chapter 7

Copyright © 2011 Pearson Education Inc. Biology: Life on Earth, 9e

Chapter 7 Capturing Solar Energy: Photosynthesis


What Is Photosynthesis?

For most organisms, energy is derived from sunlight, either directly or indirectly

Those organisms that can directly trap sunlight do so by photosynthesis

Photosynthesis is the process by which solar energy is trapped and stored as chemical energy in the bonds of a sugar– In water – protists and certain bacteria– On land – plants


An Overview of Photosynthetic Structures

cuticle

(b) Internal leaf structure

upperepidermis

mesophyllcells

lowerepidermis

chloroplastsstomabundle sheath cells

vascular bundle(vein)

stoma

outer membraneinner membranethylakoidstroma

channelinterconnectingthylakoids

(d ) Chloroplast

(a) Leaves

(c) Mesophyll cell containing chloroplasts

Fig. 7-1



Leaves and chloroplasts are adaptations for photosynthesis in plants– Leaves are flat and thin for best light penetration– Takes place in chloroplasts contained within leaf cells– Both the upper and lower surfaces of a leaf consist of a

layer of transparent cells, the epidermis


What Is Photosynthesis? Leaves and chloroplasts are adaptations for

photosynthesis in plants– The outer surface of both epidermal layers is covered by

the cuticle, a transparent, waxy, waterproof covering that reduces the evaporation of water from the leaf

– Leaves obtain CO2 for photosynthesis from the air through pores in the epidermis called stomata (singular, stoma)



Leaves and chloroplasts are adaptations for photosynthesis– Inside the leaf are layers of cells called the mesophyll,

where the chloroplasts are located and where photosynthesis occurs

– Bundle sheath cells surround the vascular bundles, which form veins in the leaf and supply water and minerals to the mesophyll



Leaves and chloroplasts are adaptations for photosynthesis – Chloroplasts are organelles with a double membrane

enclosing a fluid called the stroma– Embedded in the stroma are disk-shaped membranous

sacs called thylakoids

– Reactions that depend on light take place in the thylakoids

– Reactions of the Calvin cycle that capture carbon dioxide and produce sugar occur in the stroma



Photosynthesis consists of the light reactions and the Calvin cycle– Starting with carbon dioxide (CO2) and water (H2O),

photosynthesis converts sunlight energy into chemical energy stored in bonds of glucose and releases oxygen (O2) as a by-product

6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2

carbon water sunlight glucose oxygendioxide (sugar)


An Overview of the Relationship Between the Light Reactions and the Calvin Cycle

Fig. 7-3



Photosynthesis consists of the light reactions and the Calvin cycle – In the light reactions, chlorophyll captures light energy

and converts some into energy-carrier molecules ATP and NADPH. Water is split releasing O2

– In the reactions of the Calvin cycle, enzymes in the stroma use CO2 from the air and chemical energy from the energy-carrier molecules to synthesize a three-carbon sugar that will be used to make glucose


Light Reactions: Light Energy Converted to Chemical Energy Light is captured by pigments in chloroplasts

– The sun emits energy within a broad spectrum of electromagnetic radiation – This electromagnetic spectrum ranges from short-

wavelength gamma rays, through ultraviolet, visible, and infrared light, to long-wavelength radio waves


Light is captured by pigments in chloroplasts– Light is composed of individual packets of energy called

photons– Visible light has wavelengths with energies strong

enough to alter biological pigment molecules such as chlorophyll a

– Chlorophyll a is a key light-capturing pigment molecule in chloroplasts, absorbing violet, blue, and red light– Green light is reflected, which is why leaves appear

green

Light Reactions: Light Energy Converted to Chemical Energy


Light is captured by pigments in chloroplasts– Accessory pigments, that absorb additional

wavelengths of light energy and transfer them to chlorophyll a– Chlorophyll b - absorbs blue and red-orange light,

and appear yellow-green– Carotenoids - absorb blue and green light, and appear

yellow or orange– In autumn, more-abundant, green chlorophyll breaks

down before the carotenoids do, revealing their yellow color, which in summer is masked



The light reactions occur in association with the thylakoid membranes– Contain many photosystems

– each consists of a cluster of chlorophyll and accessory pigment molecules surrounded by various proteins

– These electron transport chains (ETC) each consist of a series of electron carrier molecules embedded in the thylakoid membrane



The hydrogen ion gradient generates ATP by chemiosmosis– The energy of electron movement through the thylakoid

membrane creates an H+ gradient that drives ATP synthesis in a process called chemiosmosis

– The generation of ATP ADP + phosphate resembles the electrical energy obtained from water flowing downhill and driving an electrical turbine



Energy is released aswater flows downhill

Energy is harnessed torotate a turbine

The energy of therotating turbine is usedto generate electricity

1

2

3

Energy Stored in a Water “Gradient” Can Be Used to Generate Electricity

Fig. 7-8


H+ are pumped to the thylakoid space

ATPsynthase

photosystem Iphotosystem II

thylakoidmembrane

lightenergy

+P

ATP

NADP+

ADP

NADPH

Calvincycle

CO2

C6H12O6

sugare–

e–

e–

e–

e–e–

1/2

2H2O

chloroplast

electron transport chain IIelectrontransportchain I

(stroma)

(thylakoid space)

thylakoid

O2

High H+ concentration is created

Flow of H+ down theirconcentration gradientpowers ATP synthesis

H+

H+ H+

H+

H+H+

H+

H+

H+

H+

H+

1

23

Fig. 7-7

Events of the Light Reactions


Review

1. Why is photosynthesis important?

2. What is the basic equation for photosynthesis?

3. What is the main light-capturing molecule in chloroplasts?

4. What are the two main end products from the light

reactions?

5. How and where are they created?


The Calvin Cycle: Chemical Energy Stored in Sugar Molecules The Calvin cycle captures carbon dioxide

– ATP and NADPH synthesized from light reactions are used to power the synthesis of a simple sugar (gyceraldehyde-3-phosphate, or G3P)

– A series of reactions occurring in the stroma– In reactions that occur outside the Calvin cycle, two G3P

molecules can be combined to form one six-carbon glucose molecule

– Glucose may then be converted to the disaccharide sucrose or linked to form starch (a storage molecule) or cellulose (a major component of plant cell walls)


C3CO2

C C C6PGA

NADP+

ATP

ADP

NADPH

6

6

6

6

C C C1G3P

C C C5G3P

ATP

ADP

3

3

C C C3RuBP

C C C1G3P

+ C C C1G3P

C C C1glucose

C C C

C C C6G3P

Calvincycle

Energy from ATPand NADPH is used to convert the sixmolecules of PGA tosix molecules of G3P

Carbon fixationcombines three CO2

with three RuBP usingthe enzyme rubisco

Using the energyfrom ATP, five of the six molecules of G3Pare converted to threemolecules of RuBP 4 One molecule of

G3P leaves the cycle

Two molecules of G3P combine toform glucose and other molecules

C C

5

3

4

2

1

The Calvin Cycle Fixes Carbon from CO2 and Produces G3P Fig. 7-9

NADP

ATP

ADPNADPH

Calvincyclelight

reactions

H2O

O2

CO2

C6H12O6

sugar

chapter 7

Documents

photosynthesis chloroplasts

pearson education

sunlight energy

chemical energy

solar energy

h2o light energy c6h12o6

water h2o

light reactions