Chapter 5 Photosynthesis slides PDF

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CHAPTER 5 PHOTOSYNTHESIS

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The interrogation of the secret life of the chloroplast began as early as in the 1800’s. The source, input, output, and fate of every reactant/product are all figured out which culminated in the Nobel award to Melvin Calvin in 1961. It’s been known for a long time that CO2 and H2O (in blue) are the reactants and O2 and glucose (in red) are the products of photosynthesis. But oxygen is in both CO2 and H2O, so the question that bothered scientists for a long time is which oxygen ends up as the oxygen gas released. The ultimate proof came from application of isotopic techniques to trace reactants when they were made radioactive (refer back to Ch 2 when we discussed the use of isotopes as a tracer in research and medicine to diagnose and treat diseases?) Say, if we incubated CO2 in which the C has been sub’ed with 14C* with leaves and found that the final product is a radioactive sugar, the most obvious interpretation is that C in CO2 is used to make the C skeleton of sugar. If we do a time-course experiment and collect samples every minute, it would be possible to identify what the intermediate metabolites are, and if long enough, even the ultimate storage form of sugar, e.g., hot potato full of radioactive starch. An even more intriguing finding happened when another reactant H2O was substituted with an isotopic glucose which contains O does not contain 18O* at the end, but the oxygen gas released as a byproduct is. So how would you interpret this result if you were the scientist? This turns out to have provided a giant leap forward in our understanding of the complex reaction.

18O*,

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Learning Objectives • Understand that energy from the sun fuels all life on earth • Describe the locations where the reactions of photosynthesis and cellular respiration occur in cells • Differentiate between the “photo” and “synthesis” reactions of photosynthesis • Explain how oxygen gas is produced in photosynthesis • Describe how CO2 is incorporated into sugars during the “synthesis” reactions of photosynthesis • Contrast C3, C4, and CAM photosynthesis • Understand which types of organisms that perform photosynthesis and cellular respiration • Compare and contrast the starting and ending materials in photosynthesis and cellular respiration

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Where does plant matter come from? Photosynthesis: the big picture. From a seed to a tree: Where does the mass come from? Photosynthetic Organisms: not just plants.

Consider that in five years a tree in a big planter can increase its weight by 150 pounds (68 kg) as it grows . Where does that 68 kg of new tree come from? When humans grow, the new tissue comes from food we eat. When plants grow, where does the new tissue come from? Figure Where does a plant come from? Although plants are the most visible organisms that have evolved the ability to capture light energy and convert it to organic matter, they are not the only organisms capable of photosynthesis. Some bacteria and many other unicellular organisms are also capable of using the energy in sunlight to produce organic materials. Figure Plants aren’t the only photosynthesizers.

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Photosynthesis: The Big Picture

“Photo” and “Synthesis”

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inputs ❑ 2 products Which example below converts kinetic into potential energy?

1. ADP + Pi → ATP 2. Sunlight + H2O+ CO2 → sugar + O2 3. ATP → ADP + Pi 4. Both 1 and 2.

There are three inputs to the process of photosynthesis: light energy (from the sun), carbon dioxide (from the atmosphere), and water (from the ground). From these three inputs, the plant produces sugar and oxygen. Photosynthesis is best understood as two separate events: a “photo” segment, during which light is captured, and a “synthesis” segment, during which sugar is synthesized. Figure Photosynthesis: the big picture. Answer: 4

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Take-home message ❑ Through photosynthesis, plants use water, the energy of sunlight, and carbon dioxide gas from the air to produce sugars and other organic materials. ❑ In the process, photosynthesizing organisms also produce oxygen, which makes all animal life possible.

Photosynthesis take place in the chloroplasts

Organelles found in plant cells If a plant part is green, then you know it is photosynthetic. Leaves are green because the cells near the surface are packed full of chloroplasts, light-harvesting organelles, which make it possible for the plant to use the energy from sunlight to make sugars (their food) and other plant tissue (much of which animals use for food) (Figure 4-12). Other plant parts, such as stems, may also contain chloroplasts (in which case they, too, are capable of photosynthesis), but most chloroplasts are located within the cells in a plant’s leaves. Figure Photosynthesis factories.

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A Closer Look at Chloroplasts

Take-home message

In plants, photosynthesis occurs in chloroplasts, green organelles packed in cells near the plants’ surfaces, especially in the leaves.

The sac-shaped organelle is filled with a fluid called the stroma. Floating in the stroma is an elaborate system of interconnected membranous structures called thylakoids, which often look like stacks of pancakes. Once inside the chloroplast, you can be in one of two places: in the stroma or inside the thylakoids.

The conversion of light energy to chemical energy—the “photo” part of photosynthesis—occurs inside the thylakoids. The production of sugars—which are made in the “synthesis” part of photosynthesis— occurs within the stroma. Figure The chloroplast.

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Light energy travels in waves: plant pigments absorb specific wavelengths

Light Energy A type of kinetic energy ❑ Made up of little energy packets called photons ❑ Different photons carry different amounts of energy, carried as waves. ❑ Length of the wave = amount of energy the photon contains. ❑

Photosynthesis is powered by light energy, a type of kinetic energy made up of little energy packets called photons, which are organized into waves. Photons can do work as they bombard surfaces such as your face (heating it) or a leaf (enabling it to build sugar from carbon dioxide and water).

Photons have various amounts of energy and the length of the wave in which they travel corresponds to the amount of energy carried by the photon.

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Electromagnetic Spectrum Range of energy that is organized into waves of different lengths. ❑ Shorter the wavelength, higher the energy.

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Visible Spectrum ❑ Range of energy humans see as light ❑ ROYGBIV ❑ Pigments = molecules that absorb light The shorter the wavelength, the more energy the light carries. Within a ray of light, there are super-high energy packets of photons (those with short wavelengths), relatively low-energy packets (those with longer wavelengths), and everything in between. This range, which is called the electromagnetic spectrum, extends from extremely short, highenergy gamma rays and X-rays, with wavelengths as short as 1 nanometer (a human hair is about 50,000 nanometers in diameter) to very long, low-energy radio waves, with wavelengths as long as a mile. Figure A spectrum of energy. Just as we can’t hear some super-high-pitched frequencies of sound (even though many dogs can), there are some wavelengths of light that are too short or too long for us to see. The light that we can see, visible light, spans all the colors of the rainbow. Humans (and some other animals) can see colors because our eyes contain light-absorbing molecules called pigments. These pigments absorb wavelengths of light within the visible range. The energy in these light waves excites electrons in the pigments, which in turn stimulates nerves in our eyes. These nerves then transmit electrical signals to our brains. We perceive different wavelengths within the visible spectrum as different colors.

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Chlorophyll ❑ Plant

pigment ❑ Absorbs certain wavelengths of energy (photons) from the sun ❑ Absorbed energy excites electrons ❑ Plant pigments can only absorb specific wavelengths of energy ❑ Therefore, plants produce several different types of pigments ❑ Chlorophyll

a Plant Pigments ❑ Chlorophyll b ❑ Carotenoids When plants use sunlight’s energy to make sugar during photosynthesis, they also use the visible portion of the electromagnetic spectrum. Chlorophyll is the pigment molecule in plants that absorbs light energy from the sun. Chlorophyll molecules are embedded in the thylakoid membranes of chloroplasts, which are found primarily in plants’ leaves. Just as light energy excites electrons in the pigments responsible for color vision in humans, electrons in a plant’s chlorophyll can become excited by certain wavelengths of light and can capture a bit of this light energy. Unlike our eyes, however, plant pigments (the energy-capturing parts of a plant) absorb and use only a portion of visible light wavelengths. Other wavelengths pass through or bounce off. Therefore, plants produce several different types of pigments in order to maximize their ability to absorb energy. Plants produce several different light-absorbing pigments: ❑ Chlorophyll a • The primary photosynthetic pigment • Efficiently absorbs blue-violet and red wavelengths ❑ Chlorophyll b • Absorbs blue and red-orange wavelengths • Reflects yellow-green wavelengths ❑ Carotenoids • Absorbs blue-violet and blue-green wavelengths • Reflects yellow, orange, and red wavelengths

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chlorophyll a, efficiently absorbs blue-violet and red wavelengths of light

Chlorophyll b, is similar in structure but absorbs blue and red-orange wavelengths

carotenoids absorbs blue-violet and bluegreen wavelengths and reflects yellow, orange, and red wavelengths.

The primary photosynthetic pigment, called chlorophyll a, efficiently absorbs blue-violet and red wavelengths of light. Every other wavelength generally travels through or bounces off this pigment. Because chlorophyll a cannot efficiently absorb green light and instead reflects those wavelengths back, our eyes and brain perceive the reflected light waves as green, and so the pigment (and the leaves in which it is found) appears green. Another pigment, chlorophyll b, is similar in structure but absorbs blue and red-orange wavelengths. Chlorophyll b reflects back yellow-green wavelengths. A related group of pigments called carotenoids absorbs blue-violet and blue-green wavelengths and reflects yellow, orange, and red wavelengths. Figure (part 1) Plant pigments. Figure (part 2) Plant pigments. Each photosynthetic pigment absorbs and reflects specific wavelengths. Why do the leaves of some trees turn beautiful colors each fall? In the late summer, cooler temperatures cause some trees to prepare for the winter by shutting down chlorophyll production and reducing their photosynthesis rates, going into a state that resembles an animal’s hibernation. Gradually, the chlorophyll a and b molecules present in the leaves are broken down and their chemical components are stored in the branches. As the amounts of chlorophyll a and b in the leaves decrease relative to the remaining carotenoids, the striking colors of the fall foliage are revealed. During the rest of the year, chlorophyll a and b are so abundant in leaves that green masks the colors of the other pigments.

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Take-home message Photosynthesis is powered by light energy, a type of kinetic energy made up from energy packets called photons. ❑ Photons hit chlorophyll and other light-absorbing molecules near the green surfaces of plants. ❑ These molecules capture the light energy and harness it to build sugar from carbon dioxide and water.

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Which pigment absorbs blue and yellow light? 1. Chlorophyll a 2. Chlorophyll b 3. Carotenoids 4. Both 1 and 2. 5. Both 1 and 3. Answer: 1

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1. Which answer is the independent variable in the graph below?

1. Time of year 2. Photosynthetic pigments 3. Amount of pigment molecules present 2. What do the colors of the bars in the graph represent? 1. 2. 3. 4.

Light wavelength absorbed by each pigment Light wavelength reflected by each pigment Distinguishes between different data sets Indicates the relative importance of the pigment in photosynthesis

1. Answer: 1 2. Answer: 2

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Photons cause electrons in chlorophyll to enter an excited state. Electron Excitation ❑ Conversion of electromagnetic energy into chemical energy of bonds between atoms ❑ Photons of specific wavelengths bump electrons up a quantum level into an excited state Two Potential Fates of Excited Electrons (1) Electron returns to resting, unexcited state. (2) Excited electrons are passed to other atoms.

An organism can use energy from the sun only if it can convert the light energy of the sun into the chemical energy in the bonds between atoms. When chlorophyll is hit by photons of certain wavelengths, the light energy bumps an electron in the chlorophyll molecule to a higher energy level, an excited state. Upon absorbing the photon, the electron briefly gains energy, and the potential energy in the chlorophyll molecule increases. An electron in a photosynthetic pigment that is excited to a higher energy state generally has one of two fates (refer to the next two slides also): (1) The electron returns to its resting, unexcited state. In the process, energy is released, some of which may be transferred to a nearby molecule, bumping electrons on that molecule to a higher energy state (and the rest of the energy is dissipated as heat) or (2) The excited electron itself is passed to another molecule.

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Figure (part 1) Capturing light energy with excited electrons. Chlorophyll electrons are excited to a higher energy state by light energy.

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Figure (part 2) Capturing light energy with excited electrons. Chlorophyll electrons are excited to a higher energy state by light energy.

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The Passing of Electrons in Their Excited State ❑ Chief

way energy moves through cells ❑ Molecules that gain electrons always carry greater energy than before receiving them • Can view this as passing of potential energy from molecule to molecu

Take-home message When chlorophyll gets hit by photons, the light energy excites an electron in the chlorophyll molecule, increasing the chlorophyll’s potential energy. ❑ The excited electrons can be passed to other atoms, moving the potential energy through the cell. ❑

The passing of electrons from molecule to molecule is one of the chief ways that energy moves through cells. Many molecules carry or accept electrons during cellular activities. All that is required is that the acceptor have a greater attraction for electrons than the molecule from which it accepts them. Suppose a large meteor hit the earth. How could smoke and soot in the atmosphere wipe out life far beyond the area of direct impact? Because any particles in the atmosphere can block the light from the sun and reduce the excitation of electrons in chlorophyll molecules, photosynthesis depends on a relatively clean atmosphere. Any reduction in the available sunlight can have serious effects on plants. Scientists believe that if a large meteor hit the earth—as one did when the dinosaurs were wiped out 65 million years ago—smoke, soot, and dust in the atmosphere could block sunlight to such an extent that plants in the region, or even possibly all of the plants on earth, could not conduct photosynthesis at high enough levels to survive. And when plants die off, all of the animals and other species that rely on them for energy die as well. As dire as it sounds, all life on earth is completely dependent on the continued excitation of electrons by sunlight.

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Photosynthesis in detail: the energy of sunlight is captured as chemical energy. FOLLOW THE ELECTRONS!

Photosynthesis is a complex process, but our understanding of this process can be greatly aided by remembering one phrase: FOLLOW THE ELECTRONS. In the slides that follow, if you feel that you are losing focus or getting lost, just remember to think about the electrons. Where are they coming from?

What are they passing through? Where are they going? And what will happen to them when they get there? Figure Overview of the "photo" portion of photosynthesis. Light energy is captured in the "photo" portion of photosynthesis. It is later used to power the building of sugar molecules.

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The “Photo” Part ❑ ❑

Sunlight → ATP AND a high-energy electron carrier. Watch this short video to get an overview of the light (photo) reaction. https://www.youtube.com/watch?v=IT6TIv1UKgQ

Two photosystems involved: In the first part of photosynthesis, the “photo” part, sunlight hits a plant and, in a three-step process, the energy in this sunlight is ultimately captured and stored in an ATP molecule and another molecule (called NADPH) that stores energy by accepting high-energy electrons. Figure a Summary of the “photo” reactions.

As these pigments absorb photons from the sunlight that hits the leaves, electrons in the pigments become excited and then return to their resting state, releasing energy. The released energy (but not the electrons) is transferred to neighboring pigment molecules. This process continues until the energy transferred among many pigment molecules makes its way to a chlorophyll a molecule at the center of the photosystem, and excites an electron there. This is where the electron journey begins. The special chlorophyll a continually loses its excited electrons to a nearby molecule, called the primary electron acceptor, which acts like an electron vacuum. Why must plants get water for photosynthesis to occur? As electrons keep getting taken away from the special chlorophyll a molecule, the electrons must be replaced. The replacement electrons come from water.

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Electrons That Leave the Photosystem Are Replenished

Where does oxygen come from?

Paul Boyer, 99, UCLA biochemist who won Nobel Prize in 1997 Dr. Boyer spent years investigating the functioning of adenosine triphosphate, the basic energy source found in all living cells. Scientists had known of the existence of ATP for decades but had no been able to determine how it was formed. https://www.bostonglobe.com/metro/obituaries/2018/06/09/paul-boy ucla-biochemist-who-won-nobelprize/HSSIMEMK1Qw3S1zggIiOeJ/story.html

As these pigments absorb photons from the sunlight that hits the leaves, electrons in the pigments become excited and then return to their resting state, releasing energy. The released energy (but not the electrons) is transferred to neighboring pigment molecules. This process continues until the energy transferred among many pigment molecules makes its way to a chlorophyll a molecule at the center of the photosystem, and excites an electron there. This is where the electron journey begins. The special chlorophyll a continually loses its excited electrons to a nearby molecule, called the primary electron acceptor, which acts like an electron vacuum. Why must plants get water for photosynthesis to occur? As electrons keep getting taken away from the special chlorophyll a molecule, the electrons must be replaced. The replacement electrons come from water.

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An Electron Transport Chain Connects the two photosystems

Product #1 of the “Photo” Portion of Photosynthesis: ATP The photosynthetic electron transpor...