PHOTOSYNTHESIS

Photosynthesis• process by which green

plants & some organisms – seaweed, algae & certain

bacteria• use light energy to convert

CO2 + water glucose

• all life on Earth, directly or indirectly, depends on photosynthesis as source of food, energy & O2

Autotrophs• self feeders

– organisms that make their own organic matter from inorganic matter

– producers• need inorganic

molecules such as CO2, H2O & minerals to make organic molecules

Heterotrophs• consumers

– other feeders

• depend on glucose as energy source

– cannot produce it

• obtained by eating plants or animals that have eaten plants

Carbon and Energy Flow

CO2 + H2O

CarbsProteinsLipids + O2

Photosynthesis

Cellular (Aerobic)Respiration(ATP Produced)

Food Chain• byproduct of

photosynthesis is O2

• humans & other animals breathe in oxygen

• used in cellular respiration

Other Benefits of Photosynthesis• humans depend on

ancient products of photosynthesis

• fossil fuels– natural gas, coal &

petroleum– for modern industrial

energy• represent remains of

organisms that relied on photosynthesis millions of years ago

Photosynthesis• plants produce

more glucose than they use

• Stored–starch & other

carbohydrates in roots, stems & leaves

Sites of Photosynthesis • leaves & green

stems • cell organelles

– chloroplasts• concentrated in

green tissue of leaf • mesophyll• green due to

presence of green pigment chlorophyll

Chloroplasts• each cell has 40-50 chloroplasts

– oval-shaped structures with double membrane

• inner membrane encloses compartment filled with stroma

• suspended in stroma are disk-shaped compartments-thylakoids– arranged vertically like stack

of plates• one stack-granum (plural,

grana) • embedded in membranes of

thylakoids are hundreds of chlorophyll molecules

Chlorophyll• light-trapping pigment

How Photosynthesis Works

• Requires

–CO2

–Water

–Sunlight

• Makes

–O2

–Glucose

How Photosynthesis Works• CO2 enters plant via

pores- stomata in leaves • water-absorbed by roots

from soil• membranes in

chloroplasts provide sites for reactions of photosynthesis

• chlorophyll molecules in thylakoids capture energy from sunlight

• chloroplasts rearrange atoms of inorganic molecules into sugars & other organic molecules

Photosynthesis• redox reaction

• 6CO2 + 12H2OC6H12O6 + 6O2 + 6H2O in presence of light

• must be an oxidation & a reduction

• water is oxidized– loses electrons &

hydrogen ions• carbon dioxide is

reduced – gains electrons &

hydrogens

Photosynthesis• relies on a flow of energy &

electrons initiated by light energy

• light energy causes electrons in chlorophyll pigments to boost electrons up & out of orbit

• hydrogens along with electrons are transferred to CO2sugar

• requires that H2O is split into H & O2

– O2 escapes to air• light drives electrons from H2O

to NADP+ which is oxidized NADPH which is reduced

Photosynthesis• 2 stages• light-dependent reactions

– chloroplasts trap light energy– convert it to chemical energy – contained in nicotinamide

adenine dinucleotide phosphate-(NADPH) & ATP

– used in second stage• light-independent reactions

– Calvin cycle– formerly called dark reactions– NADPH (electron carrier)

provides hydrogens to form glucose

• ATP provides energy

Light Dependent Reactions• convert light energy

to chemical energy & produce oxygen

• takes place- thylakoid membranes

• solar energy absorbed by chlorophyllATP + NADPH

Light Energy for Photosynthesis

• sun energy is radiation– electromagnetic energy

• travels as waves• distance between 2 waves-

wavelength• light contains many colors• each has range of

wavelengths measured in nanometers

• range of wavelengths is electromagnetic spectrum

• part seen by humans– visible light

Pigments• light absorbing molecules• built into thylakoid membranes• absorb some wavelengths & reflect

others• plants appear green because

chlorophyll-does not absorb green light– reflected back.

• as light is absorbedenergy is absorbed• chloroplasts contain several kinds of

pigments• different pigments absorb different

wavelengths of light• red & blue wavelengths are most

effective in photosynthesis• other pigments are accessory pigments• absorb different wavelengths• enhance light-absorbing capacity of a

leaf by capturing a broader spectrum of blue & red wavelengths along with yellow and orange wavelengths

Pigment Color & Maximum Absoption

• Violet:   400 - 420 nm • Indigo:   420 - 440 nm • Blue:   440 - 490 nm • Green:   490 - 570 nm • Yellow:   570 - 585 nm • Orange:   585 - 620

nm • Red:   620 - 780 nm

Chlorophylls• Chlorophyll A

– absorbs blue-violet & red light– reflects green– participates in light reactions

• Chlorophyll B– absorbs blue & orange light – reflects yellow-green – does not directly participate in

light reactions– broadens range of light plant

can use by sending its absorbed energy to chlorophyll A

Carotenoids• yellow-orange pigments• absorb blue-green

wavelengths• reflect yellow-orange• pass absorbed energy to

chlorophyll A• protective function

– absorb & dissipate excessive light energy that would damage chlorophylls

Light Energy• light behaves

as discrete packages of energy called photons

• fixed quantity of energy

Light Energy• when pigment absorbs a

photon• pigment’s electrons gains

energy• electrons are excited• unstable• electrons do not stay in

unstable state• fall back to original orbits• as electrons fall back to

ground heat is released• absorbed energy is passed

to neighboring molecules

Photosynthesis• Pigments

• Absorb light

• Excites electrons

• Energy passed to sites in cell

• Energy used to make glucose

Photosystems• chlorophyll &

other pigments are found clustered next to one another in a photosystem

• two participate in light reactions

Photosystems• photosystem I & II• each has specific

chlorophyll at reaction center

• photosystem II– chlorophyll P680

• photosystem I– chlorophyll P700

• named for type of light they absorb best

• P700 absorbs light in far red region of electromagnetic spectrum

Reaction Center• when photon strikes one

pigment molecule

• energy jumps from pigment to pigment until arrives at reaction center

• electron acceptor traps a light excited electron from reaction center chlorophyll

• passes it to electron transport chain which uses energy to make ATP & NADPH

Reaction Center

Light Reactions• during process of

making ATP & NADPH

• electrons are removed from molecules of water

• passed from photosystem II to photosystem I to NADP+

Photosystem II• water is split• oxygen atom combines

with oxygen from another split water forming molecular oxygen-O2

• each excited electron passes from photosystem II to photosystem I via electron transport chain

Photosystem I • primary electron acceptor

captures an excited electron• excited electrons are passed

through short electron transport chain to NADP+ reducing it to NADPH

• NADP+ is final electron acceptor

• electrons are stored in high state of potential energy in NADPH molecule

• NADPH, ATP and O2 are products of light reactions

ATP Formation-Chemiosmosis• uses potential energy of

hydrogen ion concentration gradient across membrane

• gradient forms when electron transport chain pumps hydrogen ions across thylakoid membrane as it passes electrons down chain that connects two photosystems

ATP Formation-Chemiosmosis

• ATP synthase (enzyme) uses energy stored by H gradient to make ATP

• ATP is produced from ADP & Pi when hydrogen ions pass out of thylakoid through ATP synthase

• photophosphorylation

ChemiosmosisH+H+

Chemiosmosis

pH 7

pH 8

Substrate-level Phosphorylation

Calvin Cycle• light independent reactions• depend on light indirectly to

obtain inputs for cycle-ATP & NADPH

• takes place in stroma of chloroplast

• cycle of reactions• makes sugar from CO2 &

energy• ATP provides chemical

energy• NADPH provides high energy

electrons for reduction of CO2 to sugar

Steps of Calvin Cycle• starting material-ribulose

bisphosphate (RuBP)• first step-carbon fixation• rubisco (an enzyme) attaches CO2 to

RuBP• Next-reduction reaction takes place• NADPH reduces 3-phosphoglyceric

acid (3-PGA) to glyceraldehye 3-phosphate (G3P) with assistance of ATP

• to do this cycle uses carbons from 3 CO2 molecules

• to complete cycle must regenerate beginning component-RuBP

• for every 3 molecules of CO2 fixed, one G3P molecule leaves cycle as product of cycle

• remaining 5 G3P molecules are rearranged using ATP to make 3 RuBP molecules

Calvin Cycle• regenerated RuBP is used

to start cycle again• process occurs repeatedly

as long as CO2, ATP & NADPH are available

• thousands of glucose molecules are produced

• used by plants to produce energy in aerobic respiration

• used as structural materials• stored

Photosynthesis Variations

• plants vary in the way they produce glucose and when

C3 Plants• use CO2 directly from air• first organic compound produced is a

3 carbon compound 3-PGA• reduce rate of photosynthesis in dry

weather• CO2 enters plants through pores in

leaves• on hot days stomata in leaves close

partially to prevent escape of water• with pores slightly open, adequate

amounts of CO2 cannot enter leaf• Calvin cycle comes to a halt• no sugar is made• in this situation rubisco adds O2 to

RuBP• 2-carbon product of this reaction is

broken down by plant cells to CO2 + H20

• Photorespiration• provides neither sugar nor

ATP

C4 Plants• have special adaptations

allowing them to save water without shutting down photosynthesis

• corn, sugar cane & crabgrass• evolved in hot, dry

environments • when hot & dry stomata are

closed• saves water• sugar is made via another

route• developed way to keep CO2

flowing without capturing it directly from air

C4 Plants• have enzymes that incorporate

carbon from CO2 into 4-C compound

• enzyme has an intense desire for CO2

• can obtain it from air spaces even when levels are very low

• 4-C compound acts as a shuttle• transfers CO2 to nearby cells -

bundle-sheath cells• found in vast quantities around

veins of leaves• CO2 levels in these cells remain

high enough for Calvin cycle to produce sugar

CAM Plants• pineapple, some cacti &

succulent plants• conserve water by opening

stomata & letting CO2 in at night

• CO2 is fixed into a 4-C compound

• saves CO2 at night & releases it in the day

• photosynthesis can take place without CO2 needing to be admitted during the day when conditions are hot and dry

Environmental Consequences of Photosynthesis

• CO2 makes up 0.03% of air• provides plants with CO2 to make sugars• important in climates• retains heat from sun that would otherwise radiate from Earth• warms the Earth• greenhouse effect

Global Warming• CO2 traps heatwarms air• maintains average temperature on

Earth about 10 degrees C warmer than without it.

• Earth may be in danger of overheating because of this greenhouse effect

• CO2 in air is increasing because of industrialization

• when oil, gas and coal are burned CO2 is released

• levels in atmosphere have increased 30% since 1850

• increasing concentrations have been linked to global warming

• slow & steady rise in surface temperature of Earth

• could have dire consequences for all life forms on Earth