Cellular Energy Photosynthesis

How do plants make energy & food? Plants use the energy from the sun to make ATP energy to make sugars glucose, sucrose, cellulose, starch, & more sun ATP sugars

Overview of Photosynthesis use sun’s energy to make ATP use CO2 & water to make sugar Occurs in chloroplasts makes a waste product oxygen (O2)  glucose + oxygen carbon dioxide sun energy + water + 6CO2 6H2O C6H12O6 6O2 sun energy  +

Overview of Photosynthesis Summarize the two main phases of photosynthesis, the light reactions and the Calvin cycle. 1. What is needed? 2. What is produced?

Overview of Photosynthesis Photosynthesis involves 2 energy conversions: Conversion of light energy into chemical energy (The Light Reactions) Storage of chemical energy in the form of sugars (The Calvin Cycle)

Chloroplasts – organelles of photosynthesis sun Leaves Chloroplasts in cell Chloroplasts contain Chlorophyll Pigments in chloroplast absorb sunlight. (All colors?)

Chloroplasts- organelles of photosynthesis Granum: a stack of thylakoids Stroma: Fluid around Grana Thylakoids: Membranes containing pigments

Chloroplasts- organelles of photosynthesis Pigments – light absorbing substances. Found in thylakoid membranes of chloroplasts. Absorb waves of visible light, reflect the color seen by your eye Chlorophyll a and b – Major pigments involved in photosynthesis. Green pigments, absorb all colors except green. Accessory pigments – absorb green. Visible in autumn, when chlorophyll breaks down.

Photosynthesis: 2 Stages Light Reactions: in thylakoid membranes Sunlight  ATP + NADPH Calvin Cycle: in stroma NADPH + ATP  Sugar

The Light Reactions Converts visible light into chemical energy carried by electrons in high-energy molecules (ATP and NADPH) Splits an H2O. Uses H, releases O. Next, we zoom in to look more closely at the thylakoids!

1. Light strikes the first photosystem (PSI), causing it to transfer excited e- to the primary electron acceptor. These e- are replaced by splitting H2O, which releases O2 as a product. 2. The excited e- travel down and e-transport chain. This process pumps H+ ions across the membrane into the thylakoid. 3. Light-excited e- in PSII are transferred to NADP+. These e- are replaced by those coming from e- transport chain. 4. The “backflow” of hydrogen ions out of the thylakoid pass through ATP synthase, powering ATP production. 2 1 4 3

The Light Reactions – Another View

The Light Reactions - Summary Light absorbed into thylakoid membrane by pigments in photosystems PSI and PSII (see figure 4.10) Light energy is transferred to the reaction center within the photosystem. H2O splits apart at PSII (2H2O  4H+ + 4e- + O2) Oxygen diffuses out of the plant Protons (H+): Transported to the thylakoid Electrons (e-): electron transport chain to PSI

The Light Reactions – Summary cont. Formation of NADPH (use e- from H2O splitting) Electrons reach PSI Used to join NADP+ with H+  NADPH This high-energy molecule will be used later for the Calvin cycle. Build-up of H+ ions Energy electrons received from reaction center is used in active transport to pump protons into thylakoid H+ build up inside thylakoid, result in potential energy difference (like a battery)

Review: Light Reaction H2O + sunlight  O2 + ATP + NADPH To see this in action, check out the YouTube video: Light Dependent Reactions http://www.youtube.com/watch?v=Oi2_n2wbB9o 2 Unstable compounds formed. Must be converted to C6H12O6 for storage!

The Calvin Cycle (Dark Reactions) Occurs in stroma of chloroplast. Energy from ATP and NADPH molecules converted to chemical bonds within glucose for storage.

1. Carbon Fixation: CO2 taken in through leaves. Combines with RuBP, a 5-Carbon sugar-phosphate, by enzyme rubisco. CO2 gas is “fixed” into an organic molecule. 6-Carbon sugar f ormed, immediately splits into two 3-C PGA molecules The Calvin Cycle 4. Remaining 5 PGAL rearranged, turned back into 3 RuBP molecules. Calvin cycle complete, can start over again. 1 4 2 2. ATP and NADPH used to rearrange 6 PGA molecules into 6 G3P (PGAL). 3. One PGAL released, remaining 5 stay in Calvin cycle. 2 PGAL  1 Glucose 3

Calvin Cycle - Summary Any Questions?? ATP + NADPH  PGAL or Glucose Carbon fixation by rubisco Rearrange molecules to produce a PGAL and get back to starting molecule (RuBP) YouTube video: The Calvin Cycle by Prentice Hall http://www.youtube.com/watch?v=_NIhg1qa_L0 Any Questions??

How are products of photosynthesis used? PGAL produced in Calvin cycle can be used in many ways: Synthesized into larger carbohydrates: glucose, sucrose Modified to make amino acids, glycerol  sugars used by leaf cell, or transported to other cells in plant.  This occurs in chloroplasts, cell, or other organisms if plant is eaten.

Rate of Photosynthesis Measured by CO2 consumed per unit time or by O2 produced. Bromthymol Blue indicator. Turns yellow in the presence of CO2. An acid-base indicator: H2O + CO2  H2CO3 (l) In water, CO2 creates carbonic acid.

Rate affected by several factors Light intensity Temperature Concentration of CO2 and O2 Photoinhibition: A reduction in the rate of photosynthesis.

Photorespiration O2 enters Calvin Cycle Enzyme Rubisco fixes CO2 in Calvin Cycle. Due to shape, rubisco can bind with either O2 or CO2. CO2 binds to rubisco  2 PGA molecules O2 binds to rubisco  1 PGA and 1 2-C acid (glycolate). Plant loses fixed carbons instead of gaining. Benefits unknown. Uses O2 and liberates CO2, which is why it is called photorespiration.

Photorespiration Effects of greenhouse gasses? Atmospheric Conditions Photosynthesis Photorespiration High CO2, Low O2 Favored Inhibited Low CO2, High O2

Photorespiration Weather and Photosyntheis In hot, dry weather, plants close leaf openings called stomates. Helps reduce water loss. No CO2 enters. Photorespiration is favored. Light reactions release oxygen high light intensities and high temperatures (above ~ 30°C) favor photorespiration.

Photorespiration: C3, C4 and CAM plants All plants carry out photosynthesis by adding CO2 to a 5-carbon sugar. Reaction is catalyzed by the enzyme RUBISCO. Results in production of PGA (starting molecule for glucose) The process is called the Calvin cycle and the pathway is called the C3 pathway. PGA has 3 carbons…. C3

Photorespiration - C3 Plants C3 Plants – Plants that use only the Calvin Cycle to fix CO2. Make up 90% of plants on Earth Wheat, rice, soy Most vulnerable to high O2 concentrations. Some plants have evolved strategies for increasing photosynthesis, and reducing photorespiration: C4 and CAM plants.

C4 Plants Limit Photorespiration The C4 cycle: Structural changes in leaf anatomy - C4 and C3 pathways are separated in different parts of the leaf - RUBISCO sequestered where the CO2 level is high, O2 level low.

C4 Plants Limit Photorespiration C4 plants are well adapted to habitats with (1) high daytime temperatures and (2)intense sunlight. Some examples: crabgrass corn (maize) sugarcane

CAM Plants Limit Photorespiration How are C4 and CAM Different?

CAM Plants Limit Photorespiration CAM plants separate their C3 and C4 cycles by time. *Note: C4 plants separate C3 and C4 cycles by location. CAM plants separate cycles by time of day. At night, CAM plants take in CO2 through their stomata CO2 is fixed into a 4-carbon molecule that accumulates in the central vacuole of the cells. In the morning, the stomata close conserving moisture The accumulated 4-C molecule leaves the vacuole and is broken down to release CO2. The CO2 is taken up into the Calvin (C3) cycle.

CAM Plants Limit Photorespiration CAM plants well adapted to conditions of (1) high daytime temperatures (2) intense sunlight, and (3) low soil moisture. Examples: cacti, aloe, pineapple, bryophyllum CAM – least efficient system. These plants grow slowly.

Evolution of C4 and CAM Plants When photosynthetic organisms first evolved, the atmosphere contained no oxygen. Rubisco did not need to differentiate. More photosynthesis occurring  more O2 in the air. Plants adapted by developing other pathways to fix carbon C4 cells in C3 plants

Photosynthesis Summary