Basic Chemistry Concepts for Anatomy & Physiology: Mixtures, Solutions, Chemical Bonds, and Reactions

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Body Organization, Homeostasis, and Basic Chemistry

Introduction to Mixtures in the Body

In anatomy and physiology, understanding the chemical basis of life is essential. Many substances in the human body exist as mixtures, which are combinations of two or more components that are physically intermixed but not chemically bonded.

Mixture: A physical combination of substances without chemical bonding.
Types of mixtures:
- Solutions: Homogeneous mixtures with evenly distributed particles.
- Colloids (Emulsions): Heterogeneous mixtures with larger particles that do not settle out.
- Suspensions: Heterogeneous mixtures with large, visible particles that settle out over time.

Classification of Mixtures

Mixtures in the body can be classified based on the size and behavior of their solute particles.

Type	Particle Size	Behavior	Example
Solution	Small (molecular size)	Do not settle out or scatter light	Mineral water, plasma
Colloid	Intermediate	Scatter light, do not settle out	Jell-O, cytosol
Suspension	Large, visible	Settle out over time	Blood (unsettled and settled), sand in water

Solutions in the Body

Properties of Solutions

Solutions are homogeneous mixtures where particles are evenly distributed. They consist of a solvent (the substance present in greatest amount, often water) and a solute (the substance dissolved in the solvent).

Solvent: The dissolving medium (e.g., water in plasma).
Solute: The dissolved substance (e.g., glucose in blood).
Solutions are usually transparent.
Examples include air (gas solution), salt solution, and blood plasma.

Concentrations of Solutions

Concentration describes the amount of solute in a given amount of solvent. Several units are used in physiology:

Percent (parts per 100 parts): E.g., 15% salt solution means 15 parts salt in 100 parts total solution.
Milligrams per deciliter (mg/dl): Mass of solute per 100 mL of solution. Example: blood glucose ≈ 80 mg/dl.
Molarity (M): Number of moles of solute per liter of solvent.
- 1 mole of a compound = its molecular weight in grams.
- Example: Glucose () has a molecular weight of 180.12 amu, so 180.12 g dissolved in enough to make 1 liter is a 1 M solution.
- 1 mole contains molecules (Avogadro's number).
- Small concentrations are expressed in millimoles (mM): .

Colloids and Suspensions

Colloids (Emulsions)

Colloids are heterogeneous mixtures with solute particles larger than those in solutions. These particles do not settle out but scatter light, giving a cloudy appearance.

Colloids can undergo sol-gel transformations (e.g., cytosol changing from liquid to gel).
Example: Jell-O, cytosol in cells.

Suspensions

Suspensions are mixtures with large, visible solutes that settle out over time.

Example: Mixture of water and sand, blood (when left to stand, red blood cells settle).

Difference Between Mixtures and Compounds

Mixtures and compounds differ in their formation and separation:

Mixtures: No chemical bonding; can be separated by physical means (straining, filtering).
Compounds: Chemical bonds present; can only be separated by breaking bonds.
Mixtures can be heterogeneous or homogeneous; compounds are always homogeneous.

Chemical Bonds

Nature of Chemical Bonds

Chemical bonds are energy relationships between electrons of atoms. They are not physical structures but describe how atoms interact to form molecules.

Electrons are the subatomic particles involved in chemical bonding.
Bonding determines whether a chemical reaction will occur and the type of bond formed.

Role of Electrons in Chemical Bonding

Electrons occupy areas around the nucleus called electron shells or energy levels. The arrangement of electrons in these shells determines chemical reactivity.

An atom can have up to 7 electron shells.
Each shell holds a specific number of electrons:
- First shell: 2 electrons
- Second shell: 8 electrons
- Third shell: 8 electrons

Valence Electrons and Chemical Reactivity

The outermost shell is called the valence shell. Electrons in this shell have the most potential energy and are involved in chemical reactions.

Atoms are stable when their valence shell is full (8 electrons, or 2 for H and He).
Atoms without a full valence shell will gain, lose, or share electrons to achieve stability.
This drive for stability is the basis for chemical bonding.

Types of Chemical Bonds

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, resulting in charged particles called ions.

Anion: Atom that gains electrons (negative charge).
Cation: Atom that loses electrons (positive charge).
Ionic compounds (e.g., salts) form crystals.

Covalent Bonds

Covalent bonds are formed by sharing electrons between atoms. The number of shared electron pairs determines if the bond is single, double, or triple.

Single bond: One pair of electrons shared.
Double bond: Two pairs shared.
Triple bond: Three pairs shared.
Covalent bonds can be polar (unequal sharing) or nonpolar (equal sharing).

Polar and Nonpolar Covalent Bonds

Polar covalent bonds occur when atoms have different electron-attracting abilities (electronegativity).

Polar molecules have partial positive and negative charges (e.g., water).
Nonpolar molecules share electrons equally.

Hydrogen Bonds

Hydrogen bonds are weak attractions between electropositive hydrogen and electronegative atoms (often oxygen or nitrogen) in another molecule.

Important for maintaining three-dimensional shapes of molecules (e.g., proteins, DNA).
Responsible for properties of water (cohesion, surface tension).

Chemical Reactions

Types of Chemical Reactions

Chemical reactions involve the formation, rearrangement, or breaking of chemical bonds. They are represented by chemical equations.

Synthesis (Combination) Reaction: Atoms or molecules combine to form larger, more complex molecules. Anabolic (building) process.
- Example:
Decomposition Reaction: Molecule is broken down into smaller molecules or atoms. Catabolic (bond-breaking) process.
- Example:
Exchange (Displacement) Reaction: Both synthesis and decomposition occur; bonds are made and broken.
- Example:

Redox (Oxidation-Reduction) Reactions

Exchange reactions involving electron transfer are called redox reactions.

Oxidation: Loss of electrons.
Reduction: Gain of electrons.
Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).
Example: Cellular respiration

Energy Flow in Chemical Reactions

Chemical reactions can be classified by energy changes:

Exergonic reactions: Release energy; products have less potential energy than reactants.
Endergonic reactions: Absorb energy; products have more potential energy than reactants.

Reversibility of Chemical Reactions

Many reactions are theoretically reversible, but in biological systems, reversibility depends on energy requirements and product removal.

Equilibrium occurs when forward and reverse reactions are balanced.
Some reactions are not reversible due to high energy barriers or product removal.

Factors Affecting Chemical Reaction Rates

The rate of chemical reactions in the body is influenced by several factors:

Temperature: Higher temperature increases reaction rate.
Concentration: Higher concentration of reactants increases rate.
Particle size: Smaller particles increase rate.
Catalysts: Substances that increase reaction rate without being changed; biological catalysts are called enzymes.

Summary Table: Types of Mixtures in the Body

Mixture Type	Distribution	Particle Size	Settling	Example
Solution	Homogeneous	Small	No	Plasma, mineral water
Colloid	Heterogeneous	Intermediate	No	Jell-O, cytosol
Suspension	Heterogeneous	Large	Yes	Blood, sand in water

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