B1.4 Photosynthesis

Question 1

State the word and chemical equation for photosynthesis

Answer 1

Carbon dioxide + water —> glucose + oxygen

6CO2 + 6H2O —> C6H12O6 + 6O2

Question 2

Describe photosynthetic organisms as the main producers of food and therefore biomass for life on Earth

Answer 2

Unlike the Venus fly trap, most plants do not catch their food. They are producers - they make their own food by photosynthesis. They use this food to grow, increasing their biomass. In turn they provide almost everything you eat, as well as building materials, paper, and medicines.

Question 3

Describe where the reactants of photosynthesis come from

Answer 3

• Carbon dioxide - diffuses from air into plant through stomata
• Water - enters roots from soil through root hair cells by osmosis

Question 4

Describe what happens to the oxygen produced during photosynthesis

Answer 4

• Some used by plant in respiration

- Rest of waste product is released back into the environment

Question 5

Describe where photosynthesis occurs

Answer 5

• takes place inside chloroplasts
• means that is mainly occurs in the leaf, but a small amount happened in green stem
• leaves + stems are green as of the pigment chlorophyll inside their chloroplasts
• light transfers energy from Sun to chlorophyll, where carbon dioxide + water react to make glucose
• glucose stores energy within its chemical bonds

Question 6

Describe the two-stage process of photosynthesis

Answer 6

Stage 1 (light dependent) - energy transferred from light splits water molecules into oxygen gas and hydrogen ions
Stage 2 (light independent) - carbon dioxide gas combines with the hydrogen ions to make glucose

Question 7

Describe what kind of reaction photosynthesis is and what that means

Answer 7

Endothermic reaction

- This means that energy must be transferred from the surroundings to keep it going

Question 8

Describe and explain what happens to the glucose produced

Answer 8

• some used in respiration immediately
• other glucose molecules converted into other sugar molecules e.g. fructose which is found in high quantities in fruit and sucrose,(made up of one molecule each of glucose and fructose) which is in the form in which glucose is transported around the plant’s tissues
• glucose not needed immediately is converted to starch for a store of energy that can be used at night for respiration when there is no photosynthesis taking place
• Chemical reactions in plants make proteins, cellulose, and fats from sugars and other substances

Question 9

Describe the process of treating the leaf for the process of testing for presence of starch with iodine

Answer 9

• take the leaf you are about to test, using forceps, place it in a beaker of boiling water for a minute to kill it
• place leaf using forceps into a boiling tube of boiling ethanol to remove all the chlorophyll
• Safety note: always use a water bath to heat ethanol as it is flammable
• Then wash leaf with water to remove ethanol + soften leaf, and spread it out on a white tile
• Add a few drops of iodine solution (mixture of iodine + potassium iodide) onto leaf
• If starch is present iodine will turn from yellow-brown to blue-black

This test is used to show that light, chlorophyll, and carbon dioxide are essential for photosynthesis.

Question 10

Describe how to destarch a plant and explain why we should do it

Answer 10

• if a plant is unable to photosynthesise, it will not be able to make starch, and so you should test for starch to see if photosynthesis has taken place
• keep the plant in the dark for a minimum of 24 hours to destarch the leaves

Question 11

How can you prove chlorophyll is needed for photosynthesis?

Answer 11

• place a destarched variegated plant in sunlight for several hours
• variegated leaves only have chlorophyll in some areas of the leaf (green and white)
• you then test one of its leaves for the presence of starch

Question 12

How can you prove light is needed for photosynthesis?

Answer 12

• take a destarched plant and cover part of one of its leaves with black card or tin foil
• light cannot reach the covered area of the leaf
• place plant in sunlight for several hours
• remove card from leaf and test for presence of starch

Question 13

How can you prove carbon dioxide is needed for photosynthesis?

Answer 13

• take destarched plant, place it inside a polythene bag
• before sealing bag add a pot of soda lime, which is a chemical that absorbs carbon dioxide and water vapour
• place plant in sunlight for several hours, before testing one of its leaves for the presence of starch

Question 14

How can you prove oxygen is given off during photosynthesis?

Answer 14

• place an upturned test tube above an aquatic plant e.g. pondweed
• put apparatus in light for maximum photosynthesis
• when you have collected a full tube of gas, place a glowing splint inside the tube
• it will relight as of the oxygen present

Question 15

Describe the rate of photosynthesis

Answer 15

• a measure of how much photosynthesis occluded in a given time
  Rate = 1/t
  T = time
  The unit ps depend on the units of time

Question 16

How does light intensity affect the rate of photosynthesis?

Answer 16

• the higher the light intensity, the faster the rate of photosynthesis
• this continues until photosynthesis reaches its maximum rate
• in very low light levels or if there is no light, photosynthesis stops

Question 17

How does carbon dioxide affect the rate of photosynthesis?

Answer 17

• one of the reactants for photosynthesis
• the greater the carbon dioxide concentration, the faster the rate of reaction
• atmosphere only contains 0.04% carbon dioxide, meaning that carbon dioxide is most commonly the limiting factor
• farmers artificially increase the levels of carbon dioxide in greenhouses to increase the rate of photosynthesis

Question 18

How does temperature affect the rate of photosynthesis?

Answer 18

• photosynthesis is a series of enzyme-controlled reactions
• so the higher the temperature, the faster the reactions occur
• but if temperature is too high the enzymes will denature + reactions will stop

Question 19

Describe how to measure the rate of photosynthesis

Answer 19

One way:

• using Elodea (pondweed)
• the volume of oxygen given of per minute is a measure of the rate of photosynthesis
• you can measure this by counting the number of bubbles produced per minute

Question 20

Describe how you can alter three key factors to study their effect on the rate of photosynthesis

Answer 20

• light intensity - place a light source at different distances from the Elodea
• carbon dioxide concentration - add different masses of potassium hydrogen carbonate powder to the water. Potassium hydrogen carbonate releases carbon dioxide to the atmosphere
• temperature - place the apparatus in water baths at different temperatures

Question 21

Describe the inverse square law and state the equation

Answer 21

• when you double distance from light source, light intensity falls by a factor of 4
  Relative light intensity = 1/distance from light source SQUARED
• doesn’t have a unit as unis cancel out (m)

Question 22

Explain the interaction of these 3 factors in limiting the rate of photosynthesis

Answer 22

• at some point, increasing the factor no longer makes a difference to rate of photosynthesis as one of the other factors (limiting factor) is now in short supply
  E.g. if there is optimum light intensity and optimum carbon dioxide, however there is a low temperature, the temperature is the limiting factor as the rate of photosynthesis is not at its optimum rate and so the temperature needs to also be increased to make it the optimum conditions for photosynthesis