Photosynthesis Lecture PDF

PHOTOSYNTHESIS  THE BASICS OF PHOTOSYNTHESIS Almost all plants are photosynthetic autotrophs, as are some bacteria and protists – Autotrophs generate their own organic matter through photosynthesis – Sunlight energy is transformed to energy stored in...

PHOTOSYNTHESIS  THE BASICS OF PHOTOSYNTHESIS Almost all plants are photosynthetic autotrophs, as are some bacteria and protists – Autotrophs generate their own organic matter through photosynthesis – Sunlight energy is transformed to energy stored in the form of chemical bonds (c) Euglena (d) Cyanobacteria (b) Kelp (a) Mosses, ferns, and flowering plants Energy can be transformed from one form to another FREE ENERGY (available for work) vs. HEAT (not available for work) THE FOOD WEB Light Energy Harvested by Plants & Other Photosynthetic Autotrophs THE SUN: MAIN SOURCE OF ENERGY FOR LIFE ON EARTH WHY ARE PLANTS GREEN? Different wavelengths of visible light are seen by the human eye as different colors. Gamma Micro- Radio X-rays UV Infrared rays waves waves Visible light Wavelength (nm) Electromagnetic Spectrum and Visible Light Gamma Infrared & rays X-rays UV Microwaves Radio waves Visible light Wavelength (nm) The feathers of male cardinals are loaded with carotenoid pigments. These pigments absorb some wavelengths of light and reflect others. l ight fl ec ted Re Sunlight minus absorbed wavelengths or colors equals the apparent color of an object. Why are plants green? t il gh c ted Transmitted light efle R WHY ARE PLANTS GREEN? Plant Cells have Green Chloroplasts The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids). Photosynthetic Pigments: The Light Receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light Chlorophyll: A Light Absorbing Pigment The Solar Panel Chemical!  Chlorophyll A and B are the two major pigments, involved in photosynthesis. Chlorophyll A is the primary pigment of photosynthesis, trapping the light energy and emitting high ­energy electrons into the two photosystems P680 and P700.  Chlorophyll B is the accessory pigment, passing the trapped energy into chlorophyll A. Thus, the main difference between chlorophyll A and B is their functions in photosynthesis.  Chlorophyll A is present in all the photosynthetic organisms on earth, giving a bluish green color to those organisms. Chlorophyll B gives a yellowish green color to organisms. Chlorophyll B is the accessory pigment in photosynthesis, trapping and passing high energy electrons to chlorophyll A.  The most absorbing wavelengths of chlorophyll A and B are 439 nm and 455 nm respectively. Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll.  THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED Chloroplasts absorb light Reflected energy and Light light convert it to chemical energy Absorbed light Transmitted Chloroplast light  Formula for PHOTOSYNTHESIS Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2 Carbon Water Glucose Oxygen dioxide gas PHOTOSYNTHESIS  Redox Reactions 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2 Equation for photosynthesis  Photosynthesis - overview Overall purpose: photosynthesis – light chemical energy complements respiration Energy for all life on earth ultimately comes from photosynthesis  Photosynthesis – chloroplast recap Outer membrane Inner membrane Thylakoid membrane Stroma Thylakoid space Intermembrane space  Photosynthesis - overview light reactions: dark reactions: – thylakoid membrane – stroma – thylakoid space H2O CO2 Light NADP+ ADP + P Calvin Light Cycle Reactions ATP NADPH Chloroplast O2 [CH2O] (sugar)  AN OVERVIEW OF PHOTOSYNTHESIS The light reactions Light convert solar Chloroplast energy to NADP chemical energy ADP Produce ATP & +P TheNADPH Calvin cycle makes Calvin Light cycle sugar from carbon reactions dioxide – ATP generated by the light reactions provides the energy for sugar synthesis – The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose The Photosynthesis Reaction is divided into two parts: Light Dark Reactions Reactions Light reactions or Dark reactions or “light “light dependent reactions” independent reactions” do not capture light energy to power need light energy to power their photosynthesis. reactions and can occur day or Light reactions occur during the night. day time. Discovered by three scientists, the dark reactions are also called the Calvin-Bensen-Bassham They take place in the thylakoids. cycle or just Calvin Cycle. Pigments in the thylakoid membranes form protein complexes called Photosystem I and Photosystem II. C B B These photosystems harvest Dark reactions occur in the photons to charge up energy stroma of chloroplasts (the carrying molecules that will space that surrounds thylakoids) power the dark reactions. and fix carbon dioxide into Energy Carrying Molecules: ATP & adenosine NADP+ triphosphate Both are energy carrier molecules used in photosynthesis and cellular “AT respiration. P” NADP+ can hold excited A P P P electrons (e-) charged from the light energy harvested by R three adenosine phosphate chlorophyll to become NADPH. = adenine + groups Eventually, NADPH passes the ribose electron it’s holding to power the dark reactions and reverts back ATP is called the “cellular to NADP+. currency” because it is used nicotinamide adenine dinucleotide to power all the reactions phosphate that take place in the cells of all living things. When ATP’s third phosphate N “NADP+ is broken off it releases ” energy that the cell can use. P P A R R ATP is made when a third phosphate group is P added to ADP (diphosphate, di = two). NADP is a very complex molecule, this is a simplification. Light Reactions The energy absorbed by the chlorophyll during the light reactions is used to power photosystem II that breaks the bonds of water absorbed through the plant’s Photosystem II roots. T PSI H I Y L A K Freed oxygen atoms bind with each O other to form the gas O. 2 I O is a byproduct of photosynthesis not D 2 used by the plant so it is released through the stomata of plants. Stomata (Greek for mouth) are little O pores in leaves that open and close to 2 let oxygen out and carbon dioxide in. Light Reactions When water molecules break apart, the remaining two hydrogen atoms have a positive charge and are called protons. These protons are kept inside the thylakoid by the thylakoid Tmembrane. H Y L H H H H H A + + + H H + + K H + + O + When there are more protons inside I ATP the thylakoid than in the stroma D maker outside, protons want to leave the crowded thylakoid. When the protons (H+) cross the membrane to leave, a protein uses their passage to power ATP The protein ATP synthase attaches production. a phosphate group to ADP (D = di or two) making it ATP (T = tri or three). Light Reactions The light energy absorbed by chlorophyll also powers photosystem I that charges up the energy carrier molecule NADP+ into NADPH. NADP NADP + H T PSI PSI NADPH then carries its energy H I over to power the dark reactions Y L or Calvin Cycle. A K O I D  NADP Light Reactions NADP + H Summary T PSI PSI Photons are absorbed by the H I pigments to power photosystem I and Y O_ photosystem II. L H H _ Photosystem II splits water A molecules into two protons (H+) K and oxygen atoms are expelled as O O2 gas through the stomata. I ATP make Protons cross the thylakoid D r membrane and power protein complex ATP synthase to make ATP. NADP+ is powered up by photosystem I to make NADPH to be used in the dark reactions. Light dependent reactions finish with charged NADPH, ATP, and released O2. Dark Reactions Also called the Calvin Cycle, the dark reactions start and end with the same products hence “cycle”. All the dark reactions take place in the stroma of the chloroplast. The Calvin Cycle starts with RuBP molecules and carbon dioxide molecules. An enzyme called Rubisco combines them into an unstable intermediate. RuBP Rubisc o CO Since the This is the reason plants take in intermediate of 2 carbon dioxide, to start the combined RuBP and Calvin Cycle and begin the CO2 is unstable it conversion of RuBP into RuBP is the starting molecule and ending glucose. quickly splits in half molecule of the Calvin Cycle. It will be and forms 2 remade at the end of the cycle so that the molecules of 3-PGA Dark Reactions The ATP and NADPH from the light reactions provide the energy to convert the two molecules of 3-PGA into their final form G3P. The left overs are reused in the light reactions to remake ATP and NADPH. 2 G3P are joined to make a glucose molecule. +  Dark Reactions Not all G3P is made into glucose. The Calvin Cycle occurs in every stroma in every chloroplast in every plant cell every second of every day. That’s a lot of reactions all happening simultaneously! The spent ATP from the Most of the G3P made during the Calvin reaction leaves ADP and a Cycle are made into RuBP, the starting phosphate group. These are molecule, with energy from ATP reused in the light reactions molecules. to make more ATP.  Chloropl Dark ast Reactions Summary The Calvin Cycle converts the carbon from carbon dioxide into glucose in the stroma. This is called carbon fixation because carbon is fixed into another form. Photosynthesis is carried out in two steps. First, in two light dependent photosystems. Second, in a light independent carbon fixation cycle called the Calvin Cycle. Through this process, the plant is able to convert sunlight, water, and CO2 into glucose (or sugar) and ATP.  Photosynthesis: An Overview The net overall equation for photosynthesis is: 6 CO2 + 6 H2O light C6H12O6 + 6 O2 Photosynthesis occurs in 2 “stages”: 1. The Light Reactions (or Light- Dependent Reactions) 2. The Calvin Cycle (or Light- Independent Reactions) Is photosynthesis an ENDERGONIC (ender means in) or EXERGONIC (exer –means out) reaction? 36 Photosynthesis: An Overview To follow the energy in photosynthesis, Light light Calvin Reactions Cycle Organic light ATP compounds thylakoids NADPH (carbs) stroma 37 Phase 2: The Calvin Cycle 38  Cellular Respiration vs. Photosynthesis Cellular Respiration: (Exergonic) Photosynthesis: (Endergonic) Photosynthesis: A Recap Based on this equation, The photosynthetic equation: how could the rate of photosynthesis be Provides the carbon to measured? produce organic The organic compound compounds during the ultimately produced Calvin Cycle during the Calvin Cycle light 6 H2O 6 CO2 6 O2 C6H12O6 Split during the Produced as a light reactions Excites byproduct of the to replace electrons splitting of electrons lost during the water during the from PSII light light reactions reactions 40