THE BASICS OF PHOTOSYNTHESIS

           Photosynthesis

            is used by plants, algae (protists), and some bacteria,

            transforms light energy into chemical energy, and

            uses carbon dioxide and water as starting materials.

           The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules.

           Organisms that use photosynthesis are

            photosynthetic autotrophs and

            the producers for most ecosystems.

 

Chloroplasts: Sites of Photosynthesis

           Chloroplasts are

            the site of photosynthesis and

            found mostly in the interior cells of leaves.

           Inside chloroplasts are interconnected, membranous sacs called thylakoids, which are suspended in a thick fluid called stroma.

           Thylakoids are concentrated in stacks called grana.

 

           The green color of chloroplasts is from chlorophyll, a light-absorbing pigment that plays a central role in converting solar energy to chemical energy.

           Stomata are tiny pores in leaves where

            carbon dioxide enters and

            oxygen exits.

 

The Simplified Equation for Photosynthesis

           In the overall equation for photosynthesis, notice that the reactants of photosynthesis are the waste products of cellular respiration.

           In photosynthesis,

            sunlight provides the energy,

            electrons are boosted uphill and added to carbon dioxide, and

            sugar is produced.

           During photosynthesis, water is split into

            hydrogen and

            oxygen.

           Hydrogen is transferred along with electrons and added to carbon dioxide to produce sugar.

           Oxygen escapes through stomata into the atmosphere.

 

A Photosynthesis Road Map

           Photosynthesis occurs in two multistep stages:

               the light reactions convert solar energy to chemical energy and

               the Calvin cycle uses the products of the light reactions to make sugar from carbon dioxide.

           The initial incorporation of carbon from the atmosphere into organic compounds is called carbon fixation.

               This lowers the amount of carbon in the air.

               Deforestation reduces the ability of the biosphere to absorb carbon by reducing the amount of photosynthetic plant life.

 

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY

           Chloroplasts

            are chemical factories powered by the sun and

            convert sunlight into chemical energy.

 

The Nature of Sunlight

           Sunlight is a type of energy called radiation, or electromagnetic energy.

           The distance between the crests of two adjacent waves is called a wavelength.

           The full range of radiation is called the electromagnetic spectrum.

 

Chloroplast Pigments

           Chloroplasts contain several pigments:

            Chlorophyll a

               absorbs mainly blue-violet and red light and
               participates directly in the light reactions.

            Chlorophyll b

               absorbs mainly blue and orange light and
               participates indirectly in the light reactions.

           Carotenoids

            absorb mainly blue-green light,

            participate indirectly in the light reactions, and

            absorb and dissipate excessive light energy that might damage chlorophyll.

           The spectacular colors of fall foliage are due partly to the yellow-orange light reflected from carotenoids.

 

How Photosystems Harvest Light Energy

           Light behaves as photons, a fixed quantity of light energy.

           Chlorophyll molecules absorb photons.

            Electrons in the pigment gain energy.

            As the electrons fall back to their ground state, energy is released as heat or light.

           In the thylakoid membrane, chlorophyll molecules are organized with other molecules into photosystems.

           A photosystem is a cluster of a few hundred pigment molecules that function as a light-gathering antenna.

           The reaction center of the photosystem consists of chlorophyll a molecules that sit next to another molecule called a primary electron acceptor, which traps the light-excited electron from chlorophyll a.

           Another team of molecules built into the thylakoid membrane then uses that trapped energy to make

            ATP and

            NADPH.

 

How the Light Reactions Generate ATP and NADPH

           Two types of photosystems cooperate in the light reactions:

            the water-splitting photosystem and

            the NADPH-producing photosystem.

           The light reactions are located in the thylakoid membrane.

           An electron transport chain

            connects the two photosystems and

            releases energy that the chloroplast uses to make ATP.

 

THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE

           The Calvin cycle

            functions like a sugar factory within a chloroplast and

            regenerates the starting material with each turn.

           Sugars are produced through four main steps in the Calvin cycle:

            Carbon fixation combines CO2 with RuBP;

            ATP and NADPH are used to energize the carbon chains, producing G3P

            Some G3P can exit the cycle, to produce anything in the plant;

            Some G3P are reconfigured to produce RuBP, and the cycle can start again.