Photosynthesis Presented by: Mrs. Knopke FUHS Science Dept.

9.2 Section Objectives – page 225 Relate the structure of chloroplasts to the events in photosynthesis. Section Objectives: Describe light-dependent reactions. Explain the reactions and products of the light- independent Calvin cycle.

5 Types of Energy 1)Chemical Energy 2)Physical Energy 3)Heat Energy 4)Electrical Energy 5)Light Energy

These 5 types of energy can be in two major forms Potential Energy: The energy is stored Kinetic or Free Energy: The energy is released or converted into one of the 5 types of free energy

ATP ATP: Adenosine Tri-phosphate ADP: Adenosine Di-phosphate AMP: Adenosine Mono-phosphate

Two Types of Reactions Endergonic: Needs energy to take place Example: Exergonic: Releases heat, light etc. Example:

Section 9.2 Summary – pages Trapping Energy from Sunlight The process that uses the sun’s energy to make simple sugars is called photosynthesis.

Section 9.2 Summary – pages Photosynthesis happens in two phases. 2. The molecules of ATP produced in the light- dependent reactions are then used to fuel the light-independent reactions that produce simple sugars. The general equation for photosynthesis is written as 6CO 2 + 6H 2 O→C 6 H 12 O 6 + 6O 2 Trapping Energy from Sunlight 1. The Light –dependent reactions convert solar energy into chemical energy

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.4

Photosynthesis is a redox reaction. –It reverses the direction of electron flow in respiration. Water is split and electrons transferred with H + from water to CO 2, reducing it to sugar. –Polar covalent bonds (unequal sharing) are converted to nonpolar covalent bonds (equal sharing). –Light boosts the potential energy of electrons as they move from water to sugar. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.3

Section 9.2 Summary – pages The chloroplast and pigments To trap the energy in the sun’s light, the thylakoid membranes contain pigments, molecules that absorb specific wavelengths of sunlight. Although a photosystem contains several kinds of pigments, the most common is chlorophyll. Chlorophyll absorbs most wavelengths of light except green.

The entire range of electromagnetic radiation is the electromagnetic spectrum. The most important segment for life is a narrow band between 380 to 750 nm, visible light. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.5

Any green part of a plant has chloroplasts. However, the leaves are the major site of photosynthesis for most plants. –There are about half a million chloroplasts per square millimeter of leaf surface. The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts. –Chlorophyll plays an important role in the absorption of light energy during photosynthesis. 2. Chloroplasts are the sites of photosynthesis in plants Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf. O 2 exits and CO 2 enters the leaf through microscopic pores, stomata, in the leaf. Veins deliver water from the roots and carry off sugar from mesophyll cells to other plant areas. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.2

A typical mesophyll cell has chloroplasts, each about 2-4 microns by 4-7 microns long. Each chloroplast has two membranes around a central aqueous space, the stroma. In the stroma are membranous sacs, the thylakoids. –These have an internal aqueous space, the thylakoid lumen or thylakoid space. –Thylakoids may be stacked into columns called grana. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.2

When light meets matter, it may be reflected, transmitted, or absorbed. –Different pigments absorb photons of different wavelengths. –A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.6

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.9

A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light. –It beams narrow wavelengths of light through a solution containing a pigment and measures the fraction of light transmitted at each wavelength. –An absorption spectrum plots a pigment’s light absorption versus wavelength. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.7

The light reaction can perform work with those wavelengths of light that are absorbed. In the thylakoid are several pigments that differ in their absorption spectrum. –Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green. –Other pigments with different structures have different absorption spectra. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.8a

Collectively, these photosynthetic pigments determine an overall action spectrum for photosynthesis. –An action spectrum measures changes in some measure of photosynthetic activity (for example, O 2 release) as the wavelength is varied. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.8b

The action spectrum of photosynthesis was first demonstrated in 1883 through an elegant experiment by Thomas Engelmann. –In this experiment, different segments of a filamentous alga were exposed to different wavelengths of light. –Areas receiving wavelengths favorable to photosynthesis should produce excess O 2. –Engelmann used the abundance of aerobic bacteria clustered along the alga as a measure of O 2 production. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 10.8c

Photosynthesis takes place in the chloroplasts of a plant cell

Thylakoid Stroma Chloroplast: Membrane bound organelle where photosynthesis takes place.

Section 9.2 Summary – pages Light-Dependent Reactions As sunlight strikes the chlorophyll molecules in a photosystem of the thylakoid membrane, the energy in the light is transferred to electrons. These highly energized, or excited, electrons are passed from chlorophyll to an electron transport chain, a series of proteins embedded in the thylakoid membrane.

Excited electrons are unstable. Generally, they drop to their ground state in a billionth of a second, releasing heat energy. Some pigments, including chlorophyll, release a photon of light, in a process called fluorescence, as well as heat. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems. A photosystem acts like a light-gathering “antenna complex” consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

Section 9.2 Summary – pages This “lost” energy can be used to form ATP from ADP, or to pump hydrogen ions into the center of the thylakoid disc. Electrons are re-energized in a second photosystem and passed down a second electron transport chain. Light-Dependent Reactions

Section 9.2 Summary – pages The electrons are transferred to the stroma of the chloroplast. To do this, an electron carrier molecule called NADP is used. NADP can combine with two excited electrons and a hydrogen ion (H + ) to become NADPH. NADPH will play an important role in the light- independent reactions. Light-Dependent Reactions

Section 9.2 Summary – pages Restoring electrons To replace the lost electrons, molecules of water are split in the first photosystem. This reaction is called photolysis. Sun Chlorophyll 2e - H2OH2O O 2 + 2H + 1 _ 2 H 2 O     2 _ 1 O 2 + 2e -

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig The light reactions use the solar power of photons absorbed by both photosystem I and photosystem II to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH.

Section 9.2 Summary – pages The oxygen produced by photolysis is released into the air and supplies the oxygen we breathe. The electrons are returned to chlorophyll. The hydrogen ions are pumped into the thylakoid, where they accumulate in high concentration. Restoring electrons

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

One of the major problems facing terrestrial plants is dehydration. At times, solutions to this problem conflict with other metabolic processes, especially photosynthesis. The stomata are not only the major route for gas exchange (CO 2 in and O 2 out), but also for the evaporative loss of water. On hot, dry days plants close the stomata to conserve water, but this causes problems for photosynthesis. 5. Alternative mechanisms of carbon fixation have evolved in hot, arid climates Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

In most plants (C 3 plants) initial fixation of CO 2 occurs via rubisco and results in a three-carbon compound, 3- phosphoglycerate. –These plants include rice, wheat, and soybeans. When their stomata are closed on a hot, dry day, CO 2 levels drop as CO 2 is consumed in the Calvin cycle. At the same time, O 2 levels rise as the light reaction converts light to chemical energy. While rubisco normally accepts CO 2, when the O 2 /CO 2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O 2 to RuBP. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

When rubisco adds O 2 to RuBP, RuBP splits into a three-carbon piece and a two- carbon piece in a process called photorespiration. –The two-carbon fragment is exported from the chloroplast and degraded to CO 2 by mitochondria and peroxisomes. –Unlike normal respiration, this process produces no ATP, nor additional organic molecules. Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

A hypothesis for the existence of photorespiraton (a inexact requirement for CO 2 versus O 2 by rubisco) is that it is evolutionary baggage. When rubisco first evolved, the atmosphere had far less O 2 and more CO 2 than it does today. –The inability of the active site of rubisco to exclude O 2 would have made little difference. Today it does make a difference. –Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day. Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

The C 4 plants fix CO 2 first in a four-carbon compound. –Several thousand plants, including sugercane and corn, use this pathway. In C 4 plants, mesophyll cells incorporate CO 2 into organic molecules. –The key enzyme, phosphoenolpyruvate carboxylase, adds CO 2 to phosphoenolpyruvate (PEP) to form oxaloacetetate. –PEP carboxylase has a very high affinity for CO 2 and can fix CO 2 efficiently when rubisco cannot - on hot, dry days with the stomata closed. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig

The mesophyll cells pump these four- carbon compounds into bundle-sheath cells. –The bundle sheath cells strip a carbon, as CO 2, from the four-carbon compound and return the three-carbon remainder to the mesophyll cells. –The bundle sheath cells then uses rubisco to start the Calvin cycle with an abundant supply of CO 2. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig

In photosynthesis, the energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds. 6. Photosynthesis is the biosphere’s metabolic foundation: a review Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig

Sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons to synthesize all the major organic molecules of cells. –About 50% of the organic material is consumed as fuel for cellular respiration in plant mitochondria. –Carbohydrate in the form of the disaccharide sucrose travels via the veins to nonphotosynthetic cells. –There, it provides fuel for respiration and the raw materials for anabolic pathways including synthesis of proteins and lipids and building the extracellular polysaccharide cellulose. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Light Reactions Takes place on thylakoids Electrons excited from chlorophyll Water is split to donate the electrons. The waste is oxygen. It is released into the air Sunlight is needed to split the water and excite the electrons 2 products go to dark reactions: –1) ATP –2) NADPH (carries electrons and hydrogen) Products goes to Calvin Cycle (Dark reactions) to help make sugar.

Light and dark reactions