Lecture 4: Environmental Variation - Energy

How do Autotrophs get Energy?

• Autotrophs assimilate radiant energy from sunlight (photosynthesis), or from inorganic compounds (chemosynthesis)

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

How do Heterotrophs get Energy?

• Heterotrophs obtain their energy by consuming organic compounds from other organisms

• This energy originated with organic compounds synthesized by autotrophs

Not all Plants are Autotrophs

• Some plants are holoparasites - have no photosynthetic pigments and get energy from other plants (heterotrophs)

• Some plants are hemiparasites - it is photosynthetic, but obtains nutrients, water, and some of its energy from the host plant

Means of Autotrophy: Photosynthesis

• Sunlight provides the energy to take up CO2 and synthesize organic compounds

• Energy is stored in the carbon-carbon bonds of organic compounds

• Most common methos of autotrophy

• Photosynthetic organisms: Archaea, bacteria, protists, algae, plants

Means of Autotrophy: Chemosynthesis

• Energy from inorganic compounds is used to produce carbohydrates

• Energy is stored in the carbon-carbon bonds of organic compounds

Steps of Photosynthesis

1. Light reaction - light is harvested and used to split water and provide electrons to make ATP and NADPH

2. Dark (or carbon) reaction - CO2 is fixed in the Calvin Cycle, and carbohydrates are synthesized

Light Response Curve

• Light compensation point is where CO2 uptake is balanced by CO2 loss by respiration (0 net photosynthesis)

• Saturation point is when photosynthesis no longer increases as light increases

Rubisco

• Carboxylase reaction: Photosynthesis

• Oxygenase reaction: O2 is taken up, carbon compounds are broken down, and CO2 is released (photorespiration) - happens more in hot conditions or when CO2 is low

Photorespiration

• Wasteful compared to photosynthesis, enzyme captures oxygen instead of CO2, consuming energy and undoing previous sugar production

• Photorespiration may protect plants from damage at high light levels

CAM Plants

• CAM minimizes water loss

• CAM plants open their stomates at night when it is cooler and humidity is higher, and close them during the day