BIO 351 Ecology - Photosynthesis and Energy

autotrophs

assimilate energy from sunlight (photosynthesis) or from inorganic compounds (chemosynthesis)

• the energy is converted into chemical energy stored in the bonds of organic molecules

heterotrophs

obtain energy by consuming organic compounds from other organisms

photosynthesis

process that turns the carbon in atmospheric CO2 + photons of light energy into complex, high energy organic compounds

how did photosynthesis change everything on Earth?

methanogenic and anoxygenic photosynthetic bacteria evolved (no O2 released) -→ marine bacteria developed ability to capture CO2 and manufacture organic molecules w/ it, using light energy captured in the chemical bonds and releasing O2 → O2 starts to build up in the atmosphere → aerobic organisms evolve → terrestrial plants evolved by capturing photosynthetic bacteria and developing a symbiotic relationship with them, they became chloroplasts living within plants

4 main reasons why plants are importan

1. trees mop of CO2 from the atmosphere by taking it up in photosynthesis and turning it into organic carbon compounds

2. forests have major effects on regional water cycles and rainfall

3. terrestrial plants have more effects in stabilizing soil and increasing rainfall penetration, preventing runoff and landslides

4. photosynthetic organisms in aquatic and terrestrial environments are the basis for (almost) all life on Earth and the foundation of (almost all food webs)

net reaction of photosynthesis

6 CO2 + 6 H2O → C6H12O6 +6 O2

what are the two main parts of photosynthesis?

• light reactions

• carbon fixation

what does light energy vary with?

varies with geographic location, time of year, wavelength, water depth, presence of other plants, leaf angle & characteristics

what is photosynthesis affected by?

affected by light intensity, wavelengths present, water availability, nutrients available in soil, and temperature

light reaction (photosynthesis)

• chloroplast inside of green photosynthetic tissue; membrane bound, precise spatial arrangement of components necessary

• after light is captured in the light reactions of photosynthesis, the energy from the light is stored in high energy chemical bonds (in ATP and NADPH)

• the energy stored in ATP and NADPH is then moved to another part of the chloroplast, an incorporated into chemical bonds in carbon compounds in the Calvin-Benson cycle

carbon fixation (calvin-benson cycle) (photosynthesis)

• O2 is captured and ‘fixed’ into organic carbon compounds, and the energy that had been captured in the light reactions is stored in the chemical bonds of those molecules

• this is now know as C3 photosynthesis - the first, most common, and simplest kind of photosynthesis

RUBISCO

essential enzyme responsible for fixing atmospheric carbon dioxide into organic molecules during the calvin cycle of photosynthesis

C3 photoshynthesis limitations

• maxes out below full sunlight - can’t keep taking up sunlight energy beyond a certain point because CO2 uptake can’t keep up

  • CO2 uptake gets more and more limited as temps get warmer

• takes a lot off rubisco to achieve high photosynthetic rates because rubisco is also used in photorespiration

photorespiration

• happens in all C3 plants

• when CO2 concentration is low, or there is a lot of O2 around, or temperatures are warm, the rubisco takes up O2 instead of CO2 in sunlight

  • no energy is captured and no carbon is fixed

relationship between photorespiration and photosynthesis

photorespiration reduces the amount of photosynthesis the plant can carry out because RUBISCO mistakenly binds oxygen instead of CO2 and creates a useless byproduct and releases previously fixed carbon back into the air as CO2, lowering overall sugar production

C4 photosynthesis

• named for 4-C organic acids

• no photorespiration occurs

• calvin cycle still fixes C, but in specialized cells away from O2

• costs extra energy to run

characteristics of plants with C4 photosynthesis

• very high maximum photosynthetic rates

• typically found in warm, dry, bright environments

• many grassland grasses, many weeds, some important crops (corn and sugar cane)

• at least 65 independent evolutionary events

does photosynthesis peak at higher or lower temperatures in C4 plants relative to C3 plants?

peaks higher

Crassulacean Acid Metabolism (CAM)

• same biochemistry as C4, but uses a different mechanism to overcome photorespiration

• initial capture of CO2 and its fixation into C compounds (via rubisco and the calvin cycle) is separated in time (not space, as in C4 plants)

• Night - stomata open, CO2 initial capture by PEP-C

• Day- stomata closed, CO2 fixation (CO2 unhooked from malate) by rubisco (calvin cycle) using light energy

characteristics of CAM plants

• pump, succulent (water-filled) leaves

• night - CO2 uptake via PEP carboxylase; stomata open

• day - stomata close, light captured in light reactions, CO2 unhooked from organic acids and fixed via rubisco (no O2 coming in, so photorespiration is minimal)

costs vs benefits CAM plants

• costs - slow photosynthetic rates; need to maintain succulent tissues

• benefits - main benefit is high WUE (water use efficiency)

various modes of heterotrophy

• foraging behavior

• digestion and assimilation efficiency

• nutritional symbioses with microbes