Ecology Lecture No. 5: Coping With Environmental Variation (Energy)
Tuesday September 25 , 2012
Autotrophs & Heterotrophs:
-Autotrophs are organisms that assimilate radiant energy from sunlight (photosynthesis), or from
inorganic compounds (chemosynthesis). This energy is then converted into chemical energy stored in
the bonds of organic molecules. Heterotrophs obtain their energy by consuming organic compounds
from other organisms. This energy originated with organic compounds synthesized by autotrophs.
-Some heterotrophs consume non-living organic matter and are known as detritivores or decomposers.
Parasites and herbivores consume live hosts, but do not necessarily kill them. Predators capture and
consume live prey animals.
Miscellaneous Forms Of Heterotrophy:
-Some plants are holoparasites –they have no photosynthetic pigments and get energy from other
plants (heterotrophs).The Dodder plant is a holoparasite that is an agricultural pest and can significantly
reduce biomass in the host plant.
-Mistletoe is an example of a hemiparasite— it is photosynthetic, but obtains nutrients, water, and some
of its energy from the host plant. Certain sea slugs have functional chloroplasts that are taken up from
the algae that they eat and enable them to capture and store energy from the sun.
-As there is no light penetrating the bottom of the ocean, organisms practise chemosynthesis
(chemolithotrophy) by obtaining energy from inorganic compounds and using them to produce
-Chemosynthesis is important in nutrient cycling bacteria, and in some ecosystems such as hydrothermal
vent communities. We often rely on chemosynthetic bacteria to liberate nutrients (such as nitrogen)
from the soil in a form that plants uptake easily.
-Photosynthesis is the process by which most autotrophs obtain their energy. Here, sunlight provides the
energy to take up CO 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.
-Photosynthesis has two major steps: The Light-dependent reaction (where light is harvested and used
to split water and provide electrons to make ATP and NADPH) and the light-independent reaction
(where CO i2 fixed in the Calvin cycle, and carbohydrates are synthesized for the plant’s benefit). Photosynthetic Rate & Terminology:
-Photosynthetic rate determines the supply of energy, which in turn influences growth and
reproduction. Environmental controls on photosynthetic rate are an important topic in physiological
-Light response curves show the influence of light levels on photosynthetic rate. The light compensation
point is where CO u2take is balanced by the CO loss2by cellular respiration. Saturation point is when
photosynthesis no longer increases as light increases.
Acclimatization In Plants:
-Plants can acclimatize to changing light intensities with shifts in light response curves. Shifts in light
saturation point involve morphological and physiological changes. For example, leaves at high light
intensity, may have thicker leaves and more chloroplasts.
-This is because as light penetrates through the leaf, it will be absorbed by chlorophylls in mesophyll
cells. Here, the extra tissue is worth the energy cost because intense light would otherwise penetrate
through the leaf and not be gained as energy. A thinner leaf would be more prevalent in low light
conditions where investing in extra tissue would be a useless undertaking.
Factors Affecting Photosynthetic Rate:
-Water availability influences CO s2pply in terrestrial plants. Low water availability causes stomata to
close, restricting CO2uptake. This is an example of an evolutionary trade-off between water
conservation and energy gain.
- Closing stomata increases the 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. Thus, temperature plays a key role in the functioning of photosynthesis.
-Nutrients can also affect photosynthesis. Most of the nitrogen in plants is associated with Rubisco and
other photosynthetic enzymes (chlorophyll also has nitrogen in its structure). Thus, higher nitrogen
levels in a leaf are correlated with higher photosynthetic rates.
-But the supply of readily available nitrogen is generally low, relative to the demand for growth and
metabolism. There is also a trade-off as increasing the nitrogen content of leaves increases the risk that
herbivores will eat them (more tasty), as plant-eating animals are also nitrogen-starved.
The Benefits & Disadvantages Of Photorespiration:
- Some metabolic processes decrease photosynthetic efficiency. For instance, Rubisco can catalyze tw