Organism live at the expense of free energy
- The maximum amount of usable energy that can be harvested from a particular reaction is the
system’s free energy change from initial to the final state
- This change in free energy (ΔG) is given by the Gibbs-Helmholtz equation at constant
temperature and pressure: ΔG= ΔH-TΔS
An energy profile of a reaction
Comparison of passive and active transport
- In passive transport, a substance diffuses spontaneously down its
concentration gradient with no need for the cell to expend energy.
- Hydrophobic molecules and very small uncharged polar molecules
diffuse directly across the membrane.
- Hydrophilic substances diffuse through transport proteins in a process
called facilitated diffusion
- In active transport, a transport protein moves substances across the
membrane “uphill” against their concentrations gradients
- Active transport requires an expenditure of energy usually supplied by
An electrogenic pump
- Proton pumps are examples of membrane proteins that store energy by generating voltage
(charge separation) across substances
- Using ATP for power, a proton pump translocates positive charge in the form of hydrogen ions.
- The voltage and H+ gradient represent a dual energy source that can be tapped by the cell to
drive other processes, such as the uptake of sugar and other nutrients. - Proton pumps are the main electrogenic pumps of plants, fungi and bacteria.
- An ATP-driven pump stores energy by concentrating a
substance (H+, in this case) on one side of the membrane.
- As the substance leaks back across the membrane through
specific proteins, it escorts other substances into the cell.
- The proton pump of the membrane is indirectly driving sucrose
accumulation by a plant cell, with the help of a protein co-
transports the two solutes.
Energy flow and chemical recycling in ecosystems
- The mitochondria of eukaryotes (including plants) use the organic products of
photosynthesis as a fuel for cellular respiration, which also consumes the oxygen
produced by photosynthesis.
- Respiration harvests the energy stored in organic molecules to generate ATP, which
powers most cellular work.
- The waste products of respiration, carbon dioxide, and water, are the very substances
that chloroplasts use as raw materials for photosynthesis
- Thus, the chemical elements essential to life are recycled. But energy is not; it flows
into an ecosystem as sunlight and leaves it as heat.
How ATP drives cellular work
- Phosphate-group transfer is the mechanism responsible for most types of cellular work
- Enzymes shift a phosphate group (P) from ATP to some other molecule and this phosphorylated
molecule undergoes a change that performs work.
- For example, ATP drives active transport by phosphorylating specialized proteins built into
- drives mechanical work by phospolyating motor proteins, such as the ones that move
organelles along cytoskeleton “tracks” in the cell;
- And drives chemical work by phosphorlyating key reactants
- The phoshorylated molecules lose the phosphate groups as work is performed leaving ADP and
inorganic phosphate as products
- Cellular respiration replenishes the ATP supply by powering the phosphorlyation of ADP. Cellular respiration and fermentation are catabolic
- Fermentation an ATP producing catabolic pathway in which both electron donors and acceptors
are organic compounds
o Can be an anaerobic process
o Results in