6.1.1 Photosynthetic organisms as producers

Photosynthesis Makes Food and Biomass

1. A producer is an organism that makes organic substances from inorganic raw materials.
2. Plants and algae are the main producers because they make glucose by photosynthesis.
3. Glucose supplies carbon and stored chemical energy for building other biological molecules.
4. Biomass is the total mass of living material in an organism or group of organisms.
5. When plants and algae grow, glucose is converted into new cell material, so photosynthesis increases biomass.
6. Consumers obtain this food and biomass directly by eating producers or indirectly by eating other consumers.

Photosynthesis Stores Light Energy

1. Photosynthesis is the endothermic process in which plants and algae use light energy to make glucose from carbon dioxide and water, releasing oxygen.
2. Endothermic means that energy is transferred into the reaction from the surroundings.
3. Chlorophyll in chloroplasts absorbs light energy.
4. The absorbed energy is transferred into chemical energy and stored in glucose.
5. Carbon dioxide enters a leaf through the stomata, while water reaches the leaf through xylem vessels.

The Photosynthesis Equations

1. The word equation is ** $$carbon\; dioxide+water\underrightarrow{light\; and\; chlorophyll}glucose+oxygen$$ **.
2. The balanced symbol equation is ** $$6C{O}_2+6H_{2}O\underrightarrow{light\; and\; chlorophyll}{C}_6{H}_{12}{O}_6+6O_2$$ **.
3. Carbon dioxide and water are the reactants, while glucose and oxygen are the products.
4. Light and chlorophyll are written over the arrow because they are required conditions rather than reactants that are used up.

Check Your Understanding

1. Photosynthetic organisms form biomass by converting glucose into substances used to make cells and tissues.
2. This biomass passes through a food chain when one organism eats another.

6.1.2a Limiting factors on the rate of photosynthesis

A Limiting Factor Sets the Rate

1. The rate of photosynthesis is the amount of photosynthetic product formed, or reactant used, per unit time.
2. A limiting factor is the factor in shortest supply that restricts the rate of a process.
3. Increasing the limiting factor increases the rate until a different factor becomes limiting.
4. The three factors assessed here are light intensity, carbon dioxide concentration and temperature.

Light and Carbon Dioxide Reach Plateaus

1. At low light intensity, light limits the rate because little energy is absorbed by chlorophyll.
2. Increasing light intensity supplies more energy, so the rate rises.
3. The curve eventually reaches a plateau, where light is no longer limiting and another factor restricts the rate.
4. At low carbon dioxide concentration, carbon dioxide limits the rate because it is a reactant.
5. Increasing carbon dioxide concentration increases the frequency with which the photosynthetic reactions can use this reactant, so the rate rises.
6. The carbon dioxide curve also plateaus when light intensity or temperature becomes limiting.

Temperature Has an Optimum

1. At low temperature, molecules have less kinetic energy, so enzyme-controlled reactions proceed slowly.
2. As temperature rises, particles move faster and successful collisions involving enzymes occur more often, so the rate increases.
3. The rate reaches an optimum, which is the temperature at which it is highest.
4. Above the optimum, bonds maintaining enzyme shape break and the active site changes shape.
5. The substrate no longer fits the active site, so fewer enzyme-substrate complexes form and the rate falls rapidly.

Graphs Show What Is Limiting

1. A rising line shows that the factor on the horizontal axis is limiting because increasing it increases the rate.
2. A plateau shows that the factor on the horizontal axis is no longer limiting.
3. A temperature graph rises to an optimum and then falls rather than forming a simple plateau.
4. When comparing points, use values from the graph and describe both the change in the factor and the change in rate.

Rate Calculations

1. Rate is calculated using ** $$rate=\frac{quantity\; changed}{time}$$ **.
2. Suitable measures include oxygen volume per minute, carbon dioxide uptake per minute or bubble count per minute.
3. Oxygen volume is more reliable than bubble count because bubbles can have different volumes.

Recall the Relationships

1. Light intensity and carbon dioxide concentration give rising curves that level off when another factor becomes limiting.
2. Temperature gives a curve with an optimum because high temperatures denature enzymes.

6.1.2b Interactions of limiting factors on photosynthesis

Limiting Factors Change With Conditions

1. Temperature, light intensity and carbon dioxide concentration interact because the factor in shortest supply sets the rate.
2. If the limiting factor is increased, the rate rises until another factor becomes limiting.
3. Increasing a factor that is not limiting produces little or no change in rate.
4. Different combinations of conditions therefore produce different maximum rates.

Compare Curves at the Same Point

1. Graphs often show rate against light intensity using separate curves for different carbon dioxide concentrations or temperatures.
2. Compare the curves at the same light intensity so only the second condition differs.
3. If the higher-carbon-dioxide curve has a higher rate, carbon dioxide limited the lower curve at that light intensity.
4. If two curves overlap at low light intensity, light is limiting both, so extra carbon dioxide has no effect.
5. If a curve plateaus, increasing light no longer helps because carbon dioxide concentration or temperature now limits the rate.

Control Conditions for Crop Yield

1. Growers can add artificial light, enrich the air with carbon dioxide and control temperature to increase photosynthesis.
2. Increasing photosynthesis can produce more glucose for growth, so crop biomass and yield may increase.
3. Heating, lighting and carbon dioxide enrichment cost money.
4. The most profitable conditions are those where the value of the extra yield exceeds the cost of controlling the environment.

Recall the Interactions

1. At low light intensity, extra carbon dioxide or warmth may have no effect because light is limiting.
2. At high light intensity, carbon dioxide concentration or temperature is more likely to become limiting.

6.1.3 Inverse square law and light intensity

Light Intensity Falls With Distance

1. Light from a lamp spreads over a larger area as distance increases, so the energy received per unit area decreases.
2. The inverse square law states that light intensity is inversely proportional to the square of the distance from the source.
3. The relationship is ** $$I\propto \frac{1}{d^2}$$ **, where $I$ is light intensity and $d$ is distance.
4. For relative light intensity, use ** $$I=\frac{1}{d^2}$$ ** when no constant of proportionality is supplied.

Distance Changes Intensity by a Square

1. If distance is multiplied by a scale factor $n$, light intensity is divided by $n^2$.
2. Doubling distance gives ** $$\frac{1}{2^2}=\frac{1}{4}$$ ** of the original intensity.
3. Tripling distance gives ** $$\frac{1}{3^2}=\frac{1}{9}$$ ** of the original intensity.
4. Halving distance gives ** $$\frac{1}{(1/2{)}^2}=4$$ ** times the original intensity.

Calculate Relative Light Intensity

1. Convert all distances to the same unit before squaring them.
2. At $d=0.20\mathrm{m}$, ** $$I=\frac{1}{(0.20{)}^2}=25{\mathrm{m}}^{-2}$$ ** as a relative value.
3. At $d=0.40\mathrm{m}$, ** $$I=\frac{1}{(0.40{)}^2}=6.25{\mathrm{m}}^{-2}$$ **.
4. The second value is one quarter of the first because the distance doubled.

Link Intensity to Photosynthesis

1. When light is the limiting factor, the rate of photosynthesis is directly proportional to light intensity.
2. Moving a lamp closer increases light intensity, so chlorophyll absorbs more energy and the rate increases.
3. The rate does not keep increasing indefinitely because carbon dioxide concentration or temperature can become limiting.
4. A graph of rate against $1/d^2$ tests the relationship more directly than a graph of rate against $d$.

Check the Calculation

1. A larger distance must give a smaller relative intensity, so use this pattern to check your answer.
2. Keep the same distance unit throughout a comparison because changing units changes the numerical relative intensity.