10.1 Cells & Human Body Systems
Key Takeaways
- A thing is living only if it shows all the characteristics of life: it needs nutrition, breathes or respires, grows, responds to its surroundings, moves, gets rid of wastes and can reproduce - one or two of these alone is not enough
- The cell is the basic unit of life: every living thing is made of one or more cells, and all the life processes happen inside them
- Every cell has a nucleus, a cell membrane and cytoplasm; plant cells add a cell wall, chloroplasts and a large vacuole, which is why plants can make their own food and animals cannot
- The digestive, circulatory, respiratory, skeletal, muscular and nervous systems each have one main job, but they always work together - during exercise, the respiratory and circulatory systems speed up to supply the muscular system with oxygen
- When ICAS shows a labelled diagram of a cell or organ, identify each labelled part first, then ask what job that part does before looking at the answer choices
Living, Non-Living and Never-Alive
Is a candle flame alive? It grows, it moves, it uses up oxygen and it can even make more flames. This is exactly the kind of puzzle ICAS loves, and deciding whether something is living or non-living is a named topic on Paper A. To answer it properly, scientists check whether something shows all the characteristics of life, not just one or two. A handy memory trick is the list MRS GREN:
- Movement - living things move all or part of themselves
- Respiration - they release energy from food (not the same as just breathing)
- Sensitivity - they respond to changes in their surroundings
- Growth - they increase in size using material from food
- Reproduction - they can produce offspring
- Excretion - they get rid of waste products
- Nutrition - they take in or make food
The candle flame fails the test: it never grows by building new body material, it has no organised structure and it cannot reproduce in the biological sense. A seed is a subtler case. It seems to do nothing at all, yet it is alive - it is respiring very slowly, and given water and warmth it will grow. Dormant things still count as living. ICAS also likes to separate non-living things that were once alive (a wooden ruler, a fossil bone, a fallen leaf) from things that were never alive (a rock, a metal spoon). A wooden desk is non-living now, but it came from a living tree - so watch the wording of the question carefully.
Cells: The Basic Unit of Life
Every living thing, from a bacterium to a blue whale, is made of cells - the smallest units that can carry out all the processes of life. That is why we call the cell the basic unit of life. Some organisms, like the amoeba, are unicellular, meaning a single cell does everything. You are multicellular: your body contains trillions of cells, organised into a tidy hierarchy that ICAS expects older students to know:
cell -> tissue -> organ -> organ system -> organism
A tissue is a group of similar cells doing one job, such as muscle tissue. An organ is made of several tissues working together, such as the heart. Organs then team up in organ systems, like the circulatory system. Muscle cell, to heart muscle tissue, to heart, to circulatory system, to you - that is the ladder, and questions often ask you to place an unfamiliar structure on the right rung.
Inside a Cell
Identifying the parts of the cell is a named topic on Paper F, so learn this table as if the labels were going to be blanked out - because in the exam, they will be.
| Part | Found in | Job |
|---|---|---|
| Nucleus | All plant and animal cells | The control centre; holds the instructions (genes) that direct everything the cell does |
| Cell membrane | All cells | A thin skin around the cell that controls what enters and leaves |
| Cytoplasm | All cells | The jelly-like substance where most of the cell's chemical activities, including respiration, take place |
| Cell wall | Plant cells only | A tough outer layer that gives the cell shape and support |
| Chloroplasts | Plant cells only (green parts) | Contain chlorophyll, the green pigment that captures light energy for photosynthesis |
| Large vacuole | Plant cells | A fluid-filled sac that stores water and keeps the cell firm |
The fastest way to tell the two cell types apart in a diagram is to look for the plant-only features: a regular boxy shape from the cell wall, green dots of chloroplasts, and a big central vacuole. Animal cells are rounder and more irregular, with only a membrane on the outside. A classic trap is the red blood cell or the root hair cell: root cells are plant cells but have no chloroplasts, because roots sit in the dark and cannot photosynthesise. Parts follow function - always ask what job the cell does.
The Human Body Systems
Your organ systems are a team. Here are the six you meet most often in ICAS:
- Digestive system - breaks food down into small soluble molecules the body can absorb
- Circulatory system - the heart, blood and blood vessels; transports oxygen, nutrients and wastes around the body
- Respiratory system - the lungs and airways; brings oxygen in and removes carbon dioxide
- Skeletal system - the bones; supports the body, protects organs and gives muscles something to pull on
- Muscular system - the muscles; produces movement by contracting
- Nervous system - the brain, spinal cord and nerves; sends fast electrical messages to control and coordinate the body
Systems working together
No system works alone, and ICAS regularly tests this with an everyday scenario. Think about what happens when you sprint for the bus. Your muscular system suddenly needs far more energy. Your respiratory system responds by breathing faster and deeper to pull in extra oxygen. Your circulatory system responds by beating the heart faster to deliver that oxygen to the muscles and carry away the carbon dioxide. Your nervous system coordinated the whole thing in the first place, and your skeletal system provided the levers. If an exam question asks why your heart rate rises during exercise, the answer is always about delivering oxygen and nutrients faster to working muscles - not because the heart is tired or excited.
Reading a diagram stimulus
When a question shows a cell or organ diagram with arrows or labels, use this routine: first name every labelled part you can, then recall each part's job, and only then read the answer choices. Most wrong options describe a real part doing the wrong job - for example, claiming the cell wall controls what enters the cell, when that is the membrane's role. Naming the parts before reading the options protects you from that trap.
A student collects four objects: a dried autumn leaf, a smooth river pebble, a bird's egg ready to hatch, and a wooden pencil. Which object is best described as living?
A diagram shows a cell with a regular rectangular shape, a dark round structure inside, many small green dots and a large fluid-filled space taking up most of the cell. The green dots are labelled P. What is structure P, and what does it do?
During a beep test at school, a student's breathing rate and heart rate both rise sharply. Which statement best explains why this happens?