Edexcel IAL Biology Unit 2 Complete Revision Guide: Membranes, Transport, Genes and Health

Cell Membrane Structure and the Fluid-Mosaic Model

• Fluid-Mosaic Model Architecture:

  • The cell-surface membrane is structured as a dynamic phospholipid bilayer containing a mosaic of embedded proteins, cholesterol molecules, glycoproteins, and glycolipids.

  • The continuous bilayer provides the fundamental structural matrix of the membrane, allowing lateral movement of lipids and proteins that contributes to membrane fluidity.

• Phospholipid Orientation and Properties:

  • Individual phospholipids consist of a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty-acid tails.

  • In an aqueous environment, phospholipids spontaneously arrange into a double layer: the polar, hydrophilic phosphate heads face outward toward the aqueous interior (cytoplasm) and exterior (extracellular fluid) of the cell, while the non-polar, hydrophobic fatty-acid tails orient inward, shielded from water.

• Role of Membrane Components:

  • Cholesterol: Intersperses between phospholipid fatty-acid tails to regulate fluidity and mechanical stability, preventing the membrane from becoming overly fluid at high temperatures or excessively rigid at low temperatures.

  • Intrinsic (Integral) Proteins: Span the entire width of the phospholipid bilayer, functioning primarily as transport proteins such as channel proteins and carrier proteins.

  • Extrinsic (Peripheral) Proteins: Confined to the inner or outer surface of the bilayer, frequently interacting with intrinsic proteins or cytoskeletal elements.

  • Glycoproteins and Glycolipids: Carbohydrate chains covalently bound to surface proteins or lipids that function in cell recognition, cellular adhesion, and as receptor sites for chemical signals.

• Membrane Permeability and Temperature Dynamics:

  • Membrane permeability increases progressively as temperature rises due to the increased kinetic energy of the phospholipids, which causes wider transient gaps in the bilayer.

  • At elevated temperatures, intrinsic and extrinsic membrane proteins undergo irreversible denaturation, destroying selective permeability and causing catastrophic structural breakdown.

  • Beetroot Practical Investigation: Beetroot tissue contains a water-soluble red pigment (betalain) housed within vacuoles; when exposed to increasing temperatures, pigment leakage into surrounding fluid increases significantly because elevated temperatures increase bilayer fluidity and denature tonoplast and plasma membrane proteins.

Transport Mechanisms Across Cell Membranes

• Passive Transport Mechanisms:

  • Simple Diffusion: The passive net movement of solute molecules down a concentration gradient from an area of higher concentration to an area of lower concentration, driven entirely by intrinsic kinetic energy without metabolic energy input.

  • Facilitated Diffusion: The passive movement of large, polar, or charged molecules across the membrane down their concentration gradient via specific intrinsic membrane transport proteins (channel proteins or carrier proteins) without ATP hydrolysis.

  • Osmosis: The specialized net diffusion of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

• Water Potential Characteristics:

  • Pure water is assigned the maximum water potential of $0\,\text{kPa}$ at standard atmospheric pressure and temperature.

  • The addition of any solute lowers the chemical potential of water, resulting in a negative water potential value ($< 0\,\text{kPa}$); higher solute concentrations yield increasingly negative water potentials.

• Active Transport:

  • The movement of specific ions or molecules across a cellular membrane against their concentration gradient (from low concentration to high concentration).

  • Requires intrinsic carrier proteins (pumps) and direct expenditure of metabolic energy derived from the hydrolysis of adenosine triphosphate ($\text{ATP}$).

• Bulk Transport Processes:

  • Endocytosis: The bulk uptake of substances into the cell via the invagination and pinching off of the cell-surface membrane to form intracellular vesicles.

  • Phagocytosis: The endocytic ingestion of large, solid particles or whole cells.

  • Pinocytosis: The endocytic non-specific uptake of extracellular liquids and dissolved solutes.

  • Exocytosis: The bulk release of substances from the cell; intracellular vesicles migrate to and fuse with the cell-surface membrane, discharging their internal contents into the extracellular space.

Protein Structure, Classification, and Enzyme Kinetics

• Amino Acid Monomers and Peptide Bonding:

  • All amino acids share a central carbon atom bonded to an amino group ($-\text{NH}_2$), a carboxyl group ($-\text{COOH}$), a hydrogen atom ($-\text{H}$), and a variable side chain ($-\text{R}$ group).

  • Condensation reactions join adjacent amino acids through the formation of covalent peptide bonds ($-\text{CONH}-$), releasing a water molecule ($\text{H}_2\text{O}$).

  • Polypeptides undergo cleavage into individual amino acids through hydrolysis reactions, requiring the addition of water.

• Hierarchical Levels of Protein Organization:

  • Primary Structure: The specific linear sequence of amino acids in a polypeptide chain, determined entirely by the genetic code.

  • Secondary Structure: Localized spatial folding or coiling of the polypeptide backbone, stabilized by hydrogen bonds between the carbonyl oxygen and amino hydrogen groups (forming structures such as $\alpha$-helices and $\beta$-pleated sheets).

  • Tertiary Structure: The overall three-dimensional conformation of a single polypeptide chain, maintained by interactions between variable $\text{R}$ groups, including hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions.

  • Quaternary Structure: The functional association of two or more distinct polypeptide subunits, or the incorporation of non-polypeptide prosthetic groups.

• Functional Classes of Proteins:

  • Globular Proteins: Compact, spherical, and generally water-soluble proteins that carry out metabolic and physiological functions (e.g., metabolic enzymes and haemoglobin).

  • Fibrous Proteins: Extended, rope-like, and insoluble proteins characterized by repeating amino-acid sequences that serve structural roles (e.g., collagen in connective tissues and keratin in skin/hair).

• Enzymatic Catalysis and Influencing Factors:

  • Enzymes act as biological catalysts that accelerate the rate of chemical reactions by lowering the required activation energy.

  • Each enzyme possesses an active site with a specific three-dimensional shape that is complementary to its substrate, allowing the formation of an enzyme-substrate ($\text{ES}$) complex.

  • Catalytic activity is directly modulated by temperature, $\text{pH}$, substrate concentration, enzyme concentration, and the presence of competitive or non-competitive inhibitors.

Nucleic Acid Structure and DNA Replication

• Deoxyribonucleic Acid (DNA) Architecture:

  • DNA is organized as a double-stranded helix consisting of two antiparallel polynucleotide chains running in opposite directions ($5' \rightarrow 3'$ and $3' \rightarrow 5'$).

  • Each nucleotide subunit is composed of a phosphate group, a deoxyribose pentose sugar, and one of four nitrogenous bases.

  • Strands are held together by complementary base pairing across the double helix: Adenine ($\text{A}$) pairs specifically with Thymine ($\text{T}$), and Cytosine ($\text{C}$) pairs specifically with Guanine ($\text{G}$).

• Ribonucleic Acid (RNA) Differences and Types:

  • RNA is single-stranded, contains a ribose pentose sugar instead of deoxyribose, and substitutes the pyrimidine base Uracil ($\text{U}$) in place of Thymine ($\text{T}$).

  • Messenger RNA (mRNA): Transcribes the genetic code from nuclear DNA and delivers it to ribosomes.

  • Transfer RNA (tRNA): Transports specific amino acids to ribosomes during translation, aligning them via anticodons.

  • Ribosomal RNA (rRNA): Forms structural and catalytic components of the ribosome.

• Semi-Conservative DNA Replication:

  • DNA replication produces two identical daughter DNA molecules, each retaining one intact original template strand and one newly synthesized strand.

  • DNA Helicase: Unwinds the double helix and breaks the hydrogen bonds between complementary base pairs, exposing single-stranded template bases.

  • DNA Polymerase: Catalyzes the condensation reactions that join incoming free phosphorylated nucleotides to the newly forming complementary strand.

• Characteristics of the Genetic Code:

  • Triplet Code: A sequence of three consecutive nitrogenous bases (a codon) codes for one specific amino acid.

  • Degenerate: Most amino acids are encoded by more than one distinct triplet codon, buffering against point mutations.

  • Non-Overlapping: Each base within the coding sequence belongs to only one codon and is read sequentially without sharing bases between adjacent triplets.

Protein Synthesis and Gene Mutations

• Transcription (Nuclear Phase):

  • Takes place inside the nucleus, where a specific section of DNA unzips to act as a template for RNA synthesis.

  • RNA polymerase aligns free RNA nucleotides along the antisense template strand through complementary base pairing and covalently links them into a primary transcript.

  • In eukaryotic organisms, this initial transcript (pre-mRNA) contains both coding sequences (exons) and non-coding sequences (introns); post-transcriptional splicing excises the introns and splices exons together to produce mature mRNA.

• Translation (Ribosomal Phase):

  • Occurs in the cytoplasm at ribosomes, where mature mRNA attaches to the small ribosomal subunit.

  • Specific tRNA molecules bearing complementary anticodon sequences bind to corresponding mRNA codons.

  • Ribosomal enzymes catalyze condensation reactions forming peptide bonds between adjacent amino acids, sequentially elongating the growing polypeptide chain until a stop codon is reached.

• Gene Mutations and Their Consequences:

  • Gene mutations represent permanent alterations in the nucleotide base sequence of DNA, including:

  • Base Substitution: One base is replaced by another; may result in a silent mutation (due to code degeneracy), a missense mutation (altering one amino acid), or a nonsense mutation (generating a premature stop codon).

  • Base Insertion: Addition of one or more extra nucleotides into the sequence.

  • Base Deletion: Removal of one or more nucleotides from the sequence.

  • Frameshift Mutations: Insertions and deletions that occur in numbers not divisible by three disrupt the entire reading frame, altering every subsequent codon downstream and typically yielding a non-functional truncated protein.

Patterns of Inheritance and Monohybrid Genetics

• Fundamental Genetic Terminology:

  • Gene: A defined length of DNA that codes for a specific polypeptide chain.

  • Allele: An alternative structural or functional form of a particular gene locus.

  • Genotype: The specific combination of alleles possessed by an organism.

  • Phenotype: The observable physical, physiological, or biochemical characteristics displayed by an organism, resulting from genotype-environment interactions.

• Dominance Relationships:

  • Dominant Alleles: Expressed in the phenotype whenever present, whether in homozygous or heterozygous states.

  • Recessive Alleles: Masked by dominant alleles; only phenotypically expressed when homozygous.

  • Codominance: Occurs when both alleles at a locus are fully and simultaneously expressed in a heterozygote (e.g., the sickle-cell allele).

  • Multiple Alleles: The existence of three or more distinct alleles for a single gene locus within a population (e.g., the ABO blood group system).

  • Sex Linkage: Genes located exclusively on a sex chromosome (typically the non-homologous region of the X chromosome) exhibit unequal phenotypic transmission patterns between males and females due to hemizygosity in males.

• Monohybrid Crosses and Test Crosses:

  • Monohybrid inheritance tracks the segregation of alleles for a single gene locus using Punnett squares.

  • A test cross involves breeding an individual displaying a dominant phenotype (unknown genotype: homozygous dominant or heterozygous) with a homozygous recessive individual to deduce the unknown genotype from offspring phenotypic ratios.

Genetic Disorders, Screening Methodologies, and Ethical Considerations

• Specific Inherited Disorders:

  • Cystic Fibrosis: An autosomal recessive condition caused by mutations in the gene encoding the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) protein; non-functional CFTR chloride ion channels disrupt osmotic balance, causing the secretion of thick, sticky mucus that obstructs respiratory and digestive systems.

  • Sickle-Cell Anaemia: Caused by a codominant allele that encodes an abnormal variant of the $\beta$-globin chain in haemoglobin; under low oxygen conditions, abnormal haemoglobin polymerizes, distorting erythrocytes into sickle shapes.

  • Huntington Disease: An autosomal dominant neurodegenerative condition characterized by progressive motor and cognitive decline, typically manifesting later in adult life.

• Screening Technologies and Genetic Counselling:

  • Genetic screening identifies asymptomatic carriers of recessive disease alleles as well as affected individuals prenatally or postnatally.

  • Genetic counselling provides individuals and prospective parents with information regarding recurrence risks, clinical prognosis, and ethical, legal, and social implications.

• Prenatal Diagnostic Methods:

  • Amniocentesis: Involves extracting amniotic fluid containing fetal cells via a transabdominal needle (typically performed around 15–20 weeks of gestation); carries a distinct procedure-related risk of miscarriage.

  • Chorionic Villus Sampling (CVS): Involves sampling placental chorionic villus tissue transcervically or transabdominally (performed earlier, around 10–14 weeks of gestation); allows earlier decision-making but carries a slightly higher risk of fetal loss compared to amniocentesis.

Examination Strategies and Technical Requirements

• Membrane Transport Protocol:

  • Explicitly state the exact direction of substance movement (e.g., "from extracellular fluid into the cytoplasm").

  • Always classify whether the process is active or passive, and explicitly state whether movement is "down" or "against" the concentration gradient.

• Osmosis and Water Potential Analysis:

  • Never state simply that "water moves in" or "water moves out."

  • State that there is a net movement of water by osmosis down a water potential gradient (from an area of higher/less negative water potential to an area of lower/more negative water potential across a partially permeable membrane).

• Genetic Problem Conventions:

  • Designate appropriate, non-ambiguous allele symbols (distinguishing upper and lower-case letters clearly).

  • Systematically state the parental phenotypes, parental genotypes, possible gamete genotypes, complete Punnett square configurations, and final phenotypic ratios of offspring.

• Mutational Impact Explanations:

  • When explaining mutations, establish a sequential causal chain:

  • Identify the precise change in the DNA base sequence.

  • Detail the altered mRNA codon sequence.

  • Explain the resulting change in the primary amino-acid sequence of the polypeptide.

  • Detail how this disrupts specific bonding (hydrogen, ionic, disulfide) and alters the tertiary structure.

  • Conclude with the loss or modification of protein function (e.g., active site no longer complementary to substrate).