IB MYP Sciences – Cells and Organisms Revision Notes

These are condensed notes for the final weeks. For full explanations and worked examples, read the cells and organisms study guide first.

The notes are for IB MYP Sciences and follow the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. They cover cells and organisms, two topics the brief lists for the MYP sciences on-screen examinations, and suit MYP years 4 and 5. There is no SL/HL split in MYP, and the notes are written for the on-screen examinations at the end of MYP year 5. MYP has no prescribed content list – schools design their own units – so check the depth of your own unit with your teacher, who will share the task-specific clarifications.

Course links: MYP Sciences hub · printable checklist · practice questions for this unit · MYP Sciences assessment revision notes.

Cell theory in three lines

1. All living things are made of one or more cells.
2. The cell is the smallest unit that carries out the processes of life.
3. New cells come only from existing cells.

Key definitions

• Eukaryotic cell: DNA inside a nucleus; membrane-bound organelles. Animals, plants, fungi, protoctists.
• Prokaryotic cell: no nucleus; no membrane-bound organelles; DNA is a loop in the cytoplasm, often with plasmids. Bacteria.
• Organelle: a structure inside a cell with a particular job.
• 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 separate.
• Differentiation: a cell developing features that suit a particular function.
• Tissue: a group of similar cells working together.
• Organ: a group of different tissues working together.
• Organ system: a group of organs working together.
• Species: organisms that can interbreed to produce fertile offspring.
• Dichotomous key: a series of paired questions that leads to the name of an organism.

Organelles at a glance

Part	Job	Animal	Plant	Bacterium
Cell membrane	Controls entry and exit	✓	✓	✓
Cytoplasm	Site of most reactions	✓	✓	✓
Ribosome	Protein synthesis	✓	✓	✓ (smaller)
Nucleus	Holds DNA; controls the cell	✓	✓	✗
Mitochondrion	Aerobic respiration	✓	✓	✗
Cell wall	Support; stops bursting	✗	✓ cellulose	✓ not cellulose
Chloroplast	Photosynthesis	✗	green parts only	✗
Permanent vacuole	Cell sap; keeps cell firm	✗	✓	✗
Plasmid	Extra small loops of DNA	✗	✗	✓

Formulas and conversions

What	Formula or fact
Magnification	magnification = image size ÷ actual size
Actual size	actual size = image size ÷ magnification
Image size	image size = actual size × magnification
Total magnification	eyepiece × objective
mm → µm	× 1000
µm → nm	× 1000
µm → mm	÷ 1000

A memory aid: I AM – Image = Actual × Magnification.

Method in steps: any magnification question

1. Write down the equation you need.
2. Convert both lengths into the same unit (µm is usually easiest).
3. Substitute and calculate.
4. Give magnification as “×number” with no unit; give sizes with a unit.
5. Ask: is the answer sensible? A cell of 3000 µm (3 mm) is visible without a microscope – check your conversion.

Method in steps: scale bar

1. Measure the scale bar with a ruler, in mm.
2. Convert to the unit on the bar’s label.
3. Magnification = measured bar length ÷ label value.
4. Measure the object, convert, and divide by that magnification.

Worked reminders

• Image 30 mm, actual 60 µm: 30 000 ÷ 60 = ×500.
• ×8000 micrograph, image 40 mm: 40 000 ÷ 8000 = 5 µm.
• Field of view 500 µm, 10 cells across: 500 ÷ 10 = 50 µm per cell.

Light versus electron microscope

	Light	Electron
Uses	light	electron beam
Magnification	up to about ×1500	can exceed ×500 000
Resolution	about 200 nm	under 1 nm
Living specimens?	yes	no (vacuum)
Colour	yes, with stains	no (added later if at all)
Cost	low	high
Shows	cells, nuclei, chloroplasts	ribosomes, inner structure of mitochondria

Specialised cells: feature → function

Cell	Feature	Why it helps
Red blood cell	no nucleus; biconcave; haemoglobin	more space to carry oxygen; large surface area
Neurone	long axon; branched endings	impulses over long distances; many connections
Sperm	tail; many mitochondria; enzymes in head	swims; energy for swimming; enters egg
Ciliated cell	cilia	moves mucus out of airways
Root hair cell	long extension; no chloroplasts	large surface area for water and ions; no light underground
Palisade cell	many chloroplasts	absorbs more light
Guard cell	pair around stoma	opens and closes the pore

Levels of organisation

cell → tissue → organ → organ system → organism

Level	Animal example	Plant example
Cell	muscle cell	palisade cell
Tissue	muscle tissue	palisade tissue
Organ	stomach	leaf
Organ system	digestive system	transport system (xylem and phloem)

Classification

Hierarchy, largest to smallest: domain → kingdom → phylum → class → order → family → genus → species.

Five kingdoms:

Kingdom	Recognise it by
Animals	multicellular; no cell wall; eat other organisms
Plants	multicellular; cellulose walls; chloroplasts
Fungi	walls not cellulose; digest food outside the cell and absorb it
Protoctists	mostly single-celled eukaryotes
Prokaryotes	single-celled; no nucleus

Three domains (Woese and colleagues, 1990): Archaea, Bacteria, Eukarya. All eukaryotes – animals, plants, fungi and protoctists – are in Eukarya.

Binomial name: Genus species – genus with a capital, species in lower case, italics (underline when handwritten). Two organisms in the same genus are more closely related than two in the same family but different genera.

Viruses: not cells, not in any kingdom, reproduce only inside host cells.

Method in steps: using or building a dichotomous key

1. Start at step 1 every time.
2. Read both options; pick the one that fits the organism in front of you.
3. Follow the arrow to the next step or name.
4. When building a key, split the group into two at each step, using features you can see or count (legs, wings, shell), never size words like “big”.
5. Check that every organism ends at its own name.

Must-know distinctions

• Prokaryotic vs eukaryotic: nucleus absent vs present.
• Magnification vs resolution: how big vs how much detail.
• Plant cell wall vs bacterial cell wall: cellulose vs not cellulose. Fungal walls are also not cellulose.
• Tissue vs organ: one type of cell vs several tissues.
• Genus vs species: first word vs second word of the binomial name.
• Chloroplast vs mitochondrion: photosynthesis vs respiration; plant cells have both.

Links to MYP concepts

“Models” is one of the related concepts the brief lists for sciences. A labelled cell diagram is a model: useful because it is simple, limited because real cells are three-dimensional and crowded. The brief’s global context “scientific and technical innovation” fits the history of microscopes.

Quick self-test

1. Convert 2.5 mm into µm.
2. Convert 750 nm into µm.
3. An image of a cell is 24 mm long at ×300. Calculate the actual length in µm.
4. An object 0.15 mm long appears 45 mm long in a drawing. Calculate the magnification.
5. A ×10 eyepiece is used with a ×10 objective. State the total magnification.
6. A bacterium 3 µm long is 9 mm long in a micrograph. Calculate the magnification.
7. Name two structures found in bacterial cells but not in animal cells.
8. Why can a light microscope not show ribosomes?
9. A leaf is made of several tissues. Which level of organisation is it?
10. Quercus robur is the English oak. Name its genus.
11. State one reason viruses are not placed in any kingdom.
12. Give one feature of a sperm cell and explain how it helps.

Answers

1. 2.5 × 1000 = 2500 µm.
2. 750 ÷ 1000 = 0.75 µm.
3. 24 mm = 24 000 µm; 24 000 ÷ 300 = 80 µm.
4. 45 ÷ 0.15 = ×300 (both already in mm).
5. 10 × 10 = ×100.
6. 9 mm = 9000 µm; 9000 ÷ 3 = ×3000.
7. Any two of: cell wall, plasmids, circular DNA free in the cytoplasm, flagellum, capsule.
8. Ribosomes are smaller than the light microscope’s resolution (about 200 nm), so they cannot be seen as separate objects.
9. An organ.
10. Quercus.
11. They are not made of cells / they can only reproduce inside a host cell.
12. For example: many mitochondria, which release energy for the tail to swim; or a tail, to swim to the egg; or enzymes in the head, to digest a way into the egg.

Where marks are usually lost

• Mixing units in the magnification equation – for example dividing 30 mm by 60 µm.
• Converting the wrong way: mm to µm is ×1000, not ÷1000.
• Writing a unit after a magnification, or forgetting the unit after an actual size.
• Rounding too early in multi-step calculations; keep full values until the final line.
• Stating that plant cells lack mitochondria.
• Saying bacteria have no DNA, or that their DNA is in a nucleus.
• Listing a specialised cell’s feature without the “so that…” link to its function.
• Putting “organ” and “tissue” in the wrong order, or calling a single cell type an organ.
• Binomial names with the wrong capitals, or not underlined when handwritten.
• In a key, using a feature that is not visible or that does not split the group into two.

Official syllabus

These notes are aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014, which lists “cells” and “organisms” among the topics explored in MYP sciences on-screen examinations.