PHIS 206 Lecture Notes - Lecture 3: Fluid Mosaic Model, Cell Membrane, Desmosome
2 May 2018
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Lecture 3: Membrane Physiology
Membrane Basics
• Plasma Membrane (surrounds the cell and sometimes surrounds things inside the cell)
o Thin layer enclosing intracellular contents
o Mechanical barrier (main job is to separate what is inside vs outside)
o Gateway for passage in and out
o Maintains differences in composition of EFC and ICF
Membrane Composition
• Phospholipid → there are many different types
o Major component of plasma membrane
o Hydrophobic (tail, nonpolar, fear of water, fatty-lover) tails
o Hydrophilic (head, water-loving, fatty-hating) head
o Fatty membrane is perfect for separating water
o One side looks like water, one side looks like fat (perfect for separating two
watery environments
Lipid Bilayer
• Arrangement
o Tails face each other (both hydrophobic)
o Heads face watery environment of ECF & ICF
• Things that are like want to be together
• If you’re watery, the membrane is a problem; if you’re fatty, there is no problem crossing
the membrane
Arrangement as Plasma Membrane
• Compartments
o ECF and ICF differ greatly
o Different ionic composition
o Different pH
o Different nutrients (level of nutrients tend to be lesser on the outside of cells)
• As long as whatever is inside the cell is relatively dissolved in the water, the membrane
will do a fantastic job of keeping whatever is inside the cell inside
• Membrane has ability to identify
Plasma Membrane
• Know fluid mosaic model
• Fluid mosaic model (Called this because the proteins can move around all they want, it’s
these proteins that make the membrane an active component)
Cell-to-Cell Attachments
• Desmosome
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o Strong mechanical connection between cells (and between the cytoskeletons of
cells) → gives cells some rigidity when they are stuck together
▪ Desmosomes are what makes cells stick together so strongly
• Tight Junction
o Prevents movement between two cells
o Barrier to migration of protein along cell surface
o “Like a belt around the cells” → protein above the belt stay above the belt and
protein below the belt stay below the belt → sometimes we want certain proteins
at certain parts of the cell
• Gap Junctions
o “Windows” between cells (the “windows” are open)
o Permit passage of small ions/molecules
o Important in some electrical tissues
o If all the cells need to do one thing at the same time, they use this → necessary for
absolute precision → allows a bunch of cells to act as one cell
▪ Like the heart, all cells need to squeeze at the same time for heartbeat,
needs gap junctions
Overview of Membrane Transport
• Permeability
o Substances may or may not be able to cross membrane (and that can change)
• Oil versus Water
o Hydrophobic substances can cross membrane with little effort; hydrophilic
substances require assistance
▪ Oxygen, CO2 → are considered fatty/like fat
• Passive versus Active
o Most crossings of membrane are passive (downhill) (high to low concentration)
o Uphill crossings require energy either directly or indirectly(ATP) (low to high
concentration)
High to Low
• Diffusion → main method of moving most substances
• Random process
• Over time, everything will spread out (high concentration to low concentration) → it just
happens and it is reliable, but it is slow
• Body tries to take advantage of diffusion
• All of the movement of good things in your cells is from diffusion
Factors Affecting Diffusion
• Concentration Difference → high to low is only driving force
• Surface Area → Opportunity to diffuse (more surface area, more opportunity to diffuse)
• Hydrophobicity → Oil and water do not mix
• Molecular size → Larger objects struggle more (the smaller your molecular size, the
easier it is to diffuse)
• Membrane Thickness → Thicker membranes slow things (most cell membranes have the
same thickness) → illnesses can change membrane thickness
find more resources at oneclass.com
find more resources at oneclass.com
