Cells 1 - Lecture 1 water and the aqueous environment PDF

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Summary

Module: Cells and Genes
Topic: Cells 1
Lecture 1: Water and the aqueous environment
By Professor George Ratcliffe
The aim of this lecture is to remind you of the biological importance of the water molecule. Water is essential for life; the structure of water; physical propert...

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Cells 1 - Lecture 1: water and the aqueous environment Water is essential for life - Water activity is the partial vapor pressure of water in a substance divided by the standard state partial vapor pressure of water - Pure distilled water was water activity level of 1 - Life decreases as water activity decreases Water use efficiency - In plants WUE is measured as CO2 assimilation per unit water transpired - Some plants close their stomata during the day and storing acquired CO2 overnight as malate Structure of water - Water is a non-linear, polar molecule - Form hydrogen bonds between O and H that are 0.28nm long Hydrogen bonding networks - Form lattices where each molecule is bonded to 4 others - 2x H-0, 2x O-H Physical properties - Water has a comparatively high melting and boiling point Biological significance of properties - High boiling point ensures water remains liquid on most of the earth - High latent heat of vaporisation means energy can be dissipated with evaporation - High density of liquid water ensures ice floats and water can remain liquid underneath - High cohesiveness of water permits transport in living organisms - High dielectric constant of water ensures its role as a solvent Water as a solvent - Water stabilises ions that carry charges - Dissociates these ions in solution - Also stabilises polar molecules or neutral molecules with polar groups - Many metabolites and macromolecules are like this - Water is the universal solvent in biology and most chemistry in cells takes place in an aqueous environment Water soluble proteins - Water can be bonded loosely to proteins or tightly - Loosely bound means they form a monolayer over the protein’s surface - Tightly bound means they can enter the structure of the protein e.g. it’s active sites and form hydrogen bonds inside the molecule

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Water is a molecular lubricant and allows great conformational flexibility in proteins Activity often increases with water content

Water as a reactant and product - Condensation/dehydration - X-OH + Y-H = X-Y + H20 - Water is released as a product - Requires input of energy so generally unfavourable - Synthesis of polymers from monomers e.g. proteins from amino acids - Hydrolysis - ATP + H2O = ADP + P - Reverse of condensation - Often energetically favourable - Oxidation - 2H2O = O2 + 4H+ + 4e- Oxidation of water by photosystem II Transmembrane water transport - Water diffuses through membranes relatively quickly and easily - Therefore cells need to counteract the osmosis flow of water to maintain a constant volume - They do this by: - Adjust the composition of the internal and external compartments to ensure a zero net flux - Enclosing cells in a rigid cell wall to combat influx of water - Extrude water with a contractile vacuole periodically...