Photosynthesis

PHOTOAUTOTROPHS

These are some of the organisms that can do photosynthesis.  These are called autotrophs because they make their own food, and the photo indicates that they use light as the source of energyThe capacity to do work or cause change. to do so.  We have a familiar forest on the bottom left here, but the bottom right picture shows a kelp forest, made of seaweed which are not plants but belonging to the eukaryotic kingdom of protists.  Above is cyanobacteria, which, despite its name, is also a protist.  It was reclassified after naming.  Oops.  The eukaryotic organelle used to so photosynthesis is the chloroplast, present in the leaf cellsThe basic structural and functional units of life. of a plant.  Don’t forget that these cells also have mitochondria.  Chloroplasts don’t do it all, mitochondria is still needed to generate usable ATPThe energy currency of cells used for muscle contraction..

CHLOROPLAST STRUCTURE

It’s important to think of where each part of photosynthesis is occurring in the cell or in the chloroplast.  The chloroplast, similar to mitochondria, has an outer and an inner membrane.  However, there is a third membrane inside that called the thylakoid membrane, that surrounds what looks like these green stacks of coins called granum.  Remember, this is a chloroplast, which is one of many contained in a eukaryotic cell, like the one here to the left.

LIGHT HAS VARIABLE ENERGY

Talking about light is like walking the border of what I understand about physics.  But, I have a simplistic understanding of it, which, I think, makes it easy for me to get some of the basicA solution with a pH above 7, having a lower concentration of H⁺ ions. concepts across. 

So.   Light is an expression of energy.  That energy can be measured in wavelengths.  Up top of this diagram here, we have a scale of wavelengths, from nanometers, on the left, to meters on the right.  The shorter wavelengths have more energy, like gamma rays and X-rays, and longer wavelengths have less energy like radiowaves and microwaves.  Light that is visible to the puny little human eye is only a very small portion of this scale ranging from 380 to 750 nm.  These wavelengths can be either absorbed or reflected by pigments like chlorophyll.

LIGHT HAS
VARIABLE ENERGY

We see chlorophyll as green, at least, if you are not color-blind.  The green color is the wavelength that is reflected by chlorophyll, the other wavelengths, or colors, are absorbed by chlorophyll.  If you want to think in terms of color instead of wavelength, go for it.  Check out this graph on the right; it is a graph of the wavelengths that are absorbed by the different chlorophyll pigments.  Notice how there is very low absorption for the blue-green wavelengths, but lots of absorption for the red, orange, yellow, and purple wavelengths.  But plants are not limited to these colors and lose out on the green-blue wavelengths.  I mean, they have energy and we want to be able to use it.  There are additional pigments in plants like carotenes, which are obviously orange, and xanthophylls that are yellow and red.  We see these pigments when the chlorophylls degrade in the Fall before the leaves drop.  For example, xanthophylls absorb the red-orange-yellow wavelengths, but not the green-blue wavelengths.  So, plants that have both these pigments can absorb almost the entire spectrum, expanding the available energy.

PHOTONS

Now that we have a grip on light energy, and how we measure it in wavelengths, let’s look a little deeper at the energy it provides.  We designate the term photons to light energy.  Think of photons this way, as a little packet of light comes in from the sun (a photon), it can excite an, thus passing on the light energy to the electron.  Electrons in chlorophyll actually skip to another shell, holding the energy from the photon.  We do something similar to this when we use these glow in the dark necklaces that you get at carnivals and such.  By cracking the necklace, or breaking it open, you excite electronsNegatively charged subatomic particles found in atoms. in the substance and the excited electron emits light.  Over time, the electron kind of fizzles out, emitting less and less energy.  You can delay this process by putting the necklace in the freezer.

THE LIGHT REACTIONS AND CARBON FIXATION

With an understanding of how the pigments in plants can absorb light energy and transfer it into an electron, we start to think of cellular respirationThe process of gas exchange, including ventilation, external and internal respiration. where the energy from excited electrons was harnessed with an electron transport chain.  Same here.  We are going to use the same machinery to harness the energy from an electron excited by a photon.  I like this overview diagram of photosynthesis.  Check out the background of the diagram, it’s the outline of a chloroplast.  We can see the outer membrane, the inner membrane, and those thylakoids that had what looked like the stacks of coins.  There are two main steps to photosynthesis: the light reactions and carbon fixation.  I like that this diagram has the light reactions on the left side of the diagram, with the reactants and products, and then the right side of the diagram shows carbon fixation.  Take a moment to look at the cycling of moleculesGroups of atoms bonded together. that we have between the two steps.  It seems that the light reactions make ATP and NADPH which then seem to be used in carbon fixation.  You may be wondering what NADPH is, and it is just another electron taxi, like NADH, however NADPH is specific to chlorophlasts whereas NADH is specific to mitochondria. 

Take a look at some of the other things associated with the light reactions like light, waterThe universal solvent essential for life., and oxygen.  In carbon fixation we seem to have an input of carbon dioxide, which we know plants require, and an out put of a molecule called G3P.

G3P

Many of us think that glucoseA simple sugar that is the main source of energy for cells. is the final product of photosynthesis, and it is, but in the short run, carbon fixation makes G3P.  G3P or glyceraldehyde 3 phosphate is a very versatile molecules that can be used for so many purposes.  Of course, G3P can be used in cellular respiration, but it can also be used to make starch, cellulose, and other organic molecules used by plants.

You already know some of  these structures

Remember that when you look at a diagram like this you want to first identify the features that you know.  Strange, somethings here seems somewhat familiar. Let’s start with location.  The membrane you see here is a thylakoid membrane that surrounds those stacks of coins called grana in a chloroplast.  The thylakoid membrane separates the inner thylakoid space from the stroma of the chloroplast.  We are going to build a proton gradient into this thylakoid space just like we built one in the intermembrane space in cellular respiration.  At the top right hand side of the diagram, we have an electron taxi, but this one has the name of NADPH.  Don’t worry about it, just pretend that the P mean photosynthesis or something.  At the bottom here we have ATP Synthase.  It doesn’t work any differently than it did in the mitochondria.  We have an electron transport chain in there, see it?  Note that it is moving protons from the stroma into the thylakoid space/  Flanking the electron transport chain are two things called photosystems.  These are new to us in this diagram.  Photosystems are quite complicated, but we can reduce them to the pigment containing proteinsLarge molecules made of amino acids with various functions in the body. in the thylakoid membrane.  We have photosystem I and II, and Photosystem II will be used first, of course.  Actually, they are so named due to their discovery.  So, some person found photosystem I and then later found photosystem II and was like: Damn!  Look how the electron transport chain is between the photosystems.  Finally, let’s locate the water we know that plants require for photosynthesis.  We can see it here on the left, and it seems to be donating two electrons to the photosystem II.  By donating these electrons, water is oxidized to molecular oxygen.  Oh!  One more thing…PHOTONS!  This is the light reactions, and we need those little packets of energy to get it going.

Let’s put this in motion.  A photon comes into Photosystem II and excites an electron.  That electron leaves the photosystem to the electron transport chain.  It is replaced by those electrons from water (apparently).  As the electron hops through the electron transport chain, protons are pumped to the thylakoid space and the electrons lose their energy as they hope from protein to protein.  But this time, there’s no one to accept the electrons from the electron transport chain.  Work with me here, because this next process kind of happens at the same time.  Another photon enters the pigments in photosystem II, exciting electrons.  These electrons are loaded into an electron taxi, NADPH.  These electrons will be shuttled to the stroma to be used in the Carbon fixation reactions in the second step of photosynthesis.  The electrons from the electron transport chain replace the electrons lost to NADPH.  All this time we’re shuffling electrons from photosystem to photosystem, we’re pumping protons to the thylakoid space.  These protons will then diffusionPassive movement of molecules from areas of high to low concentration. to the stroma through ATP synthasemembrane-embedded protein complex that adds a phosphate to ADP with energy from protons diffusing th which will harness the energy of that movementA fundamental property of life involving motion of the body or its parts. into the last bond of ATP. 

Remember that these are the light reactions, where we use light and water to make ATP, NADPH, and oxygen.  We still have to make glucose with the dark reactions.

CARBON FIXATION

Carbon fixation is known by many names like the dark reactions and the Calvin cycle.  This takes place with enzymesProteins that speed up chemical reactions in the body. in the stroma of the chloroplast.  Take a look at the inputs for carbon fixation, they are the products of the light reactions and carbon dioxide.  The carbon dioxide will enter the cells of the leave via stoma, which you can see here in the picture on the left.  Stomata are like little mouths on the bottom side of the leaves that, when open, allow the exchange of gases into and out of the leaves.

CARBON FIXATION

Carbon fixation is as complicated as the citric acid cycleseries of enzyme-catalyzed chemical reactions of central importance in all living cells for extracti of cellular respiration.  I think you’ve gotten the message that I want you to focus on inputs and outputs more than the specific steps.  Check out how carbon dioxide enters this cycle and is accepted by an enzyme named rubisco.  I just like that name because it makes me think of Nestle Crunch which is made by Nabisco.  By using the ATP and NADPH, G3P is made via reduction (the gain of electrons).  But check out how intense this cycle is – there are five G3P molecules already and always stuck in this cycle.  We make only one G3P by way of reduction, but then we spend some ATP to recycle five G3P into this molecules called RuBP, which is basically a sugar.  This is a 5-carbon sugar to which carbon dioxides will be attached to start the cycle again.

CARBON FIXATION

Carbon fixation is as complicated as the citric acidA substance that releases hydrogen ions (H⁺) in solution. cycle of cellular respiration.  I think you’ve gotten the message that I want you to focus on inputs and outputs more than the specific steps.  Check out how carbon dioxide enters this cycle and is accepted by an enzyme named rubisco.  I just like that name because it makes me think of Nestle Crunch which is made by Nabisco.  By using the ATP and NADPH, G3P is made via reduction (the gain of electrons).  But check out how intense this cycle is – there are five G3P molecules already and always stuck in this cycle.  We make only one G3P by way of reduction, but then we spend some ATP to recycle five G3P into this molecules called RuBP, which is basically a sugar.  This is a 5-carbon sugar to which carbon dioxides will be attached to start the cycle again.

EVOLUTION OF THE CALVIN CYCLE

Why do we need to know so much intense information about the two steps of photosynthesis?  Plants are green, they make sugars, what else do we really need to know?  You need to know enough to pass the quiz this module.  But, this knowledge also lets us understand plants like the pineapple plant.  It blooms this flower that is known as a multiple.  Take a look at the picture on the right, you can see the purple flowers coming out of each segment.  Each of these flowers will be pollinated and fuse together to become a pineapple fruit.  You can purchase pineapple plants at local home improvement stores.  They are common and hearty houseplants.  One thing about them is that they thrive in desert environments.  In factA statement based on direct observation that is repeatedly confirmed., they have a specific adaptation to dry climates, as they do they light reactions during the day, storing the ATP, NADPH, and oxygen gas until they open their stomata at night.  They day is so dry that opening stomata would desiccate the leaves and dehydrate the entire plant.  By opening the stomata only at night, gases can be exchanged since carbon dioxide is not needed for the light reactions, but only needed for carbon fixation.  This gives these plants an advantage in the dry arid climates of sub-tropical regions.