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? 

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.

Celluloseis 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?

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.

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/

When Evolution Goes Sideways – Sea slug hybrids, part 3

This is the third and final installment in our story of photosynthesis by an animal. E. chlorotica, a simple sea slug, has proven to be not so simple. We have seen that it is an exception to the "animals as heterotrophs" rule, as well as an exception to the "horizontal gene transfer is an activity of unicellular organisms" rule. Can we learn more from this mollusk? There must be more, otherwise the post would end right here.

Horizontal gene transfer has occurred between E. chlorotica and its algae food, V. litorea, with gain of function for photosynthesis by E. chlorotica. This is an exception to the current idea of how evolution works. More than 50 years before Darwin’s theory of natural selection, there was another natural philosopher (the term “scientist” didn’t gain popularity until the 1850’s) who had his own ideas about inheritance.

According to Lamarck, giraffes acquired their longer neck, then passed it on.

Jean Baptiste Lamarck was a retired French soldier who became a famous zoologist and botanist; he coined the terms "biologist" and "invertebrate" among other accomplishments. In the late 1700’s and early 1800’s, Lamarck suggested inheritance followed strict natural rules. His idea was called "soft inheritance" and was the first comprehensive theory of evolution. Lamarckism, or inheritance of acquired characteristics proposes that new or changed features that an organism acquired during its life would be passed on to its offspring. His classic example was the giraffe. The food was high in trees, so the giraffe strained its neck to reach the leaves, and its neck got longer. Therefore, its offspring were born with longer necks. There are two main features to Lamarckism: 1- mature organisms can change their characteristics permanently; and 2- these acquired changes are passed on to progeny.

Then came Charles Darwin. His voyage on the Beagle took place from 1831-1836 and he formulated his theory soon after that, but he found it hard to reconcile his religious beliefs with his scientific findings and theories. Therefore, he did not publish his work until 1859, when he learned that another scientist, Alfred Russell Wallace, had come to the same conclusion and was about to publish. After years of public debate and testing the theory, Darwin’s and Wallace’s theory of natural selection as the instrument of evolution won out over Lamarckism, and is now widely accepted.

The principle behind natural selection is that changes in organism characteristics are always occurring, but when changes in the environment result in a reproductive advantage for the organism that has a inherited a certain new characteristic, then that characteristic will be passed on to the next generation at a higher rate (the organism with that change is more likely to survive long enough to mate and produce offspring – if you don’t survive to mate, your genes aren’t passed on – duh!).

A prototypical example of natural selection and evolution is nylon eating bacteria. Nylon wasn’t invented until the 1940’s, so the ability to feed on it must be new as well. In the 1970’s, a strain of Flavobacterium was found producing enzymes to digest nylon. Even more amazing were the results of multiple experiments carried out with a non-nylon digesting bacterium, Pseudomonas aeruginosa. When this bug was placed in an environment where nylon was its only possible food source, it took only a few generations before every bacterium on the plate contained enzymes to metabolize nylon, and these were not the same enzymes as found in the Flavobacterium. It happened in trial after trial, showing that life can adapt to whatever the environment offers. There are many other concrete examples of natural selection in the scientific literature – it is one of the most solidly supported theories in all of science.

In an ironic twist of fate, scientists are now helping bacteria evolve the ability to process garbage into adipic acid, one of the two major chemicals in nylon. Man invented nylon, nylon-eating bacteria taught us about evolution, we used this knowledge to evolve bacteria to make nylon!






We know that natural selection takes place, causing species to diverge, converge, and move along ever changing paths. We have seen it take place and can follow the clues it has left behind. However, horizontal gene transfer has made following those paths more difficult. Using molecular methods, scientists have ways of tracking the rate of change in DNA over time. With this data they can put together family trees and charts (cladograms) to show the points at which different species diverged from one another. But if a certain organism can pick up one or more new traits in one horizontal transfer, it makes patterns for small changes harder to use in developing the evolutionary cladograms. Is a new trait the result of mutation and natural selection or horizontal gene transfer?

A simple cladogram for apes and hominids is on the left. It shows evolutionary relationships between species. The right cladogram shows things can be confusing when horizontal gene transfer (crossing arrows) takes place.




Other features have entered the debate on the nature of Darwinian evolution as well. Does natural selection proceed slowly, with very small changes over time adding up to a measurable change in some characteristic or species (called gradualism), or does a species remain stagnant for a long time, until a mutation brings some rapid, even instantaneous, change (called punctuated equilibrium)?

Gradualism has been the prevailing theory for decades now, but examples of horizontal gene transfer would argue for punctuated equilibrium, especially if it is occurring in higher eukaryotes. Instead of many small changes adding up to a measurable difference over time, horizontally transferred genes bring the potential for big changes from the time of transfer down through all subsequent generations.

Is the retention and function of chloroplasts in E. chlorotica an example of punctuated equilibrium? Definitely not - the instantaneous gain of photosynthetic ability is a sudden change, but remember, the chloroplasts themselves are not passed on to the next generation – this is not an example of horizontal gene transfer in itself. But the fact that photosynthesis-specific genes are found in the nucleus of the slug is evidence that horizontal gene transfer has taken place and that they are passed on vertically. This would argue for punctuated equilibrium over gradualism. So, could this be an exception to the commonly held idea of evolution?

What is more, the combination of heritable plant genes AND acquisition of photosynthetic ability in an already living individual smells a lot like evolution by acquired characteristic – Lamarckism! There is a change in the individual (kleptoplasty of chloroplasts) and this acquired ability is passed on to future generations (via germ line inheritance of the photosynthetic genes). That seems like a more complex, two-step version of Lamarck’s giraffes.

If true, a blow will be dealt to neo-Darwinian evolutionary theory - but not a fatal blow. Darwin himself acknowledged that a mechanism of acquired characteristic inheritance or loss of disused characteristics might exist. He called it pangenesis. His idea was deeply flawed in its proposed mechanism; Darwin thought that cells would throw off small particles (pangenes) that would carry the essence of the change throughout the body and pass the characteristic to the next generation. It sounds silly to us now, but remember that changes in environment can initiate communication between cells that are far apart, and can affect DNA function. This is called epigenetics, and we may have an opportunity to talk about it in the future. However, in the case at hand, it might turn out that Darwin was right to hedge his bet against Lamarck.

It would be nice if the story were this simple – O.K., it’s already not simple – but there has been a catch. Even though the genes for some photosynthesis elements are present in the slug’s nucleus, scientists had not been able to show that they were doing anything. Logic says they must be, since the chloroplasts are functional for so long. One could track this is based on the fact that DNA genes are converted to RNA messages before being translated into protein. If the nuclear genes are making proteins, you should be able to see their RNA transcripts - but scientists went a long time without seeing the transcripts. They surmised that the transcripts were short-lived or present in low amounts that couldn't be detected.

All this speculation has been put to rest by recent evidence from Case Western Reserve University. Investigators in late 2011 studied the transcriptosome of E. chlorotica. Whereas the genome is the sum of all genes present in an organism, a transcriptosome represents all the mRNAs present in an cell or organism, but only at a specific point in time or under a specific set of conditions.

This is real difference between genomes and transcriptosomes; genomes are basically the same in all somatic cells under all conditions. But different cell types need different gene products, and reacting to different conditions will call for changes in which genes are transcribed to mRNAs and then translated into proteins. Therefore, the transcriptosome will be different for different cells and at different times

The scientists in the Case Western study isolated mRNA from whole E. chlorotica organisms afaer they had been starved for 2 months and then exposed to sunlight for two hours. Using starved animals ensured that the dose of sunlight would stimulate expression of as many photosynthesis genes as possible.

In all 111 chloroplast transcripts from 52 different genes were identified, many encoded by the stolen cholorplasts, but many others that represent nuclear genes - once found only in the nucleus of the E. chlorotica's algal food, but now found in the nucleus of the sea slug.

This is the first direct evidence of functional lateral gene transfer in a kleptoplastic organism - and think, they only had to study a mere 98,238,204 separate sequences and 8.9 billion nucleotides of code in order to make this discover. This suggests that the copy number and the transcription rate of laterally transferred genes are low, but who cares. We now have proof of horizontal gene transfer  and the production of a true plant/animal hybrid!



Pierce, S., Fang, X., Schwartz, J., Jiang, X., Zhao, W., Curtis, N., Kocot, K., Yang, B., & Wang, J. (2011). Transcriptomic Evidence for the Expression of Horizontally Transferred Algal Nuclear Genes in the Photosynthetic Sea Slug, Elysia chlorotica Molecular Biology and Evolution, 29 (6), 1545-1556 DOI: 10.1093/molbev/msr316


For more information on Jean Baptiste Lamarck, Charles Darwin, gradualism and punctuated equilibrium or pangenesis, as well as web-based activities and experiments, go to:

Jean Baptiste Lamarck –

www.victorianweb.org/science/lamarck1.html
www.ucmp.berkeley.edu/history/lamarck.html
www.lamarck.cnrs.fr/?lang=en
http://necsi.edu/projects/evolution/lamarck/intro./lamarck_intro.html
www.britannica.com/blogs/2009/02/the-rebirth-of-lamarckism-the-rise-of-epigenetics/
http://videosift.com/video/Darwinism-vs-Lamarckism-cute-short-about-evolution
www.indiana.edu/~ensiweb/lessons/lam.darw.html
www.sciencenetlinks.com/lessons.php?DocID=387
www.liftminds.com/lesson/60/Mechanisms_of_evolution_lamarckism_principles
www.clas.ufl.edu/users/ufhatch/NSF-PLANS/4-1_ORIGIN.htm
http://thinkwell.mindbites.com/lesson/3915-biology-contrasting-lamarck-and-darwin
www.imarksweb.net/book/lamarck+vs++darwin+worksheet/
www.iteachbio.com/Life%20Science/Evolution/Earlyevoltheory.doc

Charles Darwin –
www.aboutdarwin.com/
www.blupete.com/Literature/Biographies/Science/Darwin.htm
www.darwinday.org/
www.pbs.org/wgbh/evolution/educators/lessons/lesson2/act1.html
http://teacher.scholastic.com/activities/explorations/adaptation/
www.tes.co.uk/teaching-resource/Charles-Darwin-Cartoon-Strip-activity-6077894/
http://peer.tamu.edu/curriculum_modules/ecosystems/module_1/activity.htm
www.biologyinmotion.com/evol/index.html
http://serendip.brynmawr.edu/sci_edu/waldron/
http://science.pppst.com/evolution.html
http://ats.doit.wisc.edu/biology/ev/ns/ns.htm
www.accessexcellence.org/AE/AEC/AEF/1995/wartski_natural.php
www.mhhe.com/biosci/genbio/virtual_labs/BL_12/BL_12.html

Gradualism and punctuated equilibrium –
http://necsi.edu/projects/evolution/evolution/grad+punct/evolution_grad+punct.html
www.blackwellpublishing.com/ridley/a-z/Phyletic_gradualism.asp
www.talkorigins.org/faqs/punc-eq.html
www.indiana.edu/~ensiweb/lessons/peek.html
www.youtube.com/watch?v=Islx7y62I6M
www.zartistry.net/pgvspe/index.html
www.lessonplanet.com/lesson-plans/punctuated-equilibrium
www.truthinscience.org.uk/tis2/custom/Fossils%20Lesson%20Plan.pdf
http://evolution.berkeley.edu/evosite/evo101/VIIA1bPunctuated.shtml

Pangenesis -
www.answersingenesis.org/home/area/cfol/ch2-pangenesis.asp
www.neoteny.org/2009/12/04/gemmules/
www.chemistrydaily.com/chemistry/Pangenesis
www.softchalk.com/lessonchallenge09/lesson/genetics/an_introduction_to_Mendelian_genetics_print.html

When Amazing isn’t Enough- Sea Slug Hybrids, part 2

As you undoubtedly remember, last time we talked about a fascinating exception in biology, an animal that can perform photosynthesis. The sea slug, Eylsia chlorotica, eats algae and places the intact, functional chloroplasts in its tissues by a process called kleptoplasty. From that point on, the animal can turn light and CO₂ into carbohydrates – it no longer needs to eat. You might also recall that I hinted that the mere ability to perform photosynthesis isn’t the most amazing thing about this animal. So let us jump in right there.

The average life span of our sea slug of interest is ten months. Not enough time to read War and Peace, but forever compared to the mere 24 hours allotted to the mayfly. So E. chlorotica has roughly a year to make hay while the sun shines. However, the life span of the proteins that are needed for photosynthesis is much shorter.

RuBisCO, a complex protein of photosynthesis

The single most abundant protein on earth is called RuBisCO (Ribulose-1,5-bisphosphate carboxylase oxygenase). This protein adds carbon from CO₂ to the growing carbohydrate during photosynthesis, and has a turnover rate of about 5 days. This is an abnormally long life time for a protein. Chlorophyll can have a turnover rate of a mere 10 hours in some plants. Proteins get old fast, they start to work poorly or just stop working altogether. This is especially true for proteins that work in photosynthesis, since light can damage the very proteins that harness its energy.

Scientists, under laboratory conditions, have kept E. chlorotica alive for 14 months using just water and sunlight. The take home message is that there is active photosynthesis in sea slugs for months and months, when the proteins that make photosynthesis work may need replacing in just a few hours. It makes one wonder how E. chlorotica maintains active chloroplasts for so long.

Science has considered three main possibilities, but there might be more. First, there is something unique about the V. litorea (the algae E. chlorotica eats) photosynthetic proteins that makes them extremely long-lived. This is a tenable possibility, as a few plants have chlorophyll that might never be replaced. But even with immortal chlorophyll, these plants have hundreds of other photosynthetic proteins that must be constantly replaced. So this idea must take a back seat.

Second, there might be something unique about E. chlorotica that keeps the proteins from degrading. This would be amazing, since the sea slug’s own proteins degrade just as in other animals and are replaced regularly. Again, not the strongest hypothesis. Third, E. chlorotica has managed to find a way to make photosynthetic proteins. Intriguing possibility, isn’t it?

An animal that makes RuBisCO or chlorophyll takes the idea of a plant/animal hybrid to a whole new level. It isn’t just the ability to selectively save chloroplasts from digestion and then make use of them. It would be as if the sea slug bought an old motor (the chloroplasts) and but produces replacement parts by itself. But to make the replacement parts, the instructions must be there, and this means DNA.

For our sea slug to have the proper DNA, the plant genes must be consumed, avoid digestion, and be transported to the animal cell nucleus. What is more, the genes must be incorporated into the animal's chromosomes. This is a tall order.

Chloroplasts do have some of their own DNA, since they used to be their own organism (remember endosymbiosis?), but biologists know that many of the hundreds of photosynthesis genes have been transferred to the plant nucleus and are no longer housed in the chloroplast. Therefore, just maintaining functional chloroplasts is not sufficient to produce the proteins needed to keep them active.

Perhaps the slug retains the algae nucleus after feeding. This would provide all the genes needed to produce the proteins needed for photosynthesis, as long as the animal cell can reach and read the plant DNA. Since the chloroplast is not digested, perhaps neither is the nucleus. This would be a good idea, except that scientists have starved E. chlorotica for months, and then searched the slug for plant nuclei. They haven’t found any, so it is probable that the nuclei aren’t retained.

This leaves us with the possibility that the plant genes needed for photosynthesis have been donated by the algae and added to the animal’s cell chromosomes. Don’t laugh, this happens all the time in bacteria. It is called lateral (or horizontal) gene transfer, and it can account for things like antibiotic resistance and sex change in gut bacteria (yes, bacteria can change sex). Even viruses can help accomplish horizontal gene transfer. Viruses can insert their own DNA into the infected cell’s DNA and when they cut themselves back out, they may bring more than they put in. The next infected cell is then the recipient of DNA it may not have had previously.

In vertical transmission, all DNA in the offspring

comes from the parent. In horizontal gene transfer,

the movement is between two different organisms

of the same generation; the recipient cell now has

DNA it did not have before.

Lateral gene transfer can also occur in eukaryotes, but it is usually at the primitive end of the scale. The transfer of some chloroplast and mitochondrial genes to the nucleus millions of years ago is an example of horizontal gene transfer. Horizontal gene transfer with passage of the new genes to the next generation is easy in bacteria or lower eukaryotes because they don’t reproduce through sex. In fungi, even though some progeny are produced by mating, the DNA transferred to the progeny is still the same DNA that was laterally transferred.

Sex on the other hand, means sex cells. The DNA in sex cells (gametes) is the only DNA that gets passed on to the progeny (you get half your DNA from Mom’s egg and half from Dad’s sperm). For DNA to be passed on through horizontal gene transfer, the new DNA must be transferred into either an egg or sperm, and that has to be the particular egg or sperm that participates in fertilization. This is especially difficult to imagine for E. chlorotica, as the algae is eaten, and the chloroplasts are put into the gut cells. Nothing about this leads to algae nuclear DNA getting anywhere near the sex cells. It doesn’t seem very likely - but this is exactly what happens.

Pea aphids have incorporated fungal genes to

help them blend in to their surroundings.

Scientists have found several photosynthesis-specific genes in both mature E. chlorotica that have been starved for algae for months and in immature veligers that have never fed on algae. This can only mean that the genes have been passed vertically, from parent to child, and this means that the plant DNA has entered the gamete cells. The only similar instance I can think of is the transfer of a fungal carotenoid (pigment) gene to pea aphids (ant cows, a neat story on their own) that changed the aphid’s color to match their environment, giving them a camouflage advantage. This is itself a biological exception, the only instance of an animal that produces carotenoid pigment.

Lets summarize. We have an animal that can do photosynthesis – amazing. This same animal has taken up DNA from algae, and has incorporated the new genes into its germ line cells so that they are passed on to its offspring – more amazing. Next time, we’ll talk about how one of the greatest ideas of science might be run aground by a sea slug. Could it be that a discarded version of evolution might be correct?

For more information on horizontal gene transfer, as well as web-based activities and experiments, go to:


http://www.psrast.org/hrtrintr.htm
www.genomenewsnetwork.org/articles/05_01/Lateral_gene_lit.shtml
http://amrls.cvm.msu.edu/microbiology/molecular-basis-for-antimicrobial-resistance/acquired-resistance/acquisition-of-antimicrobial-resistance-via-horizontal-gene-transfer
http://mbio.asm.org/content/2/1/e00005-11.full