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Metabolism: Biomembranes

Imagine you’re a cell.

At the age of 12 or 13 this isn’t hard; a simple image of a circle containing a filled oval. Almost like a fried egg…but with a black yolk. The yolk represents the nucleus, the white is the cytoplasm and the perimeter of the egg is the cell boundary – a.k.a the cell membrane. Fast forward a few years to GCSEs and the introduction of mitochondria. These are vital in allowing our body to function properly, but at 15 their roles are vague – by some magical process they provide energy.

At A level the difficulty increases. The illusion of magical mitochondria is destroyed as the Krebs cycle is unveiled. Two years later and University hits. Now you realise that each organelle functions via a different mechanism. Signaling pathways and a barrage of proteins with ridiculous names are fired at you. Remembering these is hard enough, so if you’re still holding up that image of a cell carrying out all its functions, your concentration level is epic. I for one am glad that cells cycle independently, and that their workings are not down to conscious decision.

But how does the cell regulate these mechanisms? What controls the influx of ions? How is the cell separated from its environment? Well the cell membrane which is composed from a phospholipid bilayer has a major part in these roles.

Formation of Lipid Bilayer

Phospholipids are made of a hydrophilic head and a hydrophobic tail. The majority of head groups have a glycerol backbone, although sphingosine has an amino alcohol backbone. The tail of the phospholipid is an acyl chain that is derived from acetyl-CoA. In biomembranes phospholipids have two acetyl chains – one saturated and one unsaturated (contains double bonds). The shorter and more unsaturated chains are the most fluid.

In water lipids would rather keep their distance from each other, so how do they get together and form a membrane? Well, it’s thanks to water. H2O molecules love bonding to each other and this attachment is so strong that they all become associated. This depletes the amount of water molecules between the phospholipids, forcing them together. However, a monolayer of phospholipids is energetically unstable as the non-polar tails face the water. The solution to this is a bilayer with the polar phospholipid heads facing out to the water, making the structure energetically stable. To prevent the exposure of the ends of the bilayer, it curves round to seal itself, forming a vesicle.

Fig 1: Diagram of the phospholipid membrane

Lipids Within the Membrane

There are many types of lipids in nature, each with a different role. The three main classes of lipids involved in biomembranes are phospholipids, glycolipids and cholesterol. The five phospholipids are identified in the table below.

The two layers of the membrane have different composition of lipids. For example, the inner leaflet of the membrane that faces the cytosol has a high ratio of phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol, as these are involved in signaling. The outer leaflet that faces the extracellular space is composed of phosphatidylcholine and glycolipids which help with cell contact.

Glycolipids and cholesterol are two other types of lipid that are found in phospholipid membranes. The steroid rings of cholesterol interact with the hydrophobic regions of the membrane making it more rigid and less permeable.

Structure

To allow for cell communication and regulate the permeability of solutes into the cell, the membrane needs a slight modification. These tasks are carried out by proteins which associate with the lipid bilayer in several ways. Insertion or attachment to the membrane depends on the proteins polar and non-polar regions. Polarity matching ensues resulting in a variety of membrane proteins:

 

Integral proteins – non-polar regions of the protein are surrounded by the hydrophobic lipid tails e.g. Rhodopsin.

 

Anchored proteins – Outer area of the protein is hydrophilic, with a hydrophobic core.

The proteins are anchored to the membrane by the process of:

a) Acylation

e.g Prenylation of Rab GTPase, enabling them to be presented to the Rab GGTase subunit. A pathophysiology of this process is Choroideremia.

b) GPI anchoring

e.g. The protein CD55. In PNH (a condition in which haemoglobin leaks from RBCs into the blood, passing into the urine at night,) a mutation in the enzyme carrying out GPI anchor synthesis occurs. This causes RBCs to lose all GPI anchored proteins (including CD55) from their cell surface.

 

REMEMBER: Not all anchored proteins remain permanently attached to the cell. Some (i.e Rab GTPase,) only “drop anchor” to allow other processes to occur within the cell. The same way a boat might drop anchor to deploy life boats if another ship was in need.

 

Peripheral proteins - Are reversibly associated with the membrane or membrane proteins.

e.g. Toxins that bind with transmembrane complexes, i.e. conotoxin from cone snails.

The proteins have structural and bioenergetics roles. Some are also ion channels or receptors which allows for cell communication.

Fig 2: Structural classification of integral membrane proteins

Fig 1: Structural classification of integral membrane proteins

Lipid Rafts

Another important feature of the plasma membrane are lipid microdomains. These are areas of the membrane in which certain phospholipids (sphingomyelin, glycolipids) and cholesterol are enriched. These rafts form due to strong van der Waals interactions and an increase in hydrogen bonding. The lipid rafts also contain many signaling proteins and more stable than the rest of the plasma membrane, suggesting and involvement in cell communication. As the lipid raft increases the thickness of the bilayer the microdomains also allow for the accommodation of proteins that have larger non-polar regions.

Other Biomembranes

Almost anything within a cell that is separated from the cytoplasm has a membrane. This includes the majority of organelles, e.g. endoplasmic reticulum, golgi body, lysosomes. Due to each organelle's specialised function the composition of its membrane will vary. For example, mitochondria have two membranes; the outer membrane - similar chemical properties to the cell membrane, and the inner membrane. Because of the mitochondria's ability to produce energy, as mentioned earlier, the inner membrane contains proteins that are specific to this process, e.g. ATP synthase and proteins that are able to perform redox reactions.

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