Chapter 8: Photosynthesis 1

Energy and Life Energy – the ability to do work No energy = no life Thermodynamics is the study of the flow and transformation of energy in the universe. Laws of Thermodynamics First law — energy can be converted from one form to another, but it cannot be created nor destroyed. Second law — energy cannot be converted without the loss of usable energy (thermal energy/heat). Example – food chains. 2

Autotrophs and Heterotrophs 1. Autotrophs – organisms that make their own food from energy from the sun or other sources - Known as producers 2. Heterotroph – organisms that need to ingest food to obtain energy. - Known as consumers 3. All organisms have to release the energy in sugars and other compounds to live. 3 Energy and Life

4 Metabolism - All of the chemical reactions in a cell. Photosynthesis —light energy from the Sun is converted to chemical energy for use by the cell. Cellular respiration —organic molecules are broken down to release energy for use by the cell

Chemical Energy and ATP ATP – Adenosine triphosphate – energy for the cell Made of adenine, ribose, and three phosphates. ATP releases energy when the bond between the second and third phosphate groups is broken, forming a molecule called adenosine diphosphate (ADP) and a free phosphate group. 5 Energy and Life

Chemical Energy and ATP Storing Energy – When bonds are formed, energy is stored. a. ADP – Adenosine diphosphate is similar to ATP, but with two phosphates instead of three. b. Energy is stored when a phosphate is added to ADP ADPATP Energy Adenosine triphosphate (ATP) Partially charged battery Fully charged battery Adenosine diphosphate (ADP) + Phosphate 6 Energy and Life

Releasing Energy a. Energy is released when bonds are broken. b.When a phosphate is removed from ATP, energy is released c. As many as two phosphates can be removed from ATP. remove one phosphate = ADP (adenosine diphosphate) remove two phosphates = AMP (adenosine monophosphate) 7 Energy and Life

Using Biochemical Energy 1.Cells use ATP for active transport, to move organelles in the cell, and to synthesize proteins and nucleic acids 2.Cells do not keep large amounts of ATP in the cell. The cell can regenerate ATP from glucose, as needed. 3. ATP is great for transferring energy, but not for storing it. 8 Energy and Life

Photosynthesis: An Overview 9 Photosynthesis – the process by which plants use sunlight to convert water and carbon dioxide into sugar and starches A. Investigating Photosynthesis 1. Van Helmont’s Experiment – wanted to know if plants grow from taking material out of the soil. - Concluded that trees gain most of their mass from water. Describe the experiment

2.Priestley’s Experiment – finds that plants release a substance that keeps a candle burning - oxygen 10 Describe the experiment

11 3. Ingenhousz’s Experiment– concludes that plants need sunlight to produce oxygen. Describe the experiment

These early investigations and the work of other scientists led to the discovery that in the presence of light, plants transform carbon dioxide and water into carbohydrates and release oxygen in the process. The Photosynthesis Equation 6 CO H 2 O ----> C 6 H 12 O O 2 carbon dioxide + water  sugar and oxygen 12 Photosynthesis: An Overview

Light Absorption and pigments 1. Photosynthesis requires light - mixture of wavelengths = ROY G BIV 2. Pigments – light absorbing molecules in the chloroplast that are organized into photosystems a. chlorophyll – principal pigment that absorbs light in the blue-violet and red regions of the visible spectrum and not the green region 1). chlorophyll a 2). chlorophyll b 13 Photosynthesis

Light Absorption and pigments b. carotenoids – accessory pigments such as carotene that absorbs other wavelengths of light = reflect orange light c. Energy absorbed by the chlorophyll molecules is transferred directly to the electrons in the chlorophyll raising their energy levels. d. It is these high energy electrons that make photosynthesis work. 14 Photosynthesis

15 Reactions of Photosynthesis Inside a Chloroplast 1. Photosynthesis takes place inside the chloroplast. a. thylakoid – saclike photosynthetic membranes where chlorophyll and other pigments are found  Site of light dependent reactions - photosystems – light collecting units in the thylakoid membrane b. granum – a stack of thylakoids c. stroma – space outside the thylakoid membrane  Site of Calvin cycle or Light Independent Reactions

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Reactions of Photosynthesis 17 Photosynthesis occurs in 2 phases: 1.Light-dependent reactions – in the thylakoid 2. Light-independent reactions – in the stroma

The light dependent reactions -The absorption of light is the first step in photosynthesis. -Chloroplasts capture light energy in the thylakoids. - These reactions produce oxygen gas and convert NADP+ and ADP into NADPH and ATP. 18 Reactions of Photosynthesis

19 Reactions of Photosynthesis The light dependent reactions 1. Light energy excites electrons in photosystem II and also causes a water molecule to split. - Releasing an electron into the electron transport system, H + into the thylakoid space, and O 2 as a waste product. 2. The excited electrons move from photosystem II 3. To an electron-acceptor molecule in the thylakoid membrane.

20 Reactions of Photosynthesis The light dependent reactions 4. The electron-acceptor molecule transfers the electrons along a series of electron-carriers to photosystem I. - This process is called electron transport and the carrier molecules are known as the electron transport chain or ETC. - As the electrons move down the chain they lose energy. This energy is used to transport protons (H+) from the stroma into the thylakoid space. 5. Photosystem I transfers the electrons 6. To a protein called ferrodoxin.

21 Reactions of Photosynthesis The light dependent reactions 7. Ferrodoxin transfers the electrons to the electron carrier NADP + 8. Forming the energy-storing molecule NADPH. Light strikes the electrons in PS I causing them to become high energy electrons that move down a second ETC until they reach NADP+ and form NADPH NADP+ (nicotinamide adenine dinucleotide phosphate) is an electron carrier that is converted into NADPH when it accepts its electrons and a proton(H+).

22 Reactions of Photosynthesis The light dependent reactions NADPH WILL THEN GET USED IN THE CALVIN CYCLE This accumulation of H+ in the thylakoid space causes a difference in charge across the membrane. It is this difference that provides the energy needed to make ATP.

The light dependent reactions ATP synthase is a protein that moves the H+ ions back to the stroma from the thylakoid space and uses their energy to convert ADP into ATP. Hydrogen Ion Movement Photosystem II Inner Thylakoid Space Thylakoid Membrane Stroma ATP synthase Electron Transport Chain Photosystem IATP Formation 23 Reactions of Photosynthesis

The light dependent reactions The light-dependent reactions produce two high energy compounds: ATP and NADPH that will provide the energy needed for the Calvin cycle. 24 Reactions of Photosynthesis

25 Reactions of Photosynthesis The light-independent reactions or the Calvin Cycle In the second phase of photosynthesis, called the Calvin cycle, energy is stored in organic molecules such as glucose.

The Calvin Cycle or Light-Independent Reactions 1. The Calvin Cycle uses ATP and NADPH from the light reactions to produce high-energy sugars. 2.These reactions do not require light and occur in the stroma and again consist of a series of steps summarized in figure The Calvin cycle uses six molecules of carbon dioxide to produce a single 6-carbon sugar molecule 26

Calvin Cycle STEP 1  Phosphoglycerate (3-phosphoglycerate or 3PG) is the first compound formed by the addition of CO2 to a 5-carbon acceptor (Rubisco). The resulting 6- carbon compound is broken into two molecules.  CO2 + 5-carbon acceptor → [6-carbon intermediate] → two phosphoglycerate.

 In the next step a phosphate group from ATP is added to each molecule of phosphoglycerate (3PG) to form an intermediate called 1,3-diphosphoglycerate. STEP 2  In the next step a pair of electrons donated by NADPH reduces 1,3-diphosphoglycerate to form glyceraldehyde phosphate.  Glyceraldehyde phosphate is a 3-carbon sugar, not the 6-carbon sugar glucose generally identified as the end product of photosynthesis. 28 Calvin Cycle

29 Calvin Cycle STEP 3 Glyceraldehyde phosphate leaves the Calvin cycle and is converted to glucose and stored as starch in the stroma of the chloroplast or used in other reactions in the cytoplasm to make sucrose for transporting to other parts of the plant. The regeneration of the initial 5-carbon acceptor in the Calvin cycle avoids wasteful reactions that use large amounts of ATP and NADPH and allows continuous CO2 fixation. In order to regenerate the 5-carbon acceptor the cycle runs three times

 The ATP and NADPH come from the light dependent reactions, the inorganic phosphate (Pi), the adenosine diphosphate (ADP) and nicotinimide adenosine diphosphate are recycled to the light reactions 30 Calvin Cycle

6 CO 2 12 ATP 12 NADPH Carbon dioxide enters the Calvin Cycle Energy is used Two Three- Carbon molecules leave to make sugars or other stuff CCCCCC The 5 carbon molecule is regenerated- Energy is used. CCCCC 6 ATP 31

Factors Affecting Photosynthesis 1. Many factors affect the rate at which photosynthesis can occur. 2. These factors include: a. Water – can slow or stop b. Temperature – enzymes work best at 0 to 35.0 C, slow or stop c. Light intensity – reaches a plateau but varies with the plant d. CO 2 – reaches a plateau but again varies with the plant 32