Energy and Nutrient Relations (General Biology Study Notes)

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Energy and Nutrient Relations

Overview

This chapter explores how organisms acquire, utilize, and optimize energy and nutrients, focusing on the diversity of energy sources, photosynthetic pathways, nutrient requirements, and the ecological and evolutionary principles that govern energy acquisition.

Energy Sources

Types of Energy Sources

Light: Utilized by photosynthetic organisms as a primary energy source.
Inorganic Molecules: Used by chemosynthetic organisms to derive energy from chemical reactions.
Organic Molecules: Consumed by heterotrophs for both energy and carbon.

Trophic Classifications

Autotrophs: Organisms that use inorganic sources for carbon and energy.
- Photosynthetic Autotrophs: Use light energy to fix carbon (e.g., plants, algae).
- Chemosynthetic Autotrophs: Use energy from inorganic chemical reactions (e.g., sulfur-oxidizing bacteria).
Heterotrophs: Organisms that use organic molecules for both carbon and energy (e.g., animals, fungi).

Major Groups and Energy Strategies

	Heterotrophic	Photosynthetic	Chemosynthetic
Prokaryotes (Bacteria, Archaea)	●	●	●
Protists	●	●
Plants		●
Fungi	●
Animals	●

Additional info: Prokaryotes are the most metabolically diverse, using all three strategies. Most plants are photosynthetic, while fungi and animals are strictly heterotrophic.

Solar-Powered Biosphere

Nature of Light

Light propagates as a wave and consists of particles called photons that carry energy.
Infrared (IR): Interacts with matter, increasing molecular motion (heat).
Ultraviolet (UV): Can damage or destroy biological molecules.
Visible Light: Wavelengths between ~400–700 nm, also called Photosynthetically Active Radiation (PAR).

Photosynthetically Active Radiation (PAR)

PAR is quantified as photon flux density (number of photons striking a surface per unit area per second).
Chlorophyll absorbs PAR to drive photosynthesis.
The amount and quality of PAR reaching an area can be altered by landscape, water, and organisms (e.g., forest canopy reduces PAR for understory plants).

Photosynthetic Pathways

General Photosynthetic Reaction

The overall equation for photosynthesis is:

C3 Photosynthesis

Used by most plants and all algae.
CO2 combines with ribulose bisphosphate (RuBP) (a 5-carbon sugar) to form phosphoglyceric acid (PGA) (a 3-carbon acid).
Carbon fixation occurs in the mesophyll cells of leaves.
Stomata must open to allow CO2 in, but this can lead to water loss.
Disadvantage: High water loss in hot, dry environments.

C4 Photosynthesis

Used by plants adapted to high light, high temperature, and dry conditions (e.g., many grasses, corn).
CO2 is initially fixed into a 4-carbon acid (using phosphoenol pyruvate (PEP)), which is then transported to bundle sheath cells where the Calvin cycle occurs.
Allows plants to keep fewer stomata open, reducing water loss.
Advantage: Greater water-use efficiency.

CAM Photosynthesis (Crassulacean Acid Metabolism)

Found in succulent plants in arid/semi-arid environments and tropical forest epiphytes.
CO2 is fixed at night (when stomata open) and stored as organic acids; during the day, CO2 is released for the Calvin cycle.
Advantage: Extremely high water-use efficiency, minimal water loss.
Disadvantage: Low rates of photosynthesis due to limited CO2 uptake at night.

Comparison of Photosynthetic Pathways

Pathway	Main Adaptation	Example Plants
C3	Efficient under cool, moist conditions; high water loss in heat	Wheat, rice, most trees
C4	Efficient under high light/temperature; conserves water	Corn, sugarcane, many grasses
CAM	Extreme water conservation; slow growth	Cacti, pineapples, some orchids

Chemosynthetic Autotrophs

Discovered in 1977 at deep-sea hydrothermal vents.
Obtain energy by oxidizing inorganic molecules (e.g., H2S, NH4+, Fe2+).
Support unique ecosystems independent of sunlight.
Can be free-living or symbiotic (e.g., with tube worms).

Chemical Composition and Nutrient Requirements

Major Elements in Biomass

Five elements make up 93–97% of the biomass of living organisms: carbon, oxygen, hydrogen, nitrogen, phosphorus.

Ecological Stoichiometry

Studies the balance of multiple chemical elements in ecological interactions.
Plants generally have a higher C:N ratio than herbivores.
Consumers must eat more plant material to obtain enough nitrogen, the limiting nutrient.

Essential Nutrients

Plants require additional elements: potassium, calcium, magnesium, sulfur, chlorine, iron, manganese, boron, zinc, copper, molybdenum.
Plants absorb most nutrients from soil via roots.
Animals require sodium and iodine, obtained from their diet.

Heterotrophs: Using Organic Molecules

Feeding Methods

Herbivores: Feed on plants.
Carnivores: Feed on animal flesh.
Detritivores: Feed on non-living organic matter (e.g., dead leaves, wood).

Herbivores

Face nutritional challenges due to low nitrogen in plants.
Must overcome plant defenses:
- Physical: Cellulose, lignin, silica (difficult to digest).
- Chemical: Toxins and digestion-reducing compounds (e.g., tannins).

Carnivores

Consume prey rich in nutrients but cannot always choose prey freely.
Engaged in a co-evolutionary arms race with prey.
Prey defenses include camouflage, anatomical/chemical defenses, and behavioral adaptations.

Detritivores

Play a key role in nutrient cycling by breaking down dead organic matter.
Detritus is rich in carbon but poor in nitrogen; fresh detritus may still contain chemical defenses.

Energy Limitation

Factors Limiting Energy Acquisition

Energy availability: Amount of energy present in the environment.
Internal intake limits: Physiological constraints on how quickly organisms can process energy.
Plants: Photosynthetic rate responds to photon flux density.
Animals: Feeding rate depends on food availability.

Photosynthetic Response Curves

Photosynthesis increases linearly with photon flux density at low light, then levels off at a maximum rate ().
Different species have different values and saturate at different light intensities ().

= maximum rate of photosynthesis = light intensity at which photosynthetic system is saturated

Animal Functional Response to Food Density

Type I: Linear increase in feeding rate with food density (little/no handling time).
Type II: Feeding rate rises at a decelerating rate, limited by handling/search time (most common).
Type III: S-shaped curve; feeding rate increases most rapidly at intermediate food densities.

Optimal Foraging Theory

Principles

Predicts that natural selection favors organisms that maximize energy acquisition efficiency.
Organisms face trade-offs due to limited energy supplies (principle of allocation).
Models how organisms optimize feeding strategies under constraints.

Optimal Foraging in Plants

Plants allocate limited energy among leaves, stems, and roots.
Soil fertility affects root:shoot biomass ratio:
- Plants in infertile soils invest more in roots (higher root:shoot ratio).
- Plants in fertile soils invest more in shoots (lower root:shoot ratio).
Example: Sorghastrum nutans decreases root:shoot ratio as soil nitrogen increases.

Summary Table: Photosynthetic Pathways

Pathway	CO2 Fixation Site	Key Adaptation	Example
C3	Mesophyll cells	Efficient in cool, moist climates	Wheat
C4	Mesophyll & bundle sheath cells	Water conservation, high light/temperature	Corn
CAM	Mesophyll cells (night/day separation)	Extreme water conservation	Cactus

Key Terms

Autotroph: Organism that produces its own food from inorganic sources.
Heterotroph: Organism that obtains energy and carbon from organic compounds.
Photosynthesis: Process by which light energy is converted to chemical energy in plants, algae, and some bacteria.
Chemosynthesis: Process by which certain organisms synthesize organic compounds using energy from inorganic chemical reactions.
Ecological Stoichiometry: Study of the balance of energy and elements in ecological interactions.
Optimal Foraging Theory: Model predicting how organisms maximize energy intake per unit time.