Photosynthesis- The Basis for Life on Earth Part I

Photosynthesis-

The Basis for Life on Earth

Part I

Photosynthesis- is the process that converts light energy into chemical energy. This chemical energy is usually a carbohydrate. Only photoautrotrops can do photosynthesis. Heterotrophs must obtain their high organic nutrients from the environment.

Glucose has more energy than carbon dioxide and water. The reaction is endergonic and will require an energy input of ATP and NADPH2

In order to keep the reaction going, the cell must regenerate ATP and NADPH

The equation below describes the Calvin cycle of photosynthesis and the chemical energy needed to make the sugar.

Regeneration of NADPH and ATP require light, and intact chloroplasts with chlorophyll. This part of photosynthesis is known as the light reaction. The hydrogen needed to reduce NADP comes from the splitting of water.

Which of the following statements is a correct distinction between autotrophs and heterotrophs ?

A) Only heterotrophs require chemical compounds from the environment. B) Cellular respiration is unique to heterotrophs. C) Only heterotrophs have mitochondria. D) Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic. E) Only heterotrophs require oxygen.

D

Autotrophs, but not heterotrophs, can nourish themselves beginning with CO2 and other nutrients that are inorganic. Photoautotrophs use light to do this and chemoautrophs uses sources of reduced inorganic chemicals for energy such as H2S.

Chlorophyll is green and reflects green light and absorbs red and blue. Chlorophyll is made from a tetrapyrole ring with Mg in the middle and a hydrocarbon tail. These pigments form photosystems found in the thylakoid membrane.

There are some other pigments in photosystems which are yellow. These yellow pigments (caroteinoids and xanthophylls) allows photosynthesis to occur in green light.

The The light reactions of photosynthesis supply the Calvin cycle with

A) light energy. B) CO2 and ATP. C) H2O and NADPH. D) ATP and NADPH. E) sugar and O2.

D

The purpose of the light reaction is to provide the Calvin cycle with the necessary chemical energy needed to reduce CO2 in the formation of a carbohydrate in the form of ATP and NADPH.

White light is mixture of different colors of light with different wave lengths and frequencies. When white light lands on a blue object, red and green light is absorbed and blue is reflected. The absorption graph shows that the pigments absorb red and blue/violet light best. This is due to the accessory pigments. Yet the action spectrum shows that a small amount of light does occur in greeen light.

Which of the following conclusions does not make sense from studying the absorption spectrum for chlorophyll a and the action spectrum for photosynthesis?A) Not all wavelengths are equally effective for photosynthesis. B) There must be accessory pigments that broaden the spectrum of light that contributes to photosynthesis. C) The red and blue areas of the spectrum are most effective in driving photosynthesis. D) Chlorophyll owes its color to the absorption of green light. E) Chlorophyll a has two absorption peaks.

D

Green pigment refects green light and does not absorb green light.

This diagram shows the location of the chloroplasts in the plant. Leaves are specialized to for photosynthesis. The leaves are usually arranged on the plant so that it maximizes exposure to the sunlight.

Grass stains are so difficult to remove from clothing because

A) chlorophyll contains the metallic ion Mg. B)  green pigments reflects blue light and most liquid detergents are blue. C) chlorophyll contains a hydrocarbon tail which repels water. D) Mg is an ion which repels the polar nature of detergent.

Chlorophyll is green and reflects green light. The hydrocarbon tail of chlorophyll repels water making it difficult to remove grass stains from clothing.

Chloroplasts have 3 membranes. The outer 2 are smooth and the inner one makes stacks of thylakoids which is a granum.

The chlorophyll and other pigments are found inside the thylakoid membrane. They have the ability to convert light energy into chemical energy.

A stack of thylakoids is called a granum. The matrix that the grana is embeded in is the stroma. It contains enzymes for carbohydrate synthesis. Below is a diagram of the thylakoid and the location of chlorophyll.

The chlorophyll molecules and accessory pigments form photosystems (I & II). Each photosystem has a reaction center (p700& p680 respectively) Once photons are absorbed by the pigments of the photosystem, the electron becomes excited and the energy is passed on from molecule to molecule until it reaches the reaction center pigment.

The p680 and p700 has the ability to pass the energized electron on to the electron transport system.

Also embedded in the thylakoid membrane is a series of proteins that have the ability to be reduced an oxidized. Each one has less reducing power than the preceeding one. Protein Q has the ability to receive an energized electron from the p680 reaction center. It then moves to PQ or plastoquione. From there it move to a cytochrome complex which is a proton pump that when reduced has the ability to pump hydrogen from the outside of the stroma to the inside of the thylakoid. From there it goes plastocyanin. By this time the e- has lost much of its free energy and must be energized by photosystem I.

Which of the following processes is most directly driven by light energy?

A) creation of a pH gradient by pumping protons across the thylakoid membrane B) carbon fixation in the stroma C) reduction of NADP+ molecules D) removal of electrons from chlorophyll molecules E) ATP synthesis

D

Light energy excites electrons in the photosystem which eventually reaches a chlorophyll pigment in the photosystem. This electron then has the ability to be moved (removed) to the electron transport chain.

The electron has now left the electron transport chain. Replacement electrons for PS II comes from the splitting of water. A managanese complex associated with PS II has the ability to split water to produce, H+, e-, and oxygen gas. The e- are shuttled to the photosystem, H+ are used to lower the pH of the thylakoid, and the oxygen gas is released to the atmosphere.

As the electron transport chain runs, there is an accumulation of H+ on the inside of the thylakoid. This is due to the splitting of water and the proton pump,PQ. As the H+ collect, the pH of the interior is lowered and there is a separation of charge across a membrane. This can now do work.

On the thylakoid membrane, there are CF complexes which contain a channel (CF0) and a large protein head (CF1). On the CF1 head, there is an enzyme ATP synthetase. This enzyme has the ability to phosphorylate ADP--->ATP as 3 H+ pass through.

This is noncyclic photophosphorylation.

Which of the following sequences correctly represents the flow of electrons during photosynthesis?

A) NADPH -> O2 -> CO2 B) NADPH -> chlorophyll -> Calvin cycle C) H2O -> photosystem I -> photosystem II D) NADPH -> electron transport chain -> O2

E) H2O -> NADPH -> Calvin cycle

E

The electrons originate from water and are released when water is split. The electrons are moved to the electron transport chain where at the end of the electron transport chain, the electrons are used to reduce NADP to form NADPH. From the the NADPH is shuttled to the Calvin cycle where they are used to reduce CO2 in the formation of a carbohydrate

The mechanism, photophosphorylation is most similar to

A) substrate-level phosphorylation in glycolysis. B) oxidative phosphorylation in cellular respiration. C) the Calvin cycle. D) carbon fixation. E) reduction of NADP+.

B

Both cell respiration and photosynthesis use chemiosmosis to generate ATP.

This is noncyclic photophosphorylation.

1. Water is split to make replacement e-, H+ and O2.

2. There are two photosystems involved.

3. NADP is reduced to NADPH.

Cyclic photophosphorylation is considered to be a more ancient biochemical pathway. It is found in most photosynthetic bacteria and all photosynthetic eukaryotes. It consist of one photosystem (PSI) and a simple electron transport chain. At the end of the electron transport chain, the electron is returned to PS I. That being the case, water is not split, nor is NADP reduced. One part of the electron carrier does pump H+ across the thylakoid membrane to make ATP. Cyclic photophosphorylation does not provide hydrogens for the reduction of carbon dioxide to make a carbohydrate. So therefore quite often the hydrogens come from H2S. In photosynthetic, eukaryotic cells, two photosystems (II & I) work together to form noncyclic photophosphorylation.

Cooperation of the two photosystems is required for

A) ATP synthesis. B) reduction of NADP+. C) cyclic photophosphorylation. D) oxidation of the reaction center of photosystem I. E) generation of a proton-motive force.

B

When two photosystems are involved, NADP is reduced, water is split, oxygen is produced. This is noncyclic photophosphorylation. Both cyclic and noncyclic photophosphorylation involve the synthesis of ATP with a generation of proton-motive force. Cyclic photophosphorylation only involves one photosystem.

Which of the following statements is a correct distinction between cyclic and noncyclic electron flow?

A) Only noncyclic electron flow produces ATP. B) In addition to ATP, cyclic electron flow also produces O2 and NADPH. C) Only cyclic electron flow utilizes light at 700

nm. D) Chemiosmosis is unique to noncyclic electron flow. E) Only cyclic electron flow can operate in the

absence of photosystem II.

E

Noncyclic photophosphorylation has two photosystems and cyclic photophosphorylation only has one photosystem as shown above.

Comparison ofcyclic versus noncyclic photophosphorylation