The last lecture by the guy who gave us lectures on lipids also gave us an overview about the cell. Therefore, while it's not 100% related to lipids, and it's got nucleic acids and stuff I haven't spoken about yet, I'm going to go over this here, because why not?
Essential Characteristics of Living Cells
According to the lecture slides, the essential characteristics are storing information in nucleic acids (well, I guess red blood cells are all just zombies then), compartmentation (i.e. separation of the cell contents from the extracellular fluid) and having controlled chemical reactions take place inside them. I wouldn't be surprised if this definition changes from person to person and textbook to textbook, though.
Information Storage
DNA encodes information yada yada yada. If you've been studying any biology whatsoever, you should know a bit about the structure of DNA, but I'll go over it later when I talk about nucleic acids.
Compartmentation
I've already spoken a helluva lot about the cell membrane in my posts here and here. I've also written a fair bit about membrane transport here. I only have one more thing to mention, and that's about the selectivity of membrane transport. Most channels and carriers are selective to only one type of molecule. For example, a K+ pore is of the perfect size for K+ to fit through. Carbonyl oxygens in the channel are perfectly spaced apart so that K+ can bond just as easily to it as it does to water, and hence K+ passes through. Na+, on the other hand, is smaller than the channel, so it will only bind to a maximum of two carbonyl oxygens easily, and thus it is more energetically favourable for it to stay in solution.
Controlled Reactions
As mentioned many times before, enzymes are often used in biological systems to catalyse reactions. There's a lot more about enzymes in my earlier post about them. Aside from allowing reactions to proceed much more quickly than they would without them, enzymes can also cause reactions to take place in small steps, preventing the release of large amounts of free energy all at once. Molecules can bind quite specifically to enzymes and to each other due to the matching up of large numbers of functional groups, both in shape and in charge (in the case of polar groups).
Energy
As you should know by now, ATP is the "energy currency" of the cell. The phosphate groups repel each other due to the negative charge, and thus the breaking of the bond between the second and third groups can release a large amount of energy that can be harnessed by the cell.
The energy required to produce ATP is produced mainly by oxidation of carbohydrates and fatty acids (though amino acids produce some energy as well). These molecules can be broken down into acetyl CoA, which can be oxidised further in the citric acid cycle. These oxidations, which may form double bonds, add hydroxyl groups or convert alcohol groups to ketone or aldehyde groups, release electrons, which can then reduce NAD+ and FAD, which are mobile electron carriers within the cell. These electrons are then sent to the electron transport chain in the inner mitochondrial membrane, where they create a proton gradient which ultimately drives the synthesis of ATP.
Another point of note: NADH binds reversibly to its enzyme, allowing it to diffuse through to the electron transport chain for oxidation. FADH2, however, remains enzyme-bound.
A Quick Note on NADPH
NADPH is a molecule similar to NADH, but it has an extra phosphate group. Like NADH, it also binds reversibly to its enzyme. Another similarity is that both NADH and NADPH release two electrons when oxidised (not sure about FADH2). However, while NADH provides electrons to the electron transport chain, NADPH is instead coupled to biosynthetic reactions, such as the synthesis of fatty acids, as I mentioned earlier on.
Well, that's it for today. When I next get a chance, I'll speak a bit more about nucleic acids.
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