1.3 - Membrane structure

Page 1

Phospolips Bilayers

Phospholipids form bilayers in water due to the ample properties of phospholipid molecules.

Hydrophilic: substances that are attracted to water

Hydrophobic: substances that are not attracted to water

Phospholipids are unusual molecules because part of the molecule is hydrophilic (phosphate group) and part of it is hydrophobic (hydrocarbon chain), this is what we call amphipathic.

The two parts of the molecule include:

Phosphate group: It is known as the phosphate head, it is attracted to the water molecules that surround the membrane.

Hydrocarbon chain: also known as the hydrocarbon tail, is not attracted to water molecules that surround the membrane.

Due to this property, the phospholipids are arranged into double layers, with the hydrophobic hydrocarbon tail facing inwards to each other and the hydrophobic head facing the water on either side.

these double layers are called phospholipid bilayers. t

They are stable structures and they form the basis of all cell membranes.

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Membrane Proteins

Membrane proteins are diverse in terms of structure position in the membrane and function.

Cell membranes have a wide number of functions, but the primary function is to form a barrier through which ions and hydrophilic molecules can't easily pass.

This is carried out by the phospholipid bilayer. Almost all either functions are carried out by proteins in the membrane. the main six are:

Hormone binding sites (hormone receptors)

Immobilized enzymes with the active site on the outside

Cell adhesion to form tight junctions between groups of cells in tissues and organs

Cell-to-cell communication

Channels for passive transport to allow hydrophilic particles across by facilitated diffusion.

Pumps for active transport that use ATP to move particles across the membrane.

Because of these varied functions membrane proteins are very diverse ins structure and in their position in the membrane. They can be divided into two groups:

Integral proteins:

Are hydrophobic on at least part of their surface

They are embedded in the hydrocarbon chain in the center of the membrane.

Many of them are transmembrane they extend across the membrane with hydrophilic parts projecting through the regions of phosphate head on either side.

Peripheral proteins:

Are hydrophilic on their surface.

They are no embedded in the hydrocarbon chain or the center of the membrane.

Most are attached to the surface of an integral protein and the attachment is reversible.

Some have a hydrocarbon chain attached to them which is inserted into the membrane to anchoring them to the membrane surface.

Membranes all have an inner face and an outer face.

Membrane proteins are oriented so that they can carry out their function correctly.

The protein content of membranes is very variable because the function of membranes varies.

The more active a membrane the higher it's protein content.

the membrane of chloroplast and mitochondria have the highest protein content about (75%)

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Cholesterol in membrane

This is a component of the animal cell membrane.

The two main components of the cell membrane are phospholipids and proteins. The animal cell membrane also contains cholesterol.

It is a form or a type of lipids, but not fat or oil.

Belongs to a group[ of substances called steroids.

Most of the cholesterol molecule is hydrophobic.

So it is attached to the hydrocarbon chain /tail in the center of the membrane.

But one end of the cholesterol molecule has a hydroxyl (-OH) group which is hydrophilic

This is attracted to the phosphate e heads on the peripheral of the membrane.

Due to these characteristics cholesterol molecules are positioned between phospholipids in the membrane.

The amount of cholesterol in an animal's body varies.

In the membrane of vesicles that hold neurotransmitters at synapses have as much as 30% of the lipid in the membrane is cholesterol.

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The role of cholesterol in membrane

Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

Cell membranes do not exactly correspond to any of the three states of matter.

Hydrophobic hydrocarbon rain is liquid

Hydrophobic phosphate head is solid

Overall the membrane is fluid as components of the membrane are free to move.

The fluidity of the animal cell membrane needs to be carefully controlled.

If too fluid: won't be able to control what goes in and out of the cell.

If not fluid enough: the movement of the sell and substances within it would be restricted.

Cholesterol disrupts the regular pattern of the hydrocarbon tails of phospholipid molecules.

Prevents them crystallizing and behaving as a solid, but also restricts molecular motion and therefore the fluidity of the membrane.

It also reduces the permeability to hydrophilic particles such as sodium ions and hydrogen ions.

Due to its shape cholesterol can help membranes to curve into a concave shape.

This helps in the formation f vesicles during endocytosis.