【普生】004 水的结构与性质

2021-09-21 20:34 作者:生命中国 0人读过 | 我要投稿

一、水分子的存在

水是生物体中含量最高的化合物，约占70％-90％。其中：水生比陆生多、幼年比老年多、男性比女性多。

自由水和结合水之间可相互转换。

二、水分子的结构

水分子中心原子为O，其价电子排布为2s22p4，sp3杂化，两对孤电子分别占据四面体的两个顶点，压缩O—H键，使水分子整体呈V型。

O的电负性较大，电子云在O—H键中偏向O，使水分子具有较强的极性，易于形成氢键。水分子既是氢键的供体，又是氢键的受体。

三、水的性质

注意这里是水的性质，而不是水分子的性质。

1.良好溶剂

水是良好溶剂，生物体中一切化学反应均需在水中进行，水分子亦作为底物或产物，参与许多反应过程。

2.内聚力与附着力

由于水中大量氢键的形成与断裂具有协同性，使得水具有较强的内聚力与附着力。

Eg：表面张力→某些昆虫能够在水面上行走，如水黾；毛细现象→植物将水分由根系向上运输。

3.比热容高

由于水中含有大量氢键，使得水具有较大的比热容，有利于维系气候温和与代谢稳定。

4.冰密度小

水在4℃时密度最大，而0℃水在固化时，由于大量氢键由动态转化为固态，每一个水分子周围形成4个氢键，与周围水分子形成空间四面体，从而令体积增大，密度减小。这使得地球上总有液态水的存在，冰浮在水面形成隔离层，隔离层之下总有生命存在。

Water's properties

◦ The polarity of water molecules results in hydrogen bonding- Hydrogen bonding is responsible for several of water's properties

▪ polarity is the result of one atom in a molecule being much more electronegative than another, causing unequal sharing of the electrons; this creates a partial positive charge and a positive negative charge

▪ Hydrogen bonds involve the partial positive charge of one molecule being attracted to the partial negative charge of another molecule.

▪ The atoms that are electronegative enough to form partial charges with hydrogen include Nitrogen, Oxygen, and Fluorine

◦ Four Important properties of water; cohesion/adhesion, specific heat, ice density, solvent properties

▪ Cohesion; The hydrogen bonding holds water close together.

· In plants, this cohesion allows water to move against gravity. Important for moving resources from roots to other parts of plants

· Surface tension- Cohesion also makes it difficult to break the surface of water; this is exemplified in nature in some animals that can walk on water

· Adhesion is the attraction of water to other substances; this includes water's adherence to cell walls which also contributes to water being able to move against gravity

▪ Specific Heat/Temperature Moderation

· Specific heat of a substance is the amount of heat that is needed for 1 g of a substance to change temperature by 1 degree Celsius. Measured in calories. 1000 calories is 1 Calorie/kcal.

· Water's hydrogen bonds mean it takes a lot of energy to change water's temperature; this gives water a high specific heat. Thus water moderates temperature within limits that are fit for life.

· Water also has a high heat of vaporization; amount of heat needed to concert 1 g of gas to liquid. This results in more temperature moderation.

· Temperature moderation occurs on large and small scales. Earth's climate is moderated by water, especially along coastal regions. It also occurs on individual scale. We, for example, sweat to take advantage of water's properties to cool off. This uses evaporative cooling; hottest molecules convert to gas first and take energy away from a surface, thus cooling it.

▪ Ice Density: Ice insulates bodies of water

· Water is less dense as a solid than as a liquid; this is very unusual and results in ice floating in water. (Again, this is caused by hydrogen bonding). If this were not the case, bodies of water like lakes and ponds would freeze solid. Due to this unusual property however, only a top layer of water is frozen, allowing organisms to continue living in the water.

▪ Solvent Properties

· Like dissolves like; polar dissolves polar and nonpolar dissolves nonpolar

· Water is very polar and thus it is very good at dissolving other polar molecules and charged molecules.

· Water acts as the main solvent for organisms. Blood for example, is mostly water. Cells, nutrients, and other such molecules exist within the context of this water.