Chapter 8: Photosynthesis

Define photic zone

well-lit upper layer of the oceans where photosynthesis can occur Where photosynthesis occurs in the ocean (100m deep)

define Photorespiration

The consumption of O2 and release of CO2. (Consumes ATP). A net energy drain: It results in the oxidation and loss, in the form of CO2, of carbon atoms that had previously been incorporated and reduced by the Calvin cycle, and second, it consumes ATP.

What does the proton pump involve?

(1) The transport of two electrons and two protons, by the diffusion of plastoquinone, from the stroma side of P2 to the luen side of the cytochrome complex and (2) the transfer of electrons within the cytochrome complex to a different molecule of plastoquinone, which results in additional protons being picked up from the stroma and subsequently released into the lumen. This accumulation on one side of the thylakoid membrane can then be used to power the synthesis of ATP by oxidative phosphorylation.

What two defenses do plant employ to avoid stresses that occur when the Calvin cycle cannot keep up with light harvesting.

1) Chemicals detoxify reactive oxygen species. (Ascorbate, carotene, and other antioxidants are able to neutralize reactive oxygen species.) 2) Prevent reactive oxygen species from forming in the first place. (Xanthophylls are pigments that slow the formation of reactive oxygen species by reducing excess light energy. They accept absorbed light energy directly from chlorophyll and then convert this energy to heat.

What is the photosynthesis equation?

6CO2+6H2O → C6H12O6 + O2

Define Pigments

Absorb some wavelengths of visible light.

Define Plastoquinone

Carries electrons from P2 to the cytochrome complex by diffusing through the membrane

Define Plastocyanin

Carries electrons from the cytochrome complex to P1 by diffusing through the thylakoid lumen.

Compare cellular respiration and photosynthesis

Cellular respiration breaks down carbohydrates in the presence of oxygen to supply the energy needs of the cell, producing carbon dioxide and water as a byproduct. Photosynthesis used carbon dioxide and water in the presence of sunlight to build carbohydrates, releasing oxygen as a byproduct.

What are the challenges to the effectiveness of photosynthesis?

Challenges to the efficiency of photosynthesis include excess light energy and the oxygenate activity of rubisco.

Photosynthesis definition in more detail

Photosynthesis is a biochemical process for building carbohydrates using energy from sunlight and carbon dioxide taken from the air.

Describe why NADPH is the reducing agent in the Calvin cycle.

Rubisco is responsible for the addition of the carbon atoms needed for the formation of carbohydrates, but by itself, rubisco does not increase the amount of energy stored in the formed bonds. For this energy increase to take place, the carbon compounds formed by rubisco must be reduced. NADPH is the reducing agent. It transfers electrons that allow carbohydrates to be synthesized from CO2.

evolution of photosynthesis

- increased oxygen Changed history on life on earth by providing organisms with a new source of energy and releasing oxygen into the atmosphere. Evolution of UV-absorbing compounds that could shield vells from the sun's damaging rays. Evolution of photosynthetic electron transport chains that use water as an electron donor. (Had to get a second photosystem for this to be able to occur) (Impacts life on earth in two ways: 1) Photosynthesis could occur anywhere there was both sunlight and sufficient water for cells to survive. 2) Using water as an electron donor results in the release of oxygen. Eukaryotic organisms gained photosynthesis by endosymbiosis.

Why is the Calvin cycle referred to as the "dark reactions"

Calvin cycle does not use sunlight directly → referred to as the 'dark' reactions of photosynthesis. Though, this cannot operate without energy input of NADPH and ATP which are both supplied by the photosynthetic electron transport chain, in which light is captured and transformed into chemical energy.

Describe how photosynthesis is a redox reaction.

Carbohydrates are synthesized from CO2 molecules during photosynthesis, yet have more energy stored in their chemical bonds than contained in the bonds of CO2 molecules. So to build carbohydrates using CO2 requires the input of energy, which comes from sunlight. The synthesis of carbohydrates from CO2 and water is a reduction-oxidation reaction. During photosynthesis, CO2 molecules are reduced (gain electrons) to form higher energy carbohydrate molecules. This requires both energy from ATP and an electron donor. The ultimate electron donor is water (the oxidation of water results in the production of electrons, protons, and O2. Thus, oxygen is formed in photosynthesis as a by product.

What are the two challenges to photosynthesis efficiency?

Challenge 1: If more light energy is absorbed than the calvin cycle can use, excess energy can damage the cell. Challenge 2: Stems from a property of rubisco; This enzyme can catalyze the addition of either carbon dioxide or oxygen to RuBP. The addition of oxygen instead of carbon dioxide can substantially reduce the amount of carbohydrate produced.

Describe the photosystems and their importance

Chlorophyll molecules are bound by their tail region to integral membrane proteins in the thylakoid membrane. These protein pigment complexes, photosystems, are the functional and structural units that absorb light energy and use it to drive electron transport.

Cyclic electron transport increases the production of ATP

In cyclic electron transport, electrons from photosystem 1 are redirected from ferredoxin back into the electron transport chain. These electrons reenter the photosynthetic ETC by plastoquinone. Why does this lead to the production of ATP? As the electrons from ferredoxin are picked up by plastoquinone, additional protons are transported from the stroma to the lumen. As a result, there are more protons in the lumen that can be used to drive the synthesis of ATP.

Where is the ETC located in Eukaryotes?

In eukaryotic cells, photosynthesis takes place in the chloroplast. Specifically, it is located in the thylakoid membrane.

Where is the ETC located in prokaryotes?

In photosynthetic prokaryotes, it is located in membranes within cytoplasm or, in some cases, directly in the plasma membrane.

Calvin cycle: The Incorporation of CO2 is catalyzed by the enzyme rubisco. Describe this.

In the first step, CO2 is added to a 5-carbon sugar called ribulose 1, aka RuBP This step is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase, or rubisco for short. An enzyme that adds CO2 to another molecule is called a carboxylase. Before rubisco can act as a carboxylase, RuBP and CO2 must diffuse into its active site. Once the active site is occupied, the addition of CO2 to RuBP proceeds spontaneously in the sense that no addition of energy is required. The product is a 6-carbon compound that breaks into two molecules of 3-phosphoglycerate. These are the first stable products of the calvin cycle.

What is the reaction center?

Most chlorophyll molecules function as antenna: Energy is transferred between chlorophyll molecules until it is finally transferred to a specially configured pair of chlorophyll molecules known as the reaction center. The reaction center is where light energy is converted into chemical energy as a result of the excited electrons transfer to an adjacent molecule. Without the antennae to gather light energy, reaction centers would sit idle much of the time, even in bright sunlight.

define photosynthesis

Photosynthesis is the major pathway by which energy and carbon are incorporated into carbohydrates.

What is photosynthesis the source of?

Photosynthesis is the source of all food we eat: Direct consumption of plant material and indirect consumption of meat. Photosynthesis is the source of all the oxygen that we breathe, as well as fuels for heating and transportation. Fossil fuels are the legacy of ancient photosynthesis

What photosynthesis is most evident to us?

Photosynthesis most evident to us: Carried out by land plants such as trees, grasses, shrubs. Though, photosynthesis also occurs among prokaryotic and eukaryotic organisms.

Define Accessory pigments

Photosystems also contain Accessory Pigments. The most notable are the orange-yellow carotenoids, which can absorb light from regions of the visible spectrum that are poorly absorbed by chlorophyll. Thus, their presence allows photosynthetic cells to absorb a broader range of visible light than would be possible with just chlorophyll lone.

Describe how Photorespiration leads to a net loss of energy and carbon

Rubisco can use both CO2 and O2. If O2 instead of CO2 diffuses into the active site of rubisco, the reaction can still proceed, although O2 is added to RuBP in place of CO2. An enzyme that adds O2 to another molecule is called an oxygenase. The result of O2 added instead of CO2 is one molecule with three carbon atoms and one molecule with only two carbon atoms. This molecule cannot be used by the calvin cycle to produce triose phosphate or RuBP.

Define photosystem II

Supplies electrons to the beginning of the electron transport chain. When P2 loses an electron, it is able to pull electrons from water.

Calvin cycle definition in detail

The biochemical pathway used in photosynthesis to synthesize carbohydrates from CO2.

Define the calvin cycle

The calvin cycle is a three step process that uses carbon dioxide to synthesize carbohydrates

What are excess carbohydrates store as?

The calvin cycle is capable of producing more carbohydrates than the cell is able to export. If these accumulate in the cell, they would cause water to enter the cell by osmosis, which would cause damage to the cell. So, instead, excess carbohydrates are converted to starch, a storage form of carbohydrates.

What are the purpose of carbohydrates produced by photosynthesis?

The carbohydrates are used both as starting points for energy synthesis of other molecules and as means of storing energy that can be converted into ATP from cellular respiration.

What do the light harvesting reactions use to produce what?

The light-harvesting reactions use sunlight to produce the ATP and NADPH required by the Calvin cycle

Define chlorophyll

The major photosynthetic pigment; it appears green because it is poor at absorbing green wavelengths. Consists of large, light absorbing "head" containing a magnesium atom at its center and a long hydrocarbon "tail".

Define visible light

The part of the electromagnetic spectrum that we can see with our eyes. The portion of the electromagnetic spectrum apparent to our eyes, and it includes the range of wavelengths used in photosynthesis. (400nm to 700nm)

What are the three main steps of the Calvin cycle?

Three Main steps: (1) Carboxylation, in which CO2 is added to a 5-carbon molecule; (2) reduction, in which energy and electrons are transferred to the compounds formed in step 1; and (3) regeneration of the 5-carbon molecule needed for carboxylation.

How can excess light cause damage?

When NADP+ is in short supply, the ETC "backs up" greatly increasing the probability of creating highly reactive forms of oxygen known as Reactive Oxygen Species. These can either be formed by the transfer of absorbed light energy from antenna chlorophyll directly to O2 or by the transfer of an electron, from O2-. Any factor that causes the rate of NADPH to fall behind the rate of light driven electron transport can lead to damage. Rate of the calvin cycle to make use of NADPH is also influenced by a number of factors that are independent of light intensity. Ex. cold temperatures cause the enzymes to function more slowly.

Who is photosynthesis carried out by?

60% of photosynthesis is carried out by terrestrial organisms, 40% takes place in the ocean. (majority of photosynthetic organisms in ocean are unicellular, ½ carried out by single-celled marine eukaryotes, while other half is carried out by photosynthetic bacteria.

Describe the regeneration phase of the calvin cycle.

A large number of reactions is needed to rearrange the carbon atoms from five 3-carbon triose phosphate molecules into three 5-carbon RuBP molecules. ATP is required for the regeneration of RuBP, raising the Calvin cycle's total energy requirements to two molecules of NADPH and three molecules of ATP for each molecule of CO2 incorporated by rubisco.

What role does ATP play in reduction?

ATP plays an important role in preparing 3-PGA for the addition of energy and electrons from NADPH. (NADPH provides most of the energy incorporated in the bonds of Carbs) These energy transfer steps result in the formation of 3-carbon carbohydrate molecules known as triose phosphates. Triose phosphates are the true products of the Calvin Cycle and they are the principal form in which carbohydrates are exported from the chloroplast during photosynthesis. Larger sugars, like glucose, are assembled from triose phosphates in the cytoplasm.

Why is starch able to be stored?

Because starch molecules are not soluble, they provide a means of carbohydrate storage that does not lead to osmosis. (Formation of starch during the day provides photosynthetic cells with a source of carbs they can use during the night)

How is light captured?

By pigments in chloroplasts Light energy absorbed by pigment molecules drives the flow of electrons through the photosynthetic ETC. The movement of electrons through the chain leads to the formation of both NADPH and ATP.

Who was Van Helmont?

Concluded that trees gain most of their mass by storing water But in actuality, photosynthesis was also responsible for this gain.

Define Cytochrome

Electrons pass between photosystem II and I. Small, reactively mobile compounds convey electrons between these protein complexes.

Define Photosystem I

Energizes electrons with a second input of light energy so they can be used to reduce NADP+. (When oxidized, not strong enough to split water, where as P2 is not strong enough reductant to form NADPH)

Can energy be transferred from one chloroplast to another?

For chlorophyll molecules with an intact chloroplast, energy can be transferred to an adjacent chlorophyll molecule instead of being lost as heat. When this happens, the energy released as an excited electron returns to its ground state and raises the energy level of an electron in an adjacent chlorophyll molecule. (very efficient)

Describe the thylakoid membrane

Form structures that resemble flattened sacs, and these sacs are grouped into structures called grana. Grana are connected to one another by membrane bridges in a way that the thylakoid membrane encloses a single interconnected compartment called the Lumen. The region surrounding the thylakoid membrane is called the Stroma. Carbohydrate synthesis takes place in the stroma, whereas sunlight is captured and transformed into chemical energy by the photosynthetic electron transport chain in the thylakoid membrane.

How does the buildup of protons in the thylakoid occur?

How the buildup of protons in thylakoid membranes occurs: First, oxidation of water releases protons and O2 into the lumen. Second, the cytochrome complex and plastoquinone together function as a proton pump that is functionally and evolutionarily related to proton pumping in the electron transport chain of cellular respiration.

What is the importance of the mitochondria?

IN mitochondria, carbohydrates are broken down to generate ATP. (ATP is needed to meet each cell's energy requirements which is why these organisms are not technically autotrophs.)

ATP synthase

In chloroplasts, as in mitochondria, ATP is synthesized by ATP synthase, a transmembrane protein powered by a proton gradient. In chloroplasts, the ATP synthase is oriented in a way that the synthesis of ATP is the result of the movement of protons from the thylakoid lumen to the stroma.

On land, where does photosynthesis most readily occur? Has this changed?

On land, photosynthesis occurs most readily in environments that are both moist and warm. (ex. Tropical rain forests, grasslands, and forests) Adaptations have been made to allow photosynthetic organisms to tolerate a wide range of environmental conditions: Ex. In very dry regions a combination of photosynthetic bacteria and unicellular algae forms an easily distrubtedlayer on the surface of the soil known as desert crust.

Photosynthesis captures just a small percentage of incoming solar energy. Explain how/why this is?

Only 1 to 2 percent of the sun's energy that lands on a leaf is converted into carbohydrates. Only visible light has the appropriate energy levels to raise the energy state of electrons in chlorophyll. Most of the sun's output is not absorbed by chlorophyll and thus cannot be used in photosynthesis. Leafs are not perfect at absorbing sunlight, 8% is reflected. ETC Captures aout 24% of the sun's usable energy arriving at the surface of a leaf. Efficiency of photosynthesis: 4%

Describe the Photosynthetic electron transport chain

The process begins with the absorption of sunlight by protein-pigment complexes. The absorbed sunlight provides the energy that drives electrons through the photosynthetic electron transport chain. In turn, the movement of electrons through this transport chain produce ATP and NADPH. ATP and NADPH are the energy sources needed to synthesize carbohydrates using CO2 in a process called the Carbon Cycle.

Describe how NADPH reduces

The reduction of 3-PGA involves two steps: (1) ATP donates a phosphate group to 3-PGA, and (2) NADPH tranders two electrons plus one proton (H+) to the phosphorylated compound, which releases one phosphate group. Because two molecules of 3-PGA are formed each time rubisco catalyzes the incorporation of one molecule of CO2, two ATP and two NADPH are required for each molecules of CO2.

Random:

Water donates electrons to one end of the ETC, whereas NADP+ accepts electrons at the other end. Enzyme that pulls electron from water is located on the lumen side of the photosystem 2. NADPH is formed when electrons are passed from P2 to a membrane associated protein called ferredoxin.

Why do we use two photosystems?

Water is an ideal source of electrons for photosynthesis. However, it takes great deal of energy to pull electrons from water, the amount of energy that a single photosystem can capture from sunlight just not enough to pull an electron from water and produce an electron donor capable of reducing NADP+. The solution to this problem is to use two photosystems arranged in series. The energy supplied by the first photosystem allows electrons to be pulled from water, and the energy supplied by the second photosystem step allows electrons to be transferred to NADP+.

Describe the process of when the reaction center is excited.

When excited, the reaction center transfers an electron to an adjacent molecule that acts as an electron acceptor. When the transfer takes place, the reaction center becomes oxidized and the adjacent electron-acceptor molecule is reduced. The result is the conversion of light energy into a chemical form. This initiates a light driven chain of redox reactions that leads to the formation of NADPH. Once the reaction center has lost an electron, it can no longer absorb light or contribute to additional electrons. Thus another electron must be delivered to take the place of the one that has entered the ETC.

what happens when visible light is absorbed by a chlorophyll molecule

When visible light is absorbed by a chlorophyll molecule, one of its electrons is elevated to a higher energy state. For chlorophyll molecules that have been extracted from chloroplasts in the lab, this absorbed light energy is rapidly released, allowing the electron to return to its initial "ground" energy state. Most of the energy is converted into heat; a small amount is remitted as light.