BIOLOGY 2B03 Chapter Notes - Chapter 3: Osmium Tetroxide, Lipid Bilayer, Biological Membrane

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BIOLOGY 2B03 - Module 3 Lecture III

Biomembranes

Define boundaries in a cell

● Cell can create a unique internal environment

● Interior and exterior of a cell are completely different

● Biomembranes also define internal microenvironments by surrounding organelles

● Membrane is to provide the mean of establishing the difference and also of

maintaining it

Characteristics:

● Biomembranes are selectively permeable

○ Very few molecules can move freely across the membranes

● Transport channels can move molecules in and out of a compartment in order to

establish or maintain molecular differences

● Membranes hold proteins that can mediate cell-to-cell interactions and adhesion

proteins

● Flexibility and dynamics of membrane allow it to change shape without breaking

○ Cells can produce through cell division and move

Endoplasmic Face: that faces outside

Cytosolic Face: that faces the cytosol inside

Lumenal Face: the face of the membrane that is the interior of organelles

● Mitochondria has two membranes; creates an intermembrane space

All of these membranes faces have a different protein makeup that defines the distinct

structure and function of that surface

TEM images of the cell surface reveal its bilayer structure

● When the looked at osmium tetroxide binds to polar group on molecules; noticed

two thin parallel lines at the cell surface

○ Molecules are aligned in a bilayer to form the cell membrane

● In an aqueous environment, the hydrophobic tails face each other, creating a

hydrophobic core

Phospholipids are the Basic Unit of Biomembranes

Phospholipids: amphipathic molecule

● In an aqueous solution, it will arrange themselves spontaneously to form a bubble

like structure; Micelle

● Micelle: when single sheet of phospholipids assembles with a hydrophilic wall

and a hydrophobic core

○ Lowest energy, the most stable state

● Higher concentration of phospholipids, the amphipathic nature of phospholipid

allows them to spontaneously assemble into a bilayer

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○ The most thermodynamically favourable structure for the bilayer is a

sealed ball

Chemical Makeup of the Phospholipid layer

Diglycerides: contain two fatty acids linked to glycerol

● Fatty acids: long hydrocarbon chains capped by carboxyl group

● Contain a phosphate group attached to the third -OH group of glycerol;

production of phospholipid

○ Phospholipids often contain additional charged groups attached to the

phosphate

■ The two fatty acids are hydrophobic or insoluble in water even

though phosphate group is hydrophilic or soluble in water

■ They spontaneously organize into bilayers, the lowest energy

configuration, when they are mixed in water

Proteins In and on the Phospholipid Bilayer

Essential components of biomembranes are the proteins associated directly or indirectly

with the phospholipid bilayer

Integral Membrane Protein: is embedded in the hydrophobic core of the bilayer, while

lipid anchored and peripheral membrane proteins are associated with one surface

● It is the collection of proteins that determine the specific functions of a

membrane

● Single membrane can have very different properties; different properties

depending on the proteins found there

● Some membranes are very dense with proteins; ex) inner mitochondrial

membrane

○ Others are relatively sparse; ex) myelin membrane that surrounds nerve

cells

Biomembranes are Fluid

Biomembranes are extraordinarily dynamic and fluid

● Allows membrane to fuse one another

● Allows membrane to be deformed without tearing

● Allows changes in shape that accompany movement and cell division

● Proteins and phospholipids move laterally through the membrane

○ Allows proteins to cluster in membrane microdomains and disperse again

● Leaflet: each layer of the phospholipid membrane

Fluidity of Cell Membrane in a Moving Cell

Filopodia: finger-like projections that extends outwards from the cell membrane

Distortion and Recovery of Cell Membrane

Example)A Nerve Cell membrane that is artificially distorted

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Document Summary

Cell can create a unique internal environment. Interior and exterior of a cell are completely different. Biomembranes also define internal microenvironments by surrounding organelles. Membrane is to provide the mean of establishing the difference and also of maintaining it. Very few molecules can move freely across the membranes. Transport channels can move molecules in and out of a compartment in order to establish or maintain molecular differences. Membranes hold proteins that can mediate cell-to-cell interactions and adhesion proteins. Flexibility and dynamics of membrane allow it to change shape without breaking. Cells can produce through cell division and move. Lumenal face: the face of the membrane that is the interior of organelles. Mitochondria has two membranes; creates an intermembrane space. All of these membranes faces have a different protein makeup that defines the distinct structure and function of that surface. Tem images of the cell surface reveal its bilayer structure.

BIOLOGY 2B03 Chapter Notes - Chapter 3: Osmium Tetroxide, Lipid Bilayer, Biological Membrane

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