ABA causes stomata to close by reversing the accumulation of potassium salts in guard cells and inducing the conversion of sugars into starch. 

The resulting fall in solute concentration leads to loss of water from the guard cell into the apoplast by osmosis, a fall in guard cell turgor, and closing of the stomatal pore. 

TLS Online TPP Program

#Id: 8755

#Unit 1. Molecules and their Interaction Relevant to Biology

Thermodynamic measurements indicate that native proteins are only marginally stable under physiological conditions. The free energy required to denature them is ∼0.4 kJ ∙ mol−1 per amino acid residue.

TLS Online TPP Program

#Id: 8756

#Unit 1. Molecules and their Interaction Relevant to Biology

Protein stability is the net balance of forces, which determine whether a protein will be in its native folded conformation or a denatured state.

TLS Online TPP Program

#Id: 8757

#Unit 1. Molecules and their Interaction Relevant to Biology

A fully folded 100-residue protein is only about 40 kJ ∙ mol−1 more stable than its unfolded form (for comparison, the energy required to break a typical hydrogen bond is ∼20 kJ ∙ mol−1).

TLS Online TPP Program

#Id: 8758

#Unit 1. Molecules and their Interaction Relevant to Biology

The combined hydrophobic and hydrophilic tendencies of individual amino acid residues in proteins can be expressed as hydropathies. The greater a side chain’s hydropathy, the more likely it is to occupy the interior of a protein.

TLS Online TPP Program

#Id: 8759

#Unit 1. Molecules and their Interaction Relevant to Biology

Hydrogen bonds, which are central features of protein structures, make only minor contributions to protein stability.

TLS Online TPP Program

#Id: 8760

#Unit 1. Molecules and their Interaction Relevant to Biology

This is because hydrogen-bonding groups in an unfolded protein form hydrogen bonds with water molecules. Thus the contribution of a hydrogen bond to the stability of a native protein is the small difference in hydrogen bonding free energies between the native and unfolded states (−2 to 8 kJ ∙ mol−1)