PHOTOSYNTHESIS OVERVIEW & 10.1

OVERVIEW The Process That Feeds the Biosphere Life on Earth is solar powered. Photosynthesis: conversion of light energy to chemical energy. Nourishes all life Organisms acquire organic compounds and carbon skeletons by one of 2 methods: autotrophic or heterotrophic nutrition.

Autotrophic Nutrition Autotrophs: “self-feeders” that sustain themselves without eating anything derived from other organisms. Known as producers, because they are a source of organic compounds for other nonautotrophic organisms. Almost all plants are autotrophs. More specifically, plants are considered photoautotrophs which are organisms that use light as a source of energy to synthesize organic substances.

Heterotrophic Nutrition Heterotrophs: organisms that are unable to make their own food, so they obtain organic material by living on compounds other organisms produce. Known as consumers, due to the fact that they consume other organisms. Animals eat plants or other molecules. Heterotrophs are completely dependent on photoautotrophs for food and oxygen.

10.1 Photosynthesis converts light energy to the chemical energy of food Quick outline: I. Chloroplasts: The Sites of Photosynthesis in Plants II. Tracking Atoms Through Photosynthesis: Scientific Inquiry a. The Splitting of Water b. Photosynthesis as a Redox Process III. The Two Stages of Photosynthesis: A Preview

I. Chloroplasts: The Sites of Photosynthesis in Plants Chloroplast: An organelle found only in plants. Found mainly in the cells of the mesophyll. Mesophyll: tissue in the interior leaf (30-40 chloroplasts in each cell) Leaves are the major sites of photosynthesis and the color of the leaves come from chlorophyll. Chlorophyll: green pigment located in the thylakoid membranes within the chloroplast.

I. Chloroplasts: The Sites of Photosynthesis in Plants Stomata: microscopic pores where carbon dioxide enters and oxygen leaves the leaf. Stroma: fluid of the chloroplast surrounding the thylakoid membrane; involved in the synthesis of organic molecules from carbon dioxide and water`. Thylakoids: flattened membrane sacs inside the chloroplast, used to convert light energy to chemical energy. Thylakoid space: interior of thylakoids. Grana: thylakoid sacs stacked in columns.

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I. Chloroplasts: The Sites of Photosynthesis in Plants Main Point: In autotrophic eukaryotes, photosynthesis occurs in chloroplasts, organelles containing thylakoids. Stack of thylakoids form grana.

II. Tracking Atoms Through Photosynthesis: Scientific Inquiry Overall photosynthetic equation known since 1800s: In the presence of light, the green parts of plants produce organic compounds and oxygen from carbon dioxide and water.

II. Tracking Atoms Through Photosynthesis: Scientific Inquiry Molecular formula: 6 CO H 2 O + Light energy  C 6 H 12 O O H 2 O Simplified by indicating the net consumption of water 6 CO H 2 O + Light energy  C 6 H 12 O O 2 Simplest possible form CO 2 + H 2 O  [CH 2 O] + O 2 NOTE: C 6 H 12 O 6 represents glucose, but the actual product is a three-carbon sugar. Glucose is used here to simplify. NOTE: [ ] indicate that CH 2 O is not an actual sugar but represents the general formula for a carbohydrate.

a. The Splitting of Water The chloroplast splits water into hydrogen and oxygen. Before scientists discovered that it was oxygen given off by plants rather than carbon dioxide, they predicted photosynthesis split carbon dioxide. The hypothesis predicted that the O 2 released in the synthesis came from CO 2. CO 2  C + O 2 and then added water to the carbon CO 2  [CH 2 O]

a. The Splitting of Water The previous idea was challenged by C. B. van Niel of Stanford University in the 1930s. By using sulfur bacteria, his findings provided evidence that plants split water as a source of electrons from hydrogen atoms, releasing oxygen as a by- product. Sulfur bacteria: CO H 2 S  [CH 2 O] H 2 O + 2S Plants: CO H 2 O  [CH 2 O] H 2 O + O 2 General: CO H 2 X  [CH 2 O] H 2 O + 2X

a. The Splitting of Water 20 years later, van Niel’s work was solidified by using oxygen- 18 as a radioactive tracer to follow the fate of oxygen atoms during photosynthesis. A significant result of shuffling of atoms during photosynthesis is the extraction of hydrogen from water and its incorporation into sugar.

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b. Photosynthesis as a Redox Process Cellular Respiration Involve redox reactions. Energy is released from sugar when electrons associated with hydrogen are transported by carriers to oxygen, forming water as a by-product. Electrons lose potential as they “fall” down the electron transport chain. Photosynthesis Involve redox reactions. Reverses the direction of the electron flow. Water is split reducing it to sugar. Electrons increase in potential caused by an energy boost from light. Recall Redox Reaction: chemical reaction involving the transfer of one or more electrons from one reactant to another; oxidation-reduction reaction.

II. Tracking Atoms Through Photosynthesis: Scientific Inquiry Main Point: Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules. Photosynthesis is a redox process: H 2 O is oxidized and CO 2 is reduced.

III. The Two Stages of Photosynthesis: A Preview Really 2 processes 1.Light reactions: The steps in photosynthesis that occur on the thylakoid membranes of the chloroplast and convert solar energy to the chemical energy of ATP and NADPH, evolving oxygen in the process. 2.Calvin Cycle: Second of the 2 major stages in photosynthesis, involving atmospheric CO 2 fixation and reduction of the fixed carbon into carbohydrate.

III. The Two Stages of Photosynthesis: A Preview 1.Light reactions The photo part of photosynthesis. Light drives a transfer of electrons and hydrogen from water to an acceptor called NADP +, which temporarily stores the energy. Water is split and O 2 is given off. NADPH is formed. ATP is generated. Photophosphorylation: process of generating ATP using chemiosmosis to power the addition of a phosphate group to ADP.

III. The Two Stages of Photosynthesis: A Preview 2. Calvin Cycle Named after Melvin Calvin. This is where sugar is produced with the help of NADPH and ATP. Carbon fixation occurs; this is the incorporation of carbon from CO 2 into an organic compound by an autotrophic organism. Sometimes referred to as the “dark-reactions” or “light- independent reactions.”

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III. The Two Stages of Photosynthesis: A Preview Main Point: The light reactions in the grana split water, releasing O 2, producing ATP, and forming NADPH. The Calvin Cycle in the stroma, forms sugar from CO 2, using ATP for energy and NADPH for reducing power.

Optional Video

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