Plant Nutrition Mechanisms: Photosynthesis and Nutrient Uptake
Core Functions of Organic Molecules
Plants require organic molecules for two primary functions:
- To obtain the energy required for vital functions.
- To increase size, develop, and form new organs.
Autotrophic Organisms
In addition to plants, other organisms are autotrophs, including many protists, Monera, and algae.
Phases of Plant Nutrition
Plant nutrition involves two main parts:
- Organic Synthesis (Photosynthesis): The process by which light energy is transformed into chemical energy, converting inorganic molecules (CO₂, H₂O) into organic molecules (sugars).
- Utilization of Organic Matter: Organic matter is degraded through cellular respiration, releasing energy (ATP) necessary for life processes.
Essential Plant Nutrients
Nutrients are categorized into two types:
-
Macronutrients: Essential elements required in large amounts:
- C, H, O, N, K, Ca, Mg, P, and S.
-
Micronutrients: Essential elements required in very small amounts:
- Fe, B, Zn, Cu, Al, Mo, Na, Cl, Si, Mn, Co, etc.
Water Absorption and Transport
Plants require intensive amounts of water because they lose up to 98% of the water absorbed by the roots through evaporation. Plants in arid climates have specialized coverings that hinder gas exchange, which is managed through structures called stomata.
Mechanisms for Inorganic Compound Uptake
Inorganic compounds enter the plant via two main mechanisms (the rate of penetration varies according to temperature, pressure, and other factors):
- Passive Transport (Diffusion): Movement down a concentration gradient.
- Active Transport: Movement requiring energy (ATP).
Water enters through the absorbent root hairs and passes through layers of intermediate cells to reach the xylem.
Understanding Water Potential
Water potential is the combined effect of solute concentration and pressure. Water moves from an area of higher water potential (less negative) to an area of lower water potential (more negative).
Transpiration and Water Flow
Transpiration, the evaporation of water from the leaves, creates a vacuum (negative pressure) that pulls water up from the roots. This phenomenon, exploiting the attraction between water molecules, is called the flow of water. Stomata regulate this process; when they close, the loss of water and the flow through the cells stop.
Capillarity and Cohesion
Capillarity leverages the strong cohesive forces between water molecules. This allows water to form continuous columns within the narrow xylem vessels (often less than one millimeter in opening), causing a continuous rise of water against gravity.
Photosynthesis: Capturing Solar Energy
The plant captures approximately 1% of the energy received from the sun. This energy is stored as chemical energy accumulated in the covalent bonds of organic molecules, such as carbohydrates.
The Process and Structures of Photosynthesis
Photosynthesis produces oxygen and organic compounds from carbon dioxide, mineral salts, and water. This process occurs in organelles called chloroplasts, which are filled with chlorophyll. Chloroplasts have two outer membranes and a third internal membrane system consisting of sacs called thylakoids. The thylakoid membranes house the chlorophyll and photosystems.
When chlorophyll absorbs light, it becomes excited and loses an electron, initiating a series of chemical reactions. Photosynthesis consists of two main parts:
Light-Dependent Reactions (Light Phase)
This phase requires light and takes place in the thylakoid membrane. It generates the energy carriers needed for the dark phase. Two key processes occur:
- Photodissociation of Water: Water molecules are split into hydrogen (H) and oxygen (O).
- Formation of ATP and NADPH: Energy and reducing power are generated.
Light-Independent Reactions (Dark Phase)
This phase takes place in the stroma of the chloroplast. The hydrogen carried by NADPH and the energy from ATP are used to reduce carbon dioxide, forming glucose. This process is known as the Calvin Cycle.
Cellular Respiration in Plants
Cellular respiration utilizes carbohydrates (often stored as starch) as fuel for the cells. In this process, carbohydrates are oxidized and transformed into CO₂ and water, releasing energy (ATP) to perform essential functions.
During the day, the plant performs both photosynthesis and cellular respiration. During the night, when light is unavailable, the plant performs only cellular respiration.
Alternative Nutritional Strategies
Plants can also obtain nutrients through heterotrophic means, such as symbiosis and parasitism.
Symbiotic Relationships
Symbiosis involves two organisms deriving mutual benefits. Two important examples are:
Rhizobium and Legumes (Nitrogen Fixation)
This is a symbiosis between a plant and bacteria. Rhizobium bacteria, which live in the soil, are attracted by the plant, absorbed by the roots, and penetrate the cells. Inside the root nodules, the bacteria fix atmospheric nitrogen, which is essential for the plant to produce organic matter.
Mycorrhizae (Fungus and Root Symbiosis)
This involves a fungus and a terrestrial plant root. The fungus effectively extends the root system and provides the plant with raw sap (water and minerals).
Carnivorous Plants
Autotrophic carnivorous plants perform photosynthesis but live in places with nutrient-poor soils (especially lacking nitrogen). Their specialized leaves have the ability to capture and feed on insects and small organisms to supplement their nutrient intake.
Parasitic Plants
Parasitic plants live on other living plants, disrupting them by absorbing necessary substances. There are two types:
- Holoparasites: These plants do not perform photosynthesis. They capture and draw all necessary sap (both crude and processed) from their host, such as Cuscuta (Dodder).
- Hemiparasites: These plants possess chlorophyll and perform photosynthesis, but they capture only crude sap (water and minerals) from their host, such as mistletoe.