HW: Complete Review questions for Section 5-1 & 5-2. Due on Friday. Photosynthesis HW: Complete Review questions for Section 5-1 & 5-2. Due on Friday.

There are two stages of Photosynthesis: Light Dependent Reactions -- Requires light Light Independent Reactions – does not require light.

Photosynthesis Photosynthesis is the process by which green plants use the energy of sunlight to produce carbohydrates. Reworded definition:

The Equation Photosynthesis can be represented by the following equation: 6H20 + 6CO2 + sunlight -> 6O2 + C6H12O6 This equation shows you what happens in photosynthesis, but not how. What is the point of doing this? What do plants get from it?

Sunlight As we mentioned yesterday, sunlight is a necessary energy source. It’s energy is used to warm the Earth. It’s energy is used to drive photosynthesis. Photosynthesis traps the energy from the sun in sugars so that heterotrophs can access the energy as well. Sunlight contains many different wavelengths. We will learn about these wavelengths and why it’s important during tomorrow’s lab.

How do they do it? How do plants carry out photosynthesis? They are three steps to the process: Step 1: Energy is captured by sunlight. Step 2: Light energy is converted to chemical energy (ATP & NADPH). Step 3: The chemical energy stored is used to make organic molecules (sugars) using CO2.

Step 1: Absorbing Light The first set of reactions, where light is absorbed, are aptly named the light-dependent reactions. They need light (are dependent upon it) in order to take place. Plants have both carotenoids and chlorophyll that absorb light. They are both needed because they absorb different wavelenths of light. (We be learning about this in detail tomorrow.)

Absorbing Energy cont’d We know plants absorb energy with chlorophyll and carotenoids, but how?? The answer is: Thylakoids When light strikes thylakoids the light energy is transferred into chlorophyll. This energy is stored in electrons until it can be transferred to ATP & NADPH.

Absorbing Energy, cont’d These excited electrons that are in the chlorophyll “jump” back to the thylakoid membrane and are used to make ATP & NADPH. There’s a slight problem with this though… When they leave they need to be replaced or the chlorophyll would run out of electrons. How are they replaced? By electrons from water. This is how water is used in plants. It’s also how water gets split from H20 to H+ (replacement electrons) and 02.

Step 2: Conversion of Light Energy Remember those excited electrons?? This is where they come into play. They are passed through an electron transport chain (kind of like a line of hot potato). Why does this create ATP, you ask?

Conversion of Light Energy The ions get moved down the chain releasing energy as they move. This energy is used to pump H+ ions across a membrane (down their gradient.) through a protein pump. The H+ passing through the protein catalyzes the creation of ATP. NADPH (a high energy electron storing molecule) is produced as well.

Summarizing the Light Reactions The light dependent reactions can be summarized as follows: Pigment molecules in thylakoids of chloroplasts absorb light energy. Electrtons in the pigments are excited by light and move through the electron transport chain in the thylakoid membrane.

Summarizing the Light Reactions The light dependent reactions summarized cont’d: These electrons are replaced by electrons from water molecules, which are split by an enzyme. Oxygen atoms from water molecules combine to form oxygen gas. Hydrogen ions accumulate inside thylkoids, setting up a concentration gradient that provides the energy to make ATP.

Step 3: Storage of Energy From the 1st and 2nd steps of photosynthesis, ATP & NADPH are produced. These steps require light energy to happen. They are light dependent. Converting the energy ATP & NADPH to sugars is not dependent upon light – they are light independent (aka dark reactions).

Storage of Energy In this step, carbon from CO2 is used build sugars. This allows the energy in ATP and NADPH to be stored for a longer time in the form of sugars. This is called “carbon dioxide fixation.” There are several different methods that plants use to fix carbon dioxide.

Storage of Energy The most common method of carbon dioxide fixation is called: The Calvin Cycle This reaction produces a three carbon sugar.