Bioenergetics B4

Photosynthesis definition & equation

• An endothermic reaction in which plants use light energy to react carbon dioxide and water to produce glucose and oxygen. • Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Where photosynthesis occurs

• Occurs inside the chloroplasts of plant cells, which contain the green pigment chlorophyll to absorb light energy.

Uses of Glucose: Respiration & Cellulose

• Respiration: Glucose is broken down during respiration to release energy for metabolic processes.

• Cellulose: Glucose molecules are linked to form cellulose to strengthen and build plant cell walls.

Uses of Glucose: Starch storage

• Glucose is converted into insoluble starch for storage. Insoluble starch is ideal because it does not affect the water potential of the cell (won't cause water to enter via osmosis).

Uses of Glucose: Fats, Oils & Amino acids

• Fats & Oils: Glucose is converted into lipids for energy storage in seeds.

• Amino acids: Glucose combines with nitrate ions absorbed from the soil to form amino acids, which join to build proteins.

Limiting Factor definition

• An environmental factor that restricts the maximum possible rate of photosynthesis when it is in short supply.

Limiting Factor: Light Intensity graph

• Initially: As light intensity increases, the rate of photosynthesis increases linearly because light is the limiting factor.

• Plateau: The graph flattens out because light is no longer the limiting factor; something else is in short supply (e.g. CO₂ concentration or temperature).

Limiting Factor: CO2 Concentration graph

• Initially: As CO₂ concentration increases, the rate increases because carbon dioxide is the limiting factor.

• Plateau: The graph flattens out because CO₂ is no longer the limiting factor; light intensity or temperature is restricting the rate.

Limiting Factor: Temperature graph

• Rising side: As temperature increases, the rate increases because enzymes and substrates gain kinetic energy, leading to more frequent successful collisions.

• Peak: The optimum temperature where enzymes work fastest.

• Falling side: At high temperatures, the rate drops to zero because the chemical bonds in enzymes break, changing the shape of the active site and denaturing them.

Photosynthesis Required Practical method

• Step 1: Place a boiling tube containing sodium hydrogencarbonate solution 10 cm away from an LED light source.

• Step 2: Put a piece of freshly cut pondweed into the tube with the cut end at the top.

• Step 3: Leave for 5 minutes to acclimatise, then start a stopwatch and count the number of oxygen bubbles produced in 1 minute.

• Step 4: Repeat twice to calculate a mean, then repeat the entire experiment at distances of 20cm, 30cm, and 40cm.

Pondweed Practical: Variables & Improveme

• Independent variable: Light intensity (altered by changing the distance of the pondweed from the light source).

• Dependent variable: Rate of photosynthesis (measured by counting bubbles or measuring the volume of oxygen gas produced).

• Control variables: CO₂ concentration (fixed by using excess sodium hydrogen-carbonate solution), temperature, and the specific species/age of pondweed.

• Improvements: Use a gas syringe or a capillary tube instead of counting bubbles to get an accurate, volume-based measurement of oxygen gas rather than counting different-sized bubbles.

The Inverse Square Law

• The intensity of light is inversely proportional to the square of the distance from the light source (Light Intensity ∝ 1 ÷ Distance²). • If you double the distance (multiply by 2), the light intensity drops to one-quarter (divide by 4).

Greenhouse Economics

• Commercial greenhouses artificially optimise limiting factors to maximise the rate of photosynthesis to increase crop yield and growth.

• Methods: Use artificial lighting (LEDs) at night, burn paraffin heaters to increase temperature and release CO₂ gas, and add fertilisers.

• Cost benefit: Farmers must balance the cost of equipment and fuel against the extra profit made from a faster, larger harvest.

Respiration definition

• An exothermic chemical reaction that occurs continuously in all living cells, breaking down nutrient molecules (like glucose) to release energy for metabolic processes.

Uses of energy from respiration

• Muscle contraction for movement, maintaining a constant body temperature (in mammals and birds), and building larger molecules from smaller ones (synthesis/anabolism).

Aerobic Respiration definition & equation

• A form of respiration that uses oxygen to completely break down glucose, releasing large amounts of energy. • Equation: Glucose + Oxygen → Carbon Dioxide + Water (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O). Takes place inside mitochondria.

Anaerobic Respiration in Animals

• Occurs when there is an insufficient supply of oxygen, resulting in the incomplete breakdown of glucose into lactic acid. • Equation: Glucose → Lactic Acid. Releases a much smaller amount of energy compared to aerobic respiration because glucose is not fully oxidised.

Anaerobic Respiration in Plants & Yeast

• Also known as fermentation, where glucose is incompletely broken down into ethanol and carbon dioxide. • Equation: Glucose → Ethanol + Carbon Dioxide. • Industrial uses: Yeast fermentation is used to make bread rise (due to CO₂ bubbles) and to produce alcoholic beverages (due to ethanol).

Body Responses to Exercise

• During exercise, the body demands more energy for increased muscle contraction.

• Response 1: Heart rate increases to pump blood faster. • Response 2: Breathing rate and breath volume increase to bring more oxygen into the lungs. • Purpose: These responses maximise the delivery of oxygen and glucose to muscle cells for aerobic respiration, while removing carbon dioxide faster.

Lactic Acid & Oxygen Debt

• Lactic Acid: Formed during strenuous exercise due to anaerobic respiration, causing muscle fatigue and pain. • Oxygen Debt: The extra amount of oxygen required by the body after exercise to completely react with and break down the accumulated lactic acid.

How Lactic Acid is removed

• Blood flowing through active muscles transports the accumulated lactic acid away to the liver. • In the liver, the lactic acid is reacted with oxygen and converted back into glucose.

Metabolism definition & examples

• Definition: The sum of all the chemical reactions that happen continuously inside a cell or the entire body of an organism.

• Synthesis examples: Converting glucose into starch, cellulose, or glycogen; forming lipid molecules from glycerol and fatty acids; joining amino acids to make proteins.

• Breakdown examples: Respiration, and breaking down excess proteins in the liver to form urea, which is excreted in urine by the kidneys.

Factors affecting Metabolic Rate

• Age, gender, genetic factors, the proportion of muscle to fat tissue in the body, and the amount of physical exercise an individual completes.