Structure of the Mammalian Lung and Gas Exchange

The mammalian lung is highly adapted for efficient gas exchange in mammals A Level Biology. Its structure includes:

Trachea: The entrance to the gas exchange system, protected by cartilage rings.

Bronchi: The trachea divides into two bronchi, composed of cartilage and smooth muscle.

Bronchioles: Bronchi branch into smaller bronchioles throughout the lungs.

Alveoli: Tiny air-filled sacs where gas exchange occurs.

Highlight: The alveoli are crucial for gas exchange, with oxygen diffusing into the bloodstream and carbon dioxide diffusing out.

The rate of gas exchange by diffusion is increased by:

1. Decreasing diffusion distance
2. Steepening the diffusion gradient
3. Increasing the surface area for exchange

Definition: Fick's law describes the rate of diffusion across a membrane, which is proportional to the concentration difference and surface area, and inversely proportional to the distance.

Adaptations of the lungs for efficient gas exchange include: • Large surface area due to millions of alveoli • Short diffusion distance - alveoli and capillaries are only one cell thick • Good blood supply to alveoli from a dense capillary network • Concentration gradient maintained by blood flow and ventilation

Protein Structure and Function

Proteins are essential biomolecules with diverse structures and functions. Their structure is determined by:

Primary structure: The sequence of amino acids joined by peptide bonds.

Secondary structure: 2D arrangement of amino acid chains, such as alpha helices or beta pleated sheets.

Tertiary structure: 3D folding of the secondary structure into a complex shape.

Quaternary structure: 3D arrangement of multiple polypeptide subunits.

Example: Hemoglobin, a globular protein, consists of four polypeptide chains and four heme groups, crucial for oxygen transport in gas exchange AQA A Level Biology.

Proteins can be fibrous (e.g., collagen) or globular (e.g., enzymes). Fibrous proteins provide structural support, while globular proteins often have metabolic functions.

DNA Replication and Protein Synthesis

DNA replication is a semi-conservative process essential for genetic continuity:

1. DNA helicase unwinds the double helix, breaking hydrogen bonds between strands.
2. Free nucleotides form hydrogen bonds with complementary bases on the template strand.
3. DNA polymerase forms phosphodiester bonds between adjacent nucleotides.

Vocabulary: Semi-conservative replication means each new DNA molecule consists of one original strand and one newly synthesized strand.

Protein synthesis occurs in two main stages:

Transcription: DNA is transcribed into mRNA in the nucleus.

Translation: mRNA is translated into a polypeptide chain at ribosomes in the cytoplasm.

Definition: The genetic code is the set of rules by which information encoded in genetic material is translated into proteins. It is non-overlapping, degenerate, and universal.

Mutations can occur during DNA replication or due to environmental factors: • Substitution: One base is replaced by another • Insertion: One or more bases are added • Deletion: One or more bases are removed

These concepts are fundamental to understanding DNA replication A level Biology OCR and AQA A Level Biology DNA replication exam questions.