Why is Photosynthesis Important?? • Provides Oxygen to almost all organisms need to break down food into energy(cell respiration-remember this for chapter 7)
Photosynthesis • Converts radiant energy into chemical energy stored in the bonds of organic compounds • Stored in plants as STARCH • Done by Autotrophs • Done in the Chloroplasts of the plant cell
I. The Chemistry of Photosynthesis • The chemical reactions of photosynthesis and respiration are opposites • Photosynthesis • CO2 + H2O+Energy C6H12O6 + O2 • Respiration • C6H12O6 + O2 CO2 + H2O + Energy
A. Energy Intermediates • ATP-ADP • NADPH- NADP+ (+) H+ • NADH- NAD+ (+) H+ • FADH2- FAD+ (+) 2H+
ATP: Adenosine Triphosphate • Three Pieces: • Adenine • Ribose • Phosphate group
The adenine and ribose combine to make Adenosine • The energy obtained from ATP comes from the breaking apart of phosphate groups • Pic Phosphorylation
B. The ATP-ADP Cycle • When the cell needs energy from a specific activity… • 1. The outermost phosphate of ATP is broken off by the enzyme ATPase • 2. This phosphate group is transferred to another molecule in the process called PHOSPHORYLATION: when a phosphate group is added to a molecule.
3. The ADP then undergoes phosphorylation with the help of ATPsynthase to form a new ATP
Turning ADP into ADP • Done with the aid of a proton pump Movement of Hydrogen Ions across the membrane …. • provides the energy needed to add a phosphate group to the ADP, creating ATP • This is done by active transport
Characteristics of Light • Pic • Humans can see light between the wavelengths 400nm and 700nm • Other organisms can see light at different wavelengths than we can!
Plants absorb light the best when the light is at a wavelength of around 450 or around 650nm • Plants do not absorb light between 500 and 600nm very well. • So, why are plants green??
Where does photosynthesis happen? • Inside Chloroplasts • Specifically within the GRANA • Stacks of flattened sacs • Each stack is made of single sacs called Thylakoids. Side Note: Single Stack is called Granum
Thylakoids • Site of light absorption, where chlorophyll is found • Membrane of double lipid layers • Embedded with proteins like ATPsynthase It is very important to keep track of where things happen in photosynthesis • Some things happen between the two thylakoid membranes, in a space calleds the thylakoid space • Some things happen on the membranes of the thylakoid, and these products are released into the Stroma: Protein rich solution around the grana
II. Stages of Photosynthesis • Divided into 3 sets of reactions: • 1. Light absorbtion by chlorophyll • 2. Light dependent reactions • A. electron transfer • B. Chemiosmosis • 3. Light independent reaction
1. Light absorbtion by chlorophyll • Radiant energy from the sun is absorbed by pigments Chlorophylls • Chlorophyll a- most like absorbtion • Chlorophyll b- some light absorbtion
In the Chloroplasts • Accessory Pigments: • Carotenoids- absorb some green and reflect yellow/orange • They and chlorophyll b send absorbed light energy to chlorophyll a
2. Light Dependent Reaction • Light Reactions Convert the light energy to chemical energy 2 Main Stages: A. Electron Transport- Converts the suns energy to electrical energy B. Chemiosmosis- Converts the electrical energy to chemical bond energy
A. Electron Transfer Sunlight strikes the chlorophyll molecules and “excites” (e-) Photosystems: are units of several hundred chlorophyll molecules and carrier molecules. The photosystems are found in the: Thylakoid Membrane
The reaction starts in Photosystem II • Sunlight strikes chlorophyll in photosystem II, and excites the e- e- move down chain of Electron Carriers(ETC)
2. e- move from high energy to a lower energy level found in photosystem I • ** Giving off energy
3. At the same time… • Light strikes photosystem I • E- are excited and move along a transport chain • E- leaving photosystem I are replaced by those from photosystem II
4. The energy is converted to energy intermediates: • A. NADPH is formed when the electron from photosystem I causes NADP+ to bond with H+ *E- lost in Photosystem II are replaced by electrons taken from water molecules When H20 splits… • hydrogen is used to make NADPH and ATP
Oxygen is given off as waste product: This is where you get all of the oxygen that you breathe!! • Pic
B. Chemiosmosis • The diffusion of chemicals through a membrane resulting in the formation of ATP • 1. During the breakdown of water, there is a build up of H+ ions in the thylakoid space
2. The movement of H+ ion across the thylakoid membrane triggers the phosphorylation of ADP to ATP Done by ATP synthase • 3. The ATP formed in the stroma is used in the 3rd reaction, during Carbon Fixation
What do we get out of the light reactions?? • ATP, produced in the stroma • NADPH, produced in the stroma • Hydrogen ions, produced in the thylakoid space, pumped into the stroma to make NADPH and ATP • Oxygen, given off as by-product(waste_
3. Light Independent Reaction: Calvin Cycle(Dark Reactions) Uses the energy formed in the light dependent reaction(ATP and NADPH) to form organic compounds. There are three possible pathways that can be followed- depends on the plants environment. • A. Calvin Cycle or C3 • B. C4 Pathway • C. CAM cycle
A. The Calvin Cycle(C3): The most common form of carbon fixation • 1. CO2 diffuses into the stroma and an enzyme on the thylakoid membrane binds to CO2 to a five carbon – Ribulose Biphosphate molecule(RUBP)= 6 carbon molecule • 2. The 6C molecule is unstable so it splits in to 2 PGA’s- Phosphoglyceric Acid(3C)
3. The 2 PGA’s get a phosphate from ATP and H from NADPH to form Phosphoglyceraldehyde(PGAL)(G3P), ADP and NADP • 4. Most of the PGAL regenerates more RuBP. This is done by using ATP. The other PGAL is used to produce carbohydrates. • Every turn of the Calvin Cycle drops off 1C. So 3 turns per G3P. Two (G3P’s) produces a glucose molecule. 6 turns total.
B. C-4 Pathway • Carbon Dioxide is bound to a compound to form a four-carbon intermediate that enters the C-3 pathway • Fixes carbon 4 times faster than the C-3 pathway- allows plants to grow more quickly • Requires more energy than the C-3
Key Feature: The light dependent reactions and the Calvin Cycle occur in different areas of the leaf.
Many desert plants are C-4 • Example Corn, Sugar Cane • C-4 occurs in sunny areas
C. CAM- Crassulacean acid metabolism • Plants take in CO2 at night and store it • Used in the daytime in The Calvin Cycle • Prevents excess water loss • Key Feature: The light dependent reactions and carbon fixation occur at different times.