Unit 4 – Plant Nutrition and Photosynthesis Notes

Plants need nutrients and light energy to grow, survive, and carry out life processes. This unit covers mineral nutrition and photosynthesis, two core topics in plant physiology.

Plants and mineral nutrition

Plants get their nutrients from the soil, water, and atmosphere. This process of acquiring and using mineral elements is called mineral nutrition. These nutrients are essential for a plant’s growth, development and metabolic functions.

Essential Mineral Elements

Scientists have identified about 17 elements that are essential for plant life. They are divided into two categories based on the amount a plant needs:

• Macronutrients: These are required in large quantities. The primary macronutrients are Nitrogen (N), Phosphorus (P), and Potassium (K). Other important ones include calcium (Ca), magnesium (Mg), and sulfur (S).
  • Nitrogen (N): A key component of proteins, nucleic acids (DNA and RNA), and chlorophyll. A deficiency leads to yellowing of older leaves.
  • Phosphorus (P): Essential for energy transfer in the form of ATP, and for the formation of cell membranes and nucleic acids. Deficiency can cause stunted growth and a dark green or purplish color in leaves.
  • Potassium (K): Plays a vital role in regulating water balance, activating enzymes, and is crucial for the opening and closing of stomata. Deficiency can cause yellowing and scorching of leaf edges.
• Micronutrients: These are required in very small amounts. They include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl). Despite being needed in small quantities, a deficiency in any of these can severely impact plant health.

How Plants Absorb Nutrients

Plants absorb mineral nutrients primarily through their root system. This happens in two main ways:

1. Passive Absorption: This process does not require the plant to expend energy. Minerals move from the soil into the root cells along a concentration gradient.
2. Active Absorption: This process requires the plant to use energy (ATP) to move minerals against a concentration gradient. This is crucial for acquiring nutrients that are in low concentration in the soil.

Mineral Deficiency Symptoms

When a plant doesn’t get enough of an essential mineral, it shows specific symptoms. These symptoms can often help in diagnosing which nutrient is lacking.

• Chlorosis: The yellowing of leaves due to a lack of chlorophyll, often caused by a deficiency in nitrogen, iron, or magnesium.
• Necrosis: The death of plant tissue, often seen as brown or black spots on leaves, caused by a lack of nutrients like calcium or potassium.
• Stunted Growth: Overall small size of the plant, often a sign of a deficiency in phosphorus.
• Wilting: A lack of potassium can impair the plant’s ability to regulate water, leading to wilting.

Nitrogen Metabolism and the Nitrogen Cycle

Nitrogen metabolism is the set of biochemical processes that plants use to absorb, convert, and assimilate nitrogen into organic compounds. While the atmosphere is about 78% nitrogen gas (N2​), most plants cannot directly use it due to the strong triple bond between the two nitrogen atoms. Therefore, plants primarily absorb nitrogen from the soil in the form of nitrate (NO3−​) and ammonium (NH4+​) ions.

The key steps are:

1. Nitrogen Fixation: The conversion of atmospheric nitrogen (N2​) into a usable form like ammonia (NH3​). This is the most crucial step as it makes nitrogen available to plants.
2. Nitrification: The process where specific soil bacteria convert ammonia to nitrites (NO2−​) and then to nitrates (NO3−​), the form most easily absorbed by plants.
3. Assimilation: Plants absorb nitrates or ammonium ions from the soil and incorporate them into organic molecules like amino acids and proteins.
4. Ammonification: When plants and animals die or excrete waste, decomposers (bacteria and fungi) convert the organic nitrogen back into ammonia.
5. Denitrification: Certain bacteria in the soil convert nitrates back into atmospheric nitrogen (N2​), completing the cycle.

Biological Nitrogen Fixation

Biological Nitrogen Fixation (BNF) is a natural process performed by specialized prokaryotes (bacteria and archaea) that convert atmospheric nitrogen (N2​) into ammonia (NH3​). This process is catalyzed by the enzyme nitrogenase and is energetically expensive, requiring a significant amount of ATP.

BNF can be categorized into:

• Symbiotic BNF: This occurs in a mutualistic relationship between nitrogen-fixing bacteria and host plants. The most well-known example is the association between Rhizobium bacteria and leguminous plants (e.g., peas, beans, clover). The bacteria live in specialized root nodules, where they fix nitrogen and provide it to the plant in exchange for carbohydrates.
• Non-symbiotic (Free-living) BNF: This is performed by free-living bacteria in the soil, such as Azotobacter (aerobic) and Clostridium (anaerobic), and by cyanobacteria (blue-green algae) like Nostoc and Anabaena.

Autotrophic Nutrition

Autotrophic nutrition is a type of nutrition in which organisms produce their own food from simple, inorganic substances like carbon dioxide and water. They are known as autotrophs and are the primary producers in most ecosystems. The most common form of autotrophic nutrition is photosynthesis, which uses light energy, though some organisms use chemical energy (chemosynthesis). In contrast, heterotrophic nutrition involves organisms obtaining food by consuming other organisms.

Photosynthesis

Photosynthesis is the process used by plants, algae, and some bacteria to convert light energy into chemical energy, which is stored in glucose. The process uses water and carbon dioxide as reactants, with oxygen being a byproduct. The overall equation for photosynthesis is:

Photosynthesis occurs in two main stages:

1. Light-Dependent Reactions: Occur in the thylakoid membranes of chloroplasts. Light energy is captured by photosynthetic pigments and used to split water molecules (photolysis) to produce oxygen, ATP (energy), and NADPH (a reducing agent).
2. Light-Independent Reactions (Calvin Cycle): Occur in the stroma of chloroplasts. The ATP and NADPH from the light-dependent reactions are used to convert carbon dioxide into glucose. This stage does not require light directly.

Photosynthetic Pigments

Photosynthetic pigments are molecules that absorb light energy from the sun. The primary pigments are chlorophylls, which give plants their green color. There are several types of chlorophyll, with chlorophyll a being the main photosynthetic pigment, while chlorophyll b and others act as accessory pigments.

Other accessory pigments include:

• Carotenoids: These pigments are responsible for the yellow, orange, and red colors in plants. They absorb light in a different part of the spectrum and transfer the energy to chlorophyll a, widening the range of light the plant can use for photosynthesis. They also protect chlorophyll from damage by excessive light.
• Phycobilins: Found in cyanobacteria and red algae, these pigments are very efficient at absorbing green light.