Effect of percentage concentration of lead compounds in rate of photosynthesis

How does the rate of photosynthesis (measured in terms of biomass) of aquatic plant Elodea depends on the percentage concentration of Lead (II) solution ?

The issue chosen for the investigation has a firm environmental aspect. Industrial waste, human interventions is a major cause of water pollution. Heavy metal poisoning is one of the major form of water pollution. Among all heavy metals, poisoning due to the presence of Lead (II) is most toxic. These heavy metal causes interferes with the growth of both plants and animals. This in turn affects the growth of the plant and thus the biomass of it. Although there are various methods to measure the rate of photosynthesis like measuring the number of bubbles, biomass is chosen as the variable here because it relates to the amount of matter which can be used to generate fuel. This investigation focuses to discuss the effect of an environmental factor – heavy metal poisoning on the amount of fuel generated which is another environmental concern studied through the lens of biology.

Photosynthesis is a process occurring in plants where carbon dioxide and water as raw materials to produce oxygen and glucose as products in presence of chlorophyll and sunlight. Chlorophyll absorbs a particular wavelength of the sunlight which enables this reaction to happen. The reaction happens in two different stages – “light dependent reaction which occurs in thylakoid and light independent reaction that occurs in stroma”.¹

The reaction of photosynthesis is given below

Carbon dioxide + Wate  \(\rightarrow\)   Glucose + Oxygen + ATP

This reaction involves the conversion of energy from sunlight into chemical energy contained within the ATP molecules (Adenosine triphosphate).

‘Biomass refers to the organic material that is used for production of energy’². This is a broad term and mainly comprises of living beings or recently dead organisms. Chemically they constitute of C, H and other volatile organic compounds. These compounds undergoes combustion to produce energy. In biology, the terms biomass and dry weight are interchangeable. To measure this, the plant is taken out and allowed to dry in an oven. This heating process makes the plant lose it’s water content. After this, the mass of the plant is recorded. This mass is considered as the dry mass or biomass of the plant.

During photosynthesis, plants produce glucose which is the food of the plant. Formation of glucose causes the plant to gain mass. This forms the basis of the process to measure the rate of photosynthesis by recording the biomass of a plant. Hence, more the rate of photosynthesis, more is the amount of glucose formed, more is the mass gained by the plant and thus more the biomass of the plant. So, the biomass is a measure of the rate of photosynthesis.

Lead enters the plant system through the permeable channels in the roots. Various factors of the soil like pH, temperature as well as properties of the roots like surface area, ‘degree of mycorrhizal transpiration’ dictates the amount of lead absorbed by the root hair cells. Lead interferes with various biological factors that influences with the growth of plant. It is primarily stored in the endodermis. Lead present in the soil exists as a cation with two units of positive charges. The cell wall of root hair cells bears negative charges which enables the Lead ions to be trapped and stored there.

Lead can bind with various enzymes that plays an active role in the Calvin cycle and other metabolic processes of plants. It also ruptures the structure of chloroplasts. Lead has strong affinity towards the negatively phosphate groups in ATP and thus they hinder the production of ATP molecules in plants. Absorption of Lead also increases the absorption of other dissolved minerals which eventually leads to the deficiency of carbon dioxide that  reduces the rate of photosynthesis. The detrimental effects of ‘lead toxicity’ is mainly observed in – reduced rates of germination, synthesis of the pigment-chlorophyll and also in lower rate of transpiration.

Percentage concentration of Lead (II)
Lead (II) has been used as this is the most toxic heavy metal among all other heavy metals that causes water pollution. The percentage concentrations used are – 1.00%, 2.00%, 3.00%, 4.00% and 5.00%. The percentages used are mass percentages. For example- 1.00% solution means that 1.00 g of Lead (II) salt is dissolved in 100 cc of tap water. To make the investigation more realistic, tap water has been used instead of distilled water. Usually, the values of concentration in water is in ppm (parts per million) but this investigation aims to use the values in % concentration as preparing solutions with concentrations expressed in ppm is not feasible.

The biomass of the plant
The mass of the plant used will be measured before it was added to the beaker and the mass was again recorded after 10 days. After 10 days, the plant was kept in an oven to allow it to dry and lose the water content. Following this, the mass of the plant was recorded again and the biomass was calculated as the difference of the initial and the final mass. The biomass is a measure of the rate of photosynthesis. More the rate of photosynthesis, more the biomass of the plant.

As discussed in the background section, lead toxicity reduces the rate of photosynthesis and thus hinders the growth of the plant. Both of these would lead to reduction in the overall mass of the plant. Thus, a negative correlation is predicted between the percentage concentration of Lead and the biomass of the plant.

Paper title – “Effects of Lead on Plant Growth, Lead Accumulation and Phytochelatin Contents of Hydroponically-Grown Sesbania Exaltata”

It is authored by Jacqueline McComb. It was published in the journal World Environment. In this article the effect of Lead on the growth of coffee plants (Sesbania Exaltata) was studied. The concentration of Lead (II) solutions were 0.1 milli mole to 20.0 milli mole. The results indicates that – “plant roots and shoots displayed symptoms of toxicity as evidenced by their decreased biomass with increasing concentrations of lead”.

• Intensity of light
  Intensity of light that the plant is exposed to has a major impact on the rate of photosynthesis. As the intensity of the light radiated upon the leaf increases, more amount of chlorophyll molecules are excited to absorb the required electromagnetic radiation and initiate the process of photosynthesis. Thus, it is necessary to ensure that the intensity of light incident on the plant is same in all cases. To do this, all plants were kept at the same location near a window where they receive the same amount of sunlight.
• Temperature
  Like all other bio-chemical reactions, photosynthesis is also influenced by the temperature at which it occurs. Higher the temperature more the rate at which it occurs. Again it must be noted that photosynthesis is an enzymatic process, so there is an optimum temperature at which the rate of photosynthesis is maximum. Thus all trials were studied over the same period so that temperature fluctuations do not have much effect on the reliability of the data collected.
• Amount of sample used
  More the mass of the plant used in the sample, more the chlorophyll and thus higher the rate of photosynthesis. Thus, the amount of plant used in all the trials were kept the same. The mass of the plant used in all the trials were 2.00 g in all cases.
• Type of sample used
  The type of sample used in all the trials are identical. Differences in the type of plants may affect the rate of photosynthesis. So, the sample used was – ‘Elodea’ in all cases.
• Time period
  Longer the time period of study more the plant photosynthesize. So, the period of investigation was kept constant at 10 days in all cases.
• Heating time
  The samples after a time period of 10 days were heated to obtain their dry mass. Thus, it is necessary to ensure that the plant reaches a constant mass and all the water is lost. For this, all the plants were heated in the oven for 30 minutes.

• Elodea plant –
• Digital mass balance-1
• Scissor – 1
• Glass beaker-100 cc – 25 pieces
• Spatula
• Lead nitrate – 50.00 g
• Graduated measuring cylinder – 1 piece with capacity of 100 cc
• Electric oven
• Tongs

• Use protective clothing like lab coats and safety gloves.
• Do not touch the electrical oven in wet hands.
• Use spatula to transfer reagents.
• Do not expose your skin to Lead salts.
• Do not consume any of the solution.
• Do not bring any eatables in the laboratory.

• A 100 cc glass beaker was taken.
• It was filled with 100 cc tap water using a graduated measuring cylinder.
• 1.00 ± 0.01 g of Lead nitrate was weighed in a digital mass balance and added to the beaker.
• The solid was dissolved in water by stirring with a glass rod.
• 2.00 g of the plant Elodea was added to the beaker. To control the mass of the plant sample a digital mass balance was used. A scissor was used to cut the leaves of the plant and adjust the mass of the plant.
• The beaker containing the plant was labelled as 1.00%.
• Same steps from 1 to 6 were done for four other beakers.
• These beakers were placed near a window.
• After 10 days, the plant was taken out and allowed to dry in air.
• To dry the sample completely and lose the water content of it, it was kept in an electric oven for 30.00 minutes.
• After that a tong was used to take the plant out and the mass of it was recorded.

The data was collected for all other concentrations in the same way. Five data sets were used for each values of concentration. For collecting data in control environment, the experiment was also repeated in 0.00% solution using only tap water.

For the plants growing in high concentrations of Lead – 4.00% and 5.00%, the leaves of the plant turned yellow indicating the occurrence of chlorosis. Some of the leaves even started to fall down. Wilting of leaves was also observed. This indicates that these concentration of Lead was highly toxic and detrimental in terms of the growth of the plant.