than the real object.
- Resolution
- The smallest distance between two points that can still be seen as two separate points.
Worked example
A drawing of a cell is 45 mm across. The real cell is 90 μm across. Calculate the magnification.
- Formula: magnification = size of image ÷ size of real object.
- Convert to the same units first: 45 mm = 45,000 μm.
- magnification = 45,000 ÷ 90.
Answer: ×500
Common mistake
Almost every lost mark here is a unit conversion, not the division. Convert both measurements to the same unit before you divide: 1 mm = 1000 μm.
Levels of organisation
Cells are the basic building blocks of all living organisms, and larger organisms are built by grouping them.
A tissue is a group of cells with a similar structure and function. Organs are aggregations of tissues performing specific functions. Organs are organised into organ systems, which work together to form organisms.
Those three sentences are the whole of this sub-topic, and they are worth learning word for word — the definitions themselves are what gets marked.
- Tissue
- A group of cells with a similar structure and function — muscle tissue, for example.
- Organ
- An aggregation of tissues performing a specific function, such as the stomach or the heart.
- Organ system
- A group of organs working together, such as the digestive system.
It is worth having a feel for the sizes as well as the order. A typical animal cell is a few tens of micrometres across; a tissue is large enough to see, measured in millimetres; an organ such as the stomach or a leaf is measured in centimetres.
How big the jump is from one level to the next varies enormously — a leaf and a heart are both organs, and neither is a fixed multiple of the tissue inside it — but the direction is always the same. That is what matters: as an organism gets larger its surface area to volume ratio falls, which is why a large organism needs a transport system and a single cell does not.
The same four levels describe a plant. Palisade mesophyll is a tissue; the leaf is an organ built from several tissues; the roots, stem and leaves together form the transport organ system; and those systems make the plant. Being able to move between an animal example and a plant one is worth practising, because a question will often give you the unfamiliar one.
Why organise at all? A single cell exchanges everything it needs straight through its own surface. Once an organism is more than a few cells thick that stops working — the surface area to volume ratio is too small — so cells specialise, group into tissues that do one thing well, and organs and systems appear to move materials between them. Organisation is the consequence of getting big.
Common mistake
The stomach is an organ, not a tissue — it contains several tissues, including muscle and glandular tissue. If something is made of more than one kind of tissue and does one specific job, it is an organ.
Before you practise
- Eukaryotic cells keep their DNA in a nucleus; prokaryotic cells have a single DNA loop free in the cytoplasm, plus plasmids.
- Every sub-cellular structure has to be paired with its function — that pairing is what gets marked.
- Plant and algal cells add a cellulose cell wall, and often chloroplasts and a permanent vacuole.
- A specialised cell answer must link the feature to what it lets the cell do — sperm, egg and ciliated epithelial cells are the named examples.
- magnification = size of image ÷ size of real object, with both in the same units.
- Learn the prefixes as powers of ten: milli is 10⁻³, micro 10⁻⁶, nano 10⁻⁹.
- An electron microscope resolves far more detail than a light microscope, which is how sub-cellular structures were first seen.