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  Science and technology education from FT Exploring.   Where and how photosynthesis occurs in a typical leaf.

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     A beautiful green summer leaf (I drew it myself with only two colored pencils).
     Warm summer days. The sun is shining. The leaves are just hanging there, doing nothing but looking pretty and wiggling in the wind.
     Wrong! It looks peaceful, but those leaves are busy. At speeds almost beyond our comprehension, thousands of chemical reactions and processes are occuring every second, inside thousands of cells.

     Busy, busy, busy, making stuff - carbohydrates that will be the building blocks to make more plant cells and the energy source for all the plant cell processes. This is photosynthesis.
     Where does all this "photosynthesizing" take place? Do all the leaf cells do it? Do they have to go to school to learn how?
     Naw, not all the leaf cells do it; and somehow, like most other living things, they just know how to do what they do without going to school. (In the case of plant cells, it's more like being programmed than "knowing".)

     Keep scrolling down to get an overview of the places and parts in a typical leaf where photosynthesis gets done. Then keep scrolling down, or click on the specific words if you are the skipping type, to get a more detailed look at the leaf parts and names and processes involved with turning solar energy, water, and carbon dioxide, into food.

The Leaf
     It's amazing what's inside a leaf. In this top section I'll give an overview of the leaf parts involved in photosynthesis. In the section below, we'll go into more detail.
     The whole leaf looks green to us, but most of the cells and cell material are colorless or clear. The green color comes from the chlorphyll molecules in the chloroplasts.
     Except for the thylakoid membranes, where the clorophyll molecules reside, the rest of my colors are just there to help you see the parts, and to make pretty pictures.
Leaf Section
     Cut out a little section of the leaf. Cut it all the way through. There are many different types of cells, specialized to do different things - all for the good of the tree, of course.
     On the top and bottom are the cuticle layer and the epidermal cells.
     In the middle, between the epidermis cells on the top and bottom, are the mesophyll cells where the chloroplasts live.
     On the bottom only, in most plants, are the stomates which let carbon dioxide in and oxygen out.
More on the Leaf Section  
Mesophyll Cells
     No, it's not a "Mess of Phils". It's a mesophyll.
     The chloroplasts, where photosynthesis occurs, are in the mesophyll cells.
      There are two kinds of mesophyll cells in our typical leaf.  The ones you see in the leaf section above, packed close together, are in the palisade parenchyma region (I can't say it either - that's biologists for you).  This is where most photosynthesis is done.  The other region is called the spongy parenchyma region.  Here the cells aren't so close.  There are roomy air spaces between them. Jump on down to the leaf section below for the purpose of the air spaces.
     Now we're getting to some small plant parts.  But you can still see chloroplasts with a regular microscope. Inside the chloroplasts, in the stoma and the grana, is where photosynthesis happens (see below).
     Promise me you will never forget the chloroplasts - fascinating little "organelles" (why not organettes?) with their own genetic material to make more of themselves.
     Each chloroplast is a little carbohydrate factory, powered by solar energy, and for which the only raw materials are carbon dioxide, water, and a few minerals.
     Out of this little factory comes food for plants and practically every other living thing on earth - including us.
     Good on yer, Chloroplast!
More on the Industrious Chloroplast  
     The little round flat pillow or pancake shaped things are called thylakoids. A stack of them is called a granum. Two or more stacks are called grana (granums would have been too easy).
     There can be from 2 to around 100 thylakoids in one granum. The little tube like strands connecting thylakoids from granum to granum are called stroma lamellae (see the chloroplast drawing below).

     Now we have finally journeyed to the place where it all starts. The chlorphylls and other pigments that start the process are here, on the outer layer of the thylakoids. Photons from sunlight hit the pigments, electrons are "knocked" loose, and off they go to energize the complicated process of photosynthesis.
     Sometimes the thylakoid is also called the photosynthetic membrane. That is easier for some of us to remember. The membrane and the space inside it (shown in yellow), is where the light or light-dependent reaction takes place. The so-called dark, or light independent reactions, take place in the stroma (shown in gray here).

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Epidermis and Cuticle:
     Hmmm? Epidermis.
     Sounds suspiciously like skin.
     As must be obvious to you, it does act a lot like skin. The epidermis cells on our leaf have stiff cell walls (which our skin doesn't). They protect the leaf, help support it and give it shape, and they keep the moisture inside. Plants can't do photosynthesis if all their water evaporates away.
     The cuticle is a hard waxy water-tight material. It is the reason water beads up so nicely on most leaves. Cuticle thickness is different in different plant species. It is usually thickest in plants that live in deserts and semi-arid climates. Why do you suppose that is?
     The cuticle is made from a material secreted by epidermis cells.
Mesophyll Cells:
     Photosynthesis happens in chloroplasts, and chloroplasts are in mesophyll cells. There can be from 1 to 50 or more chloroplasts in a single mesophyll cell. The number varies with the plant species, age, and health of the cell.

                     Continued below...
     In most plants, leaves are the main place where food is made. In order to do this, there has to be a way to move the water and minerals to the leaf cells doing photosynthesis, and a way to carry the food to other parts of the plant. A lot of water has to be delivered because most of it is lost by evaporation through the stomas.
     The transportation of water, food, and minerals is done by the veins, which are not shown in the drawing to the left. But you can see the big ones in the leaf drawing at the top of the page.
Mesophyll Cells Continued:
     The close packed cells right under the top epidermis are in the palisade parenchyma region. The cells in this area generally have the most chloroplasts.
     The palesade region is photosynthesis central, where most carbohydrate making takes place. There are small tight air spaces around these cells.  
      In the other region, called the spongy parenchyma region (don't blame me, I don't pick the names), the cells seem more loosely placed and irregularly shaped. The air spaces here are large and spacious - where the rich folk would live if rich folk lived in mesophyll cells.

Air Spaces:
     You might call it breathing. Biologists like to call it gas exchange. We animals breath in oxygen and breath out carbon dioxide. Plants, and other autotrophs, do that too (really they do), but they also need to do the opposite, get carbon dioxide in and oxygen out.
      All of these gas molecules get in and out through holes called stomas (see below).  Then they flow into the roomy air passages of the spongy parenchyma region. These passages are all connected to each other and to the mesophyll cells.  So this is where the gas molecules get into and out of the photosynthesis "factory" cells.

Air Spaces cont....
      This region of the mesophyll cells, then, is sort of, but not exactly, like our lungs and is a vital part of the photosynthesis activities of the leaf.

     The combination of guard cells and stoma is called the stomate. The stoma is just what they call the opening that connects the outside to the inside. Air flows into the air spaces through the stoma. The air brings carbon dioxide (CO2), needed for photosynthesis, with it. Oxygen (O2), a byproduct of photosyntheis, flows out through the stoma. CO2 given off by the plant as a result of repiration, may also flow out through the stoma; or it may get reused for photosynthesis.

Guard Cells:
     No air passes in or out unless the guard cells say so. Perhaps "Gate Keeper" cells would be a better name. They don't have muscles, but they can open and close. Fluid pressure is used to make the cells open and close the stoma.

Guard Cells cont...
     In a typical leaf, the guard cells are open during the day and closed at night. This can be a problem for the plants on hot dry days, because too much water is transpired, or evaporated, through the stomates. Loss of water slows down photosynthesis.

     Some plants close their stoma on hot dry days, but this also slows photosynthesis, because the cells begin to run short of carbon dioxide.
      Other plants, like the thorny cacti (more than one thorny cactus) that tend to live in extremely dry climates, have adapted by always closing the openings during the hot day time to reduce transpiration and water loss. At night when it is cooler, the stomas are opened and gas exchange takes place with less water loss.
      These plants have to make some compromising changes in the way they conduct photosynthesis for this to work. It makes them a little less efficient in some ways, but it
allows them to store water for long dry spells and to live in conditions that would kill other plants. They've got a good niche going.
     Guard cells, like the mesophylls, can have a few chloroplasts in them. No sense wasting a perfectly good solar energy collecting surface. Guard cells want to contribute in any way they can.

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The Chloroplast At Last:
     Now we are finally getting to the place where photosynthesis actually happens - the chloroplasts, each one a little carbohydrate factory.
     They are each around 5 to 10 microns across (a micron is 1 millionth of a meter). Up to 50 or more can be in some cells. Other cells may have only one.
     Each chloroplast has some, but not all of its own genetic material needed to reproduce itself.
Chloroplasts cont....
      Their genetic material is labeled in the drawing as the ribosomes and, cleverly enough, "genetic material" (I'm good with nomenclature). The rest of the chloroplast's DNA is in the mesophyll cell nucleus. This allows the mesophyll cell to have some control in the making of its own chloroplasts.
Chloroplasts cont....
     When the busy chloroplasts really get going they can make more carbohydrates than is needed just for the photosynthesis process. This "excess" carbohydrate material gets stored in the little starch grains until it is needed elsewhere.

     The outer surfaces of the thylakoids and the lamellae connecting the thylakoids are also called the photosynthetic membrane.  This membrane contains the pigment molecules of chlorophyll and carotenoid. These solar collector pigments act like antennae to capture the solar energy and start the carbohydrate making process we call photosynthesis.
      They are spread around the chloroplast to provide a lot of solar collecting surface area.
Membrane Envelople:
      The membrane envelope holds it all together and separates the chloroplast from the rest of the cell - duh! Well that's pretty obvious but it does more. The inner envelope acts like a control gate and regulator, controling the flow of necessary material and particles into, and out of, the chloroplast. The small, simple, and very important molecules of stuff like water (H2O), carbon dioxide (CO2), and oxygen (O2) can pass through this membrane (going both ways - in and out).
    The first part of the photosynthetic process, often called the light reaction or light-dependant reaction, takes place in the thylakoids. The thylakoids are stacked like pancakes or little pita breads (I love pita bread), and connected to one another by the stroma lamellae which are also part of the thylakoid membrane. The thylakoids and lamellae are thought to be one big thing, just all spread out and all connected to each other by the lamellae strands.
     More on thylakoids and stroma in the next section below.
     After the thylakoids, the photosynthesis process moves out to the stroma. The stroma is where enzymes take the carbon from carbon dioxide and combine it with hydrogen and oxygen to make simple carbohydrate molecules. This part of the process is generally referred to as the "light independent" reactions or "dark reactions".

           More on this process below...
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The Thylakoid:
     This is where the color comes from. The outer surface is called the thylakoid membrane or the photosynthetic membrane. The pigment and ATP Synthetase molecules are on the thylakoid membrane. Inside the membrane is the thylakoid space.
     Scientists generally break photosynthesis into two stages, the light reactions and the dark reactions.
     The light reactions, which are also called light dependent (because they depend on light) or energy fixing (because they capture and make energy available for the process) reactions, take place in the thylakoid space and the thylakoid membrane.

Click on the underlined links for more info and definitions.
     The dark reactions, which are also referred to as the light independent (because they don't need light) or carbon fixing reactions (because they "fix" carbon from carbon dioxide into carbohydrate molecules), occur in the stroma surrounding the thylakoids.

     There are no little blue men in the stroma. I just can't help myself sometimes.
     This picture shows three thylakoids stacked one on top of the other. A stack of thylakoids is called a granum.
      More than one granum are grana. Apparantly, the little green specks you see when you look at stacks of thylakoids through a microscope, reminded someone of "grains" of something. If I had been the first to look, we'd be calling them "pita" and "pitum", 'cause they make me think of pita bread.
Light Dependent Processes:
     The first stage of photosynthesis takes place in the thylakoid membrane and the thylakoid space. The various types of chlorophyll and carotenoid molecules are the pigments placed in the membrane. These pigments start the process by acting sort of like antennae that capture the solar energy. Light photons hit the chlorophyll or carotenoid molecules. This photon energy "knocks loose" an electron which provides the energy to other molecules to start the very complicated and fascinating photosynthesis process.
     "Okay", you say, "so those chlorophyll electrons go dancing off to provide energy for the photosynthesis process. What happens to the chlorophyll molecules? Won't they run out of electrons?"
     Good question. This is where the water comes in. There is a special enzyme molecule with the specific job of breaking apart water molecules. This procedure is called the photolysis of water. Water molecules are broken down into oxygen and hydrogen atoms. The hydrogen atoms are further broken down into a hydrogen ion and an electron.
     Guess where the electrons go. Yup. They go to the chlorophyll and other pigment molecules where they replace the missing electrons "knocked" loose by photons of light.
    Guess where the oxygen atoms go.     For the most part, they just go out! They link up with another oxygen molecule to form O2, then they flow back out to the air from whence they came. That's where the oxygen made during photosynthesis comes from. It comes from the oxygen in water molecules (H2O) and is just an unwanted byproduct as far as photosynthesis is concerned. The electrons and hydrogen ions are used, and some of the oxygen may get used for respiration, but a lot of them just go back out to the atmosphere where you and I may breath them with our very own gas exchange systems.
     The hydrogen ions left behind stay for a very short while in the thylakoid space. This space is often referred to as the hydrogen reservoir because it is a reservoir for hydrogen ions - nice and straight forward, eh? Darn sight easier to remember than thylakoid space.
Light Reactions cont...
      The hydrogen ions go zipping out of the thylakoid space, flowing through special channels in the membrane. It's the only way out. (Actually hydrogen ions are flowing both ways through the membrane during photosynthesis but we can't cover evrything - it would take pages and pages and pages.)
      The movement of these ions through the channels provides the energy to the ATP Synthetase enzymes where ATP molecules are made. ATP (adenosine triphosphate) and another molecule abbreviated NADPH (too big of a name to write) are the two energy carrying molecules that power the dark reactions (not to be confused with the dark side of the Force that powered Darth Vader) of photosynthesis Part 2 occuring in the stroma.
      There is also a steady flow of electrons, from the water, through various steps through the membrane, and finally to NADPH. This electron flow is an electric current - electricity powered by solar energy. This electric current provides energy for the making of ATP and NADPH.
     There is more, much more, but you get the idea. A lot is going on in a leaf.
     Basically, in step one, the light reactions, the energy of light is captured and transferred into ATP and NADPH, which are used to provide energy for the making of carbohydrates in stage two of photosynthesis, the carbon cycle.

Light Independent Reactions:
     Next we leave the thylakoids and head out into the stroma where the second stage of photosynthesis takes place. This stage is sometimes referred to as the carbon fixing process because during this stage carbon from carbon dioxide is "fixed" into the beginnings of simple sugar, or carbohydrate molecules. Other terms you may hear for this phase are the "dark reactions" or the "carbon cycle".
     Out here in the stroma, a bunch of different enzymes use the carbon dioxide molecules and hydrogen ions made during the light dependent phase to assemble sugar fragments which are half of a glucose molecule (just 3 carbon atoms, instead of the 6 required to make a complete glucose molecule).
     The carbon cycle, like the light reactions, is extremely complex with many steps. It is powered by the NADPH and ATP molecules formed in the light phase.

     This is as far as it goes in the chloroplast, and is the official end of photosynthesis. Ta da!

After Photosynthesis:
      Half a molecule of glucose. That's all we've got so far. Photosynthesis may be over, but it is not the end for these important molecules.
      The next step for the half-glucose molecules is to pass through the chloroplasts outer membrane into the cell. In the cell more enzymes join the 3-carbon fragments with another 3-carbon fragment, and voila! You have a glucose molecule.

     Now the glucose becomes the basic building block for a bunch of other carbohydrates, such as sucrose, lactose, ribose, cellulose and starch.
     Then they can move on into more plant cells and, if the leaf gets eaten, into animal cells. In both plants and animals they can be used to make fats, oils, amino acids, and proteins.

     I have presented a small piece of the amazing and extremely complex story of photosynthesis. I can't cover it all here, but I know you are so amazed and fascinated, you will continue to learn more in books and other resources.
(And you will send all your friends and enemies to my web site and you will buy something to help keep the site going - sneaky subliminal command)
     Glucose, the simple sugar product of photosynthesis, is the source of virtually all the energy, and most of the building materials for living organisms. We owe it all to the busy leaves and other photsynthesizing organisms.

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©Copyright 2010, 2014.  David E. Watson. All rights reserved. Everything in the Flying Turtle web site is copyrighted. For information concerning use of this material, click on the word Copyright.
Some  Definitions

  There are several types of light catching pigments in plants. Chlorophyll a is the most important photosynthetic pigment in green plants and is the one that gives most leaves their green color. It absorbs wavelengths of blue, violet, red and orange pretty well. The color it doesn't absorb very well is green. Most of the green light that hits chlorphyll a is reflected. That's why chlorophyll a looks green to us. We are seeing the green light waves that are reflected into our eyes. We don't see much blue or red, because those wavelengths are absorbed.
      Do the plants have to do without the wavelengths that chlorophyll a doesn't absorb? Not to worry. There are accessory photosynthetic pigments. These absorb other wavelengths and pass the energy on to the chlorophyll a. Chlorophyll b and chlorophyll c are two important pigments that help out. Carotenoids is another important group found in all green plants.
     Most of the time, the "non-green" absorbing pigments dominate, causing us to see green when we look at most leaves. In the fall, many plants such as deciduous trees, do something clever. The light absorbing part of chlorophyll molecules have magnesium and nitrogen atoms in them. Since you can't make chlorophyll molecules without them, these are valuable nutrients that may not always be readily available. So the plant breaks up the chlorophyll molecules and takes the magnesium and nitrogen atoms to a storage area to be saved and used to make new chlorophyll molecules in the spring. When the chlorophyll molecules are no longer reflecting green light, then some of the other pigments get a chance to shine. When we see yellow, brown, and orange leaves on the trees in the fall, we are seeing colors reflected by the carotenoids who get their brief 15 minutes of fame and glory when the chlorophyll is gone.
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ions:    Hydrogen is the simplest element. Hydrogen atoms are only made of one positively charged proton and one negatively charged electron. The positive charge of one proton exactly balances the negative charge of one electron, so a hydrogen atom has no charge. When a hydrogen proton is separated from its electron, it becomes a positively charged hydrogen ion with a charge of +1. An ion is a charged atom or group of atoms (molecule). If the ion has gained an extra electron, then it has a negative charge of -1. If the particle has become an ion by losing one electron then it has a postive charge of +1. If it has lost two electrons, it will have a charge of +2.
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photons:    Light is an interesting and hard to understand phenomenon for most of us. Physics books sometimes act like it all makes perfect sense, but come on, be honest, it's really mind-boggling. One of the hard to understand aspects of light, is that it appears to travel as either a wave or a particle depending on how you happen to be looking at it. When it travels as a particle, the little particles of energy are called photons. These little packets of electromagnetic radiation have a measurable and known amount of energy, and also a wavelength. When a pigment molecule absorbs light energy, it is absorbing photons. This added energy raises the energy level of the atoms. They get "excited" and the electrons become more energized. The more energized or excited electrons break loose from the pigments and go off to provide energy for the photosynthesis process. Thank you Mr. or Ms. photon.
     Keep reading the stuff on thylakoids above, to find out how the missing chlorophyll electrons get replaced.
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enzymes:    Enzymes are protein molecules that make things happen. During photosynthesis they grab various atoms and molecules and move them and assemble them into new molecules. Each assembly step requires a unique enzyme to perform it. There are also enzymes that take molecules apart, as in the case of water being broken down into oxygen and hydrogen.
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Back to the Thylakoid Section

A Little More on Oxygen in Plants:   What I said above about oxygen in photosynthesis is true but there is more to it than that. Oxygen is not used for the light-reaction part of the photosynthesis process, but just like animals and most other living organisms, plants need oxygen for respiration. Yes, just like animals, plant cells must "burn" surgar for energy and to do that they need oxygen. Some of the oxygen that is discharged during the photosynthesis process probably gets reabsorbed and used by the cells for respiration.
     During respiration, living cells, convert glucose to energy and release carbon dioxide as a byproduct. Just like animals, plants give off carbon dioxide. While it is true that there may be periods, such as night time, when some plants give off only carbon dioxide, they generally release more oxygen than they use during a 24 hour period. We animals would be in big trouble if it was not this way.
    Once or twice a month I get a question from someone who has heard that plants in a bedroom at night are dangerous because they give off carbon dioxide. Relax. It is a safe bet that the most dangerous thing in a bedroom, as far as carbon dioxide is concerned, is the person sleeping in there. With their bigger body mass (more respiring cells) and higher metabolism rate (how fast you burn energy per pound of living cells), humans are surely using up a lot more oxygen and producing a lot more carbon dioxide than the plants. Now if you add another person or two to the room, or maybe a dog and a few hamsters, the plant's CO2 output probably becomes negligible.
     Does this mean you should hold your breath and sleep alone? No, it is just an attempt to put things in perspective. Don't sweat the plants. A more realistic danger to children from plants in bedrooms are allergies and the possibility that small children might eat the plants or the dirt the plants are growing in.
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This page written by David Watson.

©Copyright 2009, 2014.  David E. Watson. All rights reserved. Everything in the Flying Turtle web site is copyrighted. For information concerning use of this material, click on the word Copyright.