CHAPTER 10

Photosynthesis

Sunlight as an Ultimate Energy Source

All living things need energy

Photosynthesis provides this energy Converts light energy into chemical energy

Acquired by either autotrophic or heterotrophic means

AutotrophsAutotrophs HeterotrophsHeterotrophs

Live without consuming anything from other living things Require water, soil

minerals, and CO2

Producers of the biosphere Photoautotrophs

Use light as energy sources

E.g. plants, algae, protists, and bacteria

Live on compounds produced by other organisms

Consumers of the biosphere Eat living organisms for

energy E.g. animals

Decomposers of the biosphere Breaks down dead organic

matter E.g. fungi

Anatomy of a Leaf

Stomata allow gas exchange

Veins move water from roots to leaves and sugars from leaves to roots

Chloroplasts, the site of photosynthesis, Located in the mesophyll or interior leaf tissue All green areas of

plants, concentrated in leaves

Chloroplasts

Double membrane bound organelle

Fluid filled space called the stroma

Contains multiple thylakoids, or interconnected membranous sacs Stacked into grana Chlorophyll pigment

within Gives plant characteristic

colors Captures energy for

photosynthesis

Equation of Photosynthesis

6CO2 + 6H2O + sunlight C6H1206 + 6O2

What color line is showing reduction? oxidation?

Redox Revisited

Cellular Respiration Energy from sugar as electrons from H to O2 = H2O Lose PE as fall to more electronegative oxygen Mitochondria use energy released

to make ATP

Photosynthesis H20 split and electrons to CO2 = sugar (reduction) Gain PE as bond complexity increases Requires energy = endergonic

Light provides boost

Photosynthesis: An Overview

Light reactions [photo part] Solar energy to chemical energy Light drives transfer of e -’s and H+

NADP+ NADPH (reduction or oxidation?) Create ATP using chemiosmosis to power

photophosphorylation NO sugar produced

Calvin cycle (dark reaction) [synthesis part] CO2 incorporated into organic molecules, carbon fixation

Add e -’s from NADPH and ATP to reduce into carbohydrates Makes sugar Doesn’t need light directly

Photosynthesis

Understanding Sunlight

Electromagnetic energy Exists as discrete packets of particles called photons

All wavelengths make up an electromagnetic spectrum Wavelengths are distance between crests of waves

and inversely related to amount of energy Visible light most important

to life Detectable by human eye Violet end is shortest waves Red end is longest waves

All combined = white light

Photosynthetic Pigments

Light can be reflected, transmitted, or absorbed

Chloroplasts vary in pigments Chlorophyll a, b, and

carotenoids Violet-blue and red light

most efficient for photosynthesis

Carotenoids have role in photoprotection In human eye too

Action spectrum

Excitation of Chlorophyll

Absorption of light elevates electrons of pigments to higher orbital ( PE) Pigments absorb in

specific rangeUnstable in upper

orbital so ‘fall’ back quickly Releases energy as heat

White vs black cars or clothing in the South

PhotosystemsProtein complex with a

reaction center surrounded by light-harvesting complexes Chlorophyll a always bound

with reaction center molecules

Other pigments with light-harvesting complexes Gather light from larger

surfacesPigments absorb photons

and transfer to reaction center complex

Electrons transferred to primary electron acceptor, reducing it

Two types, II and I

Light Reaction

Occurs in the thylakoids

Two Photosystems PS I absorbs at 700nm PS II at 680nm

Two electron flow patterns Linear electron flow Cyclic

Linear Electron Flow

To Calvin cycle

Comparing Chemiosmosis

Similarities ETC in membranes

pump protons across as e-’s moved to more EN carriers

ATP synthase utilizes [H+ gradient]

Differences M: e-’s from organics,

protons move out P: e-’s from H2O,

protons move in

Calvin Cycle

Anabolic reaction in the stroma

Products from light reaction are reactants for dark

(3) CO2 molecules combine to create (1) 3 carbon sugars (glyceraldehyde 3-phosphate, G3P) Cycle must occur 3 times

for 1 molecule to be madeBroken into 3 steps

Carbon fixation Reduction Regeneration of CO2

acceptor (RuBP)

CO2

RuBP

G3P

G3P G3P

3PG

Carbon Fixation

1 CO2 into stromaAttaches to ribulose bisphosphate (RuBP), a 5

carbon sugarCatalized by rubisco

Most abundant protein on Earth

Forms unstable 6 carbon molecule Immediately to (2) 3-phosphoglycerate (3PG) 2 for every 1 CO2 molecule

Reduction

3PG gains a phosphate from ATP to create 1,3-bisphosphoglycerate

NADPH reduces 1,3-bisphosphoglycerate to G3P

3 cycles (3 CO2’s) create 6 G3P Only 1 leaves (3 carbons out) Other 5 recycled (15 carbons remain)

Regeneration of CO2 Acceptor

5 G3P are rearranged into 3 RuBP (5 carbons each) Cost 3 ATP

Capable of accepting CO2 againOverall cost of cycle

9 ATP 6 NADPH 3 CO2

2 G3P to make sugars and other fuels

Review of Photosynthesis