Anderson Spring 2017 College of the Redwoods Photosynthesis Anderson Spring 2017 College of the Redwoods

How do plants get energy? 6CO2 6H2O C6H12O6 6O2 + carbon dioxide water sugar oxygen ATP energy Cellular Respiration Photosynthesis 6CO2 6H2O C6H12O6 6O2 light + carbon dioxide water sugar oxygen

Autotroph/Heterotroph Autotroph – organism that produces its own food auto = self, troph = feeder Plants (and some bacteria and algae) are photoautotrophs - uses sunlight and carbon from CO2 to synthesize energy Heterotroph – organisms that must obtain energy and carbon from food by consuming other organisms hetero = other, troph = feeder Dependent on autotrophs

Photosynthesis occurs in the mesophyll (middle layer) Leaf Cross-Section Photosynthesis occurs in the mesophyll (middle layer) CO2 exchange happens through openings on bottom of leaf, called stomata

Chloroplasts Organelle in which photosynthesis takes place Thylakoid – membrane-bound disc structure that contains chlorophyll (green) Granum – stack of thylakoids Stroma – space around granum

What is Light Energy? Longer wavelengths carry less energy; shorter carry more Sunlight emits broad range of electromagnetic radiation

Visible Light Spectrum White light – light that contains all wavelengths of light The colors we see are the light that’s NOT absorbed When we see green, all light except for green is being absorbed

Chlorophyll Pigments Chlorophyll a – absorbs from violet and red range Chlorophyll b – absorbs from blue and red-orange Carotenoid – absorbs from blue-green (looks red, orange, yellow) Not all plants are equal! (but all plant lives matter)

Photons Individual bundles of electromagnetic energy Can provide energy to electrons (makes them move) Basic unit of all light

Structure of Chlorophyll

Let’s tie all that nonsense together Plants make their own food from sunlight energy – photoautotrophs Sunlight contains tiny particles, called photons, that make up light These photons have energy! The photons in the light hit the chlorophyll pigments in plant leaves, green light is reflected, all other absorbed A reaction occurs which is the start of photosynthesis

Stages of Photosynthesis Stage 1 – Light Reactions Energy from sun light splits water to produce oxygen Reduces NADP+ to NADPH, which will then carry high energy electrons to Calvin Cycle Stage 2: Calvin Cycle Carbon dioxide and the high energy electrons from NADPH are used to create carbohydrates (glucose) Called the “dark cycle” because sunlight is not necessary

Light-Dependent Reactions Photosystem – grouping of pigment molecules (chlorophyll) and proteins Exists in thylakoid membrane Photon is “absorbed” by the chlorophyll and causes chlorophyll to become excited The electron breaks free from chlorophyll – “donates” electron The electron must be replaced – this is done by splitting water into oxygen and hydrogen ions (each water molecule releases 2 electrons) H2O  ½ O2 + 2H+

Photosystems In eukaryotes and some prokaryotes, there’s 2 photosystems: Photosystem II and Photosystem I (named by discovery, not order of function)

Photosystem II Light energy absorbed and enters reaction center – chlorophyll excited Electron captured by acceptor Water split, electrons replace donated ones from chlorophyll, O2 released as byproduct Electrons are passed to photosystem I

Photosystem I Once electrons re passed to photosystem I, electrons are captured by acceptor Excited electrons oxidize NADP+ to NADPH (another energy storing molecule)

ATP Synthase Electrochemical gradient of H+ ions created through electron transport chain ATP synthase uses gradient to diffuse H+ out of thylakoid space

End Result of Light-Dependent 2 H2O  2 O2 + 2 H2 – oxygen is the byproduct 1 NADPH 1 ATP So where does the carbon come from to make the glucose?

Calvin Cycle Reactions of photosynthesis that use energy stored from light- dependent reactions (ATP and NADPH) to form glucose CO2 enters chloroplasts by diffusion (enters cell through stomata) Calvin cycle occurs in stroma (space between membrane and thylakoids) In addition to CO2, RuBisCO (an enzyme), and RuBP (5 carbon molecule) are needed

Calvin Cycle Stage 1: Carbon fixation – CO2 and RuBP (5 carbons) create 6- carbon compound, catalyzed by RuBisCO The 6-carbon compound immediately breaks into 2 3-carbon compounds (3-PGA)

Calvin Cycle Stage 2: 3-PGA (3-carbon molecule) reduced by NADPH and ATP Results in G3P, NADP+ and ADP 1 ATP and 1 NADPH used for each 3-PGA molecule 1 G3P leaves cycle to make glucose

Calvin Cycle Stage 3: Remaining G3P molecules regenerate RuBP (5 3-carbon molecules rearrange to make 3 5-carbon molecules) 3 more ATP used, making 3 ADP And the cycle is ready for the next CO2 Takes 6 turns to make 1 glucose (or 2 with 3 CO2 to start)

How do prokaryotes do it without chloroplasts? Remember: prokaryotes lack membrane-bound organelles, such as chloroplasts Prokaryotic photosynthetic autotropic organisms have infoldings of plasma membrane for chlorophyll attachment and photosynthesis

One organism’s waste is another organism’s energy Carbon dioxide is “waste” for animals, plants need carbon dioxide for photosynthesis Oxygen is “waste” for plants, animals need oxygen for cellular respiration 6CO2 6H2O C6H12O6 6O2 + carbon dioxide water sugar oxygen 6CO2 6H2O C6H12O6 6O2 + carbon dioxide water sugar oxygen

Plants Undergo Cellular Respiration Too! Carbohydrates are energy storage molecules Plants store their glucose as starch The ATP from the light-reactions of photosynthesis is enough to fuel the Calvin cycle, not the whole plant cell Plants have mitochondria too! Cellular respiration provides more ATP When there’s little or no light, plants get their energy from cellular respiration (take in O2 and release CO2)

Do Plants Sleep? During the night, when there’s no sunlight, plants consume oxygen to undergo cellular respiration (use their carbohydrate energy stores) They will be using oxygen and releasing CO2, just like we do But! Plants take up much more CO2 during photosynthesis than they release at night So the next time a 5 year old asks you this question, you can tell them, “No, they don’t really sleep. They’re just doing other things at night that they don’t do during the day.”

Don’t listen to our President, global warming is real!