Unit 6- Photosynthesis & Cellular Respiration 6a- Photosynthesis

First Homework (New Semester) Bring a leaf to class tomorrow

Life Requirements All organisms need a constant source of energy to survive. The ultimate source of energy for most life on Earth is the Sun.

Trapping the Sun’s Energy Photosynthesis is the overall process by which solar energy is used to chemically convert water and carbon dioxide into chemical energy stored in simple sugars. Involves a complex series of chemical reactions in which the product of 1 reaction is a reactant in the next Biochemical pathway

Photosynthesis The process of photosynthesis is generally represented using a balanced chemical equation. However, this equation does not represent all of the steps that occur. Sunlight 6CO2 + 6H2O C6H12O6 + 6O2

Steps of Photosynthesis Photosynthesis can be divided into two stages Light-dependent Reaction Solar energy is absorbed by chloroplasts and two energy-storing molecules (ATP & NADPH) are produced. Calvin Cycle (Light-independent reaction) Simple sugars (glucose) are formed using CO2 & the energy stored in ATP & NADPH Previously, these reactions were called the light reaction and the dark reaction. While some text still refers to the Calvin Cycle as the dark reaction, most do not because it contributes to the misconception that the Calvin cycle occurs at night.

Structure of Chloroplasts Photosynthesis takes place in the chloroplasts Each chloroplast is surrounded by a pair of membranes. Inside the inner membrane is another system of membranes called thylakoids. Arranged as hollow disks Connected & layered to form stacks called grana Surrounding the grana is a solution called the stroma. Chloroplasts have 3 phospholipid bilayers

Where does each step take place? Light-dependent reactions take place in the membranes of the thylakoid disks Calvin Cycle takes place in the stroma

Review of Light Light from the sun- white light Can be separated into its component colors by passing the light through a prism Results in the visible spectrum (ROYGBIV) When white light strikes an object, its component colors can be reflected, transmitted, or absorbed. Pigments- compounds that absorb light Most absorb certain colors more strongly than others, so the light that is reflected no longer appears white.

The Role of Chloroplasts and Pigments The thylakoid membranes contain the pigments that can absorb certain wavelengths of sunlight. The most important pigment is chlorophyll. Absorb most wavelengths of light except for green. Green is reflected making the plants appear green. In the fall, plants reabsorb chlorophyll leaving other pigments that reflect other wavelengths of light – making the leaves appear red, yellow, or orange. In leaves of plant, chlorophyll’s much more abundant and mask colors of other pigments. In the nonphotosynthetic parts like flowers, the other pigments are visible During the growing season, chlorophyll is continually being produced and broken down & leaves appear green. As night length increases in the Fall, chlorophyll production slows down & then stops & eventually all the chlorophyll is destroyed. http://www.na.fs.fed.us/fhp/pubs/leaves/leaves.shtm

Light-Dependent Reactions Sunlight strikes the chlorophyll molecules in the thylakoid membrane. Chlorophyll absorbs the light & the solar energy is used to split water molecules (photolysis) Results in the release of oxygen (O2) as a waste product. Allows for the temporary transfer of the solar energy to electrons released by the broken bonds Electrons are passed down an Electron Transport Chain Photo- light…..lysis-split a water molecule is split (photolysis) into protons (H+), electrons, & oxygen. Electrons sent back to the chlorophyll, H+ left inside thylakoid, & the O2 diffuses out of the chloroplast & leaves the plant through stomata Photolysis 

Light Dependent Reactions The Electron Transport Chain is a series of proteins in the thylakoid membrane. As the electrons are transferred from one protein to another, some energy is released. This energy is used to make ATP & NADPH (electron carrier molecule) As the electrons are transferred from one protein to another, some energy is released which is used to… Join H+ and NADP+ (& 2e-) forming NADPH (electron carrier) Pump hydrogen ions into the center of the thylakoid disk to join ADP and an inorganic phosphate to form ATP (via ATP Synthase) ATP and NADPH will be used during the Calvin Cycle

Light Dependent Reactions

Light-dependent Reactions Solar Energy absorbed by chloroplasts Oxygen released ATP Released Products of Light Reactions (ATP and NADPH) fuel the Calvin Cycle NADPH released

Video https://www.youtube.com/watch?v=joZ1EsA5_NY (3:30) https://www.youtube.com/watch?v=sQK3Yr4Sc_k (7:35-8:20, end of light-dependent)

Calvin Cycle A series of enzyme-assisted chemical reactions that produces 3-carbon sugars Occurs in the stroma of the chloroplast Uses the ATP & NADPH that was produced during the light reactions and CO2 from the atmosphere

Calvin Cycle Carbon Fixation: An enzyme combines CO2 with a 5-carbon molecule called ribulose biphosphate (RuBP). 6-carbon molecule is very unstable, so it immediately splits into two 3-carbon molecules called 3-phosphoglycerate (3-PGA or PGA) Reduction Each 3-PGA is converted into another 3-carbon molecule, PGAL (aka G3P) in 2 steps. 1- receive a phosphate group from ATP; 2- receive a proton from NADPH ADP & NADP+ will be used again in light reaction PGAL leaves & is used to build glucose (or other carbohydrates). Regeneration Remaining PGAL molecules converted back to RuBP by the addition of a phosphate group from ATP. Enter Calvin Cycle again. Students are not responsible for this information. All they need to understand is that the Calvin Cycle uses the products of the light reaction to produce 3-carbon sugars that will be recycled to keep the cycle going or will be used to produce glucose, other carbs, or other organic molecules

The Calvin Cycle Uses Carbon Dioxide from the air Uses ATP and NADPH from light reactions to build PGAL- a 3-carbon sugar Uses another ATP to replenish RuBP Or the 3-carbon sugars can be recycled to keep the Calvin Cycle going. PGAL= G3P- glyceraldhyde 3-phosphate The 3-carbon sugars can be used to build a glucose molecule

Photosynthesis Equation Light Energy 6CO2 + 6H2O C6H12O6 + 6O2 chlorophyll Carbon Dioxide from the air – Used in the Calvin Cycle Glucose Made in the Calvin Cycle Oxygen Released during Photolysis in the Light Reactions Water Split during Photolysis in the Light Reactions Plants can use this glucose molecule for energy during Cellular Respiration. Plants can also convert this glucose molecule into other organic compounds such as proteins and fats/lipids or other carbohydrates like starch and cellulose

Cycle of Photosynthesis

3 Factors that Affect Photosynthesis Carbon Dioxide (CO2) Without CO2, the plant would not have one of the raw materials needed for photosynthesis CO2 is used in the first step of the Calvin Cycle Temperature The temperature must be in the appropriate range for the plant in order for photosynthesis to properly occur

3 Factors that Affect Photosynthesis Intensity of Light If the intensity of light is lower, the available energy for photosynthesis is lower. In a greenhouse, if the light source is further away, intensity is lower and less photosynthesis can occur If light is not available at all, the light-dependent reactions cannot occur (nor can they provide the materials used in the Calvin Cycle)

Video https://www.youtube.com/watch?v=joZ1EsA5_NY (3:30) https://www.youtube.com/watch?v=sQK3Yr4Sc_k (8:20, end of light-dependent)