The Cell and It’s Environment – SS1 Biology Lesson Note (First Term)

Living cells are surrounded by a watery environment. Substances move into and out of them by processes such as diffusion and osmosis. Diffusion is the movement of molecules from a region of higher concentration to one of lower concentration until equilibrium is reached. Osmosis is the movement of water molecules through a semi-permeable membrane.

Behavioural objectives

By the end of the lesson, students should be able to:

• describe the environment of living cells and the states of matter;
• define diffusion, state the factors that affect it and give examples of its importance in living organisms;
• define osmosis and explain hypertonic, hypotonic and isotonic solutions;
• explain plasmolysis and haemolysis;
• explain turgidity and flaccidity and their biological significance.

What is the environment of a living cell?

Living cells are surrounded by a watery environment. This may include:

• fresh or salt water in which unicellular organisms live;
• intercellular fluid that bathes the cells of higher animals.

The nature of the states of matter makes diffusion and osmosis possible. Matter is any material that has mass and occupies space. It includes all the living and non-living things around us. Matter is made up of tiny particles, which may be molecules or ions, and it can exist in three forms: solid, liquid and gas.

What is diffusion?

Diffusion is the process by which molecules of substances move from a region of higher concentration to a region of lower concentration (through the medium of air or liquid) until equilibrium is reached. The difference in the concentration of the substances in the two regions before diffusion occurs is called the concentration (or diffusion) gradient.

For example, place KMnO₄ (potassium tetraoxomanganate(VII)) in a beaker of water and allow it to stand. The purple colour starts to spread outward from the crystal. Eventually, the colour spreads evenly throughout the water.

Diffusion is also seen in the spread of the odour of perfume, in the spread of an insecticide sprayed in a room, and in the release of gases from the anus.

What factors affect diffusion?

1. Change in temperature: the higher the temperature, the faster the rate of diffusion.
2. Molecular size: the smaller the molecule, the faster the rate of diffusion.
3. State of matter: diffusion of gases is much faster than that of liquids.
4. Difference in concentration: the greater the difference in concentration of molecules, the faster the rate of diffusion.

What is the importance of diffusion in living organisms?

1. Carbon(IV) oxide moves into the leaves through the stomata during photosynthesis.
2. Oxygen moves into the leaves during respiration.
3. Water vapour moves out of the leaf during transpiration.
4. Gaseous exchange takes place in the lungs of mammals.
5. The foetus takes in oxygen and nutrients and removes waste products through the placenta.
6. Digested and soluble food moves from the villi of the small intestine into the bloodstream.
7. Small organisms such as amoeba remove waste products.

What is osmosis?

Osmosis is the movement of water molecules from a region of lower solute concentration (a dilute solution) to a region of higher solute concentration (a concentrated solution) through a semi-permeable membrane.

A permeable membrane allows molecules to pass through it freely, while a selectively permeable membrane allows only certain molecules to pass through. Osmosis occurs only when a semi-permeable membrane separates weak and strong solutions.

What happens to living cells in different solutions?

What is the importance of osmosis?

1. It aids the absorption of water from the soil into the vacuole of the root hairs.
2. It aids the movement of water from the root hairs into the cells of other parts of the plant.
3. It helps to control the opening and closing of the stomata pores.
4. It gives turgidity to plant cells, which gives support.
5. It aids the intracellular movement of water in animals.
6. It aids reabsorption of water from the kidney tubules into the blood.
7. It causes haemolysis of red blood cells.

What is plasmolysis?

Plasmolysis is the shrinkage of the vacuole and the pulling away of the cytoplasmic lining from the cell wall when a cell is placed in a hypertonic solution.

When a plant cell is placed in a hypertonic solution, exosmosis occurs. Water moves out of the cell into the surrounding fluid, so the vacuole shrinks and the cytoplasm pulls away from the cell wall. When cells are plasmolysed, the plant eventually wilts or dies.

What is haemolysis?

Haemolysis is the process whereby red blood cells (corpuscles) split and burst because too much water passes into them.

Red blood cells and blood plasma are normally isotonic, that is, they have the same osmotic concentration. If the concentration of the blood plasma falls, endosmosis occurs and water moves from the plasma into the red blood cells. Continuous absorption of water makes the cell turgid, and when it is fully stretched, it bursts.

What is turgidity and what is its biological significance?

Turgidity is the condition in which a cell absorbs plenty of water until it is fully stretched. It is seen in plants, where it keeps them erect and supports the stem, leaves and flowers.

What is flaccidity and what is its biological significance?

Flaccidity is the condition in which a plant loses water to its surroundings faster than it can absorb it. Flaccidity usually occurs when there is no water in the soil, especially during a drought. Continuous loss of water can result in the death of the plant.

Evaluation

1. Define diffusion and give two examples of it.
2. State the four factors that affect the rate of diffusion.
3. Explain the terms hypertonic, hypotonic and isotonic.
4. What is plasmolysis, and what happens to a plant cell placed in a hypertonic solution?
5. Explain why a plant becomes flaccid.

Assignment

1. Which structures must be present in a cell for osmosis to take place? A. cell (sap) vacuole and cell wall B. cell wall and cell membrane C. chloroplast and cytoplasm D. cytoplasm and cell membrane
2. The scent of a bunch of flowers spreads throughout a room. How does the scent spread? A. by conduction B. by diffusion C. by osmosis D. by transpiration
3. Which of the following environmental conditions is ideal for plant cells to remain turgid? A. Hot, dry weather B. Cold, dry weather C. Cool, humid weather D. Windy weather
4. Osmosis occurs through a membrane that can be _____ permeable. A. fully B. slowly C. differentially D. freely
5. Which of the following processes takes place when a plant cell is put in a hypotonic solution? (a) water moves into the cell and the cell bursts (b) water leaves the cell and the cell becomes flabby (c) water moves into the cell and the cell becomes turgid (d) the cell becomes plasmolysed
6. State the similarities and differences between: (a) osmosis and diffusion; (b) plasmolysis and haemolysis.