Photosynthesis

By the end of this class you should understand:

• How energy may be stored in chemical bonds• Electron transport as a means of transforming

energy• The complete process, reactants, and

products of photosynthesis• The essential purposes of the light and dark

reactions

Transforming Energy

• According to the laws of thermodynamics, energy is always lost as we do things– Moving around, producing

body heat, etc.• The total amount of energy

in all living things is constantly decreasing– Where is fresh energy

coming from?

New Energy• Life gains new energy from

the sun!• Light is a type of energy that

can move through empty space– The sun is spraying massive

amounts of light in all directions, and the Earth is getting bombarded with it

• Life took off after developing a way to directly convert this light energy

Photosynthesis

• Photosynthesis is a process where nonliving materials are converted into living materials using light energy– Please note: the light itself is

not turned into matter, only the energy is being used to do this

• The first organisms to do this were ancient prokaryotes we call cyanobacteria

Photosynthetic Organisms• The ocean is still full of

cyanobacteria!• In the ancient past, some

eukaryotic cells captured cyanobacteria and converted them into organelles called chloroplasts– These were the ancestors of

plants and algae– This probably happened

multiple times, since red algae and green algae are not closely related!

Energy Essentials

• The key to understanding energy in living things is that chemical bonds have energy stored in them– Some bonds have much more energy than others

• The third phosphate in ATP is attached to the second phosphate with a very high-energy bond– Many other bonds are also high-energy, such as the

bonds between carbon atoms in sugar

Energy Storage• Photosynthesis is essentially the process of

turning carbon dioxide (low energy) into sugar (high energy)

• This is accomplished by using the energy of sunlight– Water is also a raw material that is used up– This is why your plants get thirsty!

Two Tools of Photosynthesis

• Enzymes– Many reactions, including the

actual capture of carbon dioxide, are performed with enzymes

• Electron Transport– This is a special type of

process where energy is transformed from one kind to another

Electron Transport Analogy

• An electron transport chain works similarly to a water wheel in a river– The energy of water running

downstream is transformed into mechanical energy when it pushes on the water wheel

– An electron transport chain allows a high energy electron to move from one protein group to the next, and it transforms the energy in the process

– Allowing the electron to do work!

Cell Membranes• Electron Transport Chains

have many different proteins that must bump into each other in the right order

• They are “floating” in a cell membrane and shaped so each will only interface with the next one in line– Remember the cell

membrane is a fluid, so the proteins can move freely in two dimensions

Electron Transport

• By moving a high-energy electron through the protein structure, different types of work can be done

• One major type of action that can occur is a proton can be moved from one side of the membrane to the other– Remember, protons and

electrons are attracted to each other!

Proton Gradient• If a larger amount of

something is on one side of a membrane than the other, it is referred to as a concentration gradient– With protons, this is

specifically referred to as a proton gradient

• An electron transport chain can be used to build a proton gradient– Why would you do this?

Proton Gradient Use

• A proton gradient is a very useful tool thanks to this magic molecule: ATP Synthase

• ATP Synthase is like a turnstile that will only turn when protons move down the gradient– Also known as chemiosmosis

• As they turn the ATP synthase, the movement of the enzyme turns ADP and phosphate into ATP– This is THE most efficient way to

make ATP

ATP Synthase in Action

Photosynthesis

• Photosynthesis is made up of two stages:– Light-dependent (light) reactions– Light-independent (dark) reactions

• The light reactions actually trap the energy in a photon (particle of light)

• The dark reactions don’t actually have to take place in the dark, but they are driven by enzymes so they don’t need light– Cactuses do some of their reactions at night since it’s

too hot during the day!

Light-Dependent Reactions

• The light-dependent reactions begin when a photon hits a pigment molecule called chlorophyll– Chlorophyll molecules are

arranged into a structure called a photosystem, embedded in a chloroplast membrane

• The energy from the photon excites a photon into entering an electron transport chain

Free Electrons?• Once the chlorophyll’s electron

has been stripped from its magnesium atom, it must reload

• The way it reloads an electron is by taking one from a water molecule– This causes water molecules to

turn into oxygen, which is a waste product, and hydrogen, which is used in the proton gradient

– You’re breathing in plant farts right now!

Further Light-Dependent Reactions• The complete

photosynthesis chain has two photosystems

• At the conclusion of the light reactions the electron has released its light energy by pumping protons and is contained in a molecule called NADPH– NADPH is an electron

carrier, once it releases electrons it turns back into NADP+ and an H+

Light-Independent Reactions

• The light-independent reactions use the energy acquired in the light-dependent reactions to make energy-storing molecules– ATP has a short shelf life, and

will decay on its own– NADPH are only a short-term

solution, and having stray electrons in your cells is no bueno

Acquiring Carbon

• Carbon for the sugar is acquired by a process called carbon fixation

• Another magic enzyme performs this difficult task, this time an enzyme called Ribulose 1,5-bisphosphate carboxylase– RUBISCO

• Carbon fixation is difficult even with rubisco, so most plants just produce a ton of it– Estimated to be the most common

enzyme on the planet Earth!

Carbon Fixation

• Rubisco grabs onto a carbon dioxide and attaches it to a sugar molecule called RuBP (ribulose 1,5-bisphosphate)

• This new 6-carbon sugar immediately splits into two 3-carbon sugars– Some of these 3-carbon sugars

are re-converted back to RuBP– Extra 3-carbon sugars are used

to make other sugars and other plant/bacteria structures

Calvin-Benson Cycle

• The carbon fixation and enzymatic regeneration of RuBP is called the Calvin-Benson Cycle, or just Calvin Cycle (because no one cares about Andrew Benson)

• It is accomplished with enzymes, and with the ATP and NADPH produced during the light reactions– We will NOT be memorizing

any of that bull$#&%

Acquiring Carbon Dioxide• Carbon dioxide must be pulled in

from the air (and oxygen must be allowed to escape)

• Plants have tiny windows called stomata that can open and close to prevent excess evaporation of water

• If O2 builds up it can block rubisco, so plants can suffocate on oxygen!– Drought-resistant plants have

chemical tricks to prevent O2 buildup

Summary of Photosynthesis:

• Light hits photosystem, energizing electron• Electron goes through transport chain,

building proton gradient• Electron is trapped in NADPH; ATP Synthase

makes ATP using proton gradient• Rubisco grabs CO2 and attaches it to RuBP,

making two 3-carbon sugars• Most 3-carbon sugars are remade into RuBP

using ATP and NADPH; extras become sugar

See you in lab!

• WARNING: The photosynthesis lab will fall AFTER the first exam