Biology Chapter on Macromolecules

The Molecules of Life

• Macromolecules: The four major classes of macromolecules are carbohydrates, proteins, lipids, and nucleic acids.
• Monomers and Polymers: Polymers are large molecules constructed from repeated smaller units called monomers.
• Dehydration Synthesis and Hydrolysis: Dehydration synthesis removes a water molecule to join two monomers, while hydrolysis adds a water molecule to break a polymer into monomers.

Carbohydrates Serve as Fuel and Building Material

• Types of Carbohydrates: Monosaccharides are simple sugars, disaccharides are two monosaccharides joined together, and polysaccharides are long chains of monosaccharides.
• Glycosidic Linkages: Starch, glycogen, and cellulose are polysaccharides with different glycosidic linkages, which determine their structure and function.
  • Starch: Used for energy storage in plants.
  • Glycogen: Used for energy storage in animals.
  • Cellulose: Provides structural support in plant cell walls.
• Importance of α-glucose vs. β-glucose: α-glucose is found in starch and glycogen, while β-glucose is found in cellulose. Our digestive systems can break down α-glucose bonds but not β-glucose bonds, which is why cellulose is indigestible for humans.
• Symbiosis: Symbiotic relationships exist between animals and microbes, such as termites, to digest cellulose.

Lipids are a Diverse Group of Hydrophobic Molecules

• Types of Lipids: Fats, phospholipids, and steroids are major classes of lipids.
  • Fats: Made up of glycerol and three fatty acid chains. Often used for energy storage.
  • Phospholipids: Key components of cell membranes. Have a hydrophilic head and hydrophobic tails.
  • Steroids: Composed of four fused rings. Examples include cholesterol and hormones.
• Saturated vs. Unsaturated Fats: Saturated fats have no double bonds in their fatty acid chains, making them solid at room temperature. Unsaturated fats have one or more double bonds, making them liquid at room temperature.
• Cis vs. Trans Fats: Cis fats have hydrogen atoms on the same side of the double bond, while trans fats have them on opposite sides. Trans fats are linked to health problems.

Proteins have Many Structures, Resulting in a Wide Range of Functions

• Proteins vs. Polypeptides: Proteins are complex three-dimensional structures made up of one or more polypeptide chains. Polypeptides are unbranched polymers of amino acids.
• Peptide Bonds: Two amino acids are joined by a peptide bond, which forms through dehydration synthesis.
• Amino Acid Structure: Each amino acid has a central carbon atom bonded to a hydrogen atom, a carboxyl group, an amino group, and a unique side chain (R group).
  • R Group Properties: Amino acids can be grouped based on the chemical properties of their R groups, such as hydrophobic, hydrophilic, acidic, or basic.
• Protein Structure: The precise sequence of amino acids in a polypeptide chain determines its primary structure. This primary structure influences its secondary, tertiary, and quaternary structures.
  • Secondary Structure: Results from hydrogen bonding between backbone atoms of the polypeptide chain, forming α-helices and β-pleated sheets.
  • Tertiary Structure: Three-dimensional shape of the polypeptide chain, stabilized by interactions including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.
  • Quaternary Structure: Interaction between multiple polypeptides to form a functional protein.
• Protein Denaturation: Certain conditions, such as heat, pH changes, or the presence of salts or heavy metals, can denature proteins, causing them to lose their functional structure.

Nucleic Acids Store and Transmit Hereditary Information

• Nucleotide Structure: Nucleotides are the monomers of nucleic acids, composed of a nitrogenous base, a five-carbon sugar (ribose or deoxyribose), and a phosphate group.
• DNA Structure: Deoxyribonucleic acid (DNA) is responsible for storing and transmitting genetic information. It has a double helix structure with two antiparallel strands of nucleotides held together by hydrogen bonds between complementary base pairs (adenine-thymine, guanine-cytosine).
• RNA Structure: Ribonucleic acid (RNA) plays a key role in protein synthesis. It is typically single stranded and contains ribose sugar and the base uracil instead of thymine.

How We Study Cells

• Light Microscope: Uses lenses and visible light to magnify images.
  • Advantages: Allows viewing of living organisms and larger specimens.
  • Limitations: Limited magnification (up to 1000x) and only surface features can be observed.
• Stereoscopic (Dissecting) Microscope: Provides a three-dimensional view of larger specimens.
  • Advantages: Allows observation of larger specimens in detail.
  • Limitations: Cannot view microorganisms.
• Transmission Electron Microscope (TEM): Uses a beam of electrons to view the internal structures of cells.
  • Advantages: High resolution and can reveal internal details.
  • Limitations: Requires thin samples and must be dead specimens.
• Scanning Electron Microscope (SEM): Uses a beam of electrons to view the surface of cells and create a three-dimensional image.
  • Advantages: High resolution and produces 3D images.
  • Limitations: Requires dead specimens and only shows surface features.

A Panoramic View of the Cell

• Cell Commonalities: All cells share basic characteristics including a plasma membrane, DNA, cytoplasm, and ribosomes.
• Prokaryotic vs. Eukaryotic Cells:
  • Prokaryotes: Simple cells lacking a nucleus and membrane-bound organelles. Example: Bacteria.
  • Eukaryotes: Complex cells with a nucleus and membrane-bound organelles. Example: Animals, plants, fungi.
• Plant vs. Animal Cells:
  • Plant cells: Have cell walls, chloroplasts, and a central vacuole.
  • Animal cells: Have lysosomes.
• Cell Size Limits: There are upper and lower limits to cell size due to factors like metabolic requirements and surface area-to-volume ratio.
  • Metabolic limits: Larger cells require more resources and efficient transport mechanisms.
  • Surface area: Increased surface area is crucial for efficient exchange of materials with the environment.
• Compartmentalization: Eukaryotic cells are compartmentalized, which increases efficiency and allows for more specialized functions.

The Nucleus and Ribosomes

• Nuclear Envelope: A double membrane that encloses the nucleus, allowing regulated exchange of molecules between the nucleus and cytoplasm through nuclear pores.
• Nucleolus: A region within the nucleus where ribosomal RNA (rRNA) is synthesized and assembled with proteins to create ribosomes.
• Ribosomes: Sites of protein synthesis.
  • Free Ribosomes: Found in the cytosol, synthesize proteins that will function within the cytosol.
  • Bound Ribosomes: Associated with the endoplasmic reticulum, synthesize proteins that will be secreted, incorporated into membranes, or packaged in organelles.