Photosynthesis

BIOLOGYCONCEPTS & CONNECTIONS

Fourth Edition

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor

From PowerPoint® Lectures for Biology: Concepts & Connections

CHAPTER 7Photosynthesis:

Using Light to Make Food


• 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

Life in the Sun


• 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

AN OVERVIEW OF PHOTOSYNTHESIS

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS


• Plants, some protists, and some bacteria are photosynthetic autotrophs

– They are the ultimate producers of food consumed by virtually all organisms

7.1 Autotrophs are the producers of the biosphere


• 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


• 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

7.2 Photosynthesis occurs in chloroplasts


• The location and structure of chloroplasts

Figure 7.2

LEAF CROSS SECTION MESOPHYLL CELL

LEAF

Chloroplast

Mesophyll

CHLOROPLAST Intermembrane space

Outermembrane

Innermembrane

ThylakoidcompartmentThylakoidStroma

Granum

StromaGrana


• Stomata, open and closed

• Site of gas exchange O2, H2O and CO2

Figure 7.2x2


• The O2 liberated by photosynthesis is made from the oxygen in water molecules

7.3 Plants produce O2 gas by splitting water

Figure 7.3A


Figure 7.3B

Figure 7.3C

Experiment 1

Experiment 2

Notlabeled

Labeled

Reactants:

Products:


• 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

7.4 Photosynthesis is a redox process, as is cellular respiration

Figure 7.4A

Figure 7.4B

Reduction

Oxidation

Oxidation

Reduction


• The complete process of photosynthesis consists of two linked sets of reactions:

– the light reactions (the ‘photo’ part)

– and the Calvin cycle (the ‘synthesis’ part)

• The light reactions convert light energy into chemical energy and produce O2

• The Calvin cycle assembles sugar molecules from CO2 using the high energy products of the light reactions

7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH


• An overview of photosynthesis

Figure 7.5

Light

Chloroplast

LIGHTREACTIONS

(in grana)

CALVINCYCLE

(in stroma)

Electrons

H2O

O2

CO2

NADP+

ADP+ P

Sugar

ATP

NADPH


• Certain wavelengths of visible light drive the light reactions of photosynthesis

7.6 Visible radiation drives the light reactions

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY

Gammarays

X-rays UV Infrared Micro-waves

Radiowaves

Visible light

Wavelength (nm)Figure 7.6A


Figure 7.6B

Light

Chloroplast

Reflectedlight

Absorbedlight

Transmittedlight


• 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

7.8 In the light reactions, electron transport chains generate ATP, NADPH, and O2


• The Calvin cycle occurs in the chloroplast’s stroma

– This is where carbon fixation takes place and sugar is manufactured

7.10 ATP and NADPH power sugar synthesis in the Calvin cycle

THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS

INPUT

Figure 7.10A OUTPUT:

CALVINCYCLE


• 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


Figure 7.10B

• Details of the Calvin cycle INPUT:

Step Carbon

fixation.

In a reaction catalyzed by rubisco, 3 molecules of CO2 are fixed.

11

Step Energy

consumption

2

3 P P P6

6

2

ATP

6 ADP + P

6 NADPH

6 NADP+

6 P

G3P

Step Release of one

molecule of G3P.

3

CALVINCYCLE

3

OUTPUT: 1 PGlucoseand other compounds

G3P

Step Regeneration

of RuBP.

4

G3P

4

3 ADP

3 ATP

3CO2

5 P

RuBP 3-PGA


• A summary of the chemical processes of photo-synthesis

7.11 Review: Photosynthesis uses light energy to make food molecules

PHOTOSYNTHESIS REVIEWED AND EXTENDED

Figure 7.11

Light

Chloroplast

Photosystem IIElectron transport

chains Photosystem I

CALVIN CYCLE Stroma

Electrons

LIGHT REACTIONS CALVIN CYCLE

Cellular respiration

Cellulose

Starch

Other organic compounds


• 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


• 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

• No sugar or ATP

7.12 C4 and CAM plants have special adaptations that save water


• Photorespiration in a C3 plant

CALVIN CYCLE

2-C compound

Figure 7.12A


• Some plants have special adaptations that enable them to save water

CALVIN CYCLE

4-C compound

Figure 7.12B

– 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

3-C sugar


• The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism

CALVIN CYCLE

4-C compound

Figure 7.12C

– 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

3-C sugar

Night

Day


• 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

7.13 Human activity is causing global warming; photosynthesis moderates it

PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH’S ATMOSPHERE


Figure 7.13A & B

Sunlight

ATMOSPHERE

Radiant heat trapped by CO2 and other gases


• Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect

– Unfortunately, deforestation may cause a decline in global photosynthesis


• 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

7.14 Talking About Science: Mario Molina talks about Earth’s protective ozone 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


• Industrial chemicals called CFCs have hastened ozone breakdown, causing dangerous thinning of the ozone layer

Figure 7.14B

Sunlight

Southern tip of South America

• International restrictions on these chemicals are allowing recovery

Antarctica

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Photosynthesis

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