Biology: Macromolecules and Cellular Structure

Macromolecules

• Definition: Large molecules composed of smaller molecules (monomers) bonded together
• Four main types:
  1. Carbohydrates: Provide energy and structure (e.g., sugars, starches, cellulose)
  2. Proteins: Perform various functions (e.g., enzymes, hormones, structural proteins)
  3. Nucleic Acids: Store and transmit genetic information (e.g., DNA, RNA)
  4. Lipids: Provide energy and structure (e.g., fats, oils, cholesterol)

Cellular Structure

• Cell Membrane: Semi-permeable membrane surrounding the cell, composed of phospholipid bilayer and proteins
• Cytoplasm: Gel-like substance inside the cell membrane, containing organelles and molecules
• Nucleus: Membrane-bound organelle containing genetic material (DNA)
• Mitochondria: Organelles responsible for generating energy (ATP) through cellular respiration
• Endoplasmic Reticulum (ER): Network of membranous tubules and cisternae involved in protein synthesis and transport
• Ribosomes: Small organelles responsible for protein synthesis

DNA Replication

• Process: Creation of an exact copy of DNA before cell division
• Steps:
  1. Unwinding: Double helix is unwound, and replication fork is formed
  2. Unzipping: Hydrogen bonds between nucleotides are broken
  3. Synthesis: New nucleotides are added to template strands
  4. Elongation: New DNA strands are formed
  5. Ligation: Enzymes seal gaps between nucleotides

Biological Molecules

• Amino Acids: Building blocks of proteins (20 different types)
• Nucleotides: Building blocks of nucleic acids (A, C, G, T, and U)
• Fatty Acids: Building blocks of lipids
• Carbohydrates: Simple sugars (e.g., glucose) and complex carbohydrates (e.g., starch, cellulose)

Molecular Interactions

• Hydrogen Bonds: Weak bonds between atoms with high electronegativity (e.g., oxygen, nitrogen) and hydrogen
• Ionic Bonds: Strong bonds between atoms with large electronegativity differences
• Covalent Bonds: Strong bonds between atoms sharing electrons
• Van der Waals Forces: Weak bonds between non-polar molecules
• Hydrophobic Interactions: Attraction between non-polar molecules in aqueous environments

Macromolecules

• Composed of smaller molecules (monomers) bonded together
• Four main types: Carbohydrates, Proteins, Nucleic Acids, and Lipids
• Carbohydrates: Provide energy and structure (e.g., sugars, starches, cellulose)
• Proteins: Perform various functions (e.g., enzymes, hormones, structural proteins)
• Nucleic Acids: Store and transmit genetic information (e.g., DNA, RNA)
• Lipids: Provide energy and structure (e.g., fats, oils, cholesterol)

Cellular Structure

• Cell Membrane: Semi-permeable membrane surrounding the cell, composed of phospholipid bilayer and proteins
• Cytoplasm: Gel-like substance inside the cell membrane, containing organelles and molecules
• Nucleus: Membrane-bound organelle containing genetic material (DNA)
• Mitochondria: Organelles responsible for generating energy (ATP) through cellular respiration
• Endoplasmic Reticulum (ER): Network of membranous tubules and cisternae involved in protein synthesis and transport
• Ribosomes: Small organelles responsible for protein synthesis

DNA Replication

• Process: Creation of an exact copy of DNA before cell division
• Unwinding: Double helix is unwound, and replication fork is formed
• Unzipping: Hydrogen bonds between nucleotides are broken
• Synthesis: New nucleotides are added to template strands
• Elongation: New DNA strands are formed
• Ligation: Enzymes seal gaps between nucleotides

Biological Molecules

• Amino Acids: 20 different types, building blocks of proteins
• Nucleotides: Building blocks of nucleic acids (A, C, G, T, and U)
• Fatty Acids: Building blocks of lipids
• Carbohydrates: Simple sugars (e.g., glucose) and complex carbohydrates (e.g., starch, cellulose)

Molecular Interactions

• Hydrogen Bonds: Weak bonds between atoms with high electronegativity (e.g., oxygen, nitrogen) and hydrogen
• Ionic Bonds: Strong bonds between atoms with large electronegativity differences
• Covalent Bonds: Strong bonds between atoms sharing electrons
• Van der Waals Forces: Weak bonds between non-polar molecules
• Hydrophobic Interactions: Attraction between non-polar molecules in aqueous environments

Molecular Interactions

• Hydrogen bonds are weak electrostatic attractions between molecules, crucial for maintaining DNA structure and protein folding
• Ionic bonds are strong electrostatic attractions between oppositely charged ions, essential for protein structure and cell signaling
• Van der Waals forces are weak attractions between non-polar molecules, significant for protein-ligand interactions
• Hydrophobic interactions occur between non-polar molecules, vital for protein structure and membrane formation

Cellular Structure

• The cell membrane is a semi-permeable membrane separating the cell from its environment, composed of a phospholipid bilayer and proteins
• Cytoplasm is a gel-like substance inside the cell membrane, containing organelles and cytosol
• Organelles are specialized structures within cells, including mitochondria, ribosomes, and lysosomes
• The cytoskeleton is a network of filaments providing structural support and shape to the cell

Biological Molecules

• Carbohydrates serve as energy sources, structural components, and signaling molecules
• Monosaccharides, such as glucose, disaccharides, like sucrose, and polysaccharides, like cellulose, are types of carbohydrates
• Proteins are structural, functional, and regulatory molecules, with primary structure referring to amino acid sequence, secondary structure comprising α-helices and β-sheets, and tertiary structure describing their 3D shape
• Lipids function as energy storage, structural components, and signaling molecules, including triglycerides (fats and oils), phospholipids (cell membrane), and steroids (hormones)

DNA Replication

Initiation

• Unwinding of the double helix occurs at the origin of replication, accompanied by the binding of helicase and primase

Unwinding and Synthesis

• The separation of DNA strands enables synthesis of new strands in the 5' to 3' direction
• Synthesis occurs continuously on the leading strand and discontinuously on the lagging strand

Elongation and Editing

• Nucleotides are added to the growing strands, with proofreading and editing occurring simultaneously

Termination

• DNA replication is completed, and replication forks are resolved

Leading Strand Synthesis

• The leading strand is synthesized continuously in the 5' to 3' direction

Lagging Strand Synthesis

• The lagging strand is synthesized discontinuously in the 5' to 3' direction, resulting in the formation of Okazaki fragments
• RNA primers provide the starting point for DNA synthesis
• DNA ligase seals gaps between Okazaki fragments, forming a continuous strand

Replication Fork and Enzymes

• The replication fork is the region where DNA replication occurs, with leading and lagging strands
• Helicase unwinds the double helix, creating the replication fork
• Topoisomerase relieves tension in the DNA ahead of the replication fork
• DNA polymerase synthesizes new DNA strands, proofreading and editing simultaneously

Cell Membrane Structure

• Phospholipids are the main component of the cell membrane, consisting of a glycerol backbone, one phosphate group (hydrophilic), and two fatty acid tails (hydrophobic), making them amphipathic.
• Cholesterol is present in small amounts, has four fused hydrocarbon rings, and is a precursor to steroid hormones, also helping to regulate membrane fluidity.
• Membrane proteins are either integral (transmembrane) or peripheral, with integral proteins traversing the entire bilayer and being amphipathic, while peripheral proteins are found on the outside of the bilayer and are generally hydrophilic.

Cell Membrane Functions

• Integral proteins can assist in cell signaling or transport, with some acting as receptors, triggering secondary responses within the cell.
• Receptor proteins can transmit signals through the lipid bilayer, and drugs that bind to receptors can be either agonists or antagonists.
• Peripheral proteins can be involved in adhesion, attaching cells to other things, and act as anchors for the cytoskeleton.
• Cell membranes also contain glycoproteins, which have carbohydrate chains, used for cellular recognition.

Cell Membrane Fluidity

• The fluid mosaic model describes how the components of the cell membrane can move freely within the membrane, with the cell membrane containing many different kinds of structures.
• The fluidity of the cell membrane can be affected by temperature, cholesterol, and the degree of unsaturation of fatty acids.
• Saturated fatty acids pack more tightly than unsaturated fatty acids, which have double bonds that may introduce kinks.

Cell Wall Structure

• Peptidoglycan is a polysaccharide with peptide chains, the primary component of bacterial cell walls.
• The cell wall of archaea is also made of polysaccharides, but does not contain peptidoglycan.
• The glycocalyx is a glycolipid/glycoprotein coat found mainly on bacterial and animal epithelial cells, helping with adhesion, protection, and cell recognition.

Cell-Matrix Junctions

• Cell-matrix junctions connect the extracellular matrix (ECM) to the cytoskeleton, with two types: focal adhesions and hemidesmosomes.
• Focal adhesions connect the ECM to actin microfilaments inside the cell via integrins.
• Hemidesmosomes connect the ECM to intermediate filaments inside the cell via integrins.

Crossing Cell Membranes

• Cytosis refers to the bulk transport of large, hydrophilic molecules across the cell membrane, requiring energy (active transport mechanism).
• Endocytosis involves the cell membrane wrapping around an extracellular substance, internalizing it into the cell via a vesicle or vacuole.
• Phagocytosis, pinocytosis, and receptor-mediated endocytosis are different forms of endocytosis.
• There are three types of transport across the cell membrane: simple diffusion, facilitated transport, and active transport.
• Simple diffusion involves the diffusion of small uncharged molecules or lipid-soluble molecules directly across the cell membrane, down their concentration gradient, without using energy.
• Facilitated transport involves the diffusion of large or charged molecules across the cell membrane, down their concentration gradient, without using energy, through channel proteins.
• Active transport involves the transport of substances against their concentration gradient, requiring energy.