Photosynthesis

Light-Dependent Reactions

&Calvin Cycle

Section 8.2

Fig. 10-2

(a) Plants

(c) Unicellular protist10 µm

1.5 µm

40 µm(d) Cyanobacteria

(e) Purple sulfur bacteria

(b) Multicellular alga

• Photosynthesis occurs in plants, algae, some prokaryotes

• ENDERGONIC PROCESS-stores energy

• ANABOLIC PROCESS-Builds up molecules

BioFlix: PhotosynthesisBioFlix: Photosynthesis

Structures of Photosynthesis

• Leaves are the major locations of photosynthesis

• Chlorophyll - the green pigment within chloroplasts

• CO2 enters and O2 exits the leaf through microscopic pores called stomata

Fig. 10-3a

5 µm

Mesophyll cell

StomataCO2 O2

Chloroplast

Mesophyll

VeinLeaf cross section

• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

Component of a Chloroplast• Thylakoid – Saclike

photosynthetic membranes – Light-dependent

reactions occur here

• Granum – Stack of thylakoids

• Stroma – Region outside the thylakoid membrane– Reactions of the Calvin

Cycle occur here

DRAW THIS CHLOROPLAST DIAGRAM IN YOUR NOTES

The Photosynthesis Equation6 CO2 + 6 H2O + Light energy C6H12O6 + 6 O2

The Two Stages of Photosynthesis: A Preview

• The light reactions (in the thylakoids):

– Split H2O

– Release O2

– Reduce NADP+ to NADPH

– Generate ATP from ADP

• The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

• The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules (most importantly, glucose)

• Photosynthesis consists of the Light Reactions (the photo part) and Calvin Cycle (the synthesis part)

• Light-Dependent reaction

– Occurs in thylakoid

– Uses H2O and light to produce ATP, NADPH, and O2

– NADPH is an electron carrier

• Calvin cycle (Light-Independent Reaction)– Occurs in stroma– uses carbon dioxide, ATP, and

NADPH to produce sugars (aka food, glucose, carbohydrates, etc.)

Light-Dependent ReactionsElectron Transport Chain

• This stage of photosynthesis includes an electron transport chain (ETC). The molecules of the electron transport chain use high-energy electrons to push H+ ions from the stroma into the inner thylakoid space, producing ATP & NADPH in the process (Don’t worry about the details of the ETC). The ATP & NADPH are then used in the next stage, the light-independent reactions (Calvin Cycle).

Calvin

Cycle

Light

Fig. 10-5-1

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

Light

Fig. 10-5-2

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

Light

Fig. 10-5-3

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

Light

Fig. 10-5-4

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

[CH2O]

(sugar)

The light reactions convert solar (light) energy to the chemical energy of ATP and NADPH

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 10-7

Reflectedlight

Absorbedlight

Light

Chloroplast

Transmittedlight

Granum

• Chloroplasts are solar-powered chemical factories– Their thylakoids

transform light energy into the chemical energy of ATP & NADPH

– ATP &NADPH are then used to power the Calvin Cycle (light-independent reactions)

The Nature of Sunlight

• Light is a form of electromagnetic energy• The electromagnetic spectrum is the entire

range of electromagnetic energy, or radiation • Visible light consists of wavelengths (including

those that drive photosynthesis) that produce colors we can see

Wavelength is the distance between crests of wavesWavelength determines the type of electromagnetic energy

UV

Fig. 10-6

Visible light

InfraredMicro-waves

RadiowavesX-raysGamma

rays

103 m1 m

(109 nm)106 nm103 nm1 nm10–3 nm10–5 nm

380 450 500 550 600 650 700 750 nm

Longer wavelength

Lower energyHigher energy

Shorter wavelength

Light and Pigments

• Pigments – light absorbing chemicals

• Chlorophyll – principle pigment in plants– Chlorophyll a– Chlorophyll b– Carotenoids– Xanthophyll

Why do leaves change colors?

• Chlorophyll a

• Chlorophyll b

Why Do Leaves Change Color?

Why Do Leaves Change Color?

• Leaves change color in the Fall because chlorophyll begins to break down and disappear, exposing the other pigments – the carotenoids (orange, yellow, and reds) and xanthophylls (yellows).

Factors Affecting Photosynthesis

• Water supply

• Amount of sunlight

• Temperature

Types of Photosynthesis

• C3 Photosynthesis

• C4 Photosynthesis

• CAM Photosynthesis

C3 Photosynthesis : C3 plants.

• Called C3 because the CO2 is first incorporated into a 3-carbon compound.

• Stomata are open during the day. • Photosynthesis takes place throughout the leaf. • Adaptive Value: more efficient than C4 and CAM plants

under cool and moist conditions and under normal light because requires less machinery (fewer enzymes and no specialized anatomy)..

• Most plants are C3.

C4 Photosynthesis : C4 plants. • Called C4 because the CO2 is first incorporated into a 4-

carbon compound.

• Stomata are open during the day.

Adaptive Value: • Photosynthesizes faster than C3 plants under high light

intensity and high temperatures

• Has better Water Use Efficiency because they do not need to keep stomata open as much (less water lost by transpiration)

• C4 plants include several thousand species in at least 19 plant families. Examples: corn, sugar cane, grasses

CAM Photosynthesis : CAM plants. CAM stands for

Crassulacean Acid Metabolism • Stomata open at night (when evaporation rates are

usually lower) and are usually closed during the day. • Adaptive Value:

– Better Water Use under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).

• Examples: succulents such as cacti, pineapple, and also some orchids and bromeliads