Unit 6 Photosynthesis
Key unit competence: Explain the process of photosynthesis, the conditions it needs and the factors that limit it.
Key ideas
- Photosynthesis is the process by which green plants make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll, and release oxygen. Word equation: carbon dioxide + water → (light, chlorophyll) → glucose + oxygen. Balanced: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Photosynthesis takes place in chloroplasts, mainly in the palisade mesophyll cells of leaves. Light energy is converted into chemical energy stored in glucose.
- Two stages: the light-dependent stage (in the grana) uses light to split water (photolysis), releasing oxygen and making energy carriers (ATP); the light-independent stage (in the stroma) uses this energy to fix carbon dioxide into glucose.
- Conditions needed: light, carbon dioxide, water, chlorophyll and a suitable temperature (enzymes are involved).
- Destarching a plant (keeping it in the dark for 48 hours) removes stored starch before an experiment. Testing a leaf for starch: boil in water (kills cells), boil in ethanol in a water bath (removes chlorophyll), dip in water (softens), add iodine: blue-black shows starch was made.
- Experiments: a leaf partly covered with foil makes starch only where light reached it; a variegated leaf makes starch only in green parts (chlorophyll needed); a leaf in a flask with sodium hydroxide (absorbs CO₂) makes no starch (CO₂ needed); pondweed gives off bubbles of oxygen that relight a glowing splint.
- A limiting factor is the factor in shortest supply that limits the rate. Light intensity, carbon dioxide concentration and temperature can all limit the rate. Raising the limiting factor raises the rate until another factor becomes limiting. Greenhouses raise all three to increase yields.
- Leaf adaptations: broad, flat lamina (large surface for light), thin (short diffusion distance for gases), many chloroplasts in palisade cells near the upper surface, air spaces in spongy mesophyll, stomata for gas exchange, veins (xylem brings water, phloem removes sugars), transparent cuticle and epidermis.
- Glucose is used for respiration, stored as starch, converted to sucrose for transport, cellulose for cell walls, and with minerals into proteins and fats.
- Minerals: magnesium is part of chlorophyll; nitrogen (nitrates) is needed for proteins and chlorophyll. Lack of either causes chlorosis (yellow leaves). Fertilisers supply N, P and K, but too much runs into water and causes eutrophication (algal blooms, oxygen shortage, fish deaths).
Common misconceptions
- "Plants get their food from the soil." No: plants make their food (glucose) from CO₂ and water; the soil gives water and minerals, not food.
- "The oxygen given off comes from carbon dioxide." No: the oxygen comes from the splitting of water in the light stage.
- "Plants photosynthesise and animals respire." No: plants respire all the time; they photosynthesise only in light.
- "More light always means faster photosynthesis." No: once light is not the limiting factor, extra light does not help; CO₂ or temperature limits the rate.
Worked examples
Example 1. A destarched variegated leaf (green edges, white centre) is left in light, then tested with iodine. Predict and explain the results.
Solution
The green parts turn blue-black (starch made) and the white parts stay brown (no starch). The white parts have no chlorophyll, so they cannot absorb light for photosynthesis. This shows chlorophyll is needed.
Example 2. The rate of photosynthesis of pondweed rises as a lamp is moved closer, but stops rising when the lamp is very close. Explain.
Solution
Moving the lamp closer increases light intensity, which was the limiting factor, so the rate rises. When the lamp is very close, light is no longer limiting; another factor such as carbon dioxide concentration or temperature limits the rate.
Practice questions
Write the word equation for photosynthesis and state where it takes place.
Show answer
Carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll). It takes place in chloroplasts, mainly in palisade mesophyll cells of leaves.
Common mistake: Writing oxygen as a raw material.
Describe how to test a leaf for starch.
Show answer
Boil the leaf in water for a minute (kills cells); put it in ethanol in a hot water bath (removes chlorophyll); dip in warm water (softens); spread on a tile and add iodine: blue-black shows starch.
Common mistake: Heating ethanol over a naked flame.
State four ways in which a leaf is adapted for photosynthesis.
Show answer
Broad, flat lamina for a large surface; thin for short diffusion distance; many chloroplasts in palisade cells; air spaces in spongy layer; stomata for gas exchange; veins bring water and remove sugar; transparent epidermis.
Common mistake: Giving features without saying how they help.
Explain the term limiting factor and name three limiting factors of photosynthesis.
Show answer
The factor in shortest supply that keeps the rate low; increasing it increases the rate. Light intensity, carbon dioxide concentration and temperature.
Common mistake: Naming oxygen as a limiting factor.
Describe an experiment to show that carbon dioxide is needed for photosynthesis.
Show answer
Destarch a potted plant; enclose one leaf in a flask containing sodium hydroxide (absorbs CO₂) and another in a flask with water (control); leave in light for some hours; test both for starch: only the control leaf turns blue-black.
Common mistake: Not including a control leaf.
State three uses of the glucose made in photosynthesis.
Show answer
Respiration to release energy; stored as starch; changed to sucrose for transport; made into cellulose for cell walls; combined with minerals to make proteins and fats.
Common mistake: Saying glucose is used to make oxygen.
Explain why farmers in greenhouses may burn paraffin heaters at night.
Show answer
The heaters raise the temperature and release carbon dioxide, two limiting factors, so photosynthesis is faster when light is available and yields are higher.
Common mistake: Saying plants photosynthesise in the dark.
State two benefits and two dangers of using nitrogen fertilisers.
Show answer
Benefits: plants make more protein and chlorophyll, grow faster and give higher yields. Dangers: excess washes into rivers and lakes, causing eutrophication and fish death; it can make soil acidic; it is expensive.
Common mistake: Giving only benefits.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. When testing a leaf for starch, it is boiled in ethanol to: a. gases diffuse a short distance 2. Leaves are thin so that: b. remove the chlorophyll 3. The raw materials for photosynthesis are: c. stomata 4. Gases enter and leave the leaf through: d. carbon dioxide and water Show answer
1 → b (remove the chlorophyll) · 2 → a (gases diffuse a short distance) · 3 → d (carbon dioxide and water) · 4 → c (stomata)
- The green colour would hide the iodine result.
- Short diffusion paths speed up gas exchange.
- Glucose and oxygen are the products.
- Guard cells open and close them.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
supplies water to the leaf · absorbs carbon dioxide · sucrose in the phloem · limiting factor- The factor in shortest supply that limits the rate is the .
- Sodium hydroxide placed in a flask with a leaf .
- Sugar is transported from leaves to other parts as .
Show answer
1: limiting factor · 2: absorbs carbon dioxide · 3: sucrose in the phloem
- Raising it increases the rate.
- It shows that CO₂ is needed.
- Sucrose is soluble and less reactive.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Photosynthesis takes place mainly in the: a. keeping it in the dark for about two days 2. Destarching a plant means: b. palisade mesophyll cells 3. The oxygen released in photosynthesis comes from: c. chlorophyll 4. The green pigment that absorbs light is: d. water Show answer
1 → b (palisade mesophyll cells) · 2 → a (keeping it in the dark for about two days) · 3 → d (water) · 4 → c (chlorophyll)
- They contain many chloroplasts.
- Stored starch is used up.
- Water is split by light energy.
- It is found in chloroplasts.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
grana of the chloroplast · cytoplasm · too much fertiliser washing into water · proteins and chlorophyll- Eutrophication is caused by .
- The light-dependent stage takes place in the .
- Fertilisers containing nitrogen help plants to make .
Show answer
1: too much fertiliser washing into water · 2: grana of the chloroplast · 3: proteins and chlorophyll
- Algae grow fast, then die and decay.
- Chlorophyll is in the grana.
- Nitrogen is part of amino acids.