Showing posts with label transpiration. Show all posts
Showing posts with label transpiration. Show all posts

Leaves Suck!

We have talked about the reactions of photosynthesis
before. Basically, the plant uses the energy of the sun to
fix carbon (change it from gas to solid) by adding water
to it chemically. Then it splits them again to make energy.
In previous posts we have talked about photosynthesis – carbohydrates and made from carbon (carbo-) with water added (-hydrate, as in- when you are very thirsty, you are dehydrated). Therefore, the leaves must have a constant and reliable source of carbon (from carbon dioxide in the air) and water.

Question of the day:
How do trees move the water up into their leaves, against the force of gravity, in order to carry out photosynthesis?

Water is quite massive (1kg/L or 8.3 pounds/gallon), and a mature oak tree needs 40-60 gallons of water every day. So how does this huge amount of water get to the top of the tree? Does it travel there from someplace else? Could it be absorbed by the leaf from the air in the same way carbon dioxide is brought in? Or maybe plants don’t have to drink and they use the water they make during metabolism, like the kangaroo rat we talked about last year -they don’t drink at all and seem to get along just fine.

We might be able to eliminate one possible explanation right away – what happens when you don’t water your houseplants? Do they grow or do they die? So do you think most plants need a source of external water or could get along on the water they make during aerobic respiration? Right… I think we are down to absorption or movement from some other place on the plant, namely the roots.

Keep in mind that not every plant has to move water from its roots to its leaves, take the bromeliads for instance. Many of these plants don’t have roots, we have discussed how they have special structures that help them absorb water at the base of their leaves.

You could test other types of plants to see if water on just the leaves is enough to keep them alive. How might you do that? Cover the dirt with something that repels water and then just mist the leaves – that might do it. Try it for a while and see how the plants do.

I think that you will find that they do not thrive after the moisture in the dirt is used up. For most plants, 99% or more of the water they use must be absorbed by the roots and transported up the stem (trunk if it is a tree) to the leaves.

Celery stalks and carnations are good to show the flow
of water against gravity. Dark colors show up better.
Maybe you could have races between the two plants and
then cut them crosswise to look at the size of the vessels.
I bet the smaller ones move water faster.
To modelthe answer to our question of the day all you need is a straw. But that isn’t very illustrative or fancy – so how about cut carnation stems or celery stalks (with the leaves) in a glass of colored water. Lighter colored flowers and darker colored water works best (I use blue food coloring), but I have had students who have really gotten into this and tried to measure the time by adding one color, then switching to another and seeing how long it takes the color to change in the flower and if all the color is lost along with the water.

Over a couple of days, the color will indeed be drawn into the petals of the flower. How does the color get there? Is the water level the same? Water is moving up and taking dye with it. So you can see that it does happen – but this still doesn’t explain HOW it happens.

To answer this, you might ask what happens to the water that is being drawn up into the leaves (and flowers of the carnation model). Try putting a baggie over the end of a tree branch and tying it tight.  You will see condensation develop over a day or so. Where did this water come from?

Here are the vessels in a tree. 1) pith – it gets crushed as the tree grows 
2&3) annual growth ringsmade of water carrying xylem. Why 
do you see different rings if they are all xylem? Because spring 
xylem vessels are big, and summer xylem (less water available, so
less growth) vessels are smaller. The line is the change from 
small to big. 6) phloem– this is what carries the carbohydrate to 
the roots and other parts of the tree.
The answer is a process called transpiration (or evapotranspiration). The water evaporates from the leaves, out of pores called stomates, and this creates a negative pressure – like the negative pressure in your mouth when you suck on a straw. This negative pressure actually pulls water up from the roots through the xylem of the plant, to the leaves. In the case of the carnation flower or celery, it also pulled up the very small dye molecules in the water. This evaporative force is quite strong, but not strong enough on its own to lift that 350-500 lb.s (40-60 gallons) needed for an oak tree each day.

The water itself helps in the process. Water is a social molecule, it likes to stick to itself and to other things. It will climb up the sides of container, just look at the meniscusformed in a narrow graduated cylinder when water is added, or note how water travels up a thin capillary tube.

The capillary action comes from the water’s cohesive force, and helps the tree stay hydrated. Evapotranspiration’s negative pressure pulling water up is combined with water’s ability to climb up, and together this is enough to keep the tree’s leaves in the pink, no matter how tall it grows.

But like everything else, there are exceptions, like the plants that don’t have xylem. The non-vascular plants (like mosses and hornworts) only survive based on water absorption and capillary movement from cell to cell. Therefore, they cannot be very tall; you need vessels (xylem) to allow water movement and tall growth. The tallest of the non-vascular plants, the Polytrichummosses, may get to be two feet tall, but that’s it.

Evapotranspiration via vasculature and leaf stomates leads to another question – if water is being lost through the leaves all the time, doesn’t this hurt the plant in times of drought. Well… yes. But plants have evolved some pretty neat tricks to help out.

Some plants don’t use the most forward strategy of
photosynthesis because it would drain them of all their
water during the hottest weather. CAM plants can close
their stomates during the day and only fix carbon dioxide
at night when it is cooler. We should probably talk about
these plants in more detail later this year, they have some
mighty cool adaptations.
1) Stomatescan open and close to regulate water loss. Some plants can close their stomata completely during the hot day, and save their built up radiate energy to convert carbon dioxide and water into carbohydrates only at night, when they will lose less water. Cacti are a good example of this.

2) Leaves, especially the sun-exposed sides of leaves, are covered with a waxy substance called cuticle that greatly reduces the loss of water by diffusion through the cell wall. If water were allowed to travel through the cell membrane and wall, then it would evaporate and set up a negative osmotic and evaporative pressure that would quickly dehydrate every surface cell.

3) Here's a trick many people don’t really consider – many plants have two types of leaves! You might be able to find a tree or two with which to investigate this.

Big leaves have large surface area, so more water will be lost as compared to smaller leaves. Leaves in the direct sunlight should be structured in order to carry out the most photosynthesis, but if they are small, how can this be maximized?

Many trees have sun leaves and shade leaves. Sun leaves are smaller, thicker, have more stomata, and are located where the direct sunlight hits the tree during a good portion of the day.  Shade leaves are bigger, thinner, and have fewer stomates to reduce water loss.

Sun leaves have more layers of pallisade cells, the cells that have the most chlorophyll and do most of the photosynthesis. They are located at the ends of branches, especially on the north side, and on the crown (top) of the tree.

Sun leaves are a smaller and thicker, and they often have
fewer in and outs in their shapes. The smaller shape
reduces water loss, the thicker body provides extra layers
of cells for photosynthesis, the reduced number of cuts
and points…. I have no idea. It is a continuum, leaves that
get a good amount of light land somewhere in the middle.
Shadeleaves have to rely on lower levels of sunlight (they are in the shade), so they have even higher concentrations of chlorophyll than sun leaves, although they are thinner. They can process light more efficiently than sun leaves, so they are actually very important to the plant despite their little time in the sun.

Look at the trees around you, do some have large leave on inner branches and lower on the canopy, while having smaller leaves on top or on the ends? These are probably shade tolerant trees. They have developed the ability to still do enough photosynthesis despite low levels of light.

On the other hand, do you see a tree that has just one size of leaf (not including newly formed leaves) and only has leaves on the ends of the branches? This is probably a shade intolerant tree.

The conifers are an interesting exception, some are shade tolerant, usually the firs, while others are shade intolerant, mostly the pines. However, neither type has sun leaves and shade leaves. Their shade tolerance has more to do with their branch geometry and ability to allow just about all their leaves (needles) see the same amount of sunlight.

Next  week, we will ask if there is any limit to interspecies mating, can you cross a cat with a dog? 
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Keeping Your “Ion” The Ball – Salts and Life

Biology concepts – salts in biology, osmotic potential, action potential, transpiration


Dietary salt – crucial for survival;
Veruca Salt – not so much.
In Latin, verruca means wart, so Roald
Dahl was probably trying to tell us something
when he wrote her character into Charlie
and the Chocolate Factory.
We have learned that one of the crucial functions of water in living organisms is to help regulate the salt concentration in and between the cells (Gimme Some Dihydromonoxide). But why do living things require salts? We all know that we must have a source of salt (sal in Latin) in our diet or we die; the Romans gave it so much importance that part of a soldiers pay was to be used specifically for buying salt – his salary.  But what are its functions?

Water tends to flow from where salts are in low concentration (high water concentration) to where salts are high concentration (low water concentration). Just like other molecules, water diffuses to where its concentration is lower (It’s All In The Numbers-Sizes in Nature). Osmosis (osmo = push in Greek) is the special name given to the diffusion of water, for every other molecule it is just called diffusion.

Too much salt is destructive to cells and organisms, so water helps control the salt held in the body. On the other hand, too much water is also bad for living things (water toxicity), so salts help to control the water concentration. Together, this ratio of salt and water inside and outside of the cell leads to a controlled imbalance called the osmotic potential of the cell. Every living thing has systems to maintain this osmotic potential within a small range (osmoregulation, we will discuss this in more detail soon).


The osmotic potential is measured in units
of pressure (bars). It is equal to the amount
of water that will move in response to a
difference in solute concentration across
a membrane.
When in water, sodium chloride (NaCl, table salt) dissociates into Na+ and Cl- ions, and it is these ions, along with K+ (potassium ion from KCl) that perform many functions in living organisms. Sodium is 10x more concentrated outside the cell, while potassium is 20x more concentrated inside. The slight difference in the charges of the two ions (and the fact that most Cl- is outside cells) sets up a membrane potential in cells.

An important function of this membrane potential is in the neuron (nerve cell), as rapid reversal of the potential along the cell membrane (through ion specific channels) produces an electrical current that we know as the action potential (neural impulse). It is the rapid change in concentrations of Na+ and K+ cations (positively charged ions) inside and outside of the neurons that sends the messages from our muscles to our brains and back, as well as all the thought processes in our brain.


The action potential of the neuron is not simple.
Sodium is higher outside and potassium is higher inside.
When a signal is received (usually from another neuron),
sodium leaks in and potassium leaks out. The slight
difference in the the charge of each means that the neuron
goes from -70 mV to +40 mV. This depolarization travels
down the neuron’s membrane for the entire cell.
Salt's importance is illustrated when their concentrations get out of whack. Too little salt produces symptoms similar to dehydration, with cramping, nausea and confusion. Too much salt results in hallucinations and insanity. The classic example of too much salt intake is being lost at sea. Not having a supply of freshwater, people may start to drink seawater. The salt concentration is too high; their kidneys can’t get rid of all the excess, and the action potentials in the brain begin to misfire. People will see things that aren’t there, and will make critically bad decisions. Many end up swimming away from relative safety and subsequently drown.

We can get rid of some salt through our skin. Is your dog is happy to see you when licking your face after you arrive home, or does he just want the salt? Athletes will often eat bananas to augment their potassium stores and keep the cramps away after exercising. They should really follow that run with a bowl of lima beans; they have much more potassium.

However, munching on black licorice is alot like running a long distance. Glycyrrhizin is the main glycoside (a sugar bound to a non-carbohydrate) in licorice root and is 20x sweeter than sucrose. Glycyrrhizin prevents potassium reuptake in the kidney, so you end up urinating out most of your potassium stores. You could cramp up due to excessive snacking.

Na+ and K+ work in muscle function; cramping and paralysis may result from too little or too much salt. Your heart is a muscle, so changes in salt concentration in the cell can cause heart attacks as well. Many a mystery movie has included the injection of potassium chloride to induce a heart attack. Sodium and potassium cations help maintain proper blood pressure, proper acid/base levels, and proper movement of carbon dioxide from the blood to the lungs. There are precious few functions in which these positive ions don’t play a role.


Collagen and elastin help to make your skin and
joints pliable. O.K., maybe not this elastic – this is
the result of Ehlers-Danlos syndrome, which is
often a genetic disease.
When we think of salt, we usually think of table salt (NaCl), but there are more functions for K+ than there are for Na+, and it is present in higher concentrations in the cell. Potassium is important for the formation and crosslinking of collagen and elastin proteins. These connective tissue proteins hold all your tissues together; they keep your skin from tearing when someone pokes you in the arm, and allow your lungs to expand without ripping when you inhale. So K+ is pretty important even when not working with Na+. It is interesting then that potassium is the only major mineral nutrient for which there is not a recommended daily allowance.

Remember that we often take in these salts as NaCl or KCl. Does the Cl- play a role in organism function? – you bet it does. Chloride anion (a negatively charged ion) is used to produce the hydrochloric acid (HCl) that breaks down the food in our stomachs. Chloride also works in the immune system, hypochlorite (the same active molecule as in bleach) in the white blood cells helps to kill infectious agents and activates other immune system molecules. Chloride is required for the uptake of vitamin B12 and iron and helps control your blood pressure; therefore, Cl- isn’t just that other ion that comes in with Na+ or K+ (or Ca2+).

Chloride ion is elemental chlorine that has gained one electron. This doesn’t seem like much of a change, but it is the difference between life and death. Chlorine itself is a yellowish green gas and it can kill you in a matter of seconds. Chlorine really wants that extra electron, and it doesn’t care if it has to rip it from your lung proteins to get it. When you breathe in chlorine, it reacts with the water in your lungs to produce hydrochloric acid that eats away the cells. It will also react with almost any carbon-containing molecule and further destroy the lung tissue. It was suggested during the American Civil War that chlorine gas could be useful, but it wasn’t until World War I that it was used as a weapon.

Chlorine is poisonous, but we use it to disinfect drinking water and pools. When diluted greatly in water, chlorine does not have the strongly deleterious effect on our cells as it does as a gas, but can still react with and kill microorganisms. Chlorination of water began in the Chicago stockyards around 1908, when the decaying meat and gut bacteria were getting into the drinking water and making the residents sick. The bleach used to disinfect surfaces is much the same as the chlorine used to disinfect 75% of the drinking water in the U.S.; it’s just there in lower concentration. Now chlorine is used in pools as well, and you know it is working because your eyes get red and sting.


Did you know that plants had openings in their leaves called
stomata? Turgor pressure caused by the flow ions in and
out of the guard cells makes the stomata open or close. Their
shape changes based on the amount of water in the guard cell.
There are no exceptions to the rules of salt requirements (weird, isn’t it). All living things need to take in Na+, K+, Ca2+, and even Cl-. Plants use potassium and sodium for water balance, especially to bring morphologic changes like the blooming of flowers. These cations, along with chloride, work in the opening and closing of pores in the leaves (stomata) for the uptake of carbon dioxide and the release of oxygen and water during transpiration (Gimme Some Dihydromonoxide), and in the chemical splitting of water during photosynthesis. It seems that other organisms rely on these ions even more than animals.

All bacteria require potassium and sodium for osmotic regulation and cellular activities.
As the concentration of Na+ in a bacteria’s environment goes up, its dependence on Cl- becomes greater. Fungi, protists, and even viruses depend on salts to remain alive, even though viruses are technically not a form of life. Viruses carry nucleic acid, and salts are needed to balance the charges of the DNA or RNA so it can be stuffed into the viral package, a function within the area of molecular biology.
 

Giardia lamblia and other protozoa use salt ions
to control their osmotic potentials and for other
biochemical functions. Giardia can also change
your potassium levels by causing intense diarrhea
after drinking contaminated stream water.
Molecular biology involves replication of DNA, the transcription of DNA to RNA, and the activities of RNA translation to proteins. K+, Cl-, and Na+ are involved in all these areas. In a feedback mechanism, salt ions control the switches that turn on genes that then control the levels of the ions. If one ion is too high, it will turn on the genes that code for proteins which remove that ion from the cell. Isn’t evolution nifty?

Tightly regulating salt concentration in the cell is important for life, and we have to drink water (kangaroo rats excepted) in order to stay alive. These are the peanut butter and jelly of biology and we will start to see how they work together next time.

For more information and classroom activities on salts in biology, osmotic potential, action potentials, or chloride ion in biology, see:

Salts in biology –

Osmotic potential –

Action potential –

Chloride in biology -

stomata –
http://www.apsnet.org/edcenter/intropp/topics/Pages/OverviewOfPlantDiseases.aspx
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Gimme Some Dihydrogen Monoxide


Birds need water just like the rest of us,
but beaks make it harder. They may suck
it up like a straw or scoop it up like a bucket,
or by leaning back and letting the rain fall in.
At some point or another we've all said, “I’m about to die of thirst.” Of course we can only survive for a few short days without water, but do you know why?

Cells are full of salt water (saline), but are also crowded with proteins, carbohydrates and lipids (saline + organic molecules = cytoplasm). This suggests the importance of H2O, but it doesn’t say anything about the reasons behind its importance.

Water is the solvent (the liquid part of a solution), while the proteins and carbohydrates are the solutes (the solids dissolved in the solvent). Lipids (a type of fat) are insoluble in water; therefore, they are good for building cell membranes. They help keep what is in in, and what is out out. With a lipid membrane, our cytoplasm doesn't leak out on to the floor.


Cytoplasm isn’t water plus some organelles. As shown in
this electron micrograph, it is more like a gel, packed
with organelles, proteins, minerals, sugars, and nucleic
acids. There is water, but just enough to separate the other
constituents. Photomicrograph credit: Dr. Jeremy Burgess/Science
Photo Library.
The intracellular solutes are surrounded by water. It’s like the green jello with pineapple that your Aunt brought every Christmas, except that it's packed to the gills with pineapple. Cytoplasm is more crowded than the public pool on a 104˚F day when the ice cream vendors have gone on strike. In some cases, there may only be a few molecules of water separating different cellular components, but this water layer is crucial.

Water is the solvent in which most cellular reactions take place. Water is made up of an acid (H+) and a base (hydroxyl, OH-). Together, they are two hydrogen atoms and one oxygen, H2O! Having the H+ around keeps the bases in check, while the OH- keeps the acids in check. This helps keep the cytoplasmic pH within a small range (buffers it), about 7.35-7.45. Buffering the cytoplasm ensures that that reactions proceed in the proper direction and at the proper rate.

Water transports materials within the cell, from cell to cell, and through the blood and lymph. The partial negative and positive charges, the high surface tension, and the cohesive properties of water make it good at its jobs.


Water being sucked up in a capillary tube
uses cohesion (water sticking to water) and
adhesion (water sticking to the glass tube).
Water likes to bond to itself (cohesion) via hydrogen bonds formed between the positive H+’s of one water molecule and the negative OH-‘s of two others. Cohesion is what makes water form drops as it rains, and what gives water its strong surface tension. Surface tension is why some insects can land on water and take off again. Water striders (family Gerridae), walk on water and you can actually see the depression in the surface, like when you stand on your bed. They are helped out in this endeavor by hydrophobic (water-fearing) tiny hairs on their legs and feet.

Water also likes to hydrogen bond other surfaces; this is called adhesion. If you pour water into a small diameter glass, you can see it cling to the side (meniscus, Greek for crescent), and even seem to rise up the side of the glass (see the image above). If the glass tube is narrow enough, like in a capillary tube, the water will climb up the tube against gravity. The force that drives this is adhesion.


Water striders spread their weight over a large area to
reduce their pressure on the water. They are also helped
by the hydrophobic proteins on their legs. But mostly, the cohesive
force of the water raises the surface tension so the strider
remains on the surface.
The adhesive force is driven by the bipolar (a negative end and a positive end) nature of water, just as with cohesion. The positive H+ is attracted to any negative molecules, and the negative OH- is attracted to anything positive. Together, they are attracted to most everything, not just other water molecules.

Hydrogen bonding and the adhesion and cohesion they produce are important for plants. How does water absorbed by a redwood’s roots get to its leaves way up high? The mechanism has several features, the most important of which is suction. When water in the leaves evaporates, it creates negative pressure that actually pulls the water up from the roots through the plants vessels.

The negative pressure alone isn’t strong enough to keep the water moving against gravity, but when you add in the cohesion of water molecules to one another, and adhesion of the water molecules to the sides of the vessels, it all works out. The sum total of these actions is called transpiration, and is responsible for moving water against gravity in plants.

Water also participates in many cellular reactions, most famously photosynthesis. During the Calvin cycle of photosynthesis (dark reactions) glucose is produced, water is split into hydrogen atoms that are incorporated into the growing carbohydrate and gaseous oxygen (O2) that is released. It is this transformation of water to gas that drives transpiration.  In cellular respiration, when carbohydrates are used to produce chemical energy (ATP), the exact opposite occurs – water is formed from oxygen and hydrogen.

Other cellular reactions, such as the hydrolysis (hydro = water and lyse = split) of fats or proteins are occurring inside cells all the time. In these types of reactions, a water molecule is split into H and OH while the target molecule is also split in two; one part gains a hydrogen and the other gains a hydroxyl group. This is crucial for the normal degradation of cellular proteins by protease enzymes, amongst other things.

If that wasn’t enough, water acts as temperature buffer, helping organisms hold a more constant temperature. Water does not warm up fast and it does not cool down fast; it tends to keep an even temperature. It has a high specific heat (1 calorie/gram C˚), meaning that you must add a lot of energy in order to change its temperature. Water’s high specific heat evens out temperature fluctuations in the body and allows reactions to proceed in a controlled fashion.

Finally, many organisms use water pressure to hold their form, an example of the turgor pressure we learned about several weeks ago (Plants That Don’t Get A Good Night’s Sleep). For instance, you return home from a trip to find your plants have turned brown and are drooping in their pots. Your goldfish are belly up, and the expensive six-pack in your fridge is now a two pack – the neighbor you asked to look after them did a bang up job. If you’re lucky, the plants stand back up a few hours after a good soaking, especially if you fertilize them with your goldfish carcasses. Your plants need the water for everything we have discussed, but also because the water pressure in the cells keeps them the plant stem and leaves standing rigid.


The tube feet of starfish and other eichinoderms have a
suction cup on the end of the podia. The internal portion
is the ampulla, the tube that holds water to regulate the
tube movement.
In a similar fashion, starfish store and move water through a series of hollow tubes to form a hydrostatic skeleton. In the general sense, this type of skeleton is any fluid filled cavity surrounded by muscle, in which the actions of the muscles work against the fluid pressure in the cavity. Worms, and many other invertebrates have this type of support system.

But starfish take the concept a bit further. Not only is water used to maintain the form and structure of the animal; it makes up the water vascular system for locomotion (tube feet), food transport, and respiration. By moving water in and out of specific tubes in the different arms, the muscles contract and extend the tube feet, pushing them against a surface. The movement of water in and out of the tube feet is also the primary way to move oxygen into the tissues of the starfish, and the water pressure can be used to evert their stomach (it will protrude out their mouth and turn inside out) to surround and engulf food. Ugh!

We always knew water was crucial for life, and now we know why. Its importance is reinforced when you consider how much water there is in different organisms. Humans are about 60% water by mass, but it varies from person to person. Younger children are normally have a slightly higher percentage of water, maybe 70%, while morbidly obese people have much less water, remember that fat is stored in the absence of water (Is it Hot in Here or is it Just My Philodendron?).


The golden barrel cactus has ribs that can expand and
contract, depending on the hydration state of the plant.
It is also called a mother-in-law’s cushion….that’s just mean.
Plants require even more water. Cactuses can be more than 90% water after a good rainfall. The places where cacti grow have variable water availability, so when water is present, they must take advantage. The endangered golden barrel cactus has ribs that can expand to take in more water. In addition, the golden barrel cactus is round to reduce surface area and has a thick waxy surface, both of which reduce water loss.

Despite these dehydration prevention measures, cacti still lose water over time, and it might not be replaced for a long time. Therefore, cacti have evolved mechanisms to withstand the loss of almost 60% of their water without any negative ramifications. In this area, they are the exception. Typical flowers and trees can only withstand a 20% water loss without damage; however, this is still much better than humans can do.

No matter what your personal water percentage might be, you can only afford to lose about 5% of your water without suffering symptoms. At mild levels of dehydration (5%), you may feel groggy or get a headache. Higher levels of water loss will bring tingling in the muscles, nausea, and confusion. If the loss reaches 10-15%, there can be muscle spasms, delirium, and the kidneys may be permanently damaged (if water loss is held for a sufficient period). Held above 15%, dehydration is usually fatal. However, athletes can lose up to 30% of their body water in the short term, but it must be replenished immediately so that performance or normal function will not be compromised.

When we say normal function, we mean those functions of water we have mentioned, but also several others we haven’t. Water, along with surfactant proteins, works to keep our lungs absorbing oxygen. Water lubricates our joints and tissues to avoid friction damage. People with xerostomia (Greek, xero = dry and stoma = mouth) or xerophthalmia (dry eyes) use artificial saliva or tears to prevent damage to mucous membranes. Finally, water acts as a cushion, absorbing pressure and force to protect our organs from traumatic damage, like a punch to the gut.

Damage can come in many forms when water is low, so all living organisms require water intake to function and remain safe, right?……Or is just most organisms? Next time.

For more information, classroom activities, or laboratories about water in biology, the properties of water, transpiration, or the Calvin cycle, see:

Water in biology –

properties of water –

transpiration –

calvin cycle –
http://www.educationalrap.com/song/photosynthesis.html
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