Chapter 08 Photosynthesis Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

8.1 Overview of Photosynthesis-1 Photosynthesis converts solar energy into chemical energy of carbohydrates. Organisms that carry on photosynthesis are called autotrophs. Plants, algae, and cyanobacteria are organisms capable of photosynthesis Heterotrophs are organisms that feed on other organisms.

8.1 Overview of Photosynthesis-2 Autotrophs and heterotrophs use organic molecules produced by photosynthesis. Pigments allow photosynthetic organisms to capture solar energy. Most photosynthetic organisms contain the pigment chlorophyll. Another common pigment group is the carotenoids.

Flowering Plants as Photosynthesizers Photosynthesis occurs in the green parts of plants. Particularly leaves, contain chlorophyll and other pigments Leaves contain mesophyll tissue specialized for photosynthesis Raw materials are water and CO2

8.1 Overview of Photosynthesis-3 Water is taken up by roots and transported to leaves by veins. Carbon dioxide enters through openings in the leaves called stomata. Light energy is absorbed by chlorophyll and other pigments in the thylakoids of chloroplasts.

8.1 Overview of Photosynthesis-4 Chloroplast structure The chloroplast and its fluid-filled interior called stroma are surrounded by a double membrane. Thylakoids are a different membrane system within the stroma that form flattened sacs. Thylakoids are stacked together to form grana. Thylakoid space is formed by a continuous connection between individual thylakoids.

8.1 Overview of Photosynthesis-5 Figure 8.2(a) Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

8.1 Overview of Photosynthesis-6 Figure 8.2(b) Chloroplast: © Science Source Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

Photosynthetic Reaction Glucose and oxygen are the products of photosynthesis The oxygen given off comes from water CO2 gains hydrogen atoms and becomes a carbohydrate Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

Two Sets of Reactions-1 The two sets of reactions are called the: Photosynthesis consists of two sets of reactions Photo refers to capturing solar energy Synthesis refers to producing a carbohydrate The two sets of reactions are called the: Light Reactions (light-dependent) Calvin Reactions (light-independent) Nicotinamide adenine dinucleotide phosphate (NADP+) links these reactions

Two Sets of Reactions-2 Figure 8.3 Jump to long image description Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

8.2 Plants as Solar Energy Converters-1 During the light reactions, different pigments within the thylakoid membranes absorb energy. Solar energy can be described in terms of its wavelength and energy content.

8.2 Plants as Solar Energy Converters-2 The electromagnetic spectrum extends from very short gamma rays to very long radio waves. White or visible light is only a small portion of the spectrum. Visible light is further divided into wavelengths between 380 and 750 nm. Jump to long image description Figure 8.4 Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

Visible Light-1 Visible light contains various wavelengths The colors of visible light range from: Violet light Shortest wavelength but high energy Red light Longest wavelength but lowest energy Only about 42% of solar radiation that hits Earth’s atmosphere reaches the surface of Earth – most is in the visible-light range Higher wavelengths are screened by the ozone layer

Visible Light-2 Most photosynthetic pigments in cells are chlorophylls a and b and the carotenoids Can absorb specific various portions of visible light The absorption spectrum shown in figure on the right Figure 8.5 Jump to long image description Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

Visible Light-3 Green light is reflected and only minimally absorbed Leaves appear green Other plant pigments become noticeable in the fall when chlorophyll breaks down and the other pigments are uncovered

Light Reactions Light Reactions Take place in thylakoid membrane Light reactions consist of two pathways: Noncyclic electron pathway Cyclic electron pathway Both pathways transform solar energy to chemical energy Both pathways produce ATP Only the noncyclic pathway produces NADPH

Noncyclic Electron Pathway-1 Noncyclic electron pathway, named because electron flow is traced from water to NADP+ Uses two photosystems (Photosystems I and II) A photosystem consists of a pigment complex and electron acceptors within the thylakoid membrane The pigment complex can be described as an “antenna” for gathering solar energy

Noncyclic Electron Pathway-2 Noncyclic Electron Pathway begins with photosystem II (PSII) Pigment complex absorbs solar energy Energy passes from one pigment to another until it is concentrated in reaction center Chlorophyll a molecule Electrons in the reaction center chlorophyll become energized Escape from the reaction center and move to a nearby electron acceptor

Noncyclic Electron Pathway-3 Photosystem II would disintegrate without replacement electrons Electrons provided by splitting water Releases oxygen (O2) to atmosphere which benefits all organisms that use O2 Hydrogen ions (H+) stay in the thylakoid space Contribute to formation of hydrogen ion gradient

Noncyclic Electron Pathway-4 In PSII, an electron acceptor receives energized electrons from the reaction center It sends those electrons down an electron transport chain, (series of carriers that pass electrons from one to the other) Energy is released to pump hydrogen ions (H+) into thylakoid space forming gradient When hydrogen ions flow through ATP synthase it makes ATP

Noncyclic Electron Pathway-5 PSI comes next in noncyclic electron pathway When the photosystem I complex absorbs solar energy, energized electrons leave reaction center and are captured by a different electron acceptor Low energy PSII electrons used to replace those lost by PSI Electron acceptor in photosystem I passes its electrons to NADP+ and it becomes NADPH

Noncyclic Electron Pathway-6 Figure 8.6 Jump to long image description Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

The Organization of the Thylakoid Membrane-1 The following molecular complexes are present in the thylakoid Membrane: PS II Pigment complex and electron acceptors Water is split to replace energized electrons Oxygen (O2) is released Electron transport chain Carries electrons from PS II to PS I Uses energy to pump H+ from the stroma into thylakoid space

The Organization of the Thylakoid Membrane-2 PS I Pigment complex and electron acceptors Adjacent to enzyme that reduces NADP+ to NADPH ATP synthase complex Has a channel for H+ flow Flow drives ATP synthase to join ADP and P

The Organization of the ThylakoiMembrane-3 Jump to long image description Figure 8.7 Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

Cyclic Electron Pathway-1 Uses only photosystem I (PSI) and begins when PSI complex absorbs solar energy Energized electrons escape from the reaction center and travel down electron transport chain Released energy is stored in the form of a H+ gradient, which causes ATP production by ATP synthase Spent electrons return to PSI (cyclic) Pathway only results in ATP production

Cyclic Electron Pathway-2 Energized electrons leave the photosystem I reaction center and return to photosystem by an electron transport chain ATP from cyclic electron transport used in Calvin cycle to make carbohydrates Jump to long image description Figure 8.8 Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

ATP Production ATP Production Thylakoid space acts as a reservoir for hydrogen ions (H+) H+ from water being split within thylakoid space Pumped in by electron transport chain More H+ in thylakoid space than stroma Electrochemical gradient H+ can only flow through ATP synthase Energy powers making ATP by chemiosmosis

8.3 Plants as Carbon Dioxide Fixers-1 The Calvin Cycle (named after Melvin Calvin) Series of reactions that use CO2 from the atmosphere to produce carbohydrate Humans and most other organisms take in O2 and release CO2 Includes: Carbon dioxide fixation Carbon dioxide reduction Ribulose-1,5-bisphosphate (RuBP) regeneration

8.3 Plants as Carbon Dioxide Fixers-2 Jump to long image description Figure 8.9 Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

Fixation of Carbon Dioxide Carbon dioxide fixation is the 1st step of the Calvin cycle CO2 is attached to 5-carbon RuBP molecule This reaction occurs three times The result is a 6-carbon molecule that splits into two 3-carbon molecules 3-phoshoglycerate (3PG) RuBP Carboxylase is the enzyme that makes this happen Comparatively slow enzyme so there is a lot of it

Reduction of Carbon Dioxide-1 Each 3PG molecule undergoes reduction to G3P in two steps Energy and electrons needed for this reaction are supplied by ATP and NADPH (from light reaction)

Reduction of Carbon Dioxide-2 Page 136 Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

Regeneration of RuBP-1 Regeneration of RuBP It takes three turns of the Calvin cycle to allow one G3P to exit For every three turns of Calvin Cycle, five G3P (3-carbon molecule) are used This re-forms three RuBP (5-carbon molecule) 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)

Regeneration of RuBP-2 5×3 carbons in G3P =3×5 (carbons in RuBP) Page 137 Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

Importance of the Calvin Cycle G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules These molecules meet the plant needs The hydrocarbon skeleton of G3P can form: Fatty acids and glycerol to make plant oil Glucose phosphate (simple sugar) Fructose (+ glucose = sucrose) Starch and cellulose Amino acids

8.4 Alternate Pathways for Photosynthesis C3 Photosynthesis The leaves of C3 plants have a different structure and means of fixing CO2 than C4 plants C3 plants such as wheat, rice, and oats have mesophyll cells of leaves in parallel layers Bundle sheath cells around the plant veins do not contain chloroplasts As a result, cells using Calvin cycle exposed to CO2

C3 Photosynthesis-1 RuBP carboxylase binds O2 as well as CO2 When bound to O2, the enzyme undergoes photorespiration Wasteful reaction because it uses O2 and releases CO2, decreasing output of Calvin cycle O2 concentration in leaf rises when weather is hot and dry, because plant keeps stomata closed to conserve water

tulips: © Evelyn Jo Johnson C3 Photosynthesis-2 Figure 8.11(a) Figure 8.10(a) tulips: © Evelyn Jo Johnson Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

C4 Photosynthesis-1 In C4 plants, such as sugarcane and corn, the mesophyll cells are arranged in concentric rings around the bundle sheath cells. They also contain chloroplasts In the mesophyll cells, CO2 is initially fixed into a 4-carbon molecule The 4-carbon molecule is later broken down into a 3-carbon molecule and CO2 CO2 enters the Calvin cycle

C4 Photosynthesis-2 C4 Pathway C4 plants tend to be found in hot, dry climates In these climates, stomata tend to close to conserve water Oxygen then builds-up in the leaves But, RuBP carboxylase is not exposed to this O2 in C4 plants and photorespiration does not occur Instead, in C4 plants, the CO2 is delivered to the Calvin cycle, which is located in bundle sheath cells that are sheltered from the air spaces of the leaf

corncob: © David Frazier/Corbis RF C4 Photosynthesis-3 Figure 8.11(b) Figure 8.10(b) corncob: © David Frazier/Corbis RF Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

C4 Photosynthesis-4 When the weather is moderate, C3 plants ordinarily have the advantage. When the weather is hot and dry, C4 plants have the advantage, and can be expected to predominate. In the early summer, C3 plants such as Kentucky bluegrass predominate in lawns in the cooler parts of the United States, but by midsummer, crabgrass, a C4 plant, begins to take over.

CAM Photosynthesis-1 CAM Pathway This pathway is prevalent among most succulent plants that grow in deserts, including the cacti. CAM plants partition carbon fixation according to time. During the night, CAM plants fix CO2, forming C4 molecules. The C4 molecules are stored in large vacuoles. During daylight, C4 molecules release CO2 to the Calvin cycle.

Pineapple: © Pixtal/Age fotostock RF CAM Photosynthesis-2 Figure 8.10(c) Pineapple: © Pixtal/Age fotostock RF Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

8.5 Photosynthesis Versus Cellular Respiration-1 Both plant and animal cells carry out cellular respiration. Occurs in mitochondria Breaks glucose down Utilizes O2 and gives off CO2 Plant cells photosynthesize, but animal cells do not. Occurs in chloroplasts Builds glucose Utilizes CO2 and gives off O2 Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

8.5 Photosynthesis Versus Cellular Respiration-2 Both processes utilize an electron transport chain and chemiosmosis for ATP production. Page 139 Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

8.5 Photosynthesis Versus Cellular Respiration-3 Figure 8.12(a) Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

8.5 Photosynthesis Versus Cellular Respiration-4 Figure 8.12(b) Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.

8.5 Photosynthesis Versus Cellular Respiration-5 Figure 8.12(c) Jump to long image description Copyright © 2017 McGraw-Hill Education. Permission required for reproduction or display.