Investigating the effects of different lead chloride concentrations on the growth of cress seedlings

AS and A Level Science

A-Level Biology Coursework:

Ying-Jun Ng

Candidate number: 2135

Aim

To investigate the effect of different lead chloride concentrations on the growth of cress seedlings

Background Information

Lead (Pb) is the heaviest non-radioactive metal with an atomic number of 82 and atomic weight of 207. Lead is a soft, malleable but dense, ductile heavy metal with poor electrical conductivity. It is highly resistant to corrosion, and because of this property, it is often used to contain a variety of corrosive liquids. Lead is commonly used in building construction, paints, batteries, plumbing, ammunition and fusible alloys.

Metallic lead does occur in nature, but it is rare. Lead is usually found in ore with zinc, silver and copper, and is extracted together with these metals. Apart from how it is found naturally, lead is released into the environment by mining and smelting before it makes its way into the air, soil, and water. Lead can also remain in the environment as dust and as compounds in soil or water. Soils near roads and smelting facilities have higher levels of lead than soils in other areas because of their exposure to lead dust, which accumulates over time. Plants exposed to lead can absorb the metal dust through their leaves. Plants can also take up minimal amounts of lead from the soil. In large amounts lead is potentially life threatening and dangerous to humans and plants alike.

For humans, over exposure to lead can cause many symptoms and lead to illnesses. Lead poisoning is a medical condition caused by high levels of lead in the blood. Lead can enter the body by over exposure through inhalation, ingestion and contact. The symptoms of lead poisoning include nausea, abdominal pain, insomnia, metallic taste in the mouth, lethargy and in extreme cases seizures, comas, and even death. For plants, heavy metals kill plant life by disrupting many vital reactions of a plant. It prevents any new plants from growing so is very dangerous to the environment. The growth of a plant is dependant on the surrounding conditions of the plant. Plants will only grow when the correct conditions are provided; these are water availability, amount of carbon dioxide, amount of light intensity, presence of enzymes, and amount of chlorophyll in the plant. There need to be adequate amounts of each of these substances in order for the plant to grow.

In order for a plant to grow, energy is required. Energy needed for growth is obtained through photosynthesis. Photosynthesis (Light-dependant and light independent reactions) is a process by which green plants and photosynthetic organisms use the energy of light to convert carbon dioxide and water into the simple sugar glucose and oxygen. In doing so, photosynthesis provides the basic energy source for virtually all plants. Photosynthesis is

Light

Carbon dioxide + water glucose + oxygen

Chlorophyll

Plants utilise glucose as an energy source to grow leaves, flowers, fruits, and seeds. They also convert glucose to cellulose, the structural material used in their cell walls. Most plants produce more glucose than they actually use, so they store it in the form of starch in the roots, stems, and leaves. When required the plants can then draw on these reserves for extra energy or building materials. By changing or removing one of the factors that plant growth is dependant on plants will not grow. For example, a plant that has little access to light will grow slower than a plant that is completely exposed to light. This is because the plant has little light exposure hence light energy from the sun so it grows more slowly due to low photosynthetic rate. The presence of an enzyme will increase plant growth as photosynthesis will occur faster than usual.

Lead is not an essential component for plant growth however; it can be readily absorbed and accumulated by different parts of the plant. The rate of absorption depends on the conditions of the plant and the surrounding environment. If there is a high concentration of lead in the soil, if the plant is drained of water, or if the plant had low phosphate levels, lead will move into the plant root cells by diffusion. According to the law of diffusion, molecules will move from an area of high concentration (in this case, soil) to an area of low concentration (root cell.) The root cell would contain lots of and ions, hence high solute potential and would cause water to move into the cell by osmosis to re-establish the osmotic balance. However, the plant tissues that have absorbed lead will have a low water potential as lead blocks the cell membrane (symplast pathway) and so water cannot enter.

Small amounts of lead will not affect plant growth, but a very high lead concentration is toxic to plants. In the environment plants need to be near sources of lead emission, where concentrations are very high before the toxic effects on the plant can be observed. The main toxic effect of high concentrations of lead on a plant is the effect on plant growth. Lead will stop plant growth because lead acts as a non-competitive irreversible enzyme inhibitor. Enzymes are required for many metabolic reactions such as respiration and photosynthesis.

An enzyme is a biological catalyst, a molecule that speeds up the rate of reaction but remains unchanged at the end of the reaction. In a reaction, enzyme molecules and substrate molecules are constantly moving and frequently colliding. Enzymes are proteins and their function is determined by their complex 3-D structure. The enzyme has an area called an active site that is in a specific shape. Only one type of substrate molecule with the complementary shape will be able to bind with the enzyme and create an enzyme-substrate complex. The R groups of the amino acids in the active site interact with the substrate so that the reaction can occur readily. The products are formed from the breakdown of the substrate and the products then leave the active site of the enzyme. The active site becomes free again to bind with another substrate.

Since lead is a non-competitive irreversible inhibitor the rate of many enzyme controlled reactions within the plant and these will eventually slow down and stop. Non-competitive means that lead ions will bind onto a part of the enzyme other than the active site. This distorts the tertiary structure of the enzyme by changing the shape of the active site; this prevents any substrate from binding with the enzyme therefore no reactions will occur. It is an irreversible inhibitor since lead ions bind to the sulphydryl groups of amino acids in the enzyme. Heavy metal ions have a very high affinity for sulphur and bind very strongly to the amino acids. The lead ions cannot be removed from the enzyme and so all reactions of the enzyme will be stopped; this is known as irreversible inhibition.

An important enzyme that lead inhibits is ribulose bisphosphate carboxylase that is used to make ribulose bisphosphate (RuBP), which is used in the vital in the Calvin cycle. The Calvin cycle is the second (light independent) stage of photosynthesis. The Calvin cycle does not need light to occur, however it does need the products (reduced NADP and ATP) of the first reaction (light dependant) of photosynthesis. In this cycle carbon dioxide (a 1 carbon compound) binds with RuBP (a 5C carbon compound.) This creates an unstable intermediate (a 6 carbon compound), which quickly breaks down into two molecules of Glycerate 3-Phosphate (a 3 carbon compound.) In the presence of ATP and reduced NADP (from the light dependant stages of photosynthesis), Glycerate 3-Phosphate is reduced to 2 molecules of triose phosphate (a 3 carbon compound.) From here, many different molecules are produced from triose phosphate, e.g. glucose, amino acids, lipids, sucrose, starch, and cellulose. The remaining or other triose phosphates use ATP and the enzyme Ribulose bisphosphate carboxylase to create RuBP so the cycle can start again. As lead will inhibit Ribulose bisphosphate carboxylase the Calvin cycle cannot occur. There will be no fixation of carbon dioxide and so no glucose or other molecules will be made causing the plant to have a deficiency of these molecules.

In addition, in high concentrations lead will also inhibit chlorophyll synthesis. Chlorophyll is the green plant pigment used in photosynthesis; it is responsible for capturing the light energy needed for the light dependant stage of photosynthesis. Chlorophyll a and b, photosynthetic pigments, are arranged in light harvesting clusters called photosystems. In a photosystem, accessory pigments (chlorophyll a, b and carotenoids) surround a primary pigment molecule (chlorophyll a.) The accessory pigments absorb light energy and pass it to the primary pigment where the light energy is used to excite electrons in the Z-scheme in the light dependant stage of photosynthesis. There are two main photosystems, photosystem I where the peak absorption of chlorophyll a is at 700 nm, and photosystem II where the peak absorption of chlorophyll a is 680nm.

The light dependant stage of photosynthesis uses the light energy absorbed by the photosystems to create ATP and reduced NADP, which is later used in the light independent stage of photosynthesis. Before photosynthesis can occur water is needed. The photolysis of water occurs where a water-splitting enzyme catalyses the break down of water into two hydrogen ions, (H+), two electrons, (e-), and one molecule of oxygen (½ O2). The electrons are currently at a low energy level. However once they are transferred to photosystem II (p680) by electron carriers, the absorbed light energy excites the electrons to a higher ...