6.1 Overview of Photosynthesis

• Photosynthesis is the process by which plants, Algae, and cyanobacteria use sunlight to convert carbon dioxide and water into oxygen and carbohydrates.

• Photosynthetic organisms produce an enormous amount of carbohydrates, which are essential for life on Earth.

• The energy stored in the carbohydrates produced by photosynthetic organisms is used to sustain life on Earth, including the plants themselves, animals, and humans.

• Fossil fuels, such as coal and oil, are formed from the remains of ancient photosynthetic organisms.

• The fermentation of plant materials produces alcohol, which can be used as a fuel.

• Plants, such as flowers and mosses, typically live on land.

• Photosynthetic protists, such as Euglena, diatoms, and kelp, typically live in water.

• Cyanobacteria is a type of bacterium that lives everywhere.

Plants as Photosynthesizers

• The steps to photosynthesis are as follows:

  • Step 1: Carbon dioxide enters the leaf through small openings called stomata.

  • Step 2: Water is absorbed by the roots and moves up the stem to the leaves.

  • Step 3: CO2 and water diffuse into mesophyll cells, where they enter chloroplasts.

  • Step 4: Chlorophyll in the chloroplasts absorbs sunlight and uses the energy to convert CO2 and water into oxygen and glucose.

  • Step 5: Oxygen is released into the air.

  • Step 6: Glucose is used by the plant for energy or stored for later use.

• Raw materials for photosynthesis

  • Carbon dioxide

  • Water

• Parts of a chloroplast

  • Grana

  • Stroma

• Grana

  • Made up of thylakoids

  • Contain photosynthetic pigments, such as chlorophyll

  • Absorb solar energy

• Stroma

  • Fluid-filled space

  • Where carbon dioxide is enzymatically processed to form carbohydrates, such as glucose.

The photosynthetic Process

• Carbon dioxide (CO2) and water (H 2O), the reactants for photosynthesis, contain much less energy than the end product, carbohydrate (CH 2O).

• Sunlight provides the needed energy to allow for photosynthesis to occur.

  • During photosynthesis, electrons are removed from water molecules and energized by solar energy.

  • An enzyme called NADP+ then transfers these high-energy electrons to carbon dioxide, forming carbohydrates.

  • The electrons are accompanied by Hydrogen Ions (H+), which is why it is possible to track the movement of electrons during photosynthesis.

• The process of photosynthesis consists of light reactions and the Calvin cycle reactions.

  • Light reactions

    • Occur in the thylakoid membrane of chloroplasts

    • Use light energy to split water molecules into hydrogen and oxygen

    • Use the hydrogen to produce ATP and NADPH

  • Calvin cycle

    • Occurs in the stroma of chloroplasts

    • Uses ATP and NADPH to reduce carbon dioxide into glucose

    • Releases oxygen as a byproduct

• Reduction is the gain of electrons and hydrogen ions by a molecule.

• Oxidation is the loss of electrons and hydrogen ions by a molecule.

• Redox reactions are coupled reactions that involve oxidation and reduction.

• In a cell, reduction reactions are often coupled with oxidation reactions to produce energy.

6.2 The Light Reactions--Harvesting Energy

• During the light reactions of photosynthesis, the pigments within the thylakoid membranes absorb solar energy.

• Solar energy (also called radiant energy) can be described in terms of its wavelength and its energy content.

• The electromagnetic spectrum is a continuous range of radiant energy, with different wavelengths corresponding to different amounts of energy.

• Visible light is a form of electromagnetic radiation that can be seen by the human eye.

• Visible light is made up of different wavelengths, each of which corresponds to a different color.

• The wavelengths of visible light range from about 380 nanometers (violet) to about 700 nanometers (red).

• Less than 50% of solar radiation that hits the Earth's atmosphere reaches its surface.

• Most of the radiation that reaches the Earth's surface is within the visible light range.

• Higher-energy wavelengths are screened out by the ozone layer in the atmosphere.

• Lower-energy wavelengths are screened out by water vapor and CO2.

• Both the organic molecules within organisms and certain life processes, such as vision and photosynthesis, are adapted to utilize the radiation that is most prevalent in the environment.

Photosynthetic Pigments

• Chlorophylls and carotenoids are the main pigments found in photosynthetic cells.

• Chlorophylls absorb violet, blue, and red wavelengths of light.

• Green light is reflected by chlorophylls, which is why leaves appear green.

• Carotenoids absorb violet-blue-green wavelengths of light but not yellow-orange wavelengths.

• In the fall, when chlorophyll breaks down, carotenoids become more noticeable, giving leaves their yellow, orange, and red colors.

The Light Reactions: Capturing Solar Energy

• Light reactions capture the sun’s energy and store it in the form of a hydrogen ion (H+) gradient.

  • This hydrogen ion gradient is used to produce ATP molecules.

  

  

• The light reactions produce an important molecule called NADPH.

• The light reactions use two photosystems:

  • Photosystem I (PS I).

  • photosystem II (PS II).

  • The photosystems are named for the order in which they were discovered.

• Photosystem I (PS I) and photosystem II (PS II) are two protein complexes that use light energy to split water and generate ATP and NADPH.

• PS I and PS II each consist of a pigment complex and an electron acceptor. The pigment complex absorbs light energy and passes it to the reaction center, where it is used to excite electrons.

• PS II splits water, releasing oxygen and hydrogen ions. The hydrogen ions are used to generate ATP in the electron transport chain.

• PS I produces NADPH by passing electrons from the electron transport chain to NADP+.

The Electron Transport Chain

• PS I, PS II, and the electron transport chain are molecular complexes located within the thylakoid membrane.

• The molecular complexes are densely clustered to promote efficient electron transfer between carriers.

• The ATP synthase complex is also present.

ATP Production

• During photosynthesis, the thylakoid space acts as a reservoir for hydrogen ions (H+).

• Water is split, releasing two H+ ions into the thylakoid space.

• Electrons move from carrier to carrier down the electron transport chain, releasing energy.

• This energy is used to pump H+ ions from the stroma into the thylakoid space.

• This creates a H+ gradient, with more H+ ions in the thylakoid space than in the stroma.

• The H+ ions then flow down their concentration gradient across the thylakoid membrane at the ATP synthase complex.

• This releases energy, which causes the ATP synthase complex to change shape and produce ATP from ADP + P.

• The production of ATP captures the released energy.

NADPH Production

• NADP+ is a coenzyme that accepts electrons and a hydrogen ion to become NADPH.

• NADPH is produced in the light reactions of photosynthesis when NADP+ receives electrons and a hydrogen ion from the electron transport chain.

6.3 The Calvin Cycle Reactions--Making Sugars

• The light reactions of photosynthesis generate ATP and NADPH.

• The Calvin cycle is the second stage of photosynthesis and is powered by the previously generated ATP and NADPH.

• The Calvin cycle is also known as the light-independent reaction because it does not require an input of solar energy.

• The Calvin cycle was named after Melvin Calvin, who used the radioactive isotope 14C to discover the reactions that make up the cycle.

Overview of the Calvin Cycle

• The Calvin cycle is a series of chemical reactions that use carbon dioxide (CO2) from the atmosphere to produce glucose and other organic molecules.

• The Calvin cycle has three steps:

  • CO2 fixation: CO2 is combined with RuBP (ribulose-1,5-bisphosphate) to form two molecules of 3-phosphoglycerate (3-PGA).

  • CO2 reduction: 3-PGA is reduced using ATP and NADPH from the light reactions to form G3P (glyceraldehyde 3-phosphate).

  • Regeneration of the first substrate: G3P is used to regenerate RuBP, which can then be used to fix more CO2.

Fixation of Carbon Dioxide

• Carbon dioxide (CO2) fixation is the first step of the Calvin cycle.

• In this reaction, CO2 from the atmosphere is attached to RuBP, a 5-carbon molecule.

• The enzyme for this reaction is called RuBP carboxylase (rubisco), and the result is a 6-carbon molecule that soon thereafter splits into two 3-carbon molecules.

Reduction of Carbon Dioxide

• Reduction of CO2 is the process of using NADPH and ATP to convert carbon dioxide into carbohydrates.

• NADPH provides the electrons needed for the reduction reaction, while ATP provides the energy.

• Electrons are added to carbon dioxide one at a time, and through a series of reactions, G3P is formed.

• G3P is a carbohydrate that can be used by the plant for energy or stored for later use.

Regeneration of RuBP

• The Calvin cycle is a cyclic process, which means that some of the products of the reactions are used to regenerate the starting materials.

• The final stage of the Calvin cycle uses ATP from the light reactions to regenerate RuBP, which is the molecule that starts the cycle.

• This regeneration of RuBP allows the Calvin cycle to continue and ultimately to produce glucose from carbon dioxide and water.

The Fate of G3P

• Algae and plants have enormous biochemical capabilities compared to animal cells.

• From a G3P molecule, they can make all the molecules they need.

• A plant can utilize the hydrocarbon skeleton of G3P to form fatty acids and glycerol.

• When nitrogen is added to the hydrocarbon skeleton derived from G3P, amino acids are formed.

6.4 Variations in Photosynthesis

• Evergreen trees in cold, windy climates have small, narrow leaves that look like needles. These leaves help the trees to conserve water and energy.

• Evergreen trees in warm, wet climates have large, flat leaves to catch the rays of the sun. These leaves help the trees to photosynthesize and produce food.

C3 Photosynthesis

• C3 plants are the most common type of plant.

• They are found in moderate climates with average temperatures and rainfall.

• C3 photosynthesis is the process by which C3 plants fix carbon dioxide.

• The first detectable molecule after CO2 fixation in C3 plants is a molecule with three carbon atoms.

• C3 photosynthesis occurs in the chloroplasts of mesophyll cells.

• If the weather is hot and dry, stomata close to prevent the loss of water.

• This also prevents CO2 from entering the leaf, which reduces the efficiency of photosynthesis.

• The competition between CO2 and O2 for the active site of rubisco is called photorespiration.

• Photorespiration reduces the efficiency of photosynthesis in C3 plants in hot, dry conditions.

C4 Photosynthesis

• C4 photosynthesis is a type of photosynthesis that is used by some plants in hot, dry conditions.

• C4 photosynthesis differs from C3 photosynthesis in that it has two stages:

  • In the first stage, CO2 is fixed in the mesophyll cells by a C3 molecule and a C4 molecule forms.

  • In the second stage, the C4 molecule is modified and then pumped into bundle sheath cells. Then CO2 enters the Calvin cycle.

• C4 photosynthesis is more efficient than C3 photosynthesis in hot, dry conditions because it helps to prevent photorespiration.

• Photorespiration is a wasteful process that occurs when rubisco, the enzyme that fixes CO2, accidentally binds to O2 instead.

CAM Photosynthesis

• CAM photosynthesis is a type of photosynthesis that uses partitioning in time to conserve water.

• CAM plants open their stomata at night to take in CO2, which is then stored in vacuoles in mesophyll cells.

• During the day, the stomata close to conserve water, and the CO2 is released from the vacuoles to the Calvin cycle.

• CAM photosynthesis is less efficient than other types of photosynthesis, but it allows CAM plants to live in dry environments.

Evolutionary Trends

• C4 plants are adapted to areas of high light intensities, high temperatures, and limited rainfall.

• C4 plants are more sensitive to cold than C3 plants.

• C3 plants probably do better than C4 plants below 25°C (77°F).

• CAM plants compete well with either C3 or C4 plants in extremely arid environments.

• CAM photosynthesis is widespread and has evolved in 30 families of flowering plants, including cacti, stonecrops, orchids, and bromeliads.

• CAM photosynthesis is also found among nonflowering plants, such as some ferns and cone-bearing trees.