Cellular Physiology

      We will examine two topics concerned with cell physiology:
Membrane Transport  (Animation links: Concepts in Biochemistry - Interactive Animations;  Shockwave Animations
          The plasma membrane exists in an aqueous (water) solution. A solution is a homogeneous mixture of two or more molecules or ions. An example of a solution is seawater, a mixture of water and salts. The molecule in greatest quantity is the solvent and the molecules in smaller quantities are solutes.
       To remain alive the cell must regulate the passage of solutes between the fluid inside the cell, the intracellular fluid, and the fluid outside of the cell, the interstitial fluid. The plasma membrane’s ability to control the passage of molecules across it is called selective permeability.
       The movement of materials into and out of the cell occurs in a variety of ways that can be categorized into two basic processes, passive transport and active transport.
  Passive Transport Processes
     Diffusion is the movement of molecules from a region of high concentration to a region of lower concentration. This is expressed as the molecules moving down their concentration gradients. There are three kinds of diffusion as it relates to movement across the plasma membrane:
  Simple diffusion Biological Animations (Go to diffusion under membrane transport.)
  Molecules may move passively through the plasma membrane if they are 1) small enough to pass through, such as oxygen and carbon dioxide, or 2) can dissolve in the fatty portion of the membrane, such lipid soluble molecules including steroid hormones and fat soluble vitamins. Small ions such as Cl- can pass through the membrane if there are channels that permit their passage.
  Osmosis Biological Animations (Go to osmosis under membrane transport.)
  Osmosis is diffusion as it relates to water and its passage through a selectively permeable membrane. Although water cannot easily pass through the plasma membrane because of its lipid interior, channels called aquaporins are often present in the membrane to allow easy passage of water molecules.
  Osmosis can be confusing because it involves water molecules which are the most abundant molecules in aqueous solutions. A solution with a high concentration of solutes has a lower concentration of water then a solution with a lower concentration of solutes. Hence, water, the solvent, goes down its concentration gradient when it goes from a low concentration (of solutes) solution to the high concentration solution.
  Facilitated diffusion
  Facilitated diffusion involves the passage of molecules that are lipid-insoluble and that cannot normally pass through the plasma membrane. Passage of these molecules through the plasma membrane is made possible by the presence of proteins in the plasma membrane that act as carriers. The carriers therefore act as a kind of “ferry” that gets the molecules across the barrier created by the membrane.
  Active Transport Processes
     Active transport processes require that the cell expend energy in the form of ATP. These processes include:
  Solute pumping (active transport)
     Solute pumping involves the use of protein carriers as does facilitated diffusion but with solute pumping the transported molecules are moved against their electrochemical gradients. This requires the expenditure of ATP’s. Amino acids, some sugars, and most ions are transported by solute pumps.
     The sodium-potassium pump (Animation of Sodium-Potassium Pump ;  Animation How the Sodium Potassium Pump Works) is an important example of solute pumping. There is a larger concentration of K+ inside the cell than outside, and a larger concentration of Na+ outside the cell than inside. Both ions will go down their concentration gradients if left alone. The sodium-potassium pump maintains these gradients by pumping Na+ out of the cell while at the same time pumping K+ ions into the cell.
  Bulk transport
     Bulk transport involves the passage of substances that are too large to pass through the plasma membrane by any of the means discussed thus far. There are two categories of bulk transport:
  Exocytosis Exocytosis Animation
  Exocytosis is the means by which large amounts of substances are moved out of the cell. The substances to be released are packaged in small membranous vesicles. The vesicles migrate to the plasma membrane, fuse with it, and the substances then spill out of the cell.
  Endocytosis is the means by which large amounts of materials are brought into the cell. Substances to be brought into the cell are surrounded by the plasma membrane and the membrane then pinches off from the plasma membrane to form a membranous vesicle within the cell. There are three kinds of endocytosis:
  Phagocytosis Phagocytosis Movie
  Phagocytosis is also called “cell eating” as it involves endocytosis of large particles such as bacteria or dead or worn-out cells. The particles are engulfed by cytoplasmic extensions of the cell called pseudopods. The pseudopods completely surround the materials and pinches off from the plasma membrane to from a membranous vesicle.
  Pinocytosis is also called “cell drinking”. In this process, the plasma membrane invaginates (indents) to form a membranous vesicle containing fluid containing dissolved proteins and fats.  
  Receptor-mediated endocytosis Receptor-mediated Endocytosis
  This form of endocytosis is selective. Receptors in the plasma membrane attach to selected target molecules that may include enzymes, some hormones, cholesterol and iron. The patch of membrane containing the receptors with their attached molecules then enters the cell, as in pinocytosis, as a membranous vesicle.


Protein Synthesis Biological Animations
  Genes: The Blueprint for Protein Structure
     DNA is the master blueprint for protein synthesis. Segments of DNA encode the information for building proteins. Therefore, DNA regulates all aspects of the cell including its shape and structure, as determined by its structural proteins, and all cellular reactions and activities, as determined by its functional proteins, or enzymes.
       DNA controls protein synthesis by encoding the sequence of amino acids of a protein in the form of a sequence of three bases called a triplet. For example, a DNA base sequence of AAA specifies an amino acid called phenylalanine, while CCT calls for another amino acid called glycine.
  The Role of RNA
     The information for synthesis of protein contained in DNA is coded. The instructions for actual protein synthesis is carried out by RNA.
     Messenger RNA plays the role of copying the code contained in the triplets of DNA which is located in the nucleus and delivering the information to the protein assembly site, the ribosome, in the cytoplasm. The process of copying this code is called transcription. The bases of DNA are copied as complementary bases in the messenger RNA. Each of the four bases in the DNA sequence can pair with only one of the four bases in RNA as shown below:
  thymine pairs with adenine
  adenine pairs with uracil
  guanine pairs with cytosine
  cytosine pairs with guanine
      For example, suppose a segment of DNA had the following three triplets ATT-GCT-TAG. This information is transcribed as messenger RNA with the following base sequence UAA-CGA-AUC. Each of the new three base sequences in the messenger RNA is now called a codon because it encodes an amino acid.
     Messenger RNA delivers its encoded information to the ribosome where the process of translation occurs. Translation converts the sequence of codons contained in messenger RNA into the sequence of amino acids of a protein (technically referred to as a polypeptide). Messenger RNA moves along the ribosome in a precise direction as the codons are "read". The codons are read by transfer RNA.
        The transfer RNA molecule contains a sequence of three bases that matches the codon in messenger RNA. This sequence is called the anticodon. One part of the transfer RNA contains the anticodon, and another part combines with one of the amino acids that are used to synthesize proteins.
       As the messenger RNA moves along the ribosome the codons attach to a complementary anticodon on the transfer RNA that brings its attached amino acid into position. This amino acid is now next to the amino acid carried by the transfer RNA that preceded it. Enzymes that are part of the ribosome then promote the linking of the amino acids by a peptide bond. The messenger RNA then advances along the ribosome until the next codon is in place to receive the next transfer RNA molecule and the same steps are repeated. This process continues as the protein molecule grows in length.