Plant Vascular Systems and Photosynthesis Mechanisms
Ascent and Transportation of Crude Sap (Xylem)
The rise of crude sap takes place through the xylem vessels in woody plants, usually against gravity. This process requires the presence of several mechanisms that help the sap ascend:
Root Pressure: Root cells have a much higher concentration of solutes than the external environment. Because of this, water and salts pass into the root by osmosis.
Transpiration: This is the loss of water by evaporation from leaves and green stems. This loss produces a suction force called tension. The tension is effective due to the high cohesion of water molecules.
Tension-Cohesion Mechanism: This mechanism prevents the water column from breaking. The high adhesion of water to the vessel walls, together with cohesion, forms capillary action, aiding ascent.
Leaf Structure and Photosynthesis Efficiency
Leaves have numerous characteristics that allow them to carry out photosynthesis more effectively. The thin, elongated inner leaf consists of two main types of tissues:
Parenchyma: Both the spongy (lagoon) and the palisade cells contain chloroplasts and are therefore photosynthetic.
Vascular Tissue (Veins): These tissues are in charge of transport. They consist of two components: the xylem, composed of woody vessels, and the phloem, consisting of sieve (Liberian) vessels. Both are grouped to form a dense network of veins.
Photosynthetic Pigments and the Light Phase
Photosynthesis takes place in chloroplasts, occurring in two phases: the light phase and the dark phase. The light phase consists of a series of light-dependent chemical reactions that occur in the thylakoid membranes of plant chloroplasts.
Plants contain pigments that absorb light, including chlorophyll, xanthophylls, and carotenoids. Chlorophyll is a pigment capable of absorbing light energy from the sun; chemically, it is a lipid belonging to the group of terpenes.
Types of Chlorophyll
Existing types of chlorophyll differ in small details of their molecular structure:
Chlorophyll a: Directly involved in transforming light energy into chemical energy. Found in eukaryotes and cyanobacteria.
Chlorophyll b: Found alongside Chlorophyll a in most plants and green algae.
Chlorophyll c: Typical of diatoms and protozoa.
Transport of Elaborated Sap (Phloem)
During photosynthesis, the crude sap (transported in the xylem to the leaves) becomes elaborated sap. Elaborated sap is a solution consisting essentially of sugars, amino acids, and other nitrogenous substances.
The elaborated sap is transported throughout the plant through the vessels of the phloem. The flow of phloem sap is bidirectional (up and down) and usually ranges from production areas (or sources) to areas of consumption (or sinks).
Sinks can be storage tissues or areas of high metabolic activity, such as roots and fruits. At certain times, sinks can temporarily act as sources.