This guide introduces the basic concepts of cell biology by explaining what a cell is and how it works.
It describes the main parts of a cell, including the cell membrane, the cytoplasm, and the genetic material that controls how the cell grows and functions.
To help understand these ideas, a simple experiment using an egg is presented as a model of a single cell. By removing the shell and placing the egg in different solutions, it is possible to observe how the cell membrane allows water to move in and out.
Depending on the surrounding environment, the egg can increase in size or shrink. This activity helps students understand how cells interact with their environment and why these processes are important for life. It explains what a cell is for students and introduces a simple experiment using an egg to demonstrate how cells behave.
This activity is suitable for students who are learning the basics of cell biology. It provides a clear introduction to how cells function and interact with their environment.
- Level: Primary to Middle School (ages 8–14)
- Difficulty: Easy
- Suitable for: Students, beginners, and introductory biology courses
What is a cell? Simple explanation for students
A cell is the smallest unit of life capable of carrying out all the functions necessary for survival.
All living organisms—from bacteria to plants and animals—are made of one or more cells.
Cells can:
– grow and develop
– produce and use energy
– respond to changes in their environment
– reproduce and generate new cells
Although most cells are too small to be seen with the naked eye, they are highly organized systems containing specialized structures that work together to keep the cell alive.
Cells are essential for life because every biological process—from respiration to growth—takes place within them.
Basic structure of a cell
Most cells have three main parts that work together to maintain cellular function:
– Cell membrane: the outer boundary of the cell. It acts as a selective barrier, controlling what enters and leaves the cell and protecting the internal environment.
The cell membrane is mainly made of lipids and proteins. The lipids form a flexible barrier, while proteins help regulate the movement of substances into and out of the cell.
– Cytoplasm: The cytoplasm is the internal fluid of the cell, where many important chemical reactions take place.
It contains water, dissolved molecules, and various structures that support cellular activity. Inside the cytoplasm, cells are able to produce energy, build new molecules, and carry out essential processes needed for survival.
The cytoplasm also helps maintain the shape of the cell and allows substances to move and interact within the cellular environment.
– Genetic material (DNA): the molecule that carries the instructions for cell function, growth, and reproduction.
It contains the information that allows the cell to produce proteins, which are essential for almost all cellular processes. DNA determines how the cell behaves, how it responds to its environment, and how it can adapt to changes over time.
These instructions are carefully regulated, ensuring that each cell functions properly and performs its specific role within an organism.
To better understand how cells work, we can explore a simple experiment using an egg as a model of a cell.
Experiment: Exploring a Cell Using an Egg
Objective
To observe a real cell and understand its structure and how it behaves and responds to changes in its environment.
Background
An egg is actually a single large cell.
The shell acts as a protective outer layer. It is important to note that, unlike the egg, most animal cells do not have an external rigid structure like a shell. Animal cells are surrounded only by a flexible cell membrane. In contrast, plant cells have an additional outer layer called the cell wall, which provides extra support and protection.
Under the shell, there is a thin membrane similar to a cell membrane, which helps control what enters and leaves the cell.
The inside of the egg, including the egg white and yolk, represents the internal cell content and contains the substances needed for the cell to function.
Materials
- 1 raw egg
- Vinegar
- A glass or container
- Water
- Sugar (optional)
Procedure – Part 1: Removing the shell
To allow this large cell to interact directly with its environment, the protective shell must be removed.
1. Place the egg in a glass
2. Cover it completely with vinegar
3. Leave it for 24 hours
Observation
- The shell slowly dissolves
- The egg becomes soft and flexible
👉 What remains is a membrane surrounding the cell
Explanation
The vinegar reacts with the eggshell, which is mainly made of calcium carbonate. This reaction produces carbon dioxide gas, which can be seen as bubbles forming on the surface of the egg.
As the reaction continues, the shell slowly dissolves, leaving only the thin membrane surrounding the cell intact.
Procedure – Part 2: Testing the cell membrane (osmosis)
After removing the shell, the egg is surrounded only by a thin membrane, similar to a cell membrane.
This allows us to observe how substances move across this boundary.
Step 1 – Water
Place the shell-less egg in clean water. Handle the egg carefully, as it is soft and can break easily without the protective shell. To avoid damage, it is recommended to move the egg using a spoon.
- Measure and weigh the egg before the experiment.
- Leave the egg in the solution for several hours (at least 2–4 hours), or overnight for a stronger effect.
- Measure and weigh the egg after the experiment to observe and quantify the changes.
The egg becomes larger
Optional: Data collection.
For more advanced students, the experiment can be extended by collecting and analyzing data.
Students can measure and weigh the egg before and after the experiment, then compare the results.
If the experiment is performed in groups, the results can be combined to calculate an average value and reduce variability.(see data analysis activity).
The increase in size or weight can also be expressed as a percentage change.
This approach helps introduce basic concepts of scientific measurement, data analysis, and reproducibility.
| Condition | Weight before (g) | Weight after (g) | Change (%) | Observation |
|---|---|---|---|---|
| Water | Egg gets larger | |||
| Sugar solution | Egg gets smaller | |||
| Salt solution | Egg gets smaller |
Percentage change: ((final weight − initial weight) / initial weight) × 100
This allows students to compare results more accurately and understand how strongly different conditions affect the cell.
Step 2 – Sugar solution
Prepare a sugar solution by dissolving 3–4 teaspoons of sugar in a glass of water.
Place the egg in a solution with sugar.
The egg becomes smaller
Optional: Salt solution
Instead of sugar, a salt solution can also be used.
Salt creates a similar effect by increasing the concentration of dissolved substances in the surrounding environment. The egg may shrink even more rapidly.
This helps demonstrate how different environments affect cells in similar ways.
Scientific explanation
This happens because of osmosis.
Osmosis is the movement of water across a semi-permeable membrane.
A semi-permeable membrane allows water to pass through, but restricts many larger or dissolved substances.
Water always moves from areas with lower concentration of dissolved substances to areas with higher concentration.
In water:
The environment outside the egg contains fewer dissolved substances than the inside of the cell.
Water moves into the egg
→ The egg swells and becomes larger.
In a sugar solution:
The environment outside the egg contains more dissolved substances than the inside of the cell.
Water moves out of the egg
→ The egg shrinks.
This experiment shows that water can move across the cell membrane, while many dissolved substances cannot. This demonstrates that the membrane is selective and that cells interact with their environment.
| Condition | What is outside the egg | What happens to water | Result |
|---|---|---|---|
| Water | Few dissolved substances | Water moves into the egg | Egg gets bigger |
| Sugar solution | Many dissolved substances | Water moves out | Egg gets smaller |
| Salt solution | Many dissolved substances | Water moves out | Egg gets smaller |
Key concept: Water moves from areas with fewer dissolved substances to areas with more dissolved substances.
Conclusion
This experiment demonstrates how cells respond to their environment through the movement of water across the cell membrane.
Understanding these processes is essential in biology and helps explain how cells survive, grow, and function in different conditions.
Optional: Data analysis activity
For more advanced students, the experiment can be extended by collecting and analyzing data.
If the experiment is performed in groups, results can be compared and averaged to reduce variability and improve accuracy.
Students can measure and weigh the egg before and after the experiment, compare results between groups, and calculate average values.
| Group | Condition | Initial weight (g) | Final weight (g) | Change (%) |
|---|---|---|---|---|
| Group 1 | Water | |||
| Group 2 | Sugar | |||
| Group 3 | Salt |
“
| Condition | Average initial weight (g) | Average final weight (g) | Average change (%) |
|---|---|---|---|
| Water | |||
| Sugar | |||
| Salt |
Average: Add all values and divide by the number of measurements.
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