Subject: Integrated Science | Level: JHS (Basic 7 – 9) | Topic: The Cell | Difficulty: Beginner to Intermediate
Introduction
The cell is the basic structural and functional unit of all living organisms. Every living thing, from the smallest bacterium to a giant whale, is made of cells. Some organisms like bacteria are made of just one cell. Others, like human beings, are made of trillions of cells working together. Understanding the cell is the starting point for understanding all of Biology and much of Integrated Science.
Robert Hooke, an English scientist, first described cells in 1665 when he examined a thin slice of cork under a microscope. He saw small box-like compartments that reminded him of rooms in a monastery, which he called “cells.”
1. Types of Cells
There are two main categories of cells:
Prokaryotic Cells
These are simple cells with no distinct nucleus. The genetic material (DNA) floats freely in the cytoplasm. Bacteria are prokaryotes. They are generally smaller and simpler than eukaryotic cells.
Eukaryotic Cells
These are more complex cells with a distinct nucleus enclosed in a nuclear membrane. Plants, animals, fungi, and protists all have eukaryotic cells. At JHS level, we focus mainly on plant and animal eukaryotic cells.
2. Plant Cell vs Animal Cell
Both plant and animal cells are eukaryotic and share many features, but there are important differences.
Features found in BOTH plant and animal cells:
- Cell membrane
- Nucleus
- Cytoplasm
- Mitochondria
- Ribosomes
- Endoplasmic reticulum
- Golgi apparatus
Features found ONLY in plant cells:
- Cell wall (made of cellulose) – gives the plant cell its rigid, box-like shape
- Chloroplasts – contain chlorophyll and carry out photosynthesis
- Large central vacuole – filled with cell sap, helps maintain the shape of the cell
Features found ONLY in animal cells:
- Centrioles – involved in cell division
- Small, scattered vacuoles (not a large central one)
3. Cell Organelles and Their Functions
Organelles are the “organs” of the cell. Each one has a specific job.
Cell Membrane
A thin, flexible layer that surrounds every cell. It controls what enters and leaves the cell. It allows useful substances like glucose and oxygen to enter, and waste products like carbon dioxide to leave. It is semi-permeable, meaning it lets some things through but not others.
Nucleus
The control centre of the cell. It contains DNA (deoxyribonucleic acid), which carries the genetic information of the organism. The nucleus controls all cell activities including growth, reproduction, and protein synthesis. It is surrounded by a nuclear membrane with tiny pores.
Cytoplasm
A jelly-like fluid that fills the cell and holds all the organelles in place. Many chemical reactions of the cell take place in the cytoplasm.
Mitochondria (singular: mitochondrion)
Called the “powerhouse of the cell.” Mitochondria produce energy for the cell through a process called cellular respiration, where glucose and oxygen are used to release energy stored as ATP. Cells that need a lot of energy (like muscle cells) have more mitochondria.
Ribosomes
Tiny structures found on the endoplasmic reticulum or free in the cytoplasm. Ribosomes make proteins by translating instructions from the nucleus.
Endoplasmic Reticulum (ER)
A network of membranes that transports materials within the cell. Rough ER has ribosomes attached and is involved in protein production. Smooth ER is involved in lipid (fat) production.
Golgi Apparatus (Golgi Body)
Packages and distributes proteins and other materials to where they are needed, either within the cell or outside it. Think of it as the cell’s post office.
Vacuole
A membrane-bound sac used to store water, food, or waste products. In plant cells, the large central vacuole stores cell sap and helps keep the cell firm (turgid).
Chloroplasts (plant cells only)
Contain the green pigment chlorophyll, which captures light energy for photosynthesis. Photosynthesis uses sunlight, water, and carbon dioxide to produce glucose and oxygen. Only plant cells have chloroplasts, which is why only plants can make their own food.
Cell Wall (plant cells only)
A rigid outer layer made of cellulose that surrounds the cell membrane in plant cells. It gives the plant cell strength and a fixed shape, and prevents the cell from bursting when it takes in too much water.
4. Diffusion and Osmosis
Cells constantly exchange materials with their surroundings through the cell membrane.
Diffusion
Diffusion is the movement of molecules from an area of high concentration to an area of low concentration. This is called movement down a concentration gradient. No energy is needed for diffusion.
Example: Oxygen moves from the lungs (where it is at high concentration) into the blood (where it is at lower concentration) by diffusion.
Example: Carbon dioxide moves out of respiring cells into the blood by diffusion.
Osmosis
Osmosis is a special type of diffusion. It is the movement of water molecules through a semi-permeable membrane from a region of low solute concentration (lots of water) to a region of high solute concentration (less water).
Example: When a plant is watered, water moves by osmosis from the soil (low solute concentration) into the root cells (higher solute concentration).
Turgid: When plant cells are full of water, they become firm and turgid. This is what keeps plant leaves and stems upright.
Plasmolysis: When plant cells lose too much water (such as in a drought), the cell membrane shrinks away from the cell wall. The plant wilts.
5. Levels of Organisation in Living Things
Cells do not work alone. In complex organisms like humans and plants, cells are organized into higher levels:
- Cell – the basic unit (e.g. a muscle cell)
- Tissue – a group of similar cells working together (e.g. muscle tissue)
- Organ – a group of tissues working together (e.g. the heart)
- Organ system – a group of organs working together (e.g. the circulatory system)
- Organism – the complete living thing (e.g. a human being)
Summary
- The cell is the basic unit of all living things.
- Prokaryotic cells have no nucleus; eukaryotic cells do.
- Plant cells have a cell wall, chloroplasts, and a large vacuole. Animal cells do not.
- Key organelles include the nucleus (control), mitochondria (energy), ribosomes (protein making), and chloroplasts (photosynthesis in plants).
- Diffusion moves substances from high to low concentration; no energy needed.
- Osmosis is the movement of water through a semi-permeable membrane from low to high solute concentration.
Practice Questions
- Name THREE organelles found in a plant cell but NOT in an animal cell.
- What is the function of the mitochondria?
- Explain what happens to a plant cell when it is placed in a solution with very high salt concentration.
- A student says: “Diffusion and osmosis are exactly the same process.” Is this correct? Explain your answer.
- List the levels of organisation in a complex organism from simplest to most complex.
Answers: (1) Cell wall, chloroplasts, large central vacuole (2) Mitochondria produce energy for the cell through cellular respiration (3) Water leaves the cell by osmosis, the cell loses its turgidity, the cell membrane shrinks from the wall (plasmolysis), and the plant wilts (4) Not exactly the same. Both involve movement from high to low concentration, but osmosis specifically refers to the movement of water through a semi-permeable membrane (5) Cell, tissue, organ, organ system, organism
Next Lesson: JHS Integrated Science – Photosynthesis and Plant Nutrition