Year 9 Science · Biological Sciences · 18m read
Cells and Body Systems
Ever wonder what you're actually made of? Your entire body—from your brain to your fingernails—is built from trillions of tiny cells working together like an incredible team. You'll explore how these cells organize into tissues and organs, then discover how different body systems (like your circulatory, digestive, and nervous systems) communicate and cooperate to keep you alive and healthy, which is essential knowledge for VCE Biology and understanding how your own body functions.
Learning objective: Describe cell structure and how body systems work together to maintain life.
By the end of this lesson you can:
- I identify organelles and their functions.
- I compare plant and animal cells.
- I describe how systems are organised.
The body is a city of systems. Transport (circulatory), power supply (respiratory and digestive), waste removal (excretory) and communication (nervous) all keep the city running.
Cell structure and organisation
THE CELL — the basic unit of life.
All living things are made of cells (cell theory). Cells carry out life processes: nutrition, respiration, excretion, growth, reproduction, and response.
### Key Organelles
| Organelle | Function |
|-----------|---------|
| Cell membrane | Controls what enters/exits the cell |
| Nucleus | Contains DNA; controls cell activities |
| Cytoplasm | Jelly-like fluid; holds organelles |
| Mitochondria | Site of cellular respiration (energy production) |
| Ribosomes | Protein synthesis |
| Endoplasmic reticulum | Transport network within cell |
Plant-only organelles:
| Organelle | Function |
|-----------|---------|
| Cell wall (cellulose) | Provides rigid support |
| Chloroplasts | Site of photosynthesis (contains chlorophyll) |
| Large permanent vacuole | Stores water; maintains cell pressure (turgor) |
### Comparing Plant and Animal Cells
| Feature | Animal cell | Plant cell |
|---------|------------|-----------|
| Cell wall | Absent | Present (cellulose) |
| Chloroplasts | Absent | Present (in leaf cells) |
| Vacuole | Small, temporary | Large, permanent |
| Shape | Irregular | Regular, fixed |
### Cell Organisation (levels)
CELL → TISSUE → ORGAN → ORGAN SYSTEM → ORGANISM
- Cell: basic unit (e.g. muscle cell, nerve cell).
- Tissue: similar cells with the same function (e.g. muscle tissue).
- Organ: different tissues working together (e.g. heart = muscle + nerve + connective tissue).
- Organ system: multiple organs (e.g. circulatory system).
- Organism: the whole living thing.
### Major Body Systems
| System | Main organs | Function |
|--------|-------------|---------|
| Digestive | Mouth, stomach, intestines, liver | Break down food; absorb nutrients |
| Circulatory | Heart, arteries, veins, capillaries | Transport blood, oxygen, nutrients |
| Respiratory | Lungs, trachea, diaphragm | Gas exchange (O₂ in, CO₂ out) |
| Nervous | Brain, spinal cord, nerves | Coordination and communication |
| Skeletal/Muscular | Bones, muscles | Support, movement |
| Excretory | Kidneys, lungs, skin | Remove waste products |
| Immune | White blood cells, lymph nodes | Defend against pathogens |
### Diffusion
Substances move from HIGH concentration to LOW concentration (down the concentration gradient) without energy input.
This is how O₂ enters cells and CO₂ leaves cells in the lungs and tissues.
### Australian context
Australian researchers at the Walter and Eliza Hall Institute in Melbourne have done pioneering work on cell biology and the immune system, including the discovery of key immune cell types.
Cell memory aids
- Mitochondria: "Powerhouse of the cell" — cellular respiration, ATP production.
- Chloroplasts: Green; contain chlorophyll; ONLY in plant cells (and algae).
- Organisation levels: Cell → Tissue → Organ → System → Organism.
Worked example (Medium)
Describe how oxygen gets from the air you breathe into a cell in your leg muscle.
- Breathing. Lungs: O₂ diffuses from alveoli (high [O₂]) into blood capillaries (low [O₂]).
- Transport. O₂ binds to haemoglobin in red blood cells. Heart pumps oxygenated blood through arteries.
- Delivery. Blood reaches capillaries near leg muscle. O₂ diffuses from blood (high [O₂]) into muscle cell (low [O₂]).
- Use. Mitochondria in muscle cell use O₂ for cellular respiration, producing ATP for muscle contraction.
Answer: O₂ diffuses: alveoli → blood → muscle cell → mitochondria.
Worked example (Hard)
A student looks at two cells under a microscope. Cell A has a rigid wall, large central vacuole, and green organelles. Cell B is irregular in shape with no wall. Identify each cell and list evidence for each identification.
- Cell A. Cell wall → plant feature. Large central vacuole → plant feature. Green organelles = chloroplasts → plant feature.
- Cell A conclusion. Cell A is a PLANT CELL — specifically a leaf cell (has chloroplasts for photosynthesis).
- Cell B. No cell wall → NOT a plant cell. Irregular shape → consistent with animal cell.
- Cell B conclusion. Cell B is an ANIMAL CELL. No chloroplasts, no cell wall, irregular shape.
Answer: Cell A = plant cell (leaf); Cell B = animal cell.
Common mistakes
- Saying the nucleus "makes energy" The nucleus stores and transmits genetic information (DNA). Energy production occurs in the mitochondria. Fix: Nucleus → DNA storage, controls cell activities. Mitochondria → cellular respiration, energy (ATP) production.
- Stating that all plant cells have chloroplasts Chloroplasts are only in cells that photosynthesize — leaf mesophyll cells. Root cells, for example, are in the dark and have no chloroplasts. Fix: Say: "leaf cells" or "cells exposed to light" have chloroplasts. Root and stem cells generally do not.
- Confusing diffusion (passive) with active transport (requires energy) Diffusion is passive — no energy needed. Active transport moves substances AGAINST the concentration gradient and needs ATP. Fix: High to low concentration = diffusion (passive). Low to high concentration = active transport (needs energy).
Practice questions
Which organelle is responsible for cellular respiration (energy production)?
- Nucleus
- Mitochondria
- Ribosome
- Cell membrane
(1 marks)
List three structures found in plant cells but NOT in animal cells.
(2 marks)
Place these in order from smallest to largest: organism, cell, organ, tissue.
(2 marks)
Explain how diffusion allows oxygen to move from lung alveoli into the blood.
(3 marks)
Going Deeper — Osmosis
OSMOSIS is a special case of diffusion involving WATER across a semi-permeable membrane.
Water moves from a region of HIGH water concentration (dilute solution) to LOW water concentration (concentrated solution).
Key terms:
• HYPOTONIC solution: lower solute concentration than the cell → water enters the cell → cell swells. Plant cell: turgid (desirable). Animal cell: may burst.
• HYPERTONIC solution: higher solute concentration than cell → water leaves the cell → cell shrinks (plasmolysis in plant cells; crenation in animal cells).
• ISOTONIC solution: same concentration → no net water movement.
Example: Placing a plant cell in salty water (hypertonic solution):
The vacuole loses water → cell membrane pulls away from cell wall → cell becomes plasmolysed → plant wilts.
Placing a plant cell in pure water (hypotonic):
Water enters vacuole → cell becomes turgid → cell wall provides pressure to prevent bursting.
This is why plants wilt without water — their cells lose turgor pressure.
Osmosis is important in kidney function (water reabsorption), in intravenous drips in hospitals (isotonic saline), and in food preservation (salt or sugar draws water from bacteria).
Summary
- Cell = basic unit of life. All organisms are made of cells.
- Key organelles: nucleus (DNA), mitochondria (energy), ribosomes (protein), membrane (control).
- Plant extras: cell wall, chloroplasts, large permanent vacuole.
- Organisation: Cell → Tissue → Organ → System → Organism.
- Diffusion: high to low concentration, passive (no energy needed).