Solar energy + carbon dioxide + water → carbohydrate + oxygen Photosynthesis Converts solar energy into the potential chemical energy of a carbohydrate in this way: Solar energy + carbon dioxide + water → carbohydrate + oxygen Photosynthetic organisms include plants, algae, and certain bacteria. These organisms are called producers; they synthesize organic molecules from raw materials.

Photosynthetic organisms

Nearly all life is dependent on solar energy because: Photosynthetic organisms use solar energy to produce organic nutrients. Almost all organisms depend either directly or indirectly on these organic nutrients to sustain themselves. Photosynthetic organisms provide food for other organisms, known as consumers. The bodies of plants became the coal or other fossil fuels used today.

Structure and Function of Chloroplasts Chloroplasts are the organelles that carry on photosynthesis. Mesophyll cells in the middle of a leaf house chloroplasts Mesophyll cells are protected from drying out by a waxy cuticle. Pores called stomata allow CO2 and O2 to enter the leaf.

Mesophyll cells of a leaf

The xylem is responsible for the transport of water and soluble mineral nutrients from the roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. The phloem is concerned mainly with the transport of soluble organic material made during photosynthesis.

Structure of Chloroplasts Bounded by a double membrane. The inner membrane encloses a large central space called the stroma that houses enzymes used to reduce CO2 to carbohydrate. A membranous system of thylakoids lies within the stroma; some thylakoids are stacked into grana; thylakoids contain chlorophyll and other pigments. All thylakoids are connected – single inner compartment within choropasts called the thylakoid space Chlorophyll and other pigments absorb solar energy.

Chloroplast structure

Stepped Art Figure 7.3d,e Page 116 Overview of Photosynthesis two outer membranes inner membrane system (thylakoids connected by channels) stroma channel stacked part of thylakoid membrane Stepped Art Figure 7.3d,e Page 116

Visible Light Radiant energy from the sun (solar energy) can be described in terms of its wavelength and its energy content. The colors in visible light range from violet (the shortest wavelength and highest energy) to blue, green, yellow, orange, and red (the longest wavelength and lowest energy).

Visible Light Spectrum

Pigments (chlorophylls and carotenoids) found within photosynthesizing cells, are capable of absorbing various portions of visible light. Both chlorophyll a and chlorophyll b absorb violet, blue, and red light best. Carotenoids absorb light in the violet-blue-green range and reflect yellow or orange Leaves appear green because green light is reflected and only minimally absorbed.

In the fall when the pigment chlorophyll breaks down (chlorophyll needs warm weather to be created; is created continually during warm months), the remaining pigment (carotenoids) become unmasked, reflecting the colors like orange and yellow http://www.youtube.com/watch?v=AeypaiIoMPI

Photosynthesis is an oxidation-reduction reaction, or redox reaction for short. Oxidation is the loss of electrons; hydrogen atoms are removed from glucose. Reduction is the gain of electrons; oxygen atoms gain electrons. Remember OIL RIG (oxidation is loss, reduction is gain)

Overview of Photosynthesis A simplified overall equation for photosynthesis is: Solar energy + 6CO2 + 6H2O → C6H12O6 + 6O2 During photosynthesis, water molecules are oxidized; they lose electrons (e-) along with hydrogen ions (H+). Also, CO2 is reduced and gains electrons given up by H2O. Electrons from H2O are energized by the sun.

Enzyme involved NADP+ Nicotinamide adenine dinucleotide phosphate Accepts 2 electrons and H+ to become NADPH

The ATP cycle

Two Sets of Reactions Photosynthesis is divided into two sets of reactions, as implied by the term “photosynthesis”: “Photo” refers to the light-dependent (needs light) reactions that capture energy from the sun Photosystem II Photosystem I “Synthesis” refers to the light-independent (does not need light) reactions that produce carbohydrate (glucose). Calvin Cycle

Photosynthesis – Light-dependent and Light-independent Reactions CO2 H2O Chloroplast LIGHT REACTIONS (in thylakoids) CALVIN CYCLE (in stroma) NADP+ ADP + P ATP NADPH O Sugar Electrons Light-independent Light-dependent

Overview of Photosynthesis SUNLIGHT inside a Thylakoid O H2O H+ H+ H+ H+ e– H+ NADP+ NADPH ATP Stroma ADP + Pi H+ Light-Independent Reactions CO2 P glucose H2O carbohydrate end product (e.g., sucrose, starch, cellulose) Figure 7.3f Page 117 Overview of Photosynthesis

Animation of light-dependent reactions Simplified animated steps of light-dependent reactions Animation of overall light-dependent and animation of cyclic & noncyclic flow Overview of light-independent reactions (Calvin cycle) Another Calvin cycle animation Overall process of photosynthesis A photosynthesis rap!!!!!!!!! http://www.youtube.com/watch?v=Wi60tQa8jfE Musical lecture part I Musical lecture part II

Light-Dependent Reactions Light energy is absorbed by chlorophyll. Electrons are energized. Energized electrons move down an electron transport system, release energy, and produce ATP. NADP+ accepts electrons to produce NADPH. Water is split to produce oxygen, hydrogen ions, and electrons Photosystem II occurs first Photosystem I occurs second (II and I were named for the order in which they were discovered, NOT the order in which they occur).

Light-dependent reactions continued Both occur within the thylakoid membranes inside the chloroplast Each photosystem has a pigment system of green pigments (chlorophylls a and b) and accessory pigments. Closely packed pigments serve as an “antenna” for gathering solar energy. Electrons can follow one of two electron pathways

Electron Pathways Electrons can follow a cyclic electron pathway or a noncyclic electron pathway during the light-dependent phase of photosynthesis. The cyclic electron pathway generates ATP only. Used by some bacteria The noncyclic electron pathway generates both ATP and NADPH. Used by some bacteria and plants

Cyclic Electron Pathway The cyclic electron pathway begins after the PS I pigment complex absorbs solar energy. High-energy electrons (e-) leave the reaction-center chlorophyll a molecule and later return to it. But first, the electrons enter an electron transport system that leads to the production of ATP.

ATP Production Each time water is split, two H+ remain in the thylakoid space. As electrons move down the electron transport system, they give up energy, which is used to pump H+ from the stroma into the thylakoid space. Thus, H+ build up in the thylakoid space. The flow of H+ through an ATP synthase complex back into the stroma drives the chemiosmotic production of ATP.

The light-dependent reactions: the cyclic electron pathway

Light-dependent Photosystem II and Photosystem I - Electron transport system Stroma

Noncyclic Electron Pathway In the noncyclic electron pathway, electrons move from H2O through PS II to PS I and then on to NADP+. The PS II pigment complex first absorbs solar energy, and high-energy electrons (e-) leave the reaction-center chlorophyll a. PS II takes replacement electrons from water, which splits, releasing oxygen.

The oxygen leaves the chloroplast as oxygen gas (O2), and the hydrogen ions (H+) temporarily remain in the thylakoid space. The high-energy electrons enter an electron transport system, and energy is used to produce ATP. Low-energy electrons leaving the electron transport system enter PS I where they replace electrons that have left the PS I reaction center.

When the PS I pigment complex absorbs solar energy, high-energy electrons leaving the reaction-center chlorophyll a are captured by an electron acceptor. This time, the electron acceptor passes the electrons on to NADP+, which accepts a hydrogen ion (H+) and becomes NADPH: NADP+ + 2e- + H+ → NADPH

Results of the noncyclic electron pathway: Water is split, yielding H+, e-, and O2 ATP is produced NADP+ becomes NADPH

The light-dependent reactions: the noncyclic electron pathway

Machinery of Noncyclic Electron Flow (Light RXN) Inside THYLAKOID H2O second electron transfer chain photolysis e– e– ATP SYNTHASE first electron transfer chain NADP+ NADPH ATP PHOTOSYSTEM II PHOTOSYSTEM I ADP + Pi STROMA of Chloroplast Figure 7.13a Page 123

The Thylakoid Membrane These four complexes are in the thylakoid membrane: PS II consists of a protein complex and the light-gathering pigment complex; PS II splits water and released oxygen The cytochrome complex transports electrons between PS II and PS I, during which pumping of H+ occurs

PS I consists of a protein complex and a light-gathering pigment complex; NADP reductase is an enzyme that reduces NADP+ to NADPH ATP synthase complex has a H+ channel and a protruding ATP synthase; as H+ flows down its concentration gradient through this channel from the thylakoid space into the stroma, ATP is produced.

Light-Dependent Reaction: Arrangement of Photosystems water-splitting complex thylakoid compartment H2O 2H + 1/2O2 P680 P700 acceptor acceptor pool of electron carriers PHOTOSYSTEM II stroma PHOTOSYSTEM I Figure 7.10 Page 121

Light-dependent Photosystem II and Photosystem I - Electron transport system Stroma

LIGHT-DEPENDENT REACTION Requires Light and Water WHAT IT DOES: WHERE IT HAPPENS: SPLITS H2O Leaf Creates ATP and NADPH For Calvin Cycle Chloroplasts Inside Thylakoids Forms O2 as a byproduct

Light-Independent Reactions The light-independent reactions do not need light Take place in the stroma Reactions make up the Calvin cycle. Carbon dioxide (CO2) is taken up by the plant (from the atmosphere) ATP and NADPH (made from the light dependant reactions) add phosphates and electrons to CO2, and convert it to glyceraldehyde 3- phosphate, or G3P (PGAL) G3P is a type of sugar which a plant can easily convert to glucose. This glucose can then be used as a source of stored energy for the plant

The Calvin Cycle AKA G3P

Details about the Calvin Cycle Carbon dioxide is taken up by a 5-carbon sugar, RuBP. The resulting 6-carbon molecule breaks into two 3-carbon PGA molecules. Each 3 carbon molecule PGA is reduced to G3P/PGAL using NADPH and ATP during a redox reaction (CO2 is reduced while NADPH is oxidized). Some of the resulting G3P/PGAL is used to form glucose, the rest is used to re-form RuBP.

The Role of Glucose The formation of glucose from G3P/PGAL is of interest because glucose is used by most organisms to produce ATP to supply energy needs. Glucose can be stored as starch, or is used to as cellulose make to strengthen plant cell walls. G3P/PGAL is converted to many organic molecules besides glucose, including plant oils.

Stages of the Calvin Cycle The Calvin cycle can be divided into: Fixation of CO2; Reduction of CO2; and Regeneration of RuBP. Metabolites of the Calvin cycle include: RuBP = ribulose bisphosphate PGA = 3-phosphoglycerate PGAP = 1, 3-bisphosphoglycerate G3P/PGAL = glyceraldehyde-3-phosphate

Fixation of Carbon Dioxide The first event of the Calvin cycle is the fixation of carbon dioxide (CO2). This occurs when CO2 is attached to an organic compound. A 5-carbon molecule, RUBP, combines with CO2 to form a 6-carbon molecule. An enzyme, RuBP carboxylase, speeds this reaction.

Reduction of Carbon Dioxide The 6-carbon molecule immediately breaks down into two 3-carbon PGA molecules. Each of the two PGA undergoes reduction to G3P/PGAL in two steps. This is a redox reaction in which CO2 is reduced to carbohydrate (CH2O) and NADPH is oxidized to NADP+. Energy for this reaction comes from ATP (generated during light-dependent photosynthesis).

Regeneration of RuBP RuBP must be regenerated for the Calvin cycle to continue. For every three turns of the Calvin cycle, five molecules of G3P/PGAL are used to re-form three molecules of RuBP. This reaction also utilizes some of the ATP produced by the light-dependent reactions.

The light-independent reactions of the Calvin cycle (detailed) Or G3P

Making 1 molecule of glucose requires: 6CO2 (from atmosphere) 18ATP (from light-dependent reactions) 12 NADPH (from light-dependent reactions)

Photosynthesis, like all energy transformations, results in a loss of usable energy. Under ideal laboratory conditions, plants transform 25% of solar energy they absorb into carbohydrate. Under natural conditions, the efficiency of photosynthesis ranges from less than 1% to a maximum of 8%. Regardless, living things depend on photosynthesis as their ultimate source of chemical energy.

Calvin Cycle Overall reactants Overall products Carbon dioxide ATP NADPH Overall products Glucose ADP NADP+ Reaction pathway is cyclic and ribulose bisphosphate (RuBP) is regenerated

(Thick Fluid inside Chloroplast) DARK REACTION Requires ATP, NADPH, and CO2 WHAT IT DOES: WHERE IT HAPPENS: Produces High Energy Sugars Leaf Combines CO2, ATP and NADPH Chloroplasts In the Stroma (Thick Fluid inside Chloroplast) Forms a 6 carbon sugar (Glucose)

Summary of Photosynthesis Light-Dependent Reactions Subset RXNS: Cyclic Electron Flow (1 photosystem) Noncyclic Electron Flow (produces O2; 2 photosystems, 2 ETS) Location: Thylakoid Produces: O2, ATP, NADPH (only ATP for cyclic) Light-Independent Reactions Aka Calvin Cycle Location: Stroma Produces: glucose, ADP, NADP+

Overview of Photosynthesis

Photosynthesis Vs. Cellular Respiration Overall equation for photosynthesis is: Overall equation for cellular respiration is:

Photosynthesis Vs. Cellular Respiration Occurs in plants, algae, & some bacteria Occurs in all organisms Occurs in chloroplasts Occurs in mitochondria Glucose is produced Glucose is broken down

Photosynthesis Vs. Cellular Respiration (cont’d) Occurs in plant cells during day Occurs in plant cells day AND night Uses electron transport system Uses NADP+ and NADPH Uses NAD+ and NADH, FAD and FADH2

Photosynthesis versus cellular respiration