Photosynthesis – Process by which some organisms capture light energy and store it in organic compounds (mainly carbohydrates, sugars) Autotrophs – make their own food through photosynthesis. They are at the start of every food chain. Exs.) plants, algae, some bacteria/protozoans. Photosynthesis = “Biochemical Pathway” = a complex series of chemical reactions in which the product of 1 reaction is consumed in the next reaction. It uses CO₂ and water to make organic compounds + oxygen. Energy is stored!!!

Cellular Respiration – uses organic compounds and oxygen from photosynthesis and releases CO₂ and water. Energy is released!! See figure 6-1 on page 112. Light Reactions – initial reactions in photosynthesis (referred to as P.S. from this point forward). Begins with the absorption of light in chloroplasts. See figure 6-2 on pg Thylakoids – flattened sacs inside the chloroplast…the light reactions take place here. Granum (Grana – Plural) – stacks of thylakoids. Stroma – solution that surrounds the thylakoids…more later. Visible Spectrum of Light – from white light yields an array of colors….each color represents a different wavelength of light. See figures 6-3 and 6-4 on pg. 113.

Pigments – compounds that absorb light. Light that is reflected is the color that the pigment appears to the eye. Chlorophyll a and b – green pigments located in the membrane of the thylakoid.  Chlorophyll a - directly involved in the light reactions of P.S.  Chlorophyll b – “accessory pigment” – assists chlorophyll a.  Carotenoids – other accessory pigments – yellow, orange, red, brown. Red is the best light for P.S. See fig. 6-4, pg Photosystem – clusters of pigments in the thylakoid membrane. 2 types – photosystem I and photosystem II…..each has a different role in the light reactions. See pg. 114.

Electron Transport Electron Transport – (light reactions of P.S.) – begin when accessory pigment molecules absorb light in both photosystems. Energy from the light is quickly passed to other molecules until it reaches a specific pair of chlorophyll a molecules. The rest of the events are divided into 5 steps and are outlined on the next slide. Refer to pages 114 and 115.

Electron Transport Continued… 1. Light energy forces electrons (e-’s) to a higher energy level (“excited”) in the 2 special chlorophyll a molecules. 2. Excited electrons leave the chlorophyll a molecules. They are picked up by a molecule in the thylakoid membrane known as the Primary Electron Acceptor. 3. Electrons then enter the Electron Transport Chain = a series of molecules that move electrons through the thylakoid membrane. As the electrons flow, they lose their energy. This energy is used to pump protons (H+) into the thylakoid. Continued on next slide…..

4. At the same time, light is also being absorbed by photosystem I…..e-’s become excited begin to flow through their own e- transport chain. NOTE – e-’s lost by photosystem I are replaced by the ones flowing from photosystem II. 5. e-’s from photosystem I flow to outside of thylakoid membrane. e-’s combine with a proton (H+) and NADP+ (Nicotinamide Dinucleotide Phosphate) to yield NADPH. NOTE – Special chlorophyll a’s, from both photosystems, lose electrons, this is called OXIDATION. NADP+ accepts the e-’s, this is called REDUCTION. The whole process is known as a REDOX REACTION. NOTE #2 – as photosystem II loses e-’s, they are resupplied by the splitting of water molecules ( 2 H₂O → 4H+ + 4e- + Oxygen). This is the oxygen that we breathe! See fig. 6-6, pg Here ends the info about the Electron Transport System!

Chemiosmosis – (also part of the Light Reactions) – some protons are produced by the splitting of water; others are pumped from stroma to the inside of thylakoid. This sets up a “concentration gradient” (High concentration of H+ inside thylakoid; Low concentration of H+ outside thylakoid). This represents Potential Energy like that found in a car battery.

In Chemiosmosis: ATP Synthetase harnesses the potential energy. H+’s flow through ATP Synthetase from high to low concentration (inside thylakoid to outside). As they flow they produce energy. Energy is used to tack a phosphate onto ADP forming ATP. See fig. 6-7, pg. 116.

Summary of Light Reactions As light causes e-’s to flow from photosystems I and II, NADPH and ATP are produced. Both molecules have had energy stored in them. This energy will be used for the 2 nd set of reactions (Calvin Cycle) in Photosynthesis. Light Energy → Light Reactions → NADPH + ATP →Calvin Cycle.

Calvin Cycle Calvin Cycle – (Melvin Calvin ) – occurs in the stroma. Carbon atoms from CO₂ are bonded or “fixed” into organic compounds. This is known as CARBON FIXATION. ** The Calvin Cycle has 3 major steps. Refer to fig. 6-8, pg. 117 as you study the steps.

3 Steps of the Calvin Cycle 1. An enzyme combines CO₂ with a 5-carbon molecule called RuBP (Ribulose Bisphosphate) making a 6-carbon molecule that splits into two 3-carbon PGA molecules (phosphoglycerate). 2. Each PGA receives a phosphate group from ATP. Then NADPH donates a proton and releases a phosphate group. The resulting molecules are known as PGAL (phosphoglyceraldehyde). 3. It takes 3 turns of the Calvin Cycle. This yields 6 PGAL (5 are used to regenerate RuBP to keep the cycle going. The one PGAL that remains is built up into organic molecules that FEED THE WORLD!!! NOTE – 3 turns of Calvin Cycle uses 9 ATP and 6 NADPH from the light reactions and yields organic molecules (carbohydrates) THAT FEED THE WORLD!!!

Simple Equation for Photosynthesis  CO₂ + H₂O + Light Energy → Glucose + Oxygen

Factors that Effect the Rate of Photosynthesis 1. Light Intensity – P.S. goes faster (to a point) with more intense light. 2. CO₂ Level – P.S. goes faster (to a point) with more CO₂. 3. Temperature – P.S. goes faster (to a point) with higher temperatures.