Premed · Premed · General Biology 2
Lecture 10: Plant Transport and Nutrition
General Biology II — Organismal, Evolution & Ecology
Learning Objectives
By the end of this lecture, students will be able to:
- Explain the mechanisms of water and mineral uptake by roots
- Describe the cohesion-tension theory of xylem transport
- Explain the pressure-flow hypothesis for phloem transport
- Describe how stomata regulate gas exchange and water loss
- Identify essential plant nutrients and their roles
- Explain the importance of mycorrhizae and nitrogen-fixing bacteria in plant nutrition
Lecture Content
I. Water and Mineral Absorption by Roots
Water movement in plants is governed by water potential (Psi), defined as the tendency of water to move from one location to another. Water always flows from regions of higher water potential to regions of lower water potential. Water potential has two major components: solute potential (Psi_s), which decreases as solute concentration increases and is always negative, and pressure potential (Psi_p), which can be positive (as in turgor pressure) or negative (as in the tension within xylem). Pure water at atmospheric pressure has a water potential of 0 MPa; adding solutes makes it more negative, while positive pressure makes it less negative.
Root hair cells maintain a lower (more negative) water potential than the surrounding soil solution because of their solute content, so water enters by osmosis. Once inside the root, water travels toward the vascular cylinder by three parallel pathways. The apoplast pathway carries water through cell walls and intercellular spaces without crossing any membranes. The symplast pathway carries water through the continuous cytoplasm of connected cells, passing from one cell to the next via plasmodesmata. The transmembrane pathway involves water crossing cell membranes directly, often through aquaporin channels.
At the endodermis, the Casparian strip -- a band of suberin wax embedded in the cell walls of endodermal cells -- blocks the apoplast pathway, forcing all water and dissolved minerals to pass through the symplast by crossing at least one cell membrane. This checkpoint gives the plant selective control over which mineral ions enter the vascular cylinder. Many mineral ions are taken up by active transport, powered by proton pumps (H+-ATPase) in root cell membranes that create an electrochemical gradient. Ions then move into root cells via co-transporters and ion channels, allowing plants to regulate precisely which ions they absorb and in what quantities.
II. Xylem Transport: The Cohesion-Tension Theory
Moving water from roots to leaves is a monumental physical challenge, especially in tall trees where water must travel over 100 meters against the pull of gravity. Remarkably, xylem contains no metabolic pumps -- transport is entirely passive. The cohesion-tension theory, proposed by Dixon and Joly in 1894, explains how this is accomplished through three interconnected physical forces.
Transpiration -- the evaporation of water from leaf surfaces through open stomata -- creates a negative pressure (tension) in the leaf mesophyll cells. Cohesion, the mutual attraction of water molecules through hydrogen bonds, allows water to be pulled as a continuous column through the narrow xylem vessels, much as water can be drawn up through a straw. Adhesion, the attraction of water molecules to the hydrophilic walls of xylem vessels, helps counteract gravity and prevents the water column from pulling away from the vessel walls. The combined effect is a transpiration-driven pull that draws water upward from the roots through the xylem to the leaves.
The scale of transpiration is staggering. A large tree can transpire hundreds of liters of water per day, and approximately 97% of the water absorbed by roots is lost through transpiration. Only about 3% is actually used for photosynthesis and other metabolic processes. Under certain conditions -- high soil moisture and low transpiration rates -- root pressure provides a supplementary upward push. Ions actively pumped into the xylem create an osmotic gradient that draws water in, generating positive pressure. This can cause guttation, the appearance of water droplets on leaf edges, but root pressure alone is insufficient to drive transport in tall trees. A risk of the cohesion-tension system is cavitation, or embolism: air bubbles can form in xylem vessels under extreme tension during drought, blocking water flow. Plants have evolved mechanisms to isolate and repair cavitated vessels, maintaining the integrity of the transpiration stream.
<image>A diagram illustrating the cohesion-tension theory of xylem transport in a tree. The figure shows a full tree from roots to leaves. Panel A (leaf level): A magnified cross-section of a leaf showing a stoma with guard cells open; arrows indicate water vapor exiting (transpiration) and the resulting negative pressure (tension) pulling water from mesophyll cells into the air spaces. Panel B (stem level): A magnified view of xylem vessels showing a continuous column of water molecules with hydrogen bonds between them (cohesion) and adhesion to the vessel walls. Panel C (root level): Water entering root hairs by osmosis, moving through the cortex (apoplast and symplast pathways shown), passing the Casparian strip at the endodermis, and entering the xylem. Arrows throughout show the direction of water flow from soil to atmosphere, driven by transpiration pull.</image>
III. Stomatal Regulation
Stomata are pores on leaf surfaces, concentrated primarily on the lower epidermis, that serve as the gatekeepers of gas exchange. Each stoma is bordered by a pair of guard cells whose shape changes control whether the pore is open or closed. When guard cells accumulate potassium ions (K+), water follows by osmosis, the cells swell and become turgid, and the stoma opens. When K+ leaves the guard cells, water follows, the cells become flaccid, and the stoma closes.
Several environmental factors regulate this process. Light, particularly blue light, activates H+-ATPase proton pumps in guard cell membranes, driving K+ uptake and stomatal opening during the day. Low CO2 concentration inside the leaf signals that photosynthesis is depleting CO2 faster than it is being supplied, triggering stomatal opening to admit more. When water stress occurs, the hormone abscisic acid (ABA) is produced, triggering rapid stomatal closure to conserve water. High temperature can also induce closure, and an underlying circadian rhythm ensures that stomata typically open during the day and close at night.
Two specialized photosynthetic strategies have evolved to cope with the tension between CO2 uptake and water conservation. CAM plants (such as cacti and succulents) open their stomata at night, fixing CO2 into organic acids that are stored until daytime, when the stomata close and the stored CO2 is released internally for the Calvin cycle. This temporal separation minimizes daytime water loss. C4 plants (such as corn and sugarcane) achieve a spatial separation instead, concentrating initial CO2 fixation in mesophyll cells and passing the fixed carbon to bundle sheath cells where the Calvin cycle operates. This spatial arrangement minimizes photorespiration and maintains photosynthetic efficiency at the high temperatures where C3 plants falter.
IV. Phloem Transport: The Pressure-Flow Hypothesis
While xylem moves water upward from roots to leaves, phloem transports organic solutes -- primarily sucrose -- from organs that produce or release more sugar than they use (called sources, such as mature leaves or storage organs releasing reserves) to organs that consume or store sugar (called sinks, such as roots, developing fruits, and growing shoot tips).
The pressure-flow hypothesis, proposed by Ernst Munch in 1930, explains phloem transport through a pressure gradient generated by active loading and unloading of sucrose. At the source, companion cells actively load sucrose into sieve-tube elements via H+/sucrose co-transport. The resulting high solute concentration lowers the water potential inside the sieve tubes, drawing water in from adjacent xylem by osmosis and increasing turgor pressure. At the sink, sucrose is unloaded from the sieve-tube elements (by active or passive mechanisms, depending on the species), decreasing solute concentration and raising water potential, so water exits back into the xylem. The pressure difference between the high-pressure source end and the low-pressure sink end drives a bulk flow of phloem sap from source to sink.
An important feature of phloem transport is that it is bidirectional -- the direction of flow depends entirely on which organs are currently acting as sources and which as sinks, and these roles can change. A storage root, for instance, is a sink during the growing season as it accumulates starch, but becomes a source during sprouting when it mobilizes those reserves to fuel new growth. Unlike the passive transpiration-driven transport in xylem, phloem loading requires ATP, making sugar translocation an energy-dependent process.
<image>A diagram of the pressure-flow hypothesis for phloem transport. On the left (source — a mature leaf): sucrose is actively loaded into a sieve-tube element by a companion cell; water enters from an adjacent xylem vessel by osmosis (arrows), increasing pressure. On the right (sink — a developing fruit): sucrose is unloaded from the sieve tube; water exits to the xylem. In between, arrows show the direction of bulk flow of phloem sap from source to sink due to the pressure gradient. The xylem and phloem are shown running parallel, with water cycling between them (xylem water enters phloem at source, phloem water returns to xylem at sink). Sucrose molecules are depicted as orange dots flowing through the sieve tubes.</image>
V. Essential Plant Nutrients
Plants require 17 essential elements for normal growth and reproduction. The macronutrients, needed in relatively large amounts, include carbon, hydrogen, and oxygen (obtained from CO2 and water) along with nitrogen, phosphorus, potassium, sulfur, calcium, and magnesium, which are absorbed from the soil. The micronutrients, or trace elements, include iron, manganese, boron, zinc, copper, molybdenum, chlorine, and nickel.
Nitrogen is often the most limiting nutrient for plant growth because it is essential for amino acids, nucleic acids, and chlorophyll. It is available in soil primarily as nitrate (NO3-) and ammonium (NH4+). Phosphorus, a component of ATP, nucleic acids, and phospholipids, is frequently the second most limiting nutrient. Potassium plays critical roles in osmotic regulation (including stomatal opening) and enzyme activation. Nutrient deficiency symptoms provide diagnostic clues: nitrogen deficiency causes yellowing (chlorosis) of older leaves first, because nitrogen is mobile within the plant and is remobilized from old leaves to support new growth. Iron deficiency, by contrast, causes interveinal chlorosis in young leaves, because iron is relatively immobile. Phosphorus deficiency produces a distinctive purplish coloration. Soil pH strongly affects nutrient availability, with an optimal range of approximately pH 6-7 for most crop plants.
VI. Symbiotic Relationships in Plant Nutrition
Mycorrhizae -- mutualistic associations between fungi and plant roots -- are found in approximately 90% of all plant species and are arguably the most important symbiosis on land. In arbuscular mycorrhizae (endomycorrhizae), fungal hyphae penetrate root cortex cells and form arbuscules, highly branched structures that serve as the primary sites of nutrient exchange. In ectomycorrhizae, common in trees, fungal hyphae form a sheath around the root and grow between cortex cells without penetrating them. In both cases, the fungal mycelium extends far beyond the root system into the surrounding soil, vastly increasing the absorptive surface area available for nutrient uptake, particularly phosphorus. In return, the plant supplies the fungus with carbohydrates.
Nitrogen-fixing bacteria, most famously Rhizobium in symbiosis with legumes (beans, peas, clover), colonize root nodules where they convert atmospheric N2 into ammonia (NH3) using the enzyme nitrogenase. Because nitrogenase is destroyed by oxygen, the plant produces leghemoglobin -- an oxygen-binding protein that maintains the low-O2 environment the enzyme requires. In return, the plant provides carbon compounds to fuel the bacteria's metabolism. This partnership is ecologically and agriculturally critical: it reduces the need for synthetic nitrogen fertilizers and contributes substantially to the global nitrogen cycle.
The broader nitrogen cycle involves several microbially mediated steps. Nitrogen fixation converts N2 to NH3/NH4+. Nitrification, carried out by Nitrosomonas and Nitrobacter, converts NH4+ to NO2- and then to NO3-. Denitrification, performed by anaerobic bacteria, converts NO3- back to N2, returning nitrogen to the atmosphere. Ammonification, or decomposition, converts organic nitrogen in dead organisms back to NH4+.
Carnivorous plants have evolved an entirely different strategy for supplementing their nutrient intake. Growing in nutrient-poor habitats such as bogs and acidic soils, plants like the Venus flytrap (snap trap), sundew (sticky trap), and pitcher plant (pitfall trap) trap and digest insects to obtain nitrogen and phosphorus that the soil cannot provide.
<image>A diagram showing mycorrhizal associations and nitrogen-fixing root nodules. Panel A (Arbuscular mycorrhizae): A root cross-section showing fungal hyphae entering the root cortex and forming branching arbuscules inside cortex cells; external hyphae extend into the soil far beyond the root, absorbing phosphorus (P) and water. Arrows show P moving from soil to plant via hyphae, and sugars moving from plant to fungus. Panel B (Ectomycorrhizae): A root tip surrounded by a fungal mantle (sheath), with hyphae growing between cortex cells (Hartig net) but not penetrating them; external hyphae extend into the soil. Panel C (Root nodule): A longitudinal section of a legume root nodule showing infected cells packed with bacteroids (Rhizobium), the enzyme nitrogenase converting N2 to NH3, and leghemoglobin (pink) maintaining a low-O2 environment. Arrows show N2 entering and NH3/amino acids being transported to the plant.</image>
