photosynthesis

1) State the Word and Balanced Chemical Equations for Photosynthesis

• Photosynthesis is the process by which plants manufacture carbohydrates such as glucose, from raw materials using energy from light.
• Occurs in the chloroplasts of plant cells. Chloroplasts contain chlorophyll that converts light energy into chemical energy in molecules, for the synthesis of carbohydrates.

in the presence of
​light and chlorophyll

2) Outline the subsequent Use and Storage of the Carbohydrates made in photosynthesis

• In plant respiration (ie. in the mitochondria) to produce energy.
• Converted to starch, stored in roots, eg. potato.
• Converted to fats and oils, stored in seeds, eg. sunflower seeds.
• Make cellulose.
• Make amino acids.

3) Investigate the necessity of Chlorophyll, Light and Carbon Dioxide for Photosynthesis, using appropriate controls

3.1) Chlorophyll Is Necessary for Photosynthesis

1. Take a potted plant with variegated (green and white) leaves.
2. Destarch the plant by keeping it in complete darkness for about 48 hours.
3. Expose the plant to the sunlight for a few days and test for starch.
4. Only the green part will turn blue-black, showing presence of starch.

Independent variable – the presence or absence of chlorophyll.
Dependent variable – whether starch is present or not.
Control variables – methods and equipment used.

Test for Starch

1. Boil leaf for 2 minutes to break down cell walls, denature enzymes.
2. Warm in ethanol until leaf is colourless to extract chlorophyll.
3. Dipped in water briefly: to soften leaf.
4. Leaf is placed on a white tile and iodine is added.
5. If starch is present, colour will turn blue-black.
6. If starch is absent, it will remain yellow/orange.

3.2) Light Is Necessary for Photosynthesis

1. Destarch the plant by keeping it in darkness for 48hrs.
2. Place a stencil over part of a leaf.
3. Place the leaf in sunlight for 4-6 hours.
4. Remove the stencil and test for starch.
5. Only the part exposed to sunlight will turn blue-black, showing presence of starch.​

Independent variable – the presence or absence of light.
Dependent variable – whether starch is present or not.
Control variables – methods and equipment used.

source: science-stein

3.3) Carbon Dioxide is Necessary for Photosynthesis

1. Take two destarched potted plants.
2. Cover both the plants with bell jars and label them as A and B.
3. Inside A, keep NaHCO3 (sodium hydrogen carbonate). It produces CO2.
4. Inside B, keep soda lime. It absorbs CO2. Keep both the set-ups in the sunlight for at least 6 hours.
5. Test for starch on both plants.

Only the leaves of Plant A will turn blue-black, showing presence of starch.

                  A                                 b

source: aunibazilahbiologynotes

Independent variable – the presence or absence of carbon dioxide.
Dependent variable – whether starch is present or not.
Control variables – methods and equipment used.

4) Describe the Effects of Varying Light Intensity, Carbon Dioxide concentration and Temperature on the Rate of Photosynthesis

• Rate of photosynthesis is limited by the limiting factor.
• This is the factor which is least available to the plant.

source: biology-igcse

Light Intensity:

• As light intensity increases, the rate of photosynthesis increases.​
• But increasing the light intensity can lead to plant heating up above the optimum temperature.
  
• The limiting factor now is temperature.

 
Carbon Dioxide Concentration:

• As the amount of carbon dioxide increases, the rate of photosynthesis increases
• But increasing the amount of carbon dioxide after a certain point has not effect on rate of photosynthesis because now the limiting factors are light or temperature.

 
Temperature:

• Photosynthesis requires enzymes to carry out the reaction
  
• Enzymes have an optimum temperature, hence photosynthesis also has an optimum temperature.
• At very high temperature, these enzymes denature, thereby slowing the rate of photosynthesis.

How to reduce effects of these limiting factors:

• Place the plants in a greenhouse.
• Greenhouses trap heat, so plants are always kept at their optimum temperature.
• Artificial lighting can be used so that the plants can photosynthesize during the night as well.
• Carbon dioxide can be pumped into the greenhouse.

* 5) Investigate the effect of Gas Exchange of an aquatic plant using Hydrogencarbonate Indicator Solution

• Investigate the effect of gas exchange of an aquatic plant kept in the light and in the dark.
  
• Hydrogen carbonate indicator is used to show carbon dioxide concentration in solution.
• It is yellow in high concentrations of carbon dioxide, red in equilibrium with carbon dioxide in the atmosphere, purple in low concentrations of carbon dioxide.
• Place a leaf from a plant in a stoppered boiling tube containing some hydrogen carbonate indicator. You can then investigate the effect of light over a period of a few hours.
• Plant cells respire in the light and the dark, releasing carbon dioxide. In the light, photosynthesis can also happen, and carbon dioxide is absorbed from the air. If the light is bright enough, the rate of absorption becomes greater than the rate of release.​

leaf structure

6) Explain how the internal structure of a leaf is adapted for photosynthesis

• Plant leaves are adapted for photosynthesis to occur.
• ​This means limiting water loss, absorbing as much light energy as possible and allowing for efficient gas exchange.
  

source: pathwayz

Cuticle	Waxy layer that prevents water loss from top of the leaf, make leaf waterproof
Epidermis	Acts as a protective barrier around the leaf to prevent pathogens entering and causing ​harm to leaf. Cells are transparent to let sunlight pass through.
Palisade Mesophyll	These cells are found near the top of the leaf where most sunlight hits. Contain many chloroplasts to carry out photosynthesis.
​Spongy Mesophyll	Irregularly shaped cells that are loosely spaced, with large surface area to allow rapid diffusion of carbon dioxide, water and oxygen when the stomata are open.
Vascular Bundle	Plant's transport system. Made up of xylem and phloem.
​Xylem	Dead cells. ​Used to transport water and dissolved minerals through plant from roots via transpiration. Also has thick lignified walls made of cellulose to provide structural support.
​Phloem	Living cells. Used to transport sugars and food nutrients via translocation.
Guard cells and ​Stomata	Allow diffusion of carbon dioxide in, and water vapour and oxygen out of the leaf by diffusion. Guard cells control opening and closing of the stomata to limit water loss from the plant.

mineral requirements

7) Describe the importance of Minerals in Plants Growth

• Minerals are taken up actively by root hair cells.

Nitrogen ions	Magnesium
​Needed to build amino acids, thus protein and enzymes. ​Deficiency: plant not able to grow or repair itself. Poor growth.	​Needed to make chlorophyll. ​Deficiency: plant lack chlorophyll, photosynthesis cannot occur. Leaves yellow.

Nitrogen fertilizers:

• Provide nitrogen in the form of nitrate ions, nitrite ions or ammonium ions.
• But using fertilisers can lead to eutrophication, where fertilisers are transported by rain and leaches into water bodies e.g. pond or river.

Summary

You should know:

• the equation for photosynthesis
• the role of chlorophyll in photosynthesis
• the structure of a leaf
• how a leaf is adapted to carry out photosynthesis
  
• how a plant uses and stores the carbohydrate made in photosynthesis
  

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• why plants need nutrient ions and magnesium ions
• how to test a leaf for starch
• how to do experiment to investigate the need for chlorophyll, light and carbon dioxide for photosynthesis
• about the importance of a control in an experiment
• how to investigate the effect of light intensity on the rate of photosynthesis