It’s A Plant World, We’re Just Living In It

Biology concepts – cell walls, chloroplasts, myco-heterotrophs, holoparasites,

Life on Earth is easy. It can be boiled down to three sentences. “The mitochondria and the chloroplasts are, in a fundamental sense, the most important things on Earth. Between them, they produce oxygen and arrange for its use. In effect, they run the place.” Lewis Thomas wrote this in his award winning book, The Lives Of The Cell: Notes Of A Biology Watcher, in 1975.

Nature’s carbon recycling center. The sun’s energy is used to 
polymerize carbon (CO₂) into carbohydrates (CHO) and releases 
oxygen (O₂). Then the mitochondria use the O₂ to break down 
the CHO, resulting in chemical energy (ATP) and carbons (CO₂) 
ready to be polymerized again.

He was so right - for the organisms that use them. I guess he didn't consider the exceptions. These two organelles mesh seamlessly in their functions. One produces carbohydrate and oxygen, while consuming carbon dioxide. The other consumes carbohydrate and oxygen and produces carbon dioxide. The ultimate recyclers.

If these two organelles are the most important things for life, then doesn’t that make plants the kings of life on Earth, since they have both chloroplasts and mitochondria? Makes you feel a bit more humble now about your place in world, doesn't it.

However, this brings up an essential question – and the main focus of today’s topic and exceptions. What makes a plant cell a plant cell? Green algae have chloroplasts and mitochondria, but they aren’t plants, they belong to the kingdom Protista. We have discussed the sea slug, E. chlorotica, and its ability to photosynthesize – it is certainly not a plant. So what makes a cell a plant cell?

Leaving the chloroplast out of the equation for a minute, you could argue that a plant cell is one with a cell wall and cell membrane. That surely separates them from animal cells, since animal cells only have the cell membrane. But many bacteria, archaea, fungi, and algae have cell walls. If the argument is refined to define a plant as having a certain kind of cell wall, then we must look a little closer. Many cell walls are made of sugars, but are plant cell walls unique in their constituents?

True bacteria have two large groupings, Gram+ and Gram -,

based on their cell wall structures. The gram stain sticks to

the peptidoglycan layer, so the thick layer on G+ bacteria make

them stain deeply. The lipopolysaccharide (LPS) layer of the G-

species keeps them from staining, and is highly toxic.
Endotoxin (LPS) and causes about 70% of septic shock cases.

Bacteria cell walls are made of peptioglycan (peptido = amino acid containing, and glycan = polymer of two sugars). One of the two components is always N-acetylmuramic acid, and the other is often poly-N-acetylglucosamine, but other things can be included as well. The exception is the Mycoplasma, a group of small bacteria that don’t have a cell wall at all. Since many antibiotics function by disabling the bacterial cell wall or preventing its formation, they don’t work against mycoplasma infections like M. genitalium, which a 2011 study linked to pelvic inflammatory disease in women.

Fungal cell walls are also made of a polysaccharide (poly = many, and saccharide = sugar), in a polymer called chitin. Chitin is also the rigid polymer that makes so many insects crunch when you step on them. Chitin cell walls are defining for fungi, as many cellulose containing cell wall fungi have been moved out of the kingdom of Fungi. But this still doesn’t tell us what is unique to plant cell walls.

Plant cell walls contain cellulose, and is complex. Plant cell walls can contain up to three layers, with different sugars involved, including cellulose, hemicellulose, and pectin, and lignin. Lignin is a more rigid polysaccharide that gives strength. It is what makes bark hard, protective, and water resistant.

If the hydrogens (H) bound to the #1 and #4 carbons

up on the same side, the polymer is starch. If they

are on different sides, the polymer is cellulose.

We can digest starch: we can’t digest cellulose.

Plants make both – the part we can’t digest we call

dietary fiber.

Cellulose is made of a chain of glucoses, yet we can’t digest it. The number one carbon in glucose has an –H that is sticking up or down. If the –H sticks “down”, then it is an alpha glucose. If it sticks “up”, then it is a beta-glucose. Cellulose is linked chains of beta-glucose. Starch is linked chains and branches of alpha-glucose. Just that difference in –H position determines if it is food for us or not. Herbivores have the enzymes (and bacteria) to digest cellulose, but not us.

So is it the inclusion of cellulose that makes a plant cell wall unique? Well, no. Algal cells also use cellulose in their cell walls. You might try to argue that algae are plants, since many of them also have chloroplasts and are primary producers – but you would be wrong. Algae can be unicellular (although they can also be multicellular) while plants are all multicellular. Algae don’t have specialized reproductive cells or parts like plants do; algae reproduce by spore or from broken parts of themselves. Finally, DNA analysis shows that while plants and algae are monophyletic (one ancestor), they diverged from one another long ago.

Then there is the issue that not every plant cell has a cell wall. In angiosperms (angio = chest or vessel, and sperm = seed; plants with enclosed seeds and flowers), the gamete (sex) cells of the male in the pollen and the gamete cells of the female in the ovary do not have cell walls, at least not on all sides. The ovary contains the ovules (latin for small egg), and the pollen contains the sperm cells and the tube cell, that forms the pollen tube and delivers the sperms cells to the ovules.

After the ovules are fertilized by the sperm cells of the pollen, the ovules form the seeds, and the ovary forms the fruit. From here on in, all the daughter cells will have cell walls. For fertilization, it would make sense that the involved cells would not have a cell wall that would just get in the way of love.

The Sago Palm isn’t a palm, but is one of the most

primitive plants that reproduces with seeds. It

presents a problem to pet owners because every part

is toxic to pets, but it tastes good to them. They don’t

know not to eat it; then they bleed to death.

And even weirder, not all plants use just this strategy. Cycads (like the sago palm, which isn’t really a palm at all), and gingko biloba plants have sperm cells with flagella, long projections that whip and move them along, hopefully toward an egg cell. They don’t use a tube cell or pollen tube; these plant cells without cell walls swim. Plant cells that move, now there is an exception worth noting! Some more primitive bryophyte plants (liverworts, mosses) also have motile sperm, but the cycads and gingko are the only examples of seed plants with motile cells.

So cell walls aren’t a defining characteristic of plant cells either. Maybe it is the chloroplast that defines a plant cell --- maybe not.

As you can guess, there are exceptions going both ways. There are organisms that have chloroplasts that aren’t plants, namely the algae. But a more interesting exception are many of the protozoan Euglenids. Euglena gracilis is a prototypical euglenid that can produce carbohydrate by photosynthesis. However, most euglenids can also eat things, which makes them both autotrophic and heterotrophic.

As for the other direction, there are many plants that don’t have chloroplasts. Of the roughly 350,000 different species of plants on earth, almost 3000 of them are non-photosynthetic. Therefore, the most common characteristic that people use to tell a plant from a non-plant (photosynthesis by chlorolplasts) isn’t true for almost 1% of the species on Earth. That is a pretty big exception. That would be like saying 1% of people on earth don’t have a brain! O.K., maybe that's a bad example.

Indian Pipe is Monotropa Uniflora. Monotropa means

one turn, and uniflora means one flower. The plant is

called the ghost plant – obvious, or the corpse plant –

because it turns black as it matures. This naming thing

is easy!

Indian pipe (Montropa uniflora, or ghost plant) is one such plant. Related to the blueberry of all things, the ghost plant has gone its own way and become parasitic. It garners its nutrients and energy from the tissue of another plant. The roots of the Indian pipe penetrate the rhizoids (root-like projections) of certain types of fungi and sponge off their hard work. In fact, the fungi themselves are symbiotic, having invaded the roots of certain pine tree species.

The fungus and tree live together in a mutualistic relationship, making the fungus a mycorrhizal (myco = fungus and rrhizal = root) variety. The tree supplies the fungus with carbohydrate, and fungus supplies the tree with mineral nutrients. However, Indian pipe does not respect this mutualism and is a parasite of the fungus, taking some of the carbohydrate supplied by the tree. This makes the Indian pipe a myco-heterotrophic parasite.

Other plants without chloroplasts are holoparasitic (gain nutrients only by parasitism).  These would include the rafflesia species of the Indonesian rainforests. These plants are know for having the largest single flowers in the world, some the size of car tires! The plant doesn’t have a stem or root or leaf, it is a vine that grows inside another type of vine. Only when it is ready to flower does it bud out from the bark of the host. The flower takes nine moths to develop, and then smells like rotting flesh in order to attract fly pollinators.

Rafflesia is also known as the corpse flower, as opposed

to the corpse plant (Indian pipe). This is because it

smells like a corpse in order to attract the flies that

pollinate it. This young man is either holding his breath,

has no sense of smell, or is just really odd.

In addition to holoparasitic plants, plant cells without chloroplasts would include those same gamete cells we discussed above as not having cell walls. And neither to do most root cells. However, there are exceptions, like many of the orchids. The ghost orchid has photosynthetic roots, which is a good idea, since they grow directly on other plants; their roots are not buried in the dirt.

Maybe it is not a single characteristic that makes a plant cell a plant cell, or a plant a plant. Maybe it is the combination of cells with cell walls, central vacuoles and in most cases, chloroplasts that make it a plant. I guess it is like beauty; you can’t define it, but you know it when you see it.

Next week we will take another shot at finding a defining characteristic of plant cells, namely the plastid, the mother of all chloroplasts – might there be an exception?

Mizukami I, & Gall J (1966). Centriole replication. II. Sperm formation in the fern, Marsilea, and the cycad, Zamia. The Journal of cell biology, 29 (1), 97-111 PMID: 5950730

Nikolov LA, Tomlinson PB, Manickam S, Endress PK, Kramer EM, & Davis CC (2014). Holoparasitic Rafflesiaceae possess the most reduced endophytes and yet give rise to the world's largest flowers. Annals of botany, 114 (2), 233-42 PMID: 24942001

For more information and classroom activities on cell walls or parasitic plants, see:

Cell walls –

http://www.biology4kids.com/files/cell_wall.html

http://www.scienceprofonline.com/microbiology/bacterial-cell-wall-structure-gram-positive-negative.html

http://library.thinkquest.org/C004535/cell_wall.html

http://www.ccrc.uga.edu/~mao/intro/ouline.htm

http://micro.magnet.fsu.edu/cells/plants/cellwall.html

http://www.cellsalive.com/cells/cellwall.htm

http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit1/prostruct/cw.html

http://employees.csbsju.edu/ssaupe/biol327/lecture/cell-wall.htm

http://www.biologie.uni-hamburg.de/b-online/e26/26.htm

http://www.answers.com/topic/how-does-a-primary-cell-wall-differ-from-a-secondary-cell-wall

www.umsl.edu/~microbes/introductiontobacteria.pdf

http://www.scienceprofonline.org/microbiology/gram-negative-bacteria-cell-wall.html

http://tami-port.suite101.com/how-do-antibiotics-work-to-kill-bacteria-a74616

Parasitic plants -

http://waynesword.palomar.edu/plnov99.htm

http://www.botany.org/parasitic_plants/Hydnora_africana.php

http://www.teachervision.fen.com/dk/science/encyclopedia/parasitic-plants.html

http://www.bbc.co.uk/learningzone/clips/parasitic-plant/12796.html

http://www.ehow.com/info_8575832_two-rain-forest-illustrate-parasitism.html

http://www.moreheadplanetarium.org/index.cfm?fuseaction=page&filename=science360_flowerpower_videouse.html&print=1

http://www.montgomeryadvertiser.com/article/20120427/LIFESTYLE/304270006/Parasitic-field-dodder-very-hard-control

http://www.parasiticplants.siu.edu/

http://parasiticplants.blogspot.com/

http://www.apsnet.org/edcenter/intropp/pathogengroups/pages/parasiticplants.aspx

http://www.forestpathology.org/mistle.html

http://www.gardenbuildingsdirect.co.uk/Article/parasitic-plants 

Is It Hot In Here Or Is It Just My Philodendron?

Biology concepts – thermoregulation, pollination, tropisms, flower structure, plant communication

In many ways, plants are “smarter” than people (forgive the anthropomorphism). We can change our environment to suit our needs or move to a better environment. But plants can’t flip the light switch, can’t buy a bottle of water to quench their thirst, can’t turn on the air conditioner, and can’t hire a truck and move all their stuff to a better place.

Plants can react to many physical signals. We can sense gravity, but they can

differentiate parts of themselves with gravity – roots grow towards gravity

(positive geotropism) and stems grow away from gravity (negative geotropism).

So what can plants do given these limitations? They can make their own food (photosynthesis) – they’ve got us beat right there. They can turn to face the light (phototropism) or the sun (heliotropism). These abilities were explained by none other than Charles Darwin and his son in an elegant series of experiments in 1880.

Plant stems can grow away from gravity (negative geotropism or gravitism), while their roots grow toward gravity (positive geotropism) or water (hydrotropism). Finally, plants can twist around a wire and hold on (thigmotropism). Pretty talented, wouldn't you say? 

But wait, there's more. Plants can also communicate with one another. They alter their biochemistry to become less appealing to predatory insects or microorganisms, and their responses become better with each attack. After they develop a good defense for a particular predator, they will warn nearby members of the same species via dispersed chemicals. The warned plants then generate the best defense the first time they are attacked.

Plants can also recognize kin – and be nice to them. Research shows that plants grow less aggressively when surrounded by seedlings from the same mother plant compared to when surrounded by non-kin competitors. I wish I could get my kids to act that nicely towards one another.

Plants also commune with animals. The acacia tree has an arrangement with the ants that live on it. The tree produces hollow thorns for the ants to live in, and produces food for them to eat. In exchange, the ants protect the plant from predators such as caterpillars by attacking them. The ants will also prune away dead leaves and destroy nearby plants that might compete with their tree for light.

The acacia tree provides hollow thorns for ants to live in; the tree’s wood is so hard

that the ants can’t hollow it out on their own. The acacia wood was once used as nails. 

The acacia is related to the mimosa (sensitive plant) we discussed previously.

This is a great arrangement for both ant and tree (symbiotic mutualism), but becomes tricky during pollination. The ants will attack any insect that touches their tree; even potential pollinators.  So the acacia produces a chemical at the flower when an insect lands to feed on the nectar; it says, “this guy is O.K., don’t kill him.” Amazing - I can’t get the cats to come when I call them - and I feed them! Maybe saying someone is as dumb as a potted plant isn’t much of an insult.

Plants may be “smart” about temperature as well. They don’t regulate their own heat, and are usually the same temperature as the surrounding environment. Remember from the last post that it takes lots of energy to be an endotherm, so ectothermic plants enjoy great energy savings by adopting room temperature as their own.

A few plants can spike their temperature for a short time, usually to attract pollinators, but they can’t regulate the temperature. It is like setting a fire; it burns at as high a temperature as the fuel will allow, and then goes out.

P. selloum grows in tropical environments, but can

be found as a landscape planting in Georgia, the

Carolinas, and the gulf coast. It can grow 8 meters

(26 ft) tall and the leaves can be 1 meter (3 ft) in width.

Our exception to the rule of plant ectothermy is the philodendron. Many species of this genus can raise their temperature during the period when they produce pollen, and can regulate that temperature over a short period of time (2 days). The species Philodendron selloum (P. selloum, also called Philodendron bipinnatifidum, split leaf philodendron, tree philodendron) has been the most studied and will serve as our model.

P. selloum flowers in a structure called an inflorescence. This consists of a covering spathe and a spadix in the center. The flowers are located on the spadix, with a specific arrangement of male and female flowers, making the philodendron a monoecious plant (male and female on same plant). However, the flowers are incomplete, since each individual flower has only the male (pollen producing stamen) or female (ovule containing pistil) organs.

The flower of P. selloum is about 25 cm (10 in)

tall and the flowers are plain white, as it does

not use color to attract pollinators.

The male flowers are located on the top half of the spadix, while the middle region contains sterile male flowers, and the female flowers are located at the base. This arrangement, with the sterile gap in the middle, decreases the chances that the pollinators will pollinate a female flower on the same plant (self-pollination).

Self-pollination reduces genetic diversity as the offspring are clones of the parent (we will talk more about this next time). Also to help prevent self-pollination, the male flowers produce pollen in the first evening of the anthesis; the time period when the flower is open and fully functional. The female flowers can receive pollen on the second evening.

The spadix can reach and hold temperatures of 45 ˚C (113˚F)

and is most concentrated in the sterile male flowers. The female

flowers don’t produce heat, as this would damage the ovules.

P. selloum raises the temperature of the spadix, specifically the male flowers. The attractant is a female beetle sex pheromone that makes male beetles of a specific species think that potential mates are on this particular flower. To maximize the effect of the pheromone, the increased temperature of the spadix volatilizes the chemical (evaporates it into the air) so it can spread a greater distance. The beetles just follow their nose back to the correct plant!

The heat comes from a special reaction within the plant. Photosynthesis is actually an endergonic (energy consuming) reaction, it eats up heat, leaving the plant cooler. But respiration (creating ATP from the carbohydrates of photosynthesis) is exorgonic (heat releasing). These two processes are basically a wash, so P. selloum needs another way to generate the heat for the spadix.

Moreover, the P. selloum heat production must correlate to the time when the pollen is mature, must be localized to the spadix, and must be regulated. To do this, the philodendron has independently evolved the same trick that human babies use to stay warm!

Babies have a big surface area compared to their volume, so they tend to lose heat rapidly. This is why parents dress babies warmer than they dress themselves. To generate more heat, babies have brown fat (brown adipose tissue or BAT). BAT has more mitochondria than regular adipose (fat) tissue, and the iron in the mitochondria make this fat appear almost brown in color. The increased mitochondrial number helps to generate more heat as the fat is metabolized.

Fat is metabolized to generate heat instead of carbohydrates because it has more energy. Fat carries almost 9 kcal/gm, while carbohydrates contain only 4 kcal/gm. This is also why fat is used to store energy, it would take more than 2.5x the volume to store the same energy if it were all in the form of carbohydrate, especially since carbohydrates are connected with water when stored, while fats are not. Being fat is actually the most compact way to store energy.

Brown adipose tissue (BAT) has a centrally located nucleus and

several small lipid droplets in order to make room for the many

mitchondria. On the right, white fat cells have an offset nucleus

and are completely filled with a single lipid droplet.

To really up the heat ante, the mitochondria have an uncoupling protein (UCP) that disconnects the burning of fat from the generation of ATP. Instead of putting some of the energy into making ATP, all the energy is put toward giving off heat. Since babies aren’t coordinated enough to exercise to increase heat, and shivering isn’t that efficient, this non-shivering thermogenesis (NST) is their way to stay warm.

It was thought that adults didn’t have BAT, but recent studies indicate that most adults have some, and some people have a lot. BAT generation can actually help keep you thin, because the BAT is more readily metabolized –regular fat is a guard against bad times and the body holds on to it tightly, but BAT it is meant to be burned. New research suggests that chronic cold can stimulate BAT development, so forget your winter coat and just freeze your way into that size two.

P. selloum has developed BAT as well, an excellent example of convergent evolution (unrelated organisms develop similar characteristics). Plants use the alternative oxidase protein to uncouple fat metabolism from ATP generation instead of UCP, but the process is nearly the same. Using non-shivering thermogenesis, P. selloum can raise the temperature of the spadix to 104-113˚F and hold it there.

More amazing, P. selloum can somehow sense the ambient temperature and keep the spadix temperature 20-30˚F above that of the environment during that first evening. During the second day, the temperature is held around 80-95˚F, but is not controlled so stringently. The second evening sees a slow, regulated decrease in temperature to ambient by the third morning. It's a complex mechanism, but the payoff is survival of the species.

The whole thing is pretty smart for a plant, or for any organism. Next time, we will investigate the relationship between the pollinator beetle and P. selloum, and how limiting pollination to one species of beetle breaks a rule.



Ito K, & Seymour RS (2005). Expression of uncoupling protein and alternative oxidase depends on lipid or carbohydrate substrates in thermogenic plants. Biology letters, 1 (4), 427-30 PMID: 17148224

Dötterl, S., David, A., Boland, W., Silberbauer-Gottsberger, I., & Gottsberger, G. (2012). Evidence for Behavioral Attractiveness of Methoxylated Aromatics in a Dynastid Scarab Beetle-Pollinated Araceae Journal of Chemical Ecology, 38 (12), 1539-1543 DOI: 10.1007/s10886-012-0210-y

Technorati claim token  TTVE88EU8BQT

For more information, classroom activities, or laboratories on tropisms, pollination, plant communication, or P. selloum:

Plant tropism –

http://plantsinmotion.bio.indiana.edu/plantmotion/movements/tropism/tropisms.html

http://www.discoveryeducation.com/teachers/free-lesson-plans/the-importance-of-tropisms.cfm

http://leavingbio.net/Plant%20Responses.htm

http://www.botanical-online.com/lasplantasmovimientosvegetalesangles.htm

http://virtualastronaut.tietronix.com/textonly/act25/text-plants.html

www.the-aps.org/education/k12curric/activities/pdfs/watson.pdf

www.spacegarden.net/downloads/Gravitropism.pdf

www.sas.upenn.edu/~ltlick/Pedagogy.pdf

www.pjteaches.com/PDF/LifeScience/labs/lab13f.pd

http://www.sciencebuddies.org/science-fair-projects/project_ideas/PlantBio_p041.shtml

http://www.hcs.ohio-state.edu/hort/biology/Lab/tropismslab.html

pollination –

http://www.mbgnet.net/bioplants/pollination.html

http://www.fs.fed.us/wildflowers/pollinators/

http://www.brainpop.com/science/cellularlifeandgenetics/pollination/preview.weml

http://www.nbii.gov/portal/server.pt?open=512&objID=222&mode=2&in_hi_userid=2&cached=true

http://www.youtube.com/watch?v=ge3EM8AERV0

http://www.discoveryeducation.com/teachers/free-lesson-plans/plant-pollination.cfm

http://www.smithsonianeducation.org/educators/lesson_plans/partners_in_pollination/lesson1_main.html

http://ag.arizona.edu/pubs/insects/ahb/lsn43.html

www.ableweb.org/volumes/vol-22/12-puterbaugh.pdf

http://userwww.sfsu.edu/~biol240/labs/lab_13pollination/index.html

plant communication –

http://www.sciencedaily.com/releases/2009/06/090619171244.htm

http://www.animalintelligence.org/2007/10/15/does-plant-communication-imply-intelligence/

http://www.euromed.org.uk/plant-communication.html

http://www.csrees.usda.gov/newsroom/lgunews/plants/plant_stress_signal.html

http://www.nytimes.com/2009/12/22/science/22angi.html?em

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0013324

http://indianapublicmedia.org/amomentofscience/plant-communication-allergies-related/

http://www.usask.ca/education/coursework/mcvittiej/resources/livingthings/plants.htm

http://www.ted.com/talks/stefano_mancuso_the_roots_of_plant_intelligence.html

P. selloum –

http://www.floridata.com/ref/p/phil_bip.cfm

http://faculty.ucc.edu/biology-ombrello/pow/Philodendron.htm

http://www.exoticrainforest.com/Philodendron%20bipinnatifidum%20pc.html

http://www.pburch.net/plants/C3397731/E20060506081651/

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 model the 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. Hint - it isn't capillary action. Even in a very thin capillary tube, water will only move up a few centimeters. How could it possible move from the roots to the top of a redwood tree?

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 rings made 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 meniscus formed 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 Polytrichum mosses, 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) Stomates can 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.

Shade leaves 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? 



von Caemmerer, S., & Baker, N. (2006). The Biology of Transpiration. From Guard Cells to Globe PLANT PHYSIOLOGY, 143 (1), 3-3 DOI: 10.1104/pp.104.900213

Terashima, I. (2005). Irradiance and phenotype: comparative eco-development of sun and shade leaves in relation to photosynthetic CO2 diffusion Journal of Experimental Botany, 57 (2), 343-354 DOI: 10.1093/jxb/erj014

Christmas Trees Have Trouble Seeing The Light

Biology concepts – photoprotection, photosynthesis, non-photochemical quenching, reaction center, yule, evergreen, chlorophyll

Yule was/is a pagan celebration in midwinter. Krampus

was the spirit who came during Yule to punish children

who had misbehaved. Yule celebrations used evergreens

(note his headdress) and this has continued in the

modern Christmas celebration, but the Krampus became

paired with good Saint Nicholas, so they kind of went

the other way with that one.

Christmas trees are pagan holdovers from when early Christianity adopted December 25^th as the date of the holiday. Pagan religions used evergreens as a reminder that the Earth would bloom with life again even in the winter when the sun was scarce. The joke is on them though – biology shows us that evergreens have to protect themselves from the life giving sun.

The Romans decorated their houses with evergreen boughs in December, and in particular, the Germanic and Norse pagans celebrated the jexla or jol (respectively) festivals in late December until early January. In English, this became Yule, or Yuletide (yule time).

Evergreens, as the name suggests, stay green all winter; they don’t drop the majority of their leaves (needles) and they keep chlorophyll in their leaves all year round. Chlorophyll is green, so there you have it - evergreen.

Chlorophyll is energetically costly to make and maintain, so if there isn’t enough sunlight to make photosynthesis a net gain (once you subtract the energy needed to make and maintain leaves and chlorophyll); a good strategy might be to just drop them and start over again in the spring.

So one reason for being an evergreen would be that there is enough sun and moisture in a certain location to warrant chlorophyll production and maintenance year round. But there might be a more pressing reason for keeping leaves all year round - nutrients.

When deciduous trees lose their leaves, there are letting a lot of nutrients blow away with the wind – usually into my yard. This is costly, especially if you grow in nutrient poor soil like many evergreens do. In cold climates, leaf litter doesn’t decay fast enough to allow nutrients back into the soil, so holding on to your nitrogen is a good idea. Conifer needles tend to be lower in nitrogen than deciduous leaves, so they hold on to their nutrients better.

Conifer forests are often quite bare on the ground. This is

for a few reasons. One, the ground is usually nutrient poor

and fewer things can grow there. Two, the needles cover

the ground and reduce the light for seedlings. And three,

needles are acidic and make the ground acidic, making it

even harder for other things to grow. Fewer competitors

makes it good for the conifers.

In addition, being able to grow in acidic soil or soil that has less nitrogen and phosphorous is an evolutionary strategy for the evergreens; if fewer plants can grow there, than then will have fewer competitors for what nutrients there are and for sunlight. Some evergreen litter is designed to make soil acidic, so that fewer competitors will try and put down roots.

The result of all this is that evergreens, conifer trees and some bushes, stay green year round. But they’re just fighting to stay alive, not growing year round. It turns out that evergreens spend a lot of energy to avoid sun damage caused by the very mechanisms that allow them to gather sunlight light year round. In winter: chlorophyll + sunlight = death. Just like vampires, they have to protect themselves from the suns rays, or they burn up.

The truth is, evergreen trees in cold climates do very little photosynthesis in the winter, even if there is sunshine. They'll make carbohydrates from sunlight, water, and CO₂ (the three ingredients needed for photosynthesis) when they’re each available, but available is a relative term.

Air is always available, so CO₂ isn’t problem. The sun is at a lower angle in the winter, but it isn’t cloudy and gray every winter day; it just seems that way. So sunshine is available at least part of the time - usually daytime.

The large diameter tubes are vessels, while the narrower

ones are tracheids. Conifers only have tracheids, no vessels.

This is good for growing in cold weather environments.

When water freezes in the vessels, the width promotes gas

bubble formation. This will lock the vessel and no water can

ever be transported again. The narrow tracheids of conifers

prevent gas bubble formation.

The problem is water. Sure it’s there, but it may be solid. If the weather is cold enough to freeze the water on and in the shallow soil, it may also be cold enough to freeze the water in the tree trunk and leaves. Any photosynthetic plant, including trees, needs to split water into hydrogen and oxygen during photosynthesis. If they don’t have a source of water, then they can’t perform that little miracle that is the source of all life on Earth.

Even temperatures near freezing can slow down water movement in plants, and since cold air holds less water (humidity), more water can be lost from leaves - even the wax covered, thick leaves of conifers. The end result is that photosynthesis is just not feasible during most points of the winter.

Yet the evergreens don’t drop their leaves and keep their chlorophyll in their chloroplasts. That means that every time the sun comes out, some of the energy of the rays are caught and transferred to the photosynthetic reaction centers, including chlorophyll. If photosynthesis can’t be completed because of a lack of available water, then what happens to all that energy? It is free to bounce around and damage plant tissues, usually in the form of reactive oxygen species (ROS, see this post). Enough damage and the plant will lose its ability to function and die.

To avoid the irony and embarrassment of becoming a dead midwinter symbol of life, evolution has provided plants with certain photoprotective mechanisms. Not just evergreen plants, but all plants. It turns out that photosynthesis pathways are saturable, only so much sunlight can be used to produce energy and then carbohydrates.

The sun is unwilling to play the game; once the saturation limit is reached, it just keeps on shining. On bright summer days, just about any plant is susceptible to damage from excess energy absorption in chloroplasts. Some plants have elaborate mechanisms to change the angle of their leaves so that they receive less sunlight.

Some plants, like this Oxalis triagularis can quickly change the

angle of their leaves so that low levels of sun can be maximized

or that high levels of sun can be avoided. Evergreens can’t do t

his, so they need more mechanisms of photoprotection

Evergreens, especially conifers, can’t regulate the amount of light that shines on them, so they need additional photoprotective mechanisms. Plants of the genus Taxus, like English yew, move their chloroplasts (discussed in this post) instead of their needles.

According to a 2007 study from Japan, yew cell chloroplasts congregate in the center of the cell volume in response to low temperatures, whereas in summer they can be found along the edges closest to where the light comes in. In this way, many chloroplasts can be shielded from sunlight in the winter, so less energy will be harvested. Pretty smart.

For many evergreen plants, their chlorophyll is doomed to harvest sunlight all winter without being able to use it for photosynthesis, so what can they do with it - other than just let it damage them until they die?

It is important that the plant dissipates the light energy before it reaches the reaction center. Chlorophyll isn’t the molecule that changes sun energy to chemicoelectrical energy. Chlorophyll is the pigment that absorbs the energy. That energy can be transferred to an adjacent chlorophyll molecule and so on until it reaches the reaction center. Here, the accumulated the energy is used to split a water molecule and two electrons move into the electron transport chain. It's the reaction center that actually transduces (changes) the energy from light to chemical form.

The photosystem is made up of the reactions center and the

surrounding light harvesting complex (LHC). The LHC is made

up of many chlorophyll molecules that gather light energy and

bounce it around toward the reaction center. The reaction center

has many proteins that work together to transduce the light

energy into chemical energy by splitting water.

If the electrons are generated by the combined work of the protein complexes of the reaction center, then damage can be done because they can’t go on to fix CO2 and turn it into carbohydrate. Those electrons are free to attack any nearby proteins or lipids and break them down. In particular, they can attach themselves to oxygen and create ROS. These molecules are just itching to react with something, anything, and this leads to damage to many structures of the cell.

Plants can try to stop this by producing more antioxidants, which can absorb the electrons from ROS molecules. They might be destroyed in the process, but at least they aren’t allowing the ROS to damage something important. Evergreens ramp up antioxidant molecule production (especially glutathione and alpha-tocopherol) in the winter to prevent ROS damage.

Notice that we said above that the reaction center splits a water to generate the free electrons. Didn’t we also say that in winter, freezing conditions wouldn't allow for available water? This true, but damage to the reaction center and/or chlorophyll is possible BEFORE the point where water would be split. This energy has to be dealt with as well.

To dissipate the energy before it damages the chlorophyll or the reaction center, plants use a technique called non-photochemical quenching (NPQ). Demonstrated in a classic 1987 study, the physical positions of proteins in the photosystem (chlorophylls + reaction center) can be shifted during NPQ so that they create energy traps. In these traps, different pigments, called carotenoids (see this post for plant pigments), can accept the energy of the light.

When the energy hits a carotenoid pigment called violaxanthin, it converts it to another pigment, zeaxanthin. Zeaxanthin can’t passed energy along to the reaction center, but is good at giving it up as heat. When sunlight can be used for photosynthesis, zeaxanthin is turned back to violaxanthin and the photosystem redistributes itself so that light energy can be focused to the reaction center. This is called the xanthophyll cycle.

Cadmium is used in batteries, paint pigments, metal plating

and in the production of other metals, like copper and zinc.

Long-term exposure can lead to kidney damage, but a new

study shows the problem may be worse than that. The

experiments showed that cadmium can interrupt non-

photochemical quenching in barley. This can lead to damage

and reduced barley harvests. Barley is used in making beer –

something must be done!

The zeaxanthin NPQ mechanism (truthfully, it's much more detailed than we have talked about here) is best for quick changes in sunlight level, However, the cycle can be disconnected in winter so that zeaxanthin is maintained for a long time. In addition, a 1995 study showed that some evergreens will prevent damage by inactivated or down-regulating (making less of) proteins of the reaction center, so that the high energy electrons won’t be generated.

This would presumably create more oxygen radicals from the chlorophyll since the energy can’t be transferred to the reaction center, but since zeaxanthin is being kept all winter, that energy is dissipated too. It sounds like a lot of work, but it requires much less energy than dropping leaves in the Fall and then re-growing them in Spring. You wonder why all plants aren’t evergreens – because then we wouldn’t have a special symbol to cram presents during the holidays.

So instead of being a vampire that has to stay out of the sun, the evergreen is more like a superhero that can overcome the power of the sun – his power of photoprotection saves Christmas for us. But maybe not - next week we’ll talk about how many different ways your Christmas evergreens can kill you.

Lysenko EA, Klaus AA, Pshybytko NL, & Kusnetsov VV (2014). Cadmium accumulation in chloroplasts and its impact on chloroplastic processes in barley and maize. Photosynthesis research PMID: 25315190

Demmig, B., Winter, K., Kruger, A., & Czygan, F. (1987). Photoinhibition and Zeaxanthin Formation in Intact Leaves : A Possible Role of the Xanthophyll Cycle in the Dissipation of Excess Light Energy PLANT PHYSIOLOGY, 84 (2), 218-224 DOI: 10.1104/pp.84.2.218

Ottander C, Campbell D, Öquist G (1995). Seasonal changes in photosystem II organization and pigment composition in Pinus sylvestris. Planta, 197, 176-183.