Biology Chapter 8 (Section 8.1-8.7)

A photosystem consists of which two closely linked components?

(1) an antenna complex of hundreds of pigment molecules that gather photons and feed the captured light energy to the reaction center; and (2) a reaction center consisting of one or more chlorophyll a molecules in a matrix of protein, that passes excited electrons out of the photosystem.

Define photoelectric effect. When does it occur?

- The ability of a beam of light to excite electrons, creating an electrical current. - occurs when photons transfer energy to electrons

Structure of a chloroplast

- The internal membrane of chloroplasts, called the thylakoid membrane, is a continuous phospholipid bilayer organized into flattened sacs that are found stacked on one another in columns called grana (singular, granum). - The thylakoid membrane contains chlorophyll and other photosynthetic pigments for capturing light energy along with the machinery to make ATP. - Connections between grana are termed stroma lamella. - Surrounding the thylakoid membrane system is a semiliquid substance called stroma. The stroma houses the enzymes needed to assemble organic molecules from CO2 using energy from ATP coupled with reduction via NADPH. - In the thylakoid membrane, photosynthetic pigments are clustered together to form photosystems, which show distinct organization within the thylakoid.

Structure of chlorophylls

- These pigments contain a complex ring structure, called a porphyrin ring, with alternating single and double bonds - At the center of the ring is a magnesium atom - Different side groups attached to the outside of the ring alter the absorption properties of different types of chlorophyll

Chlorophyll b

- an accessory pigment, or secondary light-absorbing pigment, complements and adds to the light absorption of chlorophyll a. - The primary type of light-absorbing pigment in photosynthesis. Chlorophyll a absorbs light in the violet-blue and the red ranges of the visible light spectrum; chlorophyll b is an accessory pigment to chlorophyll a, absorbing light in the blue and red-orange ranges; neither pigment absorbs light in the green range, 500-600 nm.

What is the structure of a cartenoid and what is their role in photosynthesis?

- consist of carbon rings linked to chains with alternating single and double bonds - They can absorb photons with a wide range of energies, although they are not always highly efficient in transferring this energy - assist in photosynthesis by capturing energy from light composed of wavelengths that are not efficiently absorbed by chlorophylls

What is one way to study the role that pigments play in photosynthesis? What has this found?

- measure the correlation between the output of photosynthesis and the intensity of illumination—that is, how much photosynthesis is produced by how much light - Experiments on plants show that the output of photosynthesis increases linearly at low light intensities, but finally becomes saturated (no further increase) at high-intensity light. Saturation occurs because all of the light absorbing capacity of the plant is in use.

Chlorophyll a

- the main photosynthetic pigment in plants and cyanobacteria and is the only pigment that can act directly to convert light energy to chemical energy - The primary type of light-absorbing pigment in photosynthesis. Chlorophyll a absorbs light in the violet-blue and the red ranges of the visible light spectrum; does not absorb light in the green range, 500-600 nm.

What are the three stages of photosynthesis?

1. capturing energy from sunlight 2. using the energy to make ATP and to reduce the compound NADP+, an electron carrier, to NADPH; and 3. using the ATP and NADPH to power the synthesis of organic molecules from CO2 in the air.

Photosynthetic pigment arrays are thought to have evolved more than __ bya in bacteria similar to the purple and green bacteria alive today.

2

In the green sulfur bacteria, peak absorption occurs at what wavelength?

840 nm

In the purple nonsulfur bacteria, peak absorption occurs at what wavelength? Therefore what is the reaction center pigment called?

870 nm (near infrared, not visible to the human eye), and thus the reaction center pigment is called P870

Antenna complex

A complex of hundreds of pigment molecules in a photosystem that collects photons and feeds the light energy to a reaction center - also called the light harvesting complex

Action spectrum

A measure of the efficiency of different wavelengths of light for photosynthesis. In plants it corresponds to the absorption spectrum of chlorophylls.

The reaction center of purple photosynthetic bacteria is simpler than the one in chloroplasts but better understood.

A pair of bacteriochlorophyll a molecules acts as a trap for photon energy, passing an excited electron to an acceptor precisely positioned as its neighbor.

Photon

A particle of light having a discrete amount of energy.

Primary photoevent

A photon of light is captured by a pigment. This primary photoevent excites an electron within the pigment.

Cytochrome b6-f complex

A proton pump found in the thylakoid membrane. This complex uses energy from excited electrons to pump protons from the stroma into the thylakoid compartment.

Accessory pigment

A secondary light-absorbing pigment used in photosynthesis, including chlorophyll b and the carotenoids, that complement the absorption spectrum of chlorophyll a.

Reaction center

A transmembrane protein complex in a photosystem that receives energy from the antenna complex exciting an electron that is passed to an acceptor molecule.

Phycobiliprotein

A type of accessory pigment found in cyanobacteria and some algae. Complexes of phycobiliprotein are able to absorb light energy in the green range - contain a system of alternating double bonds similar to those found in other pigments and molecules that transfer electrons - can be organized to form another light-harvesting complex that can absorb green light, which is reflected by chlorophyll - These complexes are probably ecologically important to cyanobacteria, helping them to exist in low-light situations in oceans. In this habitat, green light remains because red and blue light has been absorbed by green algae closer to the surface.

Why does NADPH not form in cyclic phosphorylation in purple nonsulfur bacteria?

Absorption of a photon by chlorophyll P870 does not raise an electron to a high enough level to be passed to NADP, so they must generate reducing power in a different way.

Photosystem

An organized complex of chlorophyll, other pigments, and proteins that traps light energy as excited electrons. Plants have two linked photosystems in the thylakoid membrane of chloroplasts. Photosystem II passes an excited electron through an electron transport chain to photosystem I to replace an excited electron passed to NADPH. The electron lost from photosystem II is replaced by the oxidation of water.

Cartenoids

Any of a group of accessory pigments found in plants; in addition to absorbing light energy, these pigments act as antioxidants, scavenging potentially damaging free radicals.

In addition to their role as light absorbing molecules, what other valuable role do cartenoids play?

Carotenoids also perform a valuable role in scavenging free radicals. The oxidation-reduction reactions that occur in the chloroplast can generate destructive free radicals. Carotenoids can act as general-purpose antioxidants to lessen damage.

Chlorophyll absorption spectra

Chlorophylls absorb photons within narrow energy ranges. Two kinds of chlorophyll in plants, chlorophyll a and chlorophyll b, preferentially absorb violet-blue and red light. Neither of these pigments absorbs photons with wavelengths between about 500 and 600 nm; light of these wavelengths is reflected.

The cells of plant leaves contain organelles called _________, which carry out the photosynthetic process

Chloroplasts

General explanation of how photosystems in chloroplasts work

Each photosystem is a network of chlorophyll a molecules, accessory pigments, and associated proteins held within a protein matrix on the surface of the photosynthetic membrane. Like a magnifying glass focusing light on a precise point, a photosystem channels the excitation energy gathered by any one of its pigment molecules to a specific molecule, the reaction center chlorophyll. This molecule then passes the energy out of the photosystem as excited electrons that are put to work driving the synthesis of ATP and organic molecules.

Enhancement effect

Evidence for the action of two photosystems came from experiments that measured the rate of photosynthesis using two light beams of different wavelengths: one red and the other far-red. Using both beams produced a rate greater than the sum of the rates using individual beams of these wavelengths. This surprising result, called the enhancement effect, can be explained by a mechanism involving two photosystems acting in series (that is, one after the other), one photosystem absorbs preferentially in the red, the other in the far-red.

What happens in the photosystem of green sulfur bacteria?

Excited electrons from this photosystem can either be passed to NADPH, or returned to the chlorophyll by an electron transport chain similar to the purple bacteria. They then use electrons from hydrogen sulfide to replace those passed to NADPH.

Stroma

In chloroplasts, the semiliquid substance that surrounds the thylakoid system and that contains the enzymes needed to assemble organic molecules from CO2.

Light dependent reactions

In photosynthesis, the reactions in which light energy is captured and used in production of ATP and NADPH. In plants this involves the action of two linked photosystems.

Light independent reactions

In photosynthesis, the reactions of the Calvin cycle in which ATP and NADPH from the light-dependent reactions are used to reduce CO2 and produce organic compounds such as glucose. This involves the process of carbon fixation, or the conversion of inorganic carbon (CO2) to organic carbon (ultimately carbohydrates).

What is this weak electron donor in chloroplasts?

In plant chloroplasts, water serves as this weak electron donor. When water is oxidized in this way, oxygen is released along with two protons (H+)

Do either of these systems generate sufficient oxidizing power to oxidize H2O?

No, so they are both anoxygenic and anaerobic.

Photosystem I What is its absorption peak in nm? What is its reaction center pigment called? What does it do?

Photosystem I has an absorption peak of 700 nm, so its reaction center pigment is called P700. This photosystem can pass electrons to NADPH similarly to the photosystem found in the sulfur bacteria discussed earlier.

In plants and algae, what are the specialized different roles the two photosystems do in the overall process of oxygenic photosynthesis?

Photosystem I transfers electrons ultimately to NADP+, producing NADPH. The electrons lost from photosystem I are replaced by electrons from photosystem II. Photosystem II with its high oxidation potential can oxidize water to replace the electrons transferred to photosystem I. Thus there is an overall flow of electrons from water to NADPH.

Photosystem II What is its absorption peak in nm? What is its reaction center pigment called? What does it do?

Photosystem II, has an absorption peak of 680 nm, so its reaction center pigment is called P680. This photosystem can generate an oxidation potential high enough to oxidize water.

The thylakoid reactions take place in what four stages?

Primary photoevent Charge separation Electron transport Chemiosmosis

Carbon fixation

The conversion of CO2 into organic compounds during photosynthesis; the first stage of the dark reactions of photosynthesis, in which carbon dioxide from the air is combined with ribulose 1,5-bisphosphate.

Describe what occurs in the antenna complex.

The excitation energy resulting from the absorption of a photon passes from one pigment molecule to an adjacent molecule on its way to the reaction center. After the transfer, the excited electron in each molecule returns to the low-energy level it had before the photon was absorbed. Consequently, it is energy, not the excited electrons themselves, that passes from one pigment molecule to the next. The antenna complex funnels the energy from many electrons to the reaction center.

Electron transport

The excited electrons are shuttled along a series of electron carrier molecules embedded within the photosynthetic membrane. Several of them react by transporting protons across the membrane, generating a proton gradient. Eventually the electrons are used to reduce a final acceptor, NADPH.

Where does the light dependent reaction occur in chloroplasts?

The internal thylakoid membrane is highly organized and contains the structures involved in the light-dependent reactions

Why do these photosynthetic organisms not use a pigment like retinal (the pigment in our eyes), which has a broad absorption spectrum that covers the range of 500 to 600 nm?

The most likely hypothesis involves photoefficiency. Although retinal absorbs a broad range of wavelengths, it does so with relatively low efficiency. Chlorophyll, in contrast, absorbs in only two narrow bands, but does so with high efficiency. Therefore, plants and most other photosynthetic organisms achieve far higher overall energy capture rates with chlorophyll than with other pigments.

Chlorophyll

The primary type of light-absorbing pigment in photosynthesis. Chlorophyll a absorbs light in the violet-blue and the red ranges of the visible light spectrum; chlorophyll b is an accessory pigment to chlorophyll a, absorbing light in the blue and red-orange ranges. Neither pigment absorbs light in the green range, 500-600 nm.

What does the protein matrix do?

The protein matrix holds individual pigment molecules in orientations that are optimal for energy transfer.

Chemiosmosis

The protons that accumulate on one side of the membrane now flow back across the membrane through ATP synthase where chemiosmotic synthesis of ATP takes place, just as it does in aerobic respiration

Absorption spectrum

The relationship of absorbance vs. wavelength for a pigment molecule. This indicates which wavelengths are absorbed maximally by a pigment. For example, chlorophyll a absorbs most strongly in the violet-blue and red regions.

noncyclic photophosphorylation

The set of light-dependent reactions of the two plant photosystems, in which excited electrons are shuttled between the two photosystems, producing a proton gradient that is used for the chemiosmotic synthesis of ATP. The electrons are used to reduce NADP to NADPH. Lost electrons are replaced by the oxidation of water producing O2

Do these systems produce oxygen?

These systems do not produce oxygen and so are also anoxygenic.

What does the transferring of the electron itself allow for?

This difference allows the energy absorbed from photons to move away from the chlorophylls, and it is the key conversion of light into chemical energy.

Charge separation

This excitation energy is transferred to the reaction center, which transfers an energetic electron to an acceptor molecule, initiating electron transport.

T or F: No other structure in a plant cell is able to carry out photosynthesis

True

T or F: The linked photosystems of cyanobacteria and plant chloroplasts generate the oxidizing power necessary to oxidize H2O, allowing it to serve as a source of both electrons and protons. This production of O2 by oxygenic photosynthesis literally changed the atmosphere of the world.

True

Converting light energy into chemical energy

When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level. This light-energized electron can be transferred to the primary electron acceptor, reducing it. The oxidized chlorophyll then fills its electron "hole" by oxidizing a donor molecule. The source of this donor varies with the photosystem,.

The path of an electron in purple nonsulfur bacteria

When the P870 reaction center absorbs a photon, the excited electron is passed to an electron transport chain that passes the electrons back to the reaction center, generating a proton gradient for ATP synthesis

The color that we see is the color that is not _______—that is, it is _______.

absorbed reflected

A specific atom can absorb only certain photons of light—namely, those that correspond to the atom's available energy levels. As a result, each molecule has a characteristic _______ _________, the range and efficiency of photons it is capable of absorbing.

absorption spectrum

Why is the two stage process of using photosystem II and I called noncyclic photophosphorylation?

because the path of the electrons is not a circle—the electrons ejected from the photosystems do not return to them, but rather end up in NADPH. The photosystems are replenished with electrons obtained by splitting water.

What is this called cyclic photophosphorylation?

because this process returns the electrons back to the reaction center

Why is this protective role is not surprising?

because unlike the chlorophylls, carotenoids are found in many different kinds of organisms, including members of all three domains of life.

How do the two linked photosystems overcome the limitations of cyclic photophosphorylation?

by providing an alternative source of electrons from the oxidation of water. The oxidation of water also generates O2, thus oxygenic photosynthesis. The noncyclic transfer of electrons also produces NADPH, which can be used in the biosynthesis of carbohydrates.

The third stage is called _____ ________ because organic molecules are formed from CO2.

carbon fixation

In chloroplasts, light-harvesting complexes consist of a web of ________ molecules linked together and held tightly in the _______ membrane by a matrix of _______. Varying amounts of ________ accessory pigments may also be present.

chlorophyll thylakoid proteins cartenoid

In photosynthesis, _______ act as the photoelectric devices.

chloroplasts

In plants and algae, photosynthesis is carried out by _________, which are thought to be the evolutionary descendants of _________ _______.

chloroplasts photosynthetic bacteria

Oxygenic photosynthesis is found in which organisms?

cyanobacteria, seven groups of algae, and essentially all land plants.

These two photosystems are connected by a complex of electron carriers called the...

cytochrome/b6-f complex

The wave nature of light produces an ________ _______that differentiates light based on its wavelength

electromagnetic spectrum

In many bacteria, the plasma membrane _____ in on itself repeatedly to produce an increased _______ ______.

folds surface area

You may have heard that eating carrots can enhance vision. Is this effect is real and if so what is it probably due to?

he high content of β-carotene in carrots. This carotenoid consists of two molecules of vitamin A joined together. The oxidation of vitamin A produces retinal, the pigment used in vertebrate vision.

Short-wavelength light contains photons of higher or lower energy than long-wavelength light?

higher

Whether the photon's energy is absorbed by a certain molecule depends on (2)?

how much energy it carries (defined by its wavelength), and also on the chemical nature of the molecule it hits

Diagram Z of photosystems I and II

illustrates the two electron-energizing steps, one catalyzed by each photosystem. The horizontal axis shows the progress of the light reactions and the relative positions of the complexes, and the vertical axis shows relative energy levels of electrons. The electrons originate from water, which holds onto its electrons very tightly (redox potential = +820 mV), and end up in NADPH, which holds its electrons much more loosely (redox potential = -320 mV).

The energy content of a photon is _______ proportional to the wavelength of the light.

inversely

In the case of plants, photosynthesis takes place primarily in the _______.

leaves

The first two stages require ______ and are called the _____ _______ reactions.

light light dependent

The third stage is also called the ______ ______ reactions because they can occur either in the presence or absence of ______ as long as ____ and _____ are available.

light independent light ATP NADPH

The light-dependent reactions of photosynthesis occur in _________.

membranes

Working together, the two photosystems carry out a _______ transfer of electrons that generate both ____ and _______.

noncyclic ATP NADPH

What are the two kinds of photosynthesis?

photosynthesis that does not produce oxygen (anoxygenic) and a form that does (oxygenic)

The proteins in the green sulfur bacterial photosystem appear to be homologous to the proteins in the modern __________ ___.

photosystem I

The proteins in the purple bacterial photosystem appear to be homologous to the proteins in the modern ___________ ___.

photosystem II

In chloroplasts and all but one class of photosynthetic prokaryotes, light is captured by __________.

photosystems

Molecules that absorb light energy in the visible range are termed ______.

pigments

In photosynthetic bacteria, the _______ _______ itself is the photosynthetic membrane.

plasma membrane

Photons excite electrons in the _________ ______, which are then channeled away through the alternating carbon single-and double-bond system. Electrons not associated with a single atom or bond are said to be _________.

porphyrin ring delocalized

Anoxygenic photosynthesis is found in which four different bacterial groups?

purple bacteria, green sulfur bacteria, green nonsulfur bacteria, and heliobacteria

This equation is the _______ of the reaction for respiration. How?

reverse - In respiration, glucose is oxidized to CO2 using O2 as an electron acceptor. In photosynthesis, CO2 is reduced to glucose using electrons gained from the oxidation of water. The oxidation of H2O and the reduction of CO2 requires energy that is provided by light

How are these two types of photosynthesis similiar and how are they different?

share similarities in the types of pigments they use to trap light energy, but they differ in the arrangement and action of these pigments

In these bacteria, a _____ photosystem is used that generates ____ via ________ ______.

single ATP electron transport

The energy used by most living cells comes ultimately from the ___ and is captured by plants, algae, and bacteria through the process of ____________.

sun photosynthesis

What does the strength of the photoelectric effect depend on?

the wavelength of light

The photosystems were named I and II in the order of what?

their discovery, and not in the order in which they operate in the lightdependent reactions

For this reason, the reactions are also referred to as the _______ ______.

thylakoid reactions

Note that here in the reaction center, the excited electron is ____________, and not just the energy, as was the case in the pigment-pigment transfers of the antenna complex.

transferred

Plants have____ linked photosystems.

two

There is an overall flow of electrons from _____ to _______.

water NADPH

The _____ concept of light explains the different colors of the spectrum, whereas the ________ concept of light explains the energy transfers during photosynthesis.

wave particle