Ch 7 PHOTOSYNTHESIS 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 § Photoautotrophs use the energy of light and chloroplasts to produce organic molecules – Most plants, algae and other protists, some prokaryotes (cyanobacteria) Copyright © 2009 Pearson Education, Inc.

Chloroplast Outer and inner membranes Thylakoid Stroma Granum Thylakoid space Intermembrane space

Visible Radiation Drives the Light Reactions § Sunlight is a type of electromagnetic energy (radiation) § Visible light is a small part of the EM spectrum § Light travels in waves and is particulate – One wavelength = distance between the crests of two adjacent waves (shorter λ’s have more energy) – Photons are discrete packets of light energy (they contain a fixed quantity of light energy)

Increasing energy 10– 5 nm 10– 3 nm Gamma rays X-rays 1 nm 103 nm UV 1 m 106 nm Infrared Microwaves 103 m Radio waves Visible light 380 400 600 500 Wavelength (nm) 700 650 nm 750

Visible Radiation Drives the Light Reactions § Pigments § Are proteins § Absorb specific wavelengths of light, transmit others § The color of a pigment is the color of light most reflected or transmitted by that pigment – Ex. chlorophyll reflects/transmits green and appears green § Various pigments are built into the thylakoid membrane of the chloroplast

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 6 CO 2 + 6 H 2 O Carbon dioxide Water Light energy Photosynthesis C 6 H 12 O 6 + 6 O 2 Glucose Oxygen gas

Photosynthesis Occurs in Chloroplasts in Plant Cells § Leaves have: § Stomata = pores that allow CO 2 to enter and O 2 to exit § Veins transport water & nutrients absorbed by roots. Vein § Chloroplasts = site of photosynthesis § Contain the green pigment chlorophyll CO 2 Chloroplasts Stoma

Photosynthesis is a Redox Process, as is Cellular Respiration § A loss of electrons = oxidation § A gain of electrons = reduction § Electrons are lost and gained in the form of hydrogen § During redox reactions of photosynthesis, H 2 O donates electrons and is oxidized, CO 2 accepts electrons and is reduced Reduction 6 CO 2 + 6 H 2 O C 6 H 12 O 6 + 6 O 2 Oxidation

CO 2 H 2 O Chloroplast Light NADP+ ADP P LIGHT REACTIONS CALVIN CYCLE (in stroma) (in thylakoids) ATP NADPH O 2 Sugar

The “Photo” in Photosynthesis: THE LIGHT REACTIONS

Photosystems Capture Solar Power in the Light Reactions Photosystem § A photosystem is a functional unit that captures sunlight and helps convert it to the chemical energy of ATP & NADPH § A Photosystem has a light -harvesting complex surrounding a reaction center § chl. a and a primary eacceptor are in the reaction center Light-harvesting complexes Reaction center e– Pigmen molecule Pair of Chlorophyll a molecules

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

Photosystems Capture Solar Power in the Light Reactions § Light energy is absorbed by pigments of PS II and passed from pigment to pigment NADPH NADP + H Electron transport chain Photon Provides energy within the forphotosystem synthesis of 6 + Photon Photosystem II Stroma 1 Primary acceptor 2 e– Thylakoid membrane e– e– 3 H 2 O 1 2 Photosystem I by chemiosmosis § The energy passes to chl. a, exciting its Primary electrons acceptor e– § An excited e- from chl. a is transferred to 4 5 the primary electron acceptor P 700 P 680 Thylakoid space ATP + + O 2 + 2 H § An enzyme oxidizes water and gives the electrons to chl. a –This step releases oxygen

The Electron Transport Chain – Each photoexcited e- passes from the primary e- acceptor of PS II to PS I via an electron transport chain (ETC) – The exergonic “fall” of electrons down the ETC will help generate a H+ gradient used to generate ATP. – The ETC is a bridge between photosystems II and I

The Electron Transport Chain Photosystem II Stroma NADP+ + H+ Photon 1 Primary acceptor 2 e– Thylakoid membrane e– 4 P 700 P 680 Thylakoid space 3 H 2 O 1 2 5 O 2 + 2 H+ 6 NADPH

The Electron Transport Chain § The ETC accepts electrons from PS II § Electrons are passed along ETC and the 2 nd protein complex pumps H+ into the thylakoid space § This generates a proton (H+) gradient § H+ flows from the thylakoid space to the stroma, down its gradeint, through an ATP synthase. § ATP synthase phosphorylates ADP forming ATP § This is an energy-coupling process called chemiosmosis, where the exergonic flow of H+ powers the endergonic phosphorylation of ADP

Stroma (low H+ concentration) H+ H+ PS II H 2 O 1 2 O 2 + 2 H+ NADP+ + H+ H+ H+ ADP + P NADPH H+ ATP PS I H+ H+ H+ Electron transport chain Thylakoid space (high H+ concentration) H+ H+ H+ ATP synthase

The Light Reactions – Electrons moving down the ETC are passed to chl. a of PS I – NADP+ reductase transfers electrons from PS I to NADP+ forming NADPH

Stroma (low H+ concentration) H+ H+ PS II H 2 O 1 2 O 2 + 2 H+ NADP+ + H+ H+ H+ ADP + P NADPH H+ ATP PS I H+ H+ H+ Electron transport chain Thylakoid space (high H+ concentration) H+ H+ H+ ATP synthase

The Light Reactions § As a result of the light reactions, ATP and NADPH are 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

THE CALVIN CYCLE: CONVERTING CO 2 TO SUGAR

ATP and NADPH Power Sugar Synthesis in the Calvin Cycle § The Calvin cycle makes sugar within a chloroplast § Atmospheric CO 2, ATP, and NADPH are required to produce sugar § Using these three ingredients, a three-carbon sugar called glyceraldehyde-3 -phosphate (G 3 P) is produced § A plant cell may G 3 P to make glucose and other organic molecules CO 2 Input ATP NADPH CALVIN CYCLE Output: G 3 P

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

Step 1 Carbon fixation Input: 3 CO 2 Rubisco 1 P 3 P 6 Ru. BP CALVIN CYCLE P 3 -PGA

Step 1 Carbon fixation Input: 3 CO 2 Rubisco 1 Step 2 Reduction P 3 P P 6 Ru. BP 3 -PGA 6 ATP 6 ADP + P CALVIN 2 CYCLE 6 NADPH 6 NADP+ P 6 G 3 P

Step 1 Carbon fixation Input: 3 CO 2 Rubisco 1 Step 2 Reduction P 3 P P 6 Ru. BP 3 -PGA 6 ATP 6 ADP + P CALVIN 2 CYCLE 6 NADPH 6 NADP+ P 5 P 6 G 3 P Output: 1 P G 3 P Glucose and other compounds

Step 1 Carbon fixation Input: 3 CO 2 Rubisco 1 Step 2 Reduction P 3 P P 6 Ru. BP 3 -PGA 6 3 ADP 3 Step ATP 6 ADP + P ATP CALVIN 3 2 CYCLE 6 NADPH 3 Regeneration of Ru. BP 6 NADP+ P 5 P 6 G 3 P Output: 1 P G 3 P Glucose and other compounds

Overview of Photosynthesis

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

CO 2 H 2 O Chloroplast Light NADP+ ADP + P Photosystem II Thylakoid membranes Ru. BP CALVIN CYCLE 3 -PGA (in stroma) Electron transport chains Photosystem I ATP NADPH Stroma G 3 P O 2 Sugars LIGHT REACTIONS CALVIN CYCLE Cellular respiration Cellulose Starch Other organic compounds

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C 4 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 Ru. BP in a process called photorespiration – Photorespiration uses oxygen, produces CO 2; sugar and ATP are not produced Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C 4 and CAM plants § C 4 plants § Have a unique leaf anatomy § First stable compound is a 4 C compound § C 4 plants partially shut stomata when hot and dry to conserve water § Have PEP carboxylase to bind CO 2 at low levels (carbon fixation) § Allows plant to maintain an adequate concentration of carbon to feed into the Calvin Cycle to continue making sugar Copyright © 2009 Pearson Education, Inc.

EVOLUTION CONNECTION: Adaptations that save water in hot, dry climates evolved in C 4 and CAM plants § CAM plants – CAM plants open their stomata at night thus admitting CO 2 in w/o loss of H 2 O – CO 2 enters, and is fixed into a fourcarbon compound, (carbon fixation) – Carbon is released into the Calvin cycle during the day

CO 2 Mesophyll cell CO 2 4 -C compound CO 2 CALVIN CYCLE Bundlesheath cell 3 -C sugar C 4 plant CAM plant Night Day

Separation of Photosynthetic Pigments in Chloroplasts