5.3 The Light Reactions

A photon with a wavelength of __________ is optimal to move an electron from ground state to an excited state in Photosystem I.

700 nm ~A wavelength of 700 nm is optimal at exciting an electron from Photosystem I. This is why photosystem I is also known as P700.

Which of the following takes place as a result of photosynthetic electron transport? A. Oxygen gets reduced. B. The stroma becomes acidic. C. A proton gradient is established across the thylakoid membrane. D. ATP is formed by substrate level phosphorylation.

A proton gradient is established across the thylakoid membrane. ~During photosynthetic electron transport, protons are pumped into the lumen of the thylakoid creating a concentration gradient. This concentration gradient is used to fuel the synthesis of ATP. As protons travel down the concentration gradient (from high to low concentration) through ATP synthase, ATP is made.

Which of these statements concerning the light-dependent reactions of photosynthesis is not true? A. The energy lost from the excited electrons can be used to split carbon dioxide if it is present in large quantities. B. The reactions occur in the grana. C. If the excited electrons are not captured they will return to their ground state. D.The electrons that are excited are raised to a higher energy level and made less stable.

A. The energy lost from the excited electrons can be used to split carbon dioxide if it is present in large quantities. ~The energy released from the excited lectrons is not used to split carbon dioxide if it is present. Carbon dioxide is not involved in the light-dependent reactions of photosynthesis.

Which of the following sequences correctly represents the flow of electrons in the light-dependent reactions of photosynthesis? A. water, Photosystem II, Photosystem I, NADP B. oxygen, Photosystem I, Photosystem II, NADP C. water, Photosystem I, Photosystem II, oxygen D. water, Photosystem II, Photosystem I, oxygen.

A. water, Photosystem II, Photosystem I, NADP ~Electron flow in the light-dependent reactions of photosynthesis begins with the photolysis of water, which provides electrons to Photosystem II. Excited electrons are then passed down an electron transport chain to Photosystem I. From Photosystem I, electrons are eventually passed to NADP.

Which statement is true regarding cyclic electron flow in photosynthesis? A. Oxygen is produced from the photolysis of water. B. ATP and NADPH are produced. C. ATP is produced without increasing the amount of NADPH. D. The reducing potential of Photosystem I is increased.

ATP is produced without increasing the amount of NADPH. ~In cyclic electron flow, excited electrons are passed down an electron transport chain and eventually passed back down to chlorophyll a, rather than being used to reduce NAD+. As electrons are passed down the electron transport chain, the energy they lose is used to create a proton gradient, which is used to fuel ATP synthesis. The result of cyclic electron flow is that ATP is produced without increasing NADPH. Cyclic electron flow takes place when NADPH levels get too high.

How is photosystem II linked to photosystem I?

An electron from photosystem II is passed to photosystem I. ~Electrons that are excited from photosystem II are passed down an electron transport chain to photosystem I.

The end products of the light-dependent reactions of photosynthesis include A. ADP, H2O, and NADPH B. ATP, H2O, and CO2 C. ATP, NADPH, and O2 D. ADP, H+, and NADPH

C. ATP, NADPH, and O2 ~ATP, oxygen, and NADPH are the products of the light-dependent reactions. ATP is made from the ATP synthase complex as protons flow down a concentration gradient. Oxygen is made from the photolysis of water. NADPH is made when the electrons reduce NADP.

Which of the following is the major difference between cyclic and noncyclic photophosphorylation? A. Only noncyclic photophosphorylation produces ATP. B. Cyclic photophosphorylation reduces more NADP. C. Photosystem I is not involved in cyclic photophosphorylation. D. Photosystem II is not involved in cyclic photophosphorylation.

Photosystem II is not involved in cyclic photophosphorylation. ~Cyclic photophosphorylation occurs when there is a build up of NADPH. It involves Photosystem I only and produces ATP but no NADPH.

__________ is the source of energy for life on Earth.

The sun ~The sun serves as the source of energy for life on Earth. Photosynthetic organisms are able to use sunlight to make make complex molecules. In turn, these complex molecules serve as a source of energy for organisms.

Which of the following does not take place during cyclic photophosphorylation? A. ATP formation B. Water provides electrons to photosystem I. C. NADPH is not made. D. Oxygen is not released.

Water provides electrons to photosystem I. ~Water donates electrons to photosystem II during noncyclic photophosphorylation. During cyclic photophosphorylation, ferredoxin donates electrons to the cytochrome complex which then return the electrons to P700.

During photosynthesis, photophosphorylation and photolysis both occur in the.....

chloroplasts. ~Photophosphorylation and photolysis take place in the chloroplasts of plant cells. The chloroplasts are organelles that contain chlorophyll pigments and they are the site of photosynthesis.

A major difference between cyclic and noncyclic photophosphorylation is.....

cyclic photophosphorylation produces no NADPH. ~Noncyclic photophosphorylation involves both photosystems. An electron is excited from Photosystem II and passed down the electron transport chain in the thylakoid until it is passed to Photosystem I where it will be excited again and eventually used to form NADPH. Cyclic photophosphorylation involves only Photosystem I. Electrons are passed down a chain of proteins pumping hydrogen ions into the lumen of the thylacoid. No NADPH is formed during cyclic photophosphyorylation.

Protons in the lumen of the thylakoid A. diffuse out of the lumen to make ATP. B. diffuse into the lumen to make ATP. C. are pumped into the stroma to make ATP. D. are pumped into the intermembrane space to make ATP.

diffuse out of the lumen to make ATP. ~Protons diffuse out of the lumen of the thylakoid through a protein complex called ATP synthase. As the protons flow through ATP synthase they fuel the production of ATP.

Where would you expect to find the lowest pH in a chloroplast of an actively photosynthesizing cell?

in the lumen of the thylakoid ~The lumen of the thylakoid would have the lowest pH because a high concentration of protons is constantly being maintained within the lumen. pH is a function of proton concentration. When the proton concentration is high, the pH is low (and vice versa). During photosynthesis a proton gradient is maintained by the active pumping of protons across the thylakoid membrane into the lumen of the thylakoid.

Where is ATP produced in the light reactions?

in the stroma. ~ATP made during photosynthesis is produced in the stroma of the chloroplast. The energy released from electrons excited by light is used to pump protons into the lumen of the thylakoid. Protons within the thylakoid lumen diffuse out to the stroma through the ATP synthase enzyme complex. The movement of the protons through ATP sythase fuels the production of ATP from ADP + Pi.

When an electron from Photosystem I is excited and transferred to a primary electron acceptor, the chlorophyll a in the reaction center will be __________.

oxidized ~When the chlorophyll a in the reaction center of Photosystem I loses an excited electron it becomes oxidized. Oxidation is the loss of electrons from a molecule or atom.

Water's role in photosynthesis is to....

provide electrons to the light-dependent reactions. ~Water's role in the light-dependent reactions of photosynthesis is to provide electrons to Photosystem II after it loses excited electrons. In order to do this, water is split in a process called photolysis forming electrons, protons, and oxygen.

In eukaryotic photosynthesis, the transfer of electrons through membrane proteins provides energy to pump protons across.....

the thylakoid membrane. ~During photosynthesis, the transfer of excited electrons through a sequence of proteins provides energy to pump protons across the thylakoid membrane into the lumen. The concentration gradient of protons in the lumen of the thylakoid is used to fuel ATP sythesis.

The light-dependent reactions in plants occur in the....

thylakoid membrane. ~The light-dependent reactions in plants occur in the thylakoid membrane. The light dependent reactions require photosynthetic pigments that are found embedded in the thylakoid membrane.

The purpose of cyclic electron flow is....

to increase the ATP/ NADPH ratio. ~The purpose of cyclic electron flow is to produce ATP without producing NADPH. Thus cyclic electron flow increases the ATP/NADPH ratio.

True or false? An electron in chlorophyll a is excited to a higher energy level by light and is passed to a nearby electron acceptor molecule in the thylakoid.

true ~As one of the electrons in the chlorophyll of the antenna is excited to a higher energy level, it is transferred to an acceptor molecule to initiate electron flow.

True or false? When a chlorophyll molecule becomes excited by light, it becomes a reducing agent.

true ~When a chlorophyll molecule is excited by light, one of its electrons is boosted to a higher energy level and may be passed to an electron carrier. When a molecule gains electrons it is reduced, thus chlorophyll acted as a reducing agent.

An electron from Photosystem II is lost when it is excited by a photon of light. Photosystem II is supplied with another electron from......

water. ~The electron "hole" in Photosystem II is filled supplied from the splitting of water. Water is split into two protons, two electrons and half of an oxygen molecule by an enzyme in the thylakoid. The electrons from this split are used to replace the electrons lost by the chlorophyll a in the reaction center of Photosystem II.