Chapter 7 Membrane Structure and...

Biology – Kevin Dees

Chapter 7 Membrane 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) – This means:


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


• _________ 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!!!


Fluid Mosaic Model • In 1972__________ and _________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 (flexible) – What holds the phospholipid bilayer together?


Membrane proteins

• The __________ portion of the model

• Many, many different proteins – At least 50 different in just RBCs alone!!!!


• 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


• Integral protein • Peripheral protein • Extracellular matrix proteins • Cytoskeleton proteins • Cell-cell recognition

– Glycolipids – Glycoprotein


Functions of membrane proteins

1. Transport – passive or active 2. Enzymes – catalyze rxns 3. Signal proteins - hormones


Functions of membrane proteins

4. cell-cell recognition – glycoproteins 5. Intercellular linkages – gap junction or tight junctions 6. 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!


Membrane transport

• Selective permeability

• Transport proteins – _______________– discovered by Peter Agre in 2003 won Nobel Prize – transport of water across plasma membrane


• Transport can be : _____________requires no expenditure of cellular energy ___________requires cellular energy expense -ATP


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


____________ – 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


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


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 -


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 pump classic example of active transport


Bulk transport movement 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


• 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

Chapter 7 Membrane Structure and...

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