PHOTOSYNTHESIS AND MINERAL NUTRITION

Objectives

This blog post provides readers with the following objectives. The reader will be able to:

·            Describe the process of photosynthesis.

·            Describe the structural adaptation of the leaf for photosynthesis.

·            Explain the conditions that affect the rate of photosynthesis.

·            Explain the biochemical nature of photosynthesis

 

PHOTOSYNTHESIS

Photosynthesis is the process by which chlorophyll containing organisms (i.e., plants and algae) synthesize complex organic molecules (such as glucose) from simple inorganic substances (such as carbon dioxide and water) in the presence of light. Oxygen is given out as a by-product.

Photosynthetic organisms e.g., plants, algae are referred to as photoautotrophs. Photosynthesis goes on principally in the leaves, or through any given part of the plant that can photosynthesize. However, the leaf is the main photosynthetic organ. Sunlight is absorbed by chlorophyll, a green pigment located in the chloroplasts. 

The overall chemical reaction is

6CO₂ + 6H₂O  →  C₆H₁₂O₆ + 6O₂

Requirements for Photosynthesis

1.  Carbon dioxide and water (as raw materials for photosynthesis)

2.   Chlorophyll and sunlight (as conditions for photosynthesis).

Importance of Photosynthesis

1.  Provides food for animals.

2. The plants depend on the glucose for their growth and energy.

3. It replenishes oxygen content of the atmosphere.

4. It produces materials for protein and lipids formation.

5. Forms the base for food chains and food webs.

6. Provides oxygen as substrate for respirations.

7. It prevents greenhouse effects by to remove carbon dioxide from the atmosphere.

Adaptations of Leaf for Photosynthesis

1. Presence of impervious cuticle layer presents water and carbon dioxide from the leaf.

2. Upper epidermis is a single layer of cells on the upper surface of a leaf. It allows light to pass to the mesophyll cells.

3. The cells of palisade layer are vertically arranged immediately below the epidermis to ensures maximum light absorption.

4. Chloroplasts contain chlorophyll for absorbing sunlight.

5. The presence of network of veins allows water to reach the photosynthetic cells.

6. Presence of stomata allows carbon dioxide to enter into the leaf. 

7. The leaf is thin to ensure easily diffusion of carbon dioxide and easy penetration of light into the mesophyll cells.

8. The broad and flat lamina provides a large surface area for absorption of sunlight.

9. Presence of air spaces in the spongy mesophyll allows diffusion of carbon dioxide into the palisade cells.

Experiment on Photosynthesis

Experiment I: Testing for Starch in a Leaf

Aim: To test a leaf for starch in a leaf

Apparatus and Materials

Beaker, boiling tube, heat source, Iodine solution, ethanol, water and a green leaf.

Procedure 

Half fill a beaker with water and boil the water. 

Pluck a healthy green leaf of a plant which was in the sunlight.

Place the leaf in boiling water for one or two minutes. This denatures the enzymes and stops chemical reactions.

Now transfer the leaf to a beaker containing alcohol.

Warm it over a water bath for a few minutes to remove the chlorophyll from the leaf. The leaf becomes decolorized. 

Dip the leaf in water to remove the alcohol, soften and make it permeable.

Add iodine solution to the leaf surface.

 

Observation and Conclusion: The treated leaf after two to three minutes will turn blue-black. This indicates the presence of starch.

Experiment II: Showing that Carbon Dioxide is Necessary for Photosynthesis

Aim: To show that carbon dioxide is necessary for photosynthesis.

Apparatus and Materials

Two potted plants, polythene bags, two bell jars, concentrated sodium hydroxide solution (Soda lime), Iodine solution, water, soda lime.

Procedure

Place two potted plants in the dark for 24 hours to destarch the leaves. Pluck a leaf from the plant and test it for starch. is there any starch in the leaf? 

Enclose one leaf in a conical flask labelled A, containing soda lime, and another leaf in another conical flask containing labelled B, without soda lime. Pass the leaf stalk through a split cork which has been grease to keep the flask air tight. 

What is the role of the soda lime in flask A? Why should the flask be air tight? 

Support each flask with a clam. place the plant in sunlight for about 3 hours. Remove the leaves from the flasks and test each for starch. record our observation in each case. 

Observation: Only the leaf (one without KOH or soda lime ) will turn blue-black showing the presence of starch. This happens because KOH absorbs the CO₂ present inside one bell jar. As a result, the leaves do not get CO₂ for photosynthesis.

Conclusion: Carbon dioxide is necessary for photosynthesis.

Experiment III: Showing that Oxygen is given off during Photosynthesis

Aims: To show that oxygen is given off during photosynthesis.

Apparatus and materials: -

Beaker, short stemmed funnel, two thick wooden cubes or plasticine support, sodium hydrogen carbonate powder, test tube, water plant e.g., Elodea or Spirogyra, can be used too, glowing splint.

Procedure

Water plant is placed in a beaker of water covered with a funnel.

The funnel is placed on a support (wooden cubes) to allow free circulation of air and water.

Small amount of NaHCO₃ is added to ensure sufficient supply of CO₂ for photosynthesis.

A test-tube filled with water is inverted over the funnel 

The set-up is placed in sunlight for about 3 hours. 

Control experiment is set-up and placed in a dark cardboard for about 3 hours

Observations

Bubbles of gas were observed in the test tube of the experimental set-up. No bubbles of gas were observed in the control. The gas was tested with glowing splint which was rekindled into flames.

N/B; one property of oxygen is it supports burning. 

Conclusion: Oxygen is given off during photosynthesis

Experiment IV: Showing that Chlorophyll is Necessary for Photosynthesis

Example of Variegated Leaves

Aim: To show that chlorophyll is necessary for photosynthesis.

Apparatus and Materials

Plant that has variegated leaves, ethanol, beaker, boiling tube, water, heat source, Iodine solution.

Procedure

Detach a leaf from a variegated plant, which has been exposed to sunlight for 2-6 hours.

Make a labelled diagram of the leaf to show the green and non green parts.

Test the leaf for starch.

Make a labeled diagram to show the results.

 

Observation: The green parts (containing chlorophyll) will turn blue-black while the non-green parts (lacking chlorophyll) will stain with the color of iodine (orange- brown).

Conclusion: Starch is made only in areas of the leaf with chlorophyll, showing that chlorophyll is necessary for photosynthesis.

Experiment V: Showing that Light is Necessary for Photosynthesis

Aim: To show that light is necessary for photosynthesis.

Apparatus and Materials

Well-watered potted plant, dark paper, scissors / knife, scalpel, pins and clips

Procedure

Keep a potted plant in the dark for 24 hours.

On one of the leaves, stick black paper strips (one below and one above the leaf) with the help of clips. Make a sketch of the leaf showing the covered and uncovered parts.

Now, place this plant in sunlight for a few hours. Pluck the leaf, remove the black strips and test for the presence of starch.

Make a labeled diagram to show the results.

Observation: The entire leaf will turn blue-black except in the region that had been covered. This region did not receive light and hence no starch is present.

Conclusion: Starch is produced only in areas that received light, showing that light is necessary for photosynthesis.

Factors Affecting Rate of Photosynthesis

There are three major factors that affecting the rate of photosynthesis; Light intensity, Carbon dioxide concentration, and Temperature

These three factors are called Limiting Factors. A slight change in a limiting factor can have an adverse effect on the rate photosynthesis.

 Other factors include; chlorophyll concentration, water and pollution

Temperature

The stages of photosynthesis require enzyme activity. Temperature greatly affects enzymes activities. The rate of photosynthesis increases with increasing temperature. Temperatures ranging near 0^oC deactivate the enzymes affecting photosynthesis. Similarly, very high temperatures about 40⁰C, denature the essential enzymes prohibiting photosynthesis. The ideal temperature range is 25 to 35^oC.

Light Intensity

Light is needed for the light dependent stage of photosynthesis. The rate of photosynthesis increases linearly with increasing light intensity until it reaches its saturation point.  At this point, the rate of photosynthesis remains constant. Too much light at a high intensity can damage chloroplasts.

Light quality

Light is a combination of different wavelengths or colors. Photosynthesis occurs only in the visible spectrum, i.e., wavelength which ranges from about 400 nm to 750 nm. The rate of photosynthesis is most rapid in the red (700 nm) and blue rays (400 nm). The graph below shows how much light is absorbed by chlorophyll at each wavelength of light.

Chlorophyll Concentration

Chlorophyll affects the rate of reaction as it absorbs the light energy.  Lack of chlorophyll (or deficiency of chlorophyll) results in chlorosis or yellowing of leaves. It can occur due to disease, mineral deficiency or aging (senescence).

Carbon Dioxide Concentration 

In the atmosphere, the concentration of carbon dioxide ranges from 0.03 to 0.04 %.  It is found that 0.1% of CO₂ increases the rate of photosynthesis significantly. This is achieved in the greenhouses which are enclosed chambers where plants are grown under controlled conditions. The concentration is increased by installing gas burners which liberate carbon dioxide as the gas burns. Crops like tomatoes, lettuce are successfully grown in the greenhouses. Greenhouse crops are found to be better-yielding than those growing in natural conditions.

Water

Water is an essential factor in photosynthesis. A slight deficiency of water reduces crop yield. It also limits the quantity of carbon dioxide. This is because drying the leaves close their stomata in order to conserve water which in effect prevent carbon dioxide from entering the leaves.

Pollution  

Soot, like ozone and sulphur dioxide have an adverse effect on photosynthesis. Soot normally blocks stomata and reduce the transparency of the leaves. Pollution of water affects the hydrophytes. The capacity of water to dissolve gases like carbon dioxide and oxygen is greatly affected.

Biochemical Nature of Photosynthesis

Photosynthesis has two main stages or reactions: Light reaction (Light Dependent Reaction) and Dark reaction (Light Independent Reaction). The light reactions occur in the grana and the dark reactions take place in the stroma of the chloroplasts.

Chloroplast and Chlorophyll

Chloroplasts are double membrane organelles with an inner membrane folded into disc-shaped sacs called thylakoids. Thylakoids, containing chlorophyll and other accessory pigments, are in stacks called granum (plural: grana). Grana are connected to each other and surrounded by a gel-like material called stroma.

Chlorophyll is a light-absorbing pigment in plant cells. There are two types of chlorophyll: chlorophyll a and chlorophyll b. They absorb light in the red and blue wavelengths, making the plants leaf look green.

Light Reaction

The light reaction or stages of photosynthesis occurs in the grana of the chloroplast.

The chlorophyll molecules in the grana trap energy in the sunlight. The chlorophyll molecule becomes excited or ionized and electrons are transferred through a number of electron carriers or acceptors but finally return to the chlorophyll molecule to stabilize it.

During the electron transfer, energy is released which combine ADP and inorganic phosphate to form ATP (Adenosine triphosphate).

ADP + Pi  →   ATP

Light energy also split water molecules into hydrogen ion and hydroxyl ions in a process called photolysis of water or photochemical splitting of water.

2H₂O  →  4H⁺  +  4^e-  +  O₂

The oxygen is evolved as by-product into the atmosphere.

The hydrogen ions are converted into hydrogen atoms which are used to reduce NADP (nicotinamide adenine dinucleotide phosphate) to NADPH₂. 

 2NADP + 4H⁺ + 4^e- → NADPH₂

Both ATP and NADPH₂ are used in the dark stage

Using light energy to combine inorganic phosphate (Pi) to Adenosine diphosphate (ADP) to produce Adenosine triphosphate ATP is referred to as Photophosphorylation.

Dark Reaction (Calvin Cycle)

It involves the reduction of carbon dioxide using reduced NADP and ATP produced in the light-dependent reactions of photosynthesis.

It takes place in the stroma of the chloroplast. It was discovered by Melvin Calvin, James Bassham, and Andrew Benson at the University of California.

There are 3 stages to this process: Carbon dioxide fixation, Carbon dioxide reduction and Ribulose bisphosphate regeneration.

Carbon Dioxide Fixation

This process is called fixation because atmospheric carbon dioxide is converted into an organic compound.

Carbon dioxide combines with ribulose bisphosphate, RuBP (a 5 carbon compound) to form an unstable 6 carbon compound by enzyme ribulose bisphosphate carboxylase (Rubisco).

The unstable 6-carbon compound splits into two molecules of a 3-carbon compound called glycerate-3-phosphate.

For every 6 molecules of CO₂ entering the cycle, 12 molecules of glycerate-3-phosphate are produced.

Carbon Dioxide Reduction

Glycerate-3-phosphate is reduced into phosphoglyceraldehyde or triose phosphate using NADPH₂ and ATP produced in the light stage. 

Two molecules of triose phosphate are removed from the cycle, to be converted into glucose or fructose.

Glucose is also the monomer used in the synthesis of polysaccharides (e.g. starch).

Ribulose Bisphosphate Regeneration

In a series of complex reactions, the remaining molecules of triose phosphate are converted into 5-carbon compound ribulose monophosphate.

Ribulose monophosphate is converted into ribulose bisphosphate, using a phosphate group from ATP.

Fate of Products of Photosynthesis

The main product of photosynthesis is glucose and oxygen.

1. Glucose is used up by actively respiring cells during respiration to release energy.

2. Together with nitrates, sulphates and phosphates, glucose is used in the synthesis of amino acids which combined to form proteins.

3. Glucose can be used in the formation of fats.

4. Some of the glucose may be converted to sucrose or starch and stored in storage organs like the seeds, bulbs, tuber and fruits.

5. Oxygen, by-product may be used by the cells in respiration. The excess passed out through the stomata.

Mineral Nutrients in Plants      

Sixteen mineral nutrients are essential for plant growth and development. The mineral nutrients are dissolved in water and absorbed through a plant's roots. These elements are classified into two: macronutrients and micronutrients.

Macro Elements or Macronutrients

Elements like calcium, magnesium, nitrogen, phosphorus, carbon, hydrogen, oxygen and sulphur are required by plants in large amounts and are called major or macronutrients (at least 1 mg/g of dry matter).

Micro Elements or Micronutrients

Elements like manganese, boron, copper, zinc, molybdenum and chlorine are required in minute quantities. Hence, they are called minor, micro, rare or trace elements.

 

Function and Deficiency Effects of Micro and Macro Nutrients on Plant Growth and Development 

Nutrients	Function in plants	Effects of Deficiency
Nitrogen	-    formation of protein; nucleic acid; DNA/RNA/nitrogen bases; - formation of enzymes - it is part of the chlorophyll molecule	-   stunted growth    -   yellow on the older leaves
Phosphorus	- formation of proteins, ATP and nucleic acid - acts as buffer in the cell sap	- reddish purple leaves and stunted growth - delay in maturity
Calcium	- Cell wall formation - plant root and tip elongation - healthy growth	-  terminal buds fail to develop
Sulphur	-    formation of certain amino acids	-  stunted growth -  yellow patches on leaves
Magnesium	- Activates some enzymes - Synthesis of chlorophyll	-   internal chlorosis of older leaves
Iron	- formation of chlorophyll - enzyme systems	-  chlorosis with pale leaves
Potassium	-       activates enzymes involved in photosynthesis and protein metabolism -       play important role in stomatal opening -       helps disease resistance	-       tips and margins of leaves turn brown - weakening of straw in grain crops
Chloride	-    aids plant metabolism - it also plays a role in photosynthesis	-       reduced root growth in nutrient cultures
Boron	-  translocation of sugars across membranes - germination of pollen grains and growth of pollen tubes	-      necrosis in young leaves and stunting
Zinc	-      part of the enzyme systems which regulate plant growth -      plays an essential role in DNA transcription	-       interveinal chlorosis -       reduction in rate of shoot growth
Molybdenum	- for nitrogen fixation by rhizobia bacterial - reduction of nitrite	- interveinal chlorosis
Copper Cu+2	- copper is a catalyst in chlorophyll formation	- plants wilt and develop a bluish green cast

Effects of Minerals on Plants Using Water Culture (Hydroponics)

Hydroponics or hydro-culture is a method of growing plants using mineral nutrient solutions in water, without soil. In natural conditions, soil acts as a mineral nutrient reservoir but the soil itself is not essential to plant growth. When the required mineral nutrients are introduced into a plant's water supply artificially, soil is no longer required for the plant to thrive.

Advantages

1. No soil is needed for hydroponics 

2. Stable and high yields

3. Ease of harvesting

4. No pesticide damages

5. Plants grow healthier

Disadvantage

1. Without soil as a buffer, any failure to the system can leads to rapid plant death

2. The changes in solution composition causes change in pH which may lead to chloroses

Water Culture Experiment

Aim: To study whether nitrogen, phosphorus and magnesium are essential for plant growth.

Materials

Glass jars, jar lids, aluminium foil, non-absorbent cotton wool, maize seedlings, distilled water  

Procedure

Prepare a complete culture solution based on Sachs solution.

1000 cm^-3 (1dm^-3) distilled water

0.25g potassium nitrate

 0.25g magnesium sulphate

 0.25g potassium acid phosphate

1g calcium nitrate

2 drops iron (III) chloride solution

       

1.  Every jar is filled with different culture solutions with different mineral deficiencies: NO^3-, PO₄^3-, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, Fe²⁺. The last jar is filled with perfect culture solution in which all the minerals are present
2.   To investigate the effect Nitrogen on plant growth, omit the nitrates and use potassium chloride and calcium sulphate. To investigate whether Magnesium is really needed for plant growth, omit magnesium sulphate and use potassium sulphate. Etc.
3.     Label each jar with the missing nutrient.
4.  Take maize seedlings of same age with roughly equal size height. Carefully remove the endosperm from each grain so that the seedling has no alternate source of food.
5.   Fix one seedling into each jar through the holes in the lid or cover.
6.      Use non-absorbent cotton wool to support the plant (not too tight).
7.    Rap the outside of the glass jars with black papers to prevent light from entering the glass jars.
6.  All the jars are placed at a spot where the seedlings will receive sufficient light intensity for 4-5 weeks.

7. Observe the seedling in each jar every week and take note of the following changes; leaf color, stem width, root length, number of leaves etc.

Results: The seedling in the complete culture solution will grow healthier and stronger with dark green leaves. Whiles seedlings growing in other solutions lacking nutrients are likely to be smaller with certain disorders.  

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