IB Biology Review

IB Biology Review 3.8 & 8.2 : Photosynthesis

Relationship Between Photosynthesis and Respiration • Products of photosynthesis are reactants in respiration

What is Photosynthesis? • The conversion of light energy to chemical energy in the chemical bonds of glucose Which organisms perform photosynthesis? • Autotrophs / Producers • Some bacteria, some protists, plants

Where Does Photosynthesis Occur? • In the chloroplast What is a chloroplast? • An organelle found only in plants and photosynthetic protists • absorbs sunlight and uses that energy to drive the synthesis of glucose from carbon dioxide and water Where does photosynthesis occur in the chloroplast? • Thylakoid What is a thylakoid? • A flattened membrane sac inside the chloroplast

What chemical does photosynthesis need to occur? • Chlorophyll What is chlorophyll? • The main photosynthetic pigment that absorbs solar energy in photosynthesis • Two types • Chlorophyll A • Chlorophyll B • both absorb in the blue/violet wavelengths, but at slightly different wavelengths

Chloroplast Structure • Be able to draw these structures • Outer membrane • Inner membrane • Stroma • Granum • Lumen • Thylakoid

The Reactions of Photosynthesis • Two main sets of reactions Light Dependent Reactions • The stage of photosynthesis that occurs in the thylakoid membranes • Converts solar energy to the chemical energy of ATP and NADPH Light Independent Reactions / Calvin Reactions • The stage of photosynthesis which combines CO2 and the chemical energy obtained from the light reactions (ATP and NADPH) • Creates 3-carbon sugars which are then combined to produce the 6-carbon glucose

Overview of Photosynthesis

Photosynthesis: Step 1Light Absorption & Photolysis • Chlorophyll absorbs sunlight / energy from proton in Photosystem II • Electron donated by donated by the splitting of water (photolysis) is given energy from sunlight • Oxygen (O2) released

Photosynthesis: Step 2Chemiosmosis, Excitation and Photophosphorylation • Electron moves through the electron transport chain • Electron gives energy to proteins to pump protons into the thylakoid space • This proton gradient is used by ATPase to make ATP • Electron goes to photosystem I at end of electron transport chain • This electron is re-excited by another burst of light • Electron leaves by photosystem I • This re-excited electron is then passed to NADP+ • Produces NADPH

Comparison of Chemiosmosis in Mitochondria and Chloroplasts In both organelles • Electron transport chains pump protons (H+) across a membrane • Protons go from a region of low H+ concentration (light gray in this diagram) to one of high H+ concentration (dark gray) • Protons then diffuse back across the membrane through ATP synthase • Produces ATP

Overview of Light Dependent Reactions

Photosynthesis: Step 3Calvin Cycle Part 1: Carbon Fixation • Three different carbon molecules from CO2 from air combine with three different RuBP molecules in stroma of chloroplast • Produces six 3-phosophoglycerate molecules

Photosynthesis: Step 3Calvin Cycle Part 2: Reduction • Molecule gets phosphorylated by ATP • Molecule is reduced (gains an electron) by NADPH • Becomes G3P • For every 6 G3P molecules formed,one leaves the Calvin Cycle to make glucose • 2 G3P (3 carbon) = 1 glucose (6 carbon)

Photosynthesis: Step 3Calvin Cycle Part 3: Regeneration of RuBP • The other five G3P molecules stay in the Calvin Cycle • Get phosphorylated by ATP • Regenerates RuBP

Review of Calvin Cycle • RuBP combines with CO2 to produce 3 phosophoglycerate • After being phosphorylated by ATP, this resultant molecule is reduced by NADPH to become a molecule called G3P • For every 6 G3P molecules formed, only one leaves the Calvin Cycle to make glucose • It takes two G3P (3-carbon) molecules to make one glucose (6-carbon) molecule • The other G3P molecules stay in the Calvin Cycle and are phosphorylated by ATP to regenerate RuBP

Review of Photosynthetic Processes

Chloroplast Structure and Function • The thylakoids produce a lot of membrane surface area within the chlorophlast • Increased surface area increases space for the light dependent reactions to occur • More light dependent reactions produce more ATP and NADPH • More ATP and NADPH increase the rate of glucose production in the Calvin Cycle

What three factors limit the rate of photosynthesis? • Temperature • Gradual rise in rate • Then sudden drop as proteins denature • Light Intensity • Gradual rise in rate • Followed by a leveling off as maximum rate of photosynthesis reached • CO2 • Gradual rise in rate • Followed by a leveling off as maximum rate of photosynthesis reached

Absorption Spectrum • A graph showing how well the three photosynthetic pigments (chlorophyll a and b, and carotenoids) absorb various wavelengths of light • Shows what waves of light plants absorb • This graph has a similar overall trend as the Action Spectrum graph

Action Spectrum • A graph that shows how effective different wavelengths of light are in driving photosynthesis • Shows what waves of light plants use for photosynthesis • This graph has a similar overall trend as the Absorption Spectrum graph

Measuring Photosynthesis • Four main ways • Production of oxygen because • Oxygen is a byproduct of photosynthesis • Measure carbon dioxide uptake because • carbon dioxide is a reactant of photosynthesis • Measure increase in biomass because • products of photosynthesis are used in production of cell walls and new tissue • Measure electron flow in light reaction using • DPIP as an electron acceptor • Measuring the color change as it is reduced with a spectrophotometer

IB Exam Question 1. Which two colours of light does chlorophyll absorb most? (1 mark) A. Red and yellow B. Green and blue C. Red and green D. Red and blue • Correct answer: D

IB Exam Question 2. Explain photophosphorylation in terms of chemiosmosis. (4 marks) • Electron transport chain (ETC) causes proton/hydrogen ion pumping; • This creates a high proton gradient inside thylakoids; • Protons then pass by diffusion into the stroma through ATP synthase; • ATP synthase catalyses phosphorylation of ADP to produce ATP;

IB Exam Question 3. What is needed in photosynthesis to convert carbon dioxide into organic molecules?(1 mark) A. Light and hydrogen from the splitting of water B. Light and oxygen from the splitting of water C. ATP and hydrogen from the splitting of water D. ATP and oxygen from the splitting of water • Correct answer: C

IB Exam Question 4. Describe the reactions in the light independent part of photosynthesis. • Carbon Fixation - RuBP combines with CO2 to produce 3 phosophoglycerate • After being phosphorylated by ATP, this resultant molecule is reduced by NADPH to become a molecule called G3P • For every 6 G3P molecules formed, only one leaves the Calvin Cycle to make glucose • It takes two G3P (3-carbon) molecules to make one glucose (6-carbon) molecule • The other G3P molecules stay in the Calvin Cycle and are phosphorylated by ATP to regenerate RuBP

IB Exam Question 5. Explain how the light-independent reactions of photosynthesis rely on light-dependent reactions. (8 marks) • light-independent reaction fixes CO2; • to make glycerate 3-phosphate; • glycerate 3-phosphate / G3P becomes reduced; • phosphoglyceraldehyde / glyceraldehyde 3-phosphate; • using NADPH; • using ATP; • ATP needed to regenerate RuBP; • ATP is made in light-dependent reactions; • light causes excitation of electrons; • flow of electrons causes pumping of protons into thylakoid; • ATP formation when protons pass back across thylakoid membrane; • electrons are passed to NADP/NADP+; • NADPH produced in the light dependent reactions;

IB Exam Question 6. Describe the relationship between chloroplast structure and function.(4 marks) • The thylakoids produce a lot of membrane surface area within the chloroplast • This increased surface area increases the rate of light dependent reactions in the electron transport chain (ETC) • Faster light dependent reactions produce more ATP and NADPH • These molecules –in turn- increase the rate of glucose production in the Calvin Cycle

IB Exam Question 7. Explain the reasons for a shape of the thylakoid membranes in the chloroplast.(4 marks) • Shape leads to more surface area on the surface of the thylakoids • This leads to more photosynthetic pigments and higher rate of light reactions (photosystem 1 and 2) • This leads to more ATP and NADPH produced for the Calvin Cycle • This leads to more glucose (sugar) produced in the Calvin Cycle

IB Exam Question 8. Outline the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis. (6 marks) light: • rate of photosynthesis increases as light intensity increases; • photosynthetic rate reaches plateau at high light levels; CO2: • photosynthetic rate rises as CO2 concentration rises; • up to a maximum when rate levels off; temperature: • rate of photosynthesis increases with increase in temperature; • up to optimal level / maximum; • high temperatures reduce the rate of photosynthesis; • Some of the above points may be achieved by means of annotated diagrams or graphs.

IB Exam Question 9. Explain three ways in which the rate of photosynthesis can be measured. (4 marks) • Production of oxygen because oxygen is a by product of the reaction; • Measure carbon dioxide uptake because carbon dioxide is a raw material of the reaction; • Measure increase in biomass; because products are used in production of cell walls and new tissue; • Measure electron flow in light reaction using DPIP as an electron acceptor and measuring the color change as it is reduced with a spectrophotometer

IB Exam Question 10. Explain the reactions involving the use of light energy that occur in the thylakoids of the chloroplast.(8 marks) • Splitting of water/ photolysis provides an electron • Chlorophyll in Photosystem II absorbs light • Electron is raised to a higher energy level (is excited) • Excited electron passes from photosystem II to carriers in electron transport chain (in thylakoid membrane) • Energy from this excited electron is used to pump hydrogen ions from stroma into thylakoid space, creating high hydrogen ion concentration inside thylakoid.

IB Exam Question 11. Pigments are extracted from the leaves of a green plant. White light is then passed through the solution of pigments. What effect do the leaf pigments have on the white light? (1 mark) A. Green wavelengths are absorbed and red and blue wavelengths are transmitted. B. Red and blue wavelengths are absorbed and green wavelengths are transmitted. C. Blue wavelengths are absorbed and green and red wavelengths are transmitted. D. Green and red wavelengths are absorbed and blue wavelengths are transmitted. • Correct answer: B

IB Exam Question 12. Why is the action spectrum for photosynthesis similar to the absorption spectra of photosynthetic pigments?(1 mark) A. Photosynthetic pigments have the same optimum temperature as the enzymes used in photosynthesis. B. Plants absorb the same photosynthetic pigments for use in photosynthesis. C. Only wavelengths of light absorbed by pigments can be used in photosynthesis. D. The amount of energy absorbed by photosynthetic pigments is equal to the activation energy for photosynthesis. • Correct answer: C

IB Biology Review