Fitness of Biological Molecules (College Board AP Biology)

• All events in biology can be explained at a molecular level
  • Most of the variation between cells, between tissues and between organisms and species has a molecular explanation too
• The concept of a cell or organism having the right molecules to do its job is known as fitness
• It is sometimes helpful to think of a cell as a giant 3-D jigsaw puzzle, with molecules being the pieces
  • Although the pieces move around at great speed, colliding with each other, fitting together momentarily and falling apart again after a reaction has taken place
  • If molecules don't fit together, they bounce off each other and no reaction can take place
  • A cell has to consist of the right molecules or it won't be able to perform its function
• Variation at a molecular level gives organisms the ability to respond to different stimuli
• This provides a survival advantage to the organism, increasing the chances of reproduction in a variety of different environments

Cell Specialization Diagram

Cell specialization happens because of the molecular structure of each of the cell components

• A cell has to have the right mixture and number of molecules in order to carry out its functions
• Examples: 
  • The variation between cells depends on which genes (lengths of DNA) are being expressed
    • All somatic cells possess the organism's whole genome, with some genes being expressed (switched on) and others suppressed (switched off)
    • The expression of genes relies on molecules being exposed to each other; for example, DNA polymerase must have physical access the coding DNA strand for transcription to occur
    • Having the right suppressor protein that fits over a gene coding sequence can stop a gene being expressed
    • This can last for the whole of a human's postnatal life, in the case of fetal hemoglobin, a protein which is only needed whilst a fetus is in the uterus; just prior to birth, to prepare for the start of breathing, adult hemoglobin is expressed in red blood cells
  • The growth and development of an organism can depend on the availability of food molecules within its habitat
  • The permeability or impermeability of a cell membrane to a certain substance depends on the membrane having a specific make-up of membrane proteins as channels/carriers

Examples of Molecules Giving Cells Their Fitness

Phospholipid Types in Membranes

• The various types of phospholipid can give membranes different properties
• The extent of hydrocarbon chain saturation affects the stacking of the fatty acid chains
  • This impacts fluidity of the membrane;
  • The more saturated hydrocarbon chains in the fatty acid tails, the more rigid a membrane
  • Conversely, the more unsaturated hydrocarbon chains in the fatty acid tails, the more fluid a membrane becomes
• An engulfing white blood cell such as a monocyte or neutrophil must have a highly fluid membrane because the membrane has to flex and bend for the cell to fulfil its function of engulfing pathogens and other cellular waste
• A more rigid animal cell such as a tissue epithelial cell will have a greater composition of saturated fatty acid tails
• Cholesterol also plays a role in membrane fluidity in animal cells (plants employ other steroid-based compounds)
  • Cholesterol's structure is amphipathic
  • This property allows cholesterol to sit within and disrupt the phospholipid bilayer and makes the membrane less fluid at high temperatures
  • At low temperatures, cholesterol has the opposite effect and maintains fluidity by helping to stop membranes from freezing solid

Phospholipid Structure Affects Fitness of a Membrane

Phospholipid structure affects the fitness of a membrane; its composition makes the membrane more fluid or more rigid as needed by the cell

Different Types of Hemoglobin Affect Fitness

• Hemoglobin is a globular protein which is an oxygen-carrying pigment found in vast quantities in red blood cells
• Red blood cells are biconcave discs, meaning that they are concave on both sides
• This creates a high surface area-to-volume ratio for the diffusion of gases
• Red blood cells do not contain a nucleus
  • This provides more space inside the cell for hemoglobin so that they can transport as much oxygen as possible
• Hemoglobin is responsible for binding oxygen in the lungs and transporting the oxygen to the tissue to be used in aerobic metabolic pathways
• As oxygen is not very soluble in water and hemoglobin is, oxygen can be carried more efficiently around the body when bound to the hemoglobin
• Hemoglobin has a quaternary structure as it is made up of four polypeptide chains
  • These chains or subunits are globin proteins (two α–globins and two β–globins) and each subunit has a prosthetic heme group
  • The four globin subunits are held together by disulfide bonds and arranged so that their hydrophobic R groups are facing inwards, helping to preserve the 3-D spherical shape, and the hydrophilic R groups are facing outwards, helping to maintain solubility
  • The arrangements of the R groups is important to the functioning of hemoglobin; if changes occur to the sequence of amino acids in the subunits this can change the function of the protein, e.g.
    • In sickle cell anaemia a base substitution that results in the amino acid valine (nonpolar) replacing glutamic acid (polar) makes hemoglobin less soluble
  • The prosthetic heme group contains an iron II ion (Fe²⁺) which is able to combine reversibly with an oxygen molecule, forming oxyhemoglobin 
    • The presence of oxyhemoglobin causes oxygenated blood to appear bright red in color
    • Each hemoglobin with the four heme groups can therefore carry four oxygen molecules, or eight oxygen atoms
    • The heme group is the same for all types of hemoglobin but the globin chains can differ substantially between hemoglobins from different species 

How Hemoglobin Binds Oxygen Diagram

The binding of hemoglobin and oxygen

Adult and Fetal Hemoglobin Are Different

• The human genome contains separate genes for two types of hemoglobin, adult and fetal
• Both are different molecules that have different affinities for oxygen
  • Fetal hemoglobin must bind oxygen from the mother's blood across the placenta and so needs a higher affinity to oxygen to maximise absorption across the placenta
  • Adult hemoglobin must bind oxygen directly from the air that enters the alveoli, as oxygen concentrations are very high in the lungs, the affinity of the adult hemoglobin does not need to be as high as fetal hemoglobin
• The fetal hemoglobin gene is always switched on during growth in the uterus
• Before birth, the adult hemoglobin gene begins being expressed to prepare the newborn baby for its first breathing
• After birth, there is no further need for fetal hemoglobin, so that gene is permanently switched off for the rest of the adult's life
  • Although the gene, like any other part of the genome, is passed on in gametes for inheritance