Cell Structure

Every animal cell — from a muscle cell to a liver cell — contains the same set of basic structures, even though their shapes differ wildly:

• Nucleus. A large dense organelle containing the cell's DNA on chromosomes. Controls the cell by determining which proteins are made.
• Cytoplasm. Jelly-like material filling the cell. Site of many chemical reactions including glycolysis (the first stage of respiration). Holds the organelles in suspension.
• Cell-surface membrane. A thin layer of phospholipids and proteins surrounding the cell. Controls what enters and leaves; partially permeable (small molecules pass through, large ones do not).
• Mitochondria. Rod-shaped organelles. Site of aerobic respiration, where ATP is produced. Cells with high energy demands (muscle, sperm, root hair) have MANY mitochondria.
• Ribosomes. Tiny structures (about 20 nm) where protein synthesis takes place. Most are attached to the rough endoplasmic reticulum.

Edexcel mark-scheme keyword note. Always state the FUNCTION alongside the structure ('mitochondria are the site of aerobic respiration'); just naming the structure rarely scores full marks.

A plant cell has all the structures of an animal cell PLUS three additional features:

1. Cellulose cell wall. A rigid layer of cellulose fibres outside the cell-surface membrane. Functions:

• Gives the cell strength and a fixed shape.
• Prevents the cell from bursting when water enters by osmosis.
• Allows the cell to develop turgor pressure that keeps the plant firm and upright.

2. Chloroplasts. Green disc-shaped organelles containing the pigment chlorophyll.

• Site of photosynthesis.
• Chlorophyll absorbs light energy (especially red and blue wavelengths) and uses it to combine CO₂ and water into glucose.
• Found in cells that receive light — especially palisade mesophyll cells in the leaf — but NOT in root cells (no light underground).

3. Large permanent vacuole. A single large central sac containing cell sap (a dilute solution of sugars and salts).

• Pushes the cytoplasm outward against the cell wall.
• Keeps the cell turgid (firm).
• Plants WILT when their cells lose water and the vacuoles shrink.

Other plant-cell points to remember:

• Plants store carbohydrate as STARCH (in chloroplasts or amyloplasts). Animals store it as glycogen (in liver and muscle).
• Plant cells are usually more regular in shape than animal cells (the cell wall dictates the shape).

Edexcel pitfall. Don't draw chloroplasts in every plant cell — root cells, for example, have none. Examiner reports specifically flag this mistake.

Bacteria are prokaryotes — fundamentally different from animal and plant cells:

• No nucleus. DNA is a single circular chromosome lying free in the cytoplasm.
• Plasmids. Small extra rings of DNA may also be present. Plasmids often carry useful genes such as antibiotic resistance and can be transferred from one bacterium to another. (Plasmids are widely used in genetic engineering — they will reappear in Topic 5.)
• Cell wall. Present, but NOT made of cellulose — it is made of peptidoglycan / murein. The wall prevents osmotic bursting.
• Cell-surface membrane — encloses the cytoplasm; respiratory enzymes are attached to its inner surface (bacteria respire on the membrane because they have no mitochondria).
• Cytoplasm with ribosomes — protein synthesis still happens (bacterial ribosomes are slightly smaller than eukaryotic ones, ~70S vs ~80S).
• NO mitochondria, NO chloroplasts, NO endoplasmic reticulum, NO Golgi apparatus — no membrane-bound organelles of any kind.
• Optional features (relevant for Paper 2 P-spec depth): some bacteria have flagella (long tail-like extensions for swimming); pili (short hair-like projections for attaching to surfaces or other cells); a slime capsule for protection.

Edexcel pitfall. Don't write 'bacteria have no DNA' — they have plenty. The difference is that the DNA is NOT enclosed in a nucleus. Don't write 'cellulose cell wall' for a bacterium — examiner reports flag this every series.

Magnification tells you how many times larger the image appears than the actual object. The formula is:

$M = \dfrac{I}{A}$

where $I$ = image size and $A$ = actual size. Magnification is a ratio — it has NO units.

The formula rearranges into three useful forms:

• $M = I/A$ — find the magnification.
• $A = I/M$ — find the actual size from a magnified image.
• $I = M \times A$ — find the size of the image given the actual size and magnification.

The unit cascade. $1\,\text{mm} = 1000\,\mu\text{m} = 1\,000\,000\,\text{nm}$

Multiply by 1000 each time you go DOWN (mm → μm → nm). Divide by 1000 each time you go UP. Before substituting into $M = I/A$, image and actual size MUST be in the SAME unit.

Useful sizes to memorise:

Object	Typical size
Plant / animal cell	20-100 μm
Bacterium	1-10 μm
Mitochondrion	1-2 μm
Ribosome	~20 nm
Virus	20-300 nm

Worked example. A drawing is 60 mm wide. The actual cell is 40 μm wide. Find the magnification.

Step 1 — convert to same units: $60\,\text{mm} = 60 \times 1000 = 60\,000\,\mu\text{m}$. Step 2 — substitute: $M = 60\,000 / 40 = \times 1500$. Step 3 — no units (magnification is a ratio).

Total microscope magnification.

$M_\text{total} = M_\text{eyepiece} \times M_\text{objective}$

Example: a ×10 eyepiece with a ×40 objective gives ×400 total magnification.

Edexcel names microscopy as one of the spec required practicals. Examiners can ask about it on either paper.

Standard method — viewing onion epidermis (plant) or cheek cells (animal):

1. Prepare the slide.
   • Plant cells: peel a thin layer of epidermis from the inside of an onion bulb with forceps; lay flat on a clean glass slide.
   • Animal cells: scrape the inside of the cheek with a clean cotton bud and smear onto a glass slide.
2. Add a stain.
   • Iodine solution for plant cells — stains nuclei + starch blue-black / brown.
   • Methylene blue for animal cells — stains nuclei and cytoplasm.
3. Lower a cover slip at an angle of about 45° using a mounted needle. This avoids trapping air bubbles (which appear as dark rings and obscure cells). Soak up any excess fluid with filter paper.
4. Focus on LOW power first.
   • Always start with the lowest-power objective (×4 or ×10).
   • Use the coarse focus knob to move stage and objective close together, then move them apart while looking through the eyepiece until the image is in focus.
5. Switch to higher power.
   • Rotate the nosepiece to the higher-power objective (×40 or ×100).
   • Adjust ONLY with the fine focus knob — never coarse focus, which can crack the slide.
6. Record the total magnification.
   • $M_\text{total} = M_\text{eyepiece} \times M_\text{objective}$.
7. Draw the cell.
   • Use a sharp pencil on plain paper.
   • Continuous outlines only — no shading, no feathery sketching.
   • Draw only what you SEE, not what you remember from a textbook.
   • Aim for at least half a page in size.
8. Label.
   • Use straight ruled label lines that do NOT cross.
   • Label clearly: nucleus, cytoplasm, cell-surface membrane (animal); add cell wall, chloroplasts (if present), vacuole (plant).
9. Add a title and magnification.
   • e.g. 'Onion epidermal cell — total magnification ×400'.

Edexcel mark-scheme tip. Examiner reports specifically praise candidates who lay out their answer as a numbered list / bullet points and criticise rambling prose. Use sub-headings on extended-response practical questions whenever possible.