Membranes: Organizing the Second Solvent System | BIOL 605 | Study notes Biology

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Membranes: Organizing the Second Solvent System

Without membranes, there would be no cells. All cells are surrounded by membranes.

Membranes can be internal as well as external – in plants there are 17-20 different membrane systems

that are all distinctly different from each other.

The cell produces membranes; they are all distinct from one another and go to the correct places to

perform their various functions.

Membranes are formed from a bilayer of lipids and the two layers are distinct from one another (can

cause cell death if layers get reversed or mixed up with each other).

Plasma Membrane

An important boundary between the inside and outside of the cell (NOT the outermost

boundary – ECM)

Acts as a ‘gatekeeper’ between the internal and external environments; keeps molecules in and

out, lets certain molecules pass through

Lipid soluble molecules pass easily through membranes, water does as well even though

membranes are mostly hydrophobic

Water soluble molecules pass through membranes with more difficulty

Internal Membranes

Compartmentalize cells so specialized functions are organized at certain locations

Allow cells to organize biochemical reactions in a sequential fashion; increases the efficiency of a

cell

Organize the second solvent system

o1st solvent system = water (most numerous molecule in cells); associated with water

soluble molecules (hydrophilic molecules)

o2nd solvent system = lipids; associated with lipid soluble molecules (hydrophobic

molecules); membrane lipids are the largest part of this solvent system, though lipids

exist elsewhere in cells

oMolecules associate with one or the other of the solvent systems

General Characteristics

All true membranes are lipoic (made from lipids)

Most contain proteins

They are held together by some of the weakest bonds in biological systems – hydrophobic

interactions

Water must be present in order to have a functioning membrane

In the 1950s the existence of membranes was proven with the invention of the electron

microscope (membranes are too thin to be seen with a light microscope)

In 1972 the structure of membranes was discovered

Structure

Made from amphipathic molecules (hydrophobic tail and hydrophilic head)

Hydrophilic portion has electrical charge, hydrophobic portion is neutral

Phospholipids are hydrophobic fatty acid tails connected to a phosphorylated glycerol molecule

that is often connected to another amino acid or charged molecule (serine, choline,

ethanolamine)

Glycerol and fatty acid tails can also be attached to sugars

Sphingolipids are fatty acid tails connected to sphingosine molecules that are attached to

sugars; are hooked together by the amino acid serine

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Membranes: Organizing the Second Solvent System Without membranes, there would be no cells. All cells are surrounded by membranes. Membranes can be internal as well as external – in plants there are 17-20 different membrane systems that are all distinctly different from each other. The cell produces membranes; they are all distinct from one another and go to the correct places to perform their various functions. Membranes are formed from a bilayer of lipids and the two layers are distinct from one another (can cause cell death if layers get reversed or mixed up with each other). Plasma Membrane  An important boundary between the inside and outside of the cell (NOT the outermost boundary – ECM)  Acts as a ‘gatekeeper’ between the internal and external environments; keeps molecules in and out, lets certain molecules pass through  Lipid soluble molecules pass easily through membranes, water does as well even though membranes are mostly hydrophobic  Water soluble molecules pass through membranes with more difficulty Internal Membranes  Compartmentalize cells so specialized functions are organized at certain locations  Allow cells to organize biochemical reactions in a sequential fashion; increases the efficiency of a cell  Organize the second solvent system o 1 st^ solvent system = water (most numerous molecule in cells); associated with water soluble molecules (hydrophilic molecules) o 2 nd^ solvent system = lipids; associated with lipid soluble molecules (hydrophobic molecules); membrane lipids are the largest part of this solvent system, though lipids exist elsewhere in cells o Molecules associate with one or the other of the solvent systems General Characteristics  All true membranes are lipoic (made from lipids)  Most contain proteins  They are held together by some of the weakest bonds in biological systems – hydrophobic interactions  Water must be present in order to have a functioning membrane  In the 1950s the existence of membranes was proven with the invention of the electron microscope (membranes are too thin to be seen with a light microscope)  In 1972 the structure of membranes was discovered Structure  Made from amphipathic molecules (hydrophobic tail and hydrophilic head)  Hydrophilic portion has electrical charge, hydrophobic portion is neutral  Phospholipids are hydrophobic fatty acid tails connected to a phosphorylated glycerol molecule that is often connected to another amino acid or charged molecule (serine, choline, ethanolamine)  Glycerol and fatty acid tails can also be attached to sugars  Sphingolipids are fatty acid tails connected to sphingosine molecules that are attached to sugars; are hooked together by the amino acid serine

 Components of membranes can function as part of the cell signaling system with the cleavage of their fatty acid tails  Fatty acid tails can contain double bonds – the more double bonds present, the more kinky the tails are and this affects the fluidity of the membrane  Membranes also contain cholesterol – a mostly hydrophobic molecule that alters the fluidity of the membrane (up to a certain point makes it more fluid, then begins to make it more rigid) and is the source of steroid hormones Conformation  Gel phase – lower temperatures; hydrocarbons are tightly packed  Fluid phase – higher temperatures; bilayer ‘melts’ and more movement is allowed  Never present as flat sheets because the ends being exposed to water is unfavorable; must form closed bags (in theory, perfect spheres, but this is not the case in many biological circumstances)  Free phospholipids form ring-like structures called micelles  In 30-40 minutes a cell can completely alter the character of the membrane; it is constantly recycling o Lipids rotate, flex, bob up and down, move laterally, and can flip from one layer to the other (can cause problems since the layers are distinct) o Phospholipid translocase (Flippase) – moves lipids from one layer to the other; corrects mistakes Assembly  Built in the ER – fatty acids are inserted into the outer layer of the ER and attached to glycerol, phosphorylated, and maybe groups are also attached  As the outer layer grows, flippase moves some phospholipids to the inner layer Proteins  Integral – dissolved in the membrane. Large regions of hydrophobic (alpha helices) molecules next to large regions of hydrophilic (beta pleated sheets) molecules; weave in and out of the membrane (usually). Some beta pleated sheets violate this law and go completely through the membrane; important in molecular transport into and out of cell  Peripheral – loosely associated with membranes  Many are glycosylated with sugar trees o Dolichol , a lipid molecule, carries sugar trees and will transfer its tree to a protein when it comes across a certain amino acid sequence (Asparagine – X – Serine or Threonine)  Number of proteins in membranes varies – this affects the function of the membrane  Also move throughout membrane but more slowly because they are bigger Lipid Rafts  Mostly made of sphingolipids and cholesterol; stick together and move together throughout membranes; like icebergs; signaling and cell receptors are associated with lipid rafts.

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