Showing posts from July, 2010

Membrane potentials and action potentials

  Electrical potentials exist across membranes. Nerve and muscle cells are excitable - they generate their own electrochemical impulses at their membranes. BASIC PHYSICS OF MEMBRANE POTENTIALS A concentration  difference of ions across a selectively permeable membrane can produce a membrane potential.  Potassium diffusion potential Potassium diffusion potential: The potential difference is - 94 mV Sodium diffusion potential: +61 mV The Nernst equation describes the relation of diffusion potential to concentration difference.  EMF (millivolts = +- 61 log(concentration inside / concentration outside) where EMF is the electromotor force.  The sign of the potential is positive if the ion is negative ion, and negative if it is a positive ion. The membrane potential that prevents net diffusion of another ion in either direction through the membrane is called the Nerst potential for that ion.  The Goldman equation is used to calculate the diffusion potential when the membrane is

Transport of Ions and Molecules Through the Cell Membrane

Differences between the intracellular and the extracellular fluid is brought about by transport mechanisms of cell membranes. Extracellular fluid has a high sodium concentration and low potassium concentration.  Extracellular fluid contains a high chloride concentration The concentration of phosphates and proteins in intracellular fluid are greater than those in extracellular fluid. The cell membrane consists of a lipid bilayer with "floating" protein molecules. This is a barrier for most water-soluble substances. Lipid soluble substances can pass through the lipid bilayer. Protein molecules in the lipid bilayer form an alternative transport pathway: Channel proteins: a watery pathway for molecules to pass through them Carrier proteins: bind with specific molecules then undergo conformation changes that move molecules through the membranes. Transport through the cell membrane occurs through diffusion or active transport.  Diffusion means random movement of molecules. Dri

The DNA-Genetic system also controls cell reproduction

Control exists over growth in the cell and when the cell will divide to form new cells. Most cells are able to reproducing other cells of their own type, with the exception of RBC, striated muscle, and neurons. In the presence of sufficient nutrition, cells grow larger and larger and then through mitosis divide to form two new cells. Cellular division begins with the replication of the DNA.   DNA in the chromosome is replicated once, making an exact copy of the entire DNA before mitosis takes place. The replication of DNA is similar to RNA transcription except for the following important differences: Both strands of DNA are replicated - not only one strand of DNA Both strands of DNA helix are replicated from end to end rather than small portions of them, as in  the transcription of RNA genes. The principal enzyme for replication of DNA are the complex of several enzymes called DNA polymerase (similar to RNA polymerase). Each newly formed strand of DNA remains attached by loose

Genetic Control of Protein Synthesis, Cell Function, and Cell reproduction

Cell genes control protein synthesis. The proteins that a cell produce determine the cell function. Proteins act as catalytic enzymes and physical components of cell structure.  Nucleotides are organised to form two strands of DNA that are loosely bound to each other.  Double stranded helical DNA that are composed of 3 basic building blocks: (1) phosphoric acid, (2) deoxyribose (sugar), and (3) Four nitrogenous basis (two purines, adenine, and guanine, and two pyrimidines, thymine, and cytosine) The genetic code consist of triplets of basis. Each group of three successive basis is called a code word, and these code word control the sequence of amino acids in protein. The sequence of successive code words is called the genetic code.  DNA CODE IS TRANSFERRED TO RNA CODE BY THE PROCESS OF TRANSCRIPTION DNA controls cell function in the cytoplasm through RNA. The process where the DNA code is transferred to RNA is called transcription. The RNA diffuses from the nucleus to the nuc

Functional System of the Cell

Ingestion by the Cell - Endocytosis The cell obtains its nutrients and other substances from the surrounding fluid through Diffusion and active transport. Active transport consists of  endocytosis, the principle forms are pinocytosis and phagocytosis. Pinocytosis means the ingestion of small globules of extracellular fluid forming minute vesicles in the cell cytoplasm. The molecules attach to receptors on the membranes, call coated pits, and on the inside of this cell membrane beneath these pits is latticework of fibrillar protien called clathrin and a contractile filament of actin and myosin. After the protein molecules bind with the receptors, the membrane invaginates, and the contractile protiens surround the pit, causing the boarders to close over the attached proteins and form a pinocytotic vesicle. Phagocytosis is the ingestion of large particles like bacteria, cells, and portions of degenerated tissue. Macrophages and some white cells perform phagocytosis.   Bacteria

The Cell and its Function

Diagram of a typical  animal  ( eukaryotic ) cell, showing subcellular components. Organelles : (1)  nucleolus (2)  nucleus (3)  ribosome (4)  vesicle (5)  rough endoplasmic reticulum  (ER) (6)  Golgi apparatus (7)  Cytoskeleton (8)  smooth endoplasmic reticulum (9)  mitochondria (10)  vacuole (11)  cytoplasm (12)  lysosome (13)  centrioles  within  centrosome Eukaryotic  cells are about 15 times wider than a typical prokaryote and can be as much as 1000 times greater in volume.  Organisation of the Cell. The nucleus and the cytoplasm are separated by the nuclear membrane, The different substances that make up the cell are collectively called the protoplasm. The protoplasm is composed of: Water: 70 to 85% Electrolytes Na,K, Cl, HCO3, Ca, Mg, PO4 Proteins - 10 to 20% -  Structural proteins and Globular proteins (enzymes) Lipids - 2% - Phospholipids, cholesterol, triglycerides, and neutral fats.   Carbohydrates - 1% total body, but 3% of Muscles and 6% of Liver cells. S