Cell Membrane Structure - Essay Example

The alignment of these phospholipids along the cell membrane is tail to tail, so that these nonpolar areas form water repellant, hydrophobic regions on the inner and outer surfaces of the membrane, while the polar heads are hydrophilic and allow solvents to pass easily through. Lipids and proteins are not however likely to seep through the bilayer unless they first give up the favorable interactions with hydrogen molecules present in water before they can cross the membrane. The greater the degree to which a protein or lipid molecule is hydrophilic, the larger it is and therefore less likely to cross the cell membrane through simple diffusion. (Gray et al, 2002)

The interaction between nonpolar lipid tails that limits membrane fluidity is due to the action of the Van der Waals forces, which are increased by increasing the length of the lipid tails (Gray et al, 2002). It is the hydrophobic effect that results in the rapid self-assembly of the lipid bilayers when exposed to water since protein folding takes place through the clustering of the hydrophobic residues in the core of the protein in order to prevent contact with water. However, this structure of the cell membrane is not rigid, because the forces holding the bilayer together are weak, non-covalent interactions which allow for fluidity of the membrane.

Transport of substances across the membrane:
The outer surface of the cell membrane is rich in glycolipids, while the hydrophobic tails are embedded within the cell. The heads which are exposed outside the cell function in association with the carbohydrates attached to the integral proteins in allowing the cell to determine which materials belong to the organism and which materials are foreign, against which an immune defense must be sparked. Water, carbon dioxide, and oxygen are transported across the cell membrane through a simple process known as diffusion or Osmosis, where the difference in concentrations across the membrane stimulates movement across the membrane in order to equalize the concentrations across both sides of the membrane (www.wps.prenhall.com). This kind of transport of material across the cell membrane requires no energy from the cell.

However, the active transport of materials requires the cell to expend energy in the form of adenosine triphosphate (ATP). This may be the case when large nonlipid, soluble materials have to be transported across the membrane. Such materials are generally transported with the assistance of carrier proteins and where active transport of materials using ATP is concerned, the proteins may have to move against the concentration gradient. For example, within nerve cells, sodium – Na+ is maintained at low concentrations inside the cell while potassium – K+ is maintained at higher concentrations. However, outside the cell, the reverse is the case. Therefore when a message needs to be transported, ions pass across the membrane to transport the message, across the concentration gradient, and once the message has been communicated, they must be transported back to their starting positions, all of which requires the action of ATP, such that up to one-third of ATP use in a resting animal may be used in re-setting the Na-K pump.

ATP in respiration:
Mitchell’s Chemiosmotic hypothesis states that the respiratory chain converts redox energy into an electrochemical gradient of protons, which is the driving factor propelling the manufacture of ATP from ADP and Phosphate by utilizing F1F0 ATP Synthase, an enzyme responsible for ATP synthesis using some form of proton energy. (Dimroth et al, 1999). This proton motive force consists of two primary components – (a) the proton concentration gradient and (b) the membrane potential, and both of these are thermodynamically equivalent.

Early studies appeared to indicate that these two components are not only thermodynamically equivalent but also kinetically equivalent driving forces for the synthesis of ATP (Junesch and Graber 1991) but Dimroth et al (1999) point out that such results have been challenged on the basis that the acid bath procedure used in such studies may have contributed to the findings. Their study shows that the cell membrane is the factor responsible for generating the necessary rotational torque by the F0 motor since this torque is a prerequisite to release the tightly bound ATP molecules from the catalytic F1 sites.

The ATP synthesis model holds that it is the proton motive force existing across the inner mitochondrial membrane of the cell, which is generated by the electron transport chain, which drives the passage of protons through the membrane and causes some rotation. The angle of rotation causes changes that lead to ATP synthesis. The functioning of ATP synthase is activated during respiration.