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BIO120H1 (305)
Chapter 2

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James Thomson

Chapter 2Adaptions to the Physical Environment Water and NutrientsAll C expect pg 3136 AThe world of living organism and the physical nonliving world affect each other While being distinct from purely abiotic systems living organisms must still function within the laws of physics However the fundamental difference between the living and nonliving world is that the living organisms have a purposeful existence to survive it is the nonliving world which provides the context for this survivalWater has many unique properties which allow it to sustain life they are water stays liquid over a broad range of temperatures water has a high latent heat capacity ie it can buffer temperature changes water becomes denser when it freezes ie ice floats water is much denser and more viscose than air prevents organisms from sinking to the bottom of bodies of water water is regarded as a universal solvent ie many chemical elements which are essential to organisms survival are dissolved in water The number of hydrogen ions in an ecological system generally H ions dissolved in a body of water determine its pH or acidity Strong acids as opposed to weak acids which disassociate only partially disassociate from their hydrogen components almost completely causing higher acidity pH is a 014 logarithmic scale ie every increment of one means a change of 10x ie 1 to 2 is 10x as acidic 1 to 3 is 100x as acidic where 0 is the most acidic 14 is the most basic and 70 is regarded as neutral pure water sits around here Many acquire the inorganic nutrients nitrogen calcium potassium phosphorus are important they require as ions dissolved in water in the soil surrounding their roots Plant growth is often limited by the abundance of these nutrients Soil is made of many different particles eg clay silt sandin order of diameter smallest to largest Because surface area increases as particle size decreases soils with many smaller particles hold more water than soils with many larger particles from which water drains through quickly However water available to plants may be limited in soil consisting predominantly of clay and silt as they hold water tightly Soil with a mixture of small and large particles is often ideal for plants Water potential of a given soil is the strength of attractive forces holding water in the soil it is often called the matric potential Water potential is defined as the unit of pressure MPa mega Pascal By convention pure water has a water potential of zero Water will always move from areas of high potential to low potential Soil has a negative water potential it attracts water from a solution of pure water To overcome this plants must develop a WP lower than soil Most plants can extract water from soil with WP down to 15 MPa this is defined as the wilting coefficient or point of the soil Water in the environment or in organisms contains solutes dissolved substances Water moves from areas of low solutes to high solutes by a process called osmosis The force by which an aqueous solution attracts water is called its osmotic potential Conversely solutes move from areas of high concentration to low concentration Thu
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