Energy I. Autotrophy

Learning Goals:
* Be able to describe how energy from sunlight is used to manufacture carbohydrates by autotrophs (chemosynthesis and photosynthesis- including both light and dark reactions)
* Be able to predict how variation in environmental variables influence photosynthesis rates
* Be able to describe photosynthetic adaptations (C4, CAM photosynthetic pathways) and the environmental conditions that promoted them

Energy exists in many forms in the environment:
* Sunlight is radiant energy.
* Chemical energy is stored in the bonds of molecules.
 * Kinetic energy associated with motion; kinetic energy of air molecules measured as temperature.  Kinetic energy determines the rate of activity and metabolic energy demand.

Chemical and radiant energy are captured by organisms for growth and maintenance.
Autotrophs are organisms that assimilate energy from sunlight (photosynthesis), or from inorganic compounds (chemosynthesis).
The energy captured from radiation or oxidized chemical compounds is converted into chemical energy stored in the carbon–carbon bonds of organic molecules
Heterotrophs obtain their energy by consuming energy-rich organic compounds from other organisms; include predators, parasites, and detritivores

Chemosynthesis (chemolithotrophy) is a process that uses energy from inorganic compounds to produce carbohydrates; important in bacteria involved in nutrient cycling, ocean vent communities, and hot springs
In chemosynthesis, organisms get electrons by oxidizing the inorganic substrate (e.g. H₂S to elemental S).
The electrons are used to generate two high-energy compounds: ATP and NADPH.  Energy from ATP and NADPH is then used to take up, or “fix" CO₂ and use the carbon to make carbohydrates

Photosynthesis
two major steps:
The “light reaction”—light is harvested and used to split water and provide electrons to make ATP and NADPH.
The “dark reaction”—CO₂ is fixed in the Calvin cycle, and carbohydrates are synthesized.

Light harvesting is accomplished by chlorophyll and accessory pigments.  Chlorophyll absorbs red and blue light and reflects green.  The energy from sunlight is used to split water and provide electrons, liberating oxygen- major influence on atmospheric chemistry.

The biochemical pathway used most commonly to fix CO₂ is the Calvin cycle, catalyzed by several enzymes, and occurs in both chemosynthetic and photosynthetic organisms
A key enzyme in the Calvin cycle is ribulose 1,5 bisphosphate carboxylase / oxygenase, or “RUBISCO”, the most abundant enzyme on Earth
Net reaction:
6 CO₂ + 6 H₂O yields C₆H₁₂O₆ (carbohydrates) + 6 O₂

The rate of photosynthesis determines the supply of energy and substrates for biosynthesis, which in turn influences growth and reproduction
Environmental controls on the photosynthetic rate are an important topic in physiological ecology, and include:
1) Light- energy source to split water and generate electrons for C fixation; Some plants can acclimatize to variation in the light environment- e.g. sun/ shade leaves in trees, understory plants; Leaves grown at high light levels are thicker and have more chloroplasts than leaves grown in low light
2) Water availability (terrestrial plants); Low water availability and low plant water potentials result in stomatal closure, lowering supply of atmospheric CO2; common across nearly all terrestrial biomes
Plants adapted to dry soils tend to have higher resistance to water loss, with thick cuticles and a low degree of stomatal opening
> lowers water uptake, but potentially lowers competitive ability for that water relative to neighbors
3) Temperature- Influences photosynthetic function through enzyme activity, as well as membrane function (particularly chloroplasts)
Plants acclimatize and adapt to different temperatures by producing different forms of photosynthetic enzymes with optimal temperatures that match their environmental temperature
Temperature affects functioning of RUBISCO; Balance between CO2 uptake (carboxylase activity and oxygen (O2) uptake influenced by temperture; high temperatures favor oxygenase activity, leading to the release of CO2 and breakdown of carbohydrates and a net loss of energy, a process known as photorespiration
Photorespiration is also affected by the concentration of CO₂ relative to O₂; low CO₂ : O₂ promotes photorespiration over photosynthesis

Environmental constraints resulted in the evolution of biochemical pathways that improve the efficiency of photosynthesis
C4 photosynthetic pathway
 * named by the first stable product (4 carbon organic acid); “normal” photosynthesis is called C3 photosynthesis, using the same rationale
 * acts like a pump to increase CO₂ concentrations at the site of the Calvin cycle, minimizing photorespiration
 * first enzyme in C4 pathway (PEPc’ase) has a higher affinity for CO₂ than RUBISCO
* specialized anatomy found in C4 plants to help increase CO₂ concentrations in leaf
C4 photosynthesis evolved independently several times in different species in 18 families.
Many grass species use this pathway, including agriculturally important species such as corn, sugarcane, and sorghum, but also many grasses in temperate and tropical grasslands
C4 plants plants sensitive to low temperature and tend to have a higher light requirements

Crassulacean Acid Metabolism (CAM)- adaptation for minimizing water loss
 uses similar biochemistry as C4 pathway
* CO₂ taken up by PEP’case at night, when air is cooler and more humid, and transpiration rates are lower
* CO₂ stored as organic acid, which is broken down and released to the Calvin cycle during the day when stomates are closed
* elevated CO₂ concentrations in plant tissues lowers amount of photorespiration
CAM plants are often succulent (i.e. thick and fleshy)-helps with storage of organic acids during night time CO2 uptake
CAM is found in over 10,000 plant species belonging to 33 families; includes desert species, but also tropical epiphytes (plants that grow on aerial parts of trees)