Notes

in order for plants to grow they need

inputs of carbon dioxide water and

energy the chemical process by which

plants use these resources to

manufacture glucose the building blocks

of plants is called photosynthesis in

the process oxygen gas is produced as a

by-product the energy for photosynthesis

originates in the Sun and arrives at the

earth as sunlight this light has both a

wave and a particle nature the particles

or photons are the smallest units of

light photons oscillate along a path

which is measured as wavelengths

the light emitted from the Sun contains

photons in a wide spectrum of

wavelengths called the electromagnetic

spectrum photosynthetic organisms use

only a small portion of the

electromagnetic spectrum called visible

light

photosynthetic organisms contain

pigments that facilitate the capture of

wavelengths of light in the visible

light range the color of the pigment

comes from the wavelengths of light

reflected plants appear green because

they reflect yellow and green

wavelengths of light red and blue

wavelengths of light are absorbed by

these pigments and provide the energy

that is used for photosynthesis within

eukaryotic photosynthetic organisms also

known as photo autotrophs the chemical

reactions of photosynthesis occur within

plant cells in specialized structures

known as chloroplasts photosynthesis

consists of two sets of reactions the

light dependent reactions and the calvin

cycle within the chloroplast are small

discs like structures called thylakoids

which are surrounded by a fluid-filled

space called the stroma the reactions

that synthesize glucose the Calvin cycle

occur in the stroma the light-dependent

reactions occur in the thylakoid it is

here that conversion of light energy to

chemical energy is initiated in most

photosynthetic organisms thylakoids

contain pairs of photosystems called

photosystem 1 and photosystem 2 that

work in tandem to produce the energy

that will later be used in the stroma to

manufacture sugars

the photosystems of the thylakoid

consists of a network of accessory

pigment molecules and chlorophyll the

molecules that absorb the photons of

light within the pigment molecules the

absorbed light energy excites electrons

to a higher state photosystems will

channel the excitation energy gathered

by the pigment molecules to a reaction

center chlorophyll molecule which will

then pass the electrons to a series of

proteins located on the thylakoid

membrane photons of light strike

photosystems one and two simultaneously

we will examine what happens with the

photons striking photosystem ii first

the energized electrons are passed from

the reaction center of photosystem ii to

an electron transport chain the

electrons lost by photosystem ii are

replaced by a process called photolysis

which involves the oxidation of a water

molecule producing free electrons and

oxygen gas while this oxygen gas is a

byproduct of photosynthesis it is an

important input to the cellular

respiration pathways

as electrons pass through the electron

transport chain the energy from the

electron is used to pump hydrogen ions

from the stroma to the thylakoid

creating a concentration gradient

this gradient powers a protein called

ATP synthase which phosphorylates adp to

form ATP the low energy electrons

leaving photosystem 2 are shuttled to

photosystem one within photosystem one

low energy electrons are re-energized

and are passed through an electron

transport chain where they are used to

reduce the electron carrier nadp+ to

nadph

you

when the chloroplast is receiving a

steady supply of photons NADPH and ATP

molecules are rapidly being provided to

the metabolic pathways in the stroma

therefore the ATP and NADPH formed

during the light dependent reactions are

used in the stroma to fuel the Calvin

cycle reactions the calvin cycle

consists of a series of reactions that

reduce carbon dioxide to produce the

carbohydrate glyceraldehyde 3-phosphate

the cycle consists of three steps the

first of which is carbon fixation

in this step carbon dioxide is attached

to rib ulos one five bisphosphate

resulting in a six carbon molecule that

splits into two three carbon molecules

the second step is a sequence of

reactions using electrons from NADPH and

some of the ATP to reduce carbon dioxide

in the final step rib ulos 1-5

bisphosphate is regenerated for every

three turns of the cycle five molecules

of glyceraldehyde 3-phosphate are used

to reform three molecules of rib ulos

1-5 bisphosphate

the remaining glyceraldehyde 3-phosphate

is then used to make glucose fatty acids

or glycerol

it takes two molecules of glyceraldehyde

3-phosphate to make one molecule of

glucose phosphate thus the Calvin cycle

has to run six times to produce one

molecule of glucose these molecules can

remove their phosphate and add fructose

to form sucrose the molecule plants used

to transport carbohydrates throughout

their system glucose phosphate is also

the starting molecule for the synthesis

of starch and cellulose

plants produce sugars to use as storage

molecules and structural components for

their own benefit by utilizing the

energy of the Sun along with inputs of

water and carbon dioxide plants act as

glucose factories photosynthetic

organisms are the primary producers of

glucose on the planet they also produce

oxygen gas as a by-product and thus

serve as the foundation of life

providing food and oxygen for the

complex food webs on both land and in

the oceans