Download Lecture notes for Lecture 3 Chapters 3 (Coping with Environmental

Lecture notes for Lecture 3
Chapters 3 (Coping with Environmental Variation: Temperature and Water)
How organisms respond to their environment
- An organism has two strategies to coping with an environment: to stay and tolerate it or to leave.
While we will not talk about it so much in class, leaving is an option many animals do when they migrate
away from seasonally difficult conditions. While plants cannot migrate, their populations can move over
generations.
- The geographic area where a species lives is called its range. Within its range, environmental
conditions are such that the species can tolerate them.
- There’s a difference between a species “fundamental range” – where a species could live given the
actual environmental conditions such as climate – and it’s “realized range” – a smaller area where the
species does live. This difference often reflects other organisms (predators or competitors) that do not
allow the species to live in some parts of its fundamental range.
- If an individual organism find itself in difficult conditions, sometimes it can ‘acclimatize’ to them, but
slowly changing its physiology, morphology or behavior to adjust to the situation. A good example is
humans at high elevations. If you or I suddenly go into a high elevation environment we will find it
difficult to exercise because the thinness of the air. But over a month or so in this condition our bodies
would adjust to it. For example, our bodies would produce more red blood cells to better capture
oxygen.
- Over generations, populations can also adapt to environmental conditions. This process involves
natural selection and we will return to it in the next lecture. But the basic idea is that those individuals
in the population that best fit the environment will survive and reproduce better than other individuals
and their genes will be found in a greater proportion of the population in the next generation. Note that
an individual itself cannot adapt.
Temperature
- We will first talk about how organisms respond to temperature.
- Temperature is important because only under a certain range of temperatures can the organism
function. If it gets too hot (or cold), its enzymes cannot function. Freezing can actually puncture cell
membranes. So any organism has an optimal temperature range under which it can live.
- A few definitions of how temperature is transferred:
- Conduction. Direct transfer of heat from one object that is touching another. Hcd
- Convection. Transfer of heat into a fluid or gas, such as convection of heat away from the earth
by wind. Hcv
- Evaporation. Loss of heat by converting a liquid to a gas. He
- Radiation. Gain of heat through electromagnetic waves. Hr
- Metabolism: heat can also be given off in the breakdown of sugar. Hm
- The change of heat in an organism = delta (“change in”) H = Hm ± Hcd ± Hcv + Hr - He
- For plants, one of the major ways to lose heat is evaporation of water off of leaves, which is called
transpiration. Plants have specialized stomata cells on the bottom of leaves (why the bottom do you
think?) that open to let water out, and also allow gas exchange. The problem with transpiration for
plants is that they lose water. So plants keep very fine control of how open their stomata are in order to
control this.
- Plants have characteristics that allow them to tolerate extreme environments. For example, desert
plants need to reduce heat but retain water (so they cannot just transpire). Their leaves are away from
the ground to avoid heat gain by conductance. The branches have an open structure to increase
convection through the wind. The leaves are light color and often have tiny hairs that make them appear
white, and the leaves are held parallel to the light. so as not to absorb much radiation.
- In contrast, artic plants need to gain heat. They hug the ground to increase conductance and are thick
and bushy to decrease convection. The leaves are dark color and are held perpendicular to the ground
to maximize the amount of heat they can gain from radiation.
- Animals have an advantage in heat tolerance to plants: they can move. So many animals regulate their
temperature by moving: think of a lizard that lies on a warm rock in the morning, then digs into a cool
burrow during the noon heat.
- Animals that regulate their temperature primarily through energy exchange with the external
environment are called ectotherms. In contrast, endotherms, which include mammals and birds, rely on
internal heat generation by metabolism.
- In order to keep warm, endotherms have extensive insulation (layers of fat for mammals and fur;
feathers for birds).
- Also endotherms tend to have lower surface-to-areas ratios. The surface-to-area ratio is important
because it affects how quickly chemicals move from outside of an organisms to inside, or visa versa. For
example, the lungs have a high surface-to-areas ratios (being composed of many tiny ball-like objects)
and thus efficiently move oxygen into the body.
- Larger objects have a smaller surface-to-area ratios than smaller objects. See Slide 13 to convince
yourself of this fact. Hence endotherms tend to be quite large, and bigger than ectotherms. Ectotherms
need to have a high surface-to-area ratio so they can absorb heat, which is why insects and reptiles and
amphibians tend to be quite small, and why scientists think the dinosaurs may have been endotherms.
- Small endotherms like rodents or hummingbirds have a problem: they need to generate a lot of heat
but they lose it quickly because of their high surface-to-area ratios. So these animals also have another
strategy, which is to reduce their metabolic activity and enter a state of dormancy known as torpor.
They slow their heartrate and may expend 50% fewer calories. Torpor can occur during the night on a
daily basis or for longer periods, when it is called hibernation.
Water balance
- Water balance is obviously important for organisms on land(which usually have more than 50% their
body weight being water). But we will also emphasize that it is a problem even for organisms that live in
water, if the percentage of solutes (primarily salts) in their interior fluids is different than in the
environment.
- Just as water flows downhill, it also flows from high to low concentrations, and hence from a body that
has low amount of solutes to one with a high amount of solutes. This is also called moving from high
water potential to low water potential.
- This movement of water to lower potentials explains the turgor pressure of a plant cell. The interior of
the cell has a high amount of solutes, so water wants to flow in. This causes the cell to expand against
the cell wall which is rigid in plants. This turgor pressure thus keeps the structural components of a plant
(like the stem) rigid. When water is unavailable, turgor pressure goes down, which is what we see when
a plant wilts.
- Another interesting phenomenon of water balance in plants is “transpirational pull”. As we discussed
above, plants lose water from their leaves in the process known as transpiration. As you can see in Slide
17, the water pressure of the air has a smaller (more negative) water pressure than in the leaves, so
water naturally moves out of the leaves. The water pressure of the leaves is lower than in the stem, and
the water pressure of the stem is lower than that in the roots. So water is moving from root to stem to
leaf following the gradient in water potential.
- Another reason for this “column of water” moving from root to leaf and out into the air is that water
molecules have the property of “sticking” together, because they are polar – one side of the molecule
has a slight electively negative charge whereas the other side has a slight positive charge. This force and
water potential moves water up the stem of a tree, even one as high as a 75m sequoia.
- Given that the roots are the water gathering organs for plants, it is not surprising that plants in dry
environments invest much more in root tissue, relatively to the amount of “shoot” tissue (stem + leaves),
than do plants in wet environments (slide 19).
- The problems of water balance in aquatic animals are illustrated by teleost fishes (a very common type
of fish, representing 96% of living species). When these fish are in freshwater, they are hyper-osmotic (=
more salty). Water continually flows into the fish through its gills and to get rid of this excess water, the
fish is continually excreting urine. This means that the fish is also losing solutes (salts), and these kind of
fishes need to expend energy to filter solutes out of their urine and actively uptake salts in their gills.
- When teleost fish are in oceans, they are then hypo-osmotic, with their internal concentration of salt
lower than that of the environment. In these conditions, water tends to flow out of the fish. These fish
need to continually drink water, and they expend energy to pump salts that come in with that water out
in their urine.
- An especially difficult transition is thus faced by anadromous fish – some species of teleost fish that
hatch in freshwater but then travel downstream to the ocean, and live most of their lives in the ocean
only to return to freshwater to spawn (lay eggs). These fish most undergo dramatic physiological
changes as they move from fresh to salt water.
- Organisms that live in a terrestrial environment have adaptations to minimize water loss (hard shells or
skin, kinds of excretion (urine) that are highly concentrated). A champion animal in this regard is the
kangaroo rat, a small rodent that lives in very hot deserts. This rat never drinks! Instead in gains all of its
water from metabolism (respiration):
C6H12O6 + 6O2  6CO2 + 6 H2O + energy (ATP)
And it is very efficient in minimizing water loss in its concentrated urine. You may ask why it is necessary
to have urine at all. This is because in breaking down food, a waste product that accumulates is NH3
(ammonia), which is a poison that must be excreted.
-Similar to the idea of torpor when the temperature is low, some animals are able to desiccate when
conditions are dry. They lose ~ 80-90% of the water in their bodies and go dormant.
Energy
- Gaining energy is a central problem of all living things. There are two basic solutions. The first is to
produce energy oneself using energy stored in chemicals (chemosynthesis) or energy from the sun
(photosynthesis) to make organic compounds (compounds made with carbon-carbon bonds), like
sugars . Organisms that are able to accomplish this are called autotrophs (“trophic” means related to
nutrition).
- The other solution is to consume organic compounds that some other organism has made. Organisms
that use this strategy are called heterotrophs.
- Chemosynthesis was much more common in the beginning of life. Today chemosynthetic organisms
continue to live in deep sea trenches where there is no sun, or in places like sulfur springs, where energy,
stored in H2S, can be extracted. Another important group of chemosynthetic organisms are “nitifying”
bacteria. These bacteria convert and “fix” compounds with nitrogen in them, which means they make
nitrogen, an essential nutrient, more chemically available to other living things.
- Nevertheless, photosynthesis is the foundation of life as we know it. Review the equation for
photosynthesis and compare to that of respiration, above.
6CO2 + 6 H2O + solar energy  C6H12O6 + 6O2.
- Remember that light is a wave and light has different wavelengths. The wavelengths that
photosynthetic organisms are able to use are called photosynthetically active radiation. For example,
the most common sorts of photosynthetic pigments are the chlorophylls. Chlorophylls absorb lights in
the blue and in the red and orange wavelengths (see slide 28). They reflect wavelengths in the green
spectrum and hence appear to us to be green.
- Photosynthesis as a chemical reaction actually has two stages. The first stage is called the “light
reactions” because these steps require light and capture solar energy to make ATP, the short-term
energy storing molecule used in all living organisms. The dark (light-independent) reactions then use this
ATP to put together carbon-carbon bonds. Both these reactions occur inside chloroplasts, the
photosynthetic organelle that has its own DNA (probably because most scientists think it was its own
organisms that long ago got swallowed by other organisms).
- Looking at the light reactions more in detail, this occurs inside “thylakoid stacks”. Here light strikes the
cholorphyll molecule and excites an electron, which moves between different enzymes to the outside of
the thylakoid membrane with its negative charge. This produces a battery: a gradient where the inside
of the thylakoid stack is positive and the outside is negative. Protons (positive charge) then are attracted
to flow out of the thylakoid stack, and through an enzyme that uses their flow, like a water wheel, to
make ATP. See diagram on slide 31; this is of course a simplification of a complicated process.
- The dark reactions occur outside of the thylakoid stacks in the “stoma”. These involve a complicated
series of chemical steps called the “Calvin cycle” that slowly build a long carbon chain from a shorter one.
Note that the first step of this reaction involves an enzyme known as RuBisCo, which attached a carbon
dioxide molecule (CO2) onto the growing carbon chain. But RuBisCo is not the most efficient enzyme. In
fact sometimes (about 1/3 of the time) it actually binds with O2 instead of CO2, lowering the efficiency of
the reactions in a process known as photorespiration.
- Photorespiration is a problem for photosynthetic organisms, especially at high temperatures where
RuBisCO works even less well. The solutions that some plants have found for this problem emphasize
how living things can adapt through natural selection.
- One solution is known as C4 photosynthesis (and this contrasts with C3 photosynthesis, the normal
process described above; C3 and C4 refer to the number of carbons on the smallest molecule of the
carbon chain being built). It has a specialized leaf physiology, with special tissue known as the bundle
sheath. In C4 photosynthesis, the carbon chain is first pre-processed outside the bundle sheath, using
an enzyme called PEP that has a high affinity for CO2. The growing carbon chain is then passed inside the
bundle sheath where a chemical reaction makes CO2, and thus increases the concentration of CO2 inside
this specialized tissue. The Calvin cycle can then take place inside the bundle sheath, with RuBisCo being
more efficient because of the high CO2 concentration there. See slide 34. Again, I don’t want you to
memorize all the specific chemical reactions that are listed in this slide, but just get the general idea of
why C4 is more effective in high temperature and low CO2 environments.
- Another solution to photorespiration is known as CAM photosynthesis. This kind of photosynthesis
runs the dark reactions all at night to avoid high temperatures and reduce water loss.
- Note that C3 photosynthesis is the most efficient type of photosynthesis chemically, as it does not
waste energy getting the carbon chains ready for the Calvin cycle like C4 photosynthesis. So under low
temperature conditions, it is favored.
- Hence low temperature plant communities are dominated by plants that use the C3 pathway. In warm
and arid conditions, C4 (used by some crops like sugar cane and corn) and CAM (common in desert
plants) are more frequent. Your homework problem looks at this regional-scale pattern.
- Also note that C4 and CAM photosynthesis have evolved multiple times in different plants, and are
thus examples of convergent evolution. Convergent evolution is when organisms with different
evolutionary histories come to be similar to each other because they are adapted to the same
environment. They have evolved similar solutions to similar problems.
- Our last section is about the heterotrophs, the organisms that consume organic compounds other
organisms have made.
- There are several different kinds of heterotrophs:
- Predators consume prey, killing the prey.
- Herbivores consume plants, but only parts of the plants so they are not killed.
- Parasites live on hosts, taking nutrition from them, but do not kill the hosts.
- Detrivores eat organisms that have already died, and hence are an important part of
decomposition.
- The textbook breaks down the process of finding food by a heterotroph into three stages: finding food,
consuming food and extracting nutrition from the food (digestion).
- It notes that there is often a trade-off here. Some organisms do not work hard finding food, but then
the food is of poor nutritional quality. For example, bacteria usually consume whatever microorganisms
are near by them. In contrast, other organisms spend a lot of energy finding food -- think of a cheetah
stalking an antelope – that is of very high quality. The better the nutritional quality, the harder to find.
- In general, plant materials are of poor nutritional quality, especially leaves, because they include a lot
of cellulose, a fiber that makes up cell walls and that is difficult to digest. Also plants add “secondary
compounds” that are basically toxins, evolved to discourage herbivores. Many drugs that are derived
from plants are of such compounds. Some parts of plants, however, such as fruits and seeds, have a lot
of carbohydrates, which have a lot of energy.
- In contrast, animals are generally of higher nutritional value, including a lot of fat (more energy per
weight than carbohydrates) and proteins, high in the essential nutrient nitrogen, which is needed for
making nucleic and amino acids.
- Animals have evolved very specialized morphological structures to consume food. Think of birds’ bills
and the diversity of shapes they have. For example woodpecker bills: woodpeckers probe inside trees
for burrowing insects such as beetles. They have a very long tongue that they can shoot into the hole
they make with their beaks.
- An amazing example about how birds’ beaks are adapted to their food comes from the crossbills,
studied by Benkman (2003). Benkman shows that there are 5 types of birds in this species that differ in
the kinds of conifer cones they eat, and each type has a slightly different bill morphology. Benkman
suggests that this species of crossbill is in the process of turning into 5 separate species. We will read
this paper for homework and then discuss it again next class, when we talk about evolution and
speciation.
- Organisms also have a great variety of structures to digest food. Since, as we have discussed, plants are
of low nutritional value, animals that eat plant materials generally have much longer digestive tracts
(more surface area to do more work) than meat-eating animals. Plant-eating animals also have evolved
other means of reprocessing their food, such as cows regurgitating some food and chewing it again
(“chewing cud”) or rabbits eating their feces (“coprophagy”).
-Some animals even make tools to consume food. A famous example discovered by Jane Goodall is how
chimpanzees use a small branch to probe a termite mound and extract termites. More recently, it has
been discovered that New Caldonian crows use sticks to pull worms out of crevasses in bark. Our
reading from last week was about this, and we will discuss it in class.