Chapter 6: Photosynthesis

6-1: Capturing the Energy in Light

Energy for Life Processes Photosynthesis – Series of chemical reactions in which the product of one reaction is consumed in the next reaction.

Energy for Life Processes Photosynthesis is then labeled a biochemical pathway.

Photosynthesis Cellular Respiration Autotrophs Autotrophs and Heterotrophs Organic Compounds + Oxygen Carbon Dioxide + water

Energy for Life Processes Plants use photosynthesis to manufacture organic compounds from CO 2 and water.

Energy for Life Processes Another product of photosynthesis is O 2.

Energy for Life Processes Both Autotrophs and Heterotrophs perform cellular respiration.

Energy for Life Processes During cellular respiration, organic compounds are combined with O 2 to produce ATP.

Energy for Life Processes CO 2 and water are also released.

Light Absorption in Chloroplasts Light Reactions – The initial reactions in photosynthesis that begin with light absorption.

Light Absorption in Chloroplasts Chloroplasts – Light gathering organelle contained in autotrpohs.

Light Absorption in Chloroplasts White light from the sun contains an array of colors ranging from red to violet.

InfraredUltraviolet

Light Absorption in Chloroplasts When white light strikes an object, its component colors can be reflected, transmitted, or absorbed.

Light Absorption in Chloroplasts If the object contains a pigment, the various colors will react differently.

Light Absorption in Chloroplasts Pigment – A compound that absorbs light.

Chloroplast Pigments Chlorophyll – Light absorbing pigment found in the membrane of thylakoids.

Chloroplast Pigments Chlorophyll a absorbs red light Chlorophyll b absorbs blue light

Chloroplast Pigments Chlorophyll b is not directly involved in the light reactions therefore is considered an accessory pigment.

Electron Transport Photosystem I and Photosystem II play different roles in the light reactions.

Electron Transport The light reactions begin with accessory pigments absorbing light.

Electron Transport The acquired energy is passed until it reaches chlorophyll a molecules.

Electron Transport The events that follow can be divided into 5 steps:

Electron Transport Step 1: Light energy excites electrons in chlorophyll a molecules of photosystem II.

Electron Transport Step 2: The electrons move to a primary electron acceptor.

Electron Transport Step 3: The electrons are transferred along the electron transport chain.

Electron Transport Step 4: Light excites electrons in photosystem I.

Electron Transport Step 4: As these electrons move to an acceptor, they are replaced by electrons from photosystem II.

Electron Transport Step 5: The electrons from photosystem I go through a second chain to combine with NADP + to make NADPH.

Electron Transport The electrons in photosystem II are replaced by the splitting of water molecules.

Chemiosmosis The synthesis of ATP through the use of energy from the excited electrons in photosystem II.

Chemiosmosis This ATP is then used with NADPH to provide energy for the second set of reactions in photosynthesis.

6-2: The Calvin Cycle

Carbon Fixation In the Calvin cycle, carbon atoms from CO 2 are bonded or “fixed” into organic compounds.

Carbon Fixation There are 3 steps that occur in the stroma of the chloroplasts.

Calvin Cycle Step1: CO 2 diffuses into the stroma and combines with a 5 carbon molecule called RuBP.

Calvin Cycle Step 1: This makes a 6 carbon molecule that immediately splits into 2 3-carbon PGA molecules.

Calvin Cycle Step 2: 2 ATP and 2 NADPH molecules combine with PGA to form 2 PGAL molecules.

Calvin Cycle Step 3: Most of the PGAL is converted back into RuBP.

Calvin Cycle Step 3: To do this, more ATP is needed.

Calvin Cycle Step 3: Some of the PGAL leaves the cycle and is used to make other organic compounds

RuBP 2 Molecules of PGA 2 Molecules of PGAL CCCCC C CO 2 Step 1 CCCCCC 2 ATP 2 ADP 2 NADPH 2 NADP + CCCCCC -PP- -P Step 2 Step 3 2 Phosphate ATP ADP Organic Compounds

Balance Sheet for Photosynthesis CO 2 + H 2 O + light energy  (CH 2 O) + O 2 6CO 2 + 6H 2 O + light energy  C 6 H 12 O 6 + 6O 2

Alternate Pathways C 4 Pathway – Plants like corn, sugar cane, and crab grass use a 4 carbon molecule to save water.

Alternate Pathways CAM pathway – Cactuses, pineapples, and others open stomatas at night to conserve water.