Photosynthesis Exam Questions and Answers

سوالات

1. سوال 1بنیادی1 نمبر

   Write the word equation for photosynthesis.

   حل دیکھیںحل چھپائیں

   carbon dioxide + water → glucose + oxygen

   1. 1Reactants go on the left: carbon dioxide and water.
   2. 2Products go on the right: glucose and oxygen.
   3. 3Light and chlorophyll are conditions, not reactants, so they belong above the arrow or nowhere.
   4. 4carbon dioxide + water → glucose + oxygen
2. سوال 2بنیادی2 نمبر

   Write the balanced symbol equation for photosynthesis.

   حل دیکھیںحل چھپائیں

   6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

   1. 1Glucose is C₆H₁₂O₆, so six carbon atoms are needed on the left: 6CO₂.
   2. 2Glucose contains 12 hydrogen atoms, and each water molecule supplies 2, so 6H₂O.
   3. 3Count oxygen: left has 12 from 6CO₂ plus 6 from 6H₂O, which is 18.
   4. 4Right has 6 inside glucose, leaving 12 to come off as oxygen gas: 6O₂. Both sides now have 18.
   5. 56CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
3. سوال 3بنیادی2 نمبر

   Name the green pigment that absorbs light for photosynthesis, and the structure in the cell that contains it.

   حل دیکھیںحل چھپائیں

   Chlorophyll, contained in the chloroplasts

   1. 1The pigment is chlorophyll. (1)
   2. 2It is held in the chloroplasts, which are the organelles where photosynthesis takes place. (1)
   3. 3Chlorophyll absorbs light and reflects green, which is why leaves look green.
4. سوال 4بنیادی2 نمبر

   Name the two raw materials a plant needs for photosynthesis, and state how each one reaches the leaf.

   حل دیکھیںحل چھپائیں

   Carbon dioxide, which diffuses in through the stomata; and water, absorbed by the roots and carried up in the xylem

   1. 1Carbon dioxide diffuses from the air into the leaf through the stomata, mostly on the lower surface. (1)
   2. 2Water is absorbed from the soil by root hair cells. (1)
   3. 3It is carried up the stem and into the leaf through the xylem vessels.
   4. 4Each material needs its correct route to earn its mark; naming the substance alone is not enough here.
5. سوال 5بنیادی2 نمبر

   Is photosynthesis endothermic or exothermic? Explain your answer.

   حل دیکھیںحل چھپائیں

   Endothermic, because energy is transferred from the surroundings to the reaction as light

   1. 1An endothermic reaction takes in energy from its surroundings. (1)
   2. 2Photosynthesis takes in energy transferred by light from the Sun or a lamp. (1)
   3. 3That energy is stored in the glucose, which is why respiration can release it again later.
   4. 4Respiration is the exothermic partner reaction, and confusing the two is the usual error.
6. سوال 6بنیادی معیار3 نمبر

   A graph of rate of photosynthesis against light intensity rises steeply, then bends, then flattens. Describe the shape and name the limiting factor on each part. (3 marks)

   حل دیکھیںحل چھپائیں

   The rate rises roughly in proportion to light intensity, then levels off. Light intensity is limiting on the sloping part; on the flat part carbon dioxide concentration or temperature is limiting.

   1. 1On the first section the rate rises roughly in proportion: more light gives a faster rate. (1)
   2. 2Light intensity is the limiting factor there, because raising it raises the rate. (1)
   3. 3On the flat section extra light makes no difference, so light is now in excess and something else is limiting - carbon dioxide concentration or temperature. (1)
   4. 4The mark is for naming the factor in each region, not for describing the line.
7. سوال 7بنیادی معیار3 نمبر

   A plant is kept at constant light intensity and carbon dioxide concentration. The temperature is raised from 20 °C to 45 °C. The rate of photosynthesis rises and then falls sharply. Explain why. (3 marks)

   حل دیکھیںحل چھپائیں

   The reactions are enzyme-controlled. Rising temperature increases the rate up to the optimum because collisions become more frequent. Above the optimum the enzymes denature, the active site changes shape, and the rate falls.

   1. 1Photosynthesis is controlled by enzymes. (1)
   2. 2As the temperature rises towards the optimum, molecules have more kinetic energy and collide with the active sites more often, so the rate increases. (1)
   3. 3Above the optimum the enzymes denature: the shape of the active site changes and the substrate no longer fits. (1)
   4. 4That is why the fall is sharp rather than gradual, and why it is not reversed by cooling the plant back down.
8. سوال 8بنیادی معیار2 نمبر

   Give two ways a plant uses the glucose it makes in photosynthesis. (2 marks)

   حل دیکھیںحل چھپائیں

   Any two of: used in respiration; converted into starch for storage; used to make cellulose for cell walls; combined with nitrate ions to make amino acids; converted into lipids for storage in seeds

   1. 1Used in respiration to release energy for the plant's other processes. (1)
   2. 2Converted into starch, which is insoluble and can therefore be stored without affecting the water balance of the cell. (1)
   3. 3Other answers that earn the mark: making cellulose for cell walls, making lipids for seed stores, and combining glucose with nitrate ions from the soil to make amino acids and then proteins.
   4. 4Storing the glucose as glucose is not an acceptable answer - the point of starch is that it is insoluble.
9. سوال 9بنیادی معیار3 نمبر

   Pondweed is placed in water and the oxygen bubbles released each minute are counted as a lamp is moved to different distances. Explain why counting bubbles measures the rate of photosynthesis, and give two variables that must be controlled. (3 marks)

   حل دیکھیںحل چھپائیں

   Oxygen is a product, so bubbles per minute is a measure of the rate. Control the temperature and the carbon dioxide concentration, and use the same piece of pondweed throughout.

   1. 1Oxygen is a product of photosynthesis, so the number of bubbles released per minute rises and falls with the rate. (1)
   2. 2Control the temperature - the lamp heats the water, so without a heat shield or water bath the temperature changes along with the distance, and you cannot tell which variable caused the change. (1)
   3. 3Control the carbon dioxide concentration, for example by adding a fixed volume of sodium hydrogencarbonate solution. (1)
   4. 4Also keep the same piece of pondweed and the same counting interval. The method assumes every bubble is the same size, which is its main weakness; collecting the gas in a syringe and measuring the volume is more reliable.
10. سوال 10بنیادی معیار2 نمبر

    In the pondweed experiment the lamp is moved from 10 cm to 20 cm away. What happens to the light intensity reaching the pondweed? (2 marks)

    حل دیکھیںحل چھپائیں

    It falls to a quarter of its previous value

    1. 1Light intensity is proportional to 1/d², where d is the distance from the lamp. (1)
    2. 2Doubling the distance from 10 cm to 20 cm multiplies d² by 4.
    3. 3So the intensity falls to one quarter of what it was. (1)
    4. 4This is why results are plotted against 1/d² rather than against distance: against 1/d² the relationship shows as a straight line.
11. سوال 11مشکل4 نمبر

    Explain how the structure of a leaf is adapted for photosynthesis. (4 marks)

    حل دیکھیںحل چھپائیں

    Broad and flat for a large surface area; thin for a short diffusion path; palisade cells packed with chloroplasts near the upper surface; stomata and air spaces for gas exchange

    1. 1Broad and flat, giving a large surface area to absorb light. (1)
    2. 2Thin, so carbon dioxide has only a short distance to diffuse to the cells that need it. (1)
    3. 3Palisade mesophyll cells are column-shaped, tightly packed near the upper surface and full of chloroplasts, so they intercept most of the light. (1)
    4. 4Stomata in the lower epidermis let carbon dioxide diffuse in and oxygen out, and the air spaces of the spongy mesophyll let gases move freely inside the leaf. (1)
    5. 5Also creditworthy: a transparent cuticle and upper epidermis that let light through, and xylem in the veins delivering water to every cell.
12. سوال 12مشکل6 نمبر

    A grower produces tomatoes in a greenhouse. Explain how light intensity, carbon dioxide concentration and temperature limit the rate of photosynthesis, and how the grower can use this to increase the crop. (6 marks)

    حل دیکھیںحل چھپائیں

    A full-mark answer explains that the rate is set by whichever factor is in shortest supply, treats all three factors, links each to a specific greenhouse control, and notes that heating beyond the optimum denatures enzymes and that each measure has to pay for itself.

    1. 1Mark 1 - the principle: the rate is limited by whichever factor is in shortest supply at that moment, and raising the other two changes nothing until that one is raised.
    2. 2Mark 2 - light intensity supplies the energy for the reaction, so as it rises the rate rises. The grower can install artificial lighting to extend the day and to work through the winter.
    3. 3Mark 3 - carbon dioxide is a reactant, and air is only about 0.04% carbon dioxide, so on a bright day it is usually the factor holding the rate down. The grower can raise the concentration inside the greenhouse, for example with a carbon dioxide generator or a paraffin heater.
    4. 4Mark 4 - temperature affects the enzymes that control the reactions, so the rate rises with temperature up to an optimum. The grower can heat the greenhouse to hold that optimum.
    5. 5Mark 5 - above the optimum the enzymes denature and the rate falls, so heating past it reduces the crop as well as wasting fuel.
    6. 6Mark 6 - the judgement: each factor is worth raising only while the extra crop is worth more than the cost of the lighting, heating or carbon dioxide.

Two equations, and which one the question wants

The word equation is carbon dioxide + water → glucose + oxygen. The symbol equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. A question asking for the word equation gets no credit for the symbol version, and a question asking for a balanced symbol equation gets no credit for one where the numbers do not balance.

Light and chlorophyll belong to neither. They are conditions, not reactants: light supplies the energy and chlorophyll absorbs it, and neither is used up. Writing light on the left-hand side alongside carbon dioxide is the most common way to lose a mark that was otherwise free. Write it above the arrow, or leave it out.

Reading a limiting-factor graph

Almost every graph in this topic has the same shape: a straight rise, then a bend, then a flat line. The rise means the factor on the horizontal axis is controlling the rate. The flat part means it no longer is - it is now in excess, and something else has taken over.

The marks are for naming which factor is limiting where, not for describing the line. It goes up and then stops earns nothing. Light intensity is the limiting factor on the sloping section, and on the flat section the rate is limited by carbon dioxide concentration or temperature earns both. When the question gives you two curves at different temperatures, the one that levels off higher tells you which variable was raised.

What a six-mark answer has to do

Extended-response questions of this kind are marked in bands rather than by counting ticks, so an answer that lists light, carbon dioxide and temperature and stops sits in the bottom band however many words surround it. The bands reward explanation and connection: why each factor limits the rate, and what happens when it is changed.

The structure that works is one paragraph per factor, each naming the factor, saying why it limits the rate, and applying it to the situation in the question. The last question below is set out that way, mark by mark, so the six separate things being credited are visible. On AQA's GCSE Biology specification photosynthesis sits within Bioenergetics, and extended-response items appear in that topic on both the Biology and the Combined Science papers.