Difficulty: Introductory | Prerequisites: None, though familiarity with basic chemistry (atoms, molecules, bonds) will help.
This material sits at the foundation of virtually every biology and health-science course. It covers how molecules and water move across cell membranes, how genetic information flows from DNA to protein, and how cells reproduce. If you are joining the course late or revising from scratch, start here: most later topics (immunology, cardiovascular physiology, microbiology) assume you already have these concepts down.
Molecules move down their concentration gradient by diffusion; water does the same thing through a selective membrane, and we call it osmosis. DNA is transcribed into mRNA and then translated into proteins on ribosomes. Cells copy themselves by mitosis (body cells) or meiosis (sex cells).
Diffusion
The movement of molecules from an area of higher concentration to an area of lower concentration. Think of it as molecules spreading out until they are evenly distributed, the way a drop of ink disperses through still water.
Osmosis
The diffusion of water through a selectively permeable membrane. In simple terms, water moves toward whichever side has more dissolved solute, because the membrane lets water through but blocks larger molecules.
Hypertonic solution
A solution with a higher solute concentration than the cell's interior. The cell loses water (net flow of water out), so it shrivels.
Hypotonic solution
A solution with a lower solute concentration than the cell's interior. Water flows into the cell (net flow of water in), so the cell swells and may burst.
Isotonic solution
A solution with the same solute concentration as the cell. No net flow of water occurs, and the cell maintains its normal shape.
DNA bases
Adenine (A), Thymine (T), Cytosine (C), Guanine (G). These are the four nucleotide bases that encode genetic information in DNA. Think of them as the four letters of the genetic alphabet.
RNA bases
Uracil (U), Adenine (A), Cytosine (C), Guanine (G). RNA uses uracil in place of thymine. (Note: the source material lists "U, T, C, G," which contains an error; RNA carries U, A, C, G.)
Transcription
The process by which DNA is copied into messenger RNA (mRNA) by RNA polymerase. Think of it as making a working photocopy of a recipe from the master cookbook.
Translation
The process by which mRNA is read by a ribosome to assemble a chain of amino acids into a polypeptide (protein). tRNA molecules carry the matching amino acids to the ribosome, one codon at a time.
Mitosis
Cell division that produces two genetically identical daughter cells from one parent cell. This is how the body grows and repairs somatic (body) cells.
Meiosis
Cell division that produces four genetically unique sex cells (gametes), each with half the chromosome number. This is how eggs and sperm are made.
Diffusion is passive (no energy required). Molecules move from high to low concentration until equilibrium is reached.
Osmosis is a special case of diffusion that applies only to water crossing a selectively permeable membrane.
The direction of water movement depends on the relative solute concentration outside versus inside the cell:
Hypertonic environment: water leaves the cell, cell shrinks (crenation in animal cells, plasmolysis in plant cells).
Hypotonic environment: water enters the cell, cell swells (may lyse in animal cells; plant cells become turgid because the cell wall resists bursting).
Isotonic environment: water moves equally in both directions, no net change.
DNA stores the genetic blueprint. Its four bases pair specifically: A with T, C with G.
RNA is a working copy used in protein synthesis. It replaces thymine with uracil, so its bases are A, U, C, G.
DNA is double-stranded; RNA is usually single-stranded.
Transcription (occurs in the nucleus):
RNA polymerase binds to a promoter region on the DNA.
It reads the template strand and assembles a complementary mRNA strand.
The mRNA is processed and exported to the cytoplasm.
Translation (occurs at ribosomes in the cytoplasm):
The ribosome reads mRNA in three-base chunks called codons.
Each codon is matched by a tRNA carrying the corresponding amino acid (via its anticodon).
Amino acids are linked into a growing polypeptide chain until a stop codon is reached.
The central dogma runs one way in normal cells: DNA to RNA to protein.
Mitosis produces two identical diploid daughter cells. Used for growth, repair, and maintenance of somatic tissue.
Meiosis produces four genetically unique haploid cells (gametes). Involves two rounds of division and includes crossing over, which shuffles genetic material.
Key difference: mitosis keeps the chromosome number the same; meiosis halves it.
No numerical formulas in this section, but two process sequences are worth committing to memory:
Transcription sequence: DNA > RNA polymerase > mRNA
Translation sequence: mRNA > ribosome > tRNA (with matching anticodon) > amino acid > polypeptide strand
For tonicity, there is no formula, but the rule is straightforward: water follows solute. Whichever side of the membrane has more dissolved particles will draw water toward it.
Hospitals use isotonic saline (0.9% NaCl) for IV drips precisely because it will not cause red blood cells to swell or shrink. Getting the tonicity wrong can be fatal.
Understanding transcription and translation is the basis of modern biotechnology. mRNA vaccines, for example, deliver synthetic mRNA so your own ribosomes translate it into a target protein that trains the immune system.
Students often think osmosis applies to any molecule. It does not. Osmosis refers specifically to the movement of water across a selectively permeable membrane.
"Hyper" and "hypo" get mixed up constantly. Remember: hypertonic means more solute outside, so water leaves the cell. Hypotonic means less solute outside, so water enters.
Students sometimes believe DNA and RNA share the same bases. They do not. RNA substitutes uracil (U) for thymine (T).
Mitosis and meiosis are frequently confused. Mitosis = same (two identical cells). Meiosis = mixed and halved (four unique cells with half the chromosomes).
Expect a question asking you to predict what happens to a cell placed in a hypertonic, hypotonic, or isotonic solution.
The sequence DNA > mRNA > protein (the central dogma) is almost guaranteed to appear. Know each step and where it occurs in the cell.
Be able to distinguish mitosis from meiosis in terms of output (number of cells, ploidy, genetic variation).
RNA bases versus DNA bases is a common quick-mark question. Know that RNA uses uracil, not thymine.
True or False: Osmosis is the movement of any solute across a membrane. (False, it is the movement of water.)
Fill in the blank: In a ______ solution, water flows into the cell and the cell swells. (hypotonic)
True or False: RNA contains thymine. (False, RNA contains uracil.)
Fill in the blank: Mitosis produces ______ identical daughter cells. (two)
True or False: Translation occurs in the nucleus. (False, it occurs at ribosomes in the cytoplasm.)
Q: A red blood cell is placed in a 2% NaCl solution (normal body fluid is about 0.9% NaCl). What happens to the cell and why?
A: The 2% solution is hypertonic relative to the cell interior. Water moves out of the cell by osmosis, and the cell shrivels (crenation).
Q: Describe the path from a gene in the nucleus to a functional protein. Name each major step and where it occurs.
A: The gene (DNA) is transcribed into mRNA in the nucleus by RNA polymerase. The mRNA travels to the cytoplasm, where ribosomes translate it. tRNA molecules deliver amino acids matching each codon, and the amino acids are joined into a polypeptide chain.
Q: How does meiosis differ from mitosis in terms of the number and genetic makeup of the resulting cells?
A: Mitosis yields two genetically identical diploid cells. Meiosis yields four genetically unique haploid cells. Meiosis involves two division rounds and includes crossing over, producing genetic variation.
Q: A student writes that the bases in RNA are U, T, C, G. Identify and correct the error.
A: Thymine (T) does not appear in RNA. It should be replaced with adenine (A). The correct RNA bases are U, A, C, G.
This connects to immunology because immune cells must divide rapidly (mitosis) when the body is under attack, and understanding protein synthesis explains how antibodies are produced.
It connects to cardiovascular physiology because osmotic pressure in blood vessels governs fluid balance between the bloodstream and tissues.
It also connects to microbiology: knowing the difference between prokaryotic and eukaryotic cells (covered in the microbiology notes) builds directly on the cell-biology foundations here.
Diffusion, osmosis, osmotic pressure, tonicity, hypertonic, hypotonic, isotonic, crenation, lysis, turgid, plasmolysis, selectively permeable membrane, semipermeable membrane, concentration gradient, DNA, RNA, nucleotide bases, adenine, thymine, uracil, cytosine, guanine, base pairing, transcription, RNA polymerase, mRNA, translation, ribosome, tRNA, anticodon, codon, polypeptide, protein synthesis, central dogma, mitosis, meiosis, somatic cells, gametes, diploid, haploid, cell division, crossing over, genetic variation