Photosynthesis (Edexcel GCSE Combined Science: Biology): Flashcards
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Cards in this collection (47)
Photosynthesis
The process by which plants manufacture carbohydrates from raw materials using energy from light. It is an endothermic reaction.
Autotroph
An organism that can make complex molecules (glucose) from simple molecules (carbon dioxide and water).
What are the raw materials needed for photosynthesis?
Carbon dioxide and water.
(Light energy is needed for the reaction but is not a raw material, as it is not a substance.)
What are the products of photosynthesis?
Glucose (a carbohydrate) and oxygen (released as a waste product).
Why is photosynthesis described as an endothermic reaction?
Energy from sunlight is transferred to (taken in by) the chloroplasts during the reaction.
Write the word equation for photosynthesis.
carbon dioxide + water → glucose + oxygen
(light energy is required for the reaction)
Describe the balanced chemical equation for photosynthesis in words.
Six carbon dioxide molecules combine with six water molecules to make one glucose molecule and six oxygen molecules.
Why are plants described as producers?
They can make their own food, so they are the first organism at the start of all food chains.
List four ways a plant can use the glucose made in photosynthesis.
Used in respiration as a source of energy
Converted into starch for storage
Used to synthesise lipids for energy storage in seeds, or to form cellulose for cell walls
Combined with nitrogen and other mineral ions to produce amino acids
In photosynthesis, carbon dioxide and water are converted into glucose and .
oxygen
Plants can convert glucose into for storage.
starch
Limiting factor
Something present in the environment in such short supply that it restricts life processes (e.g. the rate of photosynthesis).
What are the three main factors that limit the rate of photosynthesis?
Temperature, light intensity and carbon dioxide concentration.
Why is water not usually considered a limiting factor for photosynthesis?
The amount needed is very small compared with the amount transpired, so there is hardly ever a situation with too little water for photosynthesis.
How does increasing light intensity affect the rate of photosynthesis?
The more light a plant receives, the faster the rate of photosynthesis, until another factor becomes limiting.
How does carbon dioxide concentration affect the rate of photosynthesis?
The more carbon dioxide present, the faster the reaction can occur, until another factor becomes limiting.
Explain how increasing temperature increases the rate of photosynthesis (below the optimum).
It increases the kinetic energy of particles, increasing the likelihood of successful collisions between reactants and enzymes, so more product is formed.
Why does the rate of photosynthesis fall at very high temperatures?
The enzymes that control photosynthesis become denatured (the active site changes shape and is no longer complementary to its substrate).
How does the number of chloroplasts affect the rate of photosynthesis?
The more chloroplasts a plant has (containing chlorophyll, which absorbs light), the faster the rate of photosynthesis.
Give three things that can reduce the number of chloroplasts or amount of chlorophyll in a plant.
Diseases such as tobacco mosaic virus
Lack of nutrients such as magnesium
Loss of leaves (fewer leaves means fewer chloroplasts)
A limiting factor is something in such short supply that it life processes.
restricts
At very high temperatures the enzymes controlling photosynthesis become .
denatured
On a graph, what does it mean when the rate of photosynthesis reaches a plateau as one factor is increased? (Higher Tier Only)
That factor is no longer limiting the rate.
Another factor (e.g. temperature or carbon dioxide concentration) has become the limiting factor instead.
At low light intensity, why does increasing the carbon dioxide concentration have no effect on the rate of photosynthesis? (Higher Tier Only)
Light intensity is the limiting factor.
Until more light is provided, increasing the carbon dioxide concentration cannot increase the rate.
On a graph of photosynthesis rate against light intensity, why does a higher carbon dioxide concentration produce a higher plateau? (Higher Tier Only)
At higher light intensities, carbon dioxide becomes the limiting factor.
Providing more carbon dioxide allows a faster maximum rate to be reached before the rate plateaus.
When the rate of photosynthesis levels off as light intensity increases, the limiting factor is no longer light but instead temperature or concentration. (Higher Tier Only)
carbon dioxide
Inverse square law (light intensity) (Higher Tier Only)
Light intensity is inversely proportional to the square of the distance from the light source: light intensity ∝ 1 ÷ distance².
How does light intensity affect the rate of photosynthesis? (Higher Tier Only)
The more light a plant receives, the faster the rate; the rate of photosynthesis is directly proportional to light intensity.
How does the distance from a light source affect the rate of photosynthesis? (Higher Tier Only)
The further away a plant is from a light source, the lower the rate of photosynthesis.
What is the relationship between light intensity and distance from the light source? (Higher Tier Only)
Light intensity is inversely proportional to the square of the distance between the plant and the light source.
What instrument is used to measure light intensity? (Higher Tier Only)
A light meter.
If the distance between a plant and a light source is doubled, what happens to the light intensity? (Higher Tier Only)
The light intensity becomes four times smaller.
Why does natural light intensity change gradually throughout the day? (Higher Tier Only)
Because the sun rises and sets, gradually changing the intensity of light reaching the plant.
Write the inverse square law relationship for light intensity. (Higher Tier Only)
light intensity ∝ 1 ÷ distance²
The rate of photosynthesis is directly proportional to . (Higher Tier Only)
light intensity
As the distance from a light source increases, the light intensity . (Higher Tier Only)
decreases
Pondweed (e.g. Elodea or Cabomba)
An aquatic plant used to investigate the rate of photosynthesis by measuring the volume of oxygen gas produced.
How is the rate of photosynthesis measured in the pondweed practical?
By measuring the volume of oxygen gas produced (collected in a gas syringe) in a set period of time.
How is light intensity changed in this investigation?
By changing the distance (d) of the light source from the plant (light intensity is proportional to 1/d²).
How is the carbon dioxide concentration changed for the pondweed?
By adding different quantities of sodium hydrogencarbonate (NaHCO₃) to the water, which dissolves to produce CO₂.
Why is a glass tank of water placed between the lamp and the boiling tube?
To absorb heat from the lamp, preventing the temperature of the solution around the plant from changing.
Why should the water be well aerated (air bubbled through) before use?
So that the oxygen given off by the plant forms bubbles and does not simply dissolve in the water.
Why is the pondweed submerged in sodium hydrogencarbonate solution?
To give the pondweed a controlled supply of carbon dioxide, a reactant in photosynthesis.
Describe the relationship seen between distance from the light source and oxygen produced.
As the distance decreases, the volume of oxygen produced increases, because as light intensity increases, the rate of photosynthesis increases.
Why does the rate stop increasing when the lamp is moved very close?
Light is no longer the limiting factor; another factor (e.g. carbon dioxide concentration) is now limiting the rate of photosynthesis.
In this practical the rate of photosynthesis is found by measuring the volume of produced per minute.
oxygen
Carbon dioxide is supplied to the pondweed using solution.
sodium hydrogencarbonate
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