Germination and Development
Now we know how seeds develop, and how they get from place
to place. But how does a seed, which can lie dormant in the soil for weeks,
months, or years, develop into a plant?
Seeds have to take in water to germinate. Drinking in water causes the seed to expand and burst its seed coat (imagine all the buttons popping off!). Inside the seed, hormonal changes cause the endosperm to break down and let the cotyledons absorb nutrients. The first thing to emerge from the seed is the embryonic root, called a radicle. Next the shoot starts to emerge, and has to push its way up through the soil to the surface where it can start getting light. This is done differently in eudicots and monocots.
In eudicots, the shoot pushes up through the soil in a bent-over, U-shaped position. The area just below the cotyledons is called the hypocotyl, and the stem area above the cotyledons is called the epicotyl. The hypocotyl acts as a battering ram for the shoot. By emerging this way, the fragile apical meristem is protected from abrasive particles in the soil.
Aboveground, the stem straightens out, the two cotyledons unfold, and the epicotyl regains its place at the top of the stem. These two first leaves are "seed leaves" and not "true leaves," since they existed in the seed. The cotyledons will nourish the seedling for a short time, but will shrivel and die quickly once true leaves have formed.
Monocots have a different way of doing things. Rather than make a battering ram out of a hypocotyl, monocots use a tunnel to get to the surface. Monocot seeds don’t come with shovels; they use the coleoptile, a protective sheath covering the embryo, for their transportation needs. The coleoptile pushes up to the surface and the shoot grows straight up through the tip of the coleoptile.
Another difference between eudicots and monocots is the way their roots grow. Eudicots usually grow a main root, called a taproot, that grows deep and other roots branch sideways from. Monocots often just have a shallow fibrous root system but no main root.
Acorus calamus is a monocot, whose roots look like this:
In this picture of, you can clearly see the taproot:
Once a seedling gets its roots going and grows some new leaves, it is pretty much established. However, it’s not ready to start a family yet; plants have juvenile and adult stages just like humans, though plants get to skip those awkward middle school years. Plant development is dictated by the all-important shoot apical meristem.
At some point determined by environmental conditions and hormones internal to the plant, the juvenile plant will become an adult. When the plant passes from its juvenile phase to its adult phase, the shoot apical meristem starts producing adult leaves that do boring adult things like balance their checkbooks and listen to talk radio. The shift to adulthood only affects new growth on the plant—leaves that already exist and were made during the juvenile phase retain their youthful look.
Two important factors for flowering, photoperiod and hormones, are discussed in their own sections. However, there are also genetic controls on flowers. When the plant becomes an adult, the apical meristem converts into a floral meristem. Somehow the apical meristem needs to develop all the flower parts. Good thing there’s an instruction booklet.
The instruction booklet for floral development is DNA, which also gives directions for how to make everything else in the plant. First, a stimulus called florigen moves up
the vascular tissue to the shoot apical meristem. This causes certain genes to be expressed.
The general idea for how genes control flowers is called the ABC model. In the ABC model, there are three switches that can be turned on in a cell to dictate what floral part develops there. The switches are really groups of genes, and gene expression is equivalent to flipping the switch on.
The A group by itself causes sepals to develop. The C group by itself causes carpels to form. When A and B are both turned on, petals form, and when B and C work together, stamens develop. Scientists have figured out what these genes do by studying mutant plants that are can’t express one of the genes. Unfortunately, the mutant plants don’t have superpowers and won’t be joining the X-Men anytime soon, but they have at least helped us figure out how flowers form.
A = sepals
A + B = petals
B + C = stamens
C = carpels
Seeds have to take in water to germinate. Drinking in water causes the seed to expand and burst its seed coat (imagine all the buttons popping off!). Inside the seed, hormonal changes cause the endosperm to break down and let the cotyledons absorb nutrients. The first thing to emerge from the seed is the embryonic root, called a radicle. Next the shoot starts to emerge, and has to push its way up through the soil to the surface where it can start getting light. This is done differently in eudicots and monocots.
In eudicots, the shoot pushes up through the soil in a bent-over, U-shaped position. The area just below the cotyledons is called the hypocotyl, and the stem area above the cotyledons is called the epicotyl. The hypocotyl acts as a battering ram for the shoot. By emerging this way, the fragile apical meristem is protected from abrasive particles in the soil.
Aboveground, the stem straightens out, the two cotyledons unfold, and the epicotyl regains its place at the top of the stem. These two first leaves are "seed leaves" and not "true leaves," since they existed in the seed. The cotyledons will nourish the seedling for a short time, but will shrivel and die quickly once true leaves have formed.
Monocots have a different way of doing things. Rather than make a battering ram out of a hypocotyl, monocots use a tunnel to get to the surface. Monocot seeds don’t come with shovels; they use the coleoptile, a protective sheath covering the embryo, for their transportation needs. The coleoptile pushes up to the surface and the shoot grows straight up through the tip of the coleoptile.
Another difference between eudicots and monocots is the way their roots grow. Eudicots usually grow a main root, called a taproot, that grows deep and other roots branch sideways from. Monocots often just have a shallow fibrous root system but no main root.
Acorus calamus is a monocot, whose roots look like this:
In this picture of, you can clearly see the taproot:
Once a seedling gets its roots going and grows some new leaves, it is pretty much established. However, it’s not ready to start a family yet; plants have juvenile and adult stages just like humans, though plants get to skip those awkward middle school years. Plant development is dictated by the all-important shoot apical meristem.
At some point determined by environmental conditions and hormones internal to the plant, the juvenile plant will become an adult. When the plant passes from its juvenile phase to its adult phase, the shoot apical meristem starts producing adult leaves that do boring adult things like balance their checkbooks and listen to talk radio. The shift to adulthood only affects new growth on the plant—leaves that already exist and were made during the juvenile phase retain their youthful look.
Two important factors for flowering, photoperiod and hormones, are discussed in their own sections. However, there are also genetic controls on flowers. When the plant becomes an adult, the apical meristem converts into a floral meristem. Somehow the apical meristem needs to develop all the flower parts. Good thing there’s an instruction booklet.
The instruction booklet for floral development is DNA, which also gives directions for how to make everything else in the plant. First, a stimulus called florigen moves up
the vascular tissue to the shoot apical meristem. This causes certain genes to be expressed.
The general idea for how genes control flowers is called the ABC model. In the ABC model, there are three switches that can be turned on in a cell to dictate what floral part develops there. The switches are really groups of genes, and gene expression is equivalent to flipping the switch on.
The A group by itself causes sepals to develop. The C group by itself causes carpels to form. When A and B are both turned on, petals form, and when B and C work together, stamens develop. Scientists have figured out what these genes do by studying mutant plants that are can’t express one of the genes. Unfortunately, the mutant plants don’t have superpowers and won’t be joining the X-Men anytime soon, but they have at least helped us figure out how flowers form.
A = sepals
A + B = petals
B + C = stamens
C = carpels
Photoperiodism
Contrary to popular belief, photoperiodism is not a modern
art movement; it refers to the day length plants need to trigger flowering. Why
is this important? As you already know, many plants require pollinators to
transfer their pollen.
Pollinators such as birds and bees are more active in summer than winter, so plants that rely on certain birds and bees are better off if they wait until the long days of summer to open up their flowers. The photoperiod is the number of hours of daylight in a 24-hour period. In the northern hemisphere, winter days have less sunlight than summer days, so they have a shorter photoperiod. The leaf is the site of light perception, and only a small amount of leaf surface needs to receive light to maintain the plant’s internal clock.
The idea that day length could influence flowering arose in the 1920s, when researchers in Maryland were trying to breed large-leaved tobacco plants of the variety Maryland Mammoth. Though the researchers manipulated temperature, moisture, and soil nutrients, they couldn’t get the plants to flower. In the winter the researchers moved the plants into a greenhouse where they wouldn’t freeze and at last, they flowered.
The researchers learned by manipulating light that the tobacco plants would flower only when there were less than 14 hours of daylight. This earned Maryland Mammoth the label, "short-day plant," because it needed a day shorter than some critical length to flower. Other short-day plants include chrysanthemums, poinsettias, Easter lilies, and some soybeans.
The opposite of a short-day plant is a long-day plant. In order to flower, these plants need light periods that are longer than a certain time. Examples of long-day plants include spinach, radishes, lettuce, and irises.
Some plants aren’t picky about their photoperiod and will flower no matter how long they get sunlight—these are called day-neutral plants, and include tomatoes, rice and dandelions. Other plants are extremely picky and require certain conditions in addition to photoperiod. Some plants that grow in northern climates need a specific photoperiod and prolonged cold temperatures to stimulate flowering. This process, called vernalization, is required of cabbage, celery, magnolias and beets.
After a few more decades of manipulating plant flowering periods, researchers realized that flowering responds to the length of the dark period (night), not the length of light. For example, one short-day plant they used was cocklebur (Xanthium strumarium), which required days that were shorter than 16 hours to flower. If researchers interrupted the light period by putting the plant in total darkness for a brief time, the plant still flowered. However, if they interrupted the dark period by briefly turning on the lights, the plant didn’t flower. The researchers concluded that the short-day plants are actually long-night plants because they need a period of uninterrupted darkness that is longer than a certain length.
The length of the night necessary for the plant to flower is called the critical period. For the Maryland Mammoth, the critical night period is ten hours because it needed a daylength shorter than 14 hours and there are 24 hours in a day.
Pollinators such as birds and bees are more active in summer than winter, so plants that rely on certain birds and bees are better off if they wait until the long days of summer to open up their flowers. The photoperiod is the number of hours of daylight in a 24-hour period. In the northern hemisphere, winter days have less sunlight than summer days, so they have a shorter photoperiod. The leaf is the site of light perception, and only a small amount of leaf surface needs to receive light to maintain the plant’s internal clock.
The idea that day length could influence flowering arose in the 1920s, when researchers in Maryland were trying to breed large-leaved tobacco plants of the variety Maryland Mammoth. Though the researchers manipulated temperature, moisture, and soil nutrients, they couldn’t get the plants to flower. In the winter the researchers moved the plants into a greenhouse where they wouldn’t freeze and at last, they flowered.
The researchers learned by manipulating light that the tobacco plants would flower only when there were less than 14 hours of daylight. This earned Maryland Mammoth the label, "short-day plant," because it needed a day shorter than some critical length to flower. Other short-day plants include chrysanthemums, poinsettias, Easter lilies, and some soybeans.
The opposite of a short-day plant is a long-day plant. In order to flower, these plants need light periods that are longer than a certain time. Examples of long-day plants include spinach, radishes, lettuce, and irises.
Some plants aren’t picky about their photoperiod and will flower no matter how long they get sunlight—these are called day-neutral plants, and include tomatoes, rice and dandelions. Other plants are extremely picky and require certain conditions in addition to photoperiod. Some plants that grow in northern climates need a specific photoperiod and prolonged cold temperatures to stimulate flowering. This process, called vernalization, is required of cabbage, celery, magnolias and beets.
After a few more decades of manipulating plant flowering periods, researchers realized that flowering responds to the length of the dark period (night), not the length of light. For example, one short-day plant they used was cocklebur (Xanthium strumarium), which required days that were shorter than 16 hours to flower. If researchers interrupted the light period by putting the plant in total darkness for a brief time, the plant still flowered. However, if they interrupted the dark period by briefly turning on the lights, the plant didn’t flower. The researchers concluded that the short-day plants are actually long-night plants because they need a period of uninterrupted darkness that is longer than a certain length.
The length of the night necessary for the plant to flower is called the critical period. For the Maryland Mammoth, the critical night period is ten hours because it needed a daylength shorter than 14 hours and there are 24 hours in a day.
.
Plant Hormones
Teenagers aren’t the only ones with raging hormones. Plants
are full of hormones too, but lucky for them they don’t get pimples. In plants,
hormones are responsible for all sorts of things, like helping the plants sense
light, forming lateral roots, and triggering flower development and
germination, just to name a few. If a plant had a facebook account, it might
write updates like "OMG my axillary branches are shooting up so fast"
or "just tricked a bee into pseudo-copulating with my flower, lol."
However, plants don’t have facebook, so they rely on hormones to be their messengers. Hormones are signaling molecules that are produced in small amounts and sent to other parts the plant body, like tiny messengers running around.
Why should anyone care about plant hormones? Plant hormones are really important in creating the green world around us, and providing the fruits we eat and other plant products we enjoy on a daily basis. Many things about plant hormones are still unknown, so it is a great field for a budding plant biologist (no pun intended…well actually it was, sorry).
Here we will discuss five types of plant hormones:
However, plants don’t have facebook, so they rely on hormones to be their messengers. Hormones are signaling molecules that are produced in small amounts and sent to other parts the plant body, like tiny messengers running around.
Why should anyone care about plant hormones? Plant hormones are really important in creating the green world around us, and providing the fruits we eat and other plant products we enjoy on a daily basis. Many things about plant hormones are still unknown, so it is a great field for a budding plant biologist (no pun intended…well actually it was, sorry).
Here we will discuss five types of plant hormones:
- Auxin
- Cytokinins
- Gibberellins
- Abscisic Acid
- Ethylene
Auxin
Scientists were interested in how plants respond to light;
if plants don’t have eyes, how do they sense where light is and which way they
should grow? It is a common observation that plants grow toward light, but for
a long time no one knew why.
One of the first people to experiment with this concept was Charles Darwin, who along with his son, Francis, was interested in figuring out how plants respond to stimuli (in this case, light). They noticed that coleoptiles, which are sheaths that protect grass stems as they germinate, bend toward light. They tried covering the coleoptile with foil and found that when covered, the coleoptile didn’t bend. When uncovered, it bent again! From this the Darwins concluded that the tip of the grass coleoptile senses light.
Even though it doesn’t seem very exciting now, in the 1800s this was just as scandalous as Lady Gaga’s meat dress. The idea that plants could do something as brilliant as respond to their environment was shocking in an age when Man was exerting control on all things wild.
Later work by another scientist, Frits Went, determined that the signal responsible for bending toward light was a mobile chemical, and Went went ahead and gave it the name auxin. These days, auxin is sometimes referred to by its chemical name, indoleacetic acid (IAA).
Auxin does a couple different things in a plant, but its main role is to work with another type of hormone (cytokinins) to stimulate elongation of stems. If auxin is helping cells elongate, it is likely found in a place where a lot of new cells are forming. Where would that be? The shoot apical meristem, of course! The shoot apical meristem is a major source of auxin, but not the only one. Developing seeds also produce auxin, which leads to fruit development. When fruits such as tomatoes are grown inside greenhouses where there are no insect pollinators, synthetic auxins are used to help fruits develop normally.
Another commercial use of auxin is in the vegetative propagation of plants from cuttings. Instead of planting seeds, people can grow some plants by just cutting a leaf or stem; spraying the detached leaf or stem with auxin induces root production, and a whole new plant is formed.
One of the first people to experiment with this concept was Charles Darwin, who along with his son, Francis, was interested in figuring out how plants respond to stimuli (in this case, light). They noticed that coleoptiles, which are sheaths that protect grass stems as they germinate, bend toward light. They tried covering the coleoptile with foil and found that when covered, the coleoptile didn’t bend. When uncovered, it bent again! From this the Darwins concluded that the tip of the grass coleoptile senses light.
Even though it doesn’t seem very exciting now, in the 1800s this was just as scandalous as Lady Gaga’s meat dress. The idea that plants could do something as brilliant as respond to their environment was shocking in an age when Man was exerting control on all things wild.
Later work by another scientist, Frits Went, determined that the signal responsible for bending toward light was a mobile chemical, and Went went ahead and gave it the name auxin. These days, auxin is sometimes referred to by its chemical name, indoleacetic acid (IAA).
Auxin does a couple different things in a plant, but its main role is to work with another type of hormone (cytokinins) to stimulate elongation of stems. If auxin is helping cells elongate, it is likely found in a place where a lot of new cells are forming. Where would that be? The shoot apical meristem, of course! The shoot apical meristem is a major source of auxin, but not the only one. Developing seeds also produce auxin, which leads to fruit development. When fruits such as tomatoes are grown inside greenhouses where there are no insect pollinators, synthetic auxins are used to help fruits develop normally.
Another commercial use of auxin is in the vegetative propagation of plants from cuttings. Instead of planting seeds, people can grow some plants by just cutting a leaf or stem; spraying the detached leaf or stem with auxin induces root production, and a whole new plant is formed.
Cytokinins
Auxin helps cells elongate, but it doesn’t work alone.
Auxin’s partner in crime is a class of hormones called cytokinins. Cytokinins
promote cell division (cytokinesis) and are produced in roots, embryos and
fruits, or wherever there is actively growing tissue. However, cytokinins need
auxin to induce cell division. The ratio of cytokinins to auxin determines where
cells will develop. If cytokinin levels increase, shoots form; if auxins
increase, roots form. By themselves, cytokinins don’t cause any new tissues to
form.
Cytokinins do a couple other things too: they help delay aging in plants by increasing the amount of new protein that is made and decreasing the amount of old protein that is demolished. Because of this, cytokinins are sprayed in flower shops to keep leaves green and cut flowers fresh.
Cytokinins do a couple other things too: they help delay aging in plants by increasing the amount of new protein that is made and decreasing the amount of old protein that is demolished. Because of this, cytokinins are sprayed in flower shops to keep leaves green and cut flowers fresh.
Gibberellins
Gibberellins are most important in stems, fruits and seeds.
In stems, they work with auxin to cause stem elongation. Gibberellins and auxin
also work in concert when fruit is developing. In fact, green seedless grapes
are usually sprayed with gibberellins to make them bigger. Maybe that’s what
Snooki sprays on her hair, too.
Seeds have the problem of not knowing when conditions are right for germination; after all, they don’t come with calendars and thermometers. Lucky for the seeds though, they do have lots of gibberellins, which are released after seeds take up water (perhaps after a heavy spring rain). After gibberellins are released, the outer layer of the endosperm releases digestive enzymes that break down nutrients in the endosperm. These nutrients feed the embryo as it germinates and grows into a seedling.
Seeds have the problem of not knowing when conditions are right for germination; after all, they don’t come with calendars and thermometers. Lucky for the seeds though, they do have lots of gibberellins, which are released after seeds take up water (perhaps after a heavy spring rain). After gibberellins are released, the outer layer of the endosperm releases digestive enzymes that break down nutrients in the endosperm. These nutrients feed the embryo as it germinates and grows into a seedling.
Abscisic Acid
It looks like a scary name, but abscisic comes from the word abscise, meaning to cut off or
to fall away. On a plant, both leaves and fruits fall off, and abscisic acid
(ABA), got its name because scientists originally thought that ABA caused
leaves and fruits to fall off. It turned out later that other hormones (see
ethylene, below) are mainly responsible for abscission, but the name stuck.
ABA does do some important things, even though it doesn’t do what it’s named for. ABA slows growth, and is the main player in seed dormancy. Since plants can’t exactly nurse their young and sing them lullabies like humans can, seeds have to be a bit more independent than a lot of animal babies. In fact, seeds are so good at taking care of themselves, they don’t even start growing until conditions are right (temperatures are warm, or there is a lot of rain, or they get free tickets to Disneyland). The abscisic acid in a seed keeps it dormant (sleeping, basically). Certain things, such as water, light, or even prolonged cold temperatures, cause the ABA to break down and cue germination of the seed.
ABA has another important role in plants: drought tolerance. When water gets scarce and leaves start wilting, ABA production is cranked up in the roots. ABA moves up the plant to the leaves. As it accumulates in the leaves, ABA causes stomata to close, preventing more water loss. When water is plentiful again, the ABA breaks down and stomata reopen.
ABA does do some important things, even though it doesn’t do what it’s named for. ABA slows growth, and is the main player in seed dormancy. Since plants can’t exactly nurse their young and sing them lullabies like humans can, seeds have to be a bit more independent than a lot of animal babies. In fact, seeds are so good at taking care of themselves, they don’t even start growing until conditions are right (temperatures are warm, or there is a lot of rain, or they get free tickets to Disneyland). The abscisic acid in a seed keeps it dormant (sleeping, basically). Certain things, such as water, light, or even prolonged cold temperatures, cause the ABA to break down and cue germination of the seed.
ABA has another important role in plants: drought tolerance. When water gets scarce and leaves start wilting, ABA production is cranked up in the roots. ABA moves up the plant to the leaves. As it accumulates in the leaves, ABA causes stomata to close, preventing more water loss. When water is plentiful again, the ABA breaks down and stomata reopen.
Ethylene
Where would we be without ethylene? We would have many
unripe fruits, for starters. And without ripe fruits we would have no
strawberry milkshakes, pineapple-mango smoothies, or Fruit Ninja.
Ethylene helps fruits ripen by making them softer, through the breakdown
components of the cell walls, and sweeter, through the conversion of starches
to sugars. Unlike the other plant hormones, ethylene is actually a gas and is
distributed through the air, not through the plant body.
One of the coolest things about ethylene is that it is released in a positive feedback loop: a little bit of ethylene causes more to be released, which causes even more to be released, and so on. A benefit of this fact is that you can take an unripe fruit (a pear, plum, or peach, for example) and put it in a paper bag with riper fruit (bananas work well for this) and ethylene will accumulate, making the unripe fruit soft and sweet
One of the coolest things about ethylene is that it is released in a positive feedback loop: a little bit of ethylene causes more to be released, which causes even more to be released, and so on. A benefit of this fact is that you can take an unripe fruit (a pear, plum, or peach, for example) and put it in a paper bag with riper fruit (bananas work well for this) and ethylene will accumulate, making the unripe fruit soft and sweet
The Light-Dependent and Light-Independent Reactions
The light reactions,
or the light-dependent reactions, are
up first. We call them either and both names. The whole process looks a little
like this:
Do not freak out or fill your head with all the complicated names in that diagram. No—stop right there. All in all, the process is simpler than it looks. In the light-dependent reactions of photosynthesis, the energy from light propels the electrons from a photosystem into a high-energy state. In plants, there are two photosystems, aptly named Photosystem I and Photosystem II, located in the thylakoid membrane of the chloroplast. The thylakoid membrane absorbs photon energy of different wavelengths of light.
Here again is our friend the chloroplast. All exposed the way he is, he kind of reminds us of a boat with green checkers in it:
Even though the two photosystems absorb different wavelengths of light, they work similarly. Each photosystem is made of many different pigments. Some of these pigments can be described as absorption pigments, and others are considered action pigments.
The absorption pigments transfer the energy from sunlight to another pigment; at each transfer, the absorption pigments pass the photon energy to another pigment that absorbs a similar or lower wavelength of light. Remember when we said that light is funky and acts like it has both particles and waves? A photon is what we call the particle-like aspect of light. In other words, a photon is the basic unit of light.
Anyway, eventually, the energy makes it to the reaction center, or action pigment. At this point, the photosystem loses a highly charged electron to adjacent oxidizing agents, or electron acceptors, in the electron transport chain. This transfer all occurs mind-bogglingly quickly at an estimated time of 200 × 10-12 seconds!2 Since the photosystem has lost an electron, it no longer has a neutral charge and has instead become a positively charged photosystem.
The positively charged photosystem creates a scenario similar to one that might occur if Twilight stars Robert Pattinson and Kristen Stewart made a surprise appearance at your local high school. You, like the electrons in the photosystem, would be attracted to their presence even if you hated them. (You would. Admit it.) The positively charged photosystem attracts electrons from water (H2O) that can then be excited by light energy. When exactly four electrons are removed from H2O, oxygen (O2) is generated. Why, you ask? If two water molecules have four hydrogens that lose four electrons, exactly four hydrogen ions (H+) and two oxygens are left. Don't believe us? Count it out for yourself.
Side note: since hydrogen normally only has 1 proton and 1 electron, the four hydrogen atoms that have each lost one electron are each referred to as H+. Since each H+ is now without an electron, there is only one proton remaining in the hydrogen atom. At some point, scientists became lazy and started equating H+ with the word proton. If you think about it, they are in fact equivalent.
Back to regularly scheduled programming. The protons are then moved into the thylakoid lumen of the chloroplast using the power of the electron transport chain. This move results in a higher concentration of protons in the lumen than in the stroma of the chloroplast.
With so much positivity around, the protons get a little upset and try to equalize their distribution in the chloroplast by moving from the lumen to the stroma to reach equilibrium (read: equal numbers of protons in both places). The rush of protons moving into the stroma is called a proton gradient. When protons move down the gradient, with down referring to the direction of the area containing fewer protons, the protons are grabbed by enzymes that bring the protons together with the electrons from the electron transport chain. This event ultimately results in the making of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) from the adenosine diphosphate (ADP) and nicotinamide adenine dinucleotide phosphate ion (NADP+) that were hanging around nearby.
Now that everyone is partying it up in the stroma, it becomes the perfect location for the next stage of photosynthesis, the light-independent reactions.
Pictures are worth the thousand words that may or may not have just whizzed by your head as you were reading. Here is a much-simplified version of the earlier picture:
Did you miss something, or do we just suck at drawing these pictures? Nope. Photosystem II is ahead of Photosystem I. You might ask, "What the heck happened, Shmoop?" Well, scientists actually discovered Photosystem II before Photosystem I, and instead of changing the names when they found the other photosystem, they just named them in reverse. We know; it's very annoying.
As you’ve probably gathered by now, the light-dependent reactions fuel the second stage of photosynthesis called the light-independent reactions. Our good buddy carbon dioxide (CO2) provides an excellent source of carbon for making carbohydrates. However, conversion of one mole (one mole is an amount equal to 6.023 × 1023 molecules) of CO2 to one mole of the carbohydrate CH2O requires a lot of energy. And we mean a lot.
Guess what? The ATP and NADPH generated in the earlier light reactions are strong reducing agents (electron donors) and are able to donate the necessary electrons to make carbohydrates. Altogether, the conversion of one mole of CO2 to one mole of CH2O requires two moles of NADPH and three moles of ATP. If you do the math, that's a heck of a lot of molecules. The cell then uses ATP and NADPH to make carbohydrates in the Calvin cycle. We could use a Calvin and Hobbes cartoon right about now.
A key player in the Calvin cycle is ribulose-1,5-bisphosphate carboxylase oxygenase (affectionately called RuBisCo—thank goodness for nicknames), an enzyme that "fixes" CO2 to a 5-carbon compound called ribulose-1,5-bisphosphate (RuBP). The oxygen in CO2 is released as H2O. Immediately after RuBisCo catalyzes the attachment of the carbon from CO2 to the 5-carbon RuBP, the new 6-carbon compound is broken down into two 3-carbon compounds called phosphoglycerate (PGA, and no, it does not know how to golf). Since these 3-carbon compounds were the first compounds to be identified in the plants, they were named C3 plants. It was originally thought that RuBisCo was catalyzing the attachment of carbon to a 2-carbon molecule to make a 3-carbon molecule. Oopsies. And we thought RuBisCo was a cookie company at first, too.
RuBisCo is actually a poor enzyme. Sorry, RuBie. It is slow at catalyzing the attachment of CO2 to RuBP. To make matters worse, RuBisCo is also capable of catalyzing another less-than-beneficial reaction. This reaction is called photorespiration, and it occurs when the concentration of CO2 drops too low relative to the concentration of O2 in the cell. Photorespiration begins when RuBisCo uses O2 instead of CO2 and adds it to RuBP.
While CO2 is eventually produced in this reaction, and O2 is consumed, the reaction does not seem to produce any useful energy forms. The origination and purpose of photorespiration is controversial and still under active study by scientists. In an attempt to overcome the deficiency of RuBisCo, the plant cell produces a whopping ton of the enzyme. If this sounds slightly masochistic, it kind of is, which is why photorespiration has been labeled an outdated evolutionary relic. However, RuBisCo is thought to be the most abundant protein on Earth.2
Right…this not a moan fest about RuBisCo. We were explaining the Calvin cycle. When RuBisCo catalyzes the attachment of CO2 to the 5-carbon RuBP, the Calvin cycle begins. Reactions are initiated to rebuild RuBP from PGA. In this process, 1 molecule of glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar, is removed from the cycle. Altogether, 1 molecule of G3P is produced using 3 molecules of CO2, 9 molecules of ATP, and 6 molecules of NADPH. This 3-carbon sugar can be exported to the cytoplasm to make sucrose (a sugar and a carbohydrate), which is then moved throughout the plant for energy use. Alternatively, sucrose can be converted into another carbohydrate, starch, and then stored in the chloroplast as a type of energy reserve. Smart, plants…real smart.
What's that? You want a picture? Sure thing; glad to be of service.
Do not freak out or fill your head with all the complicated names in that diagram. No—stop right there. All in all, the process is simpler than it looks. In the light-dependent reactions of photosynthesis, the energy from light propels the electrons from a photosystem into a high-energy state. In plants, there are two photosystems, aptly named Photosystem I and Photosystem II, located in the thylakoid membrane of the chloroplast. The thylakoid membrane absorbs photon energy of different wavelengths of light.
Here again is our friend the chloroplast. All exposed the way he is, he kind of reminds us of a boat with green checkers in it:
Even though the two photosystems absorb different wavelengths of light, they work similarly. Each photosystem is made of many different pigments. Some of these pigments can be described as absorption pigments, and others are considered action pigments.
The absorption pigments transfer the energy from sunlight to another pigment; at each transfer, the absorption pigments pass the photon energy to another pigment that absorbs a similar or lower wavelength of light. Remember when we said that light is funky and acts like it has both particles and waves? A photon is what we call the particle-like aspect of light. In other words, a photon is the basic unit of light.
Anyway, eventually, the energy makes it to the reaction center, or action pigment. At this point, the photosystem loses a highly charged electron to adjacent oxidizing agents, or electron acceptors, in the electron transport chain. This transfer all occurs mind-bogglingly quickly at an estimated time of 200 × 10-12 seconds!2 Since the photosystem has lost an electron, it no longer has a neutral charge and has instead become a positively charged photosystem.
The positively charged photosystem creates a scenario similar to one that might occur if Twilight stars Robert Pattinson and Kristen Stewart made a surprise appearance at your local high school. You, like the electrons in the photosystem, would be attracted to their presence even if you hated them. (You would. Admit it.) The positively charged photosystem attracts electrons from water (H2O) that can then be excited by light energy. When exactly four electrons are removed from H2O, oxygen (O2) is generated. Why, you ask? If two water molecules have four hydrogens that lose four electrons, exactly four hydrogen ions (H+) and two oxygens are left. Don't believe us? Count it out for yourself.
Side note: since hydrogen normally only has 1 proton and 1 electron, the four hydrogen atoms that have each lost one electron are each referred to as H+. Since each H+ is now without an electron, there is only one proton remaining in the hydrogen atom. At some point, scientists became lazy and started equating H+ with the word proton. If you think about it, they are in fact equivalent.
Back to regularly scheduled programming. The protons are then moved into the thylakoid lumen of the chloroplast using the power of the electron transport chain. This move results in a higher concentration of protons in the lumen than in the stroma of the chloroplast.
With so much positivity around, the protons get a little upset and try to equalize their distribution in the chloroplast by moving from the lumen to the stroma to reach equilibrium (read: equal numbers of protons in both places). The rush of protons moving into the stroma is called a proton gradient. When protons move down the gradient, with down referring to the direction of the area containing fewer protons, the protons are grabbed by enzymes that bring the protons together with the electrons from the electron transport chain. This event ultimately results in the making of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH) from the adenosine diphosphate (ADP) and nicotinamide adenine dinucleotide phosphate ion (NADP+) that were hanging around nearby.
Now that everyone is partying it up in the stroma, it becomes the perfect location for the next stage of photosynthesis, the light-independent reactions.
Pictures are worth the thousand words that may or may not have just whizzed by your head as you were reading. Here is a much-simplified version of the earlier picture:
Did you miss something, or do we just suck at drawing these pictures? Nope. Photosystem II is ahead of Photosystem I. You might ask, "What the heck happened, Shmoop?" Well, scientists actually discovered Photosystem II before Photosystem I, and instead of changing the names when they found the other photosystem, they just named them in reverse. We know; it's very annoying.
As you’ve probably gathered by now, the light-dependent reactions fuel the second stage of photosynthesis called the light-independent reactions. Our good buddy carbon dioxide (CO2) provides an excellent source of carbon for making carbohydrates. However, conversion of one mole (one mole is an amount equal to 6.023 × 1023 molecules) of CO2 to one mole of the carbohydrate CH2O requires a lot of energy. And we mean a lot.
Guess what? The ATP and NADPH generated in the earlier light reactions are strong reducing agents (electron donors) and are able to donate the necessary electrons to make carbohydrates. Altogether, the conversion of one mole of CO2 to one mole of CH2O requires two moles of NADPH and three moles of ATP. If you do the math, that's a heck of a lot of molecules. The cell then uses ATP and NADPH to make carbohydrates in the Calvin cycle. We could use a Calvin and Hobbes cartoon right about now.
A key player in the Calvin cycle is ribulose-1,5-bisphosphate carboxylase oxygenase (affectionately called RuBisCo—thank goodness for nicknames), an enzyme that "fixes" CO2 to a 5-carbon compound called ribulose-1,5-bisphosphate (RuBP). The oxygen in CO2 is released as H2O. Immediately after RuBisCo catalyzes the attachment of the carbon from CO2 to the 5-carbon RuBP, the new 6-carbon compound is broken down into two 3-carbon compounds called phosphoglycerate (PGA, and no, it does not know how to golf). Since these 3-carbon compounds were the first compounds to be identified in the plants, they were named C3 plants. It was originally thought that RuBisCo was catalyzing the attachment of carbon to a 2-carbon molecule to make a 3-carbon molecule. Oopsies. And we thought RuBisCo was a cookie company at first, too.
RuBisCo is actually a poor enzyme. Sorry, RuBie. It is slow at catalyzing the attachment of CO2 to RuBP. To make matters worse, RuBisCo is also capable of catalyzing another less-than-beneficial reaction. This reaction is called photorespiration, and it occurs when the concentration of CO2 drops too low relative to the concentration of O2 in the cell. Photorespiration begins when RuBisCo uses O2 instead of CO2 and adds it to RuBP.
While CO2 is eventually produced in this reaction, and O2 is consumed, the reaction does not seem to produce any useful energy forms. The origination and purpose of photorespiration is controversial and still under active study by scientists. In an attempt to overcome the deficiency of RuBisCo, the plant cell produces a whopping ton of the enzyme. If this sounds slightly masochistic, it kind of is, which is why photorespiration has been labeled an outdated evolutionary relic. However, RuBisCo is thought to be the most abundant protein on Earth.2
Right…this not a moan fest about RuBisCo. We were explaining the Calvin cycle. When RuBisCo catalyzes the attachment of CO2 to the 5-carbon RuBP, the Calvin cycle begins. Reactions are initiated to rebuild RuBP from PGA. In this process, 1 molecule of glyceraldehyde-3-phosphate (G3P), a 3-carbon sugar, is removed from the cycle. Altogether, 1 molecule of G3P is produced using 3 molecules of CO2, 9 molecules of ATP, and 6 molecules of NADPH. This 3-carbon sugar can be exported to the cytoplasm to make sucrose (a sugar and a carbohydrate), which is then moved throughout the plant for energy use. Alternatively, sucrose can be converted into another carbohydrate, starch, and then stored in the chloroplast as a type of energy reserve. Smart, plants…real smart.
What's that? You want a picture? Sure thing; glad to be of service.
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