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CHAPTER 7 Photosynthesis: Using Light to Make Food

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1 CHAPTER 7 Photosynthesis: Using Light to Make Food
Modules 7.1 – 7.5

2 Light is central to the life of a plant
Life in the Sun Light is central to the life of a plant Photosynthesis is the most important chemical process on Earth It provides food for virtually all organisms Plant cells convert light into chemical signals that affect a plant’s life cycle

3 Light can influence the architecture of a plant
Plants that get adequate light are often bushy, with deep green leaves Without enough light, plants become tall and spindly with small pale leaves Too much sunlight can damage a plant Chloroplasts and carotenoids help to prevent such damage

Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water Carbon dioxide Water Glucose Oxygen gas PHOTOSYNTHESIS

5 7.1 Autotrophs are the producers of the biosphere
Plants, some protists, and some bacteria are photosynthetic autotrophs They are the ultimate producers of food consumed by virtually all organisms

6 On land, plants such as oak trees and cacti are the predominant producers
Figure 7.1A Figure 7.1B

7 In aquatic environments, algae and photosynthetic bacteria are the main food producers
Figure 7.1C Figure 7.1D

8 7.2 Photosynthesis occurs in chloroplasts
In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts A chloroplast contains: stroma, a fluid grana, stacks of thylakoids The thylakoids contain chlorophyll Chlorophyll is the green pigment that captures light for photosynthesis

9 The location and structure of chloroplasts
LEAF CROSS SECTION MESOPHYLL CELL LEAF Mesophyll CHLOROPLAST Intermembrane space Outer membrane Granum Inner membrane Grana Stroma Thylakoid compartment Stroma Figure 7.2 Thylakoid

10 Investigating Photosynthesis
Investigations into photosynthesis began with the following question: “When a tiny seedling grows into a tall tree with a mass of several tons, where does the tree’s increase in mass come from?”

11 ______________ Experiment (1643)
Put soil in pot and took mass Took a seedling and took mass Put seed in soil...watered...waited five years... the seedling became a tree. He concluded that He determined the Van Helmont’s                                            the mass came from water the “hydrate” in the carbohydrate portion of photosynthesis

12 ___________ Experiment (1771)
Put a lit candle in a bell jar- Placed a mint plant in the jar with the candle- Concluded He determined Priestly’s The flame died out.                                                              Flame lasted longer plants release a substance needed for candle burning. plants release oxygen

13 ________________Experiment (1779)
Put aquatic plants in light... Put aquatic plants in dark... He determined: _______________ (1948) He determines Known as the Jan Ingenhousz produced oxygen No oxygen Light is needed to produce oxygen Melvin Calvin carbon’s path to make glucose Calvin’s cycle

14 7.3 Plants produce O2 gas by splitting water
The O2 liberated by photosynthesis is made from the oxygen in water Figure 7.3A

15 Experiment 1 Not labeled Experiment 2 Labeled Reactants: Products:
Figure 7.3B Reactants: Products: Figure 7.3C

16 7.4 Photosynthesis is a redox process, as is cellular respiration
Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas These electrons and H+ ions are transferred to CO2, producing sugar Reduction Oxidation Figure 7.4A Oxidation Reduction Figure 7.4B

17 7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH
The complete process of photosynthesis consists of two linked sets of reactions: the light reactions and the Calvin cycle The light reactions convert light energy to chemical energy and produce O2 The Calvin cycle assembles sugar molecules from CO2 using the energy-carrying products of the light reactions

18 LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma)
An overview of photosynthesis H2O CO2 Chloroplast Light NADP+ ADP + P LIGHT REACTIONS (in grana) CALVIN CYCLE (in stroma) ATP Electrons NADPH O2 Sugar Figure 7.5

19 7.6 Visible radiation drives the light reactions
THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY 7.6 Visible radiation drives the light reactions Certain wavelengths of visible light drive the light reactions of photosynthesis Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light Wavelength (nm) Figure 7.6A

20 Reflected light Light Chloroplast Absorbed light Transmitted light
Figure 7.6B

21 7.7 Photosystems capture solar power
Each of the many light-harvesting photosystems consists of: an “antenna” of chlorophyll and other pigment molecules that absorb light a primary electron acceptor that receives excited electrons from the reaction-center chlorophyll

22 Primary electron acceptor
PHOTOSYSTEM Photon Reaction center Pigment molecules of antenna Figure 7.7C

23 Fluorescence of isolated chlorophyll in solution
Heat Photon (fluorescence) Photon Chlorophyll molecule Figure 7.7A

24 Primary electron acceptor
Excitation of chlorophyll in a chloroplast Primary electron acceptor Other compounds Photon Chlorophyll molecule Figure 7.7B

25 7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O2
Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons The excited electrons are passed from the primary electron acceptor to electron transport chains Their energy ends up in ATP and NADPH

26 Where do the electrons come from that keep the light reactions running?
In photosystem I, electrons from the bottom of the cascade pass into its P700 chlorophyll

27 Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product
Primary electron acceptor Electron transport Primary electron acceptor Electron transport chain Photons Energy for synthesis of PHOTOSYSTEM I PHOTOSYSTEM II by chemiosmosis Figure 7.8

28 7.9 Chemiosmosis powers ATP synthesis in the light reactions
The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP In the stroma, the H+ ions combine with NADP+ to form NADPH

The production of ATP by chemiosmosis in photosynthesis Thylakoid compartment (high H+) Light Light Thylakoid membrane Antenna molecules Stroma (low H+) ELECTRON TRANSPORT CHAIN PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE Figure 7.9

30 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle
THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle occurs in the chloroplast’s stroma This is where carbon fixation takes place and sugar is manufactured INPUT CALVIN CYCLE Figure 7.10A OUTPUT:

31 The Calvin cycle constructs G3P using
carbon from atmospheric CO2 electrons and H+ from NADPH energy from ATP Energy-rich sugar is then converted into glucose

32 Details of the Calvin cycle
INPUT: 3 In a reaction catalyzed by rubisco, 3 molecules of CO2 are fixed. CO2 Step Carbon fixation. 1 1 3 P P 6 P RuBP 3-PGA 6 ATP 3 ADP Step Energy consumption and redox. 2 6 ADP + P 3 ATP CALVIN CYCLE 2 6 4 NADPH 6 NADP+ Step Release of one molecule of G3P. 3 5 P 6 P G3P G3P 3 Step Regeneration of RuBP. 4 Glucose and other compounds OUTPUT: 1 P G3P Figure 7.10B

33 7.11 Review: Photosynthesis uses light energy to make food molecules
PHOTOSYNTHESIS REVIEWED AND EXTENDED 7.11 Review: Photosynthesis uses light energy to make food molecules A summary of the chemical processes of photo-synthesis Chloroplast Light Photosystem II Electron transport chains Photosystem I CALVIN CYCLE Stroma Electrons Cellular respiration Cellulose Starch Other organic compounds LIGHT REACTIONS CALVIN CYCLE Figure 7.11

34 Many plants make more sugar than they need
The excess is stored in roots, tuber, and fruits These are a major source of food for animals

35 7.12 C4 and CAM plants have special adaptations that save water
Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle In these types of plants, stomata on the leaf surface close when the weather is hot This causes a drop in CO2 and an increase in O2 in the leaf Photorespiration may then occur

36 Photorespiration in a C3 plant
CALVIN CYCLE 2-C compound EXAMPLES: wheat, barley, potatoes and sugar beet. Figure 7.12A

37 Some plants have special adaptations that enable them to save water
Special cells in C4 plants—corn, crabgrass and sugarcane—incorporate CO2 into a four-carbon molecule This molecule can then donate CO2 to the Calvin cycle 4-C compound CALVIN CYCLE 3-C sugar Figure 7.12B


39 In C4 plants, the bundle sheath cells contain chloroplasts; carbon is fixed in mesophyll cells, then transported to bundle sheath cells where Calvin Cycle reactions occur in the absence of oxygen. 

40 The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism
They open their stomata at night and make a four-carbon compound It is used as a CO2 source by the same cell during the day 4-C compound Night Day CALVIN CYCLE 3-C sugar Figure 7.12C

7.13 Human activity is causing global warming; photosynthesis moderates it Due to the increased burning of fossil fuels, atmospheric CO2 is increasing CO2 warms Earth’s surface by trapping heat in the atmosphere This is called the greenhouse effect

42 Sunlight ATMOSPHERE Radiant heat trapped by CO2 and other gases Figure 7.13A & B

43 Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect
Unfortunately, deforestation may cause a decline in global photosynthesis

44 7.14 Talking About Science: Mario Molina talks about Earth’s protective ozone layer
Mario Molino received a Nobel Prize in 1995 for his work on the ozone layer His research focuses on how certain pollutants (greenhouse gases) damage that layer Figure 7.14A

45 The O2 in the atmosphere results from photosynthesis
Solar radiation converts O2 high in the atmosphere to ozone (O3) Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

46 International restrictions on these chemicals are allowing recovery
Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer International restrictions on these chemicals are allowing recovery Sunlight Southern tip of South America Antarctica Figure 7.14B

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