Photosynthesis

1. Heterotrophs
   1. Eat Autotrophs and other Heterotrophs for energy
   2. Gain only 10% of the energy from the organism they eat
   3. Ex.: Humans, Deer, Birds
2. Autotrophs
   1. make their own food
   2. Plants, Some Bacteria, Alagae

1. Note: Glucose isn't produced in Photosynthesis, butt rather in the equation to emphasize difference between photosynthesis and cellular respiration

Annotations:

• Note: A compound that absorbs light is called a pigment. This relates to the properties, which must be understood in order to understand how chloroplasts absorb light

1. Light Reactions: initial reactions of photosynthesis. They begin with choloplasts absorbing light.

   Annotations:

   • While some alagae can contain one large chloroplasts, a cell in plant leaf will contain 50+.   
   1. Inside membrane surrounding chloroplast, there is another system of membranes arranged as flat sacs called thylakoids. They are interconnected and layered on top of one anotherto form stacks called grana. Surrounding the thylakoids is a solution called stroma.
2. Located in membrane of thylakoids are variety of pigments, with most imp. being chloropphylls. There are several types of them, w/ most common being chlorophylls a and b.
   1. Slight diff. between a and b causes them to absorb different colors of light. But they both allow green light to reflect
   2. Only chlorophyll a is directly involved in light reactions in photosynthesis. Chlorophyll b assists chlorophyll a in capturing light energy, and therefore chlorophyll b is called an accessory pigment
   3. Other compounds in thylakoid membrane also function as accessory pigments. Accessory pigments enable plants to capture more of the energy in light
3. * During fall, many plants lose chlorophylls and take on rich hues of carotenoids

1. Chlorophylls and Carotenoids are grouped in clusters of a few 100 pigment molecules in thylakoid membrane. Each cluster is referred to as a photosystem.
   1. Two Types of Photosystems are photosystem 1 and photosystem 2. They both contain similiar pigment, but have diff. roles in light reactions.
      1. Light reactions began when accessory pigment molecules in both photosystems absorb light, which means they have aqquired energy from light waves. The aquired energy is then passed to other pigments until it reaches specific pair of chloryhll a molecules.
         1. Step 1: Light Energy forces electrons to enter higher energy level in 2 chlorophyll a molecules of photosystem 2.

            Annotations:

            • These energized electrons are said to be "excited" 
            1. Step 2: The Excited electrons have enough energy and leave chlorophyll a molecules. Because they have lost electrons, the chlorophyll a molecules have undergone an oxidation reaction.

               Annotations:

               • Each oxidation reaction must be accompanied by a refuction reaction, which means that some substance must accept the electrons that the chlorophyll a molecules have lost. The substance is known as primary electron acceptor
               1. Step 3: The primary electron acceptor then donates the electrons to the first of a series of molecules located in the thylakoid membrane, which are known as an electron transport chain (ETC) since it transports electrons from one molecule to another in a series. As the electrons move along the chain, they lose energy from when they were "excited." The energy they lost is harnessed to mover protons into the thylakoid.
                  1. Step 4: At the same time of the last step, light is absorbed by photosystem 1 and 2. Electrons move from pair of chlorophyll a molecules in photosystem 1 to another primary electron acceptor. The electrons that are lost by these chlorophyll a molecules are replaced by electrons that have come from electron transport chain from photosystem 2
                     1. Step 5: Primary electron acceptor of photosystem 1 donates electrons to diff. ETC, which then brings electrons to side of thylakoid membrane to face stroma. There, electrons combine with a protein and NADP+, and organic molecule that accepts electrons during redox reactions.

                        Annotations:

                        • A redox reaction is a reaction in which two species exchange electrons.  Also, a redox reaction causes NADP+ to be reduced down to NADPH. 
                     2. If electrons from photosystem 2 are not replaced, both electron transport chains in both photosystems will not work, and photosynthesis will not occur. The replacement electrons are provided by water molecules. An enzyme inside thlakoid splits water molecules into protons, electrons, and Oxygen

                        Annotations:

                        • For every 2 H2O molecules split, 4 electrons become abailable to replace those lost by chlorophyll molecules in photosystem 2. 
                        1. The protons produced are left in thylakoid, but oxygen diffuses out of plant, making it a by product.

1. 1. The structure of the thylakoids is described as a system of membranes stacked as flattened sacs, and their function is to house chloroplasts
   1. 2. Accessory pigments help another chlorophill capture light, therefore enabling plants to capture more light
      1. 3. Electons lost in either photosystem are replaced by water molecules, which are then broken down for use
2. 4. Protons, Electrons, and Oxygen is produced when water molecules are broken down during light reactions
   1. 5. ATP is made by ATP synthase, which harnesses potential energy along the proton concentration gradient in order to make ATP
      1. 6. If there was no concentration gradient of protons along the thlakoid membrane, ATP won't be able to be synthesized, and photosynthesis would not occur.

1. 1. The Calvin Cycle takes place in the stomata of the cloroplast.
   1. 2. The PGAL molecules are broken down and turned into ADP
      1. 3. It takes 3 turns of Calvin Cycle to produce PGAL, each turn using 2 ATP and 2 NADPH molecules
2. 4. Stomatas control passage of water out of plant and CO2 into a plant.
   1. 5. C4 plants use the C4 pathway to fix carbon by breaking into four-carbon molecules
      1. 6. The rate of photosynthesis increases and then reaches a platue as the concentration of CO2 around plant decreases because it has reached its furthest point

1. process that synthesizes ATP
2. Relies on concentration gradient of protons across the thylakoid membrane
   1. Concentration of Protons is higher in thylakoid than stroma
      1. Concentration of Protons = potential energy, which is harnessed by protein called ATP synthase (located in thylakoid membrane)

         Annotations:

         • ATP synthase is a multifunctional protein. It acts as carrier protein and enzyme
         1. ATP synthase makes ATP by adding phosphate group to ADP
            1. ATP synthase converts potential energy of proton concentration gradient into chemical energy, which is stored in ATP
               1. Together, ATP and NADPH provide energy for 2nd set of reactions in photosynthesis
                  1. The Calvin Cycle
                     1. Carbon Atoms from CO2 are bonded into organic compounds

                        Annotations:

                        • This incorporation of CO2 into organic compounds is referred to as carbon fixation
                     2. Occurs within stroma of chloroplast
                     3. Has 3 Major Steps:
                        1. Step 1: CO2 diffuses into stroma. Enzyme combines CO2 with a 5-carbon carbohydate called RuBP. Product = 6-carbon molecule that splits immediately into pair of 3-carbon molecules known as PGA
                        2. Step 2: In 2-part process, PGA = 3-carbon molecule, PGAL.
                           1. 1st, each PGA receives phosphate group from molecule of ATP.
                           2. Resulting compound then receives a proton from NADPH and releases a phosphate group, producing PGAL. These reactions also produce ADP, NADP+, and phosphate, which can be used again in light reactions to synthesis enough molecules of ATP and NADPH
                        3. Step 3: Most of PGAL is converted back into RuBP in complex series of reactions. The reactions need a phosphate group from another molecule of ATP, which is then changed into ADP. By generating the RuBP that was consumer earlier (in Step 1), these reactions allow Calvin Cycle to continue operating.

                           Annotations:

                           • Some PGAL molecules are not converted into RuBP. Instead, they leave Calvin Cycle and can be used by plant cell to make other organic compounds
                     4. Alternative Pathways to Photosynthesis
                        1. Some species "fix" carbon using alternative pathways, and then enter it into calvin cycle
                        2. Generally used by plants that live in dry and hot climates, where they loose a lot of water, most of it from stomatas

                           Annotations:

                           • Stomatas are also places where CO2 enters plant and O2 leaves. When stomata is closed, CO2 in plant decreases, and O2 rises due to Calvin Cycle
                        3. C4 Pathway: used by C4 plants. Even though CO2 level is low and O2 is high, these types of plants have an enzyme that can fix CO2 into 4-carbon compounds, which are then transported to other cells, where CO2 is released and enters Calvin Cycle
                           1. C4 plants examples: corn, sugar cane, and crabgrass
                        4. CAM Pathway: plants open their stomata at night and close them during the day. At night, CAM plants take in CO2 and fix it into a variety of organic compounds and enter it into Calvin Cycle. Since they have stomata open at night when temp. is lower, they grow slowly, but lose less water
                           1. CAM plants examples: Pineapples and cactuses
                     5. Most common pathway for photosynthesis
                     6. Plants that use it are known as C3 plants

1. affected by plant's envirnoment
   1. Light Intensity
      1. As light intensity increases, so does rate of photosynthesis
   2. CO2 Level
      1. Increased levels of CO2 increase rate of Photosynthesis
   3. Temperature
      1. As temperature increases, rate of photosynthesis increases

         Annotations:

         • Photosynthesis rate will decrease if temperature increases excessively