PHOTOSYNTHESIS

Photosynthesis

• process by which green plants and some organisms – seaweed, algae & certain bacteria • use light energy to convert CO 2 + water  glucose • all life on Earth, directly or indirectly, depends on photosynthesis as source of food, energy & O 2

Autotrophs

• self feeders – organisms that make their own organic matter from inorganic matter –

producers

• need inorganic molecules such as CO 2 , H 2 O & minerals to make organic molecules

•

Heterotrophs

consumers

– other feeders • depend on glucose as an energy source – cannot produce it • obtained by eating plants or animals that have eaten plants

Carbon and Energy Flow

CO 2 + H 2 O

Photosynthesis

Carbs Proteins Lipids + O 2

Cellular (Aerobic) Respiration ( ATP Produced )

Food Chain

• byproduct of photosynthesis is O 2 • humans & other animals breathe in oxygen • used in cellular respiration

Other Benefits of Photosynthesis

• humans also dependent on ancient products of photosynthesis • fossil fuels – natural gas, coal & petroleum • needed for modern industrial energy • complex mix of hydrocarbons • represent remains of organisms that relied on photosynthesis millions of years ago

Photosynthesis

• plants produce more glucose than they can use • stored as starch & other carbohydrates in roots, stems & leaves • can draw on these reserves for extra energy or building materials as needed

Sites of Photosynthesis

• leaves & green stems • in cell organelles – chloroplasts • concentrated in green tissue in interior 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 shaped compartments-

thylakoids

of plates

grana

)

chlorophyll

molecules

stroma

• suspended in stroma are disk – arranged vertically like stack • one stack-granum (plural, • embedded in membranes of thylakoids are hundreds of

Chlorophyll

• light-trapping pigment • other light-trapping pigments, enzymes & other molecules needed for photosynthesis are also found in thylakoid membranes

How Photosynthesis Works

• Requires –CO

2

–Water –Sunlight • Makes –O

2

–Glucose

How Photosynthesis Works

• CO 2 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

• 6CO 2 + 12H 2 O  C 6 H 12 O 6 + 6O 2 + 6H 2 O in presence of light • • must be an

oxidation reduction

& a

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 their orbit • hydrogens along with electrons are transferred to CO 2  sugar • requires that H 2 O is split into H & O 2 – O 2 escapes to air • light drives electrons from H 2 O 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 in

thylakoid membranes

• solar energy absorbed by chlorophyll  ATP + 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 defined range of wavelengths measured in nanometers • range of wavelengths is

electromagnetic spectrum

• part can be 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 absorbed  energy 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: Blue:

420 - 440 nm 440 - 490 nm

Green:

490 - 570 nm

Yellow: Orange:

nm

Red:

570 - 585 nm 585 - 620 620 - 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 • have 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 • shorter wavelengths have greater energy – violet light has 2X as much energy as red

photon

Light Energy

• when pigment absorbs a • 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 state heat is released • absorbed energy is passed to neighboring molecules

Photosynthesis

• Pigments • Absorb light • Excites electrons • Energy passed to sites in the cell • Energy used to make glucose

Photosystems

• chlorophyll & other pigments are found clustered next to one another in a

photosystem

• energy passes

rapidly

from one chlorophyll pigment molecule to another

Photosystems

• two photosystems participate in light reactions •

photosystem I & II

• each has a specific chlorophyll at reaction center • photosystem II – chlorophyll

P680

• photosystem I – chlorophyll absorb best spectrum

P700

• named for type of light they • P700 absorbs light in far red region of electromagnetic

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-O 2 • 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 a short electron transport chain to NADP + reducing it to NADPH • NADP + acceptor is final electron • electrons are stored in a high state of potential energy in NADPH molecule • NADPH, ATP and O 2 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 & P i when hydrogen ions pass out of thylakoid through

ATP synthase photophosphorylation

pH 7 H +

Chemiosmosis

H + pH 8 Chemiosmosis

Substrate-level Phosphorylation

Calvin Cycle

• light independent reactions • depend on light

indirectly

to obtain inputs for cycle-ATP & NADPH • takes place in

stroma

chloroplast of • each step controlled by different enzyme •

cycle

of reactions • makes sugar from CO 2 energy & • ATP provides chemical energy • NADPH provides high energy electrons for reduction of CO 2 to sugar

Steps of Calvin Cycle

• starting material-

ribulose bisphosphate

(RuBP) • first step-

carbon fixation

•

rubisco

(an enzyme) attaches CO 2 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 CO 2 molecules • to complete cycle must regenerate beginning component-RuBP • for every 3 molecules of CO product of cycle RuBP molecules 2 fixed, one G3P molecule leaves cycle as • remaining 5 G3P molecules are rearranged using ATP to make 3

Calvin Cycle

• regenerated RuBP is used to start Calvin cycle again • process occurs repeatedly in each chloroplast as long as CO 2 , 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

directly from air use CO 2 first organic compound produced is a 3 carbon compound 3-PGA reduce rate of photosynthesis in dry weather CO 2 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 CO 2 cannot enter leaf Calvin cycle comes to a halt no sugar is made in this situation rubisco adds O 2 RuBP to 2-carbon product of this reaction is broken down by plant cells to CO 2 + H 2 0

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 CO 2 flowing without capturing it directly from air

C4 Plants

• have enzymes that incorporate carbon from CO 2 into 4-C compound • enzyme has an intense desire for CO 2 • can obtain it from air spaces even when levels are very low • 4-C compound acts as a shuttle • transfers CO 2 to nearby cells -

bundle-sheath

cells • found in vast quantities around veins of leaves • CO 2 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 CO night 2 in at • CO 2 is fixed into a 4-C compound • saves CO 2 at night & releases it in the day • photosynthesis can take place without CO 2 needing to be admitted during the day when conditions are hot and dry

Environmental Consequences of

•

Photosynthesis

• CO 2 makes up 0.03% of air • provides plants with CO 2 to make sugars • important in climates

retains heat

from sun that would otherwise radiate from Earth • • warms the Earth

greenhouse effect

Global Warming

• CO • CO CO 2 2 2 traps heat without it. been linked to  warms air • maintains average temperature on Earth about 10oC warmer than • Earth may be in danger of overheating because of this greenhouse effect in air is increasing because of industrialization • when oil, gas and coal are burned is released • levels in atmosphere have increased 30% since 1850 • increasing concentrations have

global warming

• slow & steady rise in surface temperature of Earth • could have dire consequences for all life forms on Earth