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PHSL233 lecture 03.doc

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University of Otago
Kirk Hamilton

PHSL233 Lecture 03 Structure and Function of Epithelia II Cytoskeleton provides structural support of a cell, it is like the skeleton of the cell contained within the cytoplasm. It maintains the cell shape and protects the cell and enabling cellular motions such as developing structures such as flagella and cilia. It also plays a role in intracellular transport and cellular division. There are mainly made up of 3 filaments, microfilaments (actin), intermediate filaments and microtubules. This shows a actin filament at the brush border of an epithelial cell, actin filament are also known as microfilament. It is made up of actin proteins joined in a long chain and it is the thinnest filament. It is made up of G- actin monomer linking into a chain called F- Actin polymer, which then pair off and interwind along eachother making it microfilament. They are found in a adherens junction and microvilli etc. Although microtubules and actin filaments are found in all eucaryotes, intermediate filaments are only found in some soft tissued animals. They provide lots of strength and they are found in desomosome junctions. So 2 monomers associate with eachother creating a twisted dimer helical structure, then 2 dimers line up next to eachother forming a staggered tetramer and arranged in opposite orentiations. Then the tetramers will link end to end building up one strand of intermediate filament. Then 8 filaments come together twisting like rope giving it stronge strength. Microtubules are thickest, and they are formed by tubuline proteins joining into long tube. Structures like cilia and tail of sperm are made up of microtubules. They also provide tracks for transportation in the cytoplasma such as transporting vesicles around. They also help separate the chromosomes to opposite end during cell division thus cell motility. Cilium have this 9 pairs with 2 in the middle called a axenome structure. There are dynein arms that help slide microtubules along eachother causing the beating of cilia and ATP is used. Dynein move towards the -ve end while kinesin move towards +ve end. Membrane permeability a permeable structure allows passage of substances through it, an impermeable structure does not. Plasma membrane have a selective permeability depending on it's properties and type. The impermeable properties allows cell to set up a concentration gradient across the 2 sides of the membrane. Membrane are permeable to non-polar substances that are hydrophobic such as steroid hormones, and small uncharged molecules such as oxygen and carbondioxide. However they are impermeable to ions and molecules that is large in size even though it is uncharged, such as ions and glucose, thus they cannot cross the membrane. They are slightly permeable to small unchared polar molecules such as water and urea via diffusion. During movement of membrane of fluid mosiac model, this allows gaps to be formed in the plasa membrane so water and urea can pass through those gaps. Note that although phosphate and proteins are constantly moving, but they are only moving in their own side, thus flip flop does not occur. Unsaturated tails will also provide loose packing of membrane phosphate lipids thus providing gaps. Diffusion So when partition is removed, you will get a large movement of atom from A to B and little from B to A, thus giving a net movement of A to B so over a period of time A and B would share the same conecentration of ions. But ions are still constantly moving between B and A, just no net movement occurs. Atoms and molecules are continually moving (brownian motion) and over time they will move to a region of lower concentration gradient. The Brownian motion is the random movement motion of particles suspended in fluid (liquid or gas). So over a period of time, the atoms or molecules will spread out equally. Protein mediated transport solutes that cannot diffuse across the plasma membrane may still be able to enter a cell if the plasma membrane contains an internal membrane protein that allows a solute to pass through into the cell. They will allow a specific molecule to pass through, it has side hydrophobic amino acids on side in contact with the fatty acid tails and hydrophilic on inside pore and ends of the channel. Substances such as Na, K and gluose all have transporters embedded in the plasma membrane that span the bilayer of the membrane. Substance moved by transporter is much more faster than diffusion. Facilitated (passive) transport is a mediated movement of substance down it's electrochemical gradient with no energy required, thus both electric and concentration gradient. This includes pore-mediated, channel-mediated and carrier mediated transport. They cannot move substances against their concentration gradient. Pore-mediated transport such as a aquaporines (AQP1) which are water cahnnels that made up of four protein subunits coming together to form a pore that only water can fit through.
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