Chapter 7 Textbook Notes PDF

Preview

Summary

photosynthesis...

Description

Chapter 7 Textbook Notes Tuesday, March 9, 2021

11:53 PM

7.1 Photosynthesis fuels the biosphere Photosynthesis - the process by which plants, algae, and some protists and prokaryotes convert light energy to chemical energy that is stored in sugars made from carbon dioxide and water. - Converts CO2 and H2O to sugars and other organic molecules and release oxygen as a byproduct. Plants are autotrophs (meaning "self feeders" in Greek) in that they make their own food. - Ultimate source of organic molecules for almost all other organisms. Because they use the energy of light, plants and other photosynthesizers are specifically called photoautotrophs. - Often referred to as producers. Producers feed the consumers of the biosphere - the heterotrophs that cannot make their own food but must consume plants or animals or decompose organic material. Land autotrophs - plants Aquatic autotrophs - algae, some protists, photosynthetic prokaryotes, cyanobacteria Photosynthesis takes place in the chloroplasts. Illustrating the theme of interactions, the remarkable ability of these organelles to harness light energy and use it to drive the synthesis of organic compounds emerges from the structural organization and interactions of their component parts. - According to the widely accepted theory of endosymbiosis, chloroplasts originated from a photosynthetic prokaryote that took up residence inside a eukaryotic cell. Checkpoint 7.1 What do "self-feeding" photoautotrophs require from the environment to make their own food? Light CO2 and H2O (minerals are also required; you'll learn about -

Light, CO2, and H2O (minerals are also required; you ll learn about the needs of plants in chapter 32)

7.2 Photosynthesis occurs in chloroplasts in plant cells -

-

A leaf's green color comes from chlorophyll, a light-absorbing pigment in the chloroplasts that plays a central role in converting solar energy to chemical energy. Chloroplasts are concentrated in the cells of mesophyll, the green tissue in the interior of the leaf. CO2 enters the lead, and O2 exists, by way of tiny pores called stomata (singular stoma, meaning mouth)

-

In the chloroplast, an envelope of two membranes encloses an inner compartment, which is filled with a thick fluid called

stroma. - Suspended in the stroma is a system of interconnected membranous sacs, called thylakoids, which enclose another internal compartment, called the thylakoid space. ○ In many places, thylakoid are concentrated in stacks called grana (singular, granum) - The precise arrangements of these membranes and compartments are essential to the process of photosynthesis a classic example of the theme of structure and function. Checkpoint 7.2 How do the reactant molecules of photosynthesis reach the chloroplasts in leaves? Carbon dioxide enters leaves through stomata, and water enters the roots and is carried to leaves through veins.

7.3 Scientists traced the process of photosynthesis using isotopes Basic summary equation for photosynthesis: Light energy + 6 CO2 + 6H2O --> C6H12O6 + 6 O2 - The synthesis of sugar in photosynthesis involves numerous chemical reactions. - Working out the details of these reactions also involved the use of isotopes, in this case, radioactive isotopes. - They works for ten years to elucidate this cycle, which is not called the Calvin cycle. Checkpoint 7.3 Photosynthesis produces billions of tons of carbohydrate a year. Where does most of the mass of this huge amount of organic matter come from? Mostly from carbon dioxide in the air, which provides both the carbon and oxygen in carbohydrate. Water supplies only the hydrogen.

7.4 Photosynthesis is a redox process, as is cellular respiration -

Both photosynthesis and cell respiration illustrate the theme of the transformation of energy and matter

○

The light energy captured by chlorophyll molecules in the chloroplast provides this energy boost.

Checkpoint 7.4 Which redox process, photosynthesis or cellular respiration, is exergonic? Is endergonic? Cellular respiration; photosynthesis

7.5 Photosynthesis occurs in two stages, which are linked by ATP and NADPH Figure 7.5 A The summary equation of photosynthesis

Figure 7.5B An overview of the two stages of photosynthesis in a chloroplast.

The light reactions, which occur in the thylakoids, include the steps that convert light energy to chemical energy and release oxygen. ○ Water is split, providing a source of electrons and giving off oxygen as a by-product. ○ Light energy is absorbed by chlorophyll molecules built into the thylakoid membranes. ○ This energy is used to drive the transfer of electrons and hydrogen ions from water to the electron acceptor NADP+, reducing it to NADPH. ○ NADPH temporarily stores electrons and provides "reducing power" to the Calvin cycle. ○ Generate ATP from ADP and a phosphate group. - The Calvin cycle occurs in the stroma of the chloroplast. ○ Series of reactions that assembles sugar molecules using carbon dioxide and the energy-rich products of the light reactions. - The incorporation of carbon from carbon dioxide into organic compounds, shown in the figure as CO2 entering the Calvin cycle, is called carbon fixation. ○ After carbon fixation, the carbon compounds are reduced to sugars. - The Calvin cycle is sometimes referred to as the dark reactions, or lightindependent reactions, because none of the steps requires light directly. Photosynthesis in two stages: - Photo, from the Greek word for "light" refers to the light reactions - Synthesis, meaning "putting together" refers to sugar construction by the Calvin cycle. Checkpoint 7.5 For chloroplasts to produce sugar from carbon dioxide in the dark, they would need to be supplied with _____ and ________. ATP …. NADPH -

7.6 Visible radiation absorbed by pigments drives the light reactions The Nature of Sunlight Sunlight is a type of energy called electromagnetic energy or radiation. The distance between the crests of electromagnetic waves is called a wavelength. - The electromagnetic spectrum is the full range of electromagnetic wavelengths from vary short gamma rays to very long wavelength radio waves. Figure 7.6A The electromagnetic spectrum -

Li ht l

b h

di

t

k t

f

ll d h t

-

Light also behaves as discrete packets of energy called photons. A photon has a fixed quantity of energy, and the shorter the wavelength of light, the greater the energy of its photons. ○ This is why UV radiation can cause sunburns and skin cancer

Photosynthetic Pigments Light-absorbing molecules called pigments, built into the thylakoid membranes, absorb some wavelengths of light and reflect or transmit other wavelengths. Figure 7.6B The interaction of light with chlorophyll in a chloroplast. -

-

Different pigments absorb light of different wavelengths, and chloroplasts contain more than one type of pigment. ○ Chlorophyll a which participates directly in the light reactions absorbs

Chlorophyll a, which participates directly in the light reactions, absorbs mainly blue-violet and red light. ○ Chlorophyll b, absorbs mainly blue and orange light. Broadens the range of light that a plant can use by conveying absorbed energy to chlorophyll a, which then puts the energy to work in the light reactions. - Chloroplasts also contain pigments called carotenoids, which are various shades of yellow and orange. ○ May broaden the spectrum of colors that can drive photosynthesis. ○ Provides photoprotection, some carotenoids absorb and dissipate excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules that can damage cell molecules. Checkpoint 7.6 What color of light is least effective at driving photosynthesis? Explain. Green, because it is mostly transmitted and reflected - not absorbed - by photosynthetic pigments. ○

7.7 Photosystems capture solar energy When a pigment molecule absorbs a photon of light, one of the pigment's electrons jumps to an energy level father from the nucleus. ○ In this location, the electron has more potential energy' ○ Electron raised from a ground state to an excited state (unstable) ○ When isolated pigment molecules absorb light, their excited electrons drop back down to the ground state in a billionth of a second, releasing their excess energy as heat. - Some isolated pigments, including chlorophyll, emit light as well as heat after absorbing photons. Figure 7.7A A solution of chlorophyll glowing red when illuminated (left); an isolated chlorophyll molecule whose light-excited electron releases heat and light when it falls back to ground state (right) -

In the thylakoid membrane, chlorophyll molecules are organized into clusters called photosystems. - A photosystem contains two kinds of complexes: a reaction-center complex surrounded by a number of light-harvesting complexes. ○ A light harvesting complex consists of various pigment molecules bound to proteins. § Number and variety of pigment molecules can harvest light over a larger surface area & larger portion of the spectrum than any single molecule alone. § Together, the complexes function as a light-gathering antenna ○ The reaction-center complex contains a pair of special chlorophyll a molecules and a molecule called the primary electron acceptor, which is capable of accepting electrons and becoming reduced. § When an electron from a reaction-center chlorophyll a is boosted to a higher energy level, it is immediately captured by the primary electron acceptor. § This is the first step in the transformation of light energy to chemical energy in light reactions. Figure 7 7B A photosystem harvesting light energy and an excited electron being -

Figure 7.7B A photosystem harvesting light energy and an excited electron being passed to the primary electron acceptor

Two types of photosystems has been identified and they cooperate in light reactions ○ Photosystem I ○ Photosystem II ○ Both have a characteristic reaction-center complex. Checkpoint 7.7 Compared with a solution of isolated chlorophyll, why do intact chloroplasts not release heat and light when illuminated? In the chloroplasts, a light-excited electron from the reaction-center chlorophyll -

In the chloroplasts, a light excited electron from the reaction center chlorophyll molecules is passed to a primary electron acceptor before it can fall back to the ground state.

7.8 Two photosystems connected by an electron transport chain convert light energy to the chemical energy to the chemical energy of ATP and NADPH. Figure 7.8 A mechanical analogy of the light reactions

-

The electrons that end up reducing NADP+ to NADPH originally come from water

water. - An enzyme in the thylakoid space splits water in two electrons, two hydrogen ions, and one oxygen atom. ○ The oxygen atom immediately joins with another to form O2. ○ The electrons from water are passed, one by one, to the reaction center chlorophyll a molecules in photosystem II, replacing the photoexcited electron that was just captured by the primary electron acceptor. - From photosystem II, the electrons pass through an electron transport chain to the reaction center chlorophyll a molecules in photosystem I, again replacing photoexcited electrons that had been captured by its primary electron acceptor. - Making ATP in the light reactions involves an electron transport chain and chemiosmosis. Checkpoint 7.8 Looking at the model of the light reactions in Figure 7.8, explain why two photons of light are required in the movement of electrons from water to NADPH. One photon excited an electron from photosystem II, which is passed down an electron transfer chain to photosystem I. A second photon excited an electron from photosystem I, which is then used in the reduction of NADP+ to NADPH.

7.9 The light reactions take place within the thylakoid membranes Energy released as electrons pass down the electron transport chain powers the transport of H+ into the thylakoid space. - The concentration gradient of H+ across the thylakoid membrane drives H+ through ATP synthase, producing ATP. - Because the initial energy input is light (photo-), this chemiosmotic production of ATP is called photophosphorylation. Checkpoint 7.9 Describe the two forces moving H+ across the thylakoid membrane. 1) Energy released as electrons are passed down the electron transport chain pumps H+ into the thylakoid space 2) The concentration gradient drives H+ from the thylakoid space through ATP synthase. -

7.10 ATP and NADPH power sugar synthesis in the Calvin cycle -

The Calvin cycle is considered a cycle because the starting material is

-

regenerated after molecules enter and leave the cycle. ○ In this case, the starting material is a five-carbon sugar named ribulose biphosphate (RuBP) To make a molecule of G3P, the cycle must turn three times, incorporating three molecules of CO2.

For the synthesis of one G3P molecule, the Calvin cycle consumes nine ATP and six NADPH molecules, which were provided by the light reactions. - Photosynthesis is an emergent property of the structural organization of a chloroplast, which integrates the two stages of photosynthesis. Checkpoint 7.10 Explain why the large number of ATP and NADPH molecules used during the Calvin cycle is consistent with the value of glucose as an energy source. Glucose is a highly reduced molecule, storing lots of potential energy in its electrons. To reduce carbon dioxide to glucose, much energy and reducing power is required. -

7.11 Other methods of carbon fixation have evolved in hot, dry climates. -

-

-

-

-

Most plants use carbon dioxide directly from the air, and carbon fixation occurs when the enzyme rubisco adds carbon dioxide to RuBP. ○ Such plants are called C3 plants because the first stable product of carbon fixation is a three-carbon intermediate compound. ○ Includes crops such as soybeans, wheat, and rice. ○ Susceptible to dehydration Photorespiration is a metabolic pathway that consumes oxygen, releases carbon dioxide, and decreases photosynthetic output in plant cells. ○ Generally occurs on hot, dry days, when stomata close, oxygen accumulates in the leaf and rubisco fixes oxygen rather than carbon dioxide. ○ Does not produce sugar molecules or ATP (uses ATP) ○ Can drain away as much as 50% of the carbon fixed by the Calvin cycle. In some plant species found in hot, dry climates, alternate models of carbon fixation have evolved that minimize photorespiration and optimize the Calvin cycle. C4 plants first fix carbon dioxide into a four-carbon compound. ○ When weather is hot and dry, a C4 plant keeps its stomata mostly closed, conserving water. ○ Continues making sugars by photosynthesis using the pathway and the two types of cells shown on the left side of figure 7.11 An enzyme in the mesophyll cells has a high affinity for carbon dioxide and can fix carbon even when the carbon dioxide

fix carbon even when the carbon dioxide...