Plant Biology: Nutrition, Transport, and Gas Exchange
Plant Biology
Nutrition, Transport, and Gas Exchange
1. Introduction
Plants perform essential processes such as nutrient uptake, food digestion, gas exchange, transport, metabolism, and excretion. This document will explore these processes in detail.
2. Plant Structure and Function
Plants, such as algae and cormophytes, have specialized structures for different functions. Cormophytes possess leaves for photosynthesis, stems for support and circulation (xylem and phloem), and roots for water and nutrient absorption. Water enters roots through osmosis, while minerals are absorbed through active transport, requiring energy to move against a concentration gradient.
3. Root Structure
Roots have a complex structure consisting of the following layers:
- Epidermis: The outermost layer of young roots.
- Cortex: Composed of cortical parenchyma, which facilitates gas circulation, and the endodermis, which regulates water and salt passage.
- Pericycle: Located within the endodermis, it gives rise to lateral roots.
- Stele: Contains the phloem (for transporting sugars) and xylem (for transporting water and minerals). The root also stores reserves.
4. Crude Sap Transport
Crude sap is transported through a mechanism of tension-adhesion-cohesion, involving root pressure, transpiration, and tension-cohesion.
- Root pressure: Osmosis drives water into the root due to higher solute concentration within root cells. This creates pressure that pushes sap upwards.
- Transpiration: Water loss through evaporation from leaves creates a pulling force that draws water up from the roots through the xylem.
- Tension-cohesion: Cohesion between water molecules and adhesion to xylem walls facilitate water movement. Capillarity also plays a role.
5. Gas Exchange
Gas exchange occurs through various pathways:
- Stomata: The primary route for gas exchange in leaves.
- Root hairs: Absorb gases dissolved in soil water.
- Lenticels: Openings in woody stems that contribute to gas exchange.
During respiration, plants produce CO2. This CO2 can accumulate in the parenchyma and be used for photosynthesis when stomata are closed. During the day, with increased photosynthesis, stomata open to allow for gas exchange.
The opening and closing of stomata are regulated by ion concentration and osmosis. Potassium ion (K+) influx causes water to enter guard cells, leading to stomatal opening. Conversely, K+ efflux causes water to leave, leading to stomatal closure.
6. Light Capture and Photosynthesis
Leaves contain palisade parenchyma and spongy mesophyll for photosynthesis and gas exchange. Xylem and phloem form a network within the leaf. Photosynthesis converts light energy into organic compounds, releasing oxygen as a byproduct.
7. Translocation of Sugars
Elaborated sap (containing sugars) is transported from source to sink tissues through the phloem. This process involves active transport, osmosis, and pressure changes within the sieve tubes.
- Sugars are actively transported into companion cells.
- Sugars move from companion cells to sieve tubes via plasmodesmata. This increases solute concentration in sieve tubes, drawing water in from xylem through osmosis.
- Increased water pressure pushes the sap towards sink tissues. At the sink, sugars are actively transported out of sieve tubes, and water follows by osmosis, reducing pressure.