Topic 9- Photosynthesis

Photosynthesis

Process whereby plants use light energy (photo) to synthesise (make) glucose. 

Photosynthesis can only take place in the green parts of the plants.  

These plants cells contain chloroplats where photosynthesis takes place 

 

Why is photosynthesis important?

Coverts light energy to chemical energy which is stored in the glucose.

This chemical energy is realsed when glucose is broken down during respiration

Unused glucose plants

Unused glucose is converted into starch and stored,  

mainly in: 

• Leaves  

• Roots  

• Seeds 

 

Glucose → starch 

 

Unuse glucose animals

Unused glucose is converted into glycogen and stored mainly in: 

• Liver  

• Muscles  

 

Glucose → glycogen 

Chloroplast

A chloroplast is the organelle in plant cells where photosynthesis takes place.

• Contains pigment called chlorophyll → absorbs light energy (at certain wavelengths)

• Uses its energy to convert carbon dioxide + water into glucose + oxygen

• Found mainly in leaf cells

Word equation for photosynthesis

Carbon dioxide + water → glucose + oxygen

Chemical equation for photosynthesis

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

What type of reaction photosynthesis is 

-Sunlight transfers energy to the chlorophyll  

-Energy is absorbed for photosynthesis as it is endothermic

-Energy is stored

Definition endothermic

Means a reaction that takes in/absorbs energy from its surroundings, usually as heat

5 uses of glucose in plants 

1. Respiration → releases energy for plant processes.

2. Stored as starch → for energy storage.

3. Made into cellulose → for plant cell walls.

4. Made into fats/oils → for energy storage, especially in seeds.

5. Made into amino acids/proteins → glucose provides carbon, with nitrate ions supplying nitrogen.

Why it is an advantage to the plant to store carbohydrates as starch rather than as sugar

• Starch does not dissolve in cell water.

• Therefore, it does not lower the water potential of the cell.

• This means it does not cause water to enter by osmosis, which could make cells swell.

• Starch is also compact, so lots can be stored.

• When needed, starch can be converted back into glucose for respiration.

What is the importance of glucose being converted into starch? 

Starch is insoluble whereas glucose is solube. 

This means starch can be stored and used later on. 

Test for photosynthesis

Method

1. Boil the leaf in water for 1–2 minutes to kill it and stop chemical reactions.

2. Place the leaf in ethanol in a hot water bath to remove the chlorophyll (green pigment). Ethanol is flammable, so do not heat it directly.

3. Rinse the leaf in warm water to soften it.

4. Place the leaf on a white tile.

5. Add iodine solution to test for starch.

Results

• Blue-black: Starch is present, indicating that photosynthesis has occurred.

• Orange-brown: No starch is detected.

Conclusion

If the leaf turns blue-black, it contains starch, which suggests that the plant has been photosynthesising.

Exam tip: Remember the order: Boil → Ethanol → Rinse → Iodine.

Why would you leave a plant in a dark cupboard for around 24 hours before testing? 

To use up stored starch so any new starch is made during the experiment. 

Why do you put the leaf in boiling water? 

To kill the leaf and stop chemical reactions

Why is hot ethanol added? 

To remove chlorophyll, making the colour change easier to see. 

Why rinse the leaf in cold water? 

To soften the leaf because ethanol makes it brittle. 

What does the iodine do? 

Tests for starch. It turns blue-black if starch is present. 

In bright sunlight, why can carbon dioxide limit the rate of photosynthesis?

There is enough light but not enough CO₂.

Increasing CO₂ would increase photosynthesis.

Give 3 environmental factor, other than light intensity and carbon dioxide concentration, which can limit the rate of photosynthesis

• Temperature – Low temperatures slow down the enzymes involved in photosynthesis.

• Water availability – A lack of water reduces the rate of photosynthesis.

• Mineral availability – A lack of minerals, such as magnesium, reduces chlorophyll production and limits photosynthesis.

Validity definition

How consistent your results are

Independent Variable 

Dependent Variable

The variable that you change in an investigation

Control Variable 

The variables that you keep the same x3 minimum

Reliability  

How sure you are the results you obtained are only due to the independent variable

Method for investigating the rate of photosynthesis

Method

1. Place a piece of pondweed in a beaker of water containing sodium hydrogencarbonate (to supply carbon dioxide).

2. Position a lamp 10 cm away from the pondweed.

3. Allow the pondweed to adjust to the light.

4. Count the oxygen bubbles produced in one minute.

5. Repeat the experiment at different distances (20, 30, 40 and 50 cm).

6. Repeat each measurement three times and calculate the mean.

Conclusion investigating the rate of photosynthesis

As the distance from the lamp increases, light intensity decreases, so the rate of photosynthesis usually decreases.

Limiting factor definitionM

A limiting factor is a factor that restricts the rate of a reaction because it is in short supply. Increasing it will increase the rate of the reaction.

Why does the conclusion for investigating the rate of photosynthesis happen?

Light provides the energy needed for photosynthesis.

Increasing light intensity increases the rate of photosynthesis until another factor becomes limiting.

Evaluating investigation the rate of photosynthesis

Limitation	Improvement
Bubbles can be different sizes.	Measure the volume of oxygen using a gas syringe.
Temperature may increase due to the lamp.	Use an LED lamp and monitor temperature.
Counting bubbles can be inaccurate.	Repeat measurements and calculate a mean.

Why do we count oxygen bubbles?

Because oxygen is produced during photosynthesis, so counting bubbles estimates the rate of photosynthesis.

Why must temperature be kept constant?

Because temperature also affects the rate of photosynthesis.

Why does the rate stop increasing at high light intensity?

Because another factor, such as carbon dioxide concentration or temperature, becomes limiting

How can we calculate relative light intensity?

relative light intensity = 1/d2

d=distance from lamp

Explain the relationship between distance and mean number of bubbles released.

As distance increases, fewer bubbles are released because light intensity decreases

State 3 uses of glucose in plants 

• Respiration – To release energy.

• Storage – Converted into starch for later use.

• Growth – Used to make cellulose for cell walls.

Define limiting factor

A limiting factor is something that slows down or restricts the rate of a process,

such as photosynthesis.

What happens when there is an increase of temperature

As temperature increases, the rate of photosynthesis increases until it reaches an optimum temperature.

After this point, the rate decreases rapidly

Parts of a Leaf and their Adaptations

Part of leaf	Adaptation	Function
Waxy cuticle	Thin, transparent, waterproof layer	Reduces water loss while allowing light through
Upper epidermis	Thin and transparent	Allows sunlight to reach palisade cells
Palisade mesophyll	Contains many chloroplasts; located near the top of the leaf	Absorbs maximum light for photosynthesis
Spongy mesophyll	Loosely packed cells with large air spaces	Allows gases to diffuse easily through the leaf
Air spaces	Large spaces between cells	Allow rapid diffusion of CO₂ and O₂
Xylem	Long, hollow vessels	Transports water and mineral ions to the leaf
Phloem	Living transport tissue	Transports sugars from the leaf to other parts of the plant
Lower epidermis	Contains many stomata	Allows gas exchange
Stomata	Tiny pores, mainly on the underside of the leaf	Allow CO₂ to enter and O₂ and water vapour to leave
Guard cells	Can change shape to open and close stomata	Control gas exchange and reduce water loss

Limiting factors

Limiting factor	Effect on photosynthesis
Light intensity	More light increases photosynthesis until another factor becomes limiting.
Carbon dioxide (CO₂) concentration	More CO₂ increases photosynthesis until another factor becomes limiting.
Temperature	Low temperatures slow enzyme activity. High temperatures can denature enzymes.
Amount of chlorophyll	Less chlorophyll means less light can be absorbed.

What do plants need for photosynthesis? 

• Light – provides energy. 

• Water (H₂O) – absorbed through the roots. 

• Carbon dioxide (CO₂) – absorbed from the air. 

• Chlorophyll – absorbs light energy. 

 

Where enzymes

Organ	Enzyme produced	Function
Salivary glands	Amylase	Breaks down starch into maltose
Stomach	Protease (pepsin)	Breaks down proteins into smaller peptides
Pancreas	Amylase	Breaks down starch
Pancreas	Protease	Breaks down proteins
Pancreas	Lipase	Breaks down fats into fatty acids and glycerol
Small intestine	Maltase	Breaks down maltose into glucose
Small intestine	Peptidases	Break down peptides into amino acids

What is meant by a deficiency?  

Not having enough or lack something 

Where to plants obtain the minerals from?  

The soil 

Which minerals to plants require?  

Magnesium, Nitrates, Potassium, Calcium, Sulphur  

Mineral deficiencies in plants

Mineral	What is it needed for?	Height of shoot	Length of roots	Colour of leaves
Nitrates (N)	Making amino acids and proteins for growth	Short / stunted	Short	Older leaves turn yellow and may shrivel
Magnesium (Mg)	Making chlorophyll for photosynthesis	Short / reduced growth	Short / reduced growth	Older leaves turn yellow, often between green veins
Phosphates (P)	Making DNA, cell membranes and ATP (energy transfer)	Short / stunted	Poor root growth	Leaves may become dark green or purple
Potassium (K)	Helping enzymes work in photosynthesis and respiration	Short / reduced growth	Poor root growth	Older leaves turn yellow or brown, especially at the edges

Symptoms of mineral deficiencies

Mineral deficiency	Symptoms
Nitrates (N)	Stunted growth, short shoots and roots, older leaves turn yellow and may shrivel
Magnesium (Mg)	Yellowing between the veins of older leaves, reduced photosynthesis and growth
Phosphates (P)	Poor root growth, stunted shoots, leaves may turn dark green or purple
Potassium (K)	Poor growth, older leaves turn yellow or brown at the edges, weak stems

Phloem vs Xylem

	Phloem	Xylem
Transports what molecules?	Sucrose and amino acids	Water and mineral ions
Unidirectional or bidirectional?	Bidirectional (up and down)	Unidirectional (roots to leaves)
Living or dead tissue?	Living cells	Dead cells (vessels)
Special features?	Sieve plates with pores, companion cells provide energy for transport	Thick lignified walls for support, hollow tubes with no end walls

Xylem – Water transport

Function: Transports water and mineral ions from the roots to the leaves.

• Direction: Upwards only (unidirectional).

• Cells: Dead cells joined together to form hollow tubes.

• Special features:

  • No end walls → water flows easily.

  • Thick walls strengthened with lignin → prevent collapse and support the plant.

• Process: Water moves through the xylem in the transpiration stream.

Phloem – Food transport

Function: Transports sucrose (sugar) and amino acids around the plant.

• Direction: Upwards and downwards (bidirectional).

• Cells: Living sieve tube elements supported by companion cells.

• Special features:

  • Sieve plates with pores → allow sap to pass between cells.

  • Companion cells contain many mitochondria → provide energy for transport.

• Process: The movement of sugars and amino acids is called translocation.