Photosynthesis

I. Energy Carriers: Powering All Activities of the Cell

II. Photosynthesis: Capturing Energy from the Sun

 

I. Energy Carriers

v  To discuss photosynthesis, we must first understand how energy is transferred from one molecule to another.

o   One common method of energy transfer in the physical world depends on heat.  Ex: boiling water

o   Living organisms generally cannot use such methods but must instead rely on specialized molecules that can transfer chemical energy stored in covalent bonds in smaller, more manageable steps.

v   These molecules are called energy carriers.  The most commonly used energy carrier is ATP, which stores energy in the form of covalent bonds between phosphate groups.

o   Phosphorylation:

 

o   ATP can then contribute its stored energy to another molecule by transferring one of its phosphate groups to that molecule.  The transfer energizes the recipient molecule, enabling it to change shape or react chemically with other molecules.

 

o   NADP⁺ and NAD⁺ are two other important energy carriers.  Each can pick up high energy electrons and donate them to redox reactions.  These molecules are capable of picking up high energy electrons along with hydrogen ions to form NADPH and NADH.  They can then donate these electrons to other molecules.

II. Photosynthesis: process of repackaging the sun’s energy into the energy carriers that power the activities of the cell.  

o   Organisms that photosynthesize are called autotrophs or producers.

§  Ex: plants, algae, cyanobacteria

o   Organisms that consume other organisms are called heterotrophs or consumers.

§  Ex: herbivores, carnivores, omnivores

v  What is needed for photosynthesis?

o   6CO₂ + 6 H₂O  yields  C₆H₁₂O₆ + 6O₂

o   CO2 is present in the surrounding air (remember that we also breathe this out as a waste product during respiration). It enters the leaf through small openings called stomata.  CO2 then diffuses into chloroplasts (where photosynthesis takes place).

o   Water is absorbed by roots and enters the leaf through specialized plant cells called xylem cells.  Like CO2, it then diffuses into the chloroplasts.

v  Flowering Plants

o   Green parts of plants carry on photosynthesis

o   Leaves are structured to be specialists at photosynthesis

o   A look at chloroplasts

§  Surrounded by a double membrane

§  Thylakoids are flattened sacs inside the chloroplast

·         Stacks of thylakoids are called grana (singular=granum)

·         Thylakoid membranes contain photosynthetic pigments=where solar energy is absorbed

§  Fluid-filled interior is called the stroma

·         Stroma is where CO2 is changed to a carbohydrate

 

 

 

 

v  Photosynthesis involves 2 Reactions

o   Light Reactions

 

o   Dark Reactions

 

 

 

 

 

v Light Reactions capture energy from sunlight

o   Energy capture involves specialized pigments, the most common being chlorophyll, which accounts for the green color in most plant foliage.

o   Pigments are molecules that absorb certain wavelengths of visible light energy

§  A photon is a discrete particle of light with a fixed quantity of energy.

§  Energy is inversely proportional to wavelength. For example, photons of UV light have more energy than those of infrared.

§  Visible light drives photosynthesis. Blue and red are colors most effectively absorbed by chlorophyll, and thus most effective for photosynthesis.

§  Wavelengths that aren’t absorbed are reflected

·         Which colors are reflected by chloroplasts?

§  Each pigment has its own absorption spectrum (pattern of wavelengths it absorbs). 

§  Each pigment has its own action spectrum (how efficient it is at absorbing light).

o   In plants, pigments are embedded in the thylakoid membrane.  When exposed to light, the electrons associated with a chlorophyll molecule absorb energy from the light and become more energized.  Energized electrons are said to be “excited.”

§  Hundreds of chlorophyll molecules are arranged together in a formation called the antenna complex.

§  Reaction Center: specialized area in antenna complex with slightly different chlorophyll molecules where energy is accepted by the specialized chlorophyll and transferred to an electron transport chain.

·         Electron transport chain

 

·         Photosystem

 

o   Electron pathways involve photophosphorylation

§  Light is absorbed by a pigment, and the energy is passed to neighboring molecules.

§  Protein complexes called electron carriers pass the electrons down.

§  The first carrier is the acceptor; it gains energy from the electron.

§  That energy is used to pump protons from the stroma into the thylakoid space. (Protons are often denoted by the symbol H⁺ which allows us to keep track of ions and charged molecules that can interact in cellular situations.)

§  The electron is then passed to the next carrier, which uses the energy boost to pump in more H+.

§  As the protons accumulate, the energy is now stored in the thylakoid as a proton gradient (imbalance of proton concentration) across the thylakoid membrane.

§  The hydrogen ions flow down their gradient through ATP synthase complexes, which produces ATP. 

·         The ATP will be used by the plant in the Calvin Cycle Reactions.

o   Two types of photosystems

§  PHOTOSYSTEM II

·         Electron flow along the ETC leads to the production of ATP.

§  PHOTOSYSTEM I

·         Responsible for the production of the powerful reducing agent NADPH

o   Each photosystem contributes to photosynthesis as a single integrated process.  The movements of electrons along the ETCs of the two photosystems enable the capture of energy from sunlight, but also have an important by-product; the liberation of oxygen gas.

§  Photolysis:

o   Electrons pass down the first electron transport chain of photosystem II.  By the end of this chain, the electrons have lost energy.  They get an additional boost of energy from sunlight again in the reaction center of photosystem I.  Photosystem I eventually transfers these electrons to an ETC protein, which in turn transfers them to NADP+, forming NADPH.

 

v Dark Reactions manufacture sugars

o   Energy carriers produced by light reactions—ATP and NADPH—are used in dark reactions to make sugars from CO₂ and water.  This process is known as carbon fixation. 

o   By capturing inorganic carbon atoms from CO₂ gas and fixing them into organic compounds (sugars), dark reactions make atmospheric carbon available to plants and other living organisms.

o   Dark reactions are catalyzed by enzymes that float freely in the stroma.  The most abundant of these enzymes is called rubisco.

o   Overview of Dark Reactions:

 

 

 

o   Step 1 of the dark cycle: CO2 fixation

§  CO2 from the atmosphere attaches to RuBP (5-carbon molecule called ribulose biphosphate)

·         Results in a 6-carbon molecule that splits into two 3-carbon molecules

·         Reaction is catalyzed by rubisco.

·         Each newly formed 3-carbon molecule is called 3PG (3-phosphoglycerate).

o   Step 2 of the dark cycle: CO2 reduction

§  Each of the two 3PG molecules undergo reduction to different molecules known as G3P (glyceraldehyde 3 phosphate).

·         G3P is used to form glucose (when exported to cytoplasm and used immediately), sucrose (when exported to cytoplasm and stored), and starch (when left inside chloroplasts to accumulate)

§  Some ATP and NADPH from the light reactions are used in this process.

§  Reaction signifies the reduction of CO₂ to CH₂O.

o   Step 3 of the dark cycle: regeneration of RuBP

§  Series of steps that results in the reformation of RuBP

§  ATP from the light reactions is needed in this stage of the Calvin Cycle as well.

o   Draw the Reactions Below:

 

 

 

 

 

 

 

 

 

o   For every three turns of the carbon fixation cycle, five G3P molecules are generated.  However, molecules of RuBP have to be replaced for the cycle to continue.

§  Three turns generate enough G3P to replace three RuBP molecules and produce one additional G3P.

o   Two G3P molecules are needed to form glucose (6 turns of the cycle). 

Note: “Most plants put CO₂ directly into the Calvin cycle. Thus the first stable organic compound formed is the glyceraldehyde 3-phosphate. Since that molecule contains three carbon atoms, these plants are called C₃ plants. For all plants, hot summer weather increases the amount of water that evaporates from the plant. Plants lessen the amount of water that evaporates by keeping their stomates closed during hot, dry weather. Unfortunately, this means that once the CO₂ in their leaves reaches a low level, they must stop doing photosynthesis. Even if there is a tiny bit of CO₂ left, the enzymes used to grab it and put it into the Calvin cycle just don't have enough CO₂ to use. Typically the grass in our yards just turns brown and goes dormant. Some plants like crabgrass, corn, and sugar cane have a special modification to conserve water. These plants capture CO₂ in a different way: they do an extra step first, before doing the Calvin cycle. These plants have a special enzyme that can work better, even at very low CO₂ levels, to grab CO₂ and turn it first into oxaloacetate, which contains four carbons. Thus, these plants are called C₄ plants. The CO₂ is then released from the oxaloacetate and put into the Calvin cycle. This is why crabgrass can stay green and keep growing when all the rest of your grass is dried up and brown.

There is yet another strategy to cope with very hot, dry, desert weather and conserve water. Some plants (for example, cacti and pineapple) that live in extremely hot, dry areas like deserts, can only safely open their stomates at night when the weather is cool. Thus, there is no chance for them to get the CO₂ needed for the dark reaction during the daytime. At night when they can open their stomates and take in CO₂, these plants incorporate the CO₂ into various organic compounds to store it. In the daytime, when the light reaction is occurring and ATP is available (but the stomates must remain closed), they take the CO₂ from these organic compounds and put it into the Calvin cycle. These plants are called CAM plants, which stands for crassulacean acid metabolism after the plant family, Crassulaceae (which includes the garden plant Sedum) where this process was first discovered.”

Quoted from: http://biology.clc.uc.edu/Courses/bio104/photosyn.htm