Difficulty: Introductory to Intermediate | Prerequisites: Chapter 1 (tissue types, homeostasis)
Chapter 2 covers the four major biomolecules (carbohydrates, lipids, proteins, nucleic acids), their monomers, and their functions. It then moves into cell organelles and their roles, functional groups that affect polarity, steroid ring structure, mRNA processing (poly-A tail, 5' cap, splicing), the steps of transcription and translation, and the types of cell-to-cell junctions (desmosomes, tight junctions, gap junctions).
Nucleic acid
A biomolecule made of nucleotide monomers. Its primary function is storing and transmitting genetic information. DNA and RNA are the two types.
Nucleotide
The monomer of nucleic acids. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.
Protein
A biomolecule made of amino acid monomers. Proteins are important for enzymatic activity, structural support, transport, and signalling.
Amino acid
The monomer of proteins. There are 20 standard amino acids, linked by peptide bonds during translation.
Carbohydrate
A biomolecule made of monosaccharide monomers. Carbohydrates are the body's primary source of short-term energy.
Monosaccharide
The monomer of carbohydrates. Glucose is the most important example in human physiology.
Lipid
A hydrophobic biomolecule made up of fatty acids and glycerol (in the case of triglycerides). Lipids serve as long-term energy storage.
Think of it this way: carbohydrates are your quick-access fuel; lipids are your deep reserves.
Triglyceride
A lipid consisting of a glycerol backbone with three fatty acid tails. The body's main form of long-term energy storage, found primarily in adipose tissue.
Phospholipid
A lipid with a phosphate-containing hydrophilic head, a glycerol backbone, and two fatty acid tails. Phospholipids form the bilayer of cell membranes.
Steroid
A lipid with a characteristic ring structure: three 6-carbon rings and one 5-carbon ring. Cholesterol and testosterone are both steroids.
Cholesterol
A type of steroid found in cell membranes and used as a precursor for steroid hormones. It has the same 3 six-carbon rings + 1 five-carbon ring structure as all steroids.
Hydroxyl group (-OH)
A polar functional group. Adding a hydroxyl group to a molecule increases its polarity and allows it to form hydrogen bonds with water.
In simple terms, hydroxyl groups make molecules more water-friendly.
Methyl group (-CH3)
A nonpolar functional group. Adding methyl groups makes a molecule less polar (more hydrophobic).
Golgi apparatus
An organelle that alters, modifies, sorts, and packages proteins after they are synthesised by ribosomes.
Think of it as the post office of the cell: it processes and dispatches proteins to their correct destinations.
Smooth endoplasmic reticulum (smooth ER)
An organelle responsible for lipid synthesis and detoxification. It lacks ribosomes on its surface.
Ribosome
The organelle responsible for protein synthesis (translation). Ribosomes read mRNA and assemble amino acids into polypeptide chains.
Nucleolus
A structure within the nucleus where ribosomal RNA (rRNA) is produced. It does not synthesise carbohydrates or proteins.
Peroxisome
An organelle involved in the breakdown of fatty acids and the detoxification of hydrogen peroxide. If peroxisomes are faulty, fatty acids accumulate and hydrogen peroxide is not properly broken down.
Poly-A tail
A string of adenine nucleotides added to the 3' end of mRNA during post-transcriptional processing. Its main purpose is to provide stability and protection against degradation. Without it, the mRNA degrades more quickly in the cytoplasm, reducing protein production.
5' cap
A modified guanine nucleotide added to the 5' end of mRNA. The cap is the initiator for ribosomal binding (not the poly-A tail).
Transcription
The process of copying a gene's DNA sequence into a complementary mRNA strand. It occurs in the nucleus.
Translation
The process of reading the mRNA sequence and assembling amino acids into a protein. It occurs in the cytoplasm on ribosomes.
AUG (start codon)
The codon that signals the ribosome to begin translation. It also codes for the amino acid methionine. If a mutation changes AUG to a different codon, the ribosome may not initiate translation at all.
Desmosome
A cell junction that provides mechanical strength, holding cells together in tissues that experience high stress (heart, uterus, skin). Desmosomes do not form impermeable barriers.
Tight junction
A cell junction that forms a nearly impermeable barrier between adjacent cells, commonly found in epithelial tissue. The barrier is created by integral proteins called occludins.
Occludin
An integral protein found in tight junctions. Occludins fuse adjacent cells together, creating the seal that controls transepithelial transport.
Carbohydrates: monomer = monosaccharide. Function = short-term energy.
Lipids: composed of fatty acids + glycerol. Function = long-term energy storage. Lipids are not built from a single repeating monomer in the same way the other three are.
Proteins: monomer = amino acid. Function = enzymatic activity, structure, transport, signalling.
Nucleic acids: monomer = nucleotide. Function = genetic information storage and transmission.
Golgi apparatus: alters, modifies, sorts, and packages proteins.
Smooth ER: lipid synthesis and detoxification.
Ribosome: protein synthesis (translation).
Nucleolus: production of ribosomal RNA (rRNA). It does not synthesise carbohydrates.
Peroxisome: breakdown of fatty acids and detoxification of hydrogen peroxide. Faulty peroxisomes lead to fatty acid accumulation and hydrogen peroxide build-up.
A hydroxyl group (-OH) is polar and increases a molecule's ability to form hydrogen bonds with water.
A methyl group (-CH3) is nonpolar. Hydrocarbon chains are also nonpolar.
When asked what makes a molecule more polar, look for oxygen-containing groups (hydroxyl, carboxyl, phosphate).
All steroids share the same backbone: three 6-carbon rings and one 5-carbon ring.
Cholesterol and testosterone are both steroids and both have this ring structure.
Triglycerides and phospholipids do not have ring structures. Triglycerides have a glycerol backbone + 3 fatty acid tails. Phospholipids have a phosphate head + glycerol backbone + 2 fatty acid tails.
5' cap: added to the 5' end. Initiates ribosomal binding.
Poly-A tail: added to the 3' end. Provides stability and protection against degradation. Without it, the mRNA is degraded more quickly, reducing protein output.
Splicing: introns are removed; exons are joined together.
The poly-A tail does not affect the speed of ribosomal binding. The 5' cap handles that.
Translation occurs in the cytoplasm, not the nucleus.
The correct sequence: RNA polymerase binds to the promoter, DNA uncoils and separates, free RNA nucleotide aligns and is added to the new RNA strand, RNA synthesis begins, RNA polymerase moves down DNA, RNA elongates, elongation.
AUG is the start codon. It signals the ribosome to begin translation and codes for methionine.
If a mutation changes AUG to another codon (e.g. GUU), the ribosome may not be able to initiate translation at all. It will not simply produce a different protein.
Ribosomes are made of a large and a small subunit. Both must assemble on the mRNA for translation to proceed.
Ricin, a toxin, prevents ribosomal subunit interactions. This blocks protein synthesis entirely, eventually leading to cell death.
Blocking ribosomes does not directly affect transcription or the Golgi apparatus.
Desmosomes: mechanical strength. Found in tissues under high stress (heart, uterus, skin). They hold cells together but do not form impermeable barriers.
Tight junctions: nearly impermeable barriers between cells. Found in epithelial tissue. Created by occludin proteins. They control what passes between cells (transepithelial transport).
Exam trap: desmosomes are described with the properties of tight junctions, or vice versa. The barrier function belongs to tight junctions, not desmosomes.
Students often think the poly-A tail is responsible for ribosomal binding. It is not. The 5' cap initiates ribosomal binding. The poly-A tail provides stability and protection.
Students confuse the nucleolus with the ribosome. The nucleolus produces ribosomal RNA; the ribosome is where protein synthesis happens.
Students mix up desmosomes and tight junctions. Desmosomes provide mechanical strength. Tight junctions form impermeable barriers. The occludin proteins belong to tight junctions, not desmosomes.
Students sometimes think that if the AUG start codon is mutated, translation will still produce a different protein. In many cases, translation simply fails to initiate.
⚠️ Biomolecule matching questions (monomer + function) appear frequently. Memorise the four pairings: carbohydrate/monosaccharide/short-term energy, lipid/fatty acids + glycerol/long-term energy, protein/amino acid/enzymatic activity, nucleic acid/nucleotide/genetic information.
⚠️ Organelle mismatch questions are common. The nucleolus/carbohydrate synthesis pairing is a favourite trap.
⚠️ Know what happens when peroxisomes fail: fatty acids accumulate, hydrogen peroxide is not broken down.
⚠️ The poly-A tail vs. 5' cap distinction is heavily tested. Remember: cap = ribosomal binding, tail = stability.
⚠️ Desmosome vs. tight junction questions typically describe one junction's properties and label it as the other. Read carefully.
Fill in the blank: The monomer of nucleic acids is the __________. (Nucleotide.)
True or False: The nucleolus is responsible for carbohydrate synthesis. (False. The nucleolus produces ribosomal RNA.)
Fill in the blank: A hydroxyl group makes a molecule more __________ (polar/nonpolar). (Polar.)
True or False: The poly-A tail is the structure that initiates ribosomal binding to mRNA. (False. The 5' cap initiates ribosomal binding.)
Fill in the blank: Tight junctions form nearly impermeable barriers using integral proteins called __________. (Occludins.)
Q: Which biomolecule has nucleotides as its monomer and stores genetic information?
A: Nucleic acids. DNA and RNA are both nucleic acids built from nucleotide monomers.
Q: Which organelle is incorrectly paired: Golgi apparatus/protein modification, smooth ER/lipid synthesis, ribosome/protein synthesis, nucleolus/carbohydrate synthesis?
A: Nucleolus/carbohydrate synthesis. The nucleolus produces ribosomal RNA, not carbohydrates.
Q: If a cell has faulty peroxisomes, what would most likely happen?
A: Fatty acids would accumulate within the cell. Peroxisomes break down fatty acids and detoxify hydrogen peroxide. Without functioning peroxisomes, fatty acids build up and hydrogen peroxide is not properly neutralised.
Q: What functional group would make a molecule more polar: hydroxyl, methyl, hydrocarbon chain, or carbon double-bonded to carbon?
A: The hydroxyl group (-OH). It is the only polar functional group listed. It enables hydrogen bonding with water.
Q: A mutation prevents the poly-A tail from being added to mRNA. What happens?
A: The mRNA becomes more susceptible to degradation in the cytoplasm, reducing the amount of protein produced. The poly-A tail provides stability, not ribosomal binding (that is the 5' cap).
Q: Ricin prevents ribosomal subunit interactions. What is the most direct consequence?
A: Prevention of protein synthesis, eventually leading to cell death. Ribosomes are essential for translation. Without functional ribosomes, no new proteins are made.
Q: True or False: Desmosomes form a nearly impermeable barrier using occludins.
A: False. That describes tight junctions. Desmosomes provide mechanical strength in tissues under stress. Tight junctions use occludin proteins to form barriers in epithelial tissue.
Q: If a mutation changes the AUG start codon to GUU, will translation still produce a protein?
A: Likely not. AUG signals the ribosome to begin translation. Without it, the ribosome may not initiate translation at all.
Biomolecule structure feeds directly into Chapter 3's metabolism content: understanding that carbohydrates provide short-term energy and lipids provide long-term energy sets up glycolysis, the TCA cycle, and fat metabolism.
The mRNA processing and translation material connects forward to any genetics or molecular biology topics later in the course. Cell junctions reappear when studying epithelial barriers in the digestive, urinary, and respiratory systems.
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