Cells & Life
Cell Processes: Photosynthesis, Respiration, Diffusion & Osmosis
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓By the end of this lesson students will be able to define photosynthesis and write its balanced chemical equation.
- ✓By the end of this lesson students will be able to distinguish between aerobic and anaerobic respiration, including their equations and products.
- ✓By the end of this lesson students will be able to explain the processes of diffusion and osmosis, providing biological examples for each.
- ✓By the end of this lesson students will be able to describe the role of selectively permeable membranes in osmosis.
- ✓By the end of this lesson students will be able to compare the energy yield of aerobic and anaerobic respiration.
Key concepts
Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy, in the form of glucose (sugar). This process uses carbon dioxide and water as reactants and produces glucose and oxygen. Chlorophyll, a green pigment found in chloroplasts, absorbs the light energy needed for this reaction.
Aerobic respiration is the process of releasing energy from food (glucose) in the presence of oxygen. It occurs mainly in the mitochondria of cells and releases a large amount of energy, along with carbon dioxide and water as waste products. This is the most efficient way for organisms to get energy from their food.
Anaerobic respiration is the process of releasing energy from food (glucose) without the presence of oxygen. This process releases much less energy than aerobic respiration. In animal muscle cells, when oxygen supply is insufficient (e.g., during strenuous exercise), glucose is converted into lactic acid and a small amount of energy. Lactic acid build-up can cause muscle fatigue.
Another form of anaerobic respiration occurs in organisms like yeast. In the absence of oxygen, yeast converts glucose into ethanol (alcohol) and carbon dioxide, releasing a small amount of energy. This process is used in brewing (to produce alcohol) and baking (carbon dioxide causes dough to rise).
Diffusion is the net movement of particles (atoms, ions, or molecules) from an area of higher concentration to an area of lower concentration, down a concentration gradient. This process does not require energy from the cell and continues until the particles are evenly distributed.
Osmosis is a special type of diffusion that specifically involves the movement of water molecules. It is the net movement of water molecules from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration) across a selectively permeable membrane. This process also does not require energy from the cell.
A selectively permeable membrane (also known as a semi-permeable membrane or partially permeable membrane) is a biological membrane that allows certain molecules or ions to pass through it by means of active or passive transport. It acts as a barrier, controlling what enters and leaves a cell. This membrane is crucial for osmosis, allowing water molecules to pass through but restricting larger solute molecules.
Key facts to remember
- 1Photosynthesis converts light energy into chemical energy (glucose) in plants.
- 2The balanced equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- 3Respiration is the process of releasing energy from food.
- 4Aerobic respiration requires oxygen and releases a large amount of energy.
- 5Anaerobic respiration does not require oxygen and releases a small amount of energy.
- 6Diffusion is the movement of particles from high to low concentration.
- 7Osmosis is the diffusion of water across a selectively permeable membrane.
- 8Mitochondria are the site of aerobic respiration, and chloroplasts are the site of photosynthesis.
Worked examples
Example 1
A student observes a plant cell placed in a solution. After some time, the cell appears swollen and turgid. Explain what has happened to the cell, referring to the process involved and the concentration of the surrounding solution.
Answer
The plant cell has gained water by osmosis. This occurred because the surrounding solution had a higher water concentration (lower solute concentration) than the cytoplasm inside the plant cell. Water molecules moved from the solution, across the selectively permeable cell membrane, and into the cell, causing it to swell and become turgid.
Remember to mention the 'selectively permeable membrane' when explaining osmosis.
Example 2
Compare aerobic and anaerobic respiration in terms of oxygen requirement, energy yield, and products in human cells.
Answer
Aerobic respiration requires oxygen, releases a large amount of energy, and produces carbon dioxide and water. Anaerobic respiration (in human cells) does not require oxygen, releases a small amount of energy, and produces lactic acid.
Be specific about 'human cells' for anaerobic respiration, as yeast produces different products.
Example 3
Write the balanced chemical equation for photosynthesis and identify the reactants and products.
Answer
The balanced chemical equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (with 'Light energy' and 'Chlorophyll' typically indicated above/below the arrow). Reactants: Carbon dioxide (CO₂) and Water (H₂O). Products: Glucose (C₆H₁₂O₆) and Oxygen (O₂).
Always ensure your chemical equations are balanced and include the necessary conditions if applicable.
Common mistakes
- ✗Confusing the reactants and products of photosynthesis and respiration.
- ✗Forgetting to mention 'selectively permeable membrane' when defining osmosis.
- ✗Assuming anaerobic respiration produces more energy than aerobic respiration.
- ✗Not balancing chemical equations correctly or omitting coefficients.
- ✗Mixing up the products of anaerobic respiration in animals (lactic acid) and yeast (ethanol and carbon dioxide).
Exam tips
- ★Learn and practice writing all the chemical equations for photosynthesis and respiration until you can do them from memory.
- ★Use clear diagrams to illustrate diffusion and osmosis, showing the movement of particles and water molecules.
- ★When explaining osmosis, always explicitly state the role of the 'selectively permeable membrane'.
- ★Understand the energy implications: aerobic respiration is highly efficient, anaerobic respiration is much less efficient.
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