Using Light to Make Food Ch. 7 PHOTOSYNTHESIS Using Light to Make Food

Autotrophs Are the Producers of The Biosphere Autotrophs make their own food without using organic molecules derived from any other living thing Photoautotrophs : Autotrophs that use the energy of light to produce organic molecules Most plants, algae and other protists, and some prokaryotes are photoautotrophs (cyanobacteria) The ability to photosynthesize is directly related to the structure of chloroplasts Copyright © 2009 Pearson Education, Inc.

Figure 7.1A Forest plants.

Visible Radiation Drives the Light Reactions Sunlight is a type of electromagnetic energy (radiation) Visible light is a small part of the electromagnetic spectrum Light exhibits the properties of both waves and particles One wavelength = distance between the crests of two adjacent waves (shorter λ’s have greater energy) Light behaves as discrete packets of energy called photons (photons contain a fixed quantity of light energy)

Increasing energy 10–5 nm 10–3 nm 1 nm 103 nm 106 nm 1 m 103 m Micro- Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light Figure 7.6A The electromagnetic spectrum and the wavelengths of visible light. (A wavelength of 650 nm is illustrated.) 380 400 500 600 700 750 Wavelength (nm) 650 nm

Visible Radiation Drives the Light Reactions Pigments are proteins that absorb specific wavelengths of light and transmit others Various pigments are built into the thylakoid membrane We see the color of the wavelengths that are transmitted (not absorbed) Ex. chlorophyll transmits green

Visible Radiation Drives the Light Reactions Chloroplasts contain several different pigments and all absorb light of different wavelengths Chlorophyll a: absorbs blue violet and red light and reflects green Chlorophyll b: absorbs blue and orange and reflects yellow-green The carotenoids: absorb mainly blue-green light and reflect yellow and orange

Photosynthesis is a process that converts solar energy to chemical energy Plants use water and atmospheric carbon dioxide to produce a simple sugar and release oxygen Light energy 6 CO2 + 6 H2O C6H12O6 + 6 O2 Carbon dioxide Water Glucose Oxygen gas Photosynthesis

Photosynthesis Occurs in Chloroplasts in Plant Cells Leaves contain: Stomata = tiny pores in the leaf; allow CO2 to enter and O2 to exit Veins deliver water & nutrients absorbed by roots Chlorophyll= a light absorbing pigment responsible for the green color of plants located on thylakoid membrane Vein CO2 O2 Stoma Figure 7.2 The location and structure of chloroplasts. Chloroplasts

Chloroplast Outer and inner membranes Thylakoid Intermembrane space Stroma Granum Thylakoid space Figure 7.2 The location and structure of chloroplasts.

Photosynthesis is a Redox Process, as is Cellular Respiration Photosynthesis is a redox (oxidation-reduction) process A loss of electrons = oxidation A gain of electrons = reduction (OIL RIG) These electrons are lost and gained in the form of hydrogen In photosynthesis water loses electrons and CO2 gains electrons

Photosynthesis is a Redox Process, as is Cellular Respiration Reduction 6 CO2 + 6 H2O C6H12O6 + 6 O2 Oxidation Figure 7.4A Photosynthesis (uses light energy).

H2O CO2 Chloroplast Light NADP+ ADP  P LIGHT REACTIONS CALVIN CYCLE (in stroma) (in thylakoids) ATP Electrons NADPH O2 Sugar

The “Photo” in Photosynthesis: THE LIGHT REACTIONS

Photosystems Capture Solar Power in the Light Reactions A photosystem is a functional unit that captures sunlight and converts it to chemical energy (ATP & NADPH) A Photosystem consists of a light-harvesting complex surrounding a reaction center Light-harvesting complexes Reaction center e– Figure 7.7B Light-excited chlorophyll embedded in a photosystem: Its electron is transferred to a primary electron acceptor before it returns to ground state. Pigment molecules Pair of Chlorophyll a molecules

Photosystems Capture Solar Power in the Light Reactions Two types of photosystems exist: photosystem I and photosystem II Each type of photosystem has a characteristic reaction center Photosystem II: functions first; the chl. a of this photosystem is called P680 (it best absorbs light at 680nm or red) Photosystem I: functions next; the chl. a of this photosystem is called P700 (it best absorbs light at 700nm, also red)

Photosystems Capture Solar Power in the Light Reactions When light strikes a photosystem, energy is captured by pigments and passed from pigment to pigment within the photosystem From a pigment, energy is passed to chl. a in the reaction center where it excites electrons of chl. a An excited e- from chl. a is transferred to a primary electron acceptor This solar-powered transfer of an electron from the reaction center chl. a to the primary electron acceptor is the first step of the light reactions Electron transport chain Provides energy for synthesis of by chemiosmosis NADP+ + H+ NADPH Photon Photon Photosystem II ATP Photosystem I 6 Stroma 1 Primary acceptor Primary acceptor 2 e– e– Thylakoid mem- brane 4 5 P700 P680 Figure 7.8A Electron flow in the light reactions of photosynthesis: Both photosystems and the electron transport chain that connects them are located in the thylakoid membrane. The energy from light drives electrons from water to NADPH. Thylakoid space 3 H2O 2  1 O2 + 2 H+

The Light Reactions The primary e- acceptor passes electrons to an electron transport chain (ETC) The ETC is a bridge between photosystems II and I. The ETC also generates ATP. To fill the electron void of chl. a, it oxidizes H2O (takes electrons from water) In this is the step, water is oxidized and oxygen is released

Electron transport chain The Light Reactions Electron transport chain Provides energy for synthesis of by chemiosmosis NADP+ + H+ NADPH Photon Photon Photosystem I Photosystem II ATP 6 Stroma 1 Primary acceptor Primary acceptor 2 e– e– Thylakoid mem- brane 4 5 P700 P680 Figure 7.8A Electron flow in the light reactions of photosynthesis: Both photosystems and the electron transport chain that connects them are located in the thylakoid membrane. The energy from light drives electrons from water to NADPH. Thylakoid space 3 H2O 2  1 O2 + 2 H+

The Light Reactions As the second protein of the ETC accepts electrons it pumps H+ into the thylakoid space. Pumping H+ into the thylakoid space generates a proton (H+) gradient Protons (H+) flow from the thylakoid space to the stroma, (down their gradient) through an enzyme called ATP synthase. ATP synthase phosphorylates ADP (forming ATP) using the energy from the flow of H+ down their gradient This is an energy-coupling process where the exergonic flow of H+ powers the endergonic phosphorylation of ADP in a mechanism called: chemiosmosis

The Light Reactions ATP is produced in the stroma Photophosphorylation is the process of generating ATP from ADP & phosphate by means of a proton-motive force generated by the thylakoid membrane in the Light Reactions of photosynthesis

Two Photosystems Connected by an ETC Generate ATP and NADPH Electrons moving down the ETC are passed to P700 of Photosystem I, and ultimately to NADP+ by the enzyme NADP+ reductase This step produces NADPH in the stroma

Electron transport chain Stroma (low H+ concentration) H+ Light Light H+ ADP + P ATP H+ NADP+ + H+ NADPH H+ H2O H+ H+ H+ Figure 7.9 The production of ATP by chemiosmosis in photosynthesis. 2  1 O2 + 2 H+ H+ H+ H+ H+ H+ Photosystem II Electron transport chain Photosystem I H+ ATP synthase H+ Thylakoid space (high H+ concentration)

THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS

ATP and NADPH Power Sugar Synthesis in the Calvin Cycle The Calvin cycle makes sugar within a chloroplast Atmospheric CO2, ATP, and NADPH are required to produce sugar Using these three ingredients, a three-carbon sugar called glyceraldehyde-3-phosphate (G3P) is produced A plant cell may then use G3P to make glucose and other organic molecules CO2 ATP NADPH Input CALVIN CYCLE Figure 7.10A An overview of the Calvin cycle. Output: G3P

ATP and NADPH Power Sugar Synthesis in the Calvin Cycle The Calvin Cycle Has Three Phases: 1. Carbon Fixation- Atmospheric carbon (as CO2) is incorporated into a molecule of ribulose bisphosphate (RuBP) using the enzyme rubisco 3 molecules of CO2 are required to make 1 molecule of G3P 2. Reduction Phase – NADPH reduces 3PGA to G3P 3. Regeneration of Starting Material – RuBP is regenerated and the cycle starts again Copyright © 2009 Pearson Education, Inc.

Step Regeneration of RuBP G3P G3P Step Carbon fixation 1 Input: 3 CO2 Rubisco 1 Step Reduction 2 3 P P 6 P RuBP 3-PGA 6 ATP 3 ADP 6 ADP + P 3 ATP CALVIN CYCLE Step Release of one molecule of G3P 3 4 2 6 NADPH 6 NADP+ Figure 7.10B Details of the Calvin cycle, which takes place in the stroma of a chloroplast. 5 P 6 P Step Regeneration of RuBP 4 G3P G3P 3 Glucose and other compounds Output: 1 P G3P

PHOTOSYNTHESIS REVIEWED AND EXTENDED Copyright © 2009 Pearson Education, Inc.

H2O CO2 Chloroplast Light NADP+ ADP + P RuBP CALVIN CYCLE Electron Photosystem II RuBP CALVIN CYCLE Electron transport chains 3-PGA (in stroma) Thylakoid membranes Photosystem I ATP Stroma Figure 7.11 A summary of the chemical processes of photosynthesis. NADPH G3P Cellular respiration Cellulose Starch O2 Sugars Other organic compounds LIGHT REACTIONS CALVIN CYCLE

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants In hot climates, plant stomata close to reduce water loss so oxygen builds up Rubisco adds oxygen instead of carbon dioxide to RuBP in a process called photorespiration Photorespiration uses oxygen, produces CO2, but no sugar or ATP are produced Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants Some plants have evolved a means of carbon fixation that saves water during photosynthesis C4 plants partially shut stomata when hot and dry to conserve water This reduces CO2 levels, so contain PEP carboxylase to bind CO2 at low levels (carbon fixation); This allows plant to continue making sugar by photosynthesis Copyright © 2009 Pearson Education, Inc.

Another adaptation to hot and dry environments has evolved EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C4 and CAM plants Another adaptation to hot and dry environments has evolved CAM open their stomata at night thus admitting CO2 in w/o loss of H2O CO2 enters, and is fixed into a four-carbon compound, (carbon fixation) It is released into the Calvin cycle during the day Copyright © 2009 Pearson Education, Inc.

CALVIN CYCLE CALVIN CYCLE Mesophyll cell CO2 CO2 Night 4-C compound 4-C compound CO2 CO2 CALVIN CYCLE CALVIN CYCLE Figure 7.12 Comparison of photosynthesis in C4 and CAM plants: In both pathways, CO2 is first incorporated into a four-carbon compound, which then provides CO2 to the Calvin cycle. Bundle- sheath cell 3-C sugar 3-C sugar Day C4 plant CAM plant

Separation of Photosynthetic Pigments in Chloroplasts

Photon (fluorescence) Chlorophyll molecule Excited state e– Heat Photon Photon (fluorescence) Ground state Figure 7.7A A solution of chlorophyll glowing red when illuminated (left); a diagram of an isolated, light-excited chlorophyll molecule that releases a photon of red light (right). Chlorophyll molecule