Autotrophic Nutrition in Cormophyte Plants: Photosynthesis Process
Understanding Autotrophic Nutrition
Autotrophic nutrition in plants is a photosynthetic process where organic matter is formed from inorganic matter, using sunlight as an energy source. Plant nutrition has three phases:
- Alimentation (Feeding): This involves nutrient absorption and transport. Inorganic nutrients (crude sap) are absorbed and transported via the xylem to the cells. Organic substances are created through photosynthesis, and these organic nutrients (elaborated sap) are transported via the phloem to the entire plant.
- Metabolism: Nutrients in cells are involved in metabolic processes to produce energy (catabolism) or synthesize new molecules (anabolism).
- Excretion: This is the elimination of waste created during metabolism.
Cormophytes have specialized structures for nutrition, which provides a greater evolutionary advantage.
Nutrient Absorption
Water and mineral salts enter plants through root hairs located a few millimeters above the root cap. When they cross the epidermis, there are two ways to reach the xylem:
- Via the symplast: Most of the water and mineral ions pass through and reach the endodermis from one cell to another via plasmodesmata, which connect the cell cytoplasm.
- Via the apoplast: Much of the water and some ions circulate in the spaces between cells until they reach the endodermis membrane, where it crosses and follows the symplastic route to reach the xylem.
In the xylem, the concentration of crude sap is greater than that of the surrounding cells. As a result of osmosis, water and ions penetrate through active transport.
Crude Sap Transport
Crude sap ascends from the roots to the branches and leaves through the stem via xylem vessels. The upward movement is explained by the cohesion-tension theory:
- Cohesion: The attraction force between water molecules, due to hydrogen bonds, is enhanced by the small diameter of the vessels (capillarity).
- Tension: Transpiration, or water loss as vapor due to solar energy, creates a vacuum in the leaves that produces a pulling or tension force, pushing water up from the roots to the leaves.
Gas Exchange
Carbon dioxide (CO2) and oxygen (O2) enter through stomata and lenticels. They are disseminated through the intercellular spaces of parenchymal cells. CO2 travels towards the chloroplasts, and O2 to the mitochondria. The output of CO2 and O2 is the inverse of the input.
Stomata are formed by two guard cells. Changes in turgor pressure control the opening and closing of the stoma (ostiole). When turgid, the ostiole opens, controlling the exchange of O2, CO2, and water vapor. The active input of potassium ions (K+) in guard cells causes water to enter by osmosis, making the cells turgid and opening the ostiole, producing gas exchange and transpiration.
The mechanisms that regulate stomata aperture and closure are CO2 concentration, temperature, and light.
Photosynthesis
Chlorophyll parenchyma is responsible for photosynthesis. It is located in the leaves and green stems. Cells contain green chlorophyll in the chloroplasts.
Sunlight is captured by pigments, mainly chlorophyll, and the energy is used to split water and form organic compounds (glucose) from CO2, releasing O2. Photosynthesis occurs in two phases:
Light-dependent Phase
- Reducing power: When chlorophyll absorbs light energy, it releases excited electrons that are transported through a chain of proteins until they reach a final acceptor. Protons from water are reduced and form NADPH + H+.
- Chemical energy: As electrons move down the electron transport chain, part of the extra mass is lost as heat, and part is used to form chemical energy (ATP).
- Photolysis: The electrons lost by chlorophyll are replaced through the photolysis of water.
Light-independent Phase (Calvin Cycle)
Using ATP and NADPH + H+, CO2 is transformed into energy-rich molecules like glucose.
The global equation for photosynthesis, summarizing many reactions, is:
6CO2 + 12H2O + light -> C6H12O6 (glucose) + 6O2 + 6H2O
During the dark phase of photosynthesis, all organic molecules are formed. These molecules have Carbon, Oxygen, and Hydrogen from CO2 and H2O. They also need Nitrogen, Sulfur, and Phosphorus, derived from minerals absorbed by the roots. A more accurate equation for photosynthesis would be:
CO2 + H2O + minerals + light energy -> O2 + organic nutrients