Photosynthesis

Photosynthesis

• Process by which plants, algae, and some microorganisms use solar energy and CO2 and convert it into chemical energy

• Endergonic• Redox reaction• 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 is a Redox Reaction

Cellular Respiration

Photosynthesis

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

The Electromagnetic Spectrumand Visible 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

Pigments

Pigment 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

Absorption Spectrum of Pigments

Chloroplasts

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

photosynthetic pigments• Photosystem – in thylakoid membrane, absorbs

light and converts it to usable energy– Chlorophyll a (approx. 300 molecules)

• Reaction Center

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

– Proteins

Chloroplast

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

– Most abundant in mesophyll

Structure of a leaf

Photosynthesis Overview

• Happens in 2 stages

– Light Reactions – convert solar energy into chemical energy

• Occurs in thylakoid membrane

– Carbon Reactions/Calvin Cycle – use ATP and NADPH to reduce CO2 to glucose

• Occurs in the stroma

Photosynthesis Overview

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) passes to Photosystem I

– 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

Light Reactions: Photosystems

The Light Reactions

• Photosystem I –– Pigment molecules absorb light and transfer to

reaction center (chlorophyll a)

– 2 electrons come from first ETC (Photosystem II)

– 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)

ATP Generation – Photosystem I

The Light Reactions

• Final Outputs of Light Reactions

– Oxygen (emitted from plant)

– NADPH (used in Calvin Cycle)

– ATP (used in Calvin Cycle)

The Calvin Cycle

• Also known as: Carbon reactions, 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• Makes PGAL (2 3 carbon molecules)

– Step 2: Reduction of PGAL from step 1 to G3P• Uses NADPH and ATP from light reactions

– Step 3: G3P converted glucose– Step 4: Regeneration of RuBP

The Calvin Cycle

The Calvin Cycle

• Calvin Cycle must “turn” 6 times to make 1 glucose molecule– 1 turn for each CO2 fixed

– Calvin Cycle Uses:• 12 ATP

• 12 NADPH

• Another 6 ATP to regenerate RuBP

• Final Outputs of Calvin Cycle– C6H12O6

– O2

C-3 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 02

C3 and C4 Plant Anatomy

C4 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

C3 plant C4 plant CAM plant

Global Climate Change

Green house effect: radiant heat trapped by CO2

Global Climate Change

Global Climate Change

Global Climate Change

Global Climate Change

• What would you do to curb climate change?