Chapter 6 PHOTOSYNTHESIS

Section 1 THE LIGHT REACTION

Objectives: Explain why almost all organisms depend on photosynthesis. Describe the role of chlorophylls and other pigments in photosynthesis. Summarize the main events of the light reactions. Explain how ATP is made during the light reactions.

Obtaining Energy Organisms are classified according to how they get energy: Autotrophs: are organisms that produce complex organic compounds from simple inorganic molecules using energy from light (by photosynthesis). Heterotrophs: animals that get energy from food.

Comparing autotrophs and heterotrophs ..\..\Ch06\60014.html

Obtaining energy Photosynthesis involves a complex series of chemical reactions in which the product of one reaction is consumed in the next reaction, which is referred to as biochemical pathway.

Obtaining Energy Photosynthesis converts light energy from the sun into chemical energy in the form of organic compounds through a series of reactions known as biochemical pathways. 6CO2 + 6H2O light energy C6H12O6 + 6O2

Overview of Photosynthesis Autotrophs use photosynthesis to produce organic compounds from carbon dioxide and water.

Overview of Photosynthesis The oxygen (O2) and some of the organic compounds produced by photosynthesis are used by cells in a process called cellular respiration.

Overview of Photosynthesis, continued CELLULAR RESPIRATION The process by which cells obtain energy from carbohydrates; oxygen combines with glucose to form water and carbon dioxide. It occurs in the mitochondria.

Linking Photosynthesis and Respiration ..\..\Ch06\60375.html

Overview of Photosynthesis, continued Photosynthesis can be divided into two stages: Light Reactions Calvin Cycle

Overview of Photosynthesis, continued In the light reactions, light energy is converted to chemical energy, which is temporarily stored in ATP and the energy carrier molecule NADPH. In the Calvin Cycle, organic compounds are formed using CO2 and the chemical energy stored in ATP and NADPH.

Capturing Light Energy The light reactions begin with the absorption of light in chloroplasts, organelles found in the cells of plants, some bacteria, and algae.

Parts of a Chloroplast -Composed of membranous sacs called: -Thylakoids: contain the green pigment chlorophyll. -Thylakoids are connected and layered to form stacks called Grana. -surrounding the Grana is a solution called the stroma -They have their own DNA.

Chloroplast

Parts of a Chloroplast ..\..\Ch06\60091.html

Parts of a Chloroplast

Capturing Light Energy, continued Light and Pigments White light from the sun is composed of an array of colors called the visible spectrum. Visible spectrum ranges from red to violet. Pigments absorb certain colors of light and reflect or transmit the other colors. The light that is reflected or transmitted by the pigment no longer appears white.

Capturing Light Energy, continued Chloroplast Pigments Located in the membrane of the thylakoids of chloroplasts are several pigments, including chlorophylls (such as chlorophyll a and chlorophyll b) and carotenoids.

Capturing Light Energy, continued Only Chlorophyll a is the directly involved in the light reactions of photosynthesis. Chlorophyll b assists chlorophyll a in capturing light energy; it’s called an accessory pigment.

Spectrum of Light and Plant Pigments Visual concept 60092

Converting Light Energy To Chemical Energy The pigments are grouped in clusters of a few hundred molecules in the thylakoid membrane. Each cluster and the proteins that the pigment molecules are embedded in are referred to collectively as a photosystem. By absorbing light, pigment molecules in photosystem I and photosystem II acquire some of the energy carried by the light.

Converting Light Energy To Chemical Energy Once the pigments in the chloroplast have captured light energy, the light energy must be converted to chemical energy. This chemical energy is temporarily stored in ATP and NADPH.

Converting Light Energy To Chemical Energy, continued The light reactions begin when accessory pigment molecules in both photosystems absorb light. By absorbing light, these molecules acquire some of the energy carried by the light.

Converting Light Energy To Chemical Energy, continued In each photosystem, the acquired energy is passed quickly to the other pigment molecules until it reaches a specific pair of chlorophyll molecules a molecules. Chlorophyll a can also absorb light.

Converting Light Energy To Chemical Energy, continued The acquired energy forces electrons to enter a higher energy level in the two chlorophyll a molecules of photosystem II. These energized electrons are said to be “excited.” The excited electrons have enough energy to leave the chlorophyll a molecules.

Converting Light Energy To Chemical Energy, continued The acceptor of these electrons from photosystem II is a molecule called the primary electron acceptor, which donates the electrons to the electron transport chain. As the electrons move from molecule to molecule in this chain, they lose most of the acquired energy. The energy they lose is used to move protons into the thylakoid.

Light Reactions in Photosynthesis

Converting Light Energy To Chemical Energy, continued Light is absorbed by photosystem I at the same time it is absorbed by photosystem II. Electrons move from chlorophyll a molecules to another primary electron acceptor. The electrons lost from photosystem I are replaced by electrons that have passed through the electron transport chain from photosystem II.

Converting Light Energy To Chemical Energy, continued These electrons are then donated to another electron transport chain, which brings the electrons to the side of the thylakoid membrane that faces the stroma. In the stroma, the electrons combine with a proton and NADP+. This causes NADP+ to be reduced to NADPH.

Converting Light Energy To Chemical Energy, continued Replacing Electrons in Light Reactions Electrons from photosystem II replace electrons that leave photosystem I. Replacement electrons for photosystem II are provided by the splitting of water molecules. Oxygen produced when water molecules are split diffuses out of the chloroplast and then leaves the plant.

Converting Light Energy To Chemical Energy, continued Making ATP in Light Reactions An important part of the light reactions is the synthesis of ATP. During chemiosmosis, the movement of protons through ATP synthase into the stroma releases energy, which is used to produce ATP.

Summary of Processes in Light Reactions

Section 2 The Calvin Cycle Objectives: Summarize the main events of the Calvin cycle. Describe what happens to the compounds that are made in the Calvin cycle. Distinguish between C3, C4, and CAM plants. Summarize how the light reactions and the Calvin cycle work together to create the continuous cycle of photosynthesis. Explain how environmental factors influence photosynthesis.

The Calvin Cycle In this phase, the plants use the energy that was stored in ATP & NADPH during the light reactions to produce organic compounds.

Carbon Fixation In the Calvin cycle, CO2 is incorporated into organic compounds, a process called carbon fixation.

3-phosphoglycerate (3-PGA) Glyceraldehyde 3-phosphate (G3P) The Calvin Cycle CO2 3-phosphoglycerate (3-PGA) Glyceraldehyde 3-phosphate (G3P) Ribulose biphosphate ATP NADPH

The Calvin Cycle

3 CO2 + 6 NADPH + 5 H2O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi

The Calvin Cycle The Calvin cycle, which occurs in the stroma of the chloroplast, is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar. Most of the three-carbon sugars (G3P) generated in the Calvin cycle are converted to a five-carbon sugar (RuBP) to keep the Calvin cycle operating. But some of the three-carbon sugars leave the Calvin cycle and are used to make organic compounds, in which energy is stored for later use.

Photosynthesis