Photosynthesis

Photosynthesis

Photosynthesis

• The process by which plants, algae, and some microorganisms harness solar energy and convert it into chemical energy.

• Endergonic reaction• Redox reaction• Only done by autotrophs

• Glucose used for: fuel own plant respiration (50%), growth, make other important compounds (amino acids, cellulose, starch, sucrose)

6CO2 + 6H2O C6H12O6 + 6O2

Photosynthesis Equation

Overall equation: 6CO2 + 6H2O C6H12O6 + 6O2

Photosynthesis is a Redox ReactionRedox-reactions

Photosynthesis/Respiration Cycle

Cellularrespiration

CO2 and H2O

ATP (available for cellular tasks)

Heat energy

Atmospheric Oxygen

Light

• Light is the source of energy for photosynthesis– Made of photons—packets of kinetic energy– Part of electromagnetic spectrum– 3 types from the sun get to the earth• Ultraviolet • Visible• Infrared

Light

Pigments

• Pigment—Substance that absorbs light energy• Several types of pigments:– Chlorophyll a—most abundant, green pigment,

absorb blue/red, reflect green– Accessory Pigments:• Chlorophyll b—absorb blue/red, reflect green• Carotenes—absorb blue, reflect orange/red• Xanthophylls—absorb purple/blue/ green, reflect

yellow

PigmentsPigment Color Organisms

Major PigmentChlorophyll a

green (or yellow) plants, algae, bacteria

Accessory PigmentChlorophyll b

yellow plants, algae

Carotenoids (xanthophylls and carotenes)

orange, red, yellow plants, algae, bacteria, archaea

Structure of a leaf

Structure of a leaf

Chloroplasts

• Mainly found in cells in the LEAF– Lots of surface area to absorb light– Has abundant water– Main site of gas exchange• Exchange occurs through stomata surrounded by guard

cells

– Mainly located in mesophyll

Chloroplasts

Chloroplasts

Chloroplasts

• Stroma—inner fluid with DNA, ribosomes, fluid• Grana—Stacks of thylakoid• Thylakoid—Disks, membranes with photosynthetic

pigments• Photosystem—in thylakoid membrane– Chlorophyll a (approx. 300 molecules)

• Reaction Center

– Accessory pigments (approx. 50 molecules)• Antenna pigment to funnel light to reaction center

– Proteins

Chloroplasts

Chloroplasts

Photosynthesis Overview

• Happens in 2 stages– Light Reactions—convert solar energy into

chemical energy• Occurs in thylakoid membrane

– Carbon Reactions—use ATP and NADPH to reduce CO2 to glucose• Occurs in the stroma



Light Reactions: Photosystems

What happens if you could capture this energy?


Plants capture this energy!


The Light Reactions

• Photosystem II– Pigment molecules absorb light and transfer to reaction center

(chlorophyll a)– Water is split into 2H+ and ½ O2

– Water donates 2 electrons– Energy “excites” 2 electrons to a higher energy orbital– Chlorophyll a ejects “excited” electrons to first electron

transport chain (ETC)– ETC makes a proton gradient from stroma into the thylakoid

space– ATP synthase uses proton gradient to make ATP (chemiosmotic

phosphorylation)• Used in carbon reactions

The Light Reactions—Photosystem II

ATP Generation—Photosystem II

The Light Reactions

• Photosystem I – Pigment molecules absorb light and transfer to

reaction center (chlorophyll a)– 2 electrons come from first ETC– Energy “excites” 2 electrons to a higher energy

orbital– Chlorophyll a ejects “excited” electrons to first

electron transport chain (ETC)– Electrons are passed to NADP+ to reduce it to

NADPH (used in carbon reactions)

The Light Reactions—Photosystem I

NADPH Generation—Photosystem I

Making ATP: PhotophosphorylationH+ gradient: as electrons moved within membrane, H+ is pumped into the thylakoid space

Making ATP: Photophosphorylation

ATP produced: ATP synthase allows H+ to go down its concentration gradient, generates ATP

ETC vs. Photophosphorylation

Energy source Final Electron Acceptor

The Carbon Reactions

• Also known as: Calvin Cycle, “Dark reactions”• Occurs in the stroma• Uses ATP and NADPH to make glucose from CO2

• Calvin Cycle:– Step 1: Carbon fixation—incorporation of CO2 into an

organic molecule• CO2 combines with RuBP, using enzyme called rubisco

– Step 2: PGAL Synthesis– Step3: PGAL makes glucose– Step 4: Regeneration of RuBP

Calvin Cycle

C3 Plants

• Calvin Cycle = C3 Pathway

• All plants use Calvin Cycle, but some plants ONLY use C3 pathway– 95% of plants are this way

• Inefficient—lose some energy to heat– 30% on the best sunny day– In Photorespiration rubisco uses O2 instead of CO2 as a substrate

– Stomates open, O2 diffuses out, CO2 is used

– Hot dry climates, stomates cannot stay open—lost water, O2

builds up, photorespiration takes over

C4 Plants

• C4—adaptation to help minimize photorespiration (1% of plants)

• C4 Plants—Separate light reactions and Calvin Cycle into different cells– Light reactions and carbon fixation—mesophyll– CO2 combines with 3 carbon molecule to make 4 carbon—

C4

– C4—(malate) moves to bundle sheath cells, rest of Calvin Cycle

• Bundle sheath cells NOT exposed to O2

C3 and C4 Plant AnatomyC4 plantC3 plant

Vein

Stoma

Mesophyll cell

Bundle-sheath cell

Mesophyll cell

Stoma

Vein

Bundle-sheath cell

CAM Plants

• Occurs in desert plants (3–4% of plants)• Only open stomates at night to fix CO2, then

fix again during the day using Calvin Cycle– Store night time CO2 as malate in vacuoles

– Stomates open, malate to chloroplast, release CO2, used in Calvin Cycle

• Happens in same cells

Pathway

C3 plant C4 plant CAM plant

Global WarmingGreen house effect: radiant heat trapped by CO2

Global Warming

Ozone Depletion

Ozone: O3 (O2 converted to O3) filters out UV rays

Ozone Depletion

Chlorofluorocarbons: release chlorine which destroys ozone

Antarctica

Ozone Depletion

Chlorofluorocarbons: release chlorine which destroys ozone

Antarctica

Science

Photosynthesis