Wednesday, December 31, 2014

It May Be A New Year, But It’s The Same Old Brain

Biology concepts – learning, habit, long term potentiation, neural plasticity

50% of Americans will make at least one New Year
resolution, but a quarter of them won’t even make it
one week before relapsing. However, those who write
down a resolution are much more likely to make
changes than those who don’t make a specific
demand of themselves.
I swear, this year I’m going to get these posts written a month in advance. Really, I mean it this time. I know I said the same thing last year, but this time I’ve got a plan in place –- yeah, sure. Biology is stacked against me here; making new good habits is definitely an exception. Our brains function to make it hard to change our behaviors – but it is possible.

First things first, I am not a neurologist. I don’t even play one on TV, but we’re going to delve into some neuroanatomy and neurochemistry here. I’ll try to keep it from making your brain hurt.

Before diving into the gooey mess inside our skulls, we need to know that keeping a resolution means creating a new habit, or breaking an old habit and replacing it with a new one. But, what is a habit anyway?

A habit (from old French meaning “to hold” or “customary practice”) is an extreme form of learning, ingrained to such an extent that we do not think consciously about performing the behavior. But we still have the ability to turn the behavior on or off consciously. This is what separates a habit from an addiction. A poor man’s definition – if you have to decide to do it, it’s not a habit, and if you can’t decide not to do it, it’s an addiction.

William James was trained as a physician, but was
the first professor to start offering psychology classes
at the college level. His brother was novelist Henry
James, who wrote about the social corruption of
England versus the brash selfishness of America. His
father was a theologian who worried about the moral
evil have thinking about oneself, and Sigmund Freud was
a family friend. No wonder William went into psychology.
The philosopher and psychologist William James said, “99% of our behavior is purely automatic ….. all of our life is nothing but a mass of habits.” This is mostly true, we need to save our thinking for things that are important and undetermined, not for everyday things for which we can easily predict the outcomes and do not threaten our existence. You don’t think about putting one foot in front of the other when you walk, you look for the bus that may stop you dead in your tracks.

Habits are important, they keep us safe and alive for the most part. Good habits aren’t easy to make, while bad habits seem so simple. Bad habits are rewarded at more primitive levels of the brain, and the rewards are more tangible and shorter term. Good choices may be their own reward, but in terms of our brains, they aren’t as strong as a big ice cream sundae.

Rewards reinforce our habits and learning in a chemical sense as well. The reward centers of the brain release a neurotransmitter called dopamine, and we will see below that dopaminergic neurons are very important in learning, memory and making habits.

We need to know how our brains make habits if we want to increase our chances of keeping our resolutions. First comes intent and motivation, then comes learning, then comes making the learned behavior an unconscious act. As it turns out, there are brain centers for all these things, and they're all tangled together.

Dopaminergic neurons release, and may respond to, dopamine. They are involved in reward, learning, and in reinforcing learning to make habits. Dopaminergic neurons are located in many parts of the brain and a new study shows just how important they are in forming habits.

To help uncover the mechanisms of habit making, a mouse model has been developed that can’t form strong habits. A certain receptor was eliminated from dopaminergic neurons, and then the mice were taught new conditioned behaviors, like stepping on a lever to give them food. They could learn that the lever motion provided food, but they stopped after a while. Normal mice will learn the habit, and just keep stepping on the lever to get more and more food.

NMDA receptors contribute to LTP by allowing calcium
into the cell. This stimulates a retrograde signal that
causes the presynaptic neuron to release even more
glutamate. This stimulates more NMDA action and even
more calcium influx. This loop can literally remain
turned on for months!
The receptors in question work with dopaminergic neurons to reinforce signals and strengthen nerve firing. They are called NMDA receptors, and they respond to glutamate, an amino acid and important neurotransmitter. In the synapses (gaps, Greek; syn = together, and haptein = junction) between neurons, NMDA receptors bind glutamate and then allow sodium and calcium into the downstream neuron. These work in different ways to make the firing of the neuron stronger. Calcium in particular can keep the upstream neuron firing and keep stimulating the down-stream neuron. This leads to long-term potentiation (LTP).

LTP results in repeated firing of those neurons, from minutes to months in duration. Every time they fire, that individual pathway gets strengthened. This is the key to learning, called neural plasticity. When neural pathways are repeatedly used, they become strengthened and a behavior is learned or remembered. If they are not used, the connections fade away. Dopaminergic neurons are especially important because they can generate LTP through NMDA receptors but can use additional mechanisms as well.

Many parts of the brain are involved in habit formation, like those that link intent with action. Peter Hall at University of Waterloo near Toronto has been looking at intent and brain function, specifically, a portion of the brain called the superior prefrontal cortex (SPFC), located just behind that place on our forehead where you smack yourself when you do something stupid.

Some people have better SPFC function than others, and they find it easier to act on intentions and make behavior match intention. But good habits can increase SPFC function – see the end of the post.

Adolescent brains are maturing at an astonishing rate
during the teen years, but the maturation is uneven. This
means that they often revert to the more primitive,
emotional brain for decision making. The emotional brain
includes the reward center, so teens are more likely to make
habits based on short-term rewards. Good school work and
behavior habits are tough to develop in these befuddled brains.
The prefrontal cortex is more than just the SPFC. A 2009 study showed that the ventromedial prefrontal cortex is important in self-control, while the dorsolateral prefrontal cortex is important in meeting goals. And we all know that we need some hefty self-control to keep resolutions.

The entire prefrontal cortex is a big player here, as this is the seat of the executive function, those functions of the brain that control and manage other thinking; like planning, problem solving, resisting immediate reward, and mental flexibility. It boils down to this: the PFC is the chief weigher of risk vs. reward and is the boss decision maker – although he often listens to the primitive brain that, “wants what it wants when it wants it.”

The signaling from the PFC communicates with other brain areas that are needed for habit formation. These include the nucleus accumbens and the ventral tegmental area that are deeper and older. These just happen to be those reward centers we talked about that reinforce actions based on the pleasure they bring.

Dopaminergic signaling in the nucleus accumbens has a lot to do with LTP and plasticity. A 2012 study shows that dopamine in the nucleus accumbens works to reinforce strong signals while inhibiting weak ones. So burgeoning habits get reinforced and become strong habits, while changing habits is difficult because the signals to do so are inhibited. Plasticity isn’t an easy thing to induce.

For every resolution you make, there is an unconscious
resolution not to change. One reason that habits
(good or bad) are hard to break is because they have been
successful to this point; you aren’t dead yet. Changing a
habit means a journey into the unknown, and change is
evolutionarily dangerous; why change what has hasn’t hurt
you yet? This is why bad habits that take a long time to
manifest are so insidious – like a chain-smoking 2 yr. old.
Another reason habits are hard to break is the reinforcers; those things that trigger the behavior are a part of our everyday lives. You need to stay away from these reinforcers (temptations might be a better word) because your brain remembers those reinforcers for a long time. It stores the contexts in which the habits are triggered and can bring back the behavior of the context is encountered again. It takes time for plasticity to weaken these pathways.

It takes willpower to keep yourself out of those situations where bad habits are reinforced. It turns out that your willpower is a real thing, requiring energy to work and it can actually tire out. First proposed by Roy Baumeister in 1998, he showed that when people are asked to employ willpower to resist a temptation, it became harder for them to resist a later temptation. We all know this is true.

In addition, it seems that people with the best self-control use their willpower less often. A 2012 study of Wilhelm Hofmann from U. Chicago showed that people should set up their environments to minimize their temptations, so their willpower was energized for when it was really needed. If you want to stop gambling, don’t go to the track – duh!

Let’s put together all we have learned and get some tips from the experts (Peter Hall at University of Waterloo, B.J. Fogg at Stanford, and others) on how to keep your resolutions.

Exercise affects habit formation. A 2012 study from Brazil
shows that running rats on treadmills induced plasticity
in the habit formation portions of the brain. Proteins and
genes that control the formation and function of synapses
were affected in the striatum – which includes the
dopaminergic neurons of the ventral tegmental area.
1) Make your goal something concrete, you can’t resolve an abstraction.

2) Focus on tiny habits that can be implemented in small doses until you can build it up to something bigger. Don’t say you will learn to play the banjo – say you will learn to play one chord. Then do it over and over.

3) Don’t just say you have intent, make the implementation concrete as well. Where and when will you practice that chord on your banjo?

4) Place your new behavior directly after a good behavior that is already a habit – you will be less likely to avoid it.

5) Reward yourself – even just a nice thought about your ability to meet your goal for that day. It will help reinforce the pathways.

6) Limit your temptations, this will help degrade the pathways that lead to the behavior you wish to change and reinforce the new pathways.

7) Get some exercise – superior prefrontal cortex function in making habits and good executive function improves with physical exercise.

Next week we go back to the undulipodia. Fungi can teach us alot about evolution by looking at which ones have flagella. And one is killing off all our frogs.

Wang, L., Li, F., Wang, D., Xie, K., Wang, D., Shen, X., & Tsien, J. (2011). NMDA Receptors in Dopaminergic Neurons Are Crucial for Habit Learning Neuron, 72 (6), 1055-1066 DOI: 10.1016/j.neuron.2011.10.019

Wang, W., Dever, D., Lowe, J., Storey, G., Bhansali, A., Eck, E., Nitulescu, I., Weimer, J., & Bamford, N. (2012). Regulation of prefrontal excitatory neurotransmission by dopamine in the nucleus accumbens core The Journal of Physiology, 590 (16), 3743-3769 DOI: 10.1113/jphysiol.2012.235200

For more information, see:

NMDA receptors –

Long-term potentiation –

Neural plasticity -


Wednesday, December 17, 2014

Christmas Greenery - Friend Or Foe?

Biology concepts – toxin, botany, cancer chemotherapies, pregnancy, evergreen

Noche de Rábanos (Night of the Radishes) is celebrated in
Oaxaca, Mexico on December 23. The townspeople carve
radishes into shapes, characters or scenes and then they are
judged.  It began as a suggestion by a couple of monks to
bring people in to the market to buy the produce that the
farmers had raised, so it’s a Christmas plant tradition that
really has little to do with Christmas.
In the middle of Northern hemisphere’s winter we use plants to help celebrate Christmas, but the practice is much older than Christmas. Decorating houses with evergreen boughs was related to the pagan tradition of the circle of the seasons, a guarantee that life would again return to the land.

Different regions used different evergreens, based on their folklore and what was locally grown. The poinsettia (Euphorbia pulcherrima) wasn’t considered a winter flower at first. It is native to southern Mexico and Central America, where it carried no real significance in holiday traditions, but was used by the Aztecs for making red dye and for treating fevers (using just the sap).

The first US ambassador to Mexico, Joel Poinsett, brought them back to the states and distributed them to botanical gardens. In the 1950’s, the Tonight Show was one of the first shows to be broadcast in color, and growers offered them free poinsettia plants as set decoration near Christmas. This, with additional donations for magazine layouts and Bob Hope specials created a huge market for the flower, which is now the most popular potted plant in America.

No matter what plants your corner of the world has chosen to include in their winter holiday tradition, they bring joy and warmth and yes, a sense of returning spring. ….. And many of them can kill you. To be fair, science is finding ways that they can save us as well. Let’s look at the major Christmas evergreens and how they can ruin or save your holiday.

Christmas trees – Several species of pines are used in the United States and Europe as Christmas trees. While not lethal, my pet peeve is the itchy rash I get on my arms while putting the tree in its stand and decorating it.

Many types of firs, pines, and spruces are used for Christmas,
but I think the bonsai Christmas has been severely underused.
Think of the budget savings, not many presents will fit under
that tree. And if it catches on fire, it might not even set off
your smoke detector.
It’s called irritant contact dermatitis (ICD) and can come from the wood or needles. Many people have trouble with the sawdust from cutting pine trees. I know it won’t kill me, but since I have few allergies and therefore am not used to being itchy, this bugs me.

Many people tout the health benefits of drinking tea made from pine needles, but this could get you into trouble. Many evergreen tree species (Ponderosa pine, Lodgepole pine, the cypresses, junipers) have high levels of a chemical called isocupressic acid (ICA).

ICA causes spontaneous abortions in cattle, and perhaps humans. A 2002 study showed that progesterone levels were affected by ICA. Progesterone is important for maintaining a pregnancy through the third trimester, ie. stopping premature labor. Progesterone relaxes the uterine muscles to prevent contractions; giving high risk women progesterone gel has been show to reduce premature delivery by 45%. You can see why ICA might be a problem.

A very recent study showed that ICA inhibits the transcription of two enzymes called StAR and P450scc. And a 2005 study links these two enzymes progesterone production. This is compelling evidence that eating pine needles that contain ICA suppresses the body’s work to prevent early labor.

On the other hand, pine needle tea might just save your life. A 2014 study showed that extracts from the Taiwan white pine (Pinus morrisonicola) needle can affect some cancer cells, specifically glioblastoma cells (cancer of the support cells of the brain).

Not all Christmas trees are evergreens. The New Zealand
Christmas tree (Metrosideros excelsa, or pohutukawa) turns
red around Christmas. A 2010 study indicated that extracts
from this tree were lethal to tuberculosis organisms as well
as Staph. aureus….. you know, that bacteria that is becoming
resistant to so many antibiotics.
One chemical in the pine needles, chrysin, could not only kill cancer cells on its own, but also prevented and reversed the molecular events in glioblastoma cells that had become insensitive to the cancer drugs used to treat gliobastoma. And lucky for us, Taiwan white pine needles are very low in ICA , so we can use it to treat cattle that are pregnant and have brain cancer…. and people too.

Speaking of cancer drugs, a different kind of evergreen tree is the source of one of the most powerful cancer chemotherapeutics we have. In many parts of Britain and Europe, yew trees were used as Christmas trees instead of pines or firs. Taxol itself is found in greatest quantity in bark of the slow growing Pacific Yew.

As you have probably guessed, some people like to make tea from the bark of the Pacific Yew, so it care needs to be taken here as well. Remember that cancer drugs kill cells that are reproducing. In the case of taxol, the chemical binds to parts of the cell that help line up and pull apart the chromosomes during mitosis (called microtubules).

Since normal cells use microtubules in exactly the same way as cancer cells do for cell division, taxol inhibits their function as well. This is why people lose their hair during many cancer treatments. Cancer drug therapy is balanced on a knife edge; hopefully it kills the fast dividing cancer cells just a little bit better and faster than it kills the cells that we need, like stomach lining cells, bone marrow cells, and hair follicles.

Mistletoe – prior to the advent of the Christmas tradition in Britain, kissing bunches were popular. An evergreen would be hung in a circular hoop and couple would kiss beneath it. In some traditions, the white berries of the mistletoe (Viscum album) also were used. Each time a kiss was taken, a berry had to be pulled; when the berries were gone, the kissing was over.

On the left is the older kissing hoop, or kissing bough. The
purpose of the apples eludes me, but they may be there to keep
the hot wax from dripping on your head. The right side is our
more modern mistletoe bough. It still has the white berries, but
we don’t pick them off with each kiss anymore.
The berries of mistletoe are poisonous to humans, so I hope that the young men who had to pick the berry after a kiss didn’t just pop it in their mouth in an attempt to impress the girl or make her laugh. Both the leaves and the berries contain toxins that would require a call to Poison Control if ingested.

Different species have different cocktails of toxins, but most, including phoratoxin and tyramine, will bring you (before they kill you) nausea, vomiting, blurred vision, and dangerously low blood pressure. Some companies tout European mistletoe extract as a natural antihypertensive treatment, but safe dosages have not been worked out, so I would caution against it as an herbal medicine not taken under a doctor’s supervision.

ON THE OTHER HAND, some extracts from some species of mistletoe might save your life. A chemical called viscothionin from a Korean species of mistletoe has been shown in a 2014 series of experiments to improve liver function in people that have fatty buildups in their liver tissue. Another 2014 study showed similar benefits to the liver and other tissues in rats that had low levels of estrogen, implying that it could be of benefit to post-menopausal women.

Pancreatic cancer is staged from 1 to 4, with 1 being the least
aggressive or least progressed. Notice that the survival
timess are in MONTHS, not years. No wonder we need to
find additional drugs for this cancer and why people are looking
for drugs to make people’s live easier while they suffer with
this cancer.
In a more philosophically complicated study, mistletoe extract was given to patients with advanced, terminal pancreatic cancer. Only patients who had refused further treatment were included in the study, to see if the mistletoe extract could improve their quality of life in the time they had remaining.

It did, as witnessed by increased body weight, increased appetite, less pain, better sleep, and less fatigue during the day. Great you say, but it also prolonged their life. Again you say great….. but I wonder if is this what all the patients wanted? Improved quality of life is amazing, but if they live longer, it just means living a little longer with a horribly painful cancer. A study of the ethics of this treatment should be warranted as well.

IvyHedera helix is scientific name for English ivy, used for centuries in Christmas decorations and symbols. One of the most popular Christmas carols remains, The Holy and the Ivy, but we will focus on just the ivy here.

As an evergreen, ivy was used as a symbol of everlasting life, but there was a time when Christians in England outlawed the use of ivy as a Christmas decoration. Some thought that since it grows in the shade, it could represent the sin that takes place outside of the light of day; secrecy and debauchery must have been on their minds.

English churches used to decorate the outside with 
ivy for Christmas, but only the outside. And of, 
course this during the period when they believe
that ivy was the houseplant of the devil.
I’m not sure why they would decorate with ivy, 
most of them are covered with it anyway.
In keeping with the evil associated with ivy, all parts of the plant are toxic. Cats and horses are especially susceptible to the toxins ivy contains, including facarinol. Ingestion can lead to convulsions, breathing problems, paralysis and coma. At least in humans it takes a pretty large dose to bring on big trouble.

However, many people develop a contact dermatitis reaction to ivy. Weeping blisters are common and severe itching accompany even the slightest contact. Heaven forbid the kind of itching that might accompany the eating of ivy! A 2010 review concluded that ivy should be one of the standard botanical allergens to be tested for.

ON THE OTHER HAND, ivy extracts might save your life. The same toxin that makes you sick, falcarinol, inhibits breast cancer cells from becoming resistant to cancer drugs (2014 study). Falcarinol stops the action of proteins in the cancer cell that work to pump cancer drugs back out. Called efflux pumps, cancer cells with increased pump activity keep pumping the cancer drugs out of the cell. Ivy extract stops the efflux pump from being produced so the cancer drug can do its job.

The poinsettia has undergone many hybridizations over the years
and has been selectively bred to keep its colored bracts (leaves)
for much longer. But the purpose of the blue poinsettia escapes
me. Is it supposed to look cold? My wife says it doesn’t have to
have a purpose, it’s just pretty.
According to another recent study, a toxin from ivy leaves, hederagenin, induces colon cancer cells to kill themselves (apoptosis). I think the take home message here is that Christmas evergreens are pretty and can be meaningful, but don’t add them to the holiday meal unless instructed by a doctor.

And as for poinsettia, they have a bad reputation for being poisonous, but a child would have to eat about 500 leaves for them to be in big trouble. And they taste so bad, that no kid would eat more than one. It’s strange how some have the reputation and other that are more dangerous are not thought of as toxic.

Next week, myrrh was one of the original Christmas gifts. It had many functions in ancient times, but now we know WHY it's such a great gift.

Liu, B., Zhou, J., Li, Y., Zou, X., Wu, J., Gu, J., Yuan, J., Zhao, B., Feng, L., Jia, X., & Wang, R. (2014). Hederagenin from the leaves of ivy (Hedera helix L.) induces apoptosis in human LoVo colon cells through the mitochondrial pathway BMC Complementary and Alternative Medicine, 14 (1) DOI: 10.1186/1472-6882-14-412

Tröger W, Galun D, Reif M, Schumann A, Stanković N, & Milićević M (2014). Quality of life of patients with advanced pancreatic cancer during treatment with mistletoe: a randomized controlled trial. Deutsches Arzteblatt international, 111 (29-30) PMID: 25142075

Tsui, K., Wang, J., Wu, L., & Chiu, C. (2012). Molecular Mechanism of Isocupressic Acid Supresses MA-10 Cell Steroidogenesis Evidence-Based Complementary and Alternative Medicine, 2012, 1-12 DOI: 10.1155/2012/190107

Earl, E., Altaf, M., Murikoli, R., Swift, S., & O'Toole, R. (2010). Native New Zealand plants with inhibitory activity towards Mycobacterium tuberculosis BMC Complementary and Alternative Medicine, 10 (1) DOI: 10.1186/1472-6882-10-25

Since most of this post is about how certain plants can be poisonous, I have decided not to include links for more information on how these things can kill you. If you must know, you’ll have to look them up yourself.

Wednesday, December 10, 2014

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 25th 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.