Year 7 Science · Biological Sciences · 9m read

Cells — The Building Blocks of Life

Have you ever wondered what you're actually made of at the tiniest level? Cells are the microscopic units that make up every living thing, and while animal and plant cells share basic features like a nucleus (the control centre) and mitochondria (the powerhouse), plant cells have extras like chloroplasts for making food and a rigid cell wall. Learning what each part does — from the ribosomes that build proteins to the vacuoles that store water — gives you the foundation to understand how your own body functions and how organisms survive, which is essential for VCE biology and fields like medicine and environmental science.

Learning objective: Compare animal and plant cells and describe the function of key organelles.

By the end of this lesson you can:

• I name and describe the function of cell membrane, nucleus, cytoplasm, mitochondria, chloroplast and cell wall.
• I identify three differences between plant and animal cells.
• I explain why cells are specialised.

Cell organelles and their functions

CELL THEORY (three laws):
1. All living things are made of cells.
2. Cells are the basic unit of life.
3. New cells come from existing cells.

ORGANELLES (present in BOTH animal and plant cells):
CELL MEMBRANE — controls what enters and leaves the cell. Semi-permeable.
NUCLEUS — "control centre." Contains DNA and directs cell activities.
CYTOPLASM — jelly-like fluid filling the cell; where chemical reactions occur.
MITOCHONDRIA — "powerhouse." Produces energy (ATP) through cellular respiration.
RIBOSOMES — tiny structures that make proteins.

PLANT CELL ONLY (extras):
CELL WALL — rigid outer layer made of cellulose; provides structural support.
CHLOROPLASTS — contain chlorophyll; site of PHOTOSYNTHESIS (makes food from sunlight).
LARGE CENTRAL VACUOLE — stores water and maintains cell shape (turgor pressure).

ANIMAL CELL ONLY:
CENTRIOLES — involved in cell division.

SPECIALISED CELLS:
Different cells have different structures for their specific job.
Red blood cells: no nucleus → more space for haemoglobin.
Muscle cells: packed with mitochondria → energy for contraction.
Root hair cells: large surface area → absorbs water and minerals.

Cell as a factory. Nucleus = CEO (gives orders). Mitochondria = power plant. Ribosomes = production workers. Cell membrane = security gate. Cell wall = building walls (only in plant factories).

Cell comparison table

• Plant has, animal doesn't: Cell wall, chloroplasts, large permanent vacuole.
• Animal has, plant doesn't: Centrioles. Animal cells are irregular; plant cells are rectangular.
• Both have: Cell membrane, nucleus, cytoplasm, mitochondria, ribosomes.

Quick reference

| Organelle | Job | Plant | Animal |
|---|---|---|---|
| Nucleus | Control centre (DNA) | ✓ | ✓ |
| Cell membrane | Controls what enters/leaves | ✓ | ✓ |
| Cytoplasm | Jelly where reactions happen | ✓ | ✓ |
| Mitochondria | Respiration — energy | ✓ | ✓ |
| Cell wall | Support | ✓ | ✗ |
| Chloroplast | Photosynthesis | ✓ | ✗ |
| Large vacuole | Stores water | ✓ | small/✗ |

Worked example (Easy)

Why do leaf cells have more chloroplasts than root cells?

1. Function of chloroplasts. Chloroplasts contain chlorophyll and carry out photosynthesis — converting light energy to glucose.
2. Leaf vs root environment. Leaves are exposed to sunlight. Roots are underground and receive no light.
3. Conclusion. Leaf cells need chloroplasts to perform photosynthesis; root cells don't receive light so chloroplasts would be useless.

Answer: Leaf cells are specialised for photosynthesis and need chloroplasts; root cells have no access to light and don't need them.

Worked example (Hard)

Red blood cells have no nucleus and are packed with haemoglobin. Explain how this structure relates to their function.

1. Identify the function. Red blood cells transport oxygen from the lungs to every cell in the body.
2. Explain no nucleus. Removing the nucleus creates MORE SPACE inside the cell for haemoglobin — the protein that binds oxygen. This maximises oxygen-carrying capacity.
3. Explain haemoglobin. Haemoglobin binds to oxygen in the lungs (where oxygen concentration is high) and releases it in body tissues (where oxygen concentration is low).
4. Link structure to function. The loss of the nucleus is a structural adaptation that directly improves the cell's ability to perform its function of oxygen transport. The biconcave (disc) shape also increases surface area for gas exchange.

Answer: No nucleus → more space for haemoglobin → more oxygen carried. Structure is directly adapted to the function of oxygen transport.

Common mistakes

• Saying "mitochondria make energy" Energy cannot be "made" — this violates the law of conservation of energy. Mitochondria CONVERT chemical energy. Fix: Say: "Mitochondria convert chemical energy from glucose into a usable form (ATP) through cellular respiration." Energy is transformed, not created.
• Thinking all cells look the same Diagrams in textbooks show a "generic" cell, leading students to forget that cells are specialised. Fix: Cells are SPECIALISED for their function: nerve cells are long (to transmit signals), muscle cells have many mitochondria (for energy), root hair cells have large surface area (to absorb water).
• Confusing cell wall and cell membrane Both are outer structures, so students treat them as the same thing. Fix: Cell membrane = thin, flexible, controls what enters/exits. Found in ALL cells. Cell wall = rigid, made of cellulose, provides structural support. Found ONLY in plant cells (outside the membrane).

The microscope revolution — how cells were discovered

Cells were invisible to humans for thousands of years. It was not until the invention of the microscope in the 1600s that scientists could see the building blocks of life for the first time.

In 1665, Robert Hooke looked at a thin slice of cork under a microscope and saw tiny box-like compartments. He called them "cells" because they reminded him of the small rooms (cells) in a monastery. Hooke had discovered plant cell walls — but the cells he observed were actually dead.

Anton van Leeuwenhoek, a Dutch tradesman, built his own powerful microscopes and became the first person to observe living cells — including bacteria, which he called "animalcules." His discoveries in the 1670s were so extraordinary that many scientists refused to believe him.

By the 1830s, Schleiden and Schwann proposed cell theory: all living things are made of cells. Rudolf Virchow later added that all cells come from pre-existing cells. These three principles — the foundation of cell theory — remain unchallenged today.

Modern electron microscopes can magnify up to 2 million times, revealing structures inside cells that Hooke could never have imagined. Every advance in medicine — from antibiotics to cancer treatment — builds on the understanding of cells that began with a simple piece of cork.

Practice questions

1. Which organelle controls what enters and exits the cell?

   1. Nucleus
   2. Mitochondria
   3. Cell membrane
   4. Chloroplast

   (1 marks)

2. List THREE structural differences between a plant cell and an animal cell.

   (2 marks)

Summary

• Name and describe the function of cell membrane, nucleus, cytoplasm, mitochondria, chloroplast and cell wall.
• Identify three differences between plant and animal cells.
• Explain why cells are specialised.