PHOTOSYNTHESISCHAPTER 10

LEAF STRUCTURE

stomata – pores in lower epidermis of leaf gas exchange

mesophyll – inner-leaf tissue most chloroplasts

located in these cells veins (phloem &

xylem) phloem carries sugars

away from leaves xylem carries water to

leaves

CHLOROPLAST STRUCTURE stroma – inner fluid thylakoids –

interconnected membranous sacs contains chlorophyll

grana – stacks of thylakoids

PHOTOSYNTHESIS SUMMARY

involves redox reactions 2 stages:

light reactions – convert solar energy to chemical energy stored temporarily in ATP & NADPH

Calvin cycle – conversion of CO2 into glucose using the energy stored in ATP & NADPH

6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2

VISIBLE (WHITE) LIGHT

a mixture of wavelengths 380-750 nm when passed thru a prism, separates

into the colors of the rainbow

PIGMENTS

absorb visible light some wavelengths (colors of light) are

not absorbed but reflected or transmitted– this is the color we see (ex) leaves look green

because the pigment chlorophyll reflects and transmits green light

SPECTROPHOTOMETER

a machine that determines the wavelengths of light absorbed by a pigment by measuring the percent transmittance of each color of light

allows us to create an absorption spectrum

chlorophyll a absorbs blue-violet & red light best chlorophyll b absorbs blue & orange light best

ABSORPTION SPECTRUM FOR PHOTOSYNTHETIC PIGMENTS

EXCITED PIGMENTS – KEY TO LIGHT RXNS pigments become excited when they absorb

light because absorption of a photon of light boosts an electron to a higher energy level

if the energy is not captured, the electrons will quickly fall back to their ground state & the energy is released as heat

sometimes light is also emitted as the electrons fall back down to their ground state – the resulting after-glow is called fluorescence

PHOTOSYSTEMS

light-harvesting complexes in the thylakoid membranes consisting of pigment molecules bound to proteins

two types: photosystem II (P680) & photosystem I (P700)

create a greater surface area for absorbing light increase the range of wavelengths that can be

absorbed by the plant (due to presence of chlorophyll a, chlorophyll b, and carotenoids)

have two parts: reaction center (chlorophyll a + primary electron acceptor) surrounded by light-harvesting complexes

light strikes pigment molecules in light-harvesting complexes

energy is passed from one pigment molecule to another until it reaches chlorophyll a in the reaction center

this excites an electron in chlorophyll a which is picked up by the primary electron acceptor

ELECTRON FLOW

non-cyclic electron flow flow of electrons from PS II PS I NADP+

produces ATP and NADPH for the Calvin cycle

cyclic electron flow cycling of electrons within PS I; does not involve PS II produces additional ATP needed for Calvin cycle

NADPH concentration regulates which type occurs high [NADPH] stimulates cyclic electron flow to

balance out NADPH & ATP levels

NONCYCLIC ELECTRON FLOW

CYCLIC ELECTRON FLOW

CHEMIOSMOSIS

ATP is made during the light reactions using the same process (chemiosmosis) that makes ATP during oxidative phosphorylation of cellular respiration

SUMMARY OF THE LIGHT REACTIONS

light strikes PS II causing electrons in chlorophyll a in the reaction center to become excited

the excited electrons are picked up by the primary electron acceptor & passed down an electron transport chain which drives the synthesis of ATP by chemiosmosis

meanwhile, light strikes PS I causing electrons in chlorophyll a in the reaction center to become excited

the excited electrons are picked up by the primary electron acceptor & passed down an electron transport chain to NADP+ which is reduced to NADPH

SUMMARY OF THE LIGHT REACTIONS

the electrons lost from the chlorophyll a molecules in each photosystem are restored as follows: electrons are restored to PS II by the

splitting of H2O which produces O2 as a by-product

electrons are restored to PS I by the electron transport chain that follows PSII (PS I is the final electron acceptor for this ETC)

cont.

CALVIN CYCLE

carbon fixation – CO2 is attached to a molecule called RuBP by the enzyme rubisco forming an unstable six-carbon compound that immediately splits into two three-carbon compounds called 3-PGA

ATP & NADPH drive the conversion of 3-PGA to G3P

for every three CO2 that enter the cycle, six G3P are made – one leaves the cycle and five are recycled to regenerate RuBP (requires ATP)

glucose is made from the G3P that leaves the Calvin cycle

PHOTORESPIRATION

plants that perform the steps of photosynthesis previously discussed are called C3 plants

C3 plants use CO2 directly from the air by opening their stomata

this can be a problem on hot, dry days when plants close their stomata to reduce water loss because when stomata are closed, no CO2 can enter the leaf & no O2 can get out

the O2 build-up causes photorespiration, a process in which rubisco adds O2 to RuBP in the Calvin cycle instead of CO2

photorespiration does not produce glucose like photosynthesis or ATP like cellular respiration so it is basically a wasteful process

ADAPTATIONS TO PHOTORESPIRATION C4 photosynthesis

PEP carboxylase (which has a higher affinity for CO2 than rubisco & no affinity for O2) combines CO2 with PEP to make oxaloacetate which is converted to malate and stored in the bundle-sheath cells

CO2 is released in the bundle-sheath cells and enters the Calvin cycle

this adaptation is used in hot regions with intense sunlight where stomata partially close during the day

examples of plants that use this adaptation are corn & sugarcane

C4 PHOTOSYNTHESIS

ADAPTATIONS TO PHOTORESPIRATION CAM photosynthesis

plants take in CO2 at night and store it in organic acids

CO2 is released during the day for use in the Calvin cycle when light is available for the light reactions

this adaptation is used by plants that live in extremely arid environments like deserts

examples of plants that use this adaptation are cacti, pineapples, & succulents (water-storing plants)

cont.

COMPARISON

C4 PHOTOSYNTHESIS CAM PHOTOSYNTHESIS

spatial separation of steps

temporal separation of steps