This page needs the IB Demystified site-wide snippets. Ask the site administrator to install them.
IB DemystifiedMYP Sciences
Organisation: from cells to organisms
You are made of about 30 trillion cells, but they are not a random crowd. They are organised into tissues, organs and systems, each level doing a job the level below cannot do alone.
Recommended for MYP 2 · About 3 lessons · Criteria A, B, C and D
Figure 1. Five levels of organisation, using the circulatory system as the example.
put the levels of organisation in order and define each one
classify examples as cells, tissues, organs or organ systems
describe the main human organ systems and their functions
explain how organ systems depend on each other
identify the tissues in a leaf and describe what they do
describe differentiation and the role of stem cells
Before you start
You will use these skills. If any feel shaky, review them first.
the structures in plant and animal cells, and specialised cells (see Cells and microscopy)
the seven life processes
converting between mm and µm
Key vocabulary
Tissue
A group of similar cells working together to carry out a function.
Organ
A structure made of different tissues working together to carry out a function.
Organ system
A group of organs working together to carry out a major function.
Organism
A whole living thing, made of one or more cells.
Differentiation
The process by which a cell becomes specialised for a particular job.
Stem cell
An unspecialised cell that can divide and differentiate into specialised cells.
Understanding organisation
What is it?
In a multicellular organism, similar cells form tissues, different tissues form organs, organs form organ systems, and systems together make the organism. The heart, for example, contains muscle, nerve and connective tissue working together to pump blood.
Why does it happen?
A large organism cannot rely on each cell doing everything. As an embryo develops, cells differentiate: they switch on different genes and become specialised. Specialised cells work far better in teams, so they are grouped into tissues and organs, and systems connect them so every cell gets what it needs.
How do we know?
Anatomists dissected bodies to map organs; microscopes then showed that organs are built from distinct tissues. Today, stained tissue sections and medical scans let doctors see each level, and experiments on stem cells show how specialised cells develop.
Why does it matter?
Disease can strike at any level, from faulty cells to a failing organ, and treatments are chosen accordingly. Transplants, tissue engineering and stem-cell therapies all depend on understanding how tissues and organs are built.
What does it connect to?
Organisation links cells to every body system you will study: digestion, breathing, circulation and coordination. It connects to plant transport and photosynthesis, and in mathematics to scale and surface area to volume ratio.
Organisation in plants
Plants are organised in the same way. Their organs are roots, stems, leaves and flowers, and each is made of tissues. A leaf is an organ built for photosynthesis:
Figure 2. Cross-section of a leaf.
Epidermis (A, and the lower surface): a protective layer, covered in a waxy cuticle that reduces water loss.
Palisade layer (B): tightly packed cells full of chloroplasts, where most photosynthesis happens.
Spongy layer (C): loosely packed cells with air spaces that let gases move through the leaf.
Veins (D): xylem brings water in; phloem carries sugars away.
Stomata (E): pores, opened and closed by guard cells, that let carbon dioxide in and oxygen and water vapour out.
Organisation in the real world
Surgeons replace damaged organs through transplants, and doctors grow sheets of a patient's own skin cells to treat severe burns. Physiotherapists design training that targets particular tissues, such as muscle and tendon, and sports scientists measure how the circulatory and respiratory systems respond to exercise.
Worked examples
Example 1: classifying a structure
Is the small intestine a tissue, an organ or an organ system?
Check what it is made of: muscle tissue, lining tissue, glandular tissue and blood vessels.
Several different tissues working together for one function (digesting and absorbing food) means it is an organ.
It is one of the organs of the digestive system.
Example 2: systems working together
How does oxygen get from the air to a muscle cell in your arm?
Respiratory system: air is breathed into the lungs, and oxygen diffuses into the blood.
Circulatory system: the heart pumps the oxygen-rich blood through arteries to the arm.
Oxygen diffuses from the blood into the muscle cells, where it is used in respiration.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Show
Investigation: how do body systems respond to exercise?
Guided investigation · about 40 minutes · pairs
Research question
How do heart rate and breathing rate change during 3 minutes of step exercise and the 3 minutes afterwards?
Scientific background
Working muscles respire faster, so they need more oxygen and glucose and produce more carbon dioxide. The respiratory and circulatory systems respond together to meet this demand.
Hypothesis
Both rates will increase during exercise and decrease afterwards, because muscles need more oxygen during exercise, and less once they stop.
Independent variable
Time: at rest, after each minute of exercise, and after 1 and 3 minutes of rest.
Dependent variables
Heart rate (beats per minute, from the pulse at the wrist or neck) and breathing rate (breaths per minute).
Control variables
Same step height and stepping rhythm (use a metronome), because a harder exercise would raise the rates more.
Same person measuring, counting for the same time (15 s, multiplied by 4).
Participant rested for 5 minutes before starting.
Apparatus
Low step or bench (about 20 cm), stopwatch, metronome app.
Method
Sit quietly for 5 minutes, then measure resting heart rate and breathing rate.
Step on and off the bench in time with the metronome for 3 minutes.
Stop briefly at the end of each minute so your partner can measure both rates.
Sit down and measure again after 1 and 3 minutes of rest.
Collect class results and calculate means.
Safety. Take part only if you are well and have no medical condition that makes exercise risky; anyone can choose to be the measurer instead. Use a stable step, wear suitable shoes, and stop if you feel dizzy or unwell.
Results from one student are used in the Criterion C questions below. Then evaluate: why is it better to use class means than one student's results?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
