Water and life

BASIC CONCEPTS:

Water makes up approximately 60% of the human body. Its molecular structure allows it to act as a solvent for many of the other key molecules which enable cells to function and life to be maintained. An understanding of the distribution of water in the body, the composition of the various fluid compartments and the control of the movement of water between compartments is crucial to understanding many basic life processes.

1.1 The properties of water

Water is essential for life. The cells of living organisms are composed of around 70% water and many of the reactions essential to life occur in an aqueous environment. The chemical properties of water make it a particularly suitable medium for supporting life. Water is a polar molecule, which is to say it has an uneven distribution of charge (Fig. 1.1).

This means that it is able to interact with other polar and charged groups. Molecules or groups that interact with water are described as hydrophilic, whereas non-polar groups are described as hydrophobic.

Virtually all the molecules of life are based around the element carbon. These include:

• sugars and polysaccharides

• amino acids and proteins

• nucleotides and nucleic acids

• lipids

Polysaccharides, proteins and nucleic acids are very large molecules, termed macromolecules, and are polymers of sugars, amino acids and nucleotides respectively. Biological macromolecules contain both hydrophilic groups (such as OH, NH2 and COOH) and hydrophobic groups (for example hydrocarbons) and the relative amounts of these influence solubility (for further information see Section 3.7 in Catch Up Chemistry).

Interactions with water play an important part in determining the structure of these biological molecules. Generally speaking, hydrophilic groups tend to be exposed on the surface of a molecule or structure from where they are able to interact with water molecules. In contrast, hydrophobic groups tend to orientate themselves towards the inside of the molecule or structure where they interact with each other forming hydrophobic bonds. Interactions between hydrophobic chains of fatty acids allow the formation of cell membranes (see Chapter 4). Other molecules that are associated with membranes, such as proteins, often have hydrophobic regions which are inserted into the membrane to form an anchor.

Water is also very important as a medium of transport and forms the basis of blood. Gases dissolve in water, and this is important in allowing oxygen to be taken to cells and carbon dioxide to be removed.

1.2 Water in the human body

Approximately 60% of the weight of the human body is water – thus a 60 kg person will contain approximately 36 litres of water. Within the body the water is distributed between three main compartments. The bulk of body water (65%) is contained in the cytoplasm of cells and is known as intracellular fluid. Most of the remaining extracellular fluid is divided into the interstitial fluid (25%) which bathes the cells and the plasma (7.5%) which is contained within the blood vessels of the circulatory system. The remaining 2.5% of fluid is known as transcellular fluid and includes, for example, the water in the bladder and the contents of the gastrointestinal tract.

Intracellular fluid is separated from interstitial fluid by the plasma membrane of the cell (see Chapter 6). The ionic composition of these two compartments is dramatically different. The extracellular fluid has a similar composition to seawater and contains approximately 140 mmol Na+ and 110 mmol Cl-. Extracellular fluid also contains significant levels of bicarbonate ions. By contrast, intracellular fluid contains high levels of K+ (approximately 160 mmol compared with 4 mmol in extracellular fluid) and low levels of Na+ (10 mmol). The intracellular negative charge is provided not by Cl- but by proteins, bicarbonate and phosphate ions.

The concentration gradients of Na+ and K+ across cell membranes form the basis of many physiological processes (see Chapters 9 and 22). Ions contained within body fluids are known as electrolytes.

A general rule which applies when considering the ionic balance of any one compartment is that it should contain equivalent positive and negative charges (determined by the relative numbers of cations and anions). Each compartment is said to be electroneutral. This has significance when considering the movement of ions across membranes because, wherever possible, the body strives to ensure that movement of positively charged cations is accompanied by an equivalent negative charge in anions. When this does not happen electrical potentials are generated across membranes and this forms the basis of the function of excitable tissues (see Chapter 16).

The two components of extracellular fluid are separated from each other by the capillary wall. In most capillaries this is freely permeable to the movement of ions and small organic molecules but does not allow the passage of proteins. Thus under normal circumstances interstitial fluid contains no protein whereas both plasma and intracellular fluid are protein rich.

1.3 Test yourself

The answers are given on p. 175.

Question 1.1

Where in a biological macromolecule would hydrophobic groups generally be found?

Question 1.2

What are the three main compartments in which body water is distributed?

Question 1.3

What is the main cation of: (a) extracellular fluid; (b) intracellular fluid?

Question 1.4

Organic molecules are based around which element?

Question 1.5

Which key component of plasma does not normally pass across the capillary wall?

Proteins

BASIC CONCEPTS:

Proteins are macromolecules assembled as a sequence of amino acids. There are twenty different amino acids, giving rise to a wide range of possible proteins. According to the particular amino acid sequence, proteins will adopt different three-dimensional structures. Proteins are present in all cells and can perform many roles, including as structural elements and as enzymes. It is important to understand how the amino acid sequence of proteins can determine the properties of different proteins, and also how these properties can be altered by...