Topic 2 - Membranes, Proteins, DNA and Gene Expression

A gas-exchange surface is effective when it has a large area, a short diffusion distance and a steep concentration gradient. Fick’s law summarises this as rate ∝ surface area × concentration difference ÷ barrier thickness.

Alveoli provide a huge folded surface, a thin moist epithelium and a dense capillary network. Ventilation refreshes alveolar air and blood flow removes oxygen, maintaining the gradient.

If surface area doubles while thickness and gradient stay constant, the predicted diffusion rate doubles. If mucus thickens the barrier, diffusion slows even if oxygen concentration outside remains unchanged.

Small organisms can rely on their body surface because their surface-area-to-volume ratio is high. Larger organisms need specialised surfaces and transport systems to keep internal cells within diffusion distance.

The bilayer is fluid because phospholipids and some proteins move laterally. It is selectively permeable: small non-polar molecules cross the lipid core more readily, while ions and many polar molecules need specific channel or carrier proteins.

Models are interpretations of evidence. Electron micrographs established a very thin layered boundary; freeze-fracture images revealed particles within the bilayer, supporting embedded proteins rather than continuous protein coats. Labelling and mobility experiments showed that proteins are distributed unevenly and some can move laterally.

New data replaced simpler sandwich models with the fluid-mosaic explanation. The model can still be refined when evidence shows proteins anchored to cytoskeleton or organised into local domains.

1. Cut equal beetroot cylinders or discs and rinse until surface pigment is removed.
2. Place equal tissue amounts in equal solution volumes for the same time.
3. For temperature, use thermostatically controlled water baths; for alcohol, prepare a concentration series while holding temperature constant.
4. Remove the tissue and measure solution absorbance with a colorimeter.
5. Use a blank, repeat each condition and calculate a mean.

Higher temperature increases phospholipid movement and can denature membrane proteins; extreme heating disrupts membrane organisation. Alcohol interacts with membrane lipids and proteins, so increasing alcohol concentration can also release more pigment.

Control disc dimensions, beetroot source, rinsing, solution volume, exposure time, pH and colorimeter wavelength. Treat temperature and alcohol as separate independent-variable investigations.

Osmosis is the net movement of water across a partially permeable membrane from higher water potential to lower water potential. Water potential combines pressure effects and solute effects, so it is not simply “water concentration”.

A concentrated solution has a more negative solute potential. Water leaves a cell placed in a lower-water-potential solution, reducing turgor; water enters when the external solution has higher water potential.

If potato cylinders lose mass in a sucrose solution, the solution had a lower water potential than the potato cells. The concentration at which there is no net mass change estimates the tissue’s initial water potential.

Use equal cylinders, blot them consistently, control time and temperature, repeat at several concentrations and plot percentage mass change rather than relying on one sample.

Channels form hydrophilic pores and may be gated. Carriers bind a particular solute and change conformation; the same broad protein type can mediate facilitated diffusion or active transport depending on direction and energy coupling.

Primary structure is the amino-acid sequence. Hydrogen bonding creates secondary structure. Further folding produces tertiary structure through hydrogen bonds, ionic attractions, disulfide bonds and hydrophobic interactions; several polypeptides may form quaternary structure.

The R-group sequence fixes which interactions can form, so a primary-structure change can alter folding, shape and function. Globular proteins expose suitable hydrophilic groups to water; fibrous proteins form long structural assemblies.

In an induced-fit model, substrate binding changes the active site's shape slightly, positioning reacting groups and straining bonds. Products no longer fit and leave, so the enzyme can catalyse another reaction.

Intracellular enzymes catalyse reactions inside cells, such as steps of respiration. Extracellular enzymes are secreted and act outside cells, such as digestive enzymes hydrolysing large food molecules before absorption.

Temperature or pH outside the working range can disrupt bonds that maintain the active site. Substrate may then collide with the enzyme but fail to form a productive complex.

To investigate temperature, pH, enzyme concentration or substrate concentration, measure the initial rate of an enzyme reaction while changing one independent variable and controlling the others.

Use equal volumes, a defined mixing time and a signal that changes predictably, such as colour, gas volume or product concentration. Take several early readings, calculate a gradient and repeat at each condition.

Initial rate avoids later distortion when substrate is depleted or product accumulates. A water bath, buffer and matched enzyme volumes make the comparison fair.

A rate–temperature curve rises while collisions become more frequent, reaches an optimum and falls when the protein begins to lose structure. A plateau with increasing substrate may indicate that active sites are saturated.

A nucleotide contains a pentose sugar, phosphate group and nitrogenous base. DNA nucleotides contain deoxyribose and A, T, C or G; RNA nucleotides contain ribose and A, U, C or G.

Nucleotides join by phosphodiester bonds between the phosphate and sugars, forming a sugar–phosphate backbone. Complementary bases pair through hydrogen bonds: A with T in DNA or U in RNA, and C with G.

The base sequence stores information while the backbone gives the polymer continuity. The 2′ hydroxyl in ribose makes RNA chemically less stable than DNA and suits its shorter-lived information-carrying roles.

A DNA template with the base A pairs with a T in the complementary strand; an RNA transcript uses U instead. Replacing T with U is a molecule-level difference, not a change in the genetic message by itself.

Before cell division, DNA strands separate and each acts as a template. Free nucleotides pair by complementarity, DNA polymerase joins them into new strands, and each daughter DNA molecule contains one original and one new strand.

Helicase separates the strands; complementary nucleotides align; DNA polymerase forms phosphodiester bonds in the new strand. The two molecules then carry the same base sequence, apart from copying errors.

Meselson and Stahl grew bacteria with heavy nitrogen, moved them to light nitrogen and separated DNA by density. An intermediate-density band after one generation supported semi-conservative replication; later generations produced light and intermediate DNA.

The density pattern distinguishes semi-conservative replication from conservative and dispersive alternatives because it tracks old and newly synthesised DNA together.

The genetic code is a triplet code: each three-base codon specifies one amino acid or a start/stop signal. Codons are read sequentially and non-overlapping on mRNA.

There are more codons than amino acids, so the code is degenerate: several codons can specify the same amino acid. This can make some substitutions silent but does not make every mutation harmless.

Changing a base in a codon may leave the amino acid unchanged, replace it with another, or create a stop codon. The outcome depends on the codon and on where it occurs in the coding sequence.

Translate from the correct start codon in the correct reading frame, then stop at a stop codon. A one-base insertion can shift every downstream triplet.

The order of DNA bases determines the complementary mRNA codons made during transcription. Ribosomes read those codons during translation, and tRNAs bring amino acids in the corresponding order. Peptide bonds then join the amino acids into a polypeptide.

Changing one DNA base can alter an mRNA codon and therefore one amino acid. If that change affects folding or an active site, the protein's function may change; degeneracy means some base substitutions do not change the amino acid.

Protein synthesis has two linked stages: transcription copies a gene into mRNA, then translation reads the mRNA to assemble a polypeptide. The DNA template (antisense) strand is used for transcription; codons are read on mRNA and anticodons pair on tRNA.

During transcription, RNA polymerase opens the DNA locally and joins complementary RNA nucleotides. The mRNA leaves the nucleus and binds to a ribosome. In translation, tRNA brings amino acids whose anticodons match successive mRNA codons; peptide bonds join the chain from the start codon until a stop codon.

A template triplet TAC produces an mRNA codon AUG. The ribosome recognises AUG as the start signal and the matching tRNA brings methionine; later codons determine the rest of the sequence.

Many mutations have no observable effect because they occur outside a relevant coding sequence, produce the same amino acid, or do not alter protein function. Others cause genetic disorders when a required protein changes. Mutations affecting cell-cycle control genes can contribute to cancer.

A dominant allele determines the heterozygous phenotype; a recessive phenotype appears when no dominant allele is present. In codominance, both different alleles contribute distinctly to the heterozygous phenotype.

For monohybrid inheritance, assign allele symbols, write parental genotypes, derive gametes and combine them in a genetic cross. In a pedigree, use affected offspring, parental phenotypes and sex distribution to test possible genotypes; probabilities are not guarantees for a small family.

Red-green colour blindness is commonly caused by a recessive allele on the X chromosome. A male expresses the allele on his single X chromosome; a heterozygous female is usually a carrier. Fathers pass their X to daughters and their Y to sons, so there is no father-to-son transmission of an X-linked allele.

Cystic fibrosis is caused by recessive CFTR alleles that produce faulty chloride ion channels. Reduced chloride transport changes water movement by osmosis, making mucus unusually thick in several body systems.

In the airways, sticky mucus is harder for cilia to clear, so infection risk rises and blocked airways reduce gas exchange. In the digestive system, mucus can block pancreatic ducts and hinder enzyme delivery and nutrient absorption. In the reproductive system, thick mucus can obstruct sperm movement or passage.

Two unaffected carrier parents can each pass the recessive allele: the child-risk calculation is 1/4 affected, 1/2 carrier and 1/4 non-carrier. The inheritance probability and the physiological symptoms are linked but are not the same claim.

Genetic screening tests DNA to identify a carrier, assess an embryo before implantation, or test a foetus during pregnancy. It informs a decision; it does not by itself predict every aspect of a person's future health.

Carrier testing checks an asymptomatic person's allele status. Pre-implantation genetic diagnosis analyses embryo cells during IVF before implantation. Prenatal testing uses chorionic villus sampling or amniocentesis to obtain foetal DNA during pregnancy.

Chorionic villus sampling is earlier than amniocentesis, while both can provide information about a familial disorder. A result may help parents prepare or consider options, but sampling carries procedure risks and tests can give false results.

The social and ethical question is not simply whether screening is possible, but how its information should be used. Judgements can concern autonomy, disability, privacy, fairness, pregnancy decisions and the interests of a future child.

Different viewpoints may weigh the same result differently: a family may value preparation or avoiding a severe disorder, while another may stress the risk of pressure, discrimination, unequal access or a narrow idea of which lives are worthwhile.

A carrier result can help a couple plan, yet sharing genetic data with insurers or employers could create harm. Prenatal or embryo results may support informed choice, but false positives, false negatives and procedure risks limit certainty.