biology chapter 7 - wcjcfacultyweb.wcjc.edu/users/kevind/documents/biol_1406_files/lecture... · 1...

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Biology – Kevin Dees

Chapter 7Membrane Structure and Function


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)

– Some substances cross easier than others• Origins of life on earth??

– Membranes could have formed that contained a solution while still allowing the passage of certain materials

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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, Gorter and Grendel (Dutch) determined that phospholipids could form bilayers in an aqueous solution


• Davson and Danelli – porposed in 1935 a structure of a membrane that was basically a protein sandwich with phospholipid bread

• Their work was initially supported by electron microscope images in the 1950s

• 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, Singer and

Nicolson proposed that the membrane proteins were scattered, floating in the phospholipid bilayer– Hydrophilic portions

exposed through the membrane


Membrane fluidity• The fluid mosaic model allowed for the membranes to

be very fluid– The phospholipid bilayer –held together by hydrophobic

interactions– Membrane proteins allowed to ‘drift’ slowly– Phospholipids also ‘drift’ laterally very rapidly; rarely flip-flop

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Membrane fluidity

• The phospholipid bilayer’s fluidity is maintained by: – unsaturated

hydrocarbon tails in many membranes

– cholesterol in animal cells; reduces fluidity and acts as a temperature buffer


Membrane proteins

• The mosaic portion of the model• Many, many different proteins

– At least 50 different in just RBCs alone!!!!• Two broad categories

– Integral proteins – pass through the hydrophobic portions of the phospholipid bilayer

– Peripheral 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


Functions of membrane proteins

1. Transport – passive or active2. Enzymes – catalyze rxns3. Signal proteins - hormones

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Functions of membrane proteins

4. cell-cell recognition – glycoproteins5. Intercellular linkages – gap junction or tight junctions6. Attachment – cytoskeleton or extracellular matrix


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!

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Membrane transport

• Selective permeability• Transport proteins –

– aquaporins – discovered by Peter Agre in 2003 won Nobel Prize – transport of water across plasma membrane

• Transport can be – Passive – requires no expenditure of cellular

energy– Active – requires cellular energy expense -ATP


Passive Transport• Diffusion – the tendency of molecules of any

substance to spread out evenly into the available space

• Substances move along a concentration gradientfrom areas of high concentration to low concentration

• Net diffusion ceases when equilibrium in reached– The movement does not cease however

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Osmosis – 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


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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)


What about a single celled living thing that lives in the water??

• Contractile vacuole

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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


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).


Sodium-potassium pumpclassic example of active transport

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Bulk transportmovement of larger molecules

proteins, polysaccharides

• Exocytosis –vesicles fuse with plasma membrane of cell and release contents into extracellular space


• Endocytosis – materials enter the cell by formation of vesicles from plasma membrane– Three types

• Phagocytosis – cell eating• Pinocytosis – cell drinking• Receptor-mediated endocytosis

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• 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

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