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:
- Decreasing diffusion distance
- Steepening the diffusion gradient
- 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:
- DNA helicase unwinds the double helix, breaking hydrogen bonds between strands.
- Free nucleotides form hydrogen bonds with complementary bases on the template strand.
- 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.
