Biological Molecules: Elements, Compounds, and Water in Cells
Chemical Foundations of Life
Elements: Building Blocks of Matter
Elements are fundamental substances that cannot be broken down into simpler substances. The smallest unit of an element is an atom, which is made of only one kind of atom.
Key Elements in Living Organisms:
- Carbon
- Hydrogen
- Oxygen
- Nitrogen
- Phosphorus
- Sulfur
Compounds: Molecules of Life
Compounds are formed when two or more elements combine chemically. Most substances in living things are compounds. The smallest unit of a compound is a molecule.
For example:
- Carbon dioxide: A compound made of one carbon atom and two oxygen atoms.
- Water: A compound made of two hydrogen atoms and one oxygen atom.
Organic Compounds
Most compounds found in living things contain carbon and are known as organic compounds. These include:
- Carbohydrates
- Lipids
- Proteins
- Nucleic Acids
Carbohydrates: Energy and Structure
Carbohydrates are energy-rich organic compounds made of carbon, hydrogen, and oxygen. Examples include sugars and starches.
- Sugars: Produced when plants make their food. Fruits and some vegetables have a higher sugar content.
- Starches (Complex Carbohydrates): Large molecules formed when sugar molecules combine. Plant cells store excess energy in starch molecules. Many plant-based foods, such as potatoes, pasta, rice, and bread, contain starch. Your body breaks down starch into glucose, a sugar your cells use for energy.
Carbohydrates are also important components of some cell parts, such as cellulose in plant cell walls and cell membranes.
Lipids: Stored Energy and Cell Structure
Lipids include fats, oils, and waxes. They are energy-rich organic compounds made of carbon, hydrogen, and oxygen, and they store more energy than carbohydrates.
- Cells store energy in lipids for later use (e.g., a dormant bear lives on the energy stored in fat within its cells).
- Cell membranes are mainly made of lipids.
Proteins: Structure, Function, and Enzymes
Proteins are large organic molecules primarily made of carbon, hydrogen, oxygen, nitrogen, and sulfur (CHONS).
Structure:
- Proteins are made of smaller molecules called amino acids.
- There are 20 common amino acids.
- The order in which amino acids link together determines the type of protein.
Functions:
- Make up much of the structure of cells.
- Form part of cell membranes.
- Make up organelles within the cells.
- Enzymes (a type of protein) speed up chemical reactions in living things. For example, enzymes in saliva speed up the digestion of food by breaking down starches into sugar.
Nucleic Acids: Genetic Information Carriers
Nucleic acids are very long organic molecules made of carbon, oxygen, hydrogen, nitrogen, and phosphorus. They contain the instructions cells need to follow for the functions of life.
There are two main types:
- DNA (Deoxyribonucleic Acid): The genetic material that carries information about an organism, passed from parent to offspring. It directs cell functions and is found in chromatin within the nucleus.
- RNA (Ribonucleic Acid): Plays an important role in the production of proteins. It is found in the cytoplasm as well as the nucleus.
Inorganic Compounds
Compounds without carbon are called inorganic compounds. An example is sodium chloride (table salt).
Water: The Essential Solvent for Life
Water plays many important roles in cells:
- Most chemical reactions in cells involve substances dissolved in water.
- Water molecules actively participate in many chemical reactions within cells.
- Chemical reactions in cells could not take place without water.
- Water helps maintain a cell’s size and shape (a cell without water is like a balloon without air).
- Water changes temperature slowly, which helps prevent rapid temperature changes in cells.