The Nature of Life
Chapter 2
Outline
Attributes of Living Organisms
Chemical and Physical Bases of Life
Molecules
Bonds and Ions
Acids and Bases
Energy
Carbohydrates, Lipids, Proteins
Enzymes
Nucleic Acids
Attributes of Living Organisms
Composition and Structure
Cells - Structural units of organisms.
Cytoplasm - Interior cell matrix.
Nucleus - Houses genetic material (DNA) suspended in cytoplasm.
Cell Wall - Bounds cytoplasm.
Attributes of Living Organisms
Growth
Increase in mass accompanied by an increase in volume.
Most growth results from production in new cells and includes variation in form.
Reproduction
Producing offspring.
Always resemble parents.
Attributes of Living Organisms
Response to Stimuli
Plants respond to stimuli such as light, temperature, and gravity.
Callose and callus are two substances that may accumulate at wound sites in plant cells.
Attributes of Living Organisms
Metabolism
Collective product of all biochemical reactions in an organism.
Respiration - Energy release.
Photosynthesis - Energy harnessing.
Digestion - Large insoluble food molecules converted to smaller soluble molecules.
Assimilation - Conversion of raw materials into cell substances.
Attributes of Living Organisms
Movement
Occurs at all levels of organization.
Cytoplasmic streaming.
Organization Complexity
Molecules are organized into compartments, membranes, and other structures within cells and tissues.
Environmental Adaptation
Natural selection adapts organisms to their environment.
Chemical and Physical Bases of Life
Matter
Occupies space
Has mass
Composed of elements
Atoms - Smallest stable subdivision of an element.
Nucleus - Center of an atom.
Protons - Positively charged particles.
Neutrons - Neutral particles.
Oxygen Atom Model
Chemical and Physical Bases of Life
Atomic Number - Number of protons.
Cannot change within an element.
Atomic Mass - Combined number of protons and neutrons.
Electrons - Negative electrical charges circling the nucleus.
Orbitals - Volume of space in which a given electron occurs 90% of the time.
Chemical and Physical Bases of Life
An electron’s distance from the nucleus depends on its energy level.
Electron Shell
Outermost shell determines the atom’s reactivity.
Moving to an orbital farther away consumes energy.
Moving to an orbital closer in releases energy.
Orbital Models
Chemical and Physical Bases of Life
Isotope - Form of an element that varies in its atomic weight (Varying number of neutrons).
Radioactive isotopes are unstable and spontaneously split into smaller parts.
Molecules
Molecule - Two or more atoms bound together.
Compound - Two or more elements united in a definite ratio by chemical bonds.
Molecules are in constant motion, with a temperature increase or decrease speeding up or slowing down the atoms.
Molecules
Random collisions between molecules capable of sharing electrons are the basis for all chemical reactions.
Usually controlled by specific enzymes.
When a water molecule is formed, two hydrogen atoms become attached to an oxygen atom.
Molecules
Polarity affects atom alignment.
Water molecules form a cohesive network as the slightly positive hydrogen atoms are attracted to the slightly negative oxygen atoms.
Cohesion - Attraction of similar molecules.
Capillary movement in plants.
Adhesion - Attraction of dissimilar molecules.
Water Molecules
Chemical and Physical Bases of Life
Valence
Combining capacity of an atom or an ion.
Bonds and Ions
Bonds are forces that move molecules by attracting and holding atoms.
Number of electrons in an atom’s outermost orbital determines how many chemical bonds can be formed by that particular atom.
Bonds and Ions
Covalent Bond
Forms when two atoms complete their outermost energy level by sharing a pair of electrons.
Polar - Electrons are shared equally.
Nonpolar - Electrons are shared unequally.
Bonds and Ions
Ionic Bond
Forms when electrons in outermost orbital are completely removed from one atom and transferred to another atom.
Ions - Formed when molecules lose or gain electrons.
Bonds and Ions
Hydrogen Bond
Forms as a result of attraction between positively charged hydrogen atoms in polar molecules and negatively charged atoms in other polar molecules.
Only have 7-10% strength of covalent bonds.
Acids and Bases
Acids are chemicals that release hydrogen atoms (H+) when dissolved in water.
Bases (alkaline compounds) are compounds that release negatively charged hydroxyl ions (OH-) when dissolved in water.
pH scale represents measurement of H+ ion concentration.
7 = Neutral
<7 = Acidic
>7 = Alkaline
Energy
Energy - The capacity to perform work.
Thermodynamics - Study of energy and its conversions from one form to another.
First Law - Energy is constant. It cannot be increased or diminished, only converted from one form to another.
Second Law - Energy flow is uni-directional and there will always be less energy remaining after the conversion than existed before.
Energy
Electrons with the least potential energy are located within the single spherical orbital closest to the atom’s nucleus.
Electrons with the most potential energy are in the outermost orbital.
Monomers and Polymers
Polymers - Formed when two or more small units (monomers) bond together.
Dehydration Synthesis - Removal of water in the formation of a bond.
Hydrolysis - Occurs when hydrogen becomes attached to one monomer and a hydroxyl group to the other.
Carbohydrates
Carbohydrates are the most abundant organic compounds in nature.
Monosaccharides - Simple sugars with backbones of three to seven carbon atoms. (Glucose and Fructose)
Disaccharides - Formed when two monosaccharides bond together by dehydration synthesis. (Sucrose)
Polysaccharides - Formed when several to many monosaccharides bond together. (Cellulose)
Lipids
Lipids are fatty or oily substances that are mostly insoluble in water. (Fats and Oils)
Typically store twice as much energy as carbohydrates.
Most consist of chain with 16-18 carbon atoms.
Saturated - No double bonds.
Unsaturated - At least one double bond between carbon atoms.
Lipids
Waxes - Lipids consisting of long-chain fatty acids bonded to long chain alcohol other than glycerol.
Phospholipids - Constructed like fats, but one of the fatty acids is usually replaced by a phosphate group.
Proteins, Polypeptides, and Amino Acids
Proteins regulate chemical reactions in cells, and are usually very large and consist of one or more polypeptide chains.
Polypeptides are chains of amino acids.
Each amino acid has two functional groups plus an R group.
Amino group (-NH2)
Carboxyl group (-COOH)
Proteins, Polypeptides, and Amino Acids
Polypeptide Structure
Primary Structure - A sequence of amino acids fastened together by peptide bonds.
Secondary Structure - Coiling of polypeptide chains.
Tertiary Structure - Maintained by coils between R groups.
Quaternary Structure - Occurs when a protein has more than one kind of polypeptide.
Storage Proteins
Some plant food-storage organs store small amounts of proteins in addition to large amounts of carbohydrates.
Seeds usually contain proportionately larger amounts of proteins in addition to their complement of carbohydrates.
Enzymes
Enzymes are mostly large, complex proteins that function as organic catalysts under specific conditions.
Work by lowering energy of activation.
Temporarily bonds with potentially reactive molecules at a surface site.
Nucleic Acids
Nucleic acids are very large, complex polymers.
Vital to internal communication and cell functioning.
Deoxyribonucleic Acid (DNA) composed of nucleotides.
Nitrogenous base, five-carbon sugar, and a phosphate.
Review
Attributes of Living Organisms
Chemical and Physical Bases of Life
Molecules
Bonds and Ions
Acids and Bases
Energy
Carbohydrates, Lipids, Proteins
Enzymes
Nucleic Acids
Plant Metabolism
Chapter 10
Outline
Photosynthesis
Major Steps of Photosynthesis
Light-Dependent Reactions
Light-Independent Reactions
C4 Photosynthesis
CAM Photosynthesis
Respiration
Glycolysis
Electron Transport Chain
Enzymes and Energy Transfer
Enzymes regulate most metabolic activities.
Anabolism - Storing Energy.
Photosynthesis reactions
Catabolism - Consuming Stored Energy.
Respiration reactions
Oxidation-Reduction Reactions
Oxidation - Loss of electron(s).
Reduction - Gain of electron(s)
Usually coupled
Photosynthesis
Energy for most cellular activity involves adenosine triphosphate (ATP).
Plants make ATP using light as an energy source.
Take place in cholorpolasts and other green parts of the organisms.
6CO2+12H2O + light à C6H12O6+6O2+6H2O
Carbon Dioxide
Carbon dioxide reaches cholorplasts in the mesophyll cells by diffusing through the stomata into the leaf interior.
Use of fossil fuels, deforestation, and other human activities have added excess carbon dioxide to the atmosphere.
May enhance photosynthesis.
Plants may counter-balance by developing fewer stomata.
Water
Less than 1% of all the water absorbed by plants is used in photosynthesis.
Most of the remainder is transpired or incorporated into plant materials.
If water is in short supply, stomata usually close and thus reduce the supply of carbon dioxide available for photosynthesis.
Light
About 40% of the radiant energy received on earth is in the form of visible light.
Leaves commonly absorb about 80% of the visible light reaching them.
Light intensity varies with time of day, season, altitude, latitude, and atmospheric composition.
Considerable variation in the light intensities necessary for optimal photosynthetic rates.
Light Wavelengths
Effects of
Light and Temperature
on Photosynthesis
Chlorophyll
Several different types of chlorophyll.
Most plants contain both chlorophyll a (blue-green) and chlorophyll b (yellow-green).
Other pigments include carotenoids (yellow and orange) phycobilins (blue or red), and several other types of chlorophyll.
About 250-400 pigment molecules group as a photosynthetic unit.
Major Steps of Photosynthesis
Light Dependent Reactions
Water molecules split apart.
Electrons passed along electron transport.
ATP produced.
NADPH produced.
Major Steps of Photosynthesis
Light Independent Reactions
Calvin Cycle
Carbon dioxide combined with RuBP and then combined molecules are converted to sugars (Glucose).
Energy furnished by ATP and NADPH from Light-Dependent Reactions.
Light Dependent Reactions - In Depth
Each pigment has its own distinctive pattern of light absorption.
Light Dependent Reactions - In Depth
Two types of photosynthetic units present in most chloroplasts make up photosystems.
Photosystems I and II
Both can produce ATP.
Only organisms with both photosystem I and photosystem II can produce NADPH and oxygen as a consequence of electron flow.
Photosystems
Light Independent Reactions - In Depth
Calvin Cycle
Six molecules of CO2 combine with six molecules of RuBP with the aid of rubisco.
Resulting complexes split into twelve 3PGA molecules.
NADPH and ATP supply energy and electrons that reduce 3PGA to 12 GA3P.
Ten of the twelve GA3P molecules are restructured into six RuBP molecules.
The Calvin Cycle
Photorespiration
Stomata usually close on hot, dry days.
Closed stomata prevent carbon dioxide from entering the leaf.
When carbon dioxide levels drop below about 50 parts per million, photorespiration is initiated.
Rubisco fixes oxygen instead of carbon dioxide.
Light Independent Reactions - In Depth
4-Carbon Pathway
Plants have Kranz Anatomy.
Large chloroplast with few to no grana in the bundle sheath cells surrounding the veins.
Smaller chloroplasts with well-developed grana in the mesophyll cells.
Corn (Zea Mays) Cross-Section
4-Carbon Pathway
Plants with Kranz Anatomy produce oxaloacetic acid (4-carbon compound).
Phosphoenolpyruvate (PEP) and carbon dioxide combined in mesophyll cells with the aid of PEP carboxylase.
Provides a major reduction in photorespiration.
CAM Photosynthesis
Similar to C4 photosynthesis in that 4-carbon compounds are produced during the light-independent reactions.
However, in CAM, the organic acids accumulate at night and break down during the day, releasing carbon dioxide.
Allows plants to function well under limited water supplies, as well as high light intensity.
CAM Photosynthesis
Respiration
Respiration is essentially the release of energy from glucose molecules that are broken down to individual carbon dioxide molecules.
C6H12O6 + 6O2à 6CO2 + 6H2O + energy
Respiration
FermentationC6H12O6à 2C2H5OH + 2CO2 + ATP
C6H12O6à 2C3H6O3 + ATP
Factors Affecting the Rate of Respiration
Temperature
Water
Oxygen
Major Steps of Respiration
Glycolysis
Glucose molecule becomes a fructose molecule carrying two phosphates.
Fructose molecule is split into two GA3P molecules.
Some hydrogen, energy, and water are removed, leaving pyruvic acid.
Major Steps of Respiration
Aerobic Respiration
Citric Acid (Krebs) Cycle
O.A. + acetyl CoA + ADP+P+3NAD + FAD
à
O.A. + CoA+ATP+3NADH+H+ + FADH2+2CO2
Electron Transport
Oxidative Phosphorylation
Chemiosmosis
Assimilation and Digestion
Assimilation is the process of using organic matter produced through photosynthesis to build protoplasm and cell walls.
Digestion is the conversion of starch and other insoluble carbohydrates to soluble forms.
Nearly always hydrolysis.
Review
Photosynthesis
Major Steps of Photosynthesis
Light-Dependent Reactions
Light-Independent Reactions
C4 Photosynthesis
CAM Photosynthesis
Respiration
Glycolysis
Electron Transport Chain
. Photosynthesis-Light Harvesting
Overview
Photosynthesis is process that is most responsible for life on the earth, as we know it.
Simple organisms like chemosynthetic bacteria can function in anaerobic environments.
. However, the cosmic diversity of organisms that exist in the biosphere depends on Photosynthesis for two fundamentally important processes. These are Oxygen Evolution and Carbon Fixation. We will explore the processes which produce Oxygen and Chemical Energy first. This chemical energy is used in Carbon Fixation which will be discussed later.
. Photosynthesis means Light Building or Light Using. Solar energy is absorbed and is used to create chemical energy which is ultimately stored as Carbohydrates.
This process is complex because it involves Biophysics plus Biochemistry as well as the Ultrastructure of the Chloroplast.
. We will not explore the chemical/physical factors in great detail but will concentrate on the relationship of these processes to the structure and function of Chloroplasts & Leaves.
However, it is important to review the basic underlying processes.
. Light is a form of Electromagnetic Radiation. The Sun is the most immediate source of electromagnetic radiation (EMR).
EMR has wave-like properties.
Wavelength can be measured as the distance between one wave crest and the next. It is usually measured in meters (m) or nanometers (nm). A nanometer is 10-9 m.
Electromagnetic waves have electrical and magnetic properties
.
. Absorption Spectra of Chlorophyll a & b (Algae & Plants, plus Bacteriochlorophyll: Note the differences, especially the differences between Bacteriochlorophyll and the other two
. Absorption Spectra indicate the degree to which pigments can absorb light of different wavelengths.
The degree of absorption is indicated on the y-axis and the wavelengths of EMR are shown on the x-axis.
. Some Bacteria contain Chlorophyll that absorbs UV & IR.
Chlorophyll a & b are typically found in Land Plants and Green Algae.
Chlorophyll a is also present in Cyanobacteria
Chlorophyll a is the principal photosynthetic pigment in all three. Chlorophyll b is an accessory pigment that extends the range of wavelengths that can be absorbed by chloroplasts.
. Light has Wave and Particle-like Properties
The term Photon is used to signify its particle-like properties.
The Energy of a Photon is referred to as its Quantum.
The Quantum Energy of a Photon depends on its Frequency (n) or Wavelength (l).
.
. Solar energy output or emission of EMR peaks around 1000 nm. The production of shorter wavelengths declines gradually.
Some radiation is absorbed by the atmosphere as seen by the light intercepted at the surface of the planet.
. There is more red light available for photosynthesis than blue light.at the earth's surface.
Chlorophyll a has two prominent absorption peaks.One is in the red (660 nm)
Another is in the blue region (450 nm).
. Chlorophyll has a Green color because it absorbs red & blue light.
This means that green light is not absorbed, but is reflected or passes through most photosynthetic tissues.
That is why we see it!
. When a chlorophyll molecule absorbs a Photon of light, one of its electrons makes a transition to an "Excited State".
This means that it makes an energy transition from its "Ground State" to a higher energy "Excited State".
This represents an increase in Potential Energy.
. The excited state can immediately decay to the ground state with the consequent release of the Photon, or the production of Heat.
Excited state electrons usually decay gradually and emit lower amounts of energy compared to the energy absorbed.
. When this produces light, the process is called Fluorescence.
Fluorescence always produces light of a longer wavelength (less energetic).
If blue light was absorbed it could produce red fluorescence.
The converse would not happen.
Energy can also be dissipated by Heat production.
. Energy Transfer occurs when an Excited Chlorophyll molecule transfers its energy to another Chlorophyll molecule.
Photochemistry occurs when the energy from the excited state causes chemical reactions.
. The Photosynthetic Pigments of Eukaryotic Algae and Plants are located in Chloroplasts.
Chlorophyll a is the "central molecule" in Photosynthesis!
Chlorophyll b is present in most land plants, plus Green Algae (Chlorophyta).
Other forms of Chlorophyll are found in other Algae & Bacteria.
. Many Chlorophyll molecules are required to produce one molecule of Oxygen.
Physiological studies showed that it took approximately 2,500 chlorophyll molecules to produce one Oxygen molecule with the green alga Chlorella.
.
We will see later that chlorophyll molecules and other pigments comprise Photosynthetic Units which have Antenna Pigments and Reaction Centers.
. The amount of Energy present in the Products is enormously greater than that of the Reactants.
The Energy required to drive this process uphill is Harvested from Light.
The Light Harvesting reactions occur on/in Chloroplast Membranes called Thylakoids.
. Carbon Dioxide fixation takes place in the Stroma of the Chloroplast.
Detailed physiological studies showed that the light harvesting reactions of photosynthesis have two linked Reaction Centers.
One of these is more responsive to Red light above 680 nm.
The other responds better to Blue Light.
. These are called Photosystem I (PS I) and Photosystem II (PS II). The numbers refer to the order in which they were DISCOVERED.
PS I & PS II contain special Reaction Center Chlorophyll a molecules.
These are called P 680 and P 700 based on their absorption maxima.
Electron carriers link these Reaction Centers.
. Each of the Reaction Centers is associated with Antenna Pigment molecules which increase the chance of light absorption.
All of these all work cooperatively and are precisely located within chloroplasts.
. Light is absorbed by antenna pigments and its energy is transferred to the Reaction Center Chlorophylls (P680 or P700).
This excites an electron in each.
These electrons are transferred to electron carriers. Remember that Oxidation is the loss of electrons.
PS I loses an Electron.
. An Electron from PS II replenishes the electron lost by PS I.
An Electron is taken from Water to replace the missing PS II electron.
Water is thus Oxidized.
This leads to Oxygen Evolution.
. The PS I electron is used to Reduce NADP to NADPH (NADP effectively gains an electron).
NADPH is used to Reduce Carbon Dioxide in the Stroma of the Chloroplast.
.
. Structure and Function in the Chloroplast
The Oxidation of
Water
produces a Hydrogen Ion (Proton).
Protons are transported across the Thylakoid during Electron Transport.
Protons accumulate in the Thylakoid Lumen.
. This makes one side of the membrane Acidic because it is high in Protons.
An ATP Synthase in the Thylakoid Membrane uses the Proton Gradient to produce ATP.
The immediate products of Light Harvesting are Oxygen, ATP & Reduced NADP (NADPH).
The latter two are used to Reduce Carbon Dioxide in the Stroma.
. Organization of the Pigment Antennas and Reaction Centers
Photosynthetic Units contain Antenna Pigments & Light Harvesting Complexes.These are conceptually Organized like a Funnel with a Sponge at the bottom.
. The funnel represents the antenna pigments which absorb photons and pass along their energy through a process called Resonance Transfer.
This energy is ultimately transferred to special Chlorophyll a molecules (P 680 or P 700) that are located at the Reaction Centers which are represented by the sponges.
. There is very little energy lost during this process which is similar to the resonance of sound between closely spaced tuning forks.
The spatial location of the Reaction Center pigments is extremely important. These are contained in Light Harvesting Complexes.
. The Antenna Pigments found in Land Plants and Green Algae are Chlorophyll a & b, plus various Carotenoids. Carotenoids absorb in the blue-green region of the spectrum and broaden the range of wavelengths that can be harvested for Photosynthesis. Other organisms, like red Algae have different antenna pigments which we will study later.
. Light Harvesting Complexes are Highly Structured
The Light Harvesting Complexes which contain the Reaction Centers are rather complex. They consist of "Pigment Proteins". Each of these consists of a large complex transmembrane Protein which is associated with several Chlorophyll molecules as well as Carotenoids.
. The accumulation of
Protons in the
Thylakoid Lumen creates a charge separation that has a lot of potential energy.
ATPSynthase is able to move Protons across the Thylakoid Membrane and use the potential energy to generate ATP.
It is the Compartmentalization of the Thylakoids which facilitates this!
. The process of ATP formation associated with Electron Transport is called "Chemiosmosis". This means that cells can use ion concentration differences and differences in electric potential across membranes to make biochemical energy. This is an important biophysical/chemical concept, and it is a good example of structure/function, albeit at a submicroscopic level.
. The process that produces ATP by in this way is is called Photophosporylation.
Structure and function are intimately linked at all levels of biology. Many techniques combine to show us how the biosphere is organized right down to the molecular level.
. Electron Harvesting is very efficient. However, if the flow of absorbed energy stops, the system quickly begins to overheat and autodestruct!
Highly reactive, destructive molecules of singlet Oxygen & Superoxide can be generated. These damage the photosynthetic machinery.
. Carotenoids are found in all photosynthetic organisms. They are associated with antenna and reaction centers as well as photosynthetic membranes where the function as Accessory Pigments in light harvesting. They also have a critical role in Photoprotection.
. Quenching occurs when energy stored in chlorophyll excited states is dissipated. If this does not happen, the energy can react with molecular oxygen to produce Singlet Oxygen. This extremely reactive species can damage membrane lipids.
Carotenoids can quench excited chlorophyll molecules with some energy loss in the process.
. Consequently, the energy gained by the Carotenoids is not great enough to produce Singlet Oxygen & it is released as heat.
. Photosynthesis-Carbon Fixation-
The C3 Cycle
It has been estimated that 200 Billion tons of Carbon is fixed as Carbohydrates each year. Marine Plankton accounts for 40% of this.
The biochemical steps for this occur in the Chloroplast Stroma. Consequently, these can be called Stroma Reactions.
. Melvin Calvin elucidated the basic steps in this process and they are often referred to as the Calvin Cycle.
The first product of CO2 fixation is a 3-Carbon unit. Consequently, this can be called C3 Photosynthesis.
There are three basic events which characterize this process.
They are Carboxylation, Reduction & Regeneration.
. The first step involves the addition of CO2 to a 5-Carbon acceptor [RUBP]. This leads to the production of two 3-Carbon molecules.
The next step involves the reduction of the 3-Carbon molecule into a Carbohydrate.
The final step is a
complex series of reactions, which regenerates the
5-Carbon Acceptor
(Ribulose-1-5-bisphosphate [RUBP]).
. One CO2 is gained with each cycle. Six cycles lead to the formation of a 6-Carbon sugar.
. The Enzyme that adds CO2 to RUBP is called RUBISCO (Ribulose Bisphosphate Carboxylase/Oxygenase). It is the most abundant protein on the planet. It can represent 40% of the soluble protein in a leaf.
RUBISCO has a Dual Function. It can Carboxylate RUBP (above) or it can Oxygenate it. The latter is called Photorespiration.
. Photorespiration results in the loss of CO2 which negates CO2 fixation!
Both of these reactions involve the same active site, and the substrates (CO2 & O2) directly compete with one another.
Under typical ambient conditions the ratio between CO2 fixation & Oxygenation is 3:1.
. Photorespiration reduces the efficiency of photosynthesis by as much as 50%. It is interesting to note that the dual nature of RUBISCO is universal from orchids to photosynthetic bacteria.
The Concentrations of CO2 & O2, plus the Leaf Temperature regulate the Balance between Carboxylation (Carbon Gain) & Oxygenation (Carbon Loss).
.Carbon Dioxide concentrations are always much lower than Oxygen levels inside leaves and in the atmosphere.
Leaf Temperature rises during the day when photosynthesis occurs. This is especially true under dry, sunny conditions. This favors Oxygenation (Photorespiration).
. The kinetic properties of RUBISCO also favor Oxygenation at high temperatures.
Elevated
Temperatures favor Photorespiration
compared to CO2 Fixation by Photosynthesis!
One way to favor Carboxylation over Oxygenation would be the Elevation of CO2 levels in the vicinity of RUBISCO!
We will see two ways in which plants achieve this later!
. Algae & Cyanobacteria
CO2 Concentration in Cyanobacteria and Algae occurs via CO2 Pumps which are located in the Plasmalemma.
These proteins are produced at low CO2 levels, and they use ATP from the light harvesting reactions to concentrate CO2 which then enters the Calvin (C3) Cycle.
. The C4 Cycle
Evapo/Transpiration is the principal way in which leaves cool themselves.
The latent heat of evaporation produces a significant cooling effect.
This occurs best when the stomata are open.
However, leaf water loss leads to stomatal closure.
Since water loss is greatest when leaf temperatures are the highest, the stomata are typically closed under these circumstances. This greatly limits evaporative cooling!
Closed stomata also prevent CO2 uptake from the atmosphere.
All of these factors favor Photorespiration &
Diminish Photosynthesis!
Some plants have Carboxylating enzymes that have a higher affinity for CO2, compared to RUBISCO, especially at low CO2 concentrations & high temperatures.
. These plants use 3-Carbon acceptors like Phosphoenolpyruvate (PEP) rather than the 5-Carbon acceptor RUBP.
Carboxylation produces a 4-Carbon Acid like Malic Acid (Malate).
Consequently, this is called C4 photosynthesis.
. C4 Plants have a distinctive Leaf Anatomy compared to C3 Plants.
. C4 Leaf Anatomy
The Vascular Bundles of C4 leaves have large Photosynthetic Bundle Sheath Cells.One to three layers of Photosynthetic Mesophyll cells that surround the Bundle Sheath and radiate away from the Bundle Sheath.
. These resemble a Wreath and this has been called Kranz (wreath) Anatomy.
This is very distinctive and occurs in Monocots like Corn & Sugarcane plus some Dicots.
. The picture on the right is from a grass in the genus Poa. It also has Kranz Anatomy & C4 Photosynthesis Note the Chloroplasts in the Bundle Sheath Cells. The Stomata are precisely arranged to provide optimal diffusion paths for the Photosynthetic Mesophyll Cells. This allows for extremely efficient gas exchange with the atmosphere and the Mesophyll when the stomata are open.
. CO2 crosses the Cell Wall and Plasmalemma of a Mesophyll Cell. It is fixed by PEP Carboxylase to form a 4-Carbon Acid like Malate. This occurs in the Cytoplasm. The 4-Carbon Acid is transported via Plasmodesmata to a Bundle Sheath Cell.
. It loses one CO2 (Decarboxylation) and enters a Bundle Sheath Chloroplast where CO2 is fixed by RUBISCO using the C3 Calvin Cycle.
PEP is transported into the Mesophyll Cell where it can accept another CO2.
. This C4 process is more efficient than C3 photosynthesis because
PEP Carboxylase has a much higher affinity for CO2 than RUBISCO.
PEP Carboxylase does NOT have Oxygenase activity.
The CO2 concentration in the Bundle Sheath Chloroplasts greatly favors Carboxylation by RUBISCO & virtually eliminates Photorespiration.
. The Energy for this process comes from the Chloroplasts in the Mesophyll Cells which produce lots of ATP and NADPH.
C4 Carbon Fixation requires more energy than C3 photosynthesis. However, the increased efficiency of CO2 fixation far outweighs the increased energy requirements.
. C4 plants photosynthesize better than C3 plants under dry, hot conditions.
The greater affinity of PEP Carboxylase for its substrate means that the enzyme is saturated at low ambient CO2 levels.
. Consequently, stomata may be closed for longer periods of time with C4 plants. This obviously helps to conserve water.
C4 plants are more abundant in hot arid climates, as might be expected.
. C4 photosynthesis is not widely distributed from a taxonomic perspective. Certain families like the Gramineae (Grasses like Sugarcane & Corn), Cyperaceae (Sedges) and Chenopodiaceae (Atriplex) have many C4 species. Approximately 16 families are known to have C4 photosynthesis. This includes some Monocots and Dicots.
. CAM Photosynthesis
Crassulacean Acid Metabolism (CAM) is another ecologically significant way in which plants concentrate CO2.
This does not involve the sophisticated structural specialization seen with C4 plants.
Stomatal opening is regulated temporally so that they may Open at Night when water demand is low, thus avoiding water loss.
. CAM plants can show an 80% reduction in water loss compared to C3 plants under the same conditions. This has obvious adaptive and ecological significance!
CAM plants open their stomata at night and fix CO2 via PEP Carboxylase (a la C4 photosynthesis).
They Store Malate (4 Carbon Acid) in their Vacuoles.
. Malate is transported to Chloroplasts during the Day where Decarboxylation occurs.
The released CO2 is Fixed by RUBISCO as in C3 plants.
CAM Mesophyll Cells typically have extremely Large Storage Vacuoles and CAM Plants are Often Succulent in appearance.
. Since the Stomata are Closed during the Day, CO2 & Water can't escape.
Virtually all of the CO2 is fixed and Photorespiration is negligible.
. Stomata of CAM plants are closed during the day.
The Stomata open at night and atmospheric CO2 enters the leaf.
It is fixed by PEP Carboxylase and converted to Malate which accumulates in the Vacuole.
Malate is transported from the Vacuole and Decarboxylated.
The released CO2 can't escape because the stomata are closed.
Virtually all of the CO2 is fixed by the Calvin (C3) Cycle.
. CAM photosynthesis is common in certain families like the Crassulaceae (Crassula, Kalanchoe, Sedum), Aizoaceae (Lithops) Cactaceae and Euphorbiaceae. However, other families have CAM species. Some familiar examples of CAM plants are Pineapple (Bromeliaceae), Agave (Agavaceae) and Orchids (Orchidaceae).
. These plants are well adapted to cope with hot dry environments. CAM photosynthesis is one of their principal adaptations.
. Photosynthesis More Ecological Considerations
Leaves absorb approximately 85% of Photosynthetically Active Radiation (PAR).The Remainder is either Reflected or Transmitted. This is enriched in Green light. It is also enriched in near Far-Red light (700-750 nm).
Although this region of the spectrum is inactive in photosynthesis, it has profound effects on Plant Growth and Development! This involves the Phytochrome system and has Enormous Ecological significance which ranges from seed germination to overall growth form.
. Leaves absorb most of the light in the blue and red regions of the spectrum. Green light is reflected or transmitted, as is far-red light.
. Leaf Anatomy favors Light Absorption.
Epidermal cell walls can act like Lenses that Focus incoming Light on the photosynthetic Mesophyll cells below.
This has an amplifying effect on the amount of light reaching these cells.
. Kranz Anatomy & C-4 Photosynthesis
Present-day CO2 levels (360 ppm) are relatively low considering the history of the planet. However, it has been rising during the past 200 years. It is increasing at a rate of 1 ppm/year. This increase in CO2 levels roughly corresponds with the industrial revolution.
. It is obvious that the use and abuse of fossil fuels has lead to these elevated CO2 concentrations in the atmosphere. Prior to this CO2 levels were around 200 ppm. It has been estimated that our CO2 level will reach 600 ppm by 2020. The Greenhouse effect is a Reality!!! It will have profound environmental effects, especially for Homo stupidus!
. Heat released as long-wavelength radiation is absorbed by CO2 in the atmosphere where it accumulates as HEAT! The more CO2, the hotter it gets. You already can guess what happens next!
Unlike light, CO2 is not present at saturating levels in nature! Its ambient concentration is approximately 0.036% or 360 ppm.
. The concentration of Oxygen is 20% or 200,000 ppm.
Water Vapor is approximately 2% or 20,000 ppm!
CO2 concentration is the limiting factor for many plants to reach maximal photosynthetic levels.
Raising CO2 levels can increase the photosynthetic rate of C-3 plants.
. However, since C-4 plants concentrate CO2 inside the leaf cells, elevated CO2 levels will have much less effect them.
. C-3 Photosynthesis: Most plants have C-3 Photosynthesis in which CO2 is fixed by RUBISCO (Ribulose Bisphosphate Carboxylase/Oxygenase) which combines it with a 5-Carbon sugar Ribulose-1,5-bisphosphate. This produces two 3-Carbon molecules. Hence, comes the designation of C-3 Photosynthesis.
. RUBISCO is adversely affected by Oxygen. Ambient O2 levels result in a significant reduction in fixed CO2 through a competing process called Photorespiration. Photorespiration is favored at high temperatures when evapotranspiration would be greatest!
C-3 Leaves have no special anatomy. The Mesophyll may be uniform or have Palisade-Spongy organization. The Chloroplasts of C3 Plants have well developed Grana.
. C-3 Grass: There is a Bundle Sheath but it dooes not have the specialized Chloroplasts and the Photosynthetic Mesophyll is not tightly clustered around the Vascular Bundles. This has a Uniform Mesophyll
. C-4 Photosynthesis: Some plants use PEP Carboxylase to Fix CO2. PEP is a 3-Carbon molecule called Phosphoenol Pyruvate. PEP Carboxylase is NOT adversely affected by O2 and it has a higher afinity for CO2 compared to RUBISCO. A four carbon molecule called Oxaloacetate is produced by PEP Carboxylase (PEP + CO2 = Oxaloacetate).
. Hence, comes the designation of C-4 Photosynthesis.
Oxaloacetate is produced in Photosynthetic Mesophyll Cells and moves through the Symplast to Enlarged, Specialized Bundle Sheath Cells where CO2 is released and is fixed by RUBISCO into 3 Carbon Molecules. CO2 is effectively concentrated in the Bundle Sheath Cells.
. This favors CO2 fixation by RUBISCO.
Furthermore, the Bundle Sheath Chloroplasts lack Photosystem II which produces O2. Consequently, this is a low O2 environment in which RUBISCO operates very efficiently.
. C4 Leaves have Kranz Anatomy! The veins have Large Bundle Sheath Cells that contain specialized Chloroplasts. These have Thylakoids but lack Grana. PS II, the O2-releasing step in Photosynthesis, is assocaited with the Grana. Their absence signifies tha absence of PS II in these chloroplasts. CO2 fixation by RUBISCO occurs in the Stroma.
. Photosynthetic Mesophyll with normal Chloroplasts appear to radiate from the Bundle Sheath Cells and give the overall appearance of a wreath. These chloroplasts have PS I & II. They also contain PEP Carboxylase.
. Kranz Anatomy and C-4 Photosynthesis occur in both Dicots and Monocots. I will provide a few examples below.
C-4 Photosynthesis is considered a Xeromorphic adaptation because the stomata can be open for less time (compared to a C-3 plant) due to the efficiency of CO2 fixation. This conserves water!
.sugar cane C4
. (BS = Bundle Sheath)
note the enlarged Bundle Sheath Cells, surrounded with Photosynthetic Mesophyll
. CAM Photosynthesis
CAM (Crassulacean Acid Metabolism) plants concentrate CO2 by fixing it and storing it as a C-4 acid (Malate) at night.The captured CO2 is released by decarboxylation during the day when energy from light harvesting can be used to fix it again in the C-3 cycle.
. The Stomata remain closed during the day so that none of the CO2 is lost.
This also prevents most water loss!
The stomata open at night when temperatures are cooler and there is far less dehydration. This allows CO2 to enter with a minimal amount of water stress.
CAM plants are often Succulent.
. However, many
succulent plants are C-3 plants.
There is no distinctive anatomy associated with CAM plants.
. Their stomata are completely closed and they recycle all of their CO2 and water until suitable growth conditions occur. This is called "CAM Idling". This same process doubtlessly occurs on intact CAM plants grown under severe drought.
The graphs on the right show several responses by an Opuntia (Prickly Pear) cactus during a 24-hr. period. Most of he CO2 is fixed at night.
. Most water loss occurs at night with negligible loss during the day.
Passage of gas through
the Stoma occurs at night because that is the only time when they are open!
