Photosynthesis & Leaf Structure (Cambridge (CIE) IGCSE Co-ordinated Sciences (Double Award): Biology): Flashcards
Exam code: 0654 & 0973
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Photosynthesis
The process by which plants make carbohydrates (e.g. glucose) from carbon dioxide and water, using light energy absorbed by chlorophyll.
Write the word equation for photosynthesis.
carbon dioxide + water → glucose + oxygen
(in the presence of light and chlorophyll)
What are the raw materials needed for photosynthesis?
Carbon dioxide and water.
What are the products of photosynthesis?
Glucose and oxygen.
Photosynthesis takes place in the of plant cells.
chloroplasts
The energy used to drive photosynthesis comes from .
light / sunlight
Why is photosynthesis important?
It converts light energy into chemical energy stored in glucose, providing food for the plant and forming the basis of most food chains.
Chlorophyll
The green pigment found in chloroplasts that absorbs light energy for photosynthesis.
What is the role of chlorophyll in photosynthesis?
It absorbs light energy, which is then transferred into chemical energy for the reactions of photosynthesis.
Chlorophyll absorbs energy and transfers it to chemical energy.
light
Why do most leaves appear green?
Chlorophyll reflects green light, so leaves appear green.
Describe how chlorophyll transfers energy in photosynthesis (Extended).
Light energy is absorbed by chlorophyll.
This energy is transferred into chemical energy, which is used to make glucose from carbon dioxide and water.
Chlorophyll is found inside the of plant cells.
chloroplasts
List the ways a plant uses the glucose made in photosynthesis (Extended).
Used in respiration to release energy.
Converted to starch for storage.
Used to make cellulose for cell walls.
Converted to sucrose for transport in the phloem.
Glucose is stored in plant cells as .
starch
Why is starch a good storage molecule in plants?
It is insoluble, so it does not affect water movement (osmosis) in or out of cells, and it is compact for storage.
Glucose is used to make , which strengthens plant cell walls.
cellulose
In what form is carbohydrate transported around a plant?
As sucrose, in the phloem.
Glucose is broken down in to release energy for the plant.
respiration
What do plants use nitrate ions for (Extended)?
To make amino acids, which are then used to make proteins for growth.
What do plants use magnesium ions for?
To make chlorophyll.
Plants need ions to make amino acids and proteins.
nitrate
Plants need magnesium ions to make .
chlorophyll
Describe the signs of nitrate deficiency in a plant.
Stunted growth (due to reduced protein production).
What is the effect of magnesium deficiency in a plant?
Yellowing of the leaves (chlorosis), because chlorophyll cannot be made.
Describe how to test a leaf for the presence of starch.
Boil the leaf in water to kill it and stop reactions.
Boil it in ethanol to remove the chlorophyll (decolourise it).
Dip it in hot water to soften it.
Add iodine solution — a blue-black colour shows starch is present.
Why is the leaf boiled in ethanol when testing for starch?
To remove the chlorophyll, so the colour change with iodine can be seen clearly.
How does a variegated leaf show that chlorophyll is needed for photosynthesis?
Only the green parts (which contain chlorophyll) turn blue-black with iodine.
The white parts contain no chlorophyll and test negative for starch.
Before a photosynthesis experiment a plant is by leaving it in the dark, so existing starch is used up.
destarched
How can you show that light is needed for photosynthesis?
Destarch a plant, then cover part of a leaf with foil and leave it in the light.
Test the leaf for starch: only the exposed (uncovered) part turns blue-black, showing light is needed.
How can you show that carbon dioxide is needed for photosynthesis?
Enclose a destarched leaf in a flask with soda lime to absorb the carbon dioxide.
Test for starch: this leaf tests negative, showing carbon dioxide is needed.
Name the three environmental factors that affect the rate of photosynthesis.
Light intensity, carbon dioxide concentration, and temperature.
Limiting factor
A factor that, when in short supply, limits the rate of a process even when other factors are favourable.
As light intensity increases, the rate of photosynthesis rises until another factor becomes .
limiting
Describe how temperature affects the rate of photosynthesis.
As temperature increases, the rate of photosynthesis increases because the enzymes work faster.
Above the optimum temperature, the enzymes denature and the rate falls.
At very high temperatures the rate of photosynthesis falls because the controlling it are denatured.
enzymes
How does carbon dioxide concentration affect the rate of photosynthesis?
Increasing the carbon dioxide concentration increases the rate of photosynthesis, until another factor becomes limiting.
What does hydrogencarbonate indicator show, and what do its colours mean (Extended)?
It detects changes in carbon dioxide concentration:
Red/orange = normal CO₂ level.
Yellow = high CO₂ (respiration exceeds photosynthesis).
Purple/magenta = low CO₂ (photosynthesis exceeds respiration).
What colour does hydrogencarbonate indicator turn with a plant kept in bright light? Explain.
Purple/magenta.
Photosynthesis exceeds respiration, so carbon dioxide is removed from the solution.
What colour does hydrogencarbonate indicator turn with a plant kept in the dark? Explain.
Yellow.
Only respiration occurs, so carbon dioxide is added to the solution.
Hydrogencarbonate indicator turns when carbon dioxide levels increase.
yellow
Why is a tube with no plant used in a gas exchange experiment?
It acts as a control, showing that any colour change is caused by the plant's gas exchange and not other factors.
Write the balanced chemical (symbol) equation for photosynthesis (Extended).
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(in the presence of light and chlorophyll)
In the balanced equation for photosynthesis, molecules of carbon dioxide react with six molecules of water.
six (6)
The chemical formula for glucose is .
C₆H₁₂O₆
What are the products in the balanced equation for photosynthesis?
Glucose (C₆H₁₂O₆) and oxygen (6O₂).
Write the word equation that matches the symbol equation for photosynthesis.
carbon dioxide + water → glucose + oxygen
Describe how a leaf is adapted for photosynthesis.
Large surface area to absorb more light.
Thin, so gases diffuse only a short distance.
Many chloroplasts containing chlorophyll to absorb light.
A network of veins (xylem and phloem) to transport water and products.
Stomata to allow gas exchange.
are tiny pores in the leaf that allow gases to diffuse in and out.
Stomata
How is the palisade layer adapted for photosynthesis?
It is near the top of the leaf and packed with chloroplasts, so it absorbs the most light.
What is the role of guard cells?
They open and close the stomata to control gas exchange and water loss.
The waxy on the leaf surface reduces water loss.
cuticle
Why are leaves thin?
So that carbon dioxide has only a short distance to diffuse to the photosynthesising cells.
Waxy cuticle
A waterproof layer on the leaf surface that reduces water loss.
Upper epidermis
A thin, transparent layer of cells that lets light pass through to the cells below.
Palisade mesophyll
A layer of cells near the top of the leaf packed with chloroplasts; the main site of photosynthesis.
Spongy mesophyll
A layer of loosely packed cells with air spaces that allow gases to diffuse for gas exchange.
The mesophyll layer contains the most chloroplasts and carries out most photosynthesis.
palisade
Guard cells surround and control the opening of each .
stoma
Compare the functions of xylem and phloem in a leaf (Extended).
Xylem transports water and mineral ions into the leaf.
Phloem transports sucrose (dissolved food) out of the leaf.
Where in the leaf are most of the air spaces found, and why?
In the spongy mesophyll.
The air spaces increase the surface area for gas exchange and allow gases to diffuse to and from the cells.
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