Chapter 22: The Light Reactions

Photosynthesis uses ____ _____ to generate ____-_____ _______.

light energy, high-energy electrons

These electrons are used to create a proton-motive force that powers the synthesis of ___. The high-energy electrons are also used to form _____, biosynthetic reducing power

ATP, NADPH

The reactions powered by sunlight are called the _____ ______.

light reactions

Photosynthetic organisms are called _______, whereas organisms that obtain energy from chemical fuels only are _________.

autotrophs, heterotrophs

Photosynthesis converts _____ energy into _____ energy

light, chemical

Photosynthesis is the opposite of ______ ______ in terms of end points.

cellular respiration

Photosynthesis takes place in ________

chloroplasts

The chloroplast is a _______-_______ organelle

double-membrane

The inner membrane surrounds a space called the ______, which is the site of the dark reaction: the synthesis of glucose from ____ and ____ using ATP and NADPH formed in the ____ reactions.

stroma, CO2, H20, light

In the stroma are membranous sacs called _____ membranes. ______ membranes are the location of the light reactions of _________.

thylakoid, thylakoid, photosynthesis

Light absorption by _______ induces ______ transfer

chlorophyll, electron

Photosynthesis begins with the absorption of light by a photoreceptor molecule, also called a _______. The principal photoreceptor in the chloroplasts of green plants is ________ __.

pigment, chlorophyll a

When a photon of the appropriate energy is absorbed by a pigment, an electron in the pigment molecule jumps to a _____ energy state. The excited electron may fall to its original state, releasing the energy as ____ or ____. Another fate for the excited electron is to move to a nearby ____ that has a _____ excited state, a process called _____ ______.

higher, light, heat, molecule, lower, electron transfer

Electron transfer results in _________ ____ _______ because the initial molecule is now ______ charged and the molecule that accepted the electron is ______ charged.

photoinduced charge separation, positively, negatively

Separation of charge occurs at a site called the ______ _______.

Reaction center

Energy transfer from accessory pigments to reaction centers. (A) Light energy absorbed by accessory _______ molecules or other pigments can be transferred to reaction centers by ______ ______ ______ (black arrows), where it drives ________ charge separation. (B) Accessory pigments ______ the range of light absorption well beyond what is possible with ________ __ and b alone.

chlorophyll, resonance energy transfer, increase, chlorophyll a

Two photosystems generate a _____ ______ and _____ in oxygenic photosynthesis

proton gradient, NADPH

Photosynthesis in green plants consists of ___ ________.

two photosystems

Photosystem I generates _______ reducing power in the form of _____

biosynthetic, NADPH

Photosystem II replenishes the ________ of photosystem I while generating a ______ ______ that is used to synthesize ___.

electrons, proton gradient, ATP

The missing electrons in photosystem II are replaced by the photolysis of _______.

water

Photosystem I uses light energy to generate reduced ________, a powerful _____.

ferredoxin, reductant

The reaction center in photosystem I is called _____. The electrons from excited P700 (P700*) flow down an electron-transport chain to the ____-____ protein ferredoxin.

P700, iron-sulfur

Ferredoxin-NADP+ reductase converts ____ into _____.

NADP+, NADPH

Ferredoxin-NADP+ reductase transfers electrons from ______ to form _____, biosynthetic reducing power.

ferredoxin, NADPH

The formation of NADPH occurs on the _____ side of the ______ membrane, where it is used for ________ synthesis.

stromal, thylakoid, carbohydrate

Photosystem II transfers electrons from water to ________ and generates a ______ _______

plastoquinone, proton gradient

The reaction center in photosystem II is called _____.

P680

Excited P680 (P680*) transfers electrons from water to photosystem I in an electron-transport chain that includes _______, a chlorophyll with protons replacing magnesium; ______, which is similar to ubiquinone; and the ______ bf complex.

pheophytin, plastoquinine, cytochrome

P680, the reaction center for photosystem II, is bound by the __ and __ subunits of the photosystem.

D1, D2

Upon excitation of P680, electrons flow to _____, then to _________, and finally to reduce a mobile ________ (QH2 ).

pheophytin, plastoquinone, plastoquinone

P680+ extracts electrons from ______ _____ at the manganese center to maintain _____ balance.

water bound, redox

__________ __ links photosystem II to photosystem I

Chtochrome bf

Cytochrome bf transfers electrons from _______ (QH2 ) to ________ (Pc). Protons from plastoquinol are released into the _______ _____, and cytochrome bf pumps ___ more protons from the _____ into the _____-, generating a proton-motive force.

plastocyanin, plastoquinol, thylakoid lumen, two, stroma, lumen

P680+ is a _____ ______ that removes electrons from water. This reaction, the photolysis of water, occurs at the _____-________ complex (also called the manganese center) of photosystem II.

strong oxidant, water-oxidizing

_____ photons are required to generate ___ oxygen molecule. The ____ electrons extracted from water are used to reduce two ___ to two ___ . The ____ protons used to reduce the Q molecules come from the ____, and the four protons liberated from water are released into the _____.

Four, one, four, Q, QH2, four, stroma, lumen.

The photolysis of water is the source of ___ for all of lif

O2

A proton gradient across the ______ membrane drives ___ synthesis

thylakoid, ATP

The flow of electrons from ___ to ____ generates a proton-motive force that is used to power the synthesis of ___.

H2O, NADP+, ATP

Thylakoid membranes exposed to an artificial __ _______ synthesize ___.

pH gradient, ATP

In chloroplasts, most of the energy of the proton-motive force consists of the ______- ______, with the ________ ______ contributing little energy.

proton gradient, membrane potential

In chloroplasts, electroneutrality is maintained because ____ moves into the _____ when two ___ are pumped from the _____ into the thylakoid ______.

Mg2+, stroma, H+, stroma, lumen

In ________, the membrane potential also contributes to the proton-motive force.

mitochondria

The ATP synthase of ________ closelt resembles those of _______ and ________

chloroplasts, mitochondria, prokaryotes

The ATP synthase of the chloroplast is also called the ___-___ complex, where C stands for ______.

CF1-CF0, chloroplast

The chloroplast CF1 -CF0 complex is very similar to the ________ ______ complex.

mitochondrial F1-F0

Newly synthesized ___ is released into the stroma, where it is used in ________ synthesis.

ATP, carbohydrate

The membrane orientation of the _______ complex is _______ compared to the mitochondrial ATP synthase.

CF1-CF0, reversed

Cyclic electron flow through photosystem I leads to the production of ___ instead of _______

ATP, NADPH

If the _____ needs are met, cyclic electron flow generates ____ without forming ______.

NADPH, ATP, NADPH

The electrons of photosystem I flow from ______ through _________ to ________ and then return to P700

ferredoxin, cytochrome bf, plastocyanin

The protons pumped by cytochrome bf are used to synthesize ___.

ATP

The absorption of _____ ______ yields one ___, two _____, and ____ ATP molecules

eight protons, O2, NADPH, three

In cyclic photophosphorylation, two ______ yield one molecule of ____ but no ________.

photons, ATP, NADPH