Chapter 8: Photosynthesis

The Energy Cycle

- photosynthesis uses the products of respiration as starting substrates - respiration uses the products of photosynthesis as starting substrates

What does the cell need to build carbohydrates?

1. Energy: ATP from light-dependent reactions - drives the endergonic reactions 2. Reduction Potential: NADPH from photosystem 1 - provides a source of protons and the energetic electrons needed to bind them to carbon atoms. much of the light energy captured in photosynthesis ends up invested in the energy-rich C----H conds of sugar Calvin Cycle: - biochemical pathway that allows for carbon fixation - occurs in the stroma - uses ATP and NADPH as energy sources - incorporates CO2 into organic molecules

Photosynthesis

6CO2 + 12 H2O + light -------------> C6H12O6 + 6H2O + 6O2 carbon water glucose water oxygen dioxide 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.

Chlorophyll absorption spectra

Chlorophyll a and Chlorophyll b - absorb violet-blue and red light - do not absorb photons with wavelengths between about 500 and 600 nm - light of these wavelengths are reflected - perceived as green Chlorophyll a - the main photosynthetic pigment in plants and cyanobacteria - the only pigment that can act directly to convert light energy to chemical energy Chlorophyll b - ACCESSORY PIGMENT or secondary light-absorbing pigment - complements and adds to the light absorption of chlorophyll a - can absorb photons that chlorophyll a cannot - increases the proportion of the photons in sunlight that plants can harvest - spectrum is shifted toward the green wavelengths

The Electromagnetic Spectrum

Gamma rays (0.001 nm -1 nm) X-rays (1nm - 10nm) UV light (10nm - 1000nm) - short wavelengths; lots of energy - potentially damaging energy to organisms; potentially causing cancer; disrupts cells and DNA Visible light (400nm - 740nm) - important for photosynthesis Radio waves - bounces all around and is not harmful

Stages of Photosyntthesis

Photosynthesis takes place in three stages: 1. capturing energy from the sunlight 2. using the energy to make ATP and to reduce the compound NADP+ , an electron carrier, to NADPH 3. using the ATP and NADPH to power the synthesis of organic molecules from CO2 in the air The first two stages require light and are commonly called LIGHT-DEPENDENT REACTIONS. The third stage, the formation of organic molecules from CO2, is called CARBON FIXATION, this process takes place via a cyclic series of reaction. As long as ATP and NADPH are available, the carbon fixation reactions can occur either in the presence or in the absence of light, and so these reactions are also called the LIGHT-INDEPENDENT REACTIONS.

Light dependent reaction

Photosystem 2 acts first: - accessory pigments shuttle energy to the P680 reaction center - excited electrons from the P680 are transferred to the b6-f complex: a series of electron carriers - electron carrier molecules are embedded in the thylakoid membrane - protons are pumped into the thylakoid space to form a proton gradient - electron lost from the P680 is replaced by an electron released by the splitting of water Photosystem 1: - receives energy from an antenna complex - energy is shuttled from P700 reaction center - excited electron is transferred to a membrane-bound electron carrier - electrons are used to reduce NADP+ to NADPH - electrons lost from the P700 are replaced from the b6-f complex ATP is produced via chemiosmosis - ATP synthase is embedded in the thylakoid membrane - protons have accumulated in the thylakoid space - protons move through the stroma only though ATP synthase - ATP is produced from ADP +Pi

Phases of the Cycle

The calvin cycle has 3 phases: 1. carbon fixation RuBP + CO2 -------------------> 2 molecules PGA 2. reduction PGA is reduced to G3P 3. regeneration of RuBP G3P is used to regenerate RuBP Glucose is not a direct product of the Calvin Cycle - 2 molecules of G3P leave the cycle - each G3P contains 3 carbons - 2 G3P are used to produce 1 glucose in reactions in the cytoplasm During the Calvin Cycle energy is needed - the energy is supplied from: - 18 ATP molecules - 12 NADPH molecules

The carbon fixation reaction converts a. inorganic carbon into an organic acid. b. CO2 into glucose. c. inactive rubisco into active rubisco. d. an organic acid into CO2.

a

The colors of light that are most effective for photosynthesis are a. red, blue, and violet. b. green, yellow, and orange. c. infrared and ultraviolet. d. All colors of light are equally effective.

a

The overall process of photosynthesis a. results in the reduction of CO2 and the oxidation of H2O. b. results in the reduction of H20 and the oxidation of CO2. c. consumes O2 and produces CO2. d. produces O2 from CO2.

a

Which region of a chloroplast is associated with the capture of light energy? a. Thylakoid membrane b. Outer membrane c. Stroma d. Both a and c are correct.

a

Oxygenic Photosynthesis

a form of photosynthesis that produces oxygen - found in: - cyanobacteria - seven groups of algae - all land plants

Photon

a particle of light - acts like a discrete bundle of energy - energy content of a photon is inversely proportional to the wavelength of light: short wavelength light contains photons of higher energy than long-wavelength light a beam of light is able to remove electrons from certain molecules, creating an electrical current, this phenomenon is called the PHOTOELECTRIC EFFECT, and it occurs when photons transfer energy to electrons. - the strength of the photoelectric effect depends on the wavelength of light - short wavelengths are much more effective than the long ones in producing the effect because they have more energy

During noncyclic photosynthesis, photosystem 1 functions to ________, and photosystem 2 functions to __________. a. synthesize ATP; produce O2 b. reduce NADP+; oxidize H2O c. reduce CO2; oxidize NADPH d. restore an electron to its reaction center; gain an electron from water

b

How is a reaction center pigment in a photosystem different from a pigment in the antenna complex? a. The reaction center pigment is a chlorophyll molecule. b. The antenna complex pigment can only reflect light. c. The reaction center pigment loses an electron when it absorbs light. d. The antenna complex pigments are not attached to proteins.

c

If the Calvin cycle runs through six turns a. all of the fixed carbon will end up in the same glucose molecule. b. 12 carbons will be fixed by the process. c. enough carbon will be fixed to make one glucose, but they will not all be in the same molecule. d. one glucose will be converted into six CO2.

c

The ATP and NADPH from the light reactions are used a. in glycolysis roots. b. directly in most biochemical reactions in the cell. c. during the reactions of the Calvin cycle to produce glucose. d. to synthesize chlorophyll.

c

The light-dependent reactions of photosynthesis are responsible for the production of a. glucose b. CO2 c. ATP and NADPH d. H20

c

Carbon Fixation Reactions

carbon fixation- the incorporation of CO2 into organic molecules - occurs in the first step of the Calvin Cycle ribulose-bis-phosphate + CO2 --------------> 2 (PGA) 5 carbons 1 carbon 3 carbons The event that makes the reduction of CO2 possible is the attachment of CO2 to a highly specialized organic molecule. Photosynthetic cells produce this molecule by reassembling the bonds of two intermediates in glycolysis- fructose 6-phosphate and glyceraldehyde 3-phosphate (G3P)- to form the energy- rich 5- carbon sugar ribulose 1,5-bisphosphate (RuBP) CO2 reacts with RuBP to form a transient 6-carbon intermediate that immediately splits into two molecules of the three-carbon 3-phosphoglycerate (PGA) The reaction is catalyzed by rubisco

Photosystem Orgnaization

consists of two closely linked components 1. an ANTENNA COMPLEX - hundreds of accessory pigment molecules 2. the REACTION CENTER - one or more chlorophyll a molecules energy of electrons are transferred through the antenna complex to the reaction center At the reaction center, the energy from the antenna complex is transferred to chlorophyll a. This energy causes an electron from chlorophyll to become excited. The excited electron is transferred from chlorophyll a to an electron acceptor * water donates an electron from chlorophyll a to replace the excited electron

The excited electron from photosystem 1 a. can be returned to the reaction center to generate ATP by cyclic photophosphorylation. b. is replaced by oxidizing H2O. c. is replaced by an electron from photosystem 2. d. Both a and c are correct.

d

The overall flow of electron in the light reactions is from a. antenna pigments to the reaction center. b. H2O to CO2. c. photosystem 1 to photosystem 2. d. H2O to NADPH.

d

Which of the following are similarities between the structure and function of mitochondria and chloroplasts? a. They both create internal proton gradients by electron transport. b. They both generate CO2 by oxidation reactions. c. They both have a double membrane system. d. Both a and c are correct.

d

Where does photosynthesis take place?

in CHLOROPLASTS - chloroplasts are located in the leaves the internal membrane of chloroplasts is called the THYLAKOID MEMBRANE - a continuous phospholipid bilayer organized into flattened sacs that are found stacked on one another in columns called GRANA the thylakoid membrane contains CHLOROPHYLL and other photosynthetic pigments for capturing light energy along with machinery to make ATP. surrounding the thylakoid membrane system is a semi-liquid substance called STROMA - houses the enzymes needed to assemble organic molecules from CO2 using energy from ATP coupled with reduction via NADPH. * light dependent reactions occur in the thylakoid membrane * light independent reaction occur in the stroma

Some bacteria use a single photosystem

in sulfur bacteria only one photosystem is used for cyclic photophosphorylation 1. an electron joins a proton to produce hydrogen 2. an electron is recycled to chlorophyll- this process drives the chemiosmotic synthesis of ATP reaction center P840- wavelength of light it is best able to absorbed, not seen by the human eye, near infrared light

Photosystem

light is captured by photosystems - a network of chlorophyll a molecules, accessory pigments, and associated proteins - 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 embedded in the thylakoid membrane

Pigments

molecules that absorb light energy in the visible range - most familiar with them as dyes that impart color to clothing or other materials - the color we see is the color that is not absorbed- that is, it is reflected when a photon strikes a molecule with the amount of energy needed to excite an electron, then the molecule will absorb the photon raising the electron to a higher energy level, whether the photon's energy is absorbed depends on how much energy it carries (defined by its wavelength), and also on the chemical nature of the molecule it hits - each pigment has a characteristic ABSORPTION SPECTRUM, the range and efficiency of photons it is capable of absorbing

Light-dependent reaction

occur in four stages: 1. PRIMARY PHOTOEVENT: a photon of light is captured by a pigment. this primary photoevent excites an electron with the pigment 2. CHARGE SEPARATION: this excitation energy is transferred to the reaction center, which transfers and energetic electron to an acceptor molecule, initiating electron transport 3. ELECTRON TRANSPORT: electrons move through carriers to reduce NADP+ to NADPH 4.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

Accessory pigments

secondary pigments absorbing light wavelengths other than those absorbed by chlorophyll a - increase the range if light wavelengths that can be used in photosynthesis - include: chlorophyll b, carotenoids (plants), and phycobiloproteins (algae and cyanobacteria) - carotenoids also act as antioxidants - during photosynthesis free radicals are produced as byproducts which can damage the cell, the free radicals are absorbed by the carotenoids to make them less damaging

Discovery of Photosynthesis

the work of many scientists led to the discovery of how photosynthesis works. Jan Baptista van Helmont (1580-1644) - willow tree Joseph Priestly (1733-1804) - candle; living vegetation adds something to the air Jan Ingenhousz (1730-1799) - oxygen is released as O2 gas into the air while the carbon atom combined with water to form carbohydrates F.F. Blackman (1866-1947) - light and dark reactions

Chloroplasts have two connected photosystems light dependent reactions

working together, the two photosystems carry out a noncyclic photophosphorylation- transfer of electrons that generate both ATP and NADPH 1. Photosystem 1 - has an absorption peak of 700nm, so its reaction center is called P700 - can pass electrons to NADPH 2. Photosystem 2 - has an absorption peak of 680nm, so its reaction center is called P680 - can generate an oxidation potential high enough to oxidize water named in order of their discovery and not in the order in which they operate