Using Light to Make Food Photosynthesis Using Light to Make Food

photosynthesis Transformation of sun’s energy into useable energy (energy of chemical bonds) by autotrophs

Where? When?

Light facts Radiant energy from sun = 1 million Hiroshima bombs 1/3 of sun’s energy reflected back to space Less than 1% captured by photosynthesis

Light facts, cont’d… Described as “waves” or particles Electromagnetic spectrum shows all wavelengths Absorbtion spectrum shows photons capable of being “captured”

How to capture light

2 steps to photosynthesis Light RXN Generates NRG to drive the making of glucose Light Independent RXN Uses NRG of Light RXN to grab Carbon dioxide from the atmosphere and make glucose

Light Reaction Capture light at PSII and PSI Splitting water generates H+/e- ETC & the proton pump makes ATP (chemiosmosis or photophosphorylation) H+/e- carried by NADP for use in next step O2 released as waste

Light Independent Reaction Also known as Dark RXN, Calvin Cycle, carbon fixing cycle, and C3 cycle This is where glucose is made

*CO2 is “grabbed” by a 5-sugar compound, RuBP – carbon fixation *NADPH wagons dump H+/e- H+ are needed to build CHO molecules CO2 + H+/e- makes PGAL, then glucose

PGAL is an intermediary step in respiration Recall… Glucose breaks down to PGAL (then to Pyruvate) Therefore PGAL is a key link in the breakdown and build up of organic molecules

Most PGAL continues the cycle to regenerate RuBP so the cycle can go again **1 PGAL drops out of the cycle and goes on to make organic compounds Glucose Proteins lipids

It takes 6 turns of the cycle to make enough PGAL to make 1 molecule of glucose

Rate of photosynthesis If you change input, you change output

Stomata, open and closed Figure 7.2x2

Plant Dilemmas Keeping stomata open to bring in CO2 also means the plant is losing water Loss of water is BAD Without CO2, no glucose can be made – that’s BAD What’s a plant to do???

Alternative pathways C4 pathway & CAM pathway “stockpile” CO2 during evening or when stomata are partially closed to be used later Stomata can be closed to stop the loss of water during times of water shortage

This molecule can then donate CO2 to the Calvin cycle Special cells in C4 plants—corn and sugarcane—incorporate CO2 into a four-carbon “storage” molecule This molecule can then donate CO2 to the Calvin cycle 4-C compound CALVIN CYCLE 3-C sugar Figure 7.12B

The CAM plants—pineapples, most cacti, and succulents—employ a different mechanism They open their stomata at night and make a four-carbon compound It is used as a CO2 source by the same cell during the day 4-C compound Night Day CALVIN CYCLE 3-C sugar Figure 7.12C

Respiration Photosynthesis step by step break down of organic compounds Photosynthesis step by step build up of organic compounds

It takes energy to make energy… Flow of energy in universe is constantly cycled Stored in chemical bonds Released from chemical bonds

Sunlight ATMOSPHERE Radiant heat trapped by CO2 and other gases Figure 7.13A & B

Photosynthesis removes CO2 (greenhouse gas) and adds O2 (used to make ozone (O3)

This lets harmful UV rays thru, increasing GW Industrial chemicals called CFCs “eat ozone”, causing dangerous thinning of the ozone layer This lets harmful UV rays thru, increasing GW Sunlight International restrictions on these chemicals are allowing recovery Southern tip of South America Antarctica Figure 7.14B