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

Light can influence the architecture of a plant Plants that get adequate light are often bushy, with deep green leaves (filled with the photosynthetic machinery-chloroplasts)

7.1 Autotrophs are the producers of the biosphere Plants, some protists, and some bacteria are photosynthetic autotrophs. (Animals are not) They are the ultimate producers of food consumed by virtually all organisms

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

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

AN OVERVIEW OF PHOTOSYNTHESIS 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

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

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

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

7.2 Photosynthesis occurs in chloroplasts In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts

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

A chloroplast contains: stroma, a fluid between grana grana, stacks of thylakoids The thylakoids contain chlorophyll Chlorophyll is the green pigment that captures light for photosynthesis CHLOROPLAST Intermembrane space Outer membrane Granum Inner membrane Thylakoid compartment Stroma Thylakoid

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

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

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

Reflected light Light Chloroplast Absorbed light Transmitted light Figure 7.6B

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

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

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

Primary electron acceptor Electron transport chain Excitation of chlorophyll in a chloroplast Primary electron acceptor Electron transport chain Photon Chlorophyll molecule Figure 7.7B

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

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

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

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

ELECTRON TRANSPORT CHAIN 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

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:

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

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

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

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 drop in G3P

Some plants have special adaptations that enable them to save water Special cells in C4 plants—corn 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

PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE 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

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

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

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

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

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