Photosynthesis LA Charter School Science Partnership 3 March 2012 Nick Klein

Today’s Talk Intro Part 1: Photosynthesis – the big picture Part 2: Chemistry of photosynthesis Part 3: Biology and ocean context

Today’s Talk cont’d I’ve split this talk into three (roughly) even parts. We will take a brief (1-2min) break between sections. Please feel free to take notes, handle the props, think of questions for after the talk, etc.

Part 1: The big picture What is photosynthesis? Let’s define it! –Photosynthesis is the process by which organisms use the energy in sunlight to chemically transform carbon dioxide (CO 2 ) into organic carbon compounds such as sugars Some organisms can use energy sources other than sunlight, this is called chemosynthesis (Jason’s talk!)

Part 1: The big picture Organisms that make their own food are called autotrophs. Organisms that make food using photosynthesis are photoautotrophs All animals, including humans, are heterotrophs—we have to consume other organisms as food

Part 1: The big picture Why is photosynthesis important? –Most life on Earth depends on it –Responsible for us having oxygen to breathe! –Fossil fuels are derived from ancient photosynthesis –Photosynthesis removes CO 2 from the atmosphere

Part 1: The big picture

Photosynthesis uses sun energy to transform inorganic carbon dioxide (CO 2 ) into organic carbon compounds such as sugars, fats, and amino acids… the building blocks of life! Scientists refer to this conversion as “fixing” carbon. So, photosynthesis is sun-driven carbon fixation.

Part 1: The big picture Photosynthesis involves reduction/oxidation (redox) reactions— chemical reactions that involve the movement of electrons from one molecule to another In photosynthesis, when carbon dioxide is fixed, it is reduced (electrons are added to it) which produces organic carbon compounds

Part 1: The big picture L oss of E lectrons is O xidation goes G ain of E lectrons is R eduction

Part 1: The big picture Example: nitrogen fixation 2x nitrogen gasammonia

Part 1: Bonding on an atomic level How many electrons in nitrogen gas? NN

Part 1: Bonding on an atomic level How many electrons in two molecules of ammonia? N H H H

Part 1: The big picture In photosynthesis, photoautotrophs use the energy of sunlight to fix carbon dioxide into glucose Carbon dioxide is reduced to make the basic chemical building blocks of all life Photosynthesis is one of the most important biological processes!

Break!

What (chemically) is required for photosynthesis? –Light –Water –Carbon dioxide Part 2: Chemistry of photosynthesis 12H 2 O + 6CO 2  C 6 H 12 O 6 + 6O 2 + 6H 2 O

There are two distinct parts of photosynthesis Light-dependent (light) reactions: sun energy is absorbed and transformed into chemical energy Light-independent (dark) reactions: also called the Calvin cycle, chemical energy from the light reactions is used to fix CO 2 into glucose Part 2: Chemistry of photosynthesis

Light-dependent (light) reactions can ONLY happen when there is sunlight available Of course, the first step in transforming sun energy into chemical energy is to soak up some rays! Part 2: Chemistry of photosynthesis

Plants use pigments to absorb light energy from the sun Main pigment is chlorophyll a (shown next slide), this is responsible for plants being green There are many other accessory pigments which help plants harvest different colors of light Part 2: Chemistry of photosynthesis

anthocyanins

The pigments, along with a large number of enzymes and other chemical machinery, form photosystems Light energy is absorbed by the pigments inside the photosystem This energy is used to split water molecules into electrons, protons, and oxygen gas Part 2: Chemistry of photosynthesis

2H 2 O 4e - + 4H + + O 2 Pigments Photosystem

The electrons we get from splitting water are then excited by more sun energy captured by the pigments and enter the electron transport chain The electron transport chain is a series of different molecules each electron is passed along to. The electron loses some of its energy with each “step” it makes Part 2: Chemistry of photosynthesis

Pigments Photosystem 4e - Electron Transport Chain

After passing through the electron transport chain, the photosystem attaches the electrons to a molecule called NADPH NADPH is a reductant—it can reduce (give electrons) to other compounds Where do you think the electrons from NADPH will go? Part 2: Chemistry of photosynthesis

NADPH

Part 2: Chemistry of photosynthesis Pigments Photosystem 4e - Electron Transport Chain NADPH

The energy from the electron transport chain is used to pump protons (the H + we made from splitting water) through chemical machinery that makes ATP The high-energy phosphate bonds in ATP are broken to provide the energy for most chemical reactions that occur in biology Part 2: Chemistry of photosynthesis

ATP

Light-dependent (light reactions) summary: light is captured by pigments and used to split water, ultimately producing oxygen gas, NADPH (a reductant) and ATP (energy) Part 2: Chemistry of photosynthesis

Light-independent (dark) reactions can occur with or without light, they don’t require it NADPH and ATP from the light-dependent reactions are used to reduce CO 2 into sugars and other organic compounds Part 2: Chemistry of photosynthesis

ATP + NADPH CO 2 C 6 H 12 O 6 The Calvin Cycle (light independent reactions)

Part 2: Chemistry of photosynthesis

Light-dependent (light reactions) summary: light is captured by pigments and used to split water, ultimately producing oxygen gas, NADPH (a reductant) and ATP (energy) Light-independent reactions: ATP and NADPH are used to fix CO 2 into organic carbon (sugars) Part 2: Chemistry of photosynthesis

12H 2 O + 6CO 2  C 6 H 12 O 6 + 6O 2 + 6H 2 O C 6 H 12 O 6 + 6O 2  6H 2 O + 6CO 2 Respiration is photosynthesis backwards!

Break!

Part 3: Broader context We’ve gone over the basics and the chemistry of photosynthesis, let’s explore some of the biology and think about it in the context of the ocean and the Earth as a whole!

Part 3: Broader context

The overall trend of atmospheric CO 2 is upward (it is increasing). Why is that? Within each year, what is happening to the CO 2 ? Why might this be? The “wave” pattern is the Earth “breathing” in and out—lower CO 2 in the summer when all the plants are growing and using it for photosynthesis!

Part 3: Broader context Land plants have stomata on the undersides of their leaves They open their stomata to breathe in CO 2. When might they want to open them? When would they close them?

Part 3: Broader context

The oceans account for about half of all carbon fixation However, the oceans only have 0.2% of the biomass Fast turnover—what is the lifespan of a phytoplankton compared to a tree? What about these facts might make algae interesting to scientists?