Biology & Chemistry Essentials

Biology and Chemistry: Matter, Information, and Change

Biology explains living systems, while chemistry explains the particles and reactions that make those systems possible. Many questions become manageable when you identify what is moving, what information is being copied, or which atoms must be conserved. Use a sequence, label quantities, and check whether the final result obeys the governing rule.

Membranes and Osmosis

A cell membrane is selectively permeable: some substances cross more readily than others. Passive transport moves substances without direct cellular energy input, often down a concentration gradient. Active transport requires energy to move or maintain substances against a gradient. Osmosis is the net movement of water across a selectively permeable membrane.

Worked Example 1: Predict Cell Size

A cell contains less nonpenetrating solute than the surrounding solution. What happens to its water and size?

Step 1: Compare the two sides. The surrounding solution has more solute that cannot cross the membrane.

Step 2: Water moves in both directions, but its net movement is toward the higher nonpenetrating-solute concentration.

Step 3: Water leaves the cell, so the cell loses volume and shrinks.

Why it works: Net water movement reduces the difference in water availability across the membrane. A common mistake is saying that solute pulls water because all solute behaves identically; first confirm that the solute cannot freely cross.

From DNA to Protein

DNA stores genetic information in nucleotide sequences. During transcription, a DNA template guides production of RNA. During translation, a ribosome reads messenger RNA codons and joins amino acids into a polypeptide. The useful sequence is DNA → RNA → protein. DNA uses thymine, while RNA uses uracil.

Worked Example 2: Transcribe a Template

A short DNA template segment is TAC GGA. Write the complementary messenger RNA segment.

Step 1: Pair each DNA base with an RNA base: T pairs with A, A with U, C with G, and G with C.

Step 2: TAC becomes AUG.

Step 3: GGA becomes CCU.

Answer: The messenger RNA segment is AUG CCU.

Why it works: Complementary pairing copies the information into a form a ribosome can read. Using T in the RNA answer would ignore the chemical difference between DNA and RNA.

Inheritance as Probability

An allele is a version of a gene. A genotype names an allele combination; a phenotype is an observable outcome influenced by genotype and sometimes environment. For a simple dominant-recessive model, a heterozygote displays the dominant trait, while the recessive phenotype requires two recessive alleles. A Punnett square predicts probabilities for each offspring, not a guaranteed family pattern.

Worked Example 3: A Punnett Cross

Cross Bb with bb, where B is dominant. What fraction of offspring is expected to show the recessive phenotype?

Step 1: The Bb parent can contribute B or b. The bb parent can contribute only b.

       b     b
B     Bb    Bb
b     bb    bb

Step 2: Two of four boxes are bb.

Answer: The predicted probability is 2/4, or 50%.

Why it works: Each box represents one equally likely allele pairing under the model. Four children need not include exactly two with each outcome; probability describes repeated chances, not a fixed schedule.

Chemical Equations Conserve Atoms

A chemical reaction rearranges atoms into new combinations. Coefficients tell how many molecules or formula units participate. Subscripts belong to a substance's formula and must not be changed when balancing, because changing a subscript changes the substance itself.

Worked Example 4: Balance a Reaction

Balance N₂ + H₂ → NH₃.

Step 1: Count atoms. The left has two nitrogen atoms; the right has one.

Step 2: Place 2 before NH₃: N₂ + H₂ → 2NH₃. Nitrogen is now balanced, but the right has six hydrogen atoms.

Step 3: Place 3 before H₂: N₂ + 3H₂ → 2NH₃.

Step 4: Recount. Both sides contain two nitrogen and six hydrogen atoms.

Why it works: Ordinary reactions conserve atoms. Coefficients change the number of whole particles without redefining nitrogen, hydrogen, or ammonia.

Acids, Bases, and the pH Scale

In introductory aqueous chemistry, a pH below 7 is acidic, pH 7 is neutral at about room temperature, and a pH above 7 is basic. The scale is logarithmic: a difference of one pH unit represents a tenfold difference in hydrogen-ion concentration. Buffers resist sudden pH changes rather than making every solution neutral.

Worked Example 5: Compare Two pH Values

How does the hydrogen-ion concentration of pH 2 compare with pH 5?

Step 1: Find the difference: 5 − 2 = 3 pH units.

Step 2: Each unit represents a factor of 10, so calculate 10 × 10 × 10 = 1,000.

Step 3: The lower pH has more hydrogen ions.

Answer: The pH 2 solution has about 1,000 times the hydrogen-ion concentration of the pH 5 solution.

Why it works: The comparison is exponential, not subtraction alone. Saying “three times more acidic” treats a logarithmic scale as linear.

Common Mistakes

• Following solute instead of water: In osmosis, state the net direction of water across the membrane.
• Reversing transcription and translation: Transcription makes RNA; translation uses RNA to build protein.
• Reading probability as certainty: Each offspring is a new event.
• Changing subscripts to balance: Adjust coefficients only.
• Treating pH as linear: Convert the unit difference into a power of ten.

Knowledge Check

1. Osmosis

A cell has more nonpenetrating solute inside than outside. What is the net direction of water movement?

Answer: Into the cell. Water moves net toward the side with more nonpenetrating solute, so the cell is likely to swell.

2. Information Flow

Which process occurs at a ribosome: transcription or translation?

Answer: Translation. The ribosome reads messenger RNA and links amino acids; transcription produces the RNA copy from DNA.

3. Genetics

For Aa × Aa, what is the probability of an aa genotype?

Answer: One of four equally likely pairings is aa, so the probability is 25%. This does not guarantee one aa child in every group of four.

4. Conservation

Is 2H₂ + O₂ → 2H₂O balanced?

Answer: Yes. Each side has four hydrogen atoms and two oxygen atoms, so the count of every element is conserved.

5. pH

Compared with pH 6, how much greater is the hydrogen-ion concentration at pH 4?

Answer: The two-unit difference gives 10² = 100. The pH 4 solution has about 100 times the hydrogen-ion concentration.

Independent Practice

1. Predict what happens to a cell placed in a solution with equal nonpenetrating-solute concentration.
2. Transcribe the DNA template sequence ACG TTA into messenger RNA.
3. Draw and interpret a Punnett square for Tt × tt.
4. Balance H₂ + Cl₂ → HCl, then count each element on both sides.
5. Compare the hydrogen-ion concentrations of pH 3 and pH 7.
6. Explain why adding water lowers the concentration of a salt solution when the salt amount stays constant.

Self-check: Name the rule before calculating: selective movement, complementary pairing, allele probability, atom conservation, or powers of ten. Then verify that your answer follows that rule.