Photosynthesis

Chapter 10

Energy source Autotrophs: Producers Make own organic molecules Heterotrophs: Consumers

Photosynthesis

Photosynthesis Organisms capture energy from

sunlight Build food molecules Rich in chemical energy 6CO2 + 12H2O ⇨

C6H12O6 + 6H2O + 6O2

Photosynthesis Captures only 1% of sun’s energy Provides energy for life

Photosynthesis

Leaf structure Stoma (Stomata) opening on leaf Exchange of gases. Chloroplasts Mesophyll layer of leaf

Chloroplasts Thylakoids: Internal membranes of chloroplasts Grana: Stacks of thylakoids Chlorophyll: Green pigment Captures light for photosynthesis Membranes of thylakoids

Chloroplasts Stroma: Semi-liquid substance Surrounds thylakoids Contain enzymes Make organic molecules from

carbon dioxide

Chloroplasts

Fig. 10-3b

1 µm

Thylakoidspace

Chloroplast

GranumIntermembranespace

Innermembrane

Outermembrane

Stroma

Thylakoid

Chloroplasts Photosystem: Cluster of photosynthetic pigments Membrane of thylakoids Each pigment captures energy Photosystem then gathers energy Energy makes ATP, NADPH &

organic molecules

NADP+

Nicotinamide Adenine Dinucleotide Phosphate

Coenzyme Electron carrier Reduced during light-dependent

reactions Used later to reduce carbon Carbon dioxide forms organic molecules Photosynthesis is a redox reaction

Photophosphorylation Addition of phosphate group to

ADP Light energy

Photosynthesis Occurs in 3 stages 1. Capturing energy from sun 2. Energy makes ATP Reducing power in NADPH 3. ATP & NADPH Power synthesis of organic

molecules

Photosynthesis Light dependent reactions First 2 steps of photosynthesis Presence of light Light-independent reactions Formation of organic molecules Calvin cycle Can occur +/- light

Experimental history Jan Baptista van Helmont Plants made their own food Joseph Priestly Plants “restored” the air

Experimental history Jan Ingenhousz Sun’s energy split CO2 Carbon & Oxygen Oxygen was released into air Carbon combined with water Make carbohydrates

Experimental history Fredrick Forest Blackman 1. Initial “light” reactions are

independent of temperature 2. Second set of “dark” reactions are

independent of light Dependent on CO2 concentrations &

temperature Enzymes involved in light-independent

reactions

Experimental history C.B. van Neil Looked at light in photosynthesis Studied photosynthesis in Bacteria

C.B. van NeilCO2 + 2H2S (CH2O) + H2O +

2S

CO2 + 2H2A (CH2O) + H2O + A2

CO2 + 2H2O (CH2O) + H2O + O2

C.B. van Neil O2 produce from plant photosynthesis

comes from splitting water Not carbon dioxide Carbon Fixation: Uses electrons & H+ from splitting water Reduces carbon dioxide into organic

molecules (simple sugars). Light-independent reaction

CO2 + 2H2O (CH2O) + H2O + O2

CO2 + 2H2O (CH2O) + H2O + O2

Photosynthesis 1. Chloroplasts 2. Light-dependent reactions Sun’s energy makes NADPH & ATP 3. Light-independent reactions ATP & NADPH CO2 into organic molecules

Light

Fig. 10-5-4

H2O

Chloroplast

LightReactions

NADP+

P

ADP

i+

ATP

NADPH

O2

CalvinCycle

CO2

[CH2O]

(sugar)

Sunlight UV light from sun Source of energy when life began UV light can cause mutations in

DNA Lead to skin cancer

Light Photon: Packets of energy UV light photons have greater

energy than visible light UV light has shorter wavelengths

Light Visible light Purple shorter wavelengths More energetic photons Red longer wavelengths Less energetic photons

Spectrum

Spectrum

Absorption Spectrums Photon of energy strikes a molecule Absorbed by the molecule or lost as

heat Depends on energy in photon

(wavelength) Depends on atom’s available

energy levels Specific for each molecule

Pigments Molecules Absorb energy in visible range Chlorophylls & Carotenoids Chlorophyll a & b Absorb photons in the blue-violet

& red light

Pigments Chlorophyll a main pigment of

photosynthesis Converts light energy to chemical

energy Chlorophyll b & carotenoids are

accessory pigments Capture light energy at different

wavelengths

Pigments

Pigments

Chlorophyll b

Carotenoids Chlorophyll a

Chlorophyll structure Located in thylakoid membranes A porphyrin ring with a Mg in

center Hydrocarbon tail Photons are absorbed by the ring Absorbs photons very effectively Excites electrons in the ring

Chlorophyll structure

D:\Chapter_10\A_PowerPoint_Lectures\10_Lecture_Presentation\10_07LightAndPigments_A.html

Carotenoids Two carbon rings attached by a

carbon chain Not as efficient as the Chlorophylls Beta carotene (helps eyes) Found in carrots and yellow

veggies

Photosystems Captures the light Located on surface of the

photosynthetic membrane Chlorophyll a molecules Accessory pigments (chlorophyll b

& carotenoids) Associated proteins

Photosystems Consists of 2 components 1. Antenna (light gathering)

complex 2. Reaction center

Photosystem 1. Antenna complex Gathers photons from sun Web of Chlorophyll a molecules Held by proteins in membrane Accessory pigments carotenoids Energy is passed along the

pigments to reaction center

Photosystems 2. Reaction centers 2 special chlorophyll a molecules Accept the energy Chlorophyll a than passes the

energized electron to an acceptor Acceptor is reduced (quinone)

Photosystem

Fig. 10-12

THYLAKOID SPACE(INTERIOR OF THYLAKOID)

STROMA

e–

Pigmentmolecules

Photon

Transferof energy

Special pair ofchlorophyll amolecules

Th

yla

koid

me

mb

ran

e

Photosystem

Primaryelectronacceptor

Reaction-centercomplex

Light-harvestingcomplexes

2 photosystems Photosystem I (older) Absorbs energy at 700 nm wavelength Generates NADPH Photosystem II (newer) Absorbs energy at 680 nm wavelength Splits water (releases oxygen) Generates ATP 2 systems work together to absorb

more energy

Photosynthesis (Process) Light dependent reactions Linear electron flow Energy transfer Thylakoid membranes

Light dependent reactions Photosystem II (680 nm) Light is captured by pigments Excites an electron (unstable) Energy is transferred to reaction

center (special chlorophyll) Passes excited electron to an

acceptor molecule

Light dependent reactions PS II is oxidized Water splits (enzyme) Water donates an electron to

chlorophyll Reduces PS II Oxygen (O2) is released with 2

protons (H+)

Light dependent reactions Electron is transported to PS I (700 nm) Electron is passed along proteins in the

membrane (ETC) Protons are transported across the

membrane Protons flow back across the

membrane & through ATP synthase Generate ATP

Light dependent reactions At the same time PS I received light

energy Excites an electron Primary acceptor accepts the electron PS I is excited Electron from PS II is passed to PS I Reduces the PS I

Light dependent reactions PS I excited electron is passed to a

second ETC Ferredoxin protein NADP+ reductase catalyzes the

transfer of the electron to NADP+

Makes NADPH

Pigmentmolecules

Light

P680

e–

Primaryacceptor

2

1

e–

e–

2 H+

O2

+3

H2O

1/2

4

Pq

Pc

Cytochromecomplex

Electron transport chain

5

ATP

Photosystem I(PS I)

Light

Primaryacceptor

e–

P700

6

Fd

Electron transport chain

NADP+

reductase

NADP+

+ H+

NADPH

8

7

e–

e–

6

Fig. 10-13-5

Photosystem II(PS II)

Fig. 10-UN1

CO2

NADP+

reductase

Photosystem II

H2O

O2

ATP

Pc

Cytochromecomplex

Primaryacceptor

Primaryacceptor

Photosystem I

NADP+

+ H+

Fd

NADPH

Electron transport

chain

Electron transport

chain

O2

H2O Pq

Enhancement effect

Enhancement effect

Fig. 10-17

Light

Fd

Cytochromecomplex

ADP +

i H+

ATPP

ATPsynthase

ToCalvinCycle

STROMA(low H+ concentration)

Thylakoidmembrane

THYLAKOID SPACE(high H+ concentration)

STROMA(low H+ concentration)

Photosystem II Photosystem I

4 H+

4 H+

Pq

Pc

LightNADP+

reductase

NADP+ + H+

NADPH

+2 H+

H2OO2

e–

e–

1/21

2

3

Fig. 10-16

Key

Mitochondrion Chloroplast

CHLOROPLASTSTRUCTURE

MITOCHONDRIONSTRUCTURE

Intermembranespace

Innermembrane

Electrontransport

chain

H+ Diffusion

Matrix

Higher [H+]Lower [H+]

Stroma

ATPsynthase

ADP + P i

H+ATP

Thylakoidspace

Thylakoidmembrane

Photosystems Noncyclic photophosphorylation 2 systems work in series Produce NADPH & ATP Replaces electrons from splitting

water System II (splits water)works first

then I (NADPH)

Photosystems When more ATP is needed Plant changes direction Electron used to make NADPH in

PS I is directed to make ATP

Calvin Cycle Named for Melvin Calvin Cyclic because it regenerates it’s

starting material C3 photosynthesis First organic compound has 3

carbons

Calvin cycle Combines CO2 to make sugar Using energy from ATP Using reducing power from NADPH Occurs in stroma of chloroplast

Calvin Cycle Consists of three parts 1. Fixation of carbon dioxide 2. Reduction-forms G3P

(glyceraldehyde 3-phosphate) 3. Regeneration of RuBP (ribulose

1, 5 bisphosphate)

Calvin Cycle 3 cycles 3 CO2 molecules 1 molecule of G3P 6 NADPH 9 ATP

Fixation of carbon CO2 combines with Ribulose 1, 5 bisphosphate (RuBP) Temporary 6 carbon intermediate Splits-forms 2- three carbon molecules 3-phosphoglycerate (PGA) Large enzyme that catalyses reaction (Rubisco) Ribulose bisphosphate

carboxylase/oxygenase

Reduction Phosphate is added to 3-

phosphoglycerate 1,3 Bisphosphoglycerate NADPH reduces the molecule Glyceraldehyde 3-phosphate

(G3P)

Regeneration 5 molecules of G3P are rearranged

to make 3 RuBP Uses 3 more ATP

Fig. 10-18-3

Ribulose bisphosphate(RuBP)

3-Phosphoglycerate

Short-livedintermediate

Phase 1: Carbon fixation

(Entering oneat a time)

Rubisco

Input

CO2

P

3 6

3

3

P

PPP

ATP6

6 ADP

P P6

1,3-Bisphosphoglycerate

6

P

P6

66 NADP+

NADPH

i

Phase 2:Reduction

Glyceraldehyde-3-phosphate(G3P)

1 POutput G3P

(a sugar)

Glucose andother organiccompounds

CalvinCycle

3

3 ADP

ATP

5 P

Phase 3:Regeneration ofthe CO2 acceptor(RuBP)

G3P

Fig. 10-UN2

Regeneration ofCO2 acceptor

1 G3P (3C)

Reduction

Carbon fixation

3 CO2

CalvinCycle

6 3C

5 3C

3 5C

Calvin Cycle 3 CO2 enter cycle & combine with

RuBP Generates 3 molecules more of

RuBP & one G3P (glyceraldehyde 3-phosphate)

G3P can be made into glucose & other sugars

Calvin Cycle Enzyme mediated 5 of these enzymes need light to

be more efficient Net reaction3CO2 + 9 ATP + 6NADPH ⇨

G3P + 8Pi + 9ADP + 6NADP+

G3P G3P Converted to fructose 6-phosphate

(reverse of glycolysis) Made into sucrose Happens in cytoplasm Intense photosynthesis G3P levels rise so much some is

converted to starch

Fig. 10-21

LightReactions:

Photosystem II Electron transport chain

Photosystem I Electron transport chain

CO2

NADP+

ADP

P i+

RuBP 3-Phosphoglycerate

CalvinCycle

G3PATP

NADPHStarch(storage)

Sucrose (export)

Chloroplast

Light

H2O

O2

Photorespiration

Hot Stoma in leaf close to avoid loosing

water Carbon dioxide cannot come in Oxygen builds up inside Carbon dioxide is released G3P is not produced

Photorespiration Happens when Rubisco oxidizes RuBP

(starting molecules of Calvin cycle) Oxygen is incorporated into RuBP Undergoes reactions that release CO2

CO2 & O2 compete for same sight on the enzyme

Under conditions greater than the optimal 250C this process occurs more readily

C4 Photosynthesis Process to avoid loosing carbon dioxide Plant fixes carbon dioxide into a 4

carbon molecule (oxaloacetate) PEP carboxylase (enzyme) Oxaloacetate is converted to malate Then taken to stroma for Calvin cycle Sugarcane and corn

CAM Process to prevent loss of CO2

Plants in dry hot regions (cacti) Reverse what most plants do Open stoma at night Allows CO2to come in & water to

leave Close them during the day.

CAM Carbon fix CO2 at night into 4

carbon chains (organic acids) Use the Calvin cycle during the

day.

Fig. 10-20

CO2

Sugarcane

Mesophyllcell

CO2

C4

Bundle-sheathcell

Organic acidsrelease CO2 to Calvin cycle

CO2 incorporatedinto four-carbonorganic acids(carbon fixation)

Pineapple

Night

Day

CAM

SugarSugar

CalvinCycle

CalvinCycle

Organic acid Organic acid

(a) Spatial separation of steps (b) Temporal separation of steps

CO2 CO2

1

2