← All notesBiology · Cells & organisation⬇ Download PDF
IGCSE Biology 0610 · Chapter 2 notes · ~9 min read

Cells & organisation

What is inside animal, plant and bacterial cells and what each part does, how specialised cells are built for their jobs, the levels of organisation — and how to calculate magnification.

Animal & plant cellsBacteriaSpecialised cellsTissues & organsMagnification
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Cells are the basic units of life. Animal cells have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells also have a cell wall, chloroplasts and a vacuole.
  • Bacteria are cells with no nucleus: their DNA is a circular loop, and they have a cell wall.
  • Specialised cells have a structure that fits their function. Cells → tissues → organs → organ systems → organism.
  • Magnification = image size ÷ actual size (both in the same units).

By the end you can

  • Describe the structure and function of the parts of animal, plant and bacterial cells.
  • Relate the structure of specialised cells to their functions.
  • Order the levels of organisation: cell, tissue, organ, organ system, organism.
  • Calculate magnification, image size and actual size.
  • Convert between mm and μm. EXT

1Cell structure

Animal and plant cells

ANIMAL CELLcell membranecytoplasmmitochondrionnucleusribosomesno cell wall · no chloroplasts · no large permanent vacuole
An animal cell: no cell wall, no chloroplasts, no large permanent vacuole.
PLANT CELLcell wallcell membranenucleusmitochondrionchloroplastvacuole(cell sap)cytoplasmribosomescellulose wall · chloroplasts · large permanent vacuole
A plant cell: cell wall, chloroplasts and a large permanent vacuole.

Cell membrane — a thin layer that controls what enters and leaves the cell.

Cytoplasm — a jelly-like fluid where most chemical reactions happen.

Nucleus — contains the chromosomes (DNA) and controls the activities of the cell.

Mitochondria — where aerobic respiration takes place, releasing energy.

Ribosomes — tiny structures where proteins are made (protein synthesis).

Cell wall — made of cellulose in plants. It gives strength and support and stops the cell bursting.

Chloroplasts — contain chlorophyll, which absorbs light for photosynthesis.

Vacuole — a space full of cell sap. It keeps the cell firm (turgid) and stores substances.

Common mistake"Plant cells have a cell wall instead of a cell membrane." Plant cells have both. The membrane is inside the cell wall.
Common mistake"Cell walls control what enters the cell." Cellulose walls are fully permeable. The cell membrane controls what passes.

Bacterial cells

BACTERIAL CELL (a prokaryote)cell wallcell membranecytoplasmcircular DNA(no nucleus)plasmidribosomesflagellumno nucleus · no mitochondria · no chloroplasts
A bacterium is a prokaryote. It has no nucleus.
  • A cell wall, cell membrane, cytoplasm and ribosomes — like other cells.
  • Circular DNA loose in the cytoplasm (no nucleus) and often small rings of DNA called plasmids.
  • No mitochondria and no chloroplasts. Some have a flagellum for swimming.
StructureAnimal cellPlant cellBacterial cell
nucleusyesyesno (circular DNA instead)
cell wallnocelluloseyes (not cellulose)
chloroplastsnoyesno
mitochondriayesyesno
ribosomesyesyesyes
large vacuolenoyesno
Exam tipNew cells only come from existing cells dividing. Cells never form from non-living matter. (Cell division is covered in Inheritance, variation & selection.)

2Specialised cells

Structure fits function

(side view)Red blood cellbiconcave disc =large surface area;no nucleus: more roomfor haemoglobinCiliated cellcilia sweep mucusand trapped dirtup and away fromthe lungsSperm celltail swims to eggmitochondria giveenergy; acrosomeenzymes enter eggEgg celllarge: food storein cytoplasm;jelly coat blocksextra sperm
Four specialised animal cells. Each has features that help it do one job.
Root hair celllong, thin hair: huge surfaceto absorb water and ionsPalisade cellmany chloroplasts to absorb lighttall, at the top of the leafblue arrows = water and mineral ions moving in
Two specialised plant cells: the root hair cell and the palisade cell.

Specialised cell — a cell that has a structure adapted to its function.

CellJobAdaptations
ciliated cellmoves mucus along the airwayscilia (tiny hairs) that beat in waves; lines the trachea and bronchi
root hair cellabsorbs water and mineral ionslong, thin hair gives a large surface area; thin wall
palisade mesophyll cellphotosynthesismany chloroplasts; tall and packed at the top of the leaf
neurone (nerve cell)carries electrical impulsesvery long axon, branched ends (dendrites)
red blood cellcarries oxygenbiconcave disc, no nucleus, full of haemoglobin
sperm cellfertilises the eggtail to swim; mitochondria for energy; enzymes in the head
egg cellbecomes the embryolarge, food store in the cytoplasm; jelly coat blocks extra sperm
  • A neurone is shown in Coordination & response.
  • Gametes (sperm and egg) are haploid: they carry half the normal number of chromosomes. See Reproduction.
  • A red blood cell has no nucleus, which leaves more room for haemoglobin. A biconcave shape gives a large surface area for taking up oxygen.
Exam tipWhen a question says "explain how the cell is adapted", write feature + how it helps: "root hair cell has a long thin extension, which gives a large surface area for absorbing water."
Common mistakeLearning "palisade cells have chloroplasts" with no reason. Add: so they absorb more light for photosynthesis.

Levels of organisation

cella musclecelltissuemuscletissueorganthe heartorgan systemcirculatorysystemorganisma humancells → tissues → organs → organ systems → organism
From a single cell to a whole organism.

Tissue — a group of similar cells working together to do the same job (muscle tissue, xylem).

Organ — a structure made of different tissues that carries out a particular job (heart, stomach, leaf).

Organ system — a group of organs with related functions (digestive system, circulatory system).

Organism — a living thing made of organ systems working together.

  • The stomach is an organ made of muscle tissue, glandular tissue (makes enzymes) and epithelial tissue (lines the surface).
  • A leaf is a plant organ made of epidermis, palisade tissue, spongy tissue and xylem and phloem.
Memory trickCats Take Over Small Offices = Cell → Tissue → Organ → System → Organism.

3Size and magnification

Units

mmmillimetreμmmicrometrenmnanometre×1000×1000÷1000÷1000ant ≈ 3 mmpinhead ≈ 1 mmanimal cell ≈ 20 μmbacterium ≈ 2 μmvirus ≈ 100 nmDNA width ≈ 2 nmEach step down the units = ×10001 mm = 1000 μm · 1 μm = 1000 nm
Each step down the units multiplies by 1000.
  • 1 mm = 1000 μm (micrometres), and 1 μm = 1000 nm. EXT You must be able to convert between mm and μm.
  • To convert mm → μm, multiply by 1000. To convert μm → mm, divide by 1000.
  • A typical animal cell is about 20 μm (0.02 mm). A bacterium is about 2 μm. A virus is about 100 nm.

Calculating magnification

IAMcover what you want:M = I ÷ AI = A × MA = I ÷ MI = 36 mm (measured on the drawing)actual width A = 0.06 mm (a 60 μm cell)M = I ÷ A = 36 ÷ 0.06 = ×600Same units first! Convert, then divide.
The magnification triangle. Cover the quantity you want and read off the formula.
magnification = image size ÷ actual size
both sizes must be in the same units
image size = actual size × magnification
or actual size = image size ÷ magnification
Worked example

A drawing of a cell is 36 mm long. The real cell is 0.06 mm long. Calculate the magnification.

  1. magnification = image size ÷ actual size
  2. same units already: 36 mm and 0.06 mm
  3. 36 ÷ 0.06 = ×600
Worked example

A photograph of a cell is 45 mm wide. The magnification is ×300. What is the actual width in μm?

  1. actual size = image size ÷ magnification
  2. 45 ÷ 300 = 0.15 mm
  3. 0.15 mm × 1000 = 150 μm
Worked example

EXT A scale bar on a micrograph is 20 mm long and represents 10 μm. A cell on the micrograph is 60 mm wide. Find the actual width of the cell.

  1. convert the bar to μm: 20 mm = 20 000 μm
  2. magnification = 20 000 ÷ 10 = ×2000
  3. actual width = 60 mm ÷ 2000 = 0.03 mm
  4. 0.03 × 1000 = 30 μm
Exam tipA neat method: write both sizes in the same unit first (usually mm or μm), then divide. Always include units in the answer, but magnification itself has no unit (write ×600).
Common mistakeForgetting to convert. 36 mm ÷ 60 μm is not 0.6. Convert 60 μm = 0.06 mm first, then 36 ÷ 0.06 = 600.
Common mistakeMeasuring the drawing in cm and the actual size in mm, then dividing without converting.

4Test yourself

Test yourself — cover the answers first

Quick check

  1. Name the part of the cell where aerobic respiration happens.
  2. Which part of a plant cell contains chlorophyll?
  3. What is the cell wall of a plant made of?
  4. Give two ways a bacterial cell differs from an animal cell.
  5. Which structure controls what enters and leaves the cell?
  6. Put in order from smallest: organ, cell, organism, tissue, organ system.
  7. State the formula for magnification.
  8. EXT Convert 0.045 mm to μm.

Exam-style

  1. Describe two ways a root hair cell is adapted for its function. [2]
  2. Explain why red blood cells have no nucleus. [2]
  3. A student draws a plant cell 24 mm long. The magnification is ×120. Calculate the actual length in μm. [3]
  4. EXT Compare the structure of a plant cell and a bacterial cell. [3]
Show answers
  1. mitochondria
  2. chloroplasts
  3. cellulose
  4. no nucleus; has a cell wall; circular DNA/plasmids
  5. cell membrane
  6. cell, tissue, organ, organ system, organism
  7. magnification = image size ÷ actual size
  8. 45 μm
  9. long, thin extension gives a large surface area
    thin cell wall so water enters quickly (or: no chloroplasts, large vacuole)
  10. more room for haemoglobin
    so they carry more oxygen
  11. 24 ÷ 120
    = 0.2 mm
    = 200 μm
  12. both have cell walls, membranes, cytoplasm and ribosomes
    plant cell has a nucleus, bacterium has circular DNA free in the cytoplasm
    plant cell has chloroplasts, mitochondria and a vacuole; the bacterium does not
Practise this chapter
Cell structureCell size & magnification
Next chapterMovement into & out of cells →
Stuck on a question?

Send it to us on WhatsApp. We read every message.

Want regular help? Send this to your parent →
Share with a friendAsk on WhatsApp