Cell Structure

Biologists group all known cells into two fundamental categories based on whether their DNA is enclosed in a membrane:

Feature	Eukaryotic cell	Prokaryotic cell
Nucleus	Yes — DNA enclosed in nuclear membrane	No — DNA free in cytoplasm
Typical size	10 – 100 µm	1 – 10 µm
Membrane-bound organelles (mitochondria, chloroplasts)	Yes	No
DNA arrangement	Linear, in chromosomes	Single circular loop, plus plasmids
Cell wall	Plants and fungi only	Yes (NOT cellulose — usually peptidoglycan)
Examples	Animal, plant, fungal, protist cells	All bacteria

Eukaryotic literally means "true nucleus" (Greek eu = true, karyon = kernel/nucleus). All multicellular organisms — humans, oak trees, mushrooms — are eukaryotic. So are many unicellular ones (yeast, amoeba).

Prokaryotic means "before nucleus". These cells appeared first in evolutionary history (~3.5 billion years ago). The most common modern prokaryotes are bacteria — found everywhere on Earth, from your skin to deep-sea vents.

AQA tip. When the exam shows a labelled cell and asks "Is this cell eukaryotic or prokaryotic? Give a reason from the diagram" — your reason must point to something visible: presence/absence of a nucleus, presence of chloroplasts, or a plasmid ring. Generic answers like "it looks small" are not credit-worthy.

AQA expects you to express cell measurements in standard form (also called scientific notation). The form is:

$A \times 10^n$

where $A$ is a number between 1 and (just less than) 10, and $n$ is an integer (positive for big numbers, negative for tiny ones).

Unit ladder for cell biology:

Unit	Symbol	In metres	Used for
metre	m	1 m	Whole organisms
millimetre	mm	$1\times10^{-3}\,\text{m}$	Visible to naked eye
micrometre	µm	$1\times10^{-6}\,\text{m}$	Cells and large bacteria
nanometre	nm	$1\times10^{-9}\,\text{m}$	Ribosomes, viruses, organelles

Converting: To go from mm to µm, multiply by 1000 (×$10^3$). To go from µm to m, divide by $10^6$.

Worked example 1. Express 0.000003 m in standard form.

Move the decimal point 6 places to the right: $3 \times 10^{-6}\,\text{m}$. That's 3 µm — about the size of a typical bacterium.

Worked example 2. A human red blood cell is 7 µm across. Express this in metres in standard form.

$7\,\mu\text{m} = 7 \times 10^{-6}\,\text{m}$

Worked example 3. A ribosome is about 20 nm across. Express in µm.

20 nm = $20 \times 10^{-9}\,\text{m}$ = $2 \times 10^{-8}\,\text{m}$ = $0.02\,\mu\text{m}$.

How many bacteria fit across a eukaryotic cell? If a typical animal cell is 20 µm and a bacterium is 2 µm, then ten bacteria can line up across one animal cell — handy mental ruler for the exam.

Why does size matter biologically? Smaller cells have a larger surface area to volume ratio. Prokaryotes can exchange substances with their environment by simple diffusion — they don't need elaborate transport organelles. Large eukaryotic cells need specialised compartments (mitochondria, chloroplasts) and active transport mechanisms because diffusion alone is too slow over their bigger distances.

Beyond the bare minimum (cell membrane, cytoplasm, single DNA loop, ribosomes), bacteria may carry these AQA-named structures:

• Plasmids — small additional rings of DNA. They often carry useful genes such as antibiotic resistance. Bacteria can swap plasmids with each other, which is why resistance can spread across a population so quickly.
• Cell wall — made of peptidoglycan (not cellulose). It maintains the bacterium's shape and prevents it bursting in dilute surroundings.
• Flagellum (plural flagella) — a long tail-like whip that rotates to push the bacterium through liquid.
• Slime capsule — an optional gluey outer layer some bacteria secrete; it helps them stick to surfaces (e.g. teeth → plaque) and resist immune attack.

AQA spec note. You are NOT required to memorise the peptidoglycan chemistry, but you should know the wall is not cellulose (that is plant-only).

Common AQA exam picture. Bacterial diagrams often show one or two plasmid rings outside the main DNA loop. Don't confuse the plasmids with vacuoles or the nucleus — bacteria have neither.

Every eukaryotic cell — animal or plant — contains these five structures. Memorise the function as well as the name; AQA exam questions ask "describe the function of …" more often than they ask for a label.

Structure	Function	Memory hook
Nucleus	Contains chromosomes made of DNA; controls all the cell's activities.	The "command centre".
Cytoplasm	A jelly-like substance in which chemical reactions take place; contains the organelles.	The "factory floor".
Cell membrane	Controls what enters and leaves the cell (partially permeable).	The "border control".
Mitochondria (singular: mitochondrion)	Site of aerobic respiration — release energy from glucose.	The "power stations".
Ribosomes	Site of protein synthesis — assemble amino acids into proteins.	The "protein assembly line".

AQA tip. Cells that work hard (muscle, liver, sperm) have many mitochondria because they need more respiration energy. This link between structure and energy demand is a common exam question.

Plant cells contain everything animal cells contain PLUS three extra structures. These are the features that make plants able to photosynthesise and stand upright without bones.

Structure	Function
Cell wall (cellulose)	Tough outer layer outside the cell membrane. Provides strength; stops the cell bursting in dilute surroundings.
Chloroplasts	Contain chlorophyll (green pigment); absorb light to drive photosynthesis. Only in cells exposed to light (mostly leaves and green stems — NOT roots).
Permanent vacuole	A large sac filled with cell sap (water, sugars, salts). Keeps the cell turgid (firm), giving the plant its shape.

AQA tip. Root cells, including root-hair cells, do NOT contain chloroplasts. Why? Because they are underground — no light to absorb. If an exam diagram shows a root cell with chloroplasts, that's wrong, and AQA will sometimes test this fact in a multiple-choice question.

AQA questions often ask "explain why this cell has so many [organelle]". You're expected to link structure → activity → function:

• Lots of mitochondria → lots of aerobic respiration → lots of energy released → cell does a high-energy job. Examples: muscle cells (contract), sperm cells (swim), liver cells (synthesise/break down many molecules).
• Lots of ribosomes → lots of protein synthesis → cell makes/secretes proteins. Examples: pancreas cells (make digestive enzymes), antibody-producing white blood cells.
• Lots of chloroplasts → lots of photosynthesis → cell makes glucose. Example: palisade mesophyll cells in leaves.

Worked example. Sperm cells have many mitochondria. Explain why.

Sperm cells need a lot of energy to swim long distances to the egg. Mitochondria are the site of aerobic respiration, which releases energy. Having many mitochondria means more respiration can occur, providing the energy needed for the tail to beat.

That's a three-mark response: identifies activity, names process, links to energy.

Sperm cell (male gamete in humans):

Feature	Function
Long tail (flagellum)	Whips side-to-side to propel sperm towards the egg
Many mitochondria in midpiece	Release energy by aerobic respiration to power the tail
Acrosome (tip) full of digestive enzymes	Digests the egg's outer layer so sperm can fertilise it
Streamlined head shape	Reduces resistance through fluid
Haploid nucleus (23 chromosomes)	Fuses with egg nucleus to restore the diploid number

Nerve cell (neurone):

Feature	Function
Long axon (can be over 1 metre in humans)	Transmits electrical impulses over long distances quickly
Branched dendrites	Connect to many other neurones to receive signals
Myelin sheath (fatty insulation)	Insulates the axon; speeds up impulse transmission
Synaptic terminals (knobs at end)	Release neurotransmitters into the synapse

Muscle cell:

Feature	Function
Many mitochondria	Provide energy via aerobic respiration for contraction
Protein filaments (actin and myosin) that slide	Shorten the cell → muscle contracts
Stored glycogen	Quick fuel store for respiration during exercise

Root-hair cell — found on outermost layer of plant roots:

Feature	Function
Long, thin extension (the 'hair')	Massively increases surface area for absorption
Thin cell wall + cell membrane	Short diffusion distance for water and ions
Many mitochondria	Provide energy for active transport of mineral ions
No chloroplasts	Underground, no light needed

Xylem — transports water and dissolved mineral ions UP the plant:

Feature	Function
Cells are DEAD (no cytoplasm or nucleus)	Hollow tube allows free flow of water
Lignin (waterproof, hard substance) in walls	Strengthens the tube + waterproofs it
End walls broken down	Continuous tube from root to leaf
Rings or spirals of lignin	Strong + flexible

Phloem — transports sugars (sucrose) both UP and DOWN the plant (called translocation):

Feature	Function
Sieve tube cells joined end-to-end	Forms a continuous tube
Sieve plates between cells (perforated)	Sugars flow through holes
Companion cells alongside	Provide energy (lots of mitochondria) and control flow
Cells are ALIVE but lack a nucleus in the sieve tubes	Maximise space for sap flow

Memory: xylem = up (water, dead); phloem = both ways (sugars, alive).

AQA's most common command for this topic is "Explain how the structure of X helps it carry out its function." Structure your answer in pairs:

"Feature → because it allows / so that…"

Worked example. Explain how a root-hair cell is adapted to absorb water efficiently. (3 marks)

The cell has a long, thin extension which increases the surface area in contact with the soil (1). A larger surface area means more water can be absorbed per second by osmosis (1). The cell membrane is thin, giving a short diffusion distance (1).

Worked example. Explain why xylem is suitable for transporting water in a plant. (3 marks)

Xylem cells are dead and hollow, forming a continuous tube with no cytoplasm to obstruct water flow (1). End walls are broken down so water can move freely from root to leaf (1). Lignin in the walls makes them strong, preventing collapse, and waterproof (1).

Notice each mark = one feature linked to one effect. Never list features without the link.

Every cell in your body contains the same complete set of DNA. So why is a muscle cell different from a nerve cell? Because different genes are active.

Differentiation is the process of a cell becoming specialised. During differentiation:

1. The cell receives signals (chemical/positional) telling it what to become.
2. Specific genes are switched on; others are switched off.
3. The active genes produce mRNA, which is translated by ribosomes into proteins.
4. These proteins give the cell new structures and functions (e.g. actin/myosin in a muscle cell, or chlorophyll molecules destined for chloroplasts in a leaf cell).

After differentiation, the cell is no longer a generic cell — it's a specialised worker.

Why bother differentiating? Multicellular life needs division of labour. A single unspecialised cell couldn't perform breathing AND digestion AND sensing AND reproduction. By having different cells handle different jobs, the organism as a whole can do far more than the sum of its parts.

Animal cells:

• Most differentiate at an early stage of development (during embryonic growth).
• Once specialised, most cannot differentiate again.
• After this, cell division in an animal is mainly for growth (in young animals) or repair (to replace damaged cells).
• Some adult cells stay un-specialised — these are adult stem cells (e.g. in bone marrow, intestinal lining) — they continually replace dead cells.

Plant cells:

• Many plant cells retain the ability to differentiate throughout the plant's life.
• This happens in regions called meristems — found at root tips and shoot tips.
• Meristem cells can become any type of plant cell — this is why you can take a cutting from a plant and grow a new one.

Why the difference? Plants are stuck in one place and grow continuously. They need to make new tissue to extend roots into soil and shoots into light all their life. Animals develop a fixed body plan in childhood and mainly repair, not grow new organs.

Specialised cells dividing. Once differentiated, animal cells normally divide only for:

• Growth — embryonic and childhood development.
• Repair — replacing damaged or dead cells (e.g. skin healing).

That's it. AQA mark schemes are strict: write "growth OR repair" only.

Magnification is how many times larger an image looks than the real thing. A microscope with magnification $\times 400$ makes the image appear 400 times bigger than the actual specimen.

Resolution is the minimum distance between two points such that they can still be seen as separate. Lower resolution number = sharper detail. Below this distance, two points look like one blur.

	Light microscope	Electron microscope
Max magnification	~$\times 2000$	~$\times 1{,}500{,}000$
Resolution	~200 nm	~0.2 nm (1000× better)
Light/beam used	Visible light	Beam of electrons
Specimen	Living or dead, in colour	Dead only, vacuum chamber, black and white
Cost	Cheap, portable	Expensive, large, needs special room
Sees	Whole cells, large organelles	Individual organelles, viruses, even some molecules

Why electron microscopes resolve more. Electrons have a much shorter wavelength than visible light, and resolution is fundamentally limited by wavelength. Light wavelengths (~500 nm) can never resolve structures smaller than ~200 nm. Electrons (wavelength <0.01 nm) can resolve to ~0.2 nm.

AQA tip. A common mark-scheme distinction: "electron microscopes have HIGHER resolution and HIGHER magnification". Don't just say 'better' — use 'higher'.

The AQA equation (provided on the equation sheet from 2024):

$\text{magnification} = \dfrac{\text{image size}}{\text{actual size}}$

Rearranged forms:

• $\text{actual size} = \dfrac{\text{image size}}{\text{magnification}}$
• $\text{image size} = \text{magnification} \times \text{actual size}$

Always use the SAME UNITS for image and actual size before substituting. Then convert at the end if needed.

Worked example 1. A cell appears 20 mm long when viewed under ×500 magnification. What is the actual size in µm?

$\text{actual size} = \dfrac{20\,\text{mm}}{500} = 0.04\,\text{mm} = 40\,\mu\text{m}$

Worked example 2. A real bacterium is 2 µm long. The image shows it 5 mm long. What is the magnification?

Convert 5 mm to µm: $5\,\text{mm} = 5000\,\mu\text{m}$.

$\text{magnification} = \dfrac{5000}{2} = \times 2500$

Worked example 3. A ribosome is 20 nm wide. An electron micrograph shows it 4 mm wide. Calculate the magnification.

Convert 4 mm to nm: $4\,\text{mm} = 4\times10^{-3}\,\text{m} = 4\times10^{6}\,\text{nm} = 4{,}000{,}000\,\text{nm}$.

$\text{magnification} = \dfrac{4{,}000{,}000}{20} = \times 200{,}000$

This is only achievable with an electron microscope.

Aim: Use a light microscope to observe a variety of cells; produce a clear, labelled biological drawing with magnification stated.

Typical specimens (AQA's RP1):

• Onion epidermal cells — peel a thin layer from inside a fresh onion bulb scale.
• Cheek cells — gently scrape the inside of the cheek with a cotton bud.

Method (key steps):

1. Place specimen on a clean slide.
2. Add 1 drop of iodine solution (onion) or methylene blue (cheek) — the stain makes structures visible.
3. Carefully lower a coverslip at an angle to avoid air bubbles.
4. Place slide on microscope stage; start at lowest power objective lens (e.g. ×4).
5. Focus using the coarse adjustment knob, then fine adjustment.
6. Switch to higher power (e.g. ×10, ×40) only AFTER focusing at low power.
7. Draw what you see using a sharp pencil, with clear labels.
8. Calculate total magnification = eyepiece lens × objective lens (e.g. ×10 × ×40 = ×400).

Drawing rules (AQA expects):

• Use clear, continuous lines — no sketchy or shaded lines.
• No shading or colouring.
• Label lines should be straight and not cross.
• Write the magnification beneath the drawing.
• Include a title.

Safety: stains can stain skin and clothing — wash off iodine straight away. Methylene blue is mildly toxic — wash hands after use. Microscope slides have sharp edges.

When growing (culturing) microorganisms in a school lab, you must protect:

1. The culture from contamination by unwanted microbes in the air, on skin or on equipment.
2. People in the lab from any pathogens that might be present.

Key precautions (AQA spec essentials):

Precaution	Why it's done
Sterilise Petri dishes and agar before use (autoclave at 121 °C)	Kills any existing microbes that would interfere
Sterilise inoculating loop by passing through Bunsen flame until red-hot	Kills any microbes on the loop
Work close to a lit Bunsen flame	Creates rising convection currents — airborne microbes carried up and away
Lift Petri dish lid at a slight angle, only briefly	Reduces airborne contamination falling in
Tape lid in 4 places (NOT fully sealed)	Prevents anaerobic conditions developing (some dangerous bacteria thrive without oxygen, e.g. Clostridium)
Incubate at 25 °C in school	Lower than body temperature → reduces risk of growing human pathogens (which thrive at 37 °C)
Store inverted	Stops condensation dropping from lid onto agar and spreading bacteria

Why agar? Agar is a jelly made from seaweed. Microbes can't digest it, so they grow ON it without breaking it down. Nutrient agar contains added sugars, proteins and minerals as food.

Aim: Investigate the effect of different antiseptics (or antibiotics) on the growth of bacteria.

Method:

1. Pour sterile nutrient agar into a sterile Petri dish; let it set.
2. Spread bacterial culture evenly across the agar surface using a sterile spreader (the bacterial lawn).
3. Soak paper discs of equal size in different antiseptics/antibiotics; one disc in sterile water as the control.
4. Using sterile forceps, place discs evenly spaced on the agar.
5. Tape lid (4 places, not sealed); incubate inverted at 25 °C for 24–48 hours.
6. After incubation, measure the diameter of each clear inhibition zone with a ruler.
7. Calculate radius (= diameter ÷ 2) and area ($A = \pi r^2$).
8. Larger area = more effective antimicrobial.

The control disc soaked in water shows whether the disc material itself affects bacteria. Should give NO inhibition zone.

Bacteria reproduce by binary fission — one cell splits into two. Each division roughly doubles the population.

Growth equation:

$N = N_0 \times 2^n$

• $N_0$ = initial number of bacteria.
• $n$ = number of divisions.
• $N$ = final number after $n$ divisions.

Number of divisions = total time ÷ division time. If a bacterium divides every 20 minutes and we wait 2 hours (120 min), $n = 120/20 = 6$.

Worked example. A culture starts with 50 bacteria. They divide every 30 minutes. How many bacteria after 3 hours?

• Time = 180 min; divisions $n = 180/30 = 6$.
• $N = 50 \times 2^6 = 50 \times 64 = 3200$ bacteria.

Worked example (HT). A culture of 200 bacteria grows to 12,800 in 5 hours. What's the mean division time?

• $\dfrac{N}{N_0} = \dfrac{12800}{200} = 64 = 2^6$, so $n = 6$ divisions.
• Time per division = $5\,\text{h} / 6 \approx 0.83\,\text{h} = 50\,\text{min}$.