Photosynthesis

6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O

Synthesizes energy-rich organic molecules (glucose) from energy-poor molecules (CO2, H2O).

Uses CO2 as carbon source & light energy as energy source.

Directly or indirectly supplies energy to most living organisms.

Fig. 10.4

The major “Play-as”

Photosynthesis occurs in chloroplasts in eukaryotic organisms:

1. Light dependent reactions occur in the thylakoid membranes of the grana and yield ATP and NADPH(obtained by reducing NADP with H2O). O2 is a wasteproduct

2. The Calvin cycle occurs in the space between grana called stroma

Fig. 10.6

Photo = lightsynthesis = putting together

= production of sugar (glucose = C6H12O6) using E from solar radiation (photons), CO2 and H2O

Photons = fixed quantity of light E

Utilized by most plants, some bacteria, some protists

Nutritional categories

1. Autotrophs – require no organic nutrients. All can “fix” or reduce CO2 into glucose via the Calvin Cycle:

6CO2 + 12 NADPH + 18 ATP

C6H12O6 + 12 NADP + 18 ADP + 18 P

They can then synthesize all other organic constituentsfrom this glucose

Autotrophs

A) Photoautotrophs – use light energy to generate boththe ATP and NADPH for the Calvin cycle.(photosynthetic organisms)

B) Chemoautotrophs – cannot use light. Use respirationsof inorganic substrates like reduced sulfur compounds,nitrogen compounds, and iron to generate the ATP and NADPH for the Calvin cycle (all chemosynthetic organismsare bacteria)

2. Heterotrophs – require at least 1 organic nutrient. Heterotrophs are dependent upon autotrophs, usually the photosynthetic organisms, for a source of fixed carbon(ie carbohydrates) and other nutrients

Photoautotrophs are considered “producers” in an ecosystem. Heterotrophs are considered as “consumers”

All life is therefore either directly or indirectly dependentupon the energy of the sun

In eukaryotes, takes place in chloroplasts

Plants - chloroplasts in leaves, other green parts

Contain chlorophyll= green pigmentcaptures/absorbs light E

Electromagnetic Spectrum

Nm = nanometer = 10-9 m0.0000000001 m

Based on WavelengthShort wavelength = high E Long wavelength = low E

Fig. 10.7

Electromagnetic Spectrum

Range = wavelengths of less than 1 nm (gamma rays) to wavelengths of more than 1 km (radio waves)

gamma = high Eradio = low E

Visible LightDrives photosynthesis= light detectable by human eye380-750 nm

Ranges from violet redROY G BIV backwardsred, orange, yellow, green, blue, indigo, violet

Blue and red most important in photosynthesis

Why?Colors (wavelengths) absorbed by chlorophyll

Why is chlorophyll green?

Light can be:reflected, transmitted, absorbed

reflected - “bounces” off of pigment= color that you see

transmitted - goes through pigment

absorbed - captured by pigmentdon’t see

Fig. 10.7+8

Different pigments absorb different wavelengths of light

pigment = substance that absorb visible light

Wavelengths absorbed, disappear

black = all wavelengths absorbed

white = all wavelengths reflected/transmitted

Photosynthetic Pigments in Plants

Chlorophyll a

Chlorophyll b= yellow-green absorbs slightly different wavelength

Carotenoids= yellow & orangePhycocyanins= blue and purple

Chlorophyll a = primary pigment

Chlorophyll b, carotenoids and phycocyanins = accessory pigments

Expand range of wavelengths available for photosynthesis

Fig. 10.10

Absorption and actionspectra for photosynthesis

Chloroplast Structure

Lens-shaped

surrounded by double membrane

divided into 3 compartments by system of membranes

Chloroplast Structure1. Intermembrane space= space between the 2 outer membranes

2. Thylakoid spacethylakoids = flattened membranous sacs inside

chloroplastStacks of thylakoids = grana

chlorophyll embedded within thylakoid membrane

membrane separates thylakoid space = area inside of thylakoids from stroma

3. Stroma = thick fluid outside/surrounding thylakoids

Fig. 10.11

Photosynthesis- light (kinetic) E chemical (potential) E

E stored in bonds of glucose moleculesBreaking bonds releases E

Photosynthetic Process

2 stages1. Light Dependent Reactions - require sunlight

2. Light-Independent Reactions (Calvin Cycle) - don’t require sunlight

1. Light Reactions

convert light energy to chemical energy

energy stored in bonds of ATP & NADPH - adenosine triphosphate

- Nicotinamide adenine dinucleotide phosphate

Light energy harvesting occurs via photosytems in thylakoid membranes:

ADP + P + NADP + H2O + light energy

ATP + NADPH + O2

Involves 2 photosytems interconnected by an electron transport chain

Light Rxns.Require sunlight = light dependent rxns.

Occur in thylakoid membranes of chloroplasts

Thylakoid membranes contain photosystems= light harvesting units

consist of reaction center, antenna molecules, and e- acceptors

Example of a photosystemFig. 10.13a

Excitation of an isolated chlorophyl molecule

Fig. 10.12

Reaction Center = single, specialized chlorophyll a molecule + primary e- acceptor

Antenna molecules = all other photosynthetic pigment molecules

(chlorophyll b, carotenoids, phycocyanins)

e- acceptor molecules = molecules that accept electrons from “excited” chlorophyll molecules

How photosystems work

1. Antenna molecules absorb photons

2. Pass energy from molecule to molecule until rxn. center reached

3. Chlorophyll a molecule in rxn center donates excited e- to primary e- acceptor

Photosystems

Chlorophyll a molecule at rxn. center loses e- to primary e- acceptor

= electron transfer

e- excited

boosted to higher energy state

Transfer of e- from chlorophyll a to primary acceptor = redox rxn.

= reduction/oxidation rxn.

Reduction = gain of e- = more negative chgOxidation = loss of e- = more positive chg

primary acceptor gains e- (reduced)chlorophyll a loses e- (oxidized)= first step of light rxns.

Thylakoid membrane contains 2 types of photosystems

- photosystem I

- photosystem II

each has characteristic rxn. center

systems cooperate

Photosystem Irxn. center absorbs light having wavelength of

700 nm= P700

Photosystem IIabsorbs light having wavelength of 680 nm= P680

2 systems cooperate to generate ATP & NADPH

** = PRIMARY FUNCTION OF LIGHT REACTIONS **

Non-cyclic electron flow generates ATP and NADPH

Fig. 10.14

Non-cyclic e- flow

“mechanical analogy”

Fig. 10.15

Chemiosmosis of ATP

Fig. 10.18

Chemiosmosis in mitochondria and chloroplasts:

10.17

Light Independent Reactions

CO2 fixation via the Calvin Cycle (recall from previousNotes)

6CO2 + 12 NADPH + 18 ATP

C6H12O6 + 12 NADP + 18 ADP + 18 P

See Fig. 10.17 for normal C3 pathway:

CalvinCycle

Fig. 10.19

Calvin Cycle

6 CO2’s yield1 glucose

Occurs inStroma

Catalyzed byRubisco(RuBP)

- CO2 enter the plant leaf openings called stomata -these openings are surrounded by guard cells which when flaccid close the opening

-During dry conditions, stoma are thus closed and CO2becomes limiting

-The Rubisco enzyme then reacts with O2 rather than CO2 and photorespiration occurs instead of CO2 fixation

Photorespiration:

Photorespiration:

- Photorespiration wastes fixed carbon by convertingribulose biphosphate into only 1 glyceraldehyde phosphate plus 1 glycolic acid (CH2OHCOOH)

-this glycolic acid is removed from the cycle and is Wasted

-plants evolved photosynthetic pathways to preventthis wasteful process

C4 Plants and the C4 Photosynthetic Pathway:

-Occur/originated in tropics, Mediterranean

-Adapted to hot/arid environment

-Adaptations save water, prevent photorespiration

Initial enzyme = PEP (phosphoenolpryuvate), fixes C into 4 C molecule (oxaloacetate)

Rubisco not involved initially - eliminates photorespiration

C stored in 4 C molecule in mesophyll cells

Calvin cycle occurs in nearby cells= bundle sheath cells

Mesophyll cells shuttle C to bundle sheath cells

Allows photosynthesis to occur even if stomates closed

Examples: corn, sugarcane, Bermuda grass

C4 anatomy and pathway:Fig. 10.20

C4 can handle heat, drought, high light

C3 more efficient if water is available and under lowlight conditions

CAM Plants= Crassulacean Acid Metabolism - Cacti, pineapples, succulents

Open stomates at night- lets in CO2, minimizes water loss

CO2 incorporated into organic acids (stored)

Used in light rxns during day while stomates closed

Comparison of C4 and CAM

Fig. 10.21