PHOTOSYNTHESIS Topics 3.8 and 8.2

State that photosynthesis involves the conversion of light energy into chemical energy State that light from the Sun is composed of a range of wavelengths State that chlorophyll is the main photosynthetic pigment Outline the differences in absorption of red, blue, and green light by chlorophyll State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen

State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules Explain that the rate of photosynthesis can be measure directly by the production of oxygen or the uptake of carbon dioxide, or indirectly by an increase in biomass Outline the effects of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis

Light energy to chemical energy Sun provides light energy as a mixture of wavelengths Photosynthetic organisms use photosynthesis to convert the visible portion of the spectrum to chemical energy (glucose)

Absorb/Reflect Substances can do one of two things when struck by a particular wavelength of light: –Absorb that wavelength (energy absorbed and usable) –Reflect that wavelength (energy not absorbed and therefore color is visible) Chlorophyll is a green pigment What can you deduce about absorption/reflection of red, green, and blue light by chlorophyll?

Chlorophyll

Light-dependent reactions 1.Chlorophyll (and other pigments) absorbs light energy 2.Converts light energy to chemical energy (ATP) 3.Light energy used to accomplish photolysis of water (splitting into hydrogen and oxygen) 4.Oxygen released as waste product

Light-independent reactions 1.ATP and hydrogen used to convert (fix) carbon dioxide and water into useful organic molecules 2.6CO 2 needed to form one glucose molecule 3.This conversion of an inorganic form of an element to an organic form is known as a fixation

Summary of photosynthesis A series of reactions in which carbon dioxide and water are fixed into glucose, and oxygen is produced as a by-product The energy needed comes directly from the ATP and hydrogen created from sunlight- driven reactions

Measuring rate of photosynthesis 6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 Measuring rate of oxygen production or carbon dioxide intake is considered to be a direct measurement of photosynthetic rate as long as a correction is made for cell respiration Keeping track of the change in biomass of experimental plants is considered to be an indirect reflection of photosynthetic rate

Factors affecting rate of photosynthesis Light intensity Temperature Carbon dioxide

Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs State that photosynthesis consists of light- dependent and light-independent reactions Explain the light-dependent reactions Explain phosphorylation in terms of chemiosmosis Explain the light-independent reactions Explain the relationship between the structure of the chloroplast and its function

Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide

Chloroplast All of the photosynthetic process occurs within the chloroplast Has a double membrane (like mitochondrion) Has its own DNA (like mitochondrion) Very similar in size to a mitochondrion Most found inside leaves

Light-dependent reactions Occurs in the thylakoids of chloroplast Stack of thylakoids is called a granum Driven by light (mostly the Sun) Pigments absorbs photons of light at different wavelengths –Chlorophylls –Carotenoids

Photosystems Regions of organized pigments on the membranes of the thylakoids Include: –Chlorophyll α molecules –Accessory pigments –Protein matrix Reaction center of photosystem contains: –Pair of chlorophyll molecules –Matrix of protein –Primary electron acceptor Modern-day plants have 2 Photosystem I is most efficient at wavelength of 700 nm Photosystem II is most efficient at wavelength of 680 nm

Photosystem II (P680)Photosystem I (P700)

The light-dependent reaction 1.A photon of light is absorbed by a pigment in photosystem II and is transferred until it reaches one of the chlorophyll α (P680) molecules in the reaction center. The photon energy excites one of the chlorophyll α electrons to a higher energy state. 2.This electron is captured by the primary acceptor of the reaction center 3.Water is split by an enzyme to produce electrons, hydrogen ions, and an oxygen atom. This process is driven by light and is called photolysis. The electrons are supplied to the chlorophyll α molecules of the reaction center.

4.The excited electrons pass from the primary acceptor down an ETC losing energy at each exchange. 5.The energy lost from the electrons moving down the ETC drives chemiosmosis to bring about photophosphorylation (H+ move from thylakoid space back into stroma) of ADP to produce ATP 6.A photon of light is absorbed by a pigment in photosystem I. This energy is transferred through several accessory pigments until received by a chlorophyll α (P700) molecule. This results in an electron with a higher energy state being transferred to the primary electron acceptor. The de-energized electron from photosystem II fills the void left by the newly energized electron.

7.The electron with the higher energy state is passed down a second ETC. 8.The enzyme NADP reductase catalyzes the transfer of the electron to the energy carrier NADP+. Two electrons are required to fully reduce NADP+ to NADPH.

Final products of the light-dependent reaction NADPH and ATP (oxygen byproduct) Both supply chemical energy for the light- independent reaction

Light-independent reaction (Calvin cycle) Occurs within the stroma of the chloroplast Energy provided by ATP and NADPH produced by the light-dependent reaction 1.Ribulose biphosphate (RuBP), a 5-carbon compound, binds to an incoming CO 2 molecule in a process called carbon fixation. This fixation is catalyzed by an enzyme called RuBP carboxylase. The result is an unstable 6-carbon compound.

2.The unstable 6-carbon compound breaks down into two 3-carbon compounds called glycerate-3-phosphate 3.The molecule molecules of glycerate-3- phosphate are acted on by ATP and NADPH to form two more compounds called triose phosphate (TP). This is a reduction reaction.

4.The molecules of TP may then go one of two directions. Some leave the cycle to become sugar phosphates that may become more complex carbohydrates. Most continue in the cycle to reproduce the originating compound of the cycle, RuBP. 5.In order to regain RuBP molecules from TP, the cycle uses ATP.

Costs for production of 6 RuBP and one molecule of glucose 18 ATPs 12 NADPH

Action and absorption spectra of photosynthesis Light is electromagnetic energy which travels in rhythmic waves The entire range of radiation is referred to as the electromagnetic spectrum The part of the spectrum involved in photo. is the visible light spectrum 400 nm to 740 nm The _______ wavelengths have more energy than ________ wavelengths.

Various pigments absorb photons of light from specific wavelengths of the visible spectrum A spectrophotometer measures absorption at various light wavelengths The absorption spectrum of an individual plant is the combination of all the absorption spectra of all the pigments in its chloroplasts

Rate of photosynthesis at a particular wavelengths of visible light is referred to as the action spectrum Correlations: 1.Blue and red light show the greatest absorption and represent the peaks in the rate of photosynthesis 2.Low absorption of green light corresponds to the lower rate of photosynthesis