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Cell Membrane Structure notes for healthcare and medical student
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- Aberystwyth University (UWA)
Membrane lipids have: Polar/hydrophilic heads and nonpolar/hydrophobic fatty acid tails
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Cell Membrane StructureMembrane lipids have: Polar/hydrophilic heads and nonpolar/hydrophobic fatty acid tails
Phospholipids are the most abundant group of membrane lipids - they will always have a polar head
that contains a phosphate group and other hydrophilic atoms such as nitrogen. Hydrophilic literally
means “water-loving”. There are a huge variety of phospholipids, each with critical roles in the
overall functioning of the membrane - compositions vary between organs, between tissues within
organs, and even within a single cell, with the top (or apical) membrane of the cell very different than
the bottom (or basal) membrane.
Glycolipids: are sugar-bound lipids - these sugars make their heads polar, too.
, Cholesterol makes up about 20% of the membrane lipids. It decreases membrane permeability by
packing in between other lipids. This important shield prevents substances from entering or leaving
without permission.
A popular area of research right now is learning how cholesterol organizes where the protein
components - channels and receptors - are located within the membrane.
Fatty acids of membrane lipids
Unlike triglycerides, membrane lipids usually only have two fatty acid tails. These long carbon chains
are nonpolar and hydrophobic (hydrophobic literally means “water-fearing”).
Fatty acid tails are saturated or unsaturated, and these differences provide even more fine-tuning
and adjustment for membrane functioning.
More saturated fatty acids will provide greater stability, whereas more polyunsaturated fatty acids
provide for more flexibility in the membrane.
The unsaturated fatty acids are also crucially involved with sending signals from the membrane down
deeper into the cytoplasm of the cell.
The membrane lipids orient their hydrophobic fatty acid tails toward each and form a lipid bilayer.
Channel proteins select for substances based on size and charge.
Glucose and water are two examples of polar but uncharged molecules that pass into cells by specific
glucose and aquaporin (water) channels.
Charged ions such as sodium, potassium, and calcium all require specific channels to move through
the membrane.
Small nonpolar gases such as oxygen and carbon dioxide are among a relatively small list of
substances that can diffuse freely through the membrane; no channel needed.
Some channels have gates. For example, sodium voltage-gated channels only open when the
membrane is a particular membrane potential.
Some channels are pumps that use a lot of ATP energy! For example, the Na+/K+ ATPase pump.
Peripheral and transmembrane proteins
Peripheral proteins remain on the surface of the bilayer
Transmembrane or integral proteins pass all the way through the membrane.
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