Chapter 7- how cells capture light energy via photosynthesis

heterotrophs vs. autotrophs

Are animal cells heterotrophs or autotrophs? Plant cells?

• heterotrophs- glucose is consumed. energy is obtained by eating it

  • this applies to animal cells

• autotrophs- glucose is internally made. energy is obtained by making it.

  • this applies to plant cells

where does photosynthesis occur in plants & algae? what other parts of their cells are involved? (there are 5 to list)

• photosynthesis occurs in the chloroplast

  • also involves…

    • stomata

    • chlorophyll pigment

    • chloroplast

    • thylakoid

    • storoma

what’s a general equation for photosynthesis?

• 6CO2 + 12H2O + light energy → C6H2O

  • change in ΔG = +686 kcal/mol

light energy & how it travels

• light energy- a form of energy in the form of electric & magnetic fields

• travels as particles called photons in waves

visible light

• the range of wavelengths detected by human eyes

what can happen when light hits a molecule? (3 to list)

• pass through molecules

• bounce off molecule, changing its path toward a different direction

• absorbed by the molecule; pigments are molecules that can absorb light

  • in pigments, electrons absorb energy & raise to excited states, which are usually unstable

an electron can be released in what 3 ways?

• as heat

• as light (fluorescence)

• excited electrons in pigments can be transferred to another molecule or “captured”

linear electron flow

• photosystems II & I produce ATP & NADH using linear electron flow

Photosystems I & II 

• its relation to thylakoid membranes

• which was discovered first? which is the initial step in photosynthesis?

• does light excites pigment molecules in PSI, PSII, both, or neither?

• thylakoid membranes contain 2 distinct complexes of proteins & pigment molecules called photosystems I (PS I) & photosystems II (PS II)

• PS I was discovered first

• PS II is the initial step in photosynthesis

• light excited pigment molecules in both PS I & PS II

linear flow & what 3 things it produces

• linear flow- the combined action of PS II & PS I

  • produces…

    • O2

    • ATP

    • NADPH

role of PS II in linear electron flow (4 points to list)

• initial step in photosynthesis

• oxidizes water, generating O2 & H⁺

• releases energized electrons to the electron transport chain (ETC)

  • some energy is used to make H+ electrochemical gradient

• energy depleted electrons travel to PS I to be re-excited by light

role of PS I in linear electron flow (2 points to list)

• primary role is to make NADPH

• addition of H+ & NADP+ contributes to H+ gradient that fuels ATP synthase activity

cyclic electron flow

• cyclic electron flow produces more ATP to account for the mismatch in ATP & NADPH usage

  • mismatch caused bc linear electron flow produces ATP & NAPDH while the calvin cycle uses more ATP & NAPDH

  • favored with low NADP+ & high NADPH; also favored with low ATP

what does PS II capture & produce?

• PS II captures light energy & produces O2

  • PS II is the only known protein complex that can oxidize water, resulting in the release of O2

the ATP & NADPH generated during the light cycle reactions are used during the Calvin cycle to make what? explain the energy usage

• carbohydrates

  • the calvin cycle takes CO2 from the atmosphere & incorporates the carbon into organic molecules

    • the calvin cycle incorporates CO2 into a carbohydrate

  • the reactions of the calvin cycle require a massive input of energy

    • for every 6 CO2 incorporated, 38 ATP & 12 NADPH must be used

product of the carbon cycle

• glyceraldehyde-3-phosphate (G3P)

  • this is a carbohydrate with 3 carbon atoms that can be used in the synthesis of glucose & other organic molecules

calvin cycle’s 3 phases (list what enzyme is used in the first phase!)

• carbon fixation- CO2 is incorporated into RuBP, a 5-carbon sugar

  • the enzyme rubisco catalyzes this reaction & the 6-carbon intermediate splits into two 3-carbon molecules

• reduction & 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 the G3P is used to regenerate RuBP, allowing the cycle to continue

environmental conditions that can alter the operation of the calvin cycle (list 3)

• temp

• water availability

• light intensity

C3 plants

most plants (~90%) are called C3 plants bc the first molecule that CO2 is incorporated into (3PG) is a 3-carbon molecule

how does photosynthesis decrease the efficiency of itself?

• RUBISCO IS STUPID !!! when CO2 is low & O2 is high, it can add O2 to RuBP

• this is called photorespiration.

  • wasteful bc the loss of carbon can limit plant growth

• if C3 plants are subjected to hot & dry environments, as much as 25-50% of their photosynthetic work is reversed by photorespiration

heat-adapted plants vs. heat & dry adapted plants (list the pathway & when the stomata is open)

• heat-adapted plants

  • C4

  • stomata is always open during the day

  • bring CO2 into “waiting room”

  • open door to calvin cycle only to CO2, not O2

• heat & dry adapted plants

  • CAM

  • stomata open only at night

  • nighttime-

    • CO2 comes locked in

    • O2 is being released

  • daytime-

    • stomata closes

    • calvin cycle turned on

    • most productive sugar making will be early day when O2 is lowest