Photosynthesis And 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**.
Energy is obtained when **chlorophyll** traps light energy from the Sun.
Word equation: carbon dioxide + water → glucose + oxygen.
Glucose stores chemical energy; oxygen is a waste product.

Chlorophyll and Chloroplasts

**Chlorophyll** is a green pigment found in **chloroplasts**.
It reflects green light, giving plants their green colour.
Chlorophyll absorbs light energy and transfers it into chemical energy during carbohydrate synthesis.
Chloroplasts are the site of photosynthesis.

Use and Storage of Carbohydrates (Extended)

Glucose is converted into **starch** for storage.
Glucose is converted into **cellulose** for cell walls.
Glucose is used in **respiration** to release energy.
Glucose is converted to **sucrose** for transport in phloem.
Carbohydrates can also be converted into **lipids** and **amino acids**.

Mineral Requirements (Extended)

Plants need **nitrate ions** to make amino acids (proteins).
Plants need **magnesium ions** to make chlorophyll.
Nitrate deficiency causes **stunted growth**.
Magnesium deficiency causes **yellow leaves** (chlorosis).

Testing Leaves for Starch

Starch is stored in chloroplasts; testing for starch indicates photosynthesis.
Procedure: kill leaf in boiling water → remove chlorophyll with hot ethanol → soften in water → add **iodine solution**.
Starch turns iodine **blue-black**; no starch leaves **orange-brown**.
**Variegated leaves** show that chlorophyll is needed: only green areas turn blue-black.
**Destarch** a plant by placing in darkness for 24 hours before testing for light or CO₂ need.

Investigating Requirements for Photosynthesis

**Light**: partially cover a destarched leaf with foil; after sunlight, covered area stays orange-brown (no starch).
**Carbon dioxide**: place destarched plant in bell jar with **sodium hydroxide** (absorbs CO₂); leaf stays orange-brown.
Control plant with water in bell jar turns blue-black.
**Chlorophyll**: variegated leaf test (see above).

Environmental Factors Affecting Rate

**Light intensity**: increases rate up to a point (limiting factor).
**Carbon dioxide concentration**: increases rate up to a point.
**Temperature**: increases rate up to optimum; above optimum, enzymes denature and rate falls.
Use aquatic plants (e.g.,×Elodea×) to measure oxygen bubbles per minute.
Keep other variables constant for a fair test; use a water bath to control temperature.

Gas Exchange in Plants (Extended)

Plants respire all the time (O₂ in, CO₂ out).
During daylight, photosynthesis rate > respiration rate; net CO₂ in, O₂ out.
At night, respiration > photosynthesis; net O₂ in, CO₂ out.
**Hydrogencarbonate indicator** changes colour with CO₂ concentration: purple (low CO₂), red (medium), yellow (high).

Balanced Chemical Equation (Extended)

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (with light energy and chlorophyll).
This is the reverse of aerobic respiration.

Leaf Structure and Adaptations

Leaves have a **large surface area** to absorb light.
Leaves are **thin** for efficient gas diffusion.
**Cuticle**: waxy layer reduces water loss.
**Upper epidermis**: transparent to let light through.
**Palisade mesophyll**: packed with chloroplasts for photosynthesis.
**Spongy mesophyll**: air spaces for gas circulation.
**Stomata** (pores) allow gas exchange; **guard cells** control opening/closing.
**Vascular bundles** (xylem and phloem) transport water, minerals, and sugars.

Internal structure of a dicotyledonous leaf showing cuticle, epidermis, palisade and spongy mesophyll, air spaces, stoma and guard cells.

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

Practice questions

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1.What is the main pigment involved in photosynthesis?
Easy
- AChlorophyll
- BCarotene
- CXanthophyll
- DAnthocyanin
2.Name the two raw materials needed for photosynthesis.
Easy
3.Oxygen is a waste product of photosynthesis.
Easy
True or false?
4.Complete the word equation for photosynthesis: carbon dioxide + water → glucose + ___
Medium
carbon dioxide + water → glucose + ____
5.Which part of a leaf contains the most chloroplasts?
Medium
- ASpongy mesophyll
- BPalisade mesophyll
- CLower epidermis
- DCuticle
6.Match the leaf structure to its function.
Medium
- Cuticle
- Stomata
- Spongy mesophyll
- Reduces water loss
- Allows gas exchange
- Contains air spaces for gas diffusion
7.How many carbon dioxide molecules are needed to produce one glucose molecule in the balanced equation for photosynthesis?
Medium
- A7
- B8
- C5
- D6
8.Arrange the following steps in the correct order to test a leaf for starch: (A) boil leaf in water, (B) add iodine, (C) boil leaf in ethanol, (D) rinse leaf.
Hard
- A
- C
- D
- B

Slides

Photosynthesis

Photosynthesis is the process by which plants manufacture carbohydrates from raw materials using energy from light.
The raw materials are carbon dioxide and water.
Chlorophyll traps light energy, which is converted into chemical energy.

What are the raw materials for photosynthesis?

Glucose and oxygen
Carbon dioxide and water
Chlorophyll and light
Starch and oxygen

Word Equation for Photosynthesis

The word equation is: carbon dioxide + water → glucose + oxygen.
Carbon dioxide diffuses into leaves from the air; water is absorbed from the soil by roots.
Glucose stores chemical energy; oxygen is released as a waste product.

In photosynthesis, carbon dioxide and water are converted into $____ and ____$ .

Chlorophyll and Chloroplasts

Chlorophyll is a green pigment found inside chloroplasts.
It absorbs light energy and transfers it into chemical energy during photosynthesis.
Chlorophyll reflects green light, giving plants their green colour.

Which of these are true about chlorophyll?

Chlorophyll is found in chloroplasts.
Chlorophyll absorbs green light.
Chlorophyll transfers light energy to chemical energy.

Uses of Carbohydrates in Plants

Glucose can be converted into starch for storage, cellulose for cell walls, or used in respiration.
It can also be converted to sucrose for transport in the phloem or to lipids and amino acids.
Some glucose is used to produce nectar to attract pollinators.

What is glucose converted into for storage in plant cells?

Cellulose
Sucrose
Starch
Lipids

Minerals in Plants

Plants need nitrate ions to make amino acids for proteins.
Plants need magnesium ions to make chlorophyll.
Deficiencies: nitrate deficiency causes stunted growth; magnesium deficiency causes yellow leaves.

Which mineral ion is needed for chlorophyll production?

Nitrate
Magnesium
Phosphate
Calcium

Testing Leaves for Starch

Starch is stored in chloroplasts and indicates photosynthesis has occurred.
Test procedure: boil leaf to kill it, then place in hot ethanol to remove chlorophyll, soften in water, add iodine.
Blue-black colour shows starch present; orange-brown shows no starch.

In the starch test, a leaf is placed in $hot ____ to$ remove chlorophyll.

Investigating Need for Chlorophyll, Light, and CO₂

Use variegated leaves to show chlorophyll is needed: only green parts turn blue-black with iodine.
Destarch a plant, then cover part of a leaf with foil; only the exposed part photosynthesises.
Use sodium hydroxide to absorb CO₂; a plant without CO₂ cannot photosynthesise.

In the experiment to show light is needed for photosynthesis, why is the plant destarched first?

To remove chlorophyll
To ensure no starch is already present
To increase the rate of photosynthesis
To make the leaf softer

Environmental Factors Affecting Photosynthesis Rate

Light intensity: increases rate up to a point.
Carbon dioxide concentration: increases rate.
Temperature: increases rate until enzymes denature.
Use aquatic plants like Elodea to measure oxygen bubbles as an indicator of rate.

Which factors affect the rate of photosynthesis?

Light intensity
Wind speed
Carbon dioxide concentration
Temperature

Leaf Structure and Adaptations

Leaves have a large surface area to absorb light.
They are thin for efficient gas diffusion.
Internal structures: cuticle, epidermis, palisade mesophyll, spongy mesophyll, air spaces, stomata, guard cells, vascular bundles.

Which leaf structure is responsible for gas exchange?

Cuticle
Stomata
Palisade mesophyll
Xylem

Gas Exchange in Plants

During the day, net movement: CO₂ in, O₂ out (photosynthesis rate $> respiration)$ .
At night, net movement: O₂ in, CO₂ out $(respiration > photosynthesis)$ .
Hydrogencarbonate indicator changes colour with CO₂ concentration.

At night, the net gas exchange in a leaf $is: ____$ diffuses in $and ____$ diffuses out.

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