Membranes in cells
Membrane structure and function
Lecture 21
Electron Micrograph of Bilayer Plasma
Membranes
Membrane Function
• Surround cells and intracellular organelles
• Control the entry and exit of inorganic ions, nutrients and other
biological and non-biological compounds
• Maintain concentration gradients across inside the cells
Intracellular
Na+ (10 mM)
K+ (140 mM)
Ca++ (0.1 mM)
Extra-cellular
145 (mM)
5 (mM)
1-2 (mM)
Department of Biochemistry and Molecular Biology
New Jersey Medical School
Dr. S. Kumar
In 1925 Gorter and Grendel proposed that the unit
membrane is formed from a phospholipid bilayer
Extracellular space (aqueous)
Phosphate heads
face aqueous
solution
phospholipid
bilayer
Cytosoplasm (aqueous)
Hydrophobic tails
face inwards
Sphingosine
CH3-(CH2)12-CH=CH-CH-CH-CH2OH
OH NH2
Department of Biochemistry and Molecular
Biology
New Jersey Medical School
Dr. S. Kumar
Bilayers are asymmetric in the distribution of phospholipids
Lipid components of the plasma membrane.The outer leaflet consists
predominantly of phosphatidylcholine, sphingomyelin, and
glycolipids, whereas the inner leaflet contains
phosphatidylethanolamine, phosphatidylserine, and
phosphatidylinositol. Cholesterol is distributed in both leaflets.
Outer surface
Inner surface
Department of Biochemistry and Molecular Biology
New Jersey Medical School
Dr. S. Kumar
Red Cell
Membrane
Integral and peripheral Membrane Proteins
The fluid mosaic model of the plasma membrane:
The proteins can move freely through the lipid bilayer.
The ease with which they do this is dependent on the number of
phospholipids with unsaturated fatty acids in the phospholipids.
Lipid Rafts
The membrane contains many types of protein:
carbohydrate chain
Glycocalyx: For cell
recognition so cells group
together to form tissues
Receptor: for
recognition by
hormones
glycoprotein
peripheral protein
Enzyme
or
signalling
protein
integral protein
carrier protein
hydrophilic channel
Membrane bound proteins allow chemical processes
to occur on membranes in a sequential manner:
proteins
membrane
Cyt c
Q
I
III
II
Enzyme and transporter proteins
involved in aerobic respiration in the
inner mitochondrial membrane
IV
ATP synthase
Membranes allow cellular compartments to have different
conditions
pH 4.8
Contains digestive
enzymes, optimum
pH 4.5 - 4.8
lysosome
Membrane acts as
a barrier
pH 7.2
cytosol
Fat-soluble organic molecules can diffuse through the bilayer
but polar molecules require proteins
Fat-soluble molecules
Polar molecules
Extracellular
space
Cytosoplasm
(aqueous)
hydrophilic pore
Carrier Mediated transport
Comparison of Simple Diffusion, Facilitated Transport & Active
Transport
Property
Simple
Diffusion
Facilitated
Transport
Active
Transport
Requires special No
membrane
proteins
Yes
Yes
Highly selective
No
Yes
Yes
Transport
saturates
No
Yes
Yes
Can be inhibited
No
Yes
Yes
Hormonal
regulation
No
Yes
Yes
Uphill transport
No
No
Yes
Requires ATP
energy
No
No
Yes
Summary
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The unit membrane consists of a phospholipid bilayer
Phospholipids consist of a polar, hydrophilic phosphate head and a non-polar,
hydrophobic tail consisting of fatty acid chains.
Proteins also occur in the membrane and float freely throughout it.
The model for membrane structure is known as the fluid mosaic model.
Peripheral proteins occur on the inner or outer face of the membrane and integral
proteins extend through both lipid layers.
Membrane bound enzymes occur allowing structured metabolic pathways.
Glycoproteins form the glycocalyx and allow cell to cell recognition.
Receptor proteins can act as binding sites for hormones and other substances and
can transmit the information to the interior of the cell.
A variety of carrier proteins allow for the controlled movement of substance through
the membrane using both passive diffusion or active transport.
Non-polar, lipid soluble molecules diffuse through the phospholipid bilayer.
Ionic, polar molecules require carrier proteins to enable them to pass through the
membrane.
Membrane structure loses integrity with high temperature or presence of organic
solvents such as alcohol, thereby increasing permeability.
Lipid rafts have more sphingomyelin and cholesterol and are important in signal
transduction and viral entry into the cells