Sept 26 2013
Energy is a basic requirement for all organisms
We require it for physiological maintenance, growth, and reproduction
If energy input stops, so does biological functioning. Enzymes fail if replacement proteins can
not be made and cell membranes degrade and organelles cease to operate without energy to
maintain and repair them
Sources of energy include radiant energy which comes from the sun (chemical energy is derived
from radiant energy) and chemical energy which is stored in the food that is being consumed
(plants converted radiant energy into chemical energy). These are the types of energy that
organisms use to meet the demands of growth and maintenance.
Kinetic energy is associated with motion of molecules that make them up objects. This is
important for controlling the rate of activity and metabolic energy demand of organisms (done
through influence on rate of chemical reactions & temp)
Convert a form of energy like solar energy or chemical energy into the chemicals stored in
organic compounds. (chemical energy stored in C-C bonds of organic molecules)
Plants can assimilate radiant energy from sunlight (photosynthesis), or from inorganic
compounds (chemosynthetic archaea and bacteria)
Vast majority of autotrophic production occurs though photosynthesis, a process using sunlight
to provide the energy needed to take up CO2 and synthesize organic compounds
The remainder is produced via chemosynthesis (chemolithotrophy) which is a process that uses
energy from inorganic compounds to produce carbs (important to some key bacteria involved in
nutrient cycling and in some ecosystems such as hydrothermal vent communities). The earliest
autotrophs were likely chemosynthetic as the atmosphere was largely composed of inorganic
elements. Chemosynthesis involves organisms obtaining electrons from the inorganic substrate
(oxidation). These electrons are used to generate ATP and NADPH for the uptake of carbon
from CO2 which is then used to synthesize the carbs used for energy
Carbon is used as a measure of energy because the energy derived from photosynthesis and
chemosynthesis is stored in the C-C double bonds. Heterotrophs
Obtain energy by consuming organic compounds from other organisms. They then convert them
into usable chemical energy (ATP) via glycolysis which breaks down carbohydrates
This energy originated with organic compounds synthesized by autotrophs. Heterotrophs eat
the plants holding the energy from photosynthesis
Includes organisms that consume non living organic matter (detritovores-eat nutrients and
vitamins in the soil) as well as consumers that capture and kill their prey and organisms that
consume living organisms but do not necessarily kill them (Parasites and herbivores)
It would seem that all plants are autotrophs, all animals
and fungi are heterotrophs and archaea and bacteria can
be both autotrophs and heterotrophs. However, it is not
always so simple!
Some plants have lost photosynthetic function
and obtain their energy by parasitizing other
plants. These are called holoparasites. They have
no photosynthetic pigments and get energy from other plants (heterotrophs). Dodder is
a holoparasite that is an agricultural pest and can significantly reduce biomass in the
host plant. It attaches to the host plant by growing in spirals around the stem and
penetrates the tissue of the host, using modified roots to take up carbs.
Mistletoe is a hemiparasite. It is photosynthetic, but obtains nutrients, water and some
of its energy from the host plant.
Animals can also act as autotrophs (rare). Their photosynthetic capacity is acquired by
consuming photosynthetic organisms or by living with them in a close relationship (symbiosis).
an example is a type of sea slug which has functional chloroplasts that are taken up from the
algae that the slug eats.
Energy gain depends on the chemistry of the food, and how much effort is needed to find and
ingest the food. For example, microorganisms that feed on detritus invest little energy in
obtaining the food but the energy content is low. A cheetah hunting a gazelle invests a lot of
energy to find, chase and kill its prey but it gets an energy-rich meal. Food chemistry depends on
cell types from which it is derived (animal cells are more energy rich and have organelles that
are difficult to digest). Most heterotrophs must break down their food before it can be used as
an energy source.
Feeding methods are accordingly very diverse among heterotrophs. Prokaryotes typically absorb
food directly through their cell membranes. They excrete enzymes which break down organic
matter. Multicellular animals have evolved specialized tissues and organs for absorption,
digestion, transport, and excretion. They have tremendous diversity in morphological and
physiological feeding adaptations.
Most of the biologically available energy on earth is derived from photosynthesis Photosynthetic organisms include some archae, bacteria and protists, and most algae and
Leaves are the principal photosynthetic tissue in plants, but photosynthesis may occur in stem
and reproductive tissues as well
Involves conversion of CO2 into carboydrates used for energy storage and biosynthesis
Light and Dark Reactions
1. Light reaction-light is harvested from sunlight and used to split water and provide electrons to
make ATP and NADPH. Sunlight harvesting is accomplished by several pigments (chlorophyll and
carotenoids) Chlorophyll gives photosynthetic organisms their green appearance. These
photosynthetic pigments are embedded in a membrane which lies within chloroplast. Pigments
absorb energy from photons and that energy is used to split water. The electrons produced are
used to synthesize ATP and NADPH. Oxygen is also produced.
2. Dark reaction-CO2 is fixed in the calvin cycle, and carbohydrates are synthesized (despite
daytime occurrence in most plants) ATP and NADPH are used to fix carbon. CO2 is taken up from
the atmospher through the stomates. Rubisco catalyzes the uptake of CO2 and the synthesis of
PGA. PGA is eventually converted to glucose.
6CO2 + 6H2O -> C6H12O6 + 6O2
Photosynthetic rate determines the supply of energy, which in turn influences growth and
reproduction (ecological success)
Environmental controls on photosynthetic rate are an important topic in physiological ecology.
Light is an important determinant of rates of photosynthesis in terrestrial and aquatic habitats.
Light response curves show the
influence of light levels on
Light compensation point: Where CO2
uptake is balanced by CO2 loss by
respiration. Photosynthesis is limited
by light availability below the
Saturation point: When
photosynthesis no longer increases as
light increases (typically reached at a
level below full sunlight)
Coping with Light Variation Plants can acclimatize to changing light intensities with shifts in light response curves.
Acclimatization to different light levels involves a shift in the light saturation point.
Shifts in light saturation point involve morphological and physiological changes. These include
alterations in the thickness of leaves and variation in the number of chloroplasts available to
Photosynthetic organisms can also change
their pigments to absorb more or less sunlight and
the amounts of photosynthetic enzymes available
for the dark reactions.
Water availability influences CO2 supply in
Low water availability causes
stomates to close, restricting CO2 uptake. This is a
trade off (water conservation vs energy
Leaves at high light intensity may have thicker gain)Keeping stomates open while the tissues lose
leaves and more chloroplasts.
water can permanently impair physiological
Closing stomates not only decreases CO2 uptake, but it increases the chance of light
damage. If the Calvin cycle isn't operating, energy accumulates in the light harvesting
arrays, and can damage photosynthetic membranes. Plants have various mechanism to
dissipate this energy, including the use of carotenoids to release it as heat.
Temperature influences photosynthesis by its effects on the rates of chemical reactions and by
influencing the structural integrity of membranes/enzymes
Plants from different climate zones
have enzyme forms with different
optimal temperatures that allow
them to operate in that climate
Plants can acclimatize by
synthesizing different enzyme
Organisms from alpine
photosynthesize at temperatures
close to freezing while desert
plants may have their highest photosynthetic rate at temperatures hot enough to
denature other enzymes.
Temperature also affects membrane composition. Cold sensitivity results in loss of
membrane fluidity which inhibits the function of light harvesting molecules embedded
in the membrane Figure 5.9 Photosynthetic Responses to Temperature (Part 2)
Plants can acclimatize by synthesizing
different enzyme forms.
Nutrients can also affect photosynthesis.
Most nitrogen in plants is associated with Rubisco and other photosynthetic enzymes.
Thus, higher nitrogen levels in a leaf are correlated with higher photosynthetic rates
Why don't all plants allocate more nitrogen to their leaves to increase their