Topic 2 - Genes and Health Flashcards
How are the lungs adapted for gas exchange?
The large surface area of the alveoli, the steep concentration gradient between the alveolar air and the blood (maintained by ventilation of the alveoli and the continuous flow of blood through the lungs), and the thin walls of the alveoli and capillaries, combine to ensure rapid diffusion across the gas exchange surface.
Describe Fick’s law.
Increased surface area and a greater concentration gradient increase the rate of diffusion. smaller surface areas reduce the rate of diffusion.
Rate of diffusion is directly proportional to the (surface area x concentration difference) / thickness of the surface.
How does sticky mucus increase the chances of lung infections?
Microorganisms become trapped in the mucus in the lungs and some of these microorganisms cause illness - they are pathogens. The mucus is normally moved by cilia into the back of the mouth cavity where it is either coughed out or swallowed, reducing the risk of infection. The acid in the stomach kills most microorganisms that are swallowed.
In people with CF, the mucus layer is so sticky that the cilia cannot move it. Mucus production still continues as it would in a normal lung, and the thickened mucus build up in airways. There are low levels of oxygen in the mucus, partly because oxygen diffuses slowly through it, and partly because the epithelial cells use up more oxygen in CF patients. Harmful bacteria thrive in these anaerobic conditions.
White blood cells fight the infections within the mucus but as they die they break down and release DNA that makes the mucus even stickier. Repeated lung infections can eventually weaken the body’s ability to fight the pathogens and cause damage to the structures of the gas exchange system.
Describe the primary structure of a protein.
The primary structure of a protein is the sequence of amino acids that make up a polypeptide chain. Two amino acids join in a condensation reaction to form a dipeptide, forming a peptide bond between the two subunits. This process can be repeated to form polypeptide chains with thousands of amino acids. A protein is made up of one or more of these polypeptide chains.
Describe the secondary structure of a protein.
Within one protein molecule, there may be sections with alpha helices and other sections that contain beta-pleated sheets, and some sections may be folded or twisted in a less ordered manner. Interactions between the amino acids in the polypeptide chain cause the chain to twist and fold into a three-dimensional shape. Lengths of the chain may first coil into alpha-helices or come together in Beta pleated sheets. These features are known as the secondary structure.
Within the secondary structure of a protein, how do amino acids interact to form an alpha helix and up to how long can they be?
a. The chain of amino acids in a polypeptide chain may twist to form an alpha helix. Within the helix, hydrogen bonds form between the slightly negative C=O of the carboxylic acid and the slightly positive -NH of the amine group of different amino acids that lie above and below each other, stabilising the shape.
b. Sections of alpha-helix can be up to 35 amino acids long.
Within the secondary structure of a protein, how do amino acids interact to form Beta pleated sheets?
Amino acids within the polypeptide chain may fold back on themselves, or several lengths of the chain, each up to 15 amino acids in length, may link together with hydrogen bonds holding the parallel chains in an arrangement known as Beta pleated sheets. Each hydrogen bond is weak, but the cumulative effect of many hydrogen bonds makes the secondary structure quite stable.
Describe the tertiary structure of a protein.
Chemical bonds and hydrophobic interactions between R groups maintain this final tertiary structure of the protein. Chemical bonds may form between R groups that are close to each other in the folded structure. Hydrogen bonding, hydrophobic or hydrophilic R groups. Additionally, covalent disulphide bond may form if two cysteine (-SH groups) R groups are close to each other. Some amino acids may contain ionised R groups and so ionic bonds can form between positively and negatively charged R groups. These features allow the protein to fold into a tertiary structure.
What makes a three-dimensional structure of polypeptide chains a protein?
If the three-dimensional shape is functional, that is, the molecule is able to perform its specific function, the molecule is now described as a protein. Some proteins may only be functional if they are made up of several polypeptide chains held together.
What are the key properties of disulphide and ionic bonds within the tertiary structure of a protein?
Disulphide and ionic bonds form bonds which are much stronger than hydrogen bonds but are very sensitive to changes in ph.
What is the quaternary structure of a protein?
a. Proteins with more than one polypeptide chain have a quaternary structure, single-chain proteins stop at the tertiary level. It is the interaction between polypeptide chains.
b. The simplest case of a quaternary structure is a dimer. In a dimer, there are two polypeptide chains that constitute the protein. Each individual polypeptide chain is called a subunit. These polypeptide chains can interact usually via non-covalent bonds but sometimes covalent bonds such as disulphide bonds may hold the polypeptide chains.
c. In a dimer we have two subunits, in a trimer, we have three subunits, in a tetramer, we have four subunits, and so on.
What is a conjugated protein?
A conjugated protein has another chemical group associated with their polypeptide chains. For example, the polypeptide chains that make up myoglobin and haemoglobin are associated with an iron-containing group.
Describe globular and fibrous proteins.
a. In globular proteins the polypeptide chain is folded into a compact spherical shape. These proteins are soluble due to the hydrophilic side chains that project from the outside of the molecules and are therefore important in metabolic reactions. Enzymes are globular proteins. Their three-dimensional shape is crucial to their ability to form enzyme-substrate complexes and catalyse reactions within cells.
b. Fibrous proteins do not fold up into a ball shape but remain as long chains. Several polypeptide chains are cross-linked for additional strength. These insoluble proteins are important structural molecules. For example, collagen is a fibrous protein. Three polypeptide chains wind around each other to form a rope-like strand held together by hydrogen bonds between the chains. Each strand cross-links to other strands to produce a molecule with tremendous strength. Notice the strands are staggered avoiding the creation of any weak points along the length of the molecule.
Describe the phospholipid bilayer.
The phosphate head of the molecule is polar. This makes the phosphate head attract other polar molecules like water and therefore is hydrophilic. The fatty acid tails are non-polar and therefore hydrophobic. When added to water phospholipids become arranged with no contact between the hydrophobic tails and water. So, they may form an arrangement where their hydrophobic tails are directed out of the water, they also may become arranged into spherical clusters called micelles or form a bilayer. The bilayer is favoured as the two fatty acids are too bulky to fit into a micelle. The phospholipid bilayer is the most stable arrangement out of the three.
What is the arrangement known as the fluid mosaic model?
The cell surface membrane is not simply a phospholipid bilayer. It also contains proteins, cholesterol, glycoproteins (protein molecules with polysaccharides attached) and glycolipids (lipid molecules with polysaccharides attached). Some of the proteins span the membrane. Other proteins are found only within the inner layer or only within the outer layer. Membrane proteins have hydrophobic areas, and these are positioned within the membrane bilayer. It is thought that some of the proteins are fixed within the membrane, but others are not and can move around in the fluid phospholipid bilayer. This arrangement is known as the fluid mosaic model.
Why was the three-layer protein-lipid sandwich model rejected up and till the early 1970s?
The model was based on an electron micrograph in which the dark outer layers were thought to be proteins and the lighter region within was thought to be the lipid. However, this model does not allow the hydrophilic phosphate heads to be in contact with water, nor does it allow any non-polar hydrophobic amino acids on the outside of the membrane proteins to be kept away from water. Consideration of how lipids behaved in water, forming a bilayer because it is the most stable arrangement, was used to refine the model. Interpretation of the electron micrograph evidence changed to support the new model. The phosphate heads are more electron dense and show up as the darker edges and the lipid tails being the lighter inner part.
What did experiments in the evidence for fluid mosaic models show?
a. Experiments showed that there were two types of proteins – those that can be dissociated (separated) from the membrane quite easily by increasing the ionic strength of the solution and those (the majority) that could only be removed from the membrane by more drastic action such as adding detergents. This supported the fluid mosaic model where some peripheral proteins are loosely attached on the outside surface of the membrane whilst integral proteins are fully embedded within the phospholipids, some even spanning both layers. Several integral proteins were investigated further and shown to have regions at their ends that had some polar hydrophilic amino acids, with the middle portion being mainly composed of non-polar hydrophobic amino acids.
b. Additional evidence for integral membrane proteins came from freeze-fracture electron microscopy studies.
c. Furthermore, several experiments were carried out using labelled molecules that only attach to other specific molecules.
d. Another involved fusing mouse cells with human cells. (Read pages 67-69)
State the different ways molecules and ions moves across membranes by?
a. Diffusion.
b. Osmosis.
c. Active transport.
d. Exocytosis.
e. Endocytosis.
Define diffusion.
Diffusion is the net movement of molecules or ions from a region where they are at a higher concentration to a region of their lower concentration. Diffusion will continue until equilibrium when the particles of the substance are evenly spread throughout the whole volume. Carbon dioxide is polar, but its small size still allows rapid diffusion across the cell membrane.
Describe facilitated diffusion.
a. Hydrophilic (polar) molecules and ions that are larger than carbon dioxide cannot simply diffuse through the bilayer. They are insoluble in lipids – the hydrophobic tails of the phospholipids provide an impenetrable barrier.
b. The polar molecules and ions may diffuse through water-filled pores within channel proteins that span the membrane. There are different channel proteins for transporting different molecules. Each type of channel protein has a specific shape that permits the passage of only one particular type of ion or molecule. Some channels can be opened or closed depending on the presence or absence of a signal, which could be a specific molecule, like a hormone, or change in potential difference (voltage) across the membrane. These channels are called gated channels.
c. Some proteins which play a role in facilitated diffusion are not just simple channels but are carrier proteins. The ion or molecule binds onto a specific site on the protein. The protein changes shape and as a result the ion or molecule crosses the membrane. Movement can occur in either direction, with the net movement being dependent on the concentration difference across the membrane. Molecules move from high to low concentration due to more frequent binding to carrier proteins on the side of the membrane where the concentration is higher.
d. Diffusion, whether facilitated or not, is sometimes called passive transport. ‘Passive’ here refers to the fact that no metabolic energy is needed for the transport – the process is driven by the concentration gradient itself.
Define and describe Osmosis.
a. Osmosis is the net movement of water molecules from a solution with a lower concentration of solute to a solution with a higher concentration of solute through a partially permeable membrane.
b. Water molecules form hydrogen bonds with solute molecules, this reduces the movement of these water molecules. If more solute is present, there are fewer free water molecules able to collide with and move across the membrane. Therefore, water molecules move from a concentration of low solute to a concentration of high solute.
c. Osmosis will continue until the solutions on either side are equally concentrated or isotonic.
Describe active transport.
a. If substances need to be moved across a membrane against a concentration gradient (from low concentration to high concentration) then energy is required. As with facilitated diffusion, specific carrier proteins are also needed. Energy is supplied by the energy transfer molecule ATP. The substance to be transported across the membrane binds to the specific carrier protein. One phosphate group is removed from ATP by hydrolysis and ADP forms. Once removed, the phosphate group becomes hydrated. A lot of energy is released as bonds form between water and phosphate. This energy from ATP changes the shape of the carrier protein, causing the substance to be released on the other side of the membrane and moving it against the concentration gradient.
b. Active transport proteins are sometimes referred to as pumps, and the pumping of substances across the membranes occurs in every cell.
