Chapter 8

Last Points on the Light Reactions

- ATP and NADPH are immediately transported to the stroma of the chloroplast where they are used during the Dark Reactions - oxygen is released as a by-product of the Light Reactions

Two Major Sets of Reactions Involved in Photosynthesis

- Light-dependent reactions - Calvin cycle

CAM Photosynthesis

- The photosynthetic pathway in which carbon fixation takes place at night, and the resulting carbon acids are stored until daylight when they are broken down into pyruvate and CO2. - examples of plants that undergo this: cacti and pineapples

Photosynthesis

- a set of reactions in which plants convert light energy into glucose - plants use this glucose to generate their supply of ATP. like animals, plants breakdown the glucose to generate ATP. - during periods of active photosynthesis, plants can store extra supplies of glucose as starch. this starch can be used as a energy source during periods of stress - photosynthesis occurs in all plants, algae, and in some bacteria - autotrophs and heterotrophs - Joseph Priestly and Antoine Lavoisier

Photorespiration

- a situation in which plants lose carbon dioxide without glucose being formed. this is a very inefficient process for plants. plants can die from this. - usually occurs in warm, humid environments. - releases carbon dioxide, consumes oxygen, and produces no chemical energy or food - some plants have evolved techniques to avoid problems associated with photorespiration

Absorption Spectrum for Visible Light

- all other colors of light that we see are mixtures of the above colors - violet light has the shortest wavelength, meaning they carry the most energy - red light has the longest wavelength, meaning they carry the least amount of energy - plants are highly adapted for capturing violet and blue light. this is because it falls in the visible spectrum and is optimum for plant growth - Most plants contain a special colored chemical or pigment called chlorophyll that is used in photosynthesis. Chlorophyll is what absorbs the sun's energy and turns it into chemical energy. Not all the light energy from the sun is absorbed. Sunlight has many different colors in it.

Types of Plant Pigments

- chlorophyll - carotenoids - phycobilin

C-4 Plants

- contain special Bundle Sheath Cells which are impermeable to carbon dioxide loss. plants store carbon dioxide in these until it can be used - examples: corn, sorghum, sugarcane, millet, and switchgrass

Major Parts of a Chloroplast

- double-layered membrane - thylakoids - grana - photosystems - stroma

Chlorophyll

- green pigment found in the chloroplasts of all plants. this is the primary photosynthetic pigment. - this appears green to us because it does not absorb the green wavelengths of white light. - it is very highly adapted to capturing blue and violet light

Carotenoids

- in many plants. these pigments capture photons and pass the energy on to chlorophyll so that photosynthesis can occur - these range in color from yellow to orange to red

Overview of Photosynthesis

- in photosynthesis, plants capture the energy associated with sunlight. this energy is then used to covert atmospheric carbon dioxide into glucose. oxygen is released as a by-product of plant photosynthesis. animals depend on this oxygen for their metabolism. - light - chloroplasts -photosynthesis occurs through two major sets of reactions

Light Reactions (Light-Dependent Reactions)

- in these reactions, plants capture photons which are used to produce ATP, NADPH- a coenzyme, and oxygen - both ATP and NADPH are used in the Dark Reactions of photosynthesis - pigment molecules in the photosystem are responsible for capturing photons during these reactions - oxygen - last points on the light reactions

The Dark Reactions (Calvin Cycle)

- in these reactions, plants use atmospheric carbon dioxide to produce glucose. this glucose is used for energy and to make other substances like cellulose and starch. - the Dark Reactions are often referred to as Carbon-Fixation, meaning the process wherein photosynthetic organisms (such as plants) turn inorganic carbon into organic compounds (carbohydrates). - the carbon dioxide that is used in the Dark Reactions enters the leaves of plants through small openings known as stomates - the Dark Reactions occur in the stroma of chloroplasts - these reactions do not capture photons. however, the Dark Reactions are dependent on ATP and NADPH from the Light Reactions to occur. the Dark Reactions can occur as long as ATP and NADPH are available.

Factors That Influence the Productivity of Photosynthesis

- light intensity - water availability - photorespiration

Autotrophs

- organisms that are capable of converting light or chemical energy into sugars which are then broken down to generate ATP. - examples: plants, algae, bacteria, etc.

Heterotrophs

- organisms that must ingest food materials to obtain their sugars - these sugars are used to metabolize to produce energy - examples: dogs, birds, fish, humans, etc.

Photons

- packet of light energy - the amount of energy in light depends on the wavelength for that particular type of light. in general, light with shorter wavelengths has more energy. - in photosynthesis, plants capture photons and use this energy to produce their supply of glucose - plants are actually able to absorb many types of light - light that is not absorbed by the plant can be reflected to other nearby leaves or transmitted to leaves below.

Light

- serves as the major energy source for photosynthesis - light occurs in many forms, such as, radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays - light travels in waves - photons - absorption spectrum - plant pigments

Phosphorylation

- the metabolic process of introducing a phosphate group into an organic molecule. - this series of events is known as Noncyclic Photophosphorylation

Absorption Spectrum

- the set of wavelengths of light that a molecule absorbs. this can tell us how much energy the particular molecule can absorb from light. - plants have the ability to absorb photons from many different types of light. as a matter of fact, plants capture much of their energy from visible light. - absorption spectrum for visible light

Photolysis

- the splitting of water molecules into hydrogen ions and oxygen atoms - the hydrogen ions produced by this process are used to replace the electrons lost at PS II (at the beginning of the Light Reactions) - the oxygen atoms produced by this process bond together to produce oxygen gas (O2). This gas is released by the plant into the atmosphere.

Steps/Events in the Dark Reactions

1. 6 molecules of carbon dioxide combine with Ribulose bisphosphate (a 5 carbon sugar) to produce 3-phosphoglycerate (3PG). this reaction is catalyzed by the enzyme Rubisco. 2. PGA undergoes a series of reactions to form Glyceraldehyde-3-phosphate (G3P). ATP and NADPH from the Light Reactions participate in these reactions. 3. the majority of G3P is used to resynthesize Ribulose bisphosphate. a small amount of G3P is used to generate glucose for the plant. 4. extra supplies of glucose generated during the Dark Reactions is often stored by plants as starch.

Phycobilins

1. Phycocyanin 2. Phycoerythrin

Steps/Events in the Light Reactions

1. Sunlight, with its photons, hits PS II in chloroplasts 2. The photons strike electrons in the chlorophyll of PS II. the photons are strong enough to remove electrons from the chlorophyll at PS II. once removed from PS II, the electrons contain a small amount of energy. 3. Next, these free electrons are picked up by an electron acceptor (a protein). the electron acceptor passes the electron, along with its energy, to PS I. this involves the use of a series of electron carriers. 4. As the electrons are being passed from one carrier to another on their way to PS I, they lose some of their energy. this energy is used by the plant to generate ATP via Phosphorylation. 5. While this is occurring, sunlight is also hitting PS I. light energy removed electrons from PS I. the energy associated with these electrons is used to produce the coenzyme NADPH. this is known as Cyclic Photophosphorylation. 6. Photolysis

Equation for Photosynthesis

6CO2 + 6H2O ------> C6H12O6 + 6O2

Electron Acceptor

A reactant that gains an electron and is reduced in a reduction-oxidation reaction.

Electron Carrier

Any of various molecules that are capable of accepting one or two electrons from one molecule and donating them to another in the process of electron transport.

Joseph Priestly and Antoine Lavoisier

Discovered events in photosynthesis

Ribulose Biphosphate

Molecule that reacts with CO2 in the Calvin cycle, to produce G3P

Cyclic Photophosphorylation

The generation of ATP by cyclic electron flow.

Light Intensity

The level of light received on a plant surface.

Rubisco

The most abundant protein on earth. Performs Carbon Fixation in the Calvin Cycle.

Noncyclic Photophosphorylation

The set of light-dependent reactions of the two plant photosystems, in which excited electrons are shuttled between the two photosystems producing a proton gradient that is used for the chemiosmotic synthesis of ATP. The electrons are used to reduce NADP to NADPH. Lost electrons are replaced by the oxidation of water producing O2.

Carbon Dioxide

a colorless, odorless gas produced by burning carbon and organic compounds and by respiration. It is naturally present in air (about 0.03 percent) and is absorbed by plants in photosynthesis.

Phycocyanin

blue pigment

Photosystems

collections of chlorophyll found on the thylakoids. there are two types of photosystems (labeled as PS I and PS II). these capture different wavelengths of light

Thylakoids

flat discs in chloroplasts. these are covered by chlorophyll.

Oxygen

is produced as a by-product of the Light Reactions. this oxygen is important because it is the source of oxygen in the Earth's atmosphere

Double-Layered Membrane

nuclear envelope

Beta-Carotene

orange pigment, common carotenoid found in many plants

Phycoerythrin

red pigment in red algae

Grana

stacks of thylakoids

Phosphoglycerate

the 6-carbon substance found within the calvin cycle is called this

Water Availability

the amount of water available for growth

Stroma

the liquid portion of a chloroplasts

Chloroplasts

the photosynthetic organelles found in all plant cells