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Lecture 4

Biology 2483A Lecture Notes - Lecture 4: Hydrothermal Vent, Photorespiration, Light-Independent Reactions

Course Code
BIOL 2483A
Hugh Henry

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Assimilate radiant energy from sunlight (photosynthesis), of from inorganic compounds (chemosynthesis).
The energy is converted into chemical energy stored in the bonds of organic molecules.
Important in nutrient cycling bacteria, and in some ecosystems such as hydrothermal vent communities.
Energy from inorganic compounds is used to produce carbohydrates.
The orange is sulfur that is the end product to chemosynthetic processes.
(most autotrophes): sunlight provides the energy to take up CO2 and synthesize organic compounds
Most of the biologically available energy on Earth is derived from photosynthesis
Photosynthetic organisms include some archaea, bacteria, and protists, and most algae and plants.
Light reaction - light is harvested and used to split water and provide electrons to make ATP and NADPH.
Dark Reaction - CO2is fixed in the Calvin cycle, and the carbohydrates are synthesized.
Two Major Steps
Photosynthetic rate determines the supply of energy, which in turn influences growth and reproduction.
Environmental control on photosynthetic rate are an important topic in physiological ecology.
Light response curves show the influence of light levels on photosynthetic rate.
Cellular respiration rate = photosynthesis rate
Light compensation point: Where CO2uptake is balanced by CO2 loss by respiration.
Lecture 4: Coping with Environmental Variation : Energy
Thursday, October 1, 2015
10:02 AM
Lecture Slides Page 1

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Cellular respiration rate = photosynthesis rate
Saturation point: When photosynthesis no longer increase as light increases.
Shift to maximize performance
Plants can acclimatize to changing light intensities with shifts in light response curves.
Shifts in light saturation point involve morphological (shape and size of their leaf) and physiological (changes in the
enzymes used) changes.
If you have a lot of sunlight coming in you can use all the additional cells.
In the shade, leaves don’t have a lot of light so you don't want to spend a lot of energy in maintaining extra
Leaves at high light intensity may have thicker leaves and more chloroplasts.
Water availability influences CO2supply in terrestrial plants.
Low water availability causes stomates to close, restricting CO2 uptake.
This is trade-off: Water conservation versus energy gain.
Closing stomates increases chance of light damage: If the Calvin cycle isn't operating, energy accumulates in the
light-harvesting arrays and can damage membranes.
Plants have various mechanisms to dissipate this energy, including carotenoids.
Plants from different climate zones have enzyme forms with different optimal temperatures that allow them to
operate in that climate.
Lecture Slides Page 2

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operate in that climate.
Each enzyme becomes a specialists for the specific environment that they grow in .
Plants can acclimatize by synthesizing different enzyme forms.
You see a shift in optimal temperatures.
If you take an individual and put it in a different environment it will alter its physiology to adapt to the new
The second graph shows the second population unable to acclimatize although it is trying its best to.
Nitrogen is tied up with protein and proteins are associated with enzymes.
Thus, higher nitrogen levels in a leaf are correlated with higher photosynthetic rates.
Most nitrogen in plants is associated with rubisco and other photosynthetic enzymes.
However, nitrogen supply is low, relative to the demand for growth and metabolism
Supply and demand is imbalanced.
If you increase the nitrogen then more animals eat these high-nitrogen containing plants.
Increasing nitrogen content of leaves increases the risk that herbivores will eat them, as plant-eating
animals are also nitrogen starved.
Nutrients can also affect photosynthesis:
Some metabolic processes decrease photosynthetic efficiency.
Carboxylase reaction: photosynthesis
Photorespiration is very poor for a plant.
Instead of fixing carbon it is works backward and has a huge energetic cost.
Oxygenase reaction: O2 is taken up, carbon compounds are broken down, and CO2 is released
Rubisco can catalyze two competing reactions:
Experiments with Arabidopsis thaliana plants with a mutation that knocks out photorespiration.
This implies that photorespiration have benefits in certain conditions (too much light).
These plants die under normal light and CO2 conditions.
Hypothesis: Photo-respiration may protect plants from damage at high light levels.
Altered tobacco plants with high rates of photorespiration showed less light damage than plants with
normal or lowered photorespiration rates (Kozaki and Takeba 1996).
Does photorespiration have any benefits?
Lecture Slides Page 3
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