photosynthesis · • photosynthesis – capturing solar energy to convert to chemical energy...

Photosynthesis Capturing solar energy (sunlight) to convert it to chemical energy stored in food

Photosynthesis Outline

•  I. What is photosynthesis? •  II. Where does photosynthesis occur? •  III. What is the structure of a chloroplast? •  IV. What are the steps of photosynthesis?

Types of Reactions

• Endergonic - energy is absorbed during the reaction (energy enters)

• Exergonic - energy is released during the reaction (energy exits)

Reactants

Products

Reactants

Products

I. What is photosynthesis?

Light Energy

Chloroplast

CO2 + H2O Sugars + O2

6CO2 + 6H2O -----------------------------> C6H12O6 +6O2 + enzymes, chlorophyll

I. What is photosynthesis? Photosynthesis is… Exergonic or Endergonic?

I. What is photosynthesis (PS)? •  Life on earth is powered by sunlight ▫  Plants, algae, and bacteria produce energy by PS

•  PS occurs in the chloroplasts: light reactions in the thylakoid membranes and dark reactions in stroma

•  Two stages of PS ▫  1. Light-dependent reactions: store solar energy in

molecules of ATP and NADPH, O2 is produced from H2O ▫  2. Calvin cycle (light-independent reactions): use ATP

and NADPH to drive synthesis of organic molecules (sugars) from CO2 in air (carbon fixation)

What exactly is ATP?

• ATP = Adenosine triphosphate = an energy carrier • When the cells need energy, they use/break ATP • ATP ADP + P + Energy


What exactly is ATP?

• When the cells store energy, they make ATP • ADP + P + Energy ATP


•  Phosphate groups are negatively charged •  Repulsion acts as potential energy – like a compressed spring •  When unstable bond between 2nd and 3rd phosphate broken, energy is released

How is ATP used? •  As ATP is broken down, it

gives off usable energy to power chemical work

•  Synthesizes molecules for growth and reproduction

•  Transport work – active transport, endocytosis, and exocytosis

•  Mechanical work – muscle contraction, cilia and flagella movement, organelle movement

What is NADPH?

• NADPH acts as a shuttle for high-energy electrons

•  Think of it as an energy carrier • More on NADPH later…

II. Where does photosynthesis occur? •  Leaves!

Why broad?

Fun fact! ½ million chloroplasts/mm2

(singular = stoma)

II. Where does photosynthesis occur?

Zoom in of leaf

Cuticle

Guard cell

II. Where does photosynthesis occur?

• Plant leaves (& green parts) ▫  Specifically, in mesophyll cells (these contain

chloroplasts)

• Stomata - openings allow gases to move in (carbon dioxide) and out (oxygen)

• Veins – xylem brings water to the leaves from the roots; phloem carries away sugar

How do other leaf parts help?

Plant Stomata

Close-up View of a Stoma

(guard cells are mostly hidden)

PS Recap

•  Photosynthesis – Capturing solar energy to convert to chemical energy stored in organic compounds such as sugar

•  *Autotrophs make their own food, but aren’t 100% self-sufficient, need inorganic materials (CO2 and H2O)

•  Site of photosynthesis: ▫  Green parts → leaf → mesophyll → chloroplast

• Helpful leaf adaptations: ▫  Broad leaf, stomata, cuticle, less packed spongy

layer, proximity of vein to mesophyll

III. What is the structure of a chloroplast?

/ space

III. What is the structure of a chloroplast?

• Two membranes surround: ▫ Stroma: fluid found within the chloroplast ▫ Thylakoid Disks: (inside = thylakoid

space); membranes contain chlorophyll, the green light-capturing pigment ▫ A stack of thylakoid disks is called a

granum

Chloroplast H2O

O2 3-C Sugars

CO2

Light- Dependent Reactions

Calvin Cycle

NADPH ATP

ADP + P NADP+

Chloroplast Go to Section:

IV. What are the steps of photosynthesis?

Note: The Calvin Cycle is part of the light-independent reactions.

Light energy

(grana) (stroma)

Plants Produce O2 by Splitting H2O

Experiment 2

Reactants:

Products:

Experiment 1

Not labeled

Experiment 2

Labeled

Photosynthesis is a Redox Process •  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

Reduction

Oxidation

“LEO says GER” or OIL RIG Losing Electrons = Oxidation [oxygen] Gaining Electrons = Reduction [glucose]

Photosynthesis: Redox Redux

• H2O molecules are split using light energy ▫  H2O is oxidized ▫  Loses high-energy electrons and H+ ions ▫  NADP+ (an electron carrier) picks up these

electrons and H+ ions and becomes NADPH • ATP powers several steps in Calvin cylce • NADPH carries electrons that are used to reduce

carbon dioxide • Electrons are gained so CO2 is reduced

Step 1: Light Dependent Reactions (thylakoid: light energy → chemical energy)

1.  Light-dependent Reactions -- require light! 2.  Remember: visible light = many colors 3.  Light can be reflected, transmitted, or absorbed 4.  Pigments can absorb light of certain

wavelength (colors) better than others

Step 1: Light Dependent Reactions 1.  Thylakoid membrane houses photosynthetic

pigments that capture light energy to make ATP 2.  Most important pigments = chlorophylls

 What is being absorbed?

 What is being reflected?

 How can you relate this to plant color?

Light

Chloroplast

Reflected light

Absorbed light

Transmitted light

What’s so special about chlorophyll?

• When surrounded by other chlorophyll molecules, carotenoids, and proteins → photosystem (there are 2)

•  Light energy absorbed → transferred from chlorophyll to chlorophyll

Primary electron acceptor

Photon

Reaction center

PHOTOSYSTEM

Pigment molecules of antenna

What’s so special about chlorophyll?

• A specially positioned chlorophyll molecule has electrons that it will “give up” to a primary electron acceptor when struck by absorbed light

• Only some wavelengths of light cause the electrons to get excited (have more energy than before)

Primary electron acceptor

Other compounds

Chlorophyll molecule

Photon

Photosystem Recap

•  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

Where do the excited electrons go?

•  Primary electron acceptor passes electrons (e-) to an electron transport chain (ETC)

How do we replace lost electrons?

•  Wait, these e- need to be replaced so the ETC can keep going.

•  How do e- get replaced? ▫  Get e- from water ▫  2H2O → 4H+ +4e- + O2

▫  4e- replace e- in chlorophyll ▫  O2 diffuse out stoma ▫  4H+ stay in thylakoid

What happens in the ETC?

•  High energy electrons move through ETC and lose energy

•  This energy is used to actively transport H+ against its concentration gradient

How does ATP get made?

•  Located in the membrane near the photosystems is an enzyme called the ATP synthase

•  ATP synthase acts as a carrier protein, allowing H+ ions to diffuse down their concentration gradient, releasing energy

•  ATP synthase uses energy to create ATP

What happens in the 2nd photosystem?

•  Low energy e- from photosystem II replace lost excited e- in next photosystem

• Excited e- move through another, different ETC

•  e- at end of ETC is transferred to NADP+ to make NADPH

How ATP is Made - Recap

•  The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane ▫  The flow of H+ back through the membrane is

harnessed by ATP synthase to make ATP ▫  In the stroma, the H+ ions combine with NADP+ to

form NADPH

ATP Production by Chemiosmosis

Thylakoid compartment (high H+)

Thylakoid membrane

Stroma (low H+)

Light

Antenna molecules

Light

ELECTRON TRANSPORT CHAIN

PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

Hydrogen Ion Movement

Photosystem II

Inner Thylakoid Space

Thylakoid Membrane

Stroma

ATP synthase

Electron Transport Chain Photosystem I ATP Formation

Chloroplast

Summary of Light Reactions

Step 2: The Calvin Cycle (Light-independent rxns in stroma: CO2 to sugar)

•  The dark reactions: the Calvin Cycle •  uses 3 CO2 to build a 3-carbon carbohydrate (an

organic molecule) = G3P •  Needs electrons and H+ from NADPH

•  Is this energy-consuming or energy-releasing? •  remember that we have ATP, a high-energy

molecule from the Light Reactions, to “power” the Calvin cycle

•  So, it is energy consuming •  Carbon fixation = initial incorporation of CO2

into organic molecules (sugar molecules)

The major point of the Calvin cycle is to form new C-C bonds from piecing together CO2 molecules. Making these bonds takes energy and electrons.

1)  Carbon fixation 2)  Energy consumption and redox

3)  Release of G3P 4)  regeneration of RuBP

Photosynthesis Summary •  Light-dependent reactions – thylakoid membrane ▫  Energy absorbed from sunlight, exciting e- in

chlorophyll – lost to 1st ETC ▫  Chloroplast e- replaced by splitting H2O: e-, O2, and H+

▫  Oxygen diffuse out stomata, H+ pumped across membrane in ETC ▫  1st ETC e- replace lost chlorophyll e- in 2nd photosystem ▫  e- move through different ETC to combine with H+ to

make NADPH • Calvin cycle (light-independent) – stroma ▫  CO2 combine to make organic 3-C compound ▫  ATP energy used to drive addition of H+ from NADPH

Photosynthesis Summary

Light

Chloroplast

Photosystem II Electron transport

chains Photosystem I

CALVIN CYCLE Stroma

LIGHT REACTIONS CALVIN CYCLE

Cellular respiration Cellulose Starch

Other organic compounds

Factors that affect photosynthesis •  Light intensity ▫  ↑ light intensity, ↑ rate of PS ▫  More e- become excited, more rapid PS ▫  Eventually all e- excited, max rate

• Carbon dioxide levels ▫  ↑ carbon dioxide, ↑ rate of PS ▫  Max rate will eventually be reached

•  Temperature ▫  ↑ temperature, ↑ rate of chemical rxns ▫  Rate peaks at point where enzymes start

to become ineffective – denaturation! ▫  Stomata close – limit H2O loss and CO2

entry

What do plants do with sugar?

•  50% used for glucose in cellular respiration! ▫  C6H12O6 + 6O2 --> 6H2O + 6CO2 + ATP

• Made into sucrose, glucose, cellulose •  Store the extra sugar in… ▫  Starch ▫  Maple syrup ▫  Sap ▫  Fruits ▫  Roots ▫  Tuber

photosynthesis · • photosynthesis – capturing solar energy to convert to chemical energy...

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