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Topic 4 - Transporters
Fluid Compartments and the Distribution of Water in the Body
Plasma - There is a high content of Na+ and Cl- in the Plasma and Extracellular Fluid.
Cells - There is a high content of K+ in the Cells.
Interstitial Fluid - There are no proteins in the Interstitial Fluid.
Total Water count in the body = 40L of Water.
Intracellular Fluid = 25L of Water.
Extracellular Fluid (ECF) [Plasma + Interstitial Fluid] = 15L of Water.
Interstitial Fluid = 12L of Water.
Plasma = 3L of Water.
The Plasma membrane is selectively permeable as some materials can enter and leave, whilst others cannot.
Non-polar molecules (such as lipids and sterols), lipid-soluble and neutral molecules can usually permeate the membrane. Whilst, polar and charged molecules generally have to be transported across via transporters.
Diffusion and Electrochemical Gradients
The K+ Gradient, for example, is a crucial determinant of electrical excitability and clinical disorders involving this can result in heart arrhythmias.
Fick's Law states that the rate of diffusion across an exchange surface depends the surface area, thickness of the surface, concentration gradient and the diffusion coefficient.
Electrochemical gradients combine the chemical (concentration) and electrical gradients (voltage difference,
as well as charge of ion), and determines which direction the net movement of molecules will occur in.
Membrane pores are the simplest form of membrane transporters made from protein molecules to allow selected substances to cross the membrane.
This transport is passive, only down the concentration gradient, and tends to be specific for a certain type of molecule.
Channel Proteins - Channel Proteins form a water-filled pore or channel in the membrane, which allows charged substances, like ions, to move across the membrane. Most of these channels can also be gated,
allowing the cell to control the entry and exit of ions.
Carrier Proteins - Carrier Proteins have a binding site for a specific solute, and constantly flip between 2 states so that the site is alternatively open to opposite sides of the membrane. The substance binds on the side where it is at a high concentration and be released on the side with a lower concentration.
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