biolA 190 - ch 7

7.1

7.1

photosynthesis

captures light energy and uses this energy to synthesize carbohydrates

CO2 is

reduced

H2O is

oxidized

6CO2 + 12H2O + LIGHT ENERGY →

C6H12O6 + 6O2 + 6H2O

energy from light drives this

ENDERGONIC reaction

organisms can be categorized as

autotrophs or heterotrophs

life is primarily driven by

the photosynthetic activity of plants, algae, and cyanobacteria

chloroplasts are pr

organelles that carry out photosynthesis

they contain the pigment chlorophyll

light reactions

involve a series of energy conversions

starting with light and ending with chemical energy

stored in ATP and NADPH

ATP and NADPH provide

the energy and electrons needed to make carbohydrates during the Calvin cycle

7.2

7.2

light is a type of

electromagnetic radiation

it consists of energy in the form of electric and magnetic fields

when light encounters a molecule

1. it may pass through the molecule

2. it may bounce off the molecule changing its path in a different direction

3. it may be absorbed by the molecule; pigments are molecules that can absorb light

pigments absorb some light energy and reflect others

the wavelength of light that a pigment absorbs depends on the amount of energy needed to boost an electron to a higher orbital

after an electron absorbs energy

it is in an excited state, and it is usually unstable

excited electrons in pigments can be

transferred to another molecule or “captured“

two types of chlorophyll pigments

chlorophyll a and chlorophyll b

found in green plants and green algae

carotenoids are

another type of pigment

fruits and flowers

having different pigments allows plants to

absorb light at many different wavelengths

chlorophyll pigments absorb most strongly in

blue-violet and red parts of the visible spectrum

the thylakoid membranes contain 2 distinct complexes of proteins and pigment molecules called

PS II and PS I

light excites pigment molecules in

BOTH PS II and PS I

the combined action of PS II and PS I is termed

linear electron flow

produces O2, ATP, and NADPH

role of PS II in linear electron flow

oxidizes water

generating O2 and H+

role of PS I in linear electron flow

make NADPH

the process of ATP production in the chloroplast is called

photophosphorylation

linear electron flow produces ATP and NADPH in roughly equal amounts, however

the Calvin cycle uses more ATP than NADPH

cyclic electron flow

cyclic electron flow produces only

ATP

cyclic flow is favored when the level of NADP+ is low and

NADPH is high

also favored when ATP levels are low

7.3

7.3

PS II and PS I have 2 main components

a light-harvesting complex and a reaction center

light-harvesting complex

directly absorbs photons of light and transfers energy between pigments by a process of resonance energy tramsfer

reaction center

where the redox reaction takes place

contains P680 (special pigment molecule)

unlike other pigments, P680 releases

its high energy electron and is oxidized

P680* → P680^+ + e-

water is oxidized to replace the

electrons on P680^+

PS II is the only known protein complex that can

oxidize water

resulting in the release of O2

Z scheme

a model developed in 1960 that proposed that photosynthesis involved 2 events of light absorption

7.4

7.4

the ATP and NADPH generated during the light reactions are used during

the Calvin cycle to make carbohydrates

the reactions of the calvin cycle require

a massive input of energy

for every 6 CO2 incorporated

18 ATP and 12 NADPH must be used

the product of the calvin cycle is

G3P

a carbohydrate with 3 carbon atoms that can be used in the synthesis of glucose and other organic molecules

carbon fixation

CO2 is incorporated into RuBP, 5-carbon sugar

enzyme rubisco catalyzes this reaction

6 carbon intermediate splits into 2 3-carbon molecules

reduction and carbohydrate production

ATP is used as a source of energy

NADPH is used as a source of high energy electrons

G3P is produced

regeneration of RuBP

most of G3P is used to regenerate RuBP to continue the cycle

7.5

7.5

Environmental conditions can alter the operation of the calvin cycle

temperature

water availability

light intensity

most plants are called C3 plants because

the first molecule that CO2 is incorporated into is a 3-carbon molecule

photorespiration occurs when

rubisco adds O2

as intermediates are processed

a molecule of CO2 is released

if C3 plants are subjected to hot and dry environments

as much as 25%-50% of their photosynthetic work is reserved by photorespiration

C4 plants mace oxaloacetate (4-carbon molecule) in

the first steps of carbon fixation

mesophyll cells capture CO2 into oxaloacetate using

an enzyme that only binds to CO2

then transports the captured carbon to the bundle-sheath cells, where the Calvin cycle happens

CAM plants open their stomata at

night

CO2 is captures and stored

which is better — C3 or C4?

depends on environment

C4 and CAM plant adaptations evolved to help plants living in hot and dry environments to

conserve water and minimize photorespiration