AP Bio Chapter 8: Photosynthesis

What are the nm of visible light?

380 nm-750 nm is the area of visible light on the electromagnetic spectrum.

Who is the Calvin Cycle named for?

Melvin Calvin, a scientist who discovered the steps of the Calvin Cycle in the late 1940's.

How do the reactants of photosynthesis get in? How do the products get out?

-Carbon dioxide enters the leaf, and oxygen exits, by way of little pores called stomata. -Water absorbed by the roots of the plant is delivered to the leaves in veins. -Leaves use beins to export the product (sugar) to roots and other nonphotosynthetic parts of the plant.

Describe how light works.

-Light is a form of energy known as electromagnetic energy, also called electromagnetic radiation. Electromagnetic energy travels in waves. -The distance between crests of electromagnetic waves is called a wavelength. Wavelenths range from less than a nanometer to more than a kilometer. -Entire range of radiation- electromagnetic spectrum -For photosynthesis, we only use the visible light spectrum, which is basically the rainbow of colors (ROYGBIV). We call this visible light because we can detect it with the human eye. -Light is packaged in little particles called photons. Photons are not tangible objects, but each of them does have a fixed quantity of energy. -The shorter the wavelength, the more amount of energy in each photon (like wavelengths close to violet) -The longer the wavelength, the less amount of energy in each photon (like wavelengths close to red)

What are the parts of a chloroplast? Give a brief description.

-The dense fluid inside the chloroplasts (kind of like the cytoplasm inside of a eukaryotic cell) is called the stroma. It surrounds the grana and lamellas. -A chloroplast is surrounded by an envelope of two membranes! NOT one. -Suspended within the stroma is yet another membane system made up of sacs called thylakoids. Inisde of the thyladoids is the lumen, which is often called the "thylakoid space." Outside of the membrane is the stroma. -Grana, or granum for singular are the stacks of thylakoid membranes inside of a chloroplast. -Inside the thylakoid membranes, there are embedded structures that make photosynthesis possible. One of these is chlorophyll, the main pigment that absorbs sunlight to start photosynthesis. Photosystems II and I are also embedded within the membrane, as well as some other proteins/molecules that help conduct photosynthesis. -Lamellas are the little connectors that connect the grana stacks.

Describe the light-dependent reactions in detail, utilizing vocabulary words and definitions.

1. A photon of light strikes a pigment molecule in a light-harvesting complex of Photosystem II, boosting one of its electrons to a higher energy level. As this electron fall back to its ground state, an electron in a pigment molecule nearby is raised to an excited state, The process continues, with energy being relayed to other pigment molecules until it reaches a special pigment molecule called the P680 pair of chlorophyll a molecules. The energy from the light causes an electron from the P680 pair to become excited. 2. This excited electron is then passed to the primary electron acceptor from the P680 pair. When the P680 pair gives up its electron, it becomes P680+. 3. An enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ions, and an oxygen atom. The electrons are given one by one to the P680+ pair so that their electrons are replenished. The two H+ ions are later released into the thylakoid space (inside the thylakoid). They will eventually be pumped in there using energy lost as electrons travel down the ETC. Meanwhile, the oxygen atom combines with another oxygen atom (produced by another splitting of a water molecule) to produce O2. O2 is then released out of the plant via stomata. 4. Each energized electron that was passed to the primary electron acceptor then travels from Photosystem II to Photosystem I via an electron transport chain (ETC for short). Remember that each electron carrier becomes reduced when it recieves an electron from its less electronegative neighbor, and then becomes oxidized when it donates the electron to its other more electronegative neighbor. This is a series of becoming reduced and then oxidized. 5. Energy is lost as the electrons move down the chain, and this energy is used to synthecize ATP. The way this happens is, the energy lost as electrons move down the ETC helps power a protein that pumps H+ ions (from the split water molecule) into the thylakoid space. This contributes to a proton gradient that is used in chemiosmosis. Chemiosmosis is essentially the process of using an H+ gradient to complete cellular work, such as ATP synthesis. In this case, the chemiosmosis powers phosphorilation, which occurs when there is a gradient of H+ ions (there are many of them inside the thylakoid space). According to the laws of diffusion, it is known that substances have the tendency to move from areas of high concentration to areas of low concentration. This means that H+ ions will move down their concentration gradient and will need to move outside of the thylakoid. However, the only passageway through which they can do so is an enzyme called ATP synthase. It takes a phosphate group and adds it to ADP, creating ATP. It is essentially a small rotor that rotates to synthesize the two reactants together into ATP. 6. While all of this is occuring, light energy is being absorbed by pigments in photosystem I. The same process occurs as it did with photosystem II, this time exciting a special pigment molecule called P700. P700 then becomes P700+ as it loses its excited electron to a primary electron acceptor. How does it get replenished? The electrons that travelled down the electron transport chain from photosystem II replenish it. 7. Meanwhile, the electrons that end up going to the primary electron acceptor start to go down another electron transport chain, this one a little shorter than the first one. 8. An enzyme called NADP+ reductase reduces NADP+ by adding two electrons to it in order to create NADPH. This whole NADP+ reductase thing occurs within the stroma, and the NADPH is released into the stroma. So was the ATP, made earlier. These two products of the light-dependent reactions become the reactants of the light-independent reactions, and they are right in the stroma, where the Calvin Cycle will eventually take place. Yay!

Give me the molecular formula of photosynthesis.

6 CO2 + 6 H2O + Light --> 6 O2 + C6H12O6 *Photosynthesis equation shows that glucose is the direct product of photosynthesis, but it is NOTTTTTT!!! It is actually a 3-Carbon sugar that can be used to make glucose. *Note that the opposite equation of photosynthesis is the one for cellular respiration. *Many people (including you) make the mistake that plants only do photosynthesis and not cellular respiration. This is absolutely NOT true! Plants do photosynthesis to get their food, and they do cellular respiration to break it down. The difference between us and plants is that we need to eat other organisms to get fuel, but we are similar in the fact that we both do cellular respiration.

Chlorophyll a is often considered the main pigment in photosynthesis. Are all the accessory pigments useless?

Absolutely not! The absorption spectra of chloroplast pigments provide clues to the relative effectiveness of different wavelengths for driving photosynthesis. The more light that can be absorbed by the pigments, the more photosythesis can be done.A graph that plot's a pigment's light absorption vsersus wavelength is called an absorption spectrum. While chlorophyll a does a great job at absorbing light in the violet and red areas, chlorophyll b absorbs light best at areas close (but not exactly) the same as chlorophyll a. This means that these pigments working together will asborb a broader range of color than just one alone. There are many other accessory pigments, such as carotenoids, hydrocarbons that are orange and can absorb in that area of the spectrum. They are also in charge of photoprotection. They absorb and dissipate excessive light energy that could damage chlorophyll or interfere with oxygen, forming reactive oxidative molecules that are dangerous to the cell.

Difference between an action spectrum and an absorption spectrum?

An absorption spectrum shows the nm (wavelength) vs. the amount of absorption for each type of pigment. The action spectrum depicts the nm (wavelength) vs. the amount of photosynthesis (action) taking place at that wavelength. Usually the amount of photosymthesis matches whether the pigments absorb light in that area or not. Ex: Not much photosynthesis occurs during the green section A lot of photosynthesis occurs during the violet/red sections.

Where can chloroplasts mainly be found within the leaf?

Chloroplasts are found mainly in the cells of the mesophyll, which is just the tissue on the inside of the leaf. A typical mesophyll cell has about 30-40 chloroplasts.

How does the electron transport chain work?

During electron transport along the chain, electron carriers alternate between reduced and oxidixed states as they accept and donate electrons. Each component of the chain becomes reduced when it accepts electrons from its neighbor. Its neighbor always is less electronegative, meaning that it has less affinity for electrons. This is why it transfers the electrons down the chain. It then returns to oxidized as it passes electrons to its more electronegative other neighbor.

How is cellular respiration different than photosynthesis?

Electrons are being transferred through a series of increasingly electronegative molecules, just as in respiration, BUT: 1. Photosynthesis is ENDERGONIC, which means that when all of the reaction is taken into consideration, energy is lost. Energy is needed to synthacize the reactants in order to make the main product of glucose. Photons (light packets) power the process. 2. Cellular Respiration is EXERGONIC, which means that when all of the reaction is taken into consideration, energy is gained. Remember, in cellular respiration you are breaking down complex molecules in order to get energy as the bonds are broken. It is used to form ATP, which can be used by the cell.

What are the two ways that organisms on this earth can get fuel?

Life on Earth is solar powered. The chloroplasts inside of plants capture light energy that has travelled from the sun and converted it to chemical energy stored in sugar and other organic molecules. This process of conversion is called *photosynthesis*. Photosynthesis nourishes almost the entire living world directly or indirectly. There are two major modes through which an organism can acquire the organic compounds it uses for energy and its carbon skeletons: 1. Autotrophic Nutrition -An autotroph is a self-feeder. Auto means self, and trophos means feeder. They can sustain themselves without eating anything derived from other living things. -They produce their organic molecules from carbon dioxide and other inorganic raw materials obtained from the environment. -They are the ultimate sources of organic compounds for all nonautotrophic organisms -For this reason, scientists call autotrophs the producers of the biosphere. 2. Heterotrophic Nutrition -Heterotrophs are any organisms that are not able to make their own food. They do not photosynthesize. -They live off of compounds that are produced by other organisms. -Heterotrophs are the consumers of the biosphere. -Heterotrophs can eat plants and animals (this is what people usually think of when you say heterotroph). However, they can also feed on organisms more subtly. For instance, they can help decompose dead organisms or eat feces or fallen leaves. These types of heterotrophs are called decomposers (many types of prokaryotes (single-celled organisms) as well as fungi get their food this way). -Almost all heterotrophs, including humans, are completely dependent on autotrophs for our food (we can eat the plants themselves or eat animals that have eaten the plants, etc.). We are ALSO dependent on autotrophs for oxygen.

How does the endosymbiont theory play into the start of photosynthesis?

Photosynthesis was derived before cellular respiration because autotrophs evolved on Earth before heterotrophs (this may be why rubisco accepts both O2 and CO2 into its active site during the Calvin Cycle!) The process of photosynthesis most likely occured in a group of bacteria that had infolded regions of plasma membrane containing clusters of enzymes and photosynthetic molecules. Today, photosynthetic bacteria have infolded photosynthetic membranes that function similarly to the membranes of the chloroplast, which can be found in eukaryotic cells today. According to the *endosymbiont* theory, what is now the chloroplast was once a photosynthetic prokaryote that lived inside an ancestor of eukaryotic cells.

Explain the Calvin Cycle in all detail. Make sure to use vocab when necessary.

Remember that the Calvin Cycle is an anabolic process, which means that it takes smaller molecules and combines them into larger, more complex molecules. It consumes energy. Carbon enters the Calvin Cycle as CO2 and leaves as a sugar named G3P. For net synthesis of one G3P, the cycle needs to take place three times, fixing three molecules of CO2 (since carbon dioxide has one atom of carbon per molecule. The Calvin Cycle has three phases: 1. Carbon Fixation -The Calvin Cycle incorporates each CO2 molecule, one at a time, by attaching it to a five-carbon sugar that we call RuBP for short. The enzyme that catalyses this first step is RuBP carboxylase., also called RUBISCO. This is super important! It could literally be the most abundant protein on Earth. When carbon is incorporated into RuBP by rubisco, it is made into a six-carbon intermediate structure that is so unstable that it immediately splits in two. 2. Reduction -Each molecule of the intermediate structure that was split in two now recieves a phosphate group from ATP and two electrons from NADPH, along with a phosphate group. It then becomes G3P. 3. Regeneration of the CO2 Acceptor This is super complicated, so I won't get very detailed. But essentially five molecules of G3P are rearranged by the last steps of the Calvin Cycle in order to make RuBP again. It is now ready to accept more CO2 again, and the cycle continues. Hence the name Calvin CYCLE! What does the G3P do? It goes off through more processes in order to be transformed to glucose, but just remember that G3P is a sugar in itself. For every G3P formed, 6 NADPHs are used and so are 9 ATPs.

Which parts of a plant carry out photosynthesis?

The green parts: -Green stems -Unripened fruit -Leaves (main site of photosynthesis in most plants)

What are the two types of autotrophs?

There are two types of autotrophs: Photoautotrophs -Green plants -Algae -Photoplankton -Few unicellular eukaryotes -Basically anything that photosynthesizes normally without the need for chemical energy (they use energy from the sun/light energy) Chemoautotrophs: -They use chemical energy to produce organic compounds (Ex: bacteria in hydrothermal vents)

How to redox reactions play into photosynthesis?

Think about the photosynthesis equation below: 6 H2O + 6 CO2 + Light --> C6H12O6 + 6 O2 -The water becomes oxidized to form the O2 in the products. You can tell it is being oxidized because it loses its H+ ions, which are carrying electrons. Whenever you lose electrons in a redox reaction, you are getting oxidized. -The carbon dioxide gets reduced to form glucose. This is because it ends with hydrogen ions, which are holding electrons. Whenever you gain electrons in a redox reaction, you are getting reduced. OIL RIG--> Oxidation Is Losing, Reduction Is Gaining -The reason why carbon dioxide is needed in photosynthesis is because it eventually helps produce glucose. The reason why water is needed is because it splits to provide electrons to replenish those in P680. Its oxygen is released as a byproduct at the end of photosynthesis. -Remember that one reactant is gaining electrons and the other is losing electrons in sequential series.

How do pigments work?

When light meets matter, it can be reflected, transmitted, or absorbed. Substances that absorb light are called pigments. Different pigments absorb different wavelengths of light (different colors). Whatever color the pigment is, that is the color that it most reflects (because it is absorbing the rest of the colors). This is why when we see plants we see green. Chlorophyll a is the main pigment within most plants, and it absorbs light very well on both the red/organge and violet/indigo sections of the spectrum, but NOT in the green/lime yellow section. This is why we see this color reflected back to our eyes. Scientists are not quite sure yet why it is exactly green that gets reflected back up, but we think it is because the leaves might be absorbing a little too much energy if the leaves were black and absorbed all colors. A spectrophotometer can measure the ability of a pigment to absorb certain wavelengths.