Unit 3 Metabolism – Energy Production & Use

Unit 3Metabolism – Energy Production & Use Chapter 6 – Photosynthesis 6-2 The Light Independent Reactions – the CALVIN CYCLE pp120-4

The Calvin Cycle – Carbon Fixationon pp120-1 • What is it? The 2nd part of photosynthesis, also called the “LIGHT-INDEPENDENT REACTIONS” - it doesn’t require light • What does it need? 6CO2 molecules, NADPH and ATP from the LIGHT DEPENDENT REACTIONS – the first part of photosynthesis • What is the product? It’s called “CARBON FIXATION” because carbon atoms are “fixed” or bonded into organic molecules – C6H12O6 – glucose – the most commonly used monosaccharide to build starch, cellulose, and glycogen – BIG energy-bearing molecules called carbohydrates!

Where does it happen?In the stroma, inside the chloroplast, but outside the thylakoids http://www.youtube.com/watch?v=0UzMaoaXKaM

It takes 6 CYCLES! • The Calvin Cycle (carbon fixation cycle) has to run 6 times to make one C6H12O6 • Each time, the Calvin Cycle makes two 3-carbon molecules called G3P with the help of ATP and NADPH from the light-dependent reactions • By the end of the Calvin Cycle, one atom of carbon from one G3P molecule is “stored” to make C6H12O6 and the other 5 carbon atoms left go back into the Calvin Cycle. • The 6th time the Calvin Cycle runs, 6 carbon atoms have been “stored” to make ONEC6H12O6http://youtu.be/E_XQR800AgM

Pretty simple, huh? • The Calvin Cycle is also called the C3 Pathway because it makes the 3-carbon molecules, G3P • Not all plants use the C3 Pathway, though. It can only be used by plants that live in places with adequate water – like where we live. • Other sets of reactions, called the CAM Pathway and the C4 Pathway can be used by plants in deserts or plants that are stressed by drought in areas that usually have rainfall – like we sometimes have in our summers

Stomata – for gas exchange • Stomata are specialized cells on the structures formed by 2 guard cells on the underside of leaves that exchange gases with the atmosphere – only ONE is a stomate • O2 leaves after the Light-Dependent Reactions • CO2 enters for the Light-Independent Reactions

A stomate open on the left during plentiful water times, and on the right, one closed during a time of water shortage/drought

C4 Plants: • Field corn is a C4 plant – it can continue to go through the light-dependent and –independent reactions even during the hottest part of the day – or when our summer gets dry. • Here is how: • During the hottest part of the day, these plants keep their stomata mostly closed to prevent water vapor from escaping from them – even though they are primarily for the movement of O2 and CO2 • The light-dependent reactions keep going and keep producing O2 , but CO2 for the light-independent reactions can’t get in because the stomates are closed • C4 plants have an enzyme that produces a 4-carbon molecule that some of their cells can use to extract the CO2 needed for the light-independent reactions – the Calvin Cycle!

C4 plants can make just as many carbohydrates as C3 plants, but use only half the water! CRAB GRASS and FIELD CORN

the other way – CAM Plants • CAM is short for Crassulacean Acid Metabolism • The plants grow slower than C4 or C3 plants, BUT are really good at conserving water. • They open their stomata only at night to let in CO2 and let out O2, preventing water from also escaping in the heat of the day. • During the day, the light-dependent reactions are using water to produce hydrogen ions and O2, but the O2 is being stored until night,and will be released when the stomata are open when it isn’t so hot. • CO2 is also taken in during the night and used for the Calvin Cycle/Light—independent reactions, but these reactions slow during the day when the stomata are closed – causing CO2 to become depleted, and O2 to increase inside the plant

CAM Plants – Yucca, jade, pineapple

To summarize photosynthesis: Light-Dependent Reaction NADPH and ATP from LDR is used CO2 from atmosphere needed 6 cycles store 6 atoms of carbon to build one glucose C6H12O6 Stored for later use in plants as starch or cellulose Light-Independent Reactions / Calvin Cycle • Light must be present • Light energy absorbed by PSII and PSI • Water splits to make H+ and e- and O2 • O2 is released • H e- help NADPH form • H+ help ATP form during chemiosmosis

What affects the rate of photosynthesis? • Intensity of light – but only to a point when all electrons from available water have been used • CO2 levels – same – more is better, but only to a point • Temperature – more is NOT always better, most plants have a temperature when they cannot risk losing water vapor through open stomata, so photosynthesis slows or stops until it cools

Unit 3 Metabolism – Energy Production & Use