Lipids, Nucleic Acids, and Cell Structure
This lecture covers lipids, nucleic acids (DNA and RNA), and introduces cell structure with a focus on the differences between prokaryotic and eukaryotic cells. Key concepts include lipid classes (triglycerides, steroids, phospholipids), nucleotide structure, DNA and RNA differences, and the presence or absence of membrane-bound organelles.
Lipids Overview: Lipids are grouped together due to their hydrophobic nature, not because they are built from monomers and polymers like carbohydrates or proteins. The three main classes of lipids are triglycerides (fats and oils), steroids, and phospholipids.
Triglycerides: These consist of a glycerol molecule with three fatty acid chains linked through dehydration synthesis. They serve as energy storage, insulation, and protection. Saturated fats are solid at room temperature, while unsaturated fats are liquid (oils) due to kinks in their fatty acid chains from double bonds. Trans fats are unsaturated fats that have been altered and have no kinks and are generally solid.
Steroids: These are characterized by four interlocking rings. Cholesterol is a steroid found in plasma membranes and is a precursor for other body steroids like estrogen, testosterone, progesterone, and cortisol.
Phospholipids: They have a glycerol, two fatty acids, and a phosphate group. The phosphate head is hydrophilic, while the fatty acid tails are hydrophobic. In water, they form a bilayer with the tails facing inward and the heads facing outward, forming the basis of the plasma membrane. The presence of unsaturated fatty acids with kinks in the phospholipid tails contributes to membrane fluidity.
Nucleic Acids Overview: Nucleic acids, DNA and RNA, store and transmit hereditary information. The monomer of nucleic acids is a nucleotide. Nucleotides are composed of a phosphate group, a pentose sugar (ribose or deoxyribose), and a nitrogenous base.
Nucleotide Structure: Nucleotides have a phosphate group, a pentose sugar (ribose or deoxyribose), and a nitrogenous base. Carbons on the sugar are numbered with primes (1', 3', 5'). The nitrogenous base is attached to the 1' carbon, the next nucleotide is attached to the 3' carbon, and the phosphate group is connected to the 5' carbon.
DNA vs. RNA: DNA contains the sugar deoxyribose, while RNA contains the sugar ribose. Deoxyribose lacks an oxygen atom at the 2' carbon compared to ribose. The nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T). In RNA, uracil (U) replaces thymine (T).
Nitrogenous Bases: Nitrogenous bases are classified as pyrimidines (cytosine, thymine, uracil) which have a single ring structure, and purines (adenine, guanine) which have a double ring structure. DNA has A, G, C, and T; RNA has A, G, C, and U.
Nucleic Acid Polymerization: Nucleotides are linked together through condensation reactions (dehydration synthesis), forming a phosphodiester bond and releasing water as a byproduct. The chain is read from 5' to 3'.
DNA Structure and Function: DNA forms a double helix, stabilized by hydrogen bonds between complementary base pairs: A with T, and C with G. DNA provides instructions for its own replication and is transcribed into RNA, which is then translated into protein. DNA is very stable and needs enzymes to help with replication.
RNA Structure and Function: RNA is typically single-stranded but can fold on itself through hydrogen bonding, creating secondary and tertiary structures. RNA carries genetic instructions and has enzymatic activity. RNA is considered less stable than DNA and is thought to possibly be the original building block for life.
Cell Structure: Basic Components: All cells, both prokaryotic and eukaryotic, have a plasma membrane (selective barrier), cytosol (semi-fluid substance), chromosomes (carrying genetic information), and ribosomes (for protein synthesis). Cytoplasm refers to the entire interior of the cell, including the organelles.
Prokaryotic vs. Eukaryotic Cells: Eukaryotic cells have DNA enclosed within a membrane-bound nucleus, while prokaryotic cells have DNA concentrated in a nucleoid region without a membrane. Eukaryotic cells have membrane-bound organelles, whereas prokaryotic cells do not. Eukaryotic cells are generally larger than prokaryotic cells.
Key points
- Lipids are grouped together because they are hydrophobic or partly hydrophobic.
- Triglycerides, steroids, and phospholipids are the three main classes of lipids.
- Nucleic acids (DNA and RNA) are polymers made of nucleotide monomers.
- DNA uses deoxyribose sugar and the base thymine, while RNA uses ribose sugar and the base uracil.
- DNA is double-stranded (double helix), while RNA is typically single-stranded.
- All cells have a plasma membrane, cytosol, genetic material (DNA), and ribosomes.
- Eukaryotic cells have membrane-bound organelles, including a nucleus containing DNA, while prokaryotic cells do not.
- Eukaryotic cells are generally larger than prokaryotic cells.