Chapter 3 –Adaptations to the Physical Environment: Light, Energy and Heat
(38-45, A; 46-47, C; 48-58, A)
Green plants will primarily absorb light of wavelengths of 400 nm (violet) and 700 nm (red), belonging to the
photosynthetically active region of the spectrum. Irradiance: intensity of light of all wavelengths hitting a surface.
Albedo: proportion of light reflected by a surface (albedo of water < plants < ground, soil). Chlorophyll is a plant’s
primary pigment, it absorbs red and violet light and reflects blue and green (thus most plants are primarily green in
colour). Plants also have accessory pigments, namely: xanthophylls and carotene, which absorb and reflect different
wavelengths. Most deep-sea algae can absorb green light and reflect red and violet, this is because water is more
likely to absorb longer wavelengths (like red), especially with increasing depth.
The first stage of photosynthesis is the collection of light. This is accomplished by a plant’s photosystem (consisting
of chlorophyll and accessory pigments.) The light reactions are a series of chain reactions which eventually end
with the creation of ATP and NADPH. H2O is required for these reactions to proceed and O2 is released as a by-
product. The Calvin Cycle produces PGAL or G3P (multi-purposed energy source). Carbon fixation is the first step
in the Calvin cycle. CO2 is added to RuBP and undergoes a series of reactions catalyzed by rubisco. The cycle must
occurs three times with three CO2 molecules reacting with three RuBP molecules to make one G3P, this is because
one of the produced G3P can be used for energy needs while the rest are recycled back into RuBP.
Photorespiration may occur when oxygen levels are particularly high in the plant’s cells. The oxygen competes for
rubisco’s active site, impeding carbon fixation. Considerably less G3P is produced during photorespiration
compared to photosynthesis, making it inefficient. C3 plants can keep their levels of CO2 high by simply keeping
their stomatas open (as they often live in moist environments where water levels aren’t an issue). C4 plants (often
found in dry environments) overcome photorespiration by storing CO2 in the form of malate. CO2 is taken in and
reactions with PEP in the mesophyll layer to produce oxaloacetate which is then converted to malate and moved to
the bundle sheath cells by plasmodesmata for storage. When CO2 is required, malate is carboxylated, relasing CO2
and a pyruvate, the pyruvate travels back to the mesophyll where it is converted back to PEP. The Calcin cycle then
occurs in the bundle sheath layer, which prevents competition with oxygen for rubisco’s active site. It costs the cell
two ATP to move malate to the bundle sheath layer. CAM (crassulacean acid metabolism) plants use a similar
malate-storage method, but they don’t separate the Calvin cycle in bundle sheath cells. Instead, they only open their
stomata at night and close them during the day when they perform the Calvin cycle, blocking the entrance of oxygen
and eliminating competition.
Many plants must also combat against water loss and overheating. In cold, dry environments, plants will grow
spines and hairs to form a boundary layer of still air to capture moisture and reduce evaporation. In hot, dry
environments, plants are more inclined to grow very small leaves, allowing for maximum cooling due to increased
surface area. Equally, some will have no leaves at all and rely on their stems for photosynthesis entirely. They may
also grow waxy cuticles and recess their stomata into deep, hair-covered pits.
Gathering sufficient CO2 for photosynthesis can be a challenge for aquatic plants. Despite there being considerably
more CO2 dissolved in water than in air, much of it is in the form of bicarbonate ions. CO2 and especially
bicarbonate ions diffuse much more slowly through water than air. All surfaces of plants exposed to water have a
boundary layer of still water across which CO2 and bicarbonate ions must diffuse for photosynthesis. As CO2 is
depleted, bicarbonate ions will associate with hydrogen ions to replace replenish CO2.
Anaerobic conditions present in deep water levels (as oxygen dissolves and diffuses less readily in water than air)
pose difficulties for aquatic plants whose roots must respire. Many aquatic plants have vascular tissues which extend
out of the water and conduct air directly to the plant’s roots.
Heat and temperature are also relevant to an organism’s survival. Increased heat energy increases virtually all
physiological processes. The ration between the rate of a process and its rate at 10°C higher is called its Q10. An
entire theory – the metabolic theory of ecology – offers that temperature has an effect on metabolism, growth,
productivity, mutation, etc. However, this positive influence of temperature only extends to a certain point. At
extremely high temperatures, proteins denature and productivity and survival are jeopardized. Some thermophilic
bacteria and archea have higher proportions of amino acids which form more strong peptide bonds than the proteins
of other organisms, this prevents denaturing and allows them to tolerate hotter conditions.
Very cold temperatures can also threaten survival. To protect themselves from freezing, animals which live in very
cold environments (particularly aquatic animals) will either keep their core temperature above freezing or maintain
high levels of glycerol (or other glycoproteins) in their bodies which serve as “antifreeze” agents, lowering the
freezing points of their body fluids.
All organisms have a narrow range of environmental conditions for which they are best suited, i.e. its optimum.
This optimum is determined by an organism’s form, the structure of its proteins, lipids, etc. and its behavior. Ideal
temperature is included in an organism’s optimum. Organisms which function better at different temperatures
accommodate for their environment by having different biochemical pathways.
Heat is constantly moving about within a system, moving from high to low temperature, it does so in four ways:
radiation, emission of electromagnetic energy from a warm surface to a cooler one, e.g. the sun radiates heat to the
Earth, at night objects which were heated by the sun radiate their heat to cooler objects; conduction, the transfer of
heat between two substances in direct contact with one another, e.g. warm foot lose heat to a cold floor, a cold lizard
gains heat from warm rocks; convection, the transfer of heat by the movement of fluids (liquid or gas), e.g. a “wind
chill factor” makes you colder on a cold day as it disrupts the boundary layer of warmth around your body, current
flow either ocean or atmospheric; evaporation, the removal of heat from a substance by the evaporation of water,
e.g. a dog cools itself on a hot day by exposing its tongue from which water evaporates.
Heat budget: all of the gains and losses in heat of an organism. This includes heat lost or gained from the above
four ways along with food, water and salt available to an organism (metabolism).
All heat gains and losses occur across the surface area of an organism. Thus, a rounder organism has a lower surface
area to volume and thus a higher thermal inertia (it can resist more external temperature changes). Size also plays a
role, while larger organisms have a greater thermal inertia, they can have difficulty riding themselves of excess heat.
Organisms with a higher surface area to volume can generally only live in tropical environments (gliding snake).
Homeostasis: the ability to maintain constant internal conditions despite a changing environment, all systems
involved in homeostasis operate by a negative feedback system (i.e. a deviation from normal internal conditions
will signal a correction by the body). Homeothermy: the ability to create constant temperature within one’s cells
(e.g. humans). Poikilothermy: when one’s internal temperature is determined by external conditions (e.g. frogs).
Endotherms can generate enough heat by metabolic processes to change their internal temperature; ectotherms
regulate their internal temperature behaviorally; generally these organisms are small with low metabolic rates.
The ability to maintain one’s internal conditions at a high temperature allows organisms increased biological
activity; however, this process is costly in a metabolic sense as organisms lose heat to the environment when their
internal conditions are higher than that of their surroundings. In the short term, an endothermic homeotherm’s
survival in cold environments is based on their ability to produce heat metabolically; in the long term, it’s based on
their ability to gather food.
Torpor: a voluntary and reversible condition of low body temperature and inactivity, often entered by species living
in cold environments while they sleep; it prevents starvation.
Countercurrent circulation: a system of heat conversation generally seen in the extremities of organisms living in
cold environments. As warm blood travels to extremities through the arteries, it transfers heat to the returning veins
rather than losing it to the environment. This way, warm blood is returned to the core and it can be kept at a suitable
temperature while extremities can remain only slightly above freezing.