The Molecules of Life and Cell Division

Organizational Hierarchy of Life

Organisms are organized in a specific, hierarchical structure beginning with the smallest unit.
Atoms are the foundational building blocks which combine to form molecules.
Molecules organize into cells, the basic unit of life.
Groups of cells with similar functions form tissues.
Different tissues combined make up organs.
Multiple organs work together to form organ systems.
An organism is composed of many interacting organ systems.

Introduction to Macromolecules

The term "macro" refers to things that are large or big.
Molecules consist of two or more elements joined by a chemical bond.
Macromolecules are fundamentally referred to as polymers.
- Poly means "many."
- Mers refers to a "subunit."
Polymers are constructed from multiple subunits called monomers.
- Mono means "one."
- MERS here also implies a single subunit.
A conceptual analogy for these structures is Lego blocks, where individual blocks are monomers that can be assembled to create complex polymers.
There are four major categories of macromolecules essential to cell components: proteins, lipids, carbohydrates, and nucleic acids.

The Nature of Monomers and Polymers

Monomers are smaller molecules consisting of a single unit made of various elements.
Monomers join together chemically through covalent bonds to form larger units (polymers).
The process of joining monomers into polymers requires the application of energy.
Polymers are large, bonded macromolecules with repeated subunits.
When the chemical bonds within polymers are broken (such as during digestion), energy is released.
A real-world example of this relationship is starch and glucose:
- A single unit of glucose acts as a monomer.
- When many glucose units are linked, they form the polymer starch.
- Consuming starches (e.g., in pasta or potatoes) involves breaking those bonds via digestion back down to glucose levels, which cells then utilize to perform work.

Proteins: Building Blocks and Chemical Structure

Proteins are large polymers made of monomers called amino acids.
Amino acids linked by peptide bonds are referred to as peptides.
- Two or more amino acids linked together are peptides (shorter chains).
- Long molecules consisting of multiple amino acids and many peptide bonds are classified as proteins.
Chemical structure of an amino acid monomer:
- A central carbon ( $C$ ) atom.
- A hydrogen ( $H$ ) atom attached to the central carbon.
- An amino group (nitrogenous group), consisting of nitrogen attached to two hydrogens ( $–NH_2$ ).
- A carboxyl group, consisting of a carbon with a double bond to oxygen and a hydroxyl group ( $–COOH$ ).
- A side chain (also known as an $R$ group) made of other molecules, which varies between different amino acids.
There are 20 different amino acids that combine in various sequences to form every protein in the human body.
Amino acids are classified into two nutritional categories:
1. Essential Amino Acids: These must be acquired through food sources and diet.
2. Nonessential Amino Acids: These can be synthesized directly by the body.

Protein Folding and Hierarchy of Structure

The specific shape of a protein is crucial to its biological function.
Primary Structure: Defined by the specific sequence of amino acids (e.g., which amino acids, such as leucine, histidine, or glycine, are adjacent to one another).
Secondary Structure: Occurs when amino acids are linked by hydrogen bonds formed due to charges on the amino acids. This results in specific shapes:
- Alpha helix: A spiral-like configuration.
- Beta-pleated sheet: A structure resembling folded paper.
Tertiary Structure: Formed by the interactions and further folding between alpha helices and beta sheets.
Quaternary Structure: Formed by the interactions between more than one polypeptide chain.
If hydrogen bonds are broken, the protein loses its shape and is destroyed/denatured.

Functions of Proteins in the Human Body

Proteins exhibit the highest variety of functions among all macromolecules.
Immune System:
- Antibodies: Defense proteins produced by white blood cells to fight infections.
- Complement System: A group of proteins involved in the immune response.
Muscular System: Proteins exist as myofilaments involved in muscle flexion and growth.
Structural Components:
- Cytoskeleton: Provides the cell with shape and protection.
- Keratin: Found in skin, hair, and nails.
- Collagen: Provides skin with plumpness and a youthful appearance.
- Elastin: Provides stretchability to skin and blood vessels.
Signaling and Communication:
- Cytokines: Chemical signals or messengers that enable white blood cells to communicate to fight infection.
Transport and Homeostasis:
- Hemoglobin: Found in red blood cells to transport gases like oxygen ( $O_2$ ) and carbon dioxide ( $CO_2$ ).
- Fibrinogen: A critical clotting factor that prevents excessive bleeding and helps achieve homeostasis.
Enzymes: Every enzyme in the body is a protein, including those involved in digestion.
Cell Membrane: Proteins facilitate the movement of substances in and out of the cell.

Carbohydrates: Energy and Storage

Carbohydrates (carbs) are molecules composed of carbon ( $C$ ), hydrogen ( $H$ ), and oxygen ( $O$ ).
Chemical Formula Pattern: There is twice the amount of hydrogen compared to carbon and oxygen.
Example: Glucose has the formula $C_6H_{12}O_6$ .
Energy Value: Consuming one gram of carbohydrates provides $4\,\text{calories}$ of energy for cellular work.
Classification and Examples:
1. Monosaccharides (Single sugar units): Glucose, Fructose, Galactose. Used for direct metabolism in the mitochondria to generate $ATP$ (Adenosine Triphosphate).
2. Disaccharides (Two sugar units): Lactose, Maltose, and Sucrose (which breaks down into fructose and glucose).
3. Polysaccharides (Large polymers):
  - Starch: Found in potatoes and pasta; broken down into simpler units.
  - Glycogen: The storage form of glucose in the liver, muscles, and brain; used when sugar levels are low.
  - Cellulose: Plant-based sugar that forms cell walls. For humans, this serves as dietary fiber for digestive health.
4. Conjugated Carbohydrates: Sugars attached to lipids or proteins. These act as receptors in the cell membrane and aid in cell communication.

Lipids: Composition and Types

Lipids (fats) are composed of carbon ( $C$ ), hydrogen ( $H$ ), and oxygen ( $O$ ), but contain significantly less oxygen than carbohydrates.
Energy Value: One gram of fat provides $9\,\text{calories}$ , making it more energy-dense than carbs ( $4\,\text{cal/g}$ ) or proteins ( $4\,\text{cal/g}$ ).
Physical Properties: They are nonpolar hydrocarbons joined by covalent bonds (equal sharing). They are insoluble in water (e.g., oil and water do not mix).
Three Main Categories:
1. Triglyceride: Consists of a glycerol group (the backbone) attached to three fatty acid (hydrocarbon) chains.
2. Phospholipid: Consists of a glycerol group, two hydrocarbon fatty acid chains, and a phosphate group. These form the phospholipid bilayer of the cell membrane (plasma membrane), with phosphate groups on the outside and fatty acid chains in the center.
3. Cholesterol: A waxy substance made of a $27$ -carbon compound used for structural components and hormone synthesis.

Saturated vs. Unsaturated Fats

Saturated Fats:
- Structure: Every carbon atom in the hydrocarbon chain is attached to two hydrogen atoms.
- Characteristics: Tightly packed and straight chains.
- Physical State: Solid at room temperature (e.g., butter, bacon fat).
Unsaturated Fats:
- Structure: Contains double bonds between carbon atoms, leading to missing hydrogens.
- Characteristics: These double bonds cause "kinky" or curved hydrocarbon chains.
- Physical State: Liquid at room temperature (e.g., olive oil, vegetable oils).

Functions of Lipids in the Body

Energy Source: Provides high caloric density ( $9\,\text{calories}$ per gram).
Nutrient Absorption: Fats are necessary to absorb fat-soluble vitamins.
Brain and Nervous System: The brain is heavily composed of lipids. Neurons have axons covered in myelin, a fatty substance that acts as insulation and increases the conduction velocity of nerve signals.
Structural: Form the primary component of the cell/plasma membrane.
Hormone Synthesis: Lipids are used to create fat-based or steroid-based hormones, including:
- Testosterone
- Estrogen
- Aldosterone
Dietary Considerations: Healthy fats (monounsaturated/liquid at room temperature, nuts, fish, omega-3 fatty acids) are preferred. Excess saturated fats (solid at room temperature) can block blood vessels if consumed in high amounts without sufficient physical activity.

Nucleic Acids and Nucleotides

Nucleic acids are polymers responsible for storing and processing genetic information.
Monomers of nucleic acids are called nucleotides.
Structure of a Nucleotide:
1. Pentose Sugar: A five-carbon sugar. It is deoxyribose in $DNA$ and ribose in $RNA$ .
2. Phosphate Group.
3. Nitrogenous Base: One of four types: adenine ( $A$ ), thymine ( $T$ ), cytosine ( $C$ ), or guanine ( $G$ ). These engage in complementary base pairing.

DNA, Genes, and the Genome

DNA (Deoxyribonucleic acid): A double-stranded helix polymer containing the genetic map for an individual.
Location: Found in the nucleus of eukaryotic organisms.
Genome: The complete set of an organism's $DNA$ . Humans typically have $46$ chromosomes ( $23$ from each parent).
Genes: Specific sequences of nucleotides on the chromosomes. There are over $20,000$ genes in the human genome.
Gene Expression:
1. Transcription: Creating an $RNA$ copy (transcript) of a specific gene sequence.
2. Translation: The $RNA$ transcript is read by a ribosome to synthesize a specific protein (e.g., the gene for insulin is transcribed and then translated into the hormone insulin).

Cellular Forms of DNA

DNA: The double-stranded polymer itself.
Chromatin: The form of $DNA$ where the double helix is wrapped around protein complexes called histones. This "thread around a spool" organization allows the long polymers to fit in the nucleus and stay organized for transcription.
Chromosomes: The most condensed and tightly bound form of $DNA$ . They appear as distinct "two-armed" structures (usually colored blue in diagrams) only during cell division.

Mitosis: Somatic Cell Division

Purpose: Growth, tissue repair (e.g., healing a cut or fractured bone), and replacing old cells (e.g., red blood cells, which turnover every $100\text{--}120\,\text{days}$ ).
Result: One parent cell produces two identical daughter cells. These are diploid, meaning they have the same number of chromosomes as the parent ( $46$ in humans).
Phases of Mithosis:
1. Interphase: The longest phase. The cell grows and duplicates its $DNA$ (e.g., from $46$ to $92$ chromosomes).
2. Prophase: The nucleus disappears, and duplicated $DNA$ is released into the cytoplasm.
3. Metaphase: Chromosomes line up in the center of the cell.
4. Anaphase: Duplicated chromosomes separate and move toward opposite sides of the cell.
5. Telophase: Nuclear membranes reform around the separate sets of $DNA$ . The cell membrane begins to pinch.
6. Cytokinesis: The cells completely separate into two new identical daughter cells.

Meiosis: Reduction Division

Purpose: Production of gametes (sperm and egg cells).
Mechanism: Known as "cell reduction" because the resulting cells have half the original chromosome count.
Result: Four haploid daughter cells, each containing $23$ chromosomes in humans.
Process: Includes two series of divisions (Meiosis I and Meiosis II) following the initial $DNA$ duplication in Interphase ( $46 \rightarrow 92 \rightarrow 46 \rightarrow 23$ ).
Fertilization: When a sperm ( $23$ chromosomes) and egg ( $23$ chromosomes) combine, they form a diploid zygote ( $46$ chromosomes), which then undergoes mitosis to develop into an embryo and fetus.