Energy, ATP, and Enzymes

Energy, ATP, and Enzymes
Energy - the ability to do work, that is, to move matter against opposing forces such as gravity and friction
kinetic energy - the energy of motion.
potential energy - stored energy, the capacity to do work
Thermodynamics - the study of energy transformation The First Law of Thermodynamics - Energy can be transferred and transformed, but it can neither be created nor destroyed
The total energy of the universe is constant
Mass is a form of energy (this is only important when considering atomic reactions, so we won't dwell on it here...)
The Second Law of Thermodynamics - Every energy transfer or transformation increases the entropy of the universe
There is a trend toward randomness
Energy must be spent to retain order - this spending of energy usually releases heat, which increases the entropy elsewhere
Free Energy - the portion of a system's energy the can perform work It is called "free" energy because this is the energy which can perform work, not because there is no energy cost to the system
There still ain't no free lunch
Exergonic Reaction - a process with a net release of free energy Sometimes called spontaneous, but that doesn't mean that it will occur rapidly
Burning paper is exergonic, but paper just doesn't ignite when it is exposed to air - it requires an initial input of energy to start the reaction

First law of Thermodynamics
To understand the energy transformation we need to know the Equilibrium thermodynamics
This will provide information on energy level of a system in initial and final state.

If the final state has a lower energy level than the inItial state, then the process is energetically feasible and can be spontaneous.
That is more energy is given off than it is absorbed

The measure of the energy associated with a particular state is called Enthalpy (H): This can be summarized by the following equation

H = E + PV
Where
E= Internal Energy
P = Pressure
V = Volume ( PV is called pressure volume product

Internal energy (E) include
Velocity or translation kinetic motions of the particles,
electron energies and also absorbed radiant energy.
It is impossible to quantify the absolute internal energy of a substance and therefore it is also impossible to quantify the absolute enthalpy.

The differences in enthalpy between two states (ΔH) can be quantified by measuring the heat released or gained by the systems as it moves between the two states.

Exothermic – Process of releasing heat ,
Exothermic reaction occurs when they move from a higher enthalpy to a lower enthalpy; they have a negative change in enthalpy (-ΔH).
Endothermic-Processes that gain heat from the surroundings- These reactions occurs when the system moves from a lower to a higher enthalpy, so they have a positive enthalpy change (+ΔH)

Since Enthalpy is a measure of energy , processes that have a (–ΔH) should be spontaneous because they move from a higher energy state to a lower energy state ( Extra energy is release as heat).

Although some spontaneous have (+ΔH) moving from lower enthalpy to a higher enthalpy , they remove heat from the surroundings.

Second law of Thermodynamics
The measurement of randomness is called entropy (S)
The Second Law says that the entropy or randomness of the universe is always increasing.
If the process results in an increase in entropy in the system or its surroundings it say to have a positive entropy change (+ΔS)
However there are some spontaneous processes that have an unfavorable entropy change (-ΔS) for the system, in which case ΔS for the surroundings is then positive.

The two system properties we have defined- enthalpy and entropy determine the overall energy change as a system moves from one state to another.

Processess with a -ΔH and a +ΔS will be spontaneous, and processes with a +ΔH and a –ΔS will never be spontaneous.

The Gibbs Free Energy (G)
The Gibbs free energy (G) ;

it is a measure of the maximum energy available within the system for conversion to work( at constant temperature and pressure.

Gibbs energy is derived by combining the enthalpy (H) and the entropy (S), along with the kelvin temperature (T) in the following equation
G= H- TS

G = E+ PV- TS

ΔG = ΔH – TΔS

ΔG = Spontaneous process

+ΔG = Non spontaneous process (Not energetically feasible).

Chemical Potential
The free energy change upon addition of a unit quantity , specifically a gram molecule weight, of substance i is called the chemical potential (μi).
e.g. A solute (dissolved material) in a solvent (the liquid in which the solute is dissolved; in plants, mostly water), the chemical potential is approximately proportional to the concentration of solute.
Actually, concentration is usually corrected by some factor that depends upon concentration and other parameters to produce a corrected concentration called the activity (ai). Chemical potential of a substance can be calculated with the following relationships:
μi – RTln ai
Where : μi= chemical potential of substance i
R= gas constant (8.314Jmol-1 K-1)
T = temperature in kelvins and ai= activity of substances i


Water Potential ψ
The water potential (ψ) is the chemical potential of water in a system or part of a system , expressed in units of pressure and compared with the chemical potential (also pressure units) of pure water at atmospheric pressure and at the same temperature and height, with the chemical potential of the reference water being set at zero.
This can be summarized in the following eqn:
Ψ= (μw –μw*)/Vw
Where
Ψ= Water potential
μw= chemical potential of water in the system under consideration
μw* = Chemical potential of pure water at atmospheric pressure and at the same temperature as the system under consideration Vw = Partial molar volume of water (18cm mol-1)



The solute diffuse in response to differences in solute chemical potential , so water diffuses in response to differences in water potential.

When water potential is higher in one part (region) of a system than in another , and no impermeable barrier prevents the diffusion of water, it diffuses from the high region of water potential to the low region.
The process is spontaneous ; free energy is released to the surroundings and the system’s free energy decreases.
This released energy has the potential to do work, such as osmotically lifting water upward in stems in the phenomenon known as root pressure.

The maximum possible work is equivalent to the free energy released.

Equilibrium is only reached when the change in free energy (ΔG ) or the water potential difference (Δψ) is equal to zero.

At this point , entropy for the system and its surroundings will be at a maximum , but entropy change (ΔS) will equal zero
The combination of solute concentration differences and physical pressure are incorporated into water potential, abbreviated with the Greek letter psi( Ψ)
Ψ
Water will flow through a membrane from a solution of high water potential to a solution of low water potential
Water potential is measured in units of megapascals (MPa)
Pure water has a water potential of 0 MPa ( = 0 MPa)
The addition of solutes lowers water potential ( = -0.2 MPa for instance)
An increase in pressure (by lowering a piston for example) will raise water potential
These two forces combine to form the following equation:
=  p +  s
= total water potential
p = water potential due to pressure
May be positive or negative
s = water potential due solute concentration (also known as Osmotic Potential)
Always negative or zero
Movement of Water Through Cells - Two Routes, the Symplast and the Apoplast Symplastic Movement
Movement of water and solutes through the continuous connection of cytoplasm (though plasmodesmata)
No crossing of the plasma membrane (once it is in the symplast - however, if the solute was initially external to the cell, then it must have crossed one plasma membrane to enter the symplast)
Apoplastic Movement Movement of water and solutes through the cell walls and the intercellular spaces
No crossing of the plasma membrane
More rapid - less resistance to the flow of water
Absorption of Water and Minerals by Roots
Absorption is a surface area phenomenon - the more surface area there is, the more absorption there will be. Root hairs - extensions of the root epidermal cells to increase surface area
mycorrhizae - fungal associations with roots - greatly increase surface area
as much as three meters of fungal hyphae can extend from each centimeter of root
this is an ancient association - some of the oldest terrestrial plant fossils have fungal associations
As water is drawn into the root, dissolved minerals are also brought into the root
Water flows through the apoplast and the symplast on its way to the xylem
The majority of the water, however, travels through the apoplast
The Endodermis - The Root's Border Guard Water flowing through the apoplast contains many minerals that the plant needs - it may also contains toxins and substances that the plant may not want.  However, since the water is flowing through the apoplast, there is no way to prevent the passive transport of these toxins, until the water hits the endodermis.
Endodermis
Cells of the endodermis possess cell walls that are ringed by the Casparian Strip, a waxy layer (composed of suberin).
The Casparian Strip is a wax and therefore prevents the apoplastic flow of water
Water must pass through the plasma membrane and enter the symplast
The plasma membrane of the endodermal cells contain many transport proteins to actively transport some molecules in and others to pump other molecules out
Once water passes under the Casparian Strip in the endodermal cells, it is free to enter the apoplast again on its way to the xylem.
Transport of Xylem Sap
Xylem sap rises against gravity, without the help of any mechanical pump, to reach heights of more than 100m in the tallest trees.  How can this occur? Transpiration-Cohesion-Tension: A Mechanism to Pull Xylem Sap up the Plant
Stomata open up during the day to let CO2 in and inadvertently let H2O escape
There is a gradient in water potential, high water potential in the soil and very low water potential in the air
Water vapor leaves the air spaces of the plant via the stomates
This water is replaced by evaporation of the thin layer of water that clings to the mesophyll cells
Remember, water has strong adhesive and cohesive properties - as the water leaves, it is replaced by water clinging to the inside of the cell walls
This creates a tension (pulling) on the water in the xylem and gently pulls the water toward the direction of water loss
The cohesion of water is strong enough to transmit this pulling force all the way down to the roots
Adhesion of water to the cell wall also aids in resisting gravity
As we said before, the water column in the tallest trees can be 100m - the tension created by evaporation of water coupled with the cohesive and adhesive forces is enough to support this column against the forces of gravity
Root Pressure: A Mechanism to "Push" Xylem Sap Up the Plant At night, transpiration is almost nil.  However, the root cells continue to actively transport minerals into the stele (the root stele is basically everything surrounded by the endodermis - primarily the xylem and the phloem).
This active transport lowers the water potential within the stele
Water passively flows into the roots, pushing the water up against gravity
Water that reaches the leaves is often forced out, causing a beading of water upon the leaf tips known as guttation
In most plants, however, root pressure is not the primary mechanism for transporting the xylem
Tall trees generate almost no root pressure (the weight of the water pushing down on the xylem more than counteracts any generated root pressure)
The Control of Transpiration
Water is needed for photosynthesis - it is also lost as a product of obtaining carbon by this very same process.  How does the plant balance is requirement for water with its requirement for carbon in photosynthesis? Guard cells control the size of the stomatal openings and thus regulate gas and water exchange
Water loss by a plant through stomatal openings is known as transpiration
The efficiency of a plant can be measured by its transpiration-to-photosyntesis ratio
The amount of water lost per gram of CO2 assimilated into organic material created by photosynthesis
A typical ratio for a C3 plant is 600:1 - for a typical C4 plant it is more like 300:1.As long as plants can pull water from the soil as fast as it leaves from the leaves, there is no problem
When water loss exceeds water uptake, the plants will wilt as the leaves lose turgor pressure.The conditions that favor wilting are hot, sunny, and windy days
How Stomates Open and Close Each stoma is flanked by a pair of guard cells that are capable of changing shape, thereby widening or narrowing the gap between the two cells
When dicot guard cells take in water by osmosis, they become turgid and swell
Guard cells are not uniformly thick - this, along with a series of radically oriented cellulose microfibrils in the cell wall, cause the guard cell to buckle outwards.
As they swell, the gap between the guard cells widens
If the plant loses water, the guard cells become flaccid and the gap closes
The changes in turgor pressure result primarily through the reversible uptake of K+ ions Stomata open when guard cells accumulate K+ from neighboring epidermal cells
How does this change the water potential () in the guard cells?
Stomata close when K+ leaves the guard cells into the neighboring epidermal cells
The transport of K+ is probably coupled to the transport of H+ in an antiport system (see Fig 36.2)
Stomatal opening is triggered by light
Blue light receptors are present on the membranes of guard cells
Stimulation of the blue-light receptors stimulates an ATP-poweed proton pump on the plasma membrane
The pumping of H+ out of the cell creates and electrical potential which drives in cations like K+
Plants also observe a 24 hour cycle (a circadian rhythm)
If placed in total darkness, the plant will still open its stomates when it normally would if there was light


Adaptations to reduce transpiration loss in plants growing in dry conditions (xerophytes) Thick cuticles - prevent water loss from epidermal cells
Succulent (thick) leaves - store water
Loss of leaves/reduction of leaves to form spines - light is not limiting, so photosynthesis can be carried out by the shoot
What type of plant am I describing?
White leaves/spines - light colors reflect light and heat, thereby cooling the plant
Trachoma's (hairs) - create a more humid microenvironment to reduce evaporative water loss
Sunken stomates - like trichomes, a more humid microenvironment is created
CAM photosynthesis - stomates open during the night (when it is cooler) and fix CO2  into four-carbon acids
The light reaction occurs during the day, generating NADPH and ATP



References:

Lincoln Taiz and Eduardo Zeiger. (2006) Plant Physiology, 4th ed. Sinauer Associates, Inc. Sunderland
Horst Marschner (1986) Mineral Nutrition of Higher Plants, 2nd. Ed. Academic Press, San Diego,CA:

Lincoln Taiz and Eduardo Zeiger. (2006) Plant Physiology, 4th ed. Sinauer Associates, Inc. Sunderland
Horst Marschner (1986) Mineral Nutrition of Higher Plants, 2nd. Ed. Academic Press, San Diego, CA