The Role of Mucus in the Lungs
- Lungs allow rapid gas exchance between atmosphere and blood.
- Air drawin in via trachea due to low pressure, created by reib and diaphragm movement.
- Alveoli are sites of gas exchange
- Layer of mucus in tubes of gas exchange,.
- Any dust, debris or microroganisms enter airways trapped in the mucus removed by cilia that cover epithelial cells lining tubes.
- People with CF have drier mucus, cilia find it more difficult to remove.
- Two major effects on health.
- 1) Increases chance of lung infection. 2) Makes gas exchange less efficient.
What are Epithelial Cells?
- Form the outer surface of many animals including mammals.
- Line cavaities and tubes within the animal and cover srufaces of internal organs.
- Cells work together as a tissue known as epithelium.
- Consists of one or more layers of cells sitting on a basement membrane.
- Small intestine epithelial cells extend out from basement membrane.
- Column-shapred cells make up columnar epithelium.
- Free surface facing the intestine lumen, normally covered in microvilli which increases surface area.
- In trachea, bronchi and bronchioles there are ciliated epithelial cells with cilia on the free surface.
- Cilia beat and move substances along tube they line.
- ciliated columnar epithelium of gas exchange airways appears to be stratified, but in fact each cell in contact with basement membrane.
- Appears to have several layers because some cells have nucleus at the base of cells whils in others it is centre.
- Epithelium known as pseudostratified.
How sticky mucus increases the change of lung infections
- Microorganisms become trapped in mucus and lungs.
- Can cause illness- pathogens.
- Musucs normally moved by cilia into back of mouth cavity where either coughed out or swallowed, reducing risk of infection.
- Acid in the stomach kills most microorganisms that are swallowed.
- In CF mucus layer is so sticky that cilia cannot move the mucus.
- Mucus production still continues, airway build up layers of thickened mucus.
- Low levels of oxygen in the mucus, partly because oxygen diffuses slowly through it, partly because epithelial cells use up more oxygen in CF patients.
- Harmdful bacteria can thrive in anaerobic conditions.
- White blood cells fight infections within mucus but as they die they break down, releasing DNA which makes mucus even stickier.
- Repeated infections can eventually weaken the body's ability to fight pathogens, and cause damage to the structures of the gas exchange system.
- Gases such as oxygen cross walls of alveoli into blood system by diffusion.
- To supply enough oxygen to all the body's respiring cells, gas exchange must be rapidly.
- Fine structure of lungs helps maximise this.
Gas Exchange Surfaces
How sticky mucus might affect gas exchange
- Sticky mucus layer in bronchioles of a person with cystic fibrosis tends to block narrow airways, preventing ventilation of alveoili below blockage.
- Reduces number of alveoli providing surface area for gas exchange.
- Blockages more likley to narrow ends of airways.
- Blockages will often allow air to pass when person breathes but not when they breath out, resulting in over inflation of the lung tissue beyong blockage.
- Can damage elasticity of the lungs.
- People with CF find it difficult to take part in physical excercise because gas exchange system cannot deliver enough oxygen to muscle cells.
- Oxygen needed for the chemical processes of aerobic respiration, which release the energy used to drive the contraction of the muscles during excerise.
- People with CF become short of breth when taking excercise, but it is beneficial.
Protein Structure (Intro and Primary)
- Proteins have wide range of functions in living things.
- Antobodies, enzymes and many hormones all protein molecukes.
- Various roteins make up muscles, ligaments, tendons and hair.
- Components of cell membranes.
- All proteins composed of same basic units: amino acids.
- 20 different amino acids that occur commonly.
- Plants can make all amino acids wheras animals can make only some. (62)
- Two amino acids join in a condensation reaction to form a dipeptide, with a peptide bond forming between the two subunits.
- Process can be repeated to form polypeptide chains which may contain thousands of amino acids.
- Protein made up of one or more of polypeptide chains.
- Sequence of amino acids in polypeptide knowns as the primary structure of a protein.
Protein Structure (Secondary Structure)
Further Levels of Protein Structure
- Interactions between the amino acids in the polypeptide chains cause chain to twist and fold into 3 dimensional shape, the tertiary structure.
- The chain of amino acids may twist to form a-helix (shape like extended spring).
- Within helix, hydrogen bonds form between the C=O of the carboxylic acid and the -NH of the amine group of different amino acids, stabilising the shape.
- Several chains link together wiyh hydrogen bonds holding the parallel chains in an arrangement known as a B-pleated sheet.
- Within one protein molecule there may be sections with a-helices and other sections that contain B-pleated sheets.
Protein Structure (Tertiary,Quaternary,Conjugated)
- A polypeptide chain often bends and folds to produce a precise 3 dimensional shape.
- Chemical bonds and hydrophobic interactions between T groups maintain this final tertiary structure of the protein.
- An R group is polar when the sharing of the electrons when it is not quite even.
- Polar R groups attract other polar molecules, like water and are hydrophilic (water attracting)
- Non-polar groups are hydrophobic (water-repelling).
- Hydrophobic R groups are arranged so they face the inside of the protein, excluding water from the centre of the molecule.
- A protein may be made up of several polypeptide chains held together.
- EG: Haemoglobin, the protein found in red blood cells that carried oxygen, is made up of 4 polypeptide chains and held together in a quaternary structure.
- Only proteins with several polypeptide chains have a quaternary structure.
- Single chain proteins stop at the tertiary level.
- Some proteins have another chemical group associated with their polypeptide chains.
- EG: Polypeptide chains make up myoglobin is associated with iron-containing group.
Protein Stucture (Globular)
- Proteins can be divided into two distinct groups: globular/fibrous.
- The polypeptide chain is folded into a compact spherical shape.
- These proteins are soluble due to the hydrophilic side chains that project from the outisde of the molecules and therefore important in metabolic reactions.
- Enzymes are globular proteins.
- Their 3 dimensional shape is crucial to their ability to form enzyme substrate complexes and catalyse reactions within cells.
- 3 dimensional shapes of globular proteins are critical to their roles in binding to other substances.
- EG: Transport proteins within membrances and the oxygen-transport pigments haemoglobin (red blood cells) and myyoglobin (muscle cells).
- Antibodies are also globular and rely on their precise shapes to bind to the microorganisms that enter our bodies.
Protein Structure (Fibrous)
- Fibrous proteins do not fold up into a ball shape but remain as long chains.
- Several polypeptides chains can be cross-linked for additional strength.
- These insoluble proteins are importatn structural molecules.
- Keratin in hair/skin and collagen in the skin/tendons/bones/cartilate and blood vessels.
- Collagen is made up of three polypeptide chains that 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.
Cell Membrane Structure 1 (Phospholipid Bilayer)
- CF caused by a faulty transport protein in the surface membranes of epithelial cells.
- Bilayer-basic structure is two layers of phopholipids.
- In phospholipid there are only two fatty acids; a negatively charged phosphate group replaces the third fatty acid.
- Phosphate head of the molecule is polar; one end is slightly positive and the rest is slightly negative.
- Makes the phosphate head attract other polar moelcules, like warer, and it si therefore hydrophilic (water-attracting).
- Fatty acid tails are non-polar, hydophobic (water-repelling).
- Fats and water don't mix, so when phospholipids are added to water they arrange themselces into spherical clusters called micelles or form a bilayer.
- The bilayer is favoured by phospholipids because the two fatty acids are too bulky to fit into the interior of a micelle.
Cell Membrane Structure 2 (Phospholipid Bilayer)
- Formation of bilayers by phospholipids is of critical biological importance.
- Lipid bilayer will tend to close itself so that there are no edges with exposed hydrocarbon chains, forming compartments.
- Cells are filled with a watery or aqueous cytoplasm and are surrounded by aqueous tissue fluid.
- Cell surface membrane phospholipids tend to adopt their most stable arrangement themselves, whis is bilayer.
- Arrangement avoids the hydrophobic fatty acid tails having any contact with water on either side of the membrane but ensures the hydrophilic phosphate heads are in contact with water.
The Fluid Mosaic Model
- Cell surface also contains proteins, chloestoerol, glycoproteins (protein molecules with polysaccharides attached) and glycolipids (lipid molecules with polysaccharides attached).
- Proteins span the membrane.
- Other proteins are found within the inner layer or only within the outer layer.
- Membrane proteins have hydrophobic areas and these are positioned within the membrane bilayer.
- Thought some of the proteins are fixed within the membrane, but others are not and can move around in the fluid phospholipid bilayer.
- Arrangements known as the fluid mosaic model of membrane structure.
Evidence For The Mosaic Model (TO FINISH)
- Most widely acccepted membrain model until 197-s was a three-layer protein-lipid sanwich based on the evidence of the elextron micrographs in which the dark outer layer were thought to be proteins and the lighter region lipid.
- However, protein-liid sandwich does not allow hydrophilic phosphate heads to be in contact with water, not does it allow the non-polar hydrophobic amino acids on the outside of the membrane proteins to be kept away from water.
- Considerationn of how lipidsbehaved in water, forming a bilyaer because it is the most stable arrangements, was used to refine the model
- Interpretation of the slectron micrograph evidence changed to support the new model of the membrane structure.
- Phosphate heads are more electron dense and show up as darker edges to the membran, with the tails forming lighter inner part of sandwich.
- Experiments show that there were 3 types of protein, those that could be dissociated from membran easily by increasing ionic strength
- More phospholipids containing unsaturated fatty acids there are present in the membrane, the more fluid it is.
- The kinks in the hydrocarbon tails of the unsaturated phospholipids prevent them from packing closely together, so mor emovement is possible.
- Chlolesterol reduces the fluidity of the membrane by preventing movement of phospholipids.
- Different types of protein are found within the membrane, each type having a specific function.
- Some functions as enzymes, others as carriers and channel proteins involved in the transport of substances in and out of cells.
- Glycoproteins and glycolipids have important roles in cell to cell recognition and as receptors.
Substances Pass Through Cell Membranes?
For a cell to function correctly it needs to be able to control transport across its surface membrane. Molecules and ion move across membranes by:
- Active Transport
- Diffusion is the net movement of molecules and ions from a regions where they are at a higher concentration to a region of their lower concentration.
- Diffusion will continue until equilibrium, when the substance is evenly spread throughout the whole volume.
- Small uncharged particles diffuse across the cell memebrane, passing between the lipid molecules as they move down the concentration gradient.
- Small molecules like oxygen and carbon dioxide can diffuse rapidly across the cell membrane.
- Carbon dioxide is polar but its small size allows rapid diffusion.
Facilitated Diffusion 1
- Hydrophilic molecules and ions that are larger than carbon dioxide cannot simply diffuse through bilayer.
- Insoluble in lipids, the hydrophobic tails of the phospholipids providing and impenetrable barrier to them.
- Instead they cross the membrane with the aid of proteins in a process called facilitated diffusion.
- May diffuse through water-filled pores within channel proteins that span the membrane.
- 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 challes can be opened or closed depending on the presence or abscence of a signal, which could be a specific molecule, like a hormone, or a change in potential difference (voltage) across the membrane.
- These channels are called gated channels.
Facilitated Diffusion 2
- Some proteins play role in facilitated diffusion, not just simple channels but are carrier proteins.
- Ion or molecule indsonto a specific site on the proten.
- Protein changes chape and the ion or molecule crosses 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.
- Diffusion is sometimes called passive transport.
- Passive here refers to the fact no metabolic energy is needed for the transport.
- Net movement of water molecules from a solution with lower concentration of solute to a solution with a higher concentration of solute through a partially permeable membrane.
- Osmosis random movement of water molecules across the membrane.
- Solute molecules are present, water molecules from hydrogen bonds with them and this reduces movement of water molecules.
- More solute present there are fewer water molecules free to collide with and move across the membrane.
- With unrestricted movement of the molecules there will be diffusion of both solute and water in both directions until equilibrium is achieved.
- Presence of partially permeable membrane prevents movement of some molecules.
- Solute cannot cross the membrane but free water molecules can dffuse through the membrane to achive equilibrium.
- Water moves from left to right, where there aremore free water molecules to where there is a lower concentration of free water molecules.
- Substances need to be moved across membrane against a concentration gradient (low concentration to high concentration) then energy is required.
- Like facilitated diffusion, specific carrier proteins are needed.
- Energy comes from respiration and supplied by energy transfer molecule ATP.
- Substance to be transported binds to the carrier protein.
- Energy from ATP causes changes shape of carrier protein causing substance released on other side of membrane.
- Active transport or pumping of substances across membranes occurs in every cell.
- EG: Transport of ions across epithelial cells.
Excocytosis and Endocytosis
- Very large molecules or particles need to be transported across cell surface membranes.
- Achieved by exocytosis and endocytosis which rely on the fluid nature of the membrane.
- Exocytosis is release of substances, usually proteins or polysaccharides from the cell as vesicles (small membran bound sacs) fuse with the cell membrane.
- EG: Insulin (hormone produced by pancreas) is relased into blood by exocytosis.
- Endocytosis is the reverse process.
- Substances are taken into a cell by the creation of a vesicle.
- Part of cell membrane engulfs solid or liquid material to be transported.
- Some cases the substance to be absorbed attached to a receptor in the membrane and is then absorbed by endocytosis.
- How chloesterol is taken up into cells.
- White blood cells ingest bacteria and other foreign particles by endocytosis.