Post on 07-Aug-2020
Biology – Kevin Dees
Chapter 7 Membrane Structure and Function
Biology – Kevin Dees
The plasma membrane surrounds the living cells from their surroundings.
• Only 8 nm thick (8,000 to equal the thickness of a sheet of paper)
• Controls passage of materials in and out of cell
• Selectively permeable (selective permeability) – This means:
Biology – Kevin Dees
Brief history of the discovery of the structure of the plasma membrane
• In 1915, chemical analysis was done on RBCs and the basic components were discovered – no structure!
• Phospholipids (amphipathic molecules – remember?!!) • Proteins
• In 1925, __________ and ________(Dutch)
– determined that phospholipids could form bilayers
in an aqueous solution
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• _________ and _________– • proposed in 1935 a structure of a
membrane that was basically a protein sandwich with phospholipid bread
• But some were still skeptical – Especially about the proteins
• Large molecules • Some are amphipathic as well!!!
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Fluid Mosaic Model • In 1972__________ and _________proposed
that the membrane proteins were scattered, floating in the phospholipid bilayer – Hydrophilic portions exposed through the membrane
Biology – Kevin Dees
Membrane fluidity • The fluid mosaic model allowed for the membranes to
be very fluid (flexible) – What holds the phospholipid bilayer together?
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Membrane proteins
• The __________ portion of the model
• Many, many different proteins – At least 50 different in just RBCs alone!!!!
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• Two broad categories
– ________________proteins – pass through the hydrophobic portions of the phospholipid bilayer
– _________________proteins – attached to the surface of the membrane or to integral proteins
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• Integral protein • Peripheral protein • Extracellular matrix proteins • Cytoskeleton proteins • Cell-cell recognition
– Glycolipids – Glycoprotein
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Functions of membrane proteins
1. Transport – passive or active 2. Enzymes – catalyze rxns 3. Signal proteins - hormones
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Functions of membrane proteins
4. cell-cell recognition – glycoproteins 5. Intercellular linkages – gap junction or tight junctions 6. Attachment – cytoskeleton or extracellular matrix
Biology – Kevin Dees
Synthesis of membranes
• ER to Golgi to Vesicle to Membrane
• Inside of the membrane in the ER will become the outside of the cell membrane!
Biology – Kevin Dees
Membrane transport
• Selective permeability
• Transport proteins – _______________– discovered by Peter Agre in 2003 won Nobel Prize – transport of water across plasma membrane
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• Transport can be : _____________requires no expenditure of cellular energy ___________requires cellular energy expense -ATP
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Passive Transport • __________________– the tendency of molecules of any
substance to spread out evenly into the available space • Substances move along a ___________ ___________
from areas of high concentration to low concentration • Net diffusion ceases when _______________ in reached
– The movement does not cease however
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____________ – diffusion of water
• Solute abundance in solutions can affect the movement of water – Tonicity – the ability of a solution to cause a cell to
gain or lose water. • Hypotonic solution – less solute (more water) • Hypertonic solution – more solute (less water) • Isotonic solution – equal concentrations of
solute Hypotonic solution Hypertonic solution
Net water movement
Biology – Kevin Dees
Biology – Kevin Dees
Effects of tonicity on plant and animal cells
• Osmoregulation – maintain proper osmotic balance
• Most animal cells fare best in isotonic environment
– (not lysed)
• Most plants fare best in hypotonic environment
– (not plasmolyzed)
Biology – Kevin Dees
What about a single celled living thing that lives in the water??
• Contractile vacuole
Biology – Kevin Dees
So, if particles can move passively along a concentration gradient how do they cross a
plasma membrane?
• Diffusion through the phospholipid bilayer – Hydrophobic
molecules – Very small
uncharged particles – O2 for example
Biology – Kevin Dees
So, if particles can move passively along a concentration gradient how do they cross a
plasma membrane?
• Facilitated diffusion passively with the help of transport proteins – Channel protein – – Carrier protein -
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There are instances where particles need to be transported backwards along the concentration
gradient; from low concentration to high concentration.
• This type of transport is known as active transport because it requires the cell to expend cellular energy (ATP).
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Sodium-potassium pump classic example of active transport
Biology – Kevin Dees
Bulk transport movement of larger molecules
proteins, polysaccharides
• Exocytosis – vesicles fuse with plasma membrane of cell and release contents into extracellular space
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• Endocytosis – materials enter the cell by formation of vesicles from plasma membrane – Three types
• Phagocytosis – cell eating • Pinocytosis – cell drinking • Receptor-mediated endocytosis
Biology – Kevin Dees
Biology – Kevin Dees
Biology – Kevin Dees
• LDL in blood – low –density lipoproteins (transport form of cholesterol) – Hypercholesterolemia – LDLs are ligands whch will enter the cell after
attaching to a receptor protein. Genetic disposition for defective or missing receptor proteins.
– Causes high levels of LDL in blood (they cannot enter the cells) and may lead to deposits in the walls of blood vessels