ENERGY USE IN CELLS

Energy & Metabolism

Matter – anything that has mass and takes ups space

Energy - capacity to do work or bring about change

Matter is a form of energy [ E=mc2 ] > 4 billion kg of matter per second are converted into energy in the sun

Energy is expressed as units of work – kilojoules (kJ) or as heat energy – kilocalories (kcal)

Energy and Metabolism

Energy flows as heat energy from an object with a higher temperature to an object with lower temperature

Cells are too small to have regions with different temperatures, so biologists talk about work energy (kJ)

Organisms carry out conversions between potential energy (stored/position) and kinetic energy (motion)

Chemical energy is PE stored in chemical bonds

Thermodynamics

Thermodynamics – study of energy and its transformation◦First Law: energy cannot be created or

destroyed. It can only be transferred or converted from one form to another.

◦Mass/energy present 14 billion years ago =today’s mass/energy

◦Organisms can’t create own energy – have to capture it from the environment and transform it into a form used for biological work

Thermodynamics

Second law – When energy is converted from one form to another, some usable energy (energy available to do work) is converted to heat. Heat is the KE of randomly moving particles. This heat – the random motion of particles- cannot do work.

Amount of usable energy in the universe decreases over time

Energy is constant, but usable energy is decreasing

Thermodynamics

Less-usable energy is more diffuse and disorganized.

Entropy (S) is a measure of this disorder.Organized usable energy has low entropy.Disorganized energy (i.e. heat) has high

entropy.As a result of 2nd Law, the cell’s net energy

conversion is about 40% efficient.Organisms maintain a high degree of

organization, why don’t they “wind down?”

Organisms are open systems. They exchange energy and materials with the environment. Organization is maintained through a constant input of energy.◦Plants do photosynthesis◦Animals eat

Metabolism

Metabolism is the sum of all chemical activities in an organism.◦Anabolism – complex molecules made from

simpler substances◦Catabolism – larger molecules broken down into

smaller ones◦These are complementary processes.◦Catabolism results in overall release of energy◦Anabolism requires overall input of energy

Metabolism

In order to talk about how much usable energy is available to a cell, we talk about

Enthalpy (H) – total PE of a system, Entropy (S) – disorder and Free Energy (G) – energy available to do

work in biochemical reactionsG = H – T(S)

As entropy of a system increases, the amount of free energy decreases.

When we measure the temperature difference of a process, we are measuring the enthalpy (PE) change of the system.

If ΔT is +, then heat has been released from the system, ΔH is – (exothermic)

If Δ T is -, then heat has been absorbed by the system, ΔH is + (endothermic)

Metabolism

Exergonic reactions release energy. This means that the products have less free energy than was present in the reactants. The free energy of the system (energy available to do work) decreases and ∆G is considered to be negative. (ΔG = - )

Endergonic reactions absorb energy, the products have more free energy than the reactants, the free energy of the system increases and ∆G is positive. (ΔG = +)

Metabolism

- ∆G (exergonic) means that the reaction occurs without input of additional energy (the reaction is considered to be spontaneous).

+∆G (endergonic) means that the reaction requires input of energy to proceed (the reaction is considered to be non-spontaneous).

In organisms, endergonic reactions (reactions that are driven by energy input) are coupled with exergonic reactions (reactions that are spontaneous and release usable energy).

Free energy

Diffusion is exergonic. Diffusion goes down a concentration gradient.

There is PE in a concentration gradient. Cell uses energy to produce this concentration.

A region with a high concentration is more ordered (has lower entropy)than a region with low concentration. As particles move around randomly, they become more disordered (they now have higher entropy). As they gain higher entropy, they have less free energy. This is exergonic and the energy released can be used for cellular work.

Chemiosmosis

∆G depends on concentration of reactants and products.Any process that increases randomness (increases

entropy) can do work because free energy is increased: ∆G = ∆H – T∆S

As S (randomness) increases, ∆G becomes more negative. -∆G = exergonic

Exergonic means it is free energy- releasing. This means that energy is made available to the cell to do cellular work. Formation of ATP through chemiosmosis is an example of the use of a Hydrogen ion concentration gradient this way.