Converting Light Energy into Chemical Energy Photosynthesis Converting Light Energy into Chemical Energy

How far down the rabbit hole must we go to look at photosynthesis?

Photosynthesis Purpose of photosynthesis- To use sunlight to convert water and carbon dioxide into high-energy carbohydrates and oxygen. B. Solar energy is converted into chemical energy through photosynthesis. Light Energy Sugar + O2 CO2 + H2O

Plants use water + carbon dioxide to make sugar Plants use sugars as material for building cells and energy to run cells Humans use sugars for: Food Shelter Energy Clothing Medicines

Location of Photosynthesis Occurs in the leaves of green plants Occurs in specific cells in the middle layer of the leaf Occurs in organelles – called chloroplasts – within the cells of the middle layer of the leaf Photosynthesis Formula: (6) CO2 + (6) H2O + light energy (1) C6H12O6 + (6) O2

In the chloroplasts of photosynthetic organisms, the energy of sunlight is transformed into the energy stored in chemical bonds of carbohydrates (glucose). CO2 H2O Light Chloroplast O2 Other materials Chlorophyll absorbs Calvin Cycle: Light- Dependent Reactions Light- Independent Reactions H+ ions ATP NADPH Sugar (Glucose) C6H12O6

F. Two Reactions in Photosynthesis Light-dependent reactions convert light energy and the splitting of water to chemical energy (ATP) and release H and oxygen. Light-independent reactions use the energy from the light-dependent reaction to convert CO2 and H to sugar

G. Light Dependent Reactions 2. Occurs in the thylakoid membrane 1. Also called light reactions or photochemical reactions

Chloroplast structure

3. Sunlight is absorbed by chlorophyll pigments in photosystems located in the thylakoid membranes inside the chloroplast. Water is split into 2e-, 2 H+ and 1 O2 at Photosystem II. 4. Electrons move through the ETC from PS II to PS I. Energy from these e- transport H+ from stroma to thylakoid space.

5. Pigments in Photosystem I use energy from light to convert NADP to NADPH. 6. ATP is produced from ADP using the energy in the difference between the sides of the membrane. 7. ATP Synthase is the enzyme that converts ADP to ATP.

light dependent reaction (click on the image to be redirected to the video)

Light Reaction (recap) 1. Light Absorption Light energy absorbed by pigments generates electrons that are transferred to chlorophyll a, the primary pigment in photosystems. Photosystems are bundles of pigments responsible for absorbing sun’s light energy.

Light Reaction (recap) 2. Electron Transport Electrons are transported along the ETC within the photosystems. Electrons (-) and hydrogen ions combine with NADP+ to produce NADPH, stored energy!

ATP, adenosine triphosphate, is energy usable by cells. A few words on ATP: ATP, adenosine triphosphate, is energy usable by cells. Energy is stored and released when phosphate bonds are formed and broken. 17

Light-Independent Reaction Also called dark reactions, Calvin cycle or photochemical reactions Three phase cycle that starts and finishes with a 5-carbon molecule The net reaction of the Calvin Cycle: ATP + NADPH + Carbon Dioxide  Glucose

Calvin Cycle (Light-Independent Reaction) C. Calvin Cycle basics: Takes place in the stroma of the chloroplast The cycle gets its energy to conduct the reaction via high-energy election from NADPH and ATP The NADPH and ATP are two of the products of the light-dependent reactions

Calvin Cycle (Light-Independent Reaction) D. Calvin Cycle- a step-by-step look: 6 CO2 molecules enter the cycle from the atmosphere. The CO2 molecules combine with 6 5-carbon molecules. Result: 12 3-carbon molecules. The 12 3-C molecules are then converted into higher-energy forms.

Calvin Cycle (Light-Independent Reaction) D. Calvin Cycle- a step-by-step look: 3. Two of the 12 3-carbon molecules are removed from the cycle to be used to make glucose and other compounds. 4. The remaining 10 3-C molecules are then converted back into 6 5-C molecules. These molecules then continue through the cycle and combine with six new CO2 to begin the next cycle

LIGHT INDEPENDENT REACTIONS (Click the picture for the link)

Dark Reactions – These take place in the STROMA A. 6 CO2 molecules enter the Calvin Cycle 6 CO2 combine with 6 5-RuBP to form 6 6C molecules that split into 12 3 C (PGAL) molecules. B. 12 PGA are converted into higher energy forms. The energy for this conversion is supplied by ATP and NADPH from the light reactions. 23

Dark Reactions. C. 2 of the 12 PGAL molecules are removed and used to produce one 6C glucose molecule. D. The remaining 10 3 C molecules are converted back into six 5 C RuBP molecules to begin the cycle again. 24

A better term is “light independent” reactions. Dark Reactions. The term “dark reaction” is confusing because these reactions can occur in the light or the dark. A better term is “light independent” reactions. 25

Dark Reactions. Role of CO2 - is used to combine with RuBP to make glucose. It comes from the atmosphere. Role of RUBISCO – it is the enzyme that is used in the reaction that makes glucose. It is the most abundant protein in the world! Role of ATP and NADPH – these molecules provide the energy for these reactions. 26