PHOTOSYNTHESIS. Photosynthesis process by which green plants & some organisms –s–seaweed, algae & certain bacteria use light energy to convert CO 2 +

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• use 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 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 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 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• other light-trapping

pigments, enzymes & other molecules needed for photosynthesis are also found in thylakoid membranes

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• 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 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 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• have protective function

– absorb & dissipate excessive light energy that would damage chlorophylls

Photosynthesis• Pigments

• Absorb light

• Excites electrons

• Energy passed to sites in cell

• Energy used to make glucose

Photosystems• chlorophyll & other

pigments clustered next to one another in a photosystem

• when photon strikes one pigment molecule

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

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

Photosystems• two photosystems

participate in light reactions

• photosystems II & I

Light Reactions• make ATP & NADPH• electrons are removed from

molecules of water• oxygen escapes to air • electrons are passed from

photosystem II to photosystem I to NADP+

• light drives electrons from H2O to NADP+ which is oxidized NADPH which is reduced

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 • electron acceptor captures an

excited electron• excited electrons are passed

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

• NADP+ -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

Calvin Cycle/Dark Reactions• light independent reactions• depend on light indirectly for

inputs-ATP & NADPH• occurs-stroma of chloroplast• each step controlled by

different enzyme• 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• 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 Calvin cycle again• process occurs repeatedly

in each chloroplast 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