IB MYP Sciences – Cells and Organisms Study Guide

This study guide is for IB MYP Sciences and follows the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. It teaches cells and organisms, two of the topics the brief lists for the MYP sciences on-screen examinations, and it suits MYP years 4 and 5. MYP Sciences has no SL/HL split, so everything here applies to every student, and the page is written for the on-screen examinations the IB offers at the end of MYP year 5.

MYP has no prescribed content list – schools design their own units. This page covers a topic the IB’s brief lists, using the standard biology that school units on cells normally teach. Your teacher will share the task-specific clarifications for your unit.

Use it with the cells and organisms revision notes and the cells and organisms practice questions. For the course as a whole, see the MYP Sciences hub and the printable checklist.

What this unit covers

Area	What you must be able to do	Criterion it mainly trains
Cell theory and cell structure	Label animal, plant and bacterial cells; state the function of each part	A
Microscopy	Compare light and electron microscopes; use magnification = image size ÷ actual size	A and C
Specialised cells	Explain how the structure of a cell suits its job	A
Levels of organisation	Put cell, tissue, organ, organ system and organism in order, with examples	A
Classification	Use the classification hierarchy, binomial names and dichotomous keys	A and B

The brief names four equally weighted criteria: A Knowing and understanding, B Inquiring and designing, C Processing and evaluating, and D Reflecting on the impacts of science. Each has eight achievement levels (1–8) in four bands. In the on-screen examination, the “Knowing and understanding” task (25 marks) assesses criterion A, “Investigation skills” (50 marks) assesses criteria B and C, and “Applying science” (25 marks) assesses criterion D. Cells work can appear in all three.

Cell theory

Cell theory has three ideas:

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 the division of existing cells.

Robert Hooke used the word “cell” in 1665 after looking at thin slices of cork. Cell theory grew from many later observations, and it depended on better microscopes – science as a human endeavour.

Cell structure: animal, plant and bacterial cells

Animal and plant cells are eukaryotic: their DNA is inside a nucleus, and they contain membrane-bound organelles. Bacterial cells are prokaryotic: they have no nucleus and no membrane-bound organelles.

Structure	Function	Animal	Plant	Bacterium
Cell membrane	Controls what enters and leaves the cell	Yes	Yes	Yes
Cytoplasm	Jelly-like fluid where most chemical reactions happen	Yes	Yes	Yes
Ribosomes	Make proteins	Yes	Yes	Yes (smaller)
Nucleus	Contains DNA; controls the cell’s activities	Yes	Yes	No
Mitochondria	Site of aerobic respiration, which releases energy	Yes	Yes	No
Cell wall	Supports the cell and stops it bursting	No	Yes (cellulose)	Yes (not cellulose)
Chloroplasts	Absorb light for photosynthesis	No	In green parts	No
Permanent vacuole	Holds cell sap; keeps the cell firm	No	Yes	No
Circular DNA and plasmids	Carry the genes, free in the cytoplasm	No	No	Yes

Some bacteria also have a flagellum for movement and a slime capsule outside the wall.

Two traps: plant cells have mitochondria as well as chloroplasts, because plants respire all the time; and root cells, underground, have no chloroplasts.

Worked example 1 – identifying a cell

Question. A cell has a cell wall, ribosomes and a plasmid, but no nucleus. Identify the type of cell and give your reason.

Answer.

• A cell wall rules out an animal cell.
• No nucleus and a plasmid are features of prokaryotes.
• So it is a bacterial cell, because it has no nucleus and its DNA includes a plasmid.

Microscopy and magnification

Light and electron microscopes

Feature	Light microscope	Electron microscope
What forms the image	Light and glass lenses	A beam of electrons
Useful magnification	Up to about ×1500	Can exceed ×500 000
Resolution (smallest detail seen separately)	About 200 nm	Under 1 nm
Specimens	Living or dead	Dead only (placed in a vacuum)
Image	Can show colour (stains)	Black and white (colour may be added later)
Cost and size	Cheap, portable	Expensive, large

Magnification is how many times bigger the image is than the real object. Resolution is the ability to see two close points as separate. Enlarging a blurred image adds no detail, so a light microscope cannot show ribosomes.

Total magnification of a light microscope:

total magnification = eyepiece lens magnification × objective lens magnification

A ×10 eyepiece with a ×40 objective gives ×400.

Units

1 mm = 1000 µm        1 µm = 1000 nm

To go from mm to µm, multiply by 1000. To go from µm to nm, multiply by 1000. Typical sizes: an animal cell is about 10–30 µm across, a bacterium about 1–5 µm long, a mitochondrion about 1 µm.

The magnification equation

magnification = image size ÷ actual size
actual size   = image size ÷ magnification
image size    = actual size × magnification

Both sizes must be in the same unit before you divide. Magnification has no unit; write it as ×500, for example.

Worked example 2 – finding magnification

Question. A cheek cell is 60 µm across. In a drawing it measures 30 mm across. Calculate the magnification of the drawing.

Answer.

• Convert the image size: 30 mm × 1000 = 30 000 µm.
• Magnification = 30 000 ÷ 60 = ×500.

Worked example 3 – finding actual size

Question. An electron micrograph has a magnification of ×8000. A chloroplast in it is 40 mm long. Calculate the actual length of the chloroplast in µm.

Answer.

• Image size = 40 mm = 40 000 µm.
• Actual size = 40 000 ÷ 8000 = 5 µm.

Worked example 4 – using a scale bar

Question. A micrograph has a scale bar labelled 50 µm. The bar measures 25 mm with a ruler. A plant cell in the same image measures 42 mm long. Calculate (a) the magnification and (b) the actual length of the cell.

Answer.

• (a) Scale bar image = 25 mm = 25 000 µm. Magnification = 25 000 ÷ 50 = ×500.
• (b) Cell image = 42 mm = 42 000 µm. Actual length = 42 000 ÷ 500 = 84 µm.

A scale bar stays correct if the picture is resized; a printed “×500” does not.

Worked example 5 – estimating from the field of view

Question. At ×400, the field of view is 0.5 mm across. About 10 cheek cells fit edge to edge across it. Estimate the width of one cell.

Answer. 0.5 mm = 500 µm. Width ≈ 500 ÷ 10 = 50 µm.

Specialised cells

Cells in a multicellular organism differentiate: they develop features that suit one job. The pattern in every answer is feature → how it helps.

Cell	Key features	How the features help
Red blood cell	No nucleus; biconcave disc; full of haemoglobin	More room for haemoglobin; large surface area for oxygen to diffuse in and out
Nerve cell (neurone)	Long axon; branched endings	Carries electrical impulses over long distances; connects with many other cells
Sperm cell	Tail; many mitochondria; enzymes in the head	Swims to the egg; mitochondria release energy for swimming; enzymes digest a way into the egg
Ciliated cell (airways)	Tiny hairs called cilia	Sweep mucus and trapped dust away from the lungs
Root hair cell	Long, thin extension; no chloroplasts	Large surface area to absorb water and mineral ions; underground, so no light to use
Palisade cell (leaf)	Many chloroplasts; near the upper surface	Absorbs as much light as possible for photosynthesis
Guard cell (leaf)	A pair of cells around a pore (stoma)	Change shape to open and close the stoma, controlling gas exchange and water loss

Worked example 6 – explaining an adaptation

Question. Explain one way a root hair cell is adapted to its function.

Answer. The root hair is a long, thin extension of the cell [feature]. This gives a large surface area [effect], so the cell can absorb more water and mineral ions from the soil [link to function].

Levels of organisation

cell → tissue → organ → organ system → organism

• Tissue: a group of similar cells working together, for example muscle tissue.
• Organ: different tissues working together to do a job, for example the stomach (muscle, glandular and lining tissues) or a leaf.
• Organ system: organs working together, for example the digestive system (stomach, intestines, liver, pancreas).
• Organism: a whole living thing.

Plants have the same levels: a leaf is an organ, and xylem and phloem form a transport system. Single-celled organisms, such as bacteria and yeast, have no tissues or organs.

Classification of organisms

Scientists group organisms by shared features; modern classification also uses DNA evidence.

The hierarchy

From largest group to smallest:

domain → kingdom → phylum → class → order → family → genus → species

A species is a group of organisms that can interbreed to produce fertile offspring.

Kingdoms and domains

The five-kingdom system:

Kingdom	Main features
Animals	Multicellular; no cell walls; feed on other organisms
Plants	Multicellular; cellulose cell walls; photosynthesise
Fungi	Cell walls not made of cellulose; feed by digesting food outside their cells and absorbing it; e.g. yeast, mushrooms
Protoctists	Mostly single-celled eukaryotes, e.g. Amoeba
Prokaryotes	Single-celled; no nucleus, e.g. bacteria

In 1990 Carl Woese and colleagues proposed the three-domain system – Archaea, Bacteria and Eukarya – after chemical evidence showed that archaea differ greatly from bacteria. Viruses are not placed in any kingdom, because they are not made of cells and can reproduce only inside a host cell.

Binomial names

Carl Linnaeus, in the 18th century, set up the two-part naming system still used today. The first word is the genus (capital letter) and the second is the species (lower case). The name is written in italics, or underlined by hand.

Worked example 7 – using binomial names

Question. The lion is Panthera leo, the tiger is Panthera tigris and the domestic cat is Felis catus. Which two are most closely related? Explain.

Answer. The lion and the tiger, because they share the same genus, Panthera. The cat is in the same family (Felidae) but a different genus, so it is less closely related.

Dichotomous keys

A dichotomous key asks a series of questions, each with two answers, until you reach one name. Good keys use features you can see and measure (“wings present or absent”), not vague ones (“large or small”). Designing a key for a set of organisms is a useful criterion B skill, because you must choose features that separate every organism.

Links to MYP concepts and contexts

The brief gives models as an example of a related concept in sciences. A cell diagram is a model: flat and simplified, while real cells are three-dimensional and crowded. Say what a model shows and what it leaves out.

The global context scientific and technical innovation fits the story of microscopes. For criterion D, you might weigh up the costs and benefits of electron microscopes in a hospital.

Common errors

• Dividing sizes in different units, such as mm by µm, and getting an answer 1000 times too big or small.
• Writing magnification with a unit (“500 µm”) instead of “×500”.
• Saying plant cells have chloroplasts instead of mitochondria.
• Saying bacteria “have no DNA” – they do; it is just not in a nucleus.
• Confusing magnification with resolution.
• Writing panthera Leo – the genus takes the capital, the species does not.
• Describing a feature of a specialised cell without explaining how it helps.

Next steps

Test yourself with the revision notes, then work through the practice questions. For how the criteria and exam tasks work, read the MYP Sciences syllabus guide, the criteria in practice and investigation skills exam preparation. Related units: metabolism and evolution and interactions.

Official syllabus

This page is aligned to the International Baccalaureate Organization, Middle Years Programme Subject Brief – Sciences, from 2014. The brief lists “cells” and “organisms” among the topics explored in the MYP sciences on-screen examinations. The full MYP sciences guide is available to schools through the IB.