Photosynthesis & Leaf Structure

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Notes

Photosynthesis Overview

**Photosynthesis** is the process by which plants manufacture carbohydrates from raw materials using energy from light.
Raw materials: **carbon dioxide** and **water**.
Products: **glucose** (carbohydrate) and **oxygen** (waste product).
Energy is obtained by the pigment **chlorophyll** trapping light from the Sun.
Word equation: **carbon dioxide + water → glucose + oxygen** (in the presence of light and chlorophyll).
Balanced chemical equation (Extended): **6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂**.

Chlorophyll

**Chlorophyll** is a green pigment found in **chloroplasts** within plant cells.
It reflects green light, giving plants their green colour.
Chlorophyll absorbs light energy and transfers it into chemical energy for carbohydrate synthesis.
Photosynthesis **cannot occur** in the absence of chlorophyll.

Use & Storage of Carbohydrates

Glucose is converted into **starch** for energy storage.
Converted into **cellulose** to build cell walls.
Used in **respiration** to provide energy.
Converted to **sucrose** for transport in the phloem.
Used as **nectar** to attract insects for pollination.
Can be converted into **lipids** (energy source in seeds) and **amino acids** (for proteins) when combined with nitrogen and other mineral ions.

Minerals in Plants

Plants need mineral ions absorbed from soil by root hair cells.
**Nitrate** deficiency: stunted growth, yellowing leaves (chlorosis).
**Magnesium** is needed for chlorophyll synthesis; deficiency causes yellow leaves.
Nitrogen, phosphorus, and potassium are essential for plant growth.

Investigating Requirements for Photosynthesis

Starch test: leaf boiled in water (kill cells), then hot ethanol (remove chlorophyll), then iodine solution.
**Chlorophyll** needed: variegated leaf – only green parts turn blue-black with iodine.
**Light** needed: destarch plant, cover part of leaf with foil; covered part remains orange-brown.
**Carbon dioxide** needed: destarch two plants; one in bell jar with sodium hydroxide (absorbs CO₂) – no starch; control with water – starch present.

Investigating Rate of Photosynthesis

Use pondweed (e.g.,×Elodea×or×Cabomba×) and count oxygen bubbles per minute.
Change **light intensity**: move lamp closer/further; use a glass tank to absorb heat.
Change **temperature**: heat/cool water bath.
Change **carbon dioxide concentration**: dissolve different amounts of sodium hydrogencarbonate.
Keep all other variables constant for a fair test.

Gas Exchange in Plants

Plants respire all the time (take in O₂, release CO₂).
During daylight, photosynthesis rate > respiration rate → net intake of CO₂ and net output of O₂.
At night, only respiration occurs → net release of CO₂.
Use **hydrogencarbonate indicator** to show CO₂ concentration: yellow = high CO₂, red = medium, purple = low.

Limiting Factors (Extended)

A **limiting factor** restricts the rate of photosynthesis when in short supply.
Three main factors: **temperature**, **light intensity**, **carbon dioxide concentration**.
Water is not usually a limiting factor as the amount needed is small.
Temperature: rate increases with temperature until enzymes denature (around 40–45°C).
Light intensity: rate increases until another factor becomes limiting.
CO₂ concentration: rate increases until another factor becomes limiting.
On a graph, the rising part shows the factor on x-axis is limiting; the plateau shows another factor is limiting.

Leaf Structure & Adaptations

**Waxy cuticle**: reduces water loss.
**Upper epidermis**: transparent to let light through.
**Palisade mesophyll**: tightly packed with chloroplasts for maximum light absorption.
**Spongy mesophyll**: irregular cells with air spaces for gas diffusion.
**Stomata** (mainly lower epidermis): allow gas exchange; **guard cells** control opening.
**Vascular bundles** (xylem and phloem): transport water/minerals and sucrose.
CO₂ pathway: atmosphere → air spaces → mesophyll cells → chloroplasts.

Identifying Leaf Structures (Dicotyledonous Plant)

Identify: **chloroplasts**, **cuticle**, **guard cells**, **stomata**, **upper/lower epidermis**, **palisade mesophyll**, **spongy mesophyll**, **air spaces**, **vascular bundles** (xylem and phloem).

Cross-section of a leaf showing cuticle, epidermis, palisade and spongy mesophyll, air spaces, stomata, and vascular bundles.

Internal structure of a chloroplast: envelope, stroma, grana (stacks of thylakoids), and starch grain.

Practice questions

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1.What is the green pigment found in chloroplasts that absorbs light energy for photosynthesis?
Easy
- AChlorophyll
- BCarotene
- CXanthophyll
- DAnthocyanin
2.Which of the following is the word equation for photosynthesis?
Easy
- Acarbon dioxide + water → glucose + oxygen
- Bglucose + oxygen → carbon dioxide + water
- Ccarbon dioxide + oxygen → glucose + water
- Dglucose + water → carbon dioxide + oxygen
3.A variegated leaf is tested for starch using iodine solution. The green parts turn blue-black, but the white parts remain orange-brown. What does this show?
Medium
- AChlorophyll is necessary for photosynthesis
- BLight is necessary for photosynthesis
- CCarbon dioxide is necessary for photosynthesis
- DWater is necessary for photosynthesis
4.In an experiment to investigate the effect of light intensity on the rate of photosynthesis, pondweed is placed in water and the number of bubbles released per minute is counted. Which of the following would be the most accurate method to measure the rate?
Hard
- ACount the number of bubbles produced in one minute
- BMeasure the volume of oxygen collected in an inverted measuring cylinder over a period of time
- CMeasure the change in mass of the pondweed
- DMeasure the change in pH of the water
5.Which of the following is NOT a raw material required for photosynthesis?
Medium
- AWater
- BCarbon dioxide
- CLight energy
- DChlorophyll
6.The diagram shows a cross-section of a leaf. Which layer is the palisade mesophyll?
Easy
- AA
- BB
- CC
- DD
7.In the diagram of a chloroplast, which structure is the site of the light-dependent stage of photosynthesis?
Medium
- AStroma
- BGrana (thylakoid stacks)
- CEnvelope
- DStarch grain
8.Which of the following is a function of the waxy cuticle on a leaf?
Medium
- AAllows gas exchange
- BReduces water loss
- CAbsorbs light for photosynthesis
- DTransports water

More questions

9.The balanced chemical equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. How many molecules of carbon dioxide are required to produce one molecule of glucose?
Hard
- A1
- B6
- C12
- D3
10.In an investigation of gas exchange using hydrogencarbonate indicator, a leaf in bright light causes the indicator to turn purple. What does this indicate?
Medium
- AHigh carbon dioxide concentration
- BLow carbon dioxide concentration
- CHigh oxygen concentration
- DLow oxygen concentration
11.A student investigates the effect of temperature on photosynthesis. At 35°C, the rate is lower than at 25°C. Which is the most likely explanation?
Hard
- AEnzymes involved in photosynthesis denature at high temperatures
- BCarbon dioxide becomes less soluble at higher temperatures
- CChlorophyll is destroyed at high temperatures
- DThe plant transpires too much and wilts
12.Which of the following leaf structures is mainly responsible for allowing carbon dioxide to enter the leaf?
Medium
- ACuticle
- BStomata
- CPalisade mesophyll
- DXylem
13.What is the name of the green pigment found in chloroplasts that absorbs light energy for photosynthesis?
Easy
- AChlorophyll
- BCarotene
- CXanthophyll
- DHaemoglobin
14.The balanced chemical equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. How many molecules of oxygen are produced for every molecule of glucose?
Medium
- A1
- B6
- C12
- D3
15.A student investigates the effect of light intensity on the rate of photosynthesis in pondweed. The number of bubbles produced per minute is counted. Which of the following should be kept constant to ensure a fair test?
Medium
- ATemperature and carbon dioxide concentration
- BLight intensity and temperature
- CBubble count and light intensity
- DDistance of lamp and bubble count
16.In an experiment using hydrogencarbonate indicator, a leaf in bright light causes the indicator to turn purple. What does this indicate?
Hard
- ACarbon dioxide concentration has decreased
- BOxygen concentration has increased
- CCarbon dioxide concentration has increased
- DOxygen concentration has decreased
17.In an experiment to investigate the effect of light intensity on the rate of photosynthesis, pondweed is placed in water and the number of bubbles released per minute is counted. Which of the following would increase the accuracy of the results?
Medium
- ACollecting the gas in a measuring cylinder and measuring the volume over a fixed time
- BCounting bubbles for 10 seconds and multiplying by 6
- CUsing a brighter lamp
- DAdding sodium hydrogen carbonate to the water
18.In an experiment to investigate the effect of light intensity on the rate of photosynthesis, a student places pondweed in a beaker of water and counts the number of bubbles produced per minute. Why is a glass tank placed between the lamp and the beaker?
Medium
- ATo absorb heat from the lamp and prevent temperature change
- BTo increase the light intensity reaching the pondweed
- CTo prevent carbon dioxide from entering the water
- DTo reduce the amount of oxygen dissolving in the water
19.A student investigates the effect of carbon dioxide concentration on the rate of photosynthesis using pondweed. Sodium hydrogen carbonate is added to the water to increase carbon dioxide concentration. Which of the following is the most accurate way to measure the rate of photosynthesis?
Hard
- ACount the number of bubbles produced per minute
- BCollect the gas in a measuring cylinder and measure the volume produced per minute
- CTime how long it takes for the pondweed to change colour
- DMeasure the change in pH of the water
20.In the starch test on a leaf, why is the leaf boiled in ethanol?
Easy
- ATo kill the cells and break down cell membranes
- BTo remove chlorophyll so colour changes can be seen clearly
- CTo soften the leaf for spreading
- DTo convert starch to glucose
21.A plant is kept in a dark cupboard for 24 hours before an experiment to investigate the need for light in photosynthesis. What is the purpose of this?
Medium
- ATo kill the plant
- BTo destarch the leaves
- CTo activate chlorophyll
- DTo increase the rate of respiration
22.In an experiment to test whether carbon dioxide is necessary for photosynthesis, a destarched plant is placed in a bell jar with a beaker of sodium hydroxide. After several hours in bright light, a leaf from this plant is tested for starch. What is the expected result and why?
Hard
- AThe leaf turns blue-black because photosynthesis occurred using carbon dioxide from the air
- BThe leaf remains orange-brown because sodium hydroxide absorbed all carbon dioxide, preventing photosynthesis
- CThe leaf turns blue-black because sodium hydroxide provides carbon dioxide for photosynthesis
- DThe leaf remains orange-brown because light could not reach the leaf
23.A student sets up three boiling tubes containing hydrogencarbonate indicator and a leaf from the same plant. Tube A is placed in bright light, tube B in dim light, and tube C in darkness. After a few hours, the indicator in tube A turns purple. What does this indicate?
Medium
- AThere is a high concentration of carbon dioxide in the tube
- BThere is a low concentration of carbon dioxide in the tube
- CThere is a high concentration of oxygen in the tube
- DThere is a low concentration of oxygen in the tube
24.A gardener finds that increasing carbon dioxide concentration in a greenhouse from 400 ppm to 600 ppm increases the rate of photosynthesis, but further increase to 700 ppm does not increase the rate further. Which factor is most likely limiting the rate at 700 ppm?
Hard
- ACarbon dioxide concentration
- BLight intensity
- CWater availability
- DOxygen concentration
25.Which of the following is the correct pathway for carbon dioxide from the atmosphere to a chloroplast in a leaf?
Easy
- AAtmosphere → stomata → spongy mesophyll air spaces → leaf mesophyll cell → chloroplast
- BAtmosphere → cuticle → palisade mesophyll → chloroplast
- CAtmosphere → xylem → mesophyll cell → chloroplast
- DAtmosphere → guard cell → chloroplast
26.A plant is deficient in magnesium. What effect would this have on photosynthesis?
Medium
- APhotosynthesis would increase because magnesium is not needed
- BPhotosynthesis would decrease because chlorophyll cannot be made
- CPhotosynthesis would stop because magnesium is a raw material
- DPhotosynthesis would be unaffected because magnesium is only used in respiration
27.A student investigates the effect of temperature on the rate of photosynthesis using pondweed. At 35°C, the rate is lower than at 25°C. Which is the most likely explanation?
Hard
- AThe enzymes involved in photosynthesis have denatured at 35°C
- BThe pondweed has used up all the carbon dioxide
- CThe light intensity was lower at 35°C
- DThe oxygen produced dissolves more easily at higher temperatures
28.In an experiment to test whether carbon dioxide is necessary for photosynthesis, a destarched plant is placed in a bell jar with a beaker of sodium hydroxide. Another destarched plant is placed in a bell jar with a beaker of water. Both are left in bright light for several hours. Which result would be expected?
Medium
- ABoth leaves turn blue-black with iodine
- BThe leaf with sodium hydroxide turns blue-black, the leaf with water remains orange-brown
- CThe leaf with sodium hydroxide remains orange-brown, the leaf with water turns blue-black
- DBoth leaves remain orange-brown with iodine
29.A student investigates the effect of light intensity on the rate of photosynthesis using pondweed. The number of bubbles produced per minute is counted. Which of the following is a valid way to vary light intensity?
Medium
- AChanging the temperature of the water
- BMoving the lamp closer to or further from the beaker
- CAdding different amounts of sodium hydrogen carbonate
- DUsing a different type of pondweed
30.A student investigates the effect of light intensity on the rate of photosynthesis of pondweed. The lamp is placed at different distances from the beaker. A glass tank of water is placed between the lamp and the beaker. What is the purpose of the glass tank?
Medium
- ATo absorb heat from the lamp, keeping temperature constant
- BTo focus the light on the pondweed
- CTo prevent carbon dioxide from escaping
- DTo increase the light intensity
31.In an experiment using hydrogencarbonate indicator, a leaf in bright light causes the indicator to turn purple. A leaf in the dark causes the indicator to turn yellow. What does this show?
Hard
- AIn bright light, the rate of photosynthesis is greater than the rate of respiration, so carbon dioxide is taken up.
- BIn bright light, the rate of respiration is greater than the rate of photosynthesis, so carbon dioxide is released.
- CIn the dark, the leaf does not respire, so no carbon dioxide is released.
- DIn the dark, the leaf photosynthesises at a low rate, so carbon dioxide is taken up.
32.A student sets up three boiling tubes containing hydrogencarbonate indicator and a leaf from the same plant. Tube A is placed in bright light, tube B in dim light, and tube C in the dark. After two hours, the colours are: A = purple, B = red, C = yellow. Which tube shows the greatest net uptake of carbon dioxide?
Medium
- ATube A
- BTube B
- CTube C
- DAll tubes show the same net uptake
33.Which of the following is the balanced chemical equation for photosynthesis?
Easy
- A6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- BC₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
- CCO₂ + H₂O → C₆H₁₂O₆ + O₂
- D6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
34.A plant is kept in a dark room for 48 hours. A leaf is then partially covered with aluminium foil and the plant is placed in sunlight for 6 hours. The leaf is tested for starch. Which result would be expected?
Hard
- AThe covered area remains orange-brown; the exposed area turns blue-black.
- BThe covered area turns blue-black; the exposed area remains orange-brown.
- CBoth areas turn blue-black.
- DBoth areas remain orange-brown.
35.In an experiment to test whether carbon dioxide is necessary for photosynthesis, a destarched plant is placed in a bell jar with a beaker of sodium hydroxide. Another destarched plant is placed in a bell jar with a beaker of water (control). Both are left in bright light for several hours. What result would show that carbon dioxide is necessary?
Medium
- AThe leaf from the plant with sodium hydroxide remains orange-brown, and the leaf from the control turns blue-black.
- BThe leaf from the plant with sodium hydroxide turns blue-black, and the leaf from the control remains orange-brown.
- CBoth leaves turn blue-black.
- DBoth leaves remain orange-brown.
36.The graph shows the effect of light intensity on the rate of photosynthesis at two different carbon dioxide concentrations. At point X, the rate is the same for both concentrations. What is the limiting factor at point X?
Hard
- ALight intensity
- BCarbon dioxide concentration
- CTemperature
- DWater availability
37.A student measures the rate of photosynthesis of pondweed by counting bubbles per minute. At 20°C and high light intensity, the bubble count is 30 per minute. When the temperature is raised to 35°C, the bubble count drops to 15 per minute. What is the most likely explanation?
Medium
- AThe enzymes involved in photosynthesis have started to denature.
- BThe rate of respiration has increased, using up more oxygen.
- CThe light intensity has decreased.
- DThe carbon dioxide concentration has increased.
38.A farmer increases the carbon dioxide concentration in his greenhouse from 400 ppm to 800 ppm. The rate of photosynthesis increases initially but then levels off. Which factor is most likely limiting the rate after the increase?
Hard
- ALight intensity
- BCarbon dioxide concentration
- COxygen concentration
- DWater availability
39.The diagram shows a chloroplast. Which part is the site of the light-dependent stage of photosynthesis?
Medium
- AGrana (thylakoid membranes)
- BStroma
- COuter membrane
- DStarch grain
40.A gardener notices that the leaves of his tomato plants are turning yellow. Which mineral deficiency is most likely?
Medium
- AMagnesium deficiency
- BNitrate deficiency
- CPhosphate deficiency
- DPotassium deficiency

Slides

What is Photosynthesis?

Green plants make glucose from carbon dioxide and water.
Oxygen is released as a waste product.
The process requires light energy trapped by chlorophyll.
Photosynthesis: manufacture of carbohydrates from raw materials using light energy.

What are the raw materials required for photosynthesis?

Carbon dioxide and water
Oxygen and glucose
Light and chlorophyll
Carbon dioxide and oxygen

Chlorophyll

Chlorophyll is a green pigment found in chloroplasts.
It reflects green light, giving plants their colour.
It absorbs light energy and transfers it into chemical energy for making carbohydrates.
Photosynthesis cannot occur without chlorophyll.

Chlorophyll is found in ____ within plant cells. It transfers energy from ____ into chemical energy.

Uses of Carbohydrates from Photosynthesis

Converted to starch for energy storage.
Converted to cellulose for cell walls.
Used in respiration to release energy.
Converted to sucrose for transport in phloem.
Can be turned into lipids and amino acids (with nitrogen).

Which of these are uses of carbohydrates produced in photosynthesis?

Converted to starch for storage
Used to make DNA directly
Converted to cellulose for cell walls
Used in respiration to provide energy

Leaf Structure

The leaf has a waxy cuticle to reduce water loss.
Upper epidermis is transparent to let light through.
Palisade mesophyll cells are packed with chloroplasts for photosynthesis.
Spongy mesophyll has air spaces for gas exchange.
Stomata (with guard cells) allow CO₂ in and O₂ out.

Which leaf layer is tightly packed with chloroplasts for maximum light absorption?

Spongy mesophyll
Palisade mesophyll
Upper epidermis
Waxy cuticle

How CO₂ Reaches Chloroplasts

CO₂ diffuses from the atmosphere into leaf air spaces through stomata.
Then it diffuses into mesophyll cells and finally into chloroplasts.
This pathway is driven by diffusion down a concentration gradient.

Carbon dioxide enters the leaf through ____ and diffuses through ____ spaces to reach mesophyll cells.

Investigating Photosynthesis – Starch Test

Starch is stored in chloroplasts where photosynthesis occurs.
Test for starch: boil leaf in water, then in hot ethanol to remove chlorophyll, soften in water, add iodine.
Blue-black colour indicates starch (photosynthesis occurred); orange-brown means no starch.
Variegated leaves show that chlorophyll is needed – only green parts turn blue-black.

Why is a leaf boiled in ethanol during the starch test?

To kill the cells
To remove chlorophyll
To soften the leaf
To extract starch

Investigating Light and CO₂ Needs

Destarch a plant by placing it in the dark for 24 hours.
For light: cover part of a leaf with foil; after light exposure, test for starch – covered area stays orange-brown.
For CO₂: use two plants in bell jars – one with sodium hydroxide (absorbs CO₂), one with water (control). Only the control produces starch.

Which of these are needed for a plant to produce starch in a leaf?

Chlorophyll
Light
Carbon dioxide
Oxygen

Rate of Photosynthesis – Pondweed Experiment

Use pondweed (Elodea or Cabomba) to measure oxygen bubbles.
Count bubbles per minute or collect oxygen volume.
Change one factor at a time: light intensity (move lamp), temperature (heat water), CO₂ concentration (add sodium hydrogen carbonate).
Keep other factors constant for a fair test.

In the pondweed experiment, what is used to change carbon dioxide concentration?

Sodium hydroxide
Sodium hydrogen carbonate
Ethanol
Iodine

Limiting Factors of Photosynthesis (Extended)

Limiting factor: something in short supply that restricts the rate.
Three main factors: temperature, light intensity, carbon dioxide concentration.
Water is not usually limiting as the amount needed is small.
Rate increases with factor until another factor becomes limiting.

Which of these is NOT usually a limiting factor for photosynthesis?

Temperature
Light intensity
Carbon dioxide concentration
Water availability

Balanced Chemical Equation (Extended)

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Light energy is converted to chemical energy in glucose bonds.
This is the reverse of aerobic respiration.

The balanced equation for photosynthesis is: 6CO₂ + 6H₂O → ____ + 6O₂.

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