CHAPTER 6 PHOTOSYNTHESIS

OBJECTIVES 1. Explain how the structure of a chloroplast relates to its function 2. Describe the job of pigments in photosynthesis 3. Summarize the events of the light reactions 4. Explain how ATP is made during the light reactions 5. Summarize the events of the dark reactions (Calvin cycle) 6. Explain how some plants use alternative ways to fix carbon (CAM and C 4 ) 7. Know the chemical equation for photosynthesis

ENERGY FOR LIFE PROCESSES PHOTOSYNTHESIS- process by which light energy is transferred to chemical energy RECALL: Autotrophs- organisms that manufacture their own food>> PLANTS - most use photosynthesis Heterotrophs- cannot make their own food - must eat autotrophs or other heterotrophs

Ex: A caterpillar (heterotroph) eats grass (autotroph); a bird (heterotroph) eats the caterpillar - ALL LIFE DEPENDS ON AUTOTROPHS Photosynthesis is an example of a BIOCHEMICAL PATHWAY- a complex set of reactions in which the product of one reaction is consumed (used) in the next reaction - Autotrophs use photosynthesis to manufacture organic compounds from CO 2 and water

- during this reaction, O 2 is released While only autotrophs perform photosynthesis, both autotrophs and heterotrophs perform a process called CELLULAR RESPIRATION - in cellular respiration, organic compounds are combined with O 2 to produce ATP - CO 2 and water are given off as wastes - the products of photosynthesis (organic compounds and O 2 ) are reactants in cellular respiration

LIGHT ABSORPTION AND CHLOROPLASTS LIGHT REACTIONS- initial reactions in photosynthesis - begin with light absorption in chloroplasts - a cell may have 50 or more chloroplasts CHLOROPLAST STRUCTURE 2 Membranes: outer and inner - inside the inner membrane is another system of membranes, arranged in flattened sacs called THYLAKOIDS

- thylakoids are interconnected - GRANA- stacks of thylakoids - STROMA- solution surrounding thylakoids DRAW A CHLOROPLAST!!!

LIGHT Light from the sun appears white, but is made up of a variety of colors - you can separate light by passing it through a prism - the resulting colors ranging from red at one end to violet at the other is called the VISIBLE SPECTRUM Light travels though space as waves of energy, similar to waves that travel through a body of water

- waves are measured in WAVELENGTHS- the distance beween crests in a wave - different colors in the spectrum have different wavelengths PIGMENT- a compound that absorbs light - most pigments absorb some colors more strongly than others - when pigments absorb colors, they subtract those colors from the visible spectrum - the light that is reflected or transmitted by the pigment no longer appears white

Ex: A shirt reflects or transmits the color yellow and absorbs all other colors WHAT COLOR IS THE SHIRT? YELLOW, of course visible spectrum

INTRO. TO PHOTOSYNTHESIS Photosynthesis can be divided into 3 sets of reactions: 1. Light absorption by chlorophyll 2. Light reactions (light-depended reactions) 3. Dark reactions (light-independent reactions)

PIGMENTS IN CHLOROPLASTS The most important pigments in chloroplasts are called CHLOROPHYLLS - the most common types are chlorophyll a and chlorophyll b - these 2 types absorb different colors of light CHLOROPHYLL a- absorbs less blue light but more red light CHLOROPHYLL b- absorbs more blue light but less red light

- neither absorbs much green light; green light is reflected or transmitted - this is why most leaves are green ONLY chlorophyll a is directly involved in the light reactions - Chlorophyll b is called an ACCESSORY PIGMENT because it assists chlorophyll a

Other accessory pigments are yellow, orange, and blue CAROTENOIDS - carotenoids absorb colors that chlorophyll a cannot, and enables plants to capture more of the energy in light - during the fall, plants lose their chlorophylls and take on the colors of their carotenoids, which is why the leaves change colors

LIGHT REACTIONS/LIGHT DEPENDENT REACTIONS & LIGHT ABSORPTION BY CHLOROPHYLL PHOTOSYSTEM- clusters of pigment molecules in the thylakoid membrane - there are 2 types: photosystem I and photosystem II Light reactions begin when accessory pigment molecules in both photosystems absorb light - the molecules acquire the energy that was carried by the light waves

- the energy is passed to the other pigments until it reaches a certain pair of chlorophyll a molecules called the REACTION CENTER

STEPS OF PHOTOSYSTEM II REACTIONS - photosystem II replaces electrons lost in photosystem I 1. Light energy forces electrons to enter a higher energy level in the reaction center (2 chlorophyll a molecules) - these energized electrons are said to be “excited”

2. The electrons now have enough energy to leave the reaction center - since the reaction center has lost the electrons, another substance must pick up those electrons 3. Electrons are picked up a PRIMARY ELECTRON ACCEPTOR 4. The primary electron acceptor gives the electrons to a series of molecules in the thylakoid membrane - the series of molecules is called the ELECTRON TRANSPORT CHAIN

***NOTE: Photosystem II loses electrons that MUST BE REPLACED. The replacement comes from the splitting of 2 water molecules. (remember that water is a necessary ingredient for photosynthesis) ***NOTE: When H 2 O is split, O 2 is released (remember that O 2 is a product of photosynthesis)

STEPS OF PHOTOSYSTEM I REACTIONS - remember, occurs at the same time as photosystem II reactions 1. At the same time light is absorbed by photosystem II, light is also absorbed by photosystem I - electrons move from the reaction center in photosystem I to a primary electron acceptor

- the electrons that are lost by the reaction center are replaced by the electrons that have gone through the electron transport chain in photosystem II 2. The primary electron acceptor of photosystem I donates electrons to a different electron transport chain - the chain brings the electrons to the part of the thylakoid membrane that faces the stroma

- there electrons combine with a proton and NADP +, another electron acceptor - this causes NADP + to be reduced to NADPH THIS COMPOUND WILL BE USED IN THE DARK REACTIONS

CHEMIOSMOSIS CHEMIOSMOSIS- process by which chemicals pass through a membrane resulting in ATP formation STEPS IN CHEMIOSMOSIS: 1. As electrons are moved along the electron transport chain, hydrogen ions (from the splitting of H 2 O) are built up on one side of the thylakoid membrane as the electrons are moved to the other side of the membrane

- this is like a battery, with positive charges on one side of the membrane and negative charges on the other side of the membrane - this creates a large amount of potential energy (like a battery has a large amount of potential energy) 2. As the H + ions build up, they become higher in concentration - remember, electrons are in high concentrations on the other side of the membrane

3. When the H + concentration is at its highest, a special protein called ATP synthase opens and the H + ions diffuse through, being pulled by the very negative charge on the other side 4. This releases enough energy to make ATP from ADP - this ATP will also be used in the dark reactions

DARK REACTIONS/LIGHT INDEPENDENT REACTIONS 1. The dark reactions do not need light to occur, however, they need the products of the light reactions (ATP and NADPH) - the light reactions must occur BEFORE the dark reactions 2. The dark reactions occur in the STROMA - What is the stroma again? - solution inside chloroplast surrounding thylakoid membrane

3. The dark reactions “fix” or put the carbon from CO 2 into organic carbohydrate molecules 4. Carbon fixation is by the Calvin Cycle (dark reactions; also called C 3 cycle) - the Calvin Cycle must make 6 complete turns to produce a molecule of glucose - named after American scientist Melvin Calvin

STEPS OF CALVIN CYCLE STEP 1 CO 2 diffuses into the stroma and it is bonded to a 5- carbon compound called RuBP to form a 6-carbon compound. In 3 turns, 3 CO 2 molecules bind to 3 RuBP molecules. STEP 2 The 6-carbon compounds immediately split to form 2 molecules of a compound called PGA. There is a total of 6 PGAs formed here (2 for each of the 3 turns) - PGA is a 3-carbon compound, hence the name C 3 pathway

STEP 3 Next these PGA compounds are converted to PGAL compounds. In order to perform this step the ATP and NADPH from the light reactions are needed. - this step results in ADP (which can go back to the light reactions to pick up more P and make ATP) and NADP (which can also go back to light reactions to pick up more H to make NADPH)

STEP 4 The remaining PGAL can do one of two things: - 6 PGAL are used to make more RuBP to keep the dark reactions going - 2 PGAL can join together to make a glucose molecule; glucose is then used by the plant to make other carbohydrates (1 PGAL for every 3 turns)

Glucose is not the only organic compound formed - some plants use PGAL to form a variety of organic compounds such as amino acids, lipids, and carbohydrates

DRAW THE CALVIN CYCLE

OTHER CARBON-FIXING PATHWAYS - The Calvin cycle is the most common method of carbon fixation - Some plant species living in hot, dry climates have evolved different biochemical pathways for carbon fixation - Most water loss in plants occurs through small pores called STOMATA, found on the underside of leaves

- In hot, dry conditions, plants can partially close their stomata and reduce loss of water - However, when stomata are closed, CO 2 cannot enter the plant and O 2 cannot leave the plant - These conditions cause the Calvin Cycle to not operate properly - Plants need other methods of carbon fixation

C 4 PATHWAY In the C 4 pathway, CO 2 is bound to an intermediate compound before it enters the Calvin Cycle - This produces a 4-carbon compound (hence the name C 4 ) - Plants that use this pathway are called C 4 plants - The 4-carbon compound then undergoes a series of reactions until the CO 2 is finally transferred to the RuBP - It then enters the Calvin Cycle

ADVANTAGES OF C 4 : - Allows plants to photosynthesize in very hot and dry climates - Allows plants to fix carbon 4 times faster and thus grow faster in hot dry conditions DISADVANTAGES OF C 4 : - Requires more energy, but these plants grow where there is always abundant sunlight Ex: corn, sugarcane, crabgrass

CAM PATHWAY In the CAM pathway, CO 2 is brought in at night, when conditions are cooler - used by plants that live in very dry conditions - stomata are closed during the day to prevent water loss - the CO 2 taken in at night is stored in a compound called Crassulacean acid (giving it the name Crassulacean Acid Metabolism)

- the acid then releases the CO 2 into the stroma of the leaves during the day where it then bonds with RuBP and enters the Calvin Cycle. ADVANTAGES OF CAM: - growth without water loss DISADVANTAGES OF CAM - also uses extra energy; only helps plants in dry climates Ex: cactuses, pineapples

SUMMARY OF PHOTOSYNTHESIS Balanced equation for photosynthesis: 6 CO H 2 O + light energy  C 6 H 12 O O 2 - the 6 CO 2 are fixed during the dark reactions into the resulting carbohydrates - the 6 H 2 O are split in the light reactions (remember, H + are picked up by NADP and the electrons restore photosystem I)

- the 6 O 2 are released when water is split - the C 6 H 12 O 6 is a product, which can be used directly by the plant or used to make more advanced carbohydrates THE END