6.1 Photosynthesis (Cambridge (CIE) O Level Biology): Flashcards
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Define photosynthesis.
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Define photosynthesis.
The process by which plants make carbohydrates from raw materials (carbon dioxide and water) using energy from light.
What are the two raw materials needed for photosynthesis?
Carbon dioxide and water.
What are the products of photosynthesis?
Glucose and oxygen (oxygen is released as a waste product).
The green pigment absorbs the light energy needed for photosynthesis.
The green pigment chlorophyll absorbs the light energy needed for photosynthesis.
Give the word equation for photosynthesis.
carbon dioxide + water → glucose + oxygen (using light energy and chlorophyll).
Give the balanced chemical equation for photosynthesis.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
True or False?
Light energy is a raw material for photosynthesis.
False.
Light energy is not a substance, so it is not a raw material. The raw materials are carbon dioxide and water.
During photosynthesis, light energy is converted into energy stored in glucose.
During photosynthesis, light energy is converted into chemical energy stored in glucose.
True or False?
The photosynthesis equation is the exact reverse of the aerobic respiration equation.
True.
If you have learned one equation, you also know the other.
Why is oxygen described as a waste product of photosynthesis?
It is produced during the reaction but is not needed by the plant for photosynthesis, so it is released.
Define chlorophyll.
The green pigment found in chloroplasts that absorbs light energy for photosynthesis.
Where in a plant cell is chlorophyll found?
Inside the chloroplasts.
What does chlorophyll do in photosynthesis?
It absorbs light energy and transfers it into stored chemical energy in glucose molecules.
Chlorophyll transfers light energy into chemical energy stored in the bonds of molecules.
Chlorophyll transfers light energy into chemical energy stored in the bonds of glucose molecules.
Why are most plants green?
Because their chloroplasts contain the green pigment chlorophyll.
Chlorophyll is a green found inside chloroplasts.
Chlorophyll is a green pigment found inside chloroplasts.
True or False?
Chlorophyll absorbs light energy and converts it into chemical energy.
True.
It absorbs light energy and transfers it into chemical energy stored in glucose.
What can the glucose produced using chlorophyll be converted into?
Other carbohydrates such as sucrose and starch.
Define the chloroplast.
The organelle in plant cells that contains chlorophyll and is the site of photosynthesis.
True or False?
Chlorophyll gives out light energy during photosynthesis.
False.
Chlorophyll absorbs light energy; it does not give it out.
In what form do plants store carbohydrate?
As starch, which acts as an energy store in plant cells.
What is glucose converted into to build plant cell walls?
Cellulose.
How is glucose transported around a plant?
It is converted to sucrose for transport in the phloem.
Glucose is used in to release energy for cell activities.
Glucose is used in respiration to release energy for cell activities.
Besides carbohydrates, what two types of molecule can plants make from glucose?
Lipids and amino acids (used to build proteins).
Plants store carbohydrate as , whereas animals store it as glycogen.
Plants store carbohydrate as starch, whereas animals store it as glycogen.
Define cellulose.
A carbohydrate made from glucose that is used to build plant cell walls.
True or False?
Plants convert glucose into starch for energy storage.
True.
Starch is an insoluble energy store made from glucose in plant cells.
Why is glucose converted to sucrose before being transported in a plant?
Sucrose is the form in which sugar is transported around the plant in the phloem.
Give three uses of the glucose made in photosynthesis.
Used in respiration to release energy
Converted to starch for storage
Converted to cellulose to build cell walls
Why is a leaf tested for starch rather than glucose to show photosynthesis?
Glucose is quickly used or converted into other substances, but starch is stored in the chloroplasts, making it a reliable indicator.
Why is a leaf boiled in ethanol when testing it for starch?
To remove the chlorophyll, so the colour change with iodine can be seen clearly.
Why must the Bunsen burner be turned off before adding ethanol?
Ethanol is extremely flammable, so a naked flame is a fire hazard.
How is a variegated leaf used to show chlorophyll is needed for photosynthesis?
Only the green (chlorophyll-containing) parts turn blue-black with iodine; the white areas stay orange-brown because no starch is made there.
Before testing whether light is needed, a plant is destarched by leaving it in the for 24 hours.
Before testing whether light is needed, a plant is destarched by leaving it in the dark for 24 hours.
In the 'need for light' experiment, why is part of a leaf covered with aluminium foil?
To block light from that area; the covered part stays orange-brown (no starch) while the exposed part turns blue-black.
What is used to absorb carbon dioxide in the 'need for carbon dioxide' experiment?
Sodium hydroxide, placed in a beaker inside the bell jar.
Why is a plant with a beaker of water used in the 'need for carbon dioxide' experiment?
It acts as the experimental control, showing it is the absence of carbon dioxide (not another factor) that stops photosynthesis.
A leaf that has photosynthesised turns when iodine is added, showing starch is present.
A leaf that has photosynthesised turns blue-black when iodine is added, showing starch is present.
Why is a leaf first dipped in boiling water during the starch test?
To kill the cells and break down the cell membranes (and later to soften the leaf).
Name three factors that affect the rate of photosynthesis.
Light intensity
Carbon dioxide concentration
Temperature
How does light intensity affect the rate of photosynthesis?
As light intensity increases, the rate increases, because photosynthesis requires light energy.
How does carbon dioxide concentration affect the rate of photosynthesis?
As carbon dioxide concentration increases, the rate increases, because carbon dioxide is a reactant.
Why does the rate of photosynthesis fall at very high temperatures?
The enzymes controlling photosynthesis denature, so the rate decreases and can reach zero.
As temperature rises towards the optimum, molecules gain more energy and collide more often.
As temperature rises towards the optimum, molecules gain more kinetic energy and collide more often.
True or False?
Photosynthesis is controlled by enzymes.
True.
Photosynthesis is an enzyme-controlled reaction, which is why temperature affects its rate.
Why does increasing light intensity increase the rate of photosynthesis?
More light energy is available for the reaction, so it can proceed faster.
Carbon dioxide is a in photosynthesis, so increasing it raises the rate.
Carbon dioxide is a reactant in photosynthesis, so increasing it raises the rate.
True or False?
Increasing temperature always increases the rate of photosynthesis.
False.
Only up to a point — beyond the optimum, the enzymes denature and the rate falls.
Why is temperature important for photosynthesis?
Photosynthesis is enzyme-controlled, and temperature affects the rate of enzyme-controlled reactions.
How can the rate of photosynthesis be measured in an aquatic plant?
By measuring the volume of oxygen produced, or counting the bubbles of oxygen released per minute.
Name an aquatic plant commonly used to investigate photosynthesis.
Elodea or Cabomba (both types of pondweed).
How is light intensity changed in the pondweed experiment?
By moving a lamp to different distances from the pondweed — further away gives a lower light intensity.
The more oxygen produced per minute, the faster the rate of photosynthesis.
The more oxygen bubbles produced per minute, the faster the rate of photosynthesis.
Why is collecting oxygen in a test tube more accurate than counting bubbles?
It measures the volume of gas, so the results are not affected by differences in bubble size.
Why is a glass tank of water placed between the lamp and the beaker?
To absorb heat from the lamp, keeping the temperature constant so only light intensity changes.
Carbon dioxide concentration is varied by dissolving different amounts of in the water.
Carbon dioxide concentration is varied by dissolving different amounts of sodium hydrogencarbonate in the water.
When describing the oxygen produced, what should you refer to instead of 'amount'?
The volume of gas or the number of bubbles.
Define hydrogencarbonate indicator.
A pH indicator that shows the carbon dioxide concentration in a solution, used to measure the rate of photosynthesis.
True or False?
A submerged plant photosynthesising faster than it respires turns hydrogencarbonate indicator purple.
True.
Carbon dioxide is used up faster than it is produced, turning the indicator purple.
Which gases does a plant take in and release during the day in bright light?
A net intake of carbon dioxide and a net release of oxygen, because photosynthesis exceeds respiration.
Which gases does a plant take in and release at night?
It takes in oxygen and releases carbon dioxide, because it only respires (no photosynthesis).
Plants respire , but they only photosynthesise in the light.
Plants respire all the time, but they only photosynthesise in the light.
Why does a plant have a net release of oxygen on a bright day?
It photosynthesises faster than it respires, so more oxygen is made than is used up.
Define hydrogencarbonate indicator.
An indicator that changes colour with carbon dioxide concentration, used to investigate net gas exchange in plants.
Why can hydrogencarbonate indicator detect carbon dioxide?
Carbon dioxide is an acidic gas when dissolved in water, which changes the indicator's pH and colour.
True or False?
At night a plant releases more oxygen than carbon dioxide.
False.
At night the plant only respires, so it takes in oxygen and releases carbon dioxide.
During the day, a plant photosynthesises faster than it .
During the day, a plant photosynthesises faster than it respires.
How is the effect of light on net gas exchange investigated?
Leaves are placed in stoppered tubes with hydrogencarbonate indicator, and the colour change is observed under different light conditions.
Do plants respire during the day?
Yes — plants respire all the time, but during the day photosynthesis occurs at a faster rate.
Define a limiting factor.
An environmental factor that limits the rate of a reaction (such as photosynthesis) when it is in shortest supply.
Name the three main limiting factors of photosynthesis.
Temperature
Light intensity
Carbon dioxide concentration
Why is water not usually considered a limiting factor of photosynthesis?
Water is involved in many plant processes, so it affects those before it directly limits photosynthesis.
On a graph, how do you identify the limiting factor where the line is still rising?
The limiting factor is the variable shown on the x-axis.
On a graph, what does a horizontal (levelled-off) line tell you about the limiting factor?
The limiting factor is something other than the x-axis variable (one of temperature, light intensity or carbon dioxide).
A limiting factor is whichever required factor is in supply at a given time.
A limiting factor is whichever required factor is in shortest supply at a given time.
True or False?
Increasing light intensity will always increase the rate of photosynthesis.
False.
Only until another factor (such as carbon dioxide or temperature) becomes the limiting factor.
What happens to the rate of photosynthesis at very high temperatures, and why?
It decreases, because the enzymes controlling photosynthesis denature.
When a light-intensity graph levels off, the limiting factor could be temperature or .
When a light-intensity graph levels off, the limiting factor could be temperature or carbon dioxide concentration.
What conditions allow a plant to photosynthesise at its maximum rate?
Unlimited light, carbon dioxide and water in a warm environment.
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