Chapter 3: Adaptations to Terrestrial Environments & Photosynthesis

Indigenous Lands Acknowledgement and Course Context

• University of Arizona acknowledges its location on Indigenous lands and territories.
• Arizona is home to 22 federally recognized tribes; Tucson specifically hosts the O’odham and Yaqui nations.
• Emphasis on sustainable relationships with sovereign Native Nations and Indigenous communities through education, partnerships, and community service.
• Context note: Ethical framing for ecological study and resource use; interconnectedness of people, place, and environment.

Module Overview and Key Dates (Housekeeping)

• Module 1: Life & Environment, Chapter 3.
• Required quick quizzes by Friday:
  • Chapter 2 short quiz (opens right after class on Thursday, closes Friday at 11:59 pm).
  • Chapter 3 short quiz (opens right after class on Thursday, closes Friday at 11:59 pm).

Recap: Water, Life, and Aquatic Constraints

• Water has properties favorable to life but presents challenges in aquatic environments.
• Gas diffusion in water limits uptake of gases by aquatic animals.
• Temperature sets the upper and lower bounds for aquatic life; some taxa are more temperature-tolerant than others.

Chapter 3 Learning Objectives (Adaptations to Terrestrial Environments)

• Terrestrial plants obtain nutrients and water largely from soil and rely on sunlight for photosynthesis.
• Terrestrial animals must balance water, salt, and nitrogen under varied temperatures.
• Adaptations to different temperatures enable terrestrial life across diverse climates.

Soil Nutrients: Forms, Availability, and Soil Chemistry

• Nutrients are obtained as ions dissolved in soil water; key ions include:
  • Ammonium: ext{NH}_4^+\,,
  • Nitrate: ext{NO}_3^-\,,
  • Phosphate: ext{PO}_4^{3-}\,
  • Calcium: ext{Ca}^{2+}\,
  • Potassium: ext{K}^+\,
• Availability varies with soil temperature, pH, and the presence of other ions.

Water Potential and Matric (Matrix) Potential

• Water potential is a measure of water’s potential energy and influences water movement in soil.
• Matric potential (Ψm): potential energy due to attractive forces between water and soil particles; arises from electrical charges between water and soil.
• Matric potential is expressed in megapascals (MPa).
• Key reference points:
  • In saturated soil, Ψm ≈ 0 MPa; most water is not held tightly by soil.
  • When gravity drains soil water, Ψm drops to about −0.01 MPa.
  • Field capacity is the maximum water held by soil against gravity.
  • As plants extract water, Ψm becomes more negative.
  • Wilting point occurs around Ψm ≈ −1.5 MPa; beyond this, most plants cannot extract water.

Drought Tolerance in Plants: Creosote Bushes

• Creosote bushes tolerate extreme drought stress, operating at about −5 MPa (≈ −50 bars) water potential.
• Example: Creosote King Clone (Larrea tridentata) is ~11,700 years old and extremely drought-tolerant.

Soil Texture, Structure, and Water-Holding Capacity

• Soil water-holding capacity increases with surface area; soil texture components:
  • Sand: diameters > 0.05 mm
  • Silt: diameters 0.002–0.05 mm
  • Clay: diameters < 0.002 mm
• Smaller particles (e.g., clay) provide greater surface area relative to volume, allowing more water to be held, but water is bound tightly.
• Soil structure affects water retention; loam soils are among the best for plant growth.
• Figures illustrate how different textures (sand, loam, clay) relate to field capacity and wilting point.

Soil Texture and Water Content: Conceptual Overview

• Percent composition (e.g., clay, silt, sand) influences water-holding capacity and availability.
• Loam and loamy textures balance water retention with aeration, supporting diverse plant communities.

Osmosis and Water Uptake by Roots

• Osmosis: passive movement of solvent (e.g., water) through a semipermeable membrane from hypotonic to hypertonic solutions.
• In plants, osmotic uptake drives water from soil into root cells.
• Root pressure (osmotic potential in roots) helps push water into the xylem but is not sufficient to move water to the upper portions of tall plants on its own.

Xylem Water Transport: Root Pressure, Cohesion, and Transpiration

• Water is transported from roots to leaves via the xylem.
• Root pressure can raise water to about ~20 m above the soil, but tall trees (>100 m) rely on other forces.
• Cohesion: hydrogen bonds between water molecules create a cohesive column that can be pulled upward.
• Transpiration: evaporation of water from leaf surfaces creates negative pressure (tension) that pulls the water column up the xylem.

Cohesion-Tension Theory: Mechanism of Water Movement

• Theory I: Stomata & guard cells regulate gas exchange; transpiration lowers leaf water potential, generating tension that pulls water up the xylem against gravity and the root’s osmotic potential.
• Theory II: Water movement is driven by cohesion and tension created by transpiration; stomata act as gateways for CO2 uptake and water loss.
• Key concepts:
  • Transpiration is the escape of water vapor from leaves; cooling effect accompanies water loss.
  • Cohesion keeps water molecules attached to each other within the xylem.
  • The combination enables water transport to leaves despite gravity.

Recap: Cohesion-Tension Theory and Visual Aids

• Transpiration-driven tension creates a negative pressure in the water column; cohesion maintains a continuous column.
• The theory explains how plants transport water to great heights without energy expenditure by acting like a suction pump.
• A recommended video demonstrates the concept (link provided in class materials).

Chapter 3 Learning Objectives Revisited (Summary)

• Terrestrial plants obtain nutrients and water from soil; sunlight powers photosynthesis.
• Terrestrial environments impose challenges for animals to balance water, salt, and nitrogen; temperature adaptations are vital for global distribution.
• Adaptive strategies like C3, C4, and CAM photosynthesis optimize water use and carbon gain under different climates.

Visual and Humor Note: Light-hearted Reminders

• A slide encourages reflection on the idea that trees contribute oxygen and support life, while the ecosystem services they provide are complex and essential.

Early History of Photosynthesis and Biosphere Context

• Timeline highlights:
  • 4.6 billion years ago: formation of Earth.
  • 3.4 billion years ago: first photosynthetic bacteria; they absorbed near-infrared light and produced sulfur compounds rather than oxygen.
  • 2.4–2.3 billion years ago: first rock evidence of atmospheric oxygen.
  • 2.7 billion years ago: cyanobacteria became oxygen producers.
  • 1.2 billion years ago: red and brown algae appear; phycobilin pigments and chlorophyll present.
  • 0.75 billion years ago: green algae appear.
  • 0.475 billion years ago: first land plants (mosses and liverworts) emerge.
  • 0.423 billion years ago: vascular plants appear.
• Biosphere is a closed system; oxygen is produced via water-splitting during light harvesting; light is the primary energy source for the biosphere.

Photosynthesis: Foundations of Food Webs

• Photosynthesis converts light energy into chemical energy stored as sugars; plants (producers) form the base of food webs.
• Simplified schematic: Sunlight, water, and inorganic nutrients support producers; consumers and decomposers form subsequent trophic levels.

Light and Energy: Solar Radiation and PAR

• Sunlight is absorbed, reflected, or transmitted by plants.
• Plants appear green because chlorophyll reflects green wavelengths; pigments absorb specific wavelengths for photosynthesis.
• Electromagnetic radiation from the Sun consists of photons; photon energy increases with frequency and decreases with wavelength.
• Photosynthetically Active Radiation (PAR) ranges from 400 nm to 700 nm, aligning with chlorophyll absorption bands.

Chloroplasts: Structure and Pigments

• Chloroplasts are the organelles where photosynthesis occurs in plants and algae.
• Internal structure:
  • Thylakoid membranes stacked into grana; site of light reactions.
  • Stroma: fluid surrounding thylakoids; site of the Calvin cycle.
• Key pigments: Chlorophylls and carotenoids absorb light; Chlorophyll a is essential for photosynthesis across plants.

Leaf Anatomy and Gas Exchange: Stomata and Guard Cells

• Leaves contain stomata, pores controlled by guard cells.
• Stomata regulate CO2 uptake for photosynthesis and water loss via transpiration.
• Guard cells change shape to open/close stomata, influencing gas exchange and leaf temperature.
• Open stomata enable CO2 entry but increase water loss; closed stomata conserve water but limit carbon uptake.

Photosynthesis: Calvin Cycle, C3 Pathway

• Overall reaction (C3):
  • Gas exchange: 6\,\mathrm{CO2} + 6\,\mathrm{H2O} + \text{photons} \rightarrow \mathrm{C6H{12}O6} + 6\,\mathrm{O2}
  • In plants, the Calvin cycle fixes CO2 using the enzyme Rubisco to form glyceraldehyde-3-phosphate (G3P) intermediates that are ultimately used to synthesize glucose.
• Two major stages:
  • Light reactions (photophosphorylation): capture light energy to produce ATP and NADPH; water is split, releasing O2.
  • Calvin cycle (dark reactions): uses ATP and NADPH to fix CO2 into sugars.

The Calvin Cycle and Rubisco

• Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is the most abundant enzyme on Earth and catalyzes carbon fixation.
• Primary carboxylation reaction (C3 pathway):
  • \text{CO}_2 + \text{RuBP} \rightarrow 2\,\text{3-PGA}
• Rubisco also catalyzes oxygenation (photorespiration) when O2 competes with CO2 for RuBP:
  • This side reaction reduces photosynthetic efficiency, especially under hot, dry conditions when stomata close and CO2 is limited.
• Efficiency note: Rubisco carboxylates roughly 3 molecules of CO2 per second, in contrast to many enzymes that process thousands of substrate molecules per second.

Photorespiration and Hot, Dry Conditions

• When stomata close to conserve water in hot/dry climates, CO2 becomes limited inside the leaf while O2 accumulates, increasing photorespiration.
• Photorespiration oxidizes carbohydrates to CO2 and H2O, consuming ATP and NADPH without producing sugar, thus reducing growth efficiency.
• Notation: C3 photosynthesis is less optimal under high temperature and water stress due to Rubisco’s oxygenase activity.

Alternative Carbon-Fixation Pathways to Improve Water Use and Carbon Gain

• Water balance and carbon gain can be improved via:
  • 1) Modified photosynthetic pathways (C4) – spatial separation of pathways within the leaf.
  • 2) Temporal separation of pathways (CAM) – night-time CO2 uptake with day-time fixation under closed stomata.
• C3 is optimized for cool, wet conditions; C4 and CAM are advantageous in warm, arid conditions where water is limiting.

C4 Photosynthesis: Spatial Separation and Water Use Efficiency

• Key features:
  • Initial CO2 fixation by PEP carboxylase in mesophyll cells forms a 4-carbon acid (oxaloacetate, OAA).
  • OAA is converted to malate/aspartate and transported to bundle-sheath cells.
  • CO2 is released in bundle-sheath cells for fixation by the Calvin cycle, reducing photorespiration.
• Advantages: Higher water-use efficiency and better performance under high light and high temperatures; however, energetically more demanding when water is not limiting.
• Evolutionary note: C4 pathway has evolved convergently around 45 times in 19 plant families, with ~8000 species using C4 photosynthesis.
• Proportion: C4 plants account for roughly 25% of global land photosynthesis; they are typically more light-demanding.

CAM Photosynthesis: Temporal Separation and Maximum Water Use Efficiency

• Key features:
  • Night: CO2 uptake via open stomata; CO2 is fixed into a 4-carbon acid (e.g., malate) and stored in vacuoles.
  • Day: Stomata stay closed; CO2 is released from the stored acids to the Calvin cycle for sugar production.
• CAM plants are highly water-use efficient, enabling survival in extremely dry environments.

Leaf Size, Energy Balance, and Water Loss

• Leaf size influences energy balance and transpiration:
  • Large leaves increase transpiration and risk overheating; small leaves reduce water loss and can dissipate heat through boundary layers and vein architecture.
  • High vein density supports rapid water transport and tissue hydration; small leaves help prevent embolisms in water-stressed environments.
• Boundary layer: still air near leaf surface reduces evaporative water loss.

Leaf:Root Ratios and Rooting Strategy in Deserts

• Desert and semiarid plants often allocate a large fraction of biomass to roots (≈90% of biomass in some desert plants) to maximize water uptake after brief rainfall events.
• In contrast, coniferous forests may allocate ~25% of biomass to roots.
• Shallow roots enable rapid water uptake after rainfall; deep roots access deeper water sources during dry periods.

Essential Nutrients and Liebig’s Principle

• Liebig’s classical assertion: certain elements are essential for plant life; deficiency in any essential element halts growth.
• Essential nutrient balance is critical for photosynthesis, growth, and homeostasis.

Photosynthesis: Summary of Key Points

• Light from the sun is the primary energy source for life on Earth; photosynthesis anchors food webs.
• Land plants must obtain nutrients and water from the soil to fuel photosynthesis and growth.
• Carbon uptake requires water loss; water must be available to sustain photosynthetic activity.
• Rubisco is extremely abundant but not highly efficient due to its oxygenase activity; governs the balance between carbon fixation and photorespiration.
• C4 and CAM photosynthesis enhance water-use efficiency, especially under warm and dry conditions; C3 plants are better suited to cool, wet environments.
• The plant’s photosynthetic strategy is tightly linked to leaf anatomy, stomatal regulation, and environmental conditions.

Homeostasis in Organisms: Negative Feedback and Set Points

• Homeostasis: the ability to maintain stable internal conditions despite external fluctuations.
• Negative feedbacks restore a system to a desired set point when deviations occur (e.g., mammalian temperature regulation via hypothalamus; metabolism adjusts with body temperature).

Salt/Nutrient Balance and Plant Physiology

• Land plants transpire large amounts of water for biomass production; salts and nutrient balance must be maintained.
• Roots function as a kidney-like organ, expending energy to pump excess salts back into the soil, maintaining osmotic balance and cellular function.

Animal Water and Salt Balance: Examples and Mechanisms

• Desert kangaroo rat: nocturnal activity to reduce water loss, subterranean behavior during the day.
• Notable renal adaptations: large loop of Henle and large kidneys enabling high water retention.
• Marine iguana and other saline drinkers manage salt balance using specialized excretory organs (e.g., salt glands). These adaptations reflect trade-offs between water conservation and salt excretion.

Nitrogen Balance in Animals: Form and Trade-Offs

• Nitrogenous wastes exist in different forms with varying energy and water costs:
  • Ammonia (NH3): simplest form; high water loss, common in aquatic organisms.
  • Urea: moderate energy cost; less water loss; common in mammals and many terrestrial vertebrates.
  • Uric acid (C5H4N4O3): high energy cost to produce, extremely water-efficient, common in birds and reptiles.
• Trade-offs among water availability and energy expenditure influence nitrogen waste pathways across taxa.

Adaptations Against Freezing: Notothenioids

• Notothenioids (≈122 species in Antarctic waters) exhibit:
  • Antifreeze glycoproteins in blood to prevent ice formation.
  • Extremely low hemoglobin concentrations (<1% of normal red blood cell hemoglobin).
  • Optimal body temperature range: approximately −2°C to 4°C; mortality outside this range (temperatures above 5–7°C) is high.

Thermoregulation: Emerging Threats and Bat Mortality

• White-nose syndrome (WNS) is caused by the fungus Pseudogymnoascus destructans and has caused extensive bat mortality.
• Native to Europe and Western Siberia; in North America, 11 bat species are susceptible with over 6 million bat deaths recorded since 2006 (NY introduction).
• Affected regions include 33 US states and 7 Canadian provinces.
• Potential biocontrols discussed include Rhodococcus rhodochrous, though field results have been limited.
• Notable susceptible bat species include: Big brown bat (Eptesicus fuscus), Indiana bat (Myotis sodalis), Virginia big-eared bat (Corynorhinus townsendii virginianus), and several Myotis species (e.g., Myotis grisescens, Myotis lucifugus, Myotis septentrionalis).

Practical and Ethical Implications

• The contents highlight the importance of understanding ecological limits (water, nutrients, temperature) in the context of climate variability and habitat change.
• Ethically, acknowledging Indigenous lands centers the responsibility of sustainable resource management and inclusive stewardship of ecosystems.
• Conservation and biosecurity implications (e.g., bat health and WNS) require integrative approaches combining ecology, physiology, and policy.
• Agricultural and forestry practices benefit from understanding C3/C4/CAM dynamics to improve water-use efficiency in crops under drought scenarios.

Connections to Foundational Principles and Real-World Relevance

• Water potential, soil texture, and matric potential connect to fundamental physics of fluids and plant physiology.
• Osmosis, diffusion, cohesion, and transpiration link molecular interactions to organism-level processes like water transport and leaf cooling.
• Photosynthesis foundationally drives energy flow in ecosystems and underpins food webs; understanding Rubisco limitations informs crop improvement strategies.
• Adaptive strategies (C4, CAM) illustrate evolutionary responses to water scarcity and high-temperature environments, informing agricultural resilience in arid regions.
• Notable natural histories (photosynthesis timeline, evolution of oxygenic photosynthesis) provide context for current atmospheric chemistry and climate.

Quick Reference: Key Numerical and Conceptual Highlights

• Wilting point: around \(-1.5) MPa; field capacity: 0 MPa; saturated soil: Ψm ≈ 0 MPa; gravity drainage can yield Ψm ≈ \(-0.01) MPa.
• Drought stress example: Creosote bush at ~-5 MPa (≈ -50 bars); King Clone age ≈ 11,700 years.
• Soil texture categories: sand > 0.05 mm; silt 0.002–0.05 mm; clay < 0.002 mm.
• Proportions and evolutionary notes:
  • C4 plants account for ~25% of all land photosynthesis and evolved ~45 times across 19 angiosperm families; ~8000 species using C4.
  • CAM plants represent a highly water-use-efficient pathway;
  • CAM and C4 are often less energy-efficient than C3 when water is not limiting.
• History snapshot: Photosynthesis arose around 3.4 Ga in bacteria; first atmospheric oxygen appears around 2.4–2.3 Ga; land plants arise about 0.475 Ga.
• Photosynthesis equation (overall):
  6\,\mathrm{CO2} + 6\,\mathrm{H2O} + h\nu \rightarrow \mathrm{C6H{12}O6} + 6\,\mathrm{O2}
• Calvin cycle (C3) carboxylation:\text{CO_2} + \text{RuBP} \xrightarrow{\text{Rubisco}} 2\,\text{3-PGA}
• C4 initial fixation: \text{CO_2} + \text{PEP} \rightarrow \text{OAA (4C)}
• CAM night-time fixation: CO2 fixed to form 4C acids (e.g., malate); day-time decarboxylation feeds Calvin cycle while stomata are closed.
• Rubisco dual activity: Carboxylase (CO2 fixation) vs Oxygenase (O2 uptake), with photorespiration as a costly side reaction.
• Noteworthy organisms and adaptations: Notothenioids with antifreeze glycoproteins; marine iguana salt glands; desert kangaroo rat renal adaptations; WNS impact on bats.

References to Class Materials

• Figures and videos referenced (e.g., cohesion-tension theory recap and related YouTube video) provide visual support for water transport concepts.
• Textbook references: Ecology: The Economy of Nature, Seventh Edition; various diagrams (Figure 3.3, Figure 3.4) illustrate soil texture and water-holding capacity.