THE CELL AND IT’S ENVIRONMENT Lesson Note
Learning Objectives
This SS1 Biology lesson note covers THE CELL AND IT’S ENVIRONMENT for First Term, Week 5.
Lesson Note
Learning Objectives
By the end of the lesson, students should be able to:
Explain the relationship between a cell and its environment.
Define diffusion, osmosis and active transport.
Explain how substances move in and out of cells.
State the differences between diffusion, osmosis and active transport.
Explain the importance of the cell membrane in controlling substances entering and leaving the cell.
Describe the effects of different solutions on plant and animal cells.
Explain plasmolysis and deplasmolysis.
State the importance of these processes to living organisms.
A. The Cell and Its Environment
A cell is constantly interacting with its surroundings. It needs to obtain useful substances such as water, oxygen and nutrients, while waste products such as carbon dioxide must be removed.
The cell membrane controls the movement of substances into and out of the cell. It is described as selectively permeable because it allows some substances to pass through more easily than others.
Movement of substances across the cell membrane occurs mainly through:
1. Diffusion
2. Osmosis
3. Active transport
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration until the particles become more evenly distributed.
Diffusion does not require energy from the cell, so it is a form of passive transport.
Examples of diffusion
Oxygen moving from the air in the alveoli into the blood.
Carbon dioxide moving from body cells into the blood.
Movement of dissolved substances within cells.
Factors affecting the rate of diffusion
*Difference in concentration
*Temperature
*Surface area
*Distance or thickness of the membrane
*Size of particles
A greater concentration difference generally results in faster diffusion.
2. Osmosis
Osmosis is the net movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (more concentrated solution) through a selectively permeable membrane.
Osmosis is important because cells need water to carry out many metabolic activities.
Examples
*Water entering root hair cells from the soil.
*Water entering plant cells.
*Movement of water between cells and their surroundings.
Importance of Osmosis in Plants
Osmosis is very important in plants because it:
1. Enables root hair cells to absorb water from the soil.
2. Helps plant cells become turgid.
3. Provides support to soft plant tissues.
4. Helps in the opening and closing of stomata.
5. Assists the movement of water between cells.
A plant cell that gains water becomes turgid, which helps the plant remain firm.
Osmosis in Animal Cells
Animal cells do not have a cell wall, so changes in the surrounding water concentration can cause significant changes in cell size.
Dilute solution
When an animal cell is placed in a very dilute solution, water enters the cell by osmosis. The cell may swell and eventually burst.
Concentrated solution
When an animal cell is placed in a concentrated solution, water leaves the cell by osmosis. The cell becomes smaller and shrivelled.
Osmosis in Plant Cells
Plant cells have a cell wall, which provides additional support.
In a dilute solution
Water enters the plant cell by osmosis. The cell becomes turgid.
In a concentrated solution
Water leaves the cell. The cell membrane pulls away from the cell wall.
This condition is called plasmolysis.
B. Plasmolysis
Plasmolysis is the condition in which the cell membrane of a plant cell pulls away from the cell wall because the cell has lost water by osmosis.
It occurs when a plant cell is placed in a solution more concentrated than the cell sap.
Effects of plasmolysis
1. The cell loses water.
2. The cytoplasm contracts.
3. The cell membrane moves away from the cell wall.
4. The plant may lose firmness and wilt if many cells become plasmolysed.
De-plasmolysis
De-plasmolysis is the reversal of plasmolysis when a plasmolysed plant cell gains water by osmosis.
When placed in a sufficiently dilute solution, water enters the cell, causing the cell membrane to move back towards the cell wall and the cell becomes turgid again.
D. Active Transport
Active transport is the movement of substances from a region of lower concentration to a region of higher concentration through a cell membrane using energy from the cell.
Unlike diffusion and osmosis, active transport requires energy, usually supplied by ATP.
Examples
*Absorption of mineral salts by root hair cells.
*Absorption of some nutrients by cells lining the small intestine.
Importance of active transport
It allows cells to take in substances even when their concentration is lower outside the cell than inside.
E. Differences Between Diffusion, Osmosis and Active Transport
Diffusion Osmosis Active Transport
DIFFUSION
Movement of particles from high to low concentration
Does not require cellular energy
May involve gases or dissolved substances
May occur without a selectively permeable membrane
Passive process
OSMOSIS
Movement of water molecules from high water concentration to low water concentration
Does not require cellular energy
Involves water molecules
Requires a selectively permeable membrane
Passive process
ACTIVE TRANSPORT
Movement of substances from low to high concentration
Requires cellular energy
May involve ions and other substances
Usually involves membrane proteins
Active process
F. Importance of Movement of Substances in Cells
Movement of substances across cell membranes is essential because it:
Provides cells with water.
Allows oxygen to enter cells.
Removes carbon dioxide and other waste products.
Enables plants to absorb mineral salts.
Helps maintain the correct internal conditions of cells.
Supports growth and metabolism.
Helps maintain water balance in organisms.
Enables cells to obtain nutrients.
Summary
Cells continuously exchange substances with their environment. The cell membrane regulates this exchange. Diffusion involves the movement of particles from higher to lower concentration, while osmosis involves the movement of water through a selectively permeable membrane. Active transport moves substances against the concentration gradient and requires energy. Osmosis can cause plant cells to become turgid, plasmolysed, or undergo deplasmolysis, depending on the surrounding solution.
Evaluation
OBJECTIVE QUESTIONS
1. The movement of particles from a region of higher concentration to a region of lower concentration is called
A. Osmosis
B. Diffusion
C. Active transport
D. Transpiration
2. Osmosis involves the movement of
A. oxygen molecules
B. glucose molecules
C. water molecules
D. protein molecules
3. The structure that controls the movement of substances into and out of a cell is the
A. nucleus
B. cell wall
C. cell membrane
D. mitochondrion
4. Which of the following processes requires energy from the cell?
A. Diffusion
B. Osmosis
C. Active transport
D. Passive transport
5. Active transport moves substances from
A. high concentration to low concentration
B. low concentration to high concentration
C. high water concentration to low water concentration
D. the cell wall to the cytoplasm only
6. A plant cell becomes turgid when it
A. loses water
B. gains water
C. loses minerals
D. loses oxygen
7. The pulling away of the cell membrane from the cell wall due to water loss is called
A. diffusion
B. turgidity
C. plasmolysis
D. germination
8. Deplasmolysis occurs when a plasmolysed plant cell
A. loses more water
B. gains water
C. loses oxygen
D. gains carbon dioxide
9. Which of the following is an example of active transport in plants?
A. Absorption of mineral salts by root hair cells
B. Movement of oxygen into leaves
C. Loss of water vapour through stomata
D. Movement of carbon dioxide into a leaf
10. Which of the following is a passive process?
A. Active transport
B. Diffusion
C. Protein synthesis
D. Cell division
THEORY QUESTIONS
1a. Define diffusion, osmosis and active transport.
1b. State four differences between diffusion and osmosis.
2a. Explain what happens to a plant cell when it is placed in a concentrated solution.
2b. What is plasmolysis? Explain how it occurs.
2c. State five importance of the movement of substances across the cell membrane.