IGCSE Biology Photosynthesis

IGCSE Biology Photosynthesis

Photosynthesis
– a process by which plants manufacture carbohydrates using sunlight, carbon dioxide and water
– by product : oxygen
– occurs inside chloroplasts which posses chlorophyll

Uses of Glucose
– is able to be combined to create starch for storage
– can be used to release energy during respiration
– is able to be transformed into fructose for the fruit
– can be converted into fat and oil for the seeds
– can be joined together to make cellulose for the cell wall
– can be combined with nutrients from the soil to make protein
Structures of leaves
1. Epidermis
●covers both upper and lower surfaces
●transparent: allows sunlight to pass into mesophyll cells
●protesct cells against mechanical injuries, fugal and bacteria invasions

2. Cuticle
●covers the epidermis
●a water-proof layer which prevents excess water loss by evaporation

3. Palisade mesophyll
●made up of tightly packed cylindrical shped cells to absorb more sunlight
●contain many chloroplasts to obtain a maximum rate of photosynthesis

4. Spongy mesophyll
●cells are irregular and loosely packed
●many air spaces for rapid diffusion of gases into and out of the cells
●fewer chloroplasts

5. Epidermal cells
●no chloroplasts
●transparent

6. Stoma / Stomata
●surrounded by guard cells which control the opening and closing of the stomata
●guard cells have many molecules
●fewer stomata on the upper surface of a lead to reduce water loss

7. Vascular bundles
●for the transport of solution containing chemical which platns need
●contains xylem and phloem

8. Xylem
●dead cell
●transports water and minerals from the roots to the leave from down to up – one direction
●provides mechanical support for the plant

9. Phloem
●transports nutrients away from the leaves – bi-direction

Requirements for photosynthesis
1. carbon dioxide in atmosphere
●diffuses through stoma -> intercellular space -> 1. diffuses into mesophyll cells 2.
dissolves into film of water on mesophyll cells

2. water
●xylem -> water -> moves from xylem into mesophyll by osmosis

3. oxygen
●for respiration -> diffuses into atmosphere

Transport of photosynthetic products
In dicotyledonous plants
●some glucose molecules can be oxidised to provide energy by respiration
●glucose molecules are quickly built into large starch molecules
●starch is converted to sucrose in the dark
●sucrose is transported to other parts ot eh plant by phloem

Compensation point
●respiration goes on all the time
●measure the photosynthesis rate above the compensation point
●eg. rate of photosynthesis = rate of respiration, no net exchange of carbon dioxide and
oxygen
●usually related to light intensity or carbon dioxide level
●meets the x axis

Limiting factor
●something present in the environment in such short supply that it restricts life processes
●light intensity, temperature, carbon dioxide concentration

1. Light intensity
a. initially, the rate of photosynthesis increases rapily with the increase in light intensity and hence more energy
b. increase stops when the light intensity reaches a saturation point stomata begin to prevent water loss
carbon dioxide does not enter the leaf slows down the rate of photosynthesis
c. light is no longer the limiting factor as other factor limits the rate, eg. carbon dioxide concentration after the saturation point

2. Carbon dioxide concentration
a. increase in carbon dioxide concentration due to the increase in raw materials and hence increases the rate of photosynthesis
b. when carbon dioxide concentration becomes the limiting factor, the final rate will level off at a higher rate
c. at the saturation point, photosynthetic rate is limited by the ability of the leaf to process the carbon dioxide that is delivered to it
d. limitation may be caused by insufficient light energy to drive the maximum rate of photosynthesis or by rate at which enymes catalyse the steps in photosynthetic carbon dioxide metabolism

3. Temperature
a. Lower than optimum temperature : temperature gradually increases and hence increasing the rate of photosynthesis – more collisions between substrate and enzymes
b. Slightly above the optimum temperature, the rate of photosynthesis slows down as water loss (evaporation) through the stomata (transpiration) – stomata begin to close – reduce the amount
of carbon dioxide in the leaf
c. High temperature – enzymes responsible for photosynthesis are denatured, slows down the rate of photosynthesis

How Greenhouse affects the rate of photosynthesis
● the glass wall helps to trap heat inside and maintain a closed system
● light intensity and carbon dioxide concentration can be changed easily
● can increase production of crops and improve their quality
● can also avoid the damage caused by insects and changes in the environmental conditions

Detection of starch produced from photosynthesis
1. Boil a leaf in water for a minute
● kills the lead and makes it permeable
2. Transfer the leaf to a boiling tube with some alcohol
Turn off the Bunsen burner (ALCOHOL / ETHANOL: FLAMMABLE)
Put the tube in a beaker of hot water for 10 minutes
Alcohol dissolves the chlorophyll thus decolorises the leaf

3. After decolorising the leaf, transfer it from the tube to hot water
Boil the lead in ethanol, makes it brittle, so water helps to soften it

4. Spread the leaf on a white tile
Add a few drops of iodine solution onto its surface
Observe color change, positive result: color change from brown to blue-black

Destarch
●make sure that starch found is produced during the experiment
●make sure before performing the experiment, it does not carry any starch

To show that light is necessary for photosynthesis
●masking the leaf with black paper or aluminium foil
●by comparing the masked and unmasked areas to find out

To show that carbon dioxide is necessary for photosynthesis
●potassium hydroxide or sodium hydroxide can be used to remove carbon dioxide from air
●set up A (controlled set up) – insert one of the leaves into a conical flask containing water
●set up B (experimental set up) – insert one of the leaves into a conical flask containing potassium hydroxide
● no photosynthesis can take place in the leaf in flask B as carbon dioxide is absent

To show that chlorophyll is necessary for photosynthesis
Test with variegated leaves
1. Destarch a variegated plant by putting it in the dark for 48 hours
2. Expose the whole plant to bright light for about 5 hours
3. Pick a leaf and test the leaf for strach

Nitrate
●important for the synthesis of protein and growth
●protein is made up of amino acids which contain nitrogen
●nitrogen is mainly absorbed in the form of nitrate and ammonium salts
●to build proteins, plants first make amino acids by combining sugars, made during
photosynthesis, with nitrate
●amino aids are joined together to build proteins which are used to make cytoplasm and enzymes

Deficiency disease
●reduced growth
●upper leaves turn pale green and bottom leaves become yellow or dead
●thinner stem
●smaller roots

Magnesium
●important for the synthesis of chlorophyll
●each chlorophyll molecule contains one magnesium atom
●plants need chlorophyll to trap light to provide energy during photosynthesis

Deficiency disease
●leaves turn yellow from the bottom of the stem upwards
●plant growth will suffer

Uses of nitrogen fertilizers in agriculture
●to increase crop yeilds
●intensive farming (repeatedly using the same land for crops) to remove nitrates from the soil, thus nitrates are needed to be replaces to prevent a drop in yeild
● consumption of nitrates > supply of nitrates

Nitrates can be replaced by:
●applying animal manure
●crop rotation
a. growing leguminous plants (eg. peas beans) and clover every two or three hours
b. theses plants develop root nodules containing nitrogen fixing bacteria, and the roots are ploughed into the soil, boosting nitrate levels
c. adding artificial fertilisers (eg. ammonium nitrate)

Dangers of overuse of nitrogen fertilisers
●applying too much nirogen fertiliser can result in water being drawn out of the plant roots by osmosis, wilting and death of plants occur
● leaching of excessive amounts of nutrients from the soil into lakes and rivers can lead to eutrophication

Eutrophication
1. Leaching
● nitrates are soluble, leached out of the soil by heavy rainfall and are carried into the nearest water system (eg. river)
2. Rapid algal growth
●presence of extra nitrates promotes growth of algae
3. Death of algae
●surface algal growth blocks light for algae below as the surface algae grow more quickly – > die
4. Death of bacteria
● as the algae die, they are decomposed by bacteria -> the bacteria respire aerobically, using up oxygen
5. Death of aquatic animals
● fish and other aquatic animals die from lack of oxygen
● fertiliser runoff from agricultural and residential land contributes plant nutrients to water bodies
● high nitrate levels in drinking water are also toxic, particularly for infants, small children, and pregnant women