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Lecture 4

Biology 2483A Lecture Notes - Lecture 4: Heterotroph, Rubisco, Photorespiration


Department
Biology
Course Code
BIOL 2483A
Professor
Lisa Henry
Lecture
4

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Lecture 4: Coping with
Environmental Variation Energy
Autotrophs: assimilated radiant energy from sunlight (photosynthesis), or from inorganic
compounds (chemosynthesis)
The energy is converted into chemical energy stored in the bonds of organic
molecules
Photosynthesis (most autotrophs): 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
Chemosynthesis (chemolithotrophy): energy from inorganic compounds is use to
produce carbohydrates
-Chemosynthesis is important in nutrient cycling bacteria, and in some
ecosystems such as hydrothermal vent communities
Heterotrophs: obtain their energy by consuming organic compounds from other
organisms
This energy originated with organic compounds synthesized by autotrophs
Some consume non-living organic matter
Parasites and herbivores consume live hosts, but don’t kill them
Predators capture and consume live prey animals
Some plants are holoparasites: they have no photosynthetic pigments and get
energy from other plants (heterotrophs)
- Dodder is a holoparasites that is an agricultural pest and can significantly
reduce biomass in the host plant
Hemiparasite: photosynthetic, but obtains nutrients, water and some of its energy
from the host plant – ex. Mistletoe
- Seas slugs have functional chloroplasts that are taken up from the algae that
the slug eats
Photosynthesis
1) Light reaction: light is harvested and used to split water and provide electrons to
make ATP and NADPH
2) Dark reaction: CO2 is fixed in the Calvin cycle, the carbohydrates are synthesized
can take place in light but doesn’t need it
Photosynthetic rate determines the supply of energy, which in turn influences
growth and reproduction

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Environmental controls on photosynthetic rate are an important topic in
physiological ecology
Light response curves: show the influence of light levels on photosynthetic rate
Light compensation point: where CO2 uptake is balanced by CO2 loss by
respiration
- When you put a plant in the dark, it respires like humans do (they give off
CO2)
- This is the point where photosynthesis balances
compensation
Saturation point: where photosynthesis no longer
increases as light increases
The graph: dark you have negative net because you’re respiring
You no longer have an increase in photosynthesis as you increase light at the
saturation point
Plants can acclimatize to changing light intensities with shifts in light response
curves - Shifts in light saturation point involve morphological and
physiological changes
For the graph
At the bottom left: the light compensation point of the green is
lower than the other two
Because it shifts in higher light environment
Plants grown in higher light environment need more energy, so they respire more
in the dark
They can attain a lot higher rate of photosynthesis once the lights go on
Red is energy hog because more energy is built up – more machinery
Leaves
Leaves at high intensity may have thicker leaves and more
chloroplasts  morphological features can be
responsible for this
Palisade – full of chlorophyll
Multiple layers but in the shade leaf there is less
The sun plant has more light coming through so it will penetrate through the first
layer and down to the other
In the shade, the light is dim so most of the light coming down is absorbed by the
first layer
Water availability influences CO2 supply in terrestrial plants
- Low water availability causes stomates to close (so it can conserve water),
restricting CO2 uptake
- This is a trade-off  water conservation vs. energy gain
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