How Cells Acquire Energy

Chapter 6

Light is central to the life of a plantPhotosynthesis is the most important

chemical process on Earth It provides food for

virtually all organismsPlant cells convert light

into chemical signals that affect a plant’s life cycle

Life in the Sun

PhotosynthesisAn Overview

Chloroplasts are made up of; Stroma -liquid

Thylakoids- disks containing chlorophyll

Two stages of Photosynthesis Light-dependent reactions - need

sunlight

Light-independent reactions- do not need light

Figure 7.3B

Figure 7.3C

Experiment 1

Experiment 2

Notlabeled

Labeled

Reactants:

Products:

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

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

Energy and Materials for the Reactions

Light Dependent Sunlight energy is absorbed and

converted to ATP energy

Water molecules are split - releasing oxygen

NADP (coenzyme and electron carrier) picks up hydrogen and electrons to form NADPH

Energy and Materials for the Reactions

Light-Independent ATP donates energy to the cycle

NADP donates hydrogen

CO2 donates Carbon and Oxygen

Glucose (C6H12O6) made from CO2 and Hydrogens

Different sites in chloroplast Stroma

Granum (in stroma) - stacks of thylakoids

Absorption Spectra

2 kinds of chlorophyll absorb wavelengths of visible light

The Chemical Basis of Color

Pigments Chlorophylls a

& b-green

Carotenoids-orange

Phycobilins - yellow

What Happens to the Absorbed Energy?

Light energy is passed from pigment to pigment until it reaches the reactioh center

Light-Dependent Reactions

In Thylakoid membrane (granum) Pigments absorb photon energy

Transfer of electrons and hydrogen through electron transport system

ATP and NADPH formation

Replacement of electrons to pigments

Cyclic and Noncyclic Electron Flow

Cyclic Pathway

P700

Type I photosystem

Electron transfer creates ATP only

Noncyclic Pathway

P680 Electrons not cycled

Type II and I photosystems

Electrons from water molecules passed along

Photolysis- breaking water

ATP and NADPH made

ATP Formation in Chloroplasts

ATP Formation in Chloroplasts

ATP Formation in Chloroplasts

Oxygen released from photolysis of water

H+ concentration and electric gradients form across the thylakoid membrane

Flow of ions from thylakoid compartment into the stroma drives ATP formation12H20 + 6CO2 ---> 602 + C6H12O6 + 6H2O

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

Light-Independent Reactions

“Synthesis” of sugar

Calvin-Benson Cycle

Calvin-Benson Cycle

Carbon Fixation RuBP (ribulose biphosphate) attaches to

carbon dioxide

RuBP carboxylase ( RUBisCO)-enzyme that puts together RuBP and carbon dioxide

PGA (Phosphoglyceraldehyde) - The 3 carbon sugar produced

Stroma Diffusion of CO2 and O2 through pores called

stomates

Fixing Carbon

C3 Plants -normal photosynthesis Kentucky blue

grass, crops

C4 Plants- hide the carbon dioxide in a 4 carbon sugar deeper in the leaf so when they need carbon dioxide they have it Crabgrass

Fixing CarbonThe C3 Pathway

Evergreen trees and shrubs

and nonwoody plants of

temperate zones

Fixing CarbonThe C4 Pathway

Grasses and othertropical plants - Corn, crabgrass,

and sugarcane

Fixing Carbon CAM Plants

Hot dry climates -cacti, pineapple,

orchids, and Succulents

Stomates only open at night

Autotrophs, Humans, and The Biosphere

Satellite imagesRed-orange signifies chlorophyll concentrations

In Conclusion

Plants and other autotrophs use carbon dioxide as their source of carbon and sunlight as the energy source

Photosynthesis is the main process by which carbon and energy enter the web of life

Photosynthesis has two stages: the light-dependent and the light-independent reactions

In Conclusion

Light-dependent reactions proceed in the thylakoids

Light-independent reactions occur in the stroma

Chlorophyll a is the main photosynthetic pigment

Accessory pigments absorb wavelengths that chlorophyll a cannot

In Conclusion

In chloroplasts, photosynthetic pigments are part of photosystems

Thylakoid membranes have photosystems I & II

The light-independent reactions in the stroma deal with the Calvin-Benson cycle

In Conclusion

Summary of photosynthesis

In Conclusion

Photorespiration predominates in C3

plants

C4 plants fix carbon twice and CAM plants

fix carbon at night

developed by M. Roig