Photosynthesis Chapter 10 Biology – Campbell Reece

Autotrophs vs. Heterotrophs  Autotroph (from auto, self, and trophos, feed) – produce their own organic molecules from CO 2 and other inorganic raw materials from the environment –A.K.A. Producers  Heterotroph (hetero means other or different) – obtain their nutrients by eating other organisms –A.K.A. Consumers

Autotrophs  Plants are photoautotrophs because they use light as a source of energy  Photosynthesis – the conversion of light energy into chemical energy –Occurs in plants, algae, and in some prokaryotes (cyanobacteria)  Some autotrophs are chemoautotrophs which obtain their energy from inorganic substances, such as sulfur or ammonia

Chloroplasts  Are the sites of photosynthesis in plants  The leaves are the major sites of photosynthesis, with about ½ million chloroplasts per square millimeter  Chlorophyll is the green pigment in the chloroplasts  Chloroplasts are mainly found in the mesophyll of the leaf

Chloroplasts  Mesophyll – the tissue in the interior of the leaf  Stomata – microscopic pores in the surface of the leave (carbon dioxide enters, oxygen exits)

Inside a Chloroplast  thylakoids – saclike photosynthetic membranes (chlorophyll resides here)  grana – stacks of thylakoids  stroma – space outside the thylakoids

The Basics of Photosynthesis  Overall equation: –6CO H light energy → C 6 H 12 O 6 + 6O 2 + 6H 2 O  The carbohydrate C 6 H 12 O 6 is glucose  Water is on both sides of the equation because some is consumed & some is produced  Simplified equation: –6CO 2 + 6H light energy → C 6 H 12 O 6 + 6O 2

Photosynthesis in a nutshell  Plants take in energy from the sun (absorbed by the chlorophyll); water through the roots; CO 2 through the stomata  Plants convert the light energy into chemical energy in the bonds of glucose, C 6 H 12 O 6 and produce O 2 as a waste product

Where does the O 2 come from?  Hypothesis #1: O 2 comes from CO 2 –Step 1: CO 2 → C + O 2 –Step 2: C + H 2 O → CH 2 O  Hypothesis #2: O 2 comes from H 2 O –Experiment 1: CO 2 + 2H 2 O → CH 2 O + H 2 O +O 2 –Experiment 2: CO 2 + 2H 2 O → CH 2 O + H 2 O +O 2

Where does the O 2 come from?

Two Processes  Light reactions – convert solar energy to chemical energy –NADPH –ATP (through photophosphorylation) –Oxygen is produced in this process  Calvin Cycle (dark or light-independent reactions)– convert CO 2 to carbohydrate –Named for Melvin Calvin who identified the steps of the cycle

Two Processes

Light Reactions: A Closer Look  Light is absorbed by the pigments in the leaf –chlorophyll a (blue-green) – primary pigment involved in the light reactions –chlorophyll b (yellow-green) – an accessory pigment –carotenoids (yellow and orange) – other accessory pigments

Photosystems  A light-gathering “antenna complex” consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoids  When energy is absorbed by any antenna molecule, the energy is passed from pigment molecule to pigment molecule until it reaches a particular chlorophyll a molecule in the reaction center

Photosystems  Sharing the reaction center with the chlorophyll a is the primary electron acceptor, which traps the high-energy electron

Photosystems  Light drives the synthesis of NADPH and ATP by energizing the two photosystems (photosystems I and II) in the thylakoid membranes  Two possible routes for electron flow: –Cyclic –Linear (the predominant route)

Linear Electron Flow 1. When photosystem II absorbs light, an excited electron in the P680 is captured by the PEA 2. Water is split to replace the electron in the P680; oxygen is formed 3. Each photoexcited electron passes from the PEA of photosystem II to photosystem I through an electron transport chain

Linear Electron Flow 4. As electrons flow down the chain, the energy released is used to produce ATP (photophosphorylation) 5. When the electron reaches the bottom of the ETC, it goes to the P700, which moved an electron to the PEA of photosystem I 6. The electron in the PEA then moves down another ETC to produce NADPH (an electron carrier)

Linear Electron Flow

Cyclic Electron Flow  Uses photosystem I only  There is no production of NADPH or oxygen  But it does generate ATP  Why? –More ATP is used in the Calvin Cycle than NADPH, so extra ATP needs to be produced

Cyclic Electron Flow

Light Reactions in a nutshell  Reactants: –Light energy –water  Products: –ATP –NADPH –Oxygen (as a by-product)

The Calvin Cycle  Carbon enters the Calvin cycle in the form of CO 2 and leaves in the form of sugar (anabolic process)  ATP is used as an energy source  NADPH is used to add high-energy electrons to make the sugar  The cycle must take place 3 times to produce one molecule of the 3-carbon sugar glyceraldehyde-3-phosphate (G3P)

3 phases of the Calvin Cycle  Phase 1: Carbon Fixation –Each CO 2 molecule is attached to a five- carbon sugar, RuBP –The enzyme used, rubisco, is the most abundant protein in plants (and probably on Earth) –An intermediate molecule is formed

3 phases of the Calvin Cycle  Phase 2: Reduction –A phosphate group (from an ATP) is added to the intermediate –A pair of electrons (from the NADPH) is added to the intermediate to form G3P –G3P is a sugar –6 molecules of G3P are produced for every 3 CO 2, 5 of them are recycled to make RuBP, the other 1 leaves the cycle

3 phases of the Calvin Cycle  Phase 3: Regeneration of CO 2 acceptor (RuBP) –The carbon skeletons of the 5 molecules of G3P that stayed in the cycle are rearranged to make 3 molecules of RuBP –The RuBP is now ready to receive CO 2 again –The cycle continues…

3 Phases of the Calvin Cycle

Calvin Cycle in a nutshell  Reactants: –3 CO 2 –9 ATP –6 NADPH  Products: –1 G3P (6 are produced, but only one comes out of the cycle) –The G3P can then be combined with another G3P to make glucose or another carbohydrate

C 3 Plants  The first product of the Calvin Cycle is a 3-carbon compound  On dry, hot days, the stomata close to conserve water, reducing the rate of photosynthesis  CO 2 levels decrease, O 2 levels increase  Photorespiration – O 2 is consumed in the Calvin Cycle (rubisco can accept O 2 ), but no sugar is made

C 4 and CAM Plants  C 4 Plants –Minimize photorespiration and enhance sugar production –Keep CO 2 concentrations high enough for rubisco to accept CO 2 instead of O 2  CAM Plants –Take up CO 2 at night and incorporate it into organic acids which are stored until daytime –Then the CO 2 is released from the organic acids to be converted to sugar

C 4 and CAM Plants

After Photosynthesis is complete…  The sugar produced: –supplies the plant with chemical energy –is used to make all the major organic molecules (lipids, proteins, etc.) –is consumed as fuel (in the form of sucrose) for cellular respiration –is used to make cellulose for the cell walls –is stored as starch

A Review of Photosynthesis Photosynthesis  Light reactions: –Are carried out in the thylakoid membranes –Convert light energy to the chemical energy of ATP and NADPH –Split H 2 O and release O 2  Calvin Cycle: –Take place in the stroma –Use ATP and NADPH to convert CO 2 to the sugar G3P –Return ADP, P i, and NADP + to the light reactions