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Biological systems are highly ordered, and yet,

Disorder reigns!

Entropy rules!

Everything’s falling

apart!

The Second Law of Thermodynamics•The total disorder (entropy) always increases.

Examples of entropy increasing:•Solutes diffuse from areas of higher to lower concentration.•Heat flows from a warmer body to a cooler one.•Complex structures degrade into simpler parts.•Batteries lose their charge over time

“castles made of sand, fall into the sea, eventually”Jimi Hendrix

•A process can happen spontaneously, (without the input of energy), if it increases the entropy of the system and its surroundings.

Things change spontaneously so as to reduce the capacityfor further change.

•Temperature gradients tend to dissipate

•Complex structures tend to fall apart into simpler pieces

•Solute concentration tends to become equal everywhere

EquilibriumA system is at equilibrium when it has no capacity for further spontaneous change.

Diffusion - a spontaneous process leading to the net movement of a substance from a region of higher to lower concentration.

Diffusion results from the random thermal motions of molecules

Diffusion is involved in many plant processes:• gas exchange by leaves• nutrient movement to root surface• the leaking of solutes out of the vacuole• osmosis, the movement of water from regions of

higher to lower water concentrations

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Figure 3.7

So, if disorder is the natural tendency, how do biological systems maintain such highly ordered states?

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They use energy to do the work of maintaining order!

Energy is required to counter the universal tendency for disorder (entropy) to increase.

•to maintain solute concentration gradients

•to maintain thermal or electrical gradients

•to rebuild complex structures and molecules

What is energy?1) The capacity to do work

2) That which is required to displace an object againsta force - mechanical, electrical, osmotic, chemical potential

Work and Energy have the same units: calories or Joules

Some examples of the work that plants do (the ways they use energy).•Growing roots through soil - mechanical work•Raising water against gravity - mechanical work•Moving charged solutes against membrane electrical gradients•Concentrating solutes in a compartment - osmotic work•Synthesizing complex molecules - chemical work

Units of energy are joules, J

How much is 1 Joule? •about 1/4 calorie•enough energy to heat 1/4 gram of water 10C

Plants convert sunlight energy into chemical energy (ATP), then use that energy to do work.

The different compounds made by plants and animals have different energy contents.

proteins and carbohydrates ≈ 17 kJ/gfats ≈ 37 kJ/g

Why do great white sharks (usually) spit out surfers but eat seals?

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Yum, energy dense blubber!

Yuck, another skinny human

More on units

We’ll use the International System of Scientific Units

Distance in meters not feet or inches!Mass in kilograms not pounds or ounces!Time in seconds

So area is m2, volume is m3, velocity is m s-1

Temperatures will be in either Celsius (0C) or Kelvin (absolute temperature scale, 0K = 0C + 273)

Water freezes at 32oF, 00C, or 273 0K.

For quantity we’ll use moles. A mole is 6.02 x 1023 of anything.

For concentration we’ll use moles/volume (liters m3, etc.).

Molarity, M, is moles per liter

Exponents are your friends!

“milli”, m 10-3 millimeter, 1/1000 m“micro”, µ 10-6 microgram, a millionth gram“kilo”, k 103 kiloliter, 1000 liters“mega”, M 106 megabyte, a million bytes

Some important derived units

Variable name fundamental units also expressed asforce Newton, N 1 kg m s-2

energy joule, J kg m2 s-2 force times distance(work) N . m

pressure pascal, Pa kg m-1 s-2 force per area N m-2

power watt, W kg m2 s-3 energy per time J s-1

Example applicationsEnergy •How much energy does a plant expend in pumping nutrient ions from the soil solution into root cells?•What’s a better strategy for dealing with herbivores - making chemical defenses or suffering the losses and regrowing leaves? •How much energy is in a photon of light? Does it matter whatcolor the light is?

Power •What is the power (energy/time) of light striking a leaf in full sun? •How does this compare to the rate of energy production as carbohydrates are made in the chloroplast?

Pressure •What positive pressure (turgor) is required to expand a plant cell? •At what pressure does a leaf wilt?•What negative pressures (tension) exist in the xylem cells of a transpiring plant? •At what tension do embolisms form?

The gas constant, R

Remember PV = nRT?

R is the constant that makes the relationship amongP, V, n, and T work.

Values and units for R8.314 J mol-1 K-1

8.314 m3 Pa mol-1 K-1

We’ll use R a lot!