I mentioned in one of my earlier post (Nov 1) that my favorite part of the cell is the cell membrane. Well, I was happy to rediscover this paragraph while I was rereading Lewis Thomas' "Lives of a Cell" the other day...
"It takes a membrane to make sense out of disorder in biology. You have to be able to catch energy and hold it, storing precisely the needed amount and releasing it in measured shares. A cell does this, and so do the organelles inside. Each assemblage is poised in the flow of solar energy, tapping off energy from metabolic surrogates of the sun. To stay alive, you have to be able to hold out against equilibrium, maintain imbalance, bank against entropy, and you can only transact this business with membranes in our kind of world".
All I can say is "Amen" to this. Lewis Thomas speaks exactly what I think about cell membranes. What do you think?
Sunday, December 7, 2008
Complementarity between structure and function at the cellular level
One recurring theme in biology is the complementarity between structure and function. This can be observed starting from the subcellular level up to the organismic level. Since my blog is about cells, I’ll focus first on the cellular level. Then in a future post, I will talk about this on the subcellular level.
Since you have already seen how a nerve cell and an intestinal cell look like (Nov 20and 22), I’ll just focus on these two first.
A nerve cell’s function is communication, so its many processes are arranged in such a way that it can receive as well as send as much information as possible. The dendrites, the receiving ends, are highly branched, while the axon, the sending end, reaches out as far as possible.
An intestinal cell on the other hand is mainly absorptive in its function. Thus the “frills” or microvilli that I mentioned before are designed to increase the absorptive surface of this cell.
We can look other cells in a future post or you can start looking at other cells in your books and try to see if you can determine the function of the cell based on its structure.
Since you have already seen how a nerve cell and an intestinal cell look like (Nov 20and 22), I’ll just focus on these two first.
A nerve cell’s function is communication, so its many processes are arranged in such a way that it can receive as well as send as much information as possible. The dendrites, the receiving ends, are highly branched, while the axon, the sending end, reaches out as far as possible.
An intestinal cell on the other hand is mainly absorptive in its function. Thus the “frills” or microvilli that I mentioned before are designed to increase the absorptive surface of this cell.
We can look other cells in a future post or you can start looking at other cells in your books and try to see if you can determine the function of the cell based on its structure.
Friday, December 5, 2008
"Lives of a Cell"
Today I thought I will diverge from my usual post about some cell lessons. Today I would like to share a favorite quotation from an author who made me think about cells in a different way. I'm talking about Lewis Thomas who wrote "Lives of A Cell" sometime in 1971.
I'm lucky I was able to listen to Lewis Thomas in person when I was studying in the US in 1979 - 1980. He was as fascinating in person as in his book.
The following lines are found in the 1st page of his book.
"I have been trying to think of the earth as a kind of organism, but it is no go, I cannot think of it this way. It is too big, too complex, with too many working parts lacking visible connections... If not like an organism, what is it like, what is it most like? Then, satisfactorily it came to me: it is most like a single cell".
- Lewis Thomas
"Lives of A Cell"
What do you think? Why did Lewis Thomas think of the earth as like a single cell?
I'm lucky I was able to listen to Lewis Thomas in person when I was studying in the US in 1979 - 1980. He was as fascinating in person as in his book.
The following lines are found in the 1st page of his book.
"I have been trying to think of the earth as a kind of organism, but it is no go, I cannot think of it this way. It is too big, too complex, with too many working parts lacking visible connections... If not like an organism, what is it like, what is it most like? Then, satisfactorily it came to me: it is most like a single cell".
- Lewis Thomas
"Lives of A Cell"
What do you think? Why did Lewis Thomas think of the earth as like a single cell?
Thursday, December 4, 2008
Another correcting misconception day – about cholesterol
“Cholesterol is a steroid that is harmful to the body because it causes heart ailments”. This is a very common misconception found in many biology textbooks. It is always mentioned in relation to the chemicals making up cells.
The correction is: Cholesterol is only harmful if present in large amounts. Cholesterol per se is not harmful. In fact it is needed for synthesis of steroid hormones and for maintaining the fluidity of cell membranes.
We have to be aware that there are two kinds of cholesterol in our body: LDL (low density lipoprotein) cholesterol or the “bad” cholesterol and HDL (high density lipoprotein) or the “good” cholesterol. When too much LDL or “bad” cholesterol circulates in the blood, it can clog arteries and increase our risk of heart attack and stroke. HDL or “good” cholesterol on the other hand helps remove the “bad” cholesterol from our arteries.
We also have to be aware that most (about 75 %) of the cholesterol we have is actually produced naturally by our own liver. Only a small amount (25%) comes from the food that we eat. Unfortunately, many people inherit genes from their parents or even grandparents that cause them to make too much LDL. Inheritance plus the kind of food that we eat can therefore cause high LDLs in our blood and this is the one that triggers ailments of our circulatory system.
If high blood cholesterol runs in our family, therefore, a change in lifestyle (like watching our diet and refraining from smoking) plus some medications will probably be needed.
One more thing, if high blood LDL runs in our family, then it is important to watch the diet even of children because deposit of fatty plaques in the arteries starts even in childhood and slowly builds up as we grow older.
The correction is: Cholesterol is only harmful if present in large amounts. Cholesterol per se is not harmful. In fact it is needed for synthesis of steroid hormones and for maintaining the fluidity of cell membranes.
We have to be aware that there are two kinds of cholesterol in our body: LDL (low density lipoprotein) cholesterol or the “bad” cholesterol and HDL (high density lipoprotein) or the “good” cholesterol. When too much LDL or “bad” cholesterol circulates in the blood, it can clog arteries and increase our risk of heart attack and stroke. HDL or “good” cholesterol on the other hand helps remove the “bad” cholesterol from our arteries.
We also have to be aware that most (about 75 %) of the cholesterol we have is actually produced naturally by our own liver. Only a small amount (25%) comes from the food that we eat. Unfortunately, many people inherit genes from their parents or even grandparents that cause them to make too much LDL. Inheritance plus the kind of food that we eat can therefore cause high LDLs in our blood and this is the one that triggers ailments of our circulatory system.
If high blood cholesterol runs in our family, therefore, a change in lifestyle (like watching our diet and refraining from smoking) plus some medications will probably be needed.
One more thing, if high blood LDL runs in our family, then it is important to watch the diet even of children because deposit of fatty plaques in the arteries starts even in childhood and slowly builds up as we grow older.
Wednesday, December 3, 2008
Cell design 101.5, skin cell or keratinocyte
Several kinds of cells make up our skin. However, today, I’m just going to talk about the main cell type or the keratinocyte.
Keratinocytes start from the basal layer of our skin’s epidermis. Here, they divide several times to produce more keratinocytes. Several of them are later pushed up slowly to the surface of our skin. As they move up, they lose their ability to divide but become more specialized by accumulating keratin filaments in their cytoplasm. As more keratin accumulate in their cytoplasm, some are also secreted out into the surroundings of the cell and create a barrier. Thus, nutrients can no longer move into the cells, and they die. The topmost cells of our skin are therefore dead keratinocytes. They are later removed from our skin surface and will be replaced by new cells coming from the basal layer of our skin. Keratin plus other molecules joined with them make our skin waterproof.
The whole process of skin renewal takes about 20 – 30 days. That means every 20 -30 days we have new cells on the surface of our skin. If one has the skin disease, psoriasis however, new skin is produced in less than 20 days. Thus, people with psoriasis have portions of “bumpy” skin.
By the way, formation of new keratinocytes by mitosis usually takes place at night while we sleep. This might explain why our skin suffers if have too many late nights.
Keratinocytes start from the basal layer of our skin’s epidermis. Here, they divide several times to produce more keratinocytes. Several of them are later pushed up slowly to the surface of our skin. As they move up, they lose their ability to divide but become more specialized by accumulating keratin filaments in their cytoplasm. As more keratin accumulate in their cytoplasm, some are also secreted out into the surroundings of the cell and create a barrier. Thus, nutrients can no longer move into the cells, and they die. The topmost cells of our skin are therefore dead keratinocytes. They are later removed from our skin surface and will be replaced by new cells coming from the basal layer of our skin. Keratin plus other molecules joined with them make our skin waterproof.
The whole process of skin renewal takes about 20 – 30 days. That means every 20 -30 days we have new cells on the surface of our skin. If one has the skin disease, psoriasis however, new skin is produced in less than 20 days. Thus, people with psoriasis have portions of “bumpy” skin.
By the way, formation of new keratinocytes by mitosis usually takes place at night while we sleep. This might explain why our skin suffers if have too many late nights.
Tuesday, December 2, 2008
microtubules
So far I have only mentioned the functions of the microfilaments as part of the cell’s bones and muscles or cytoskeleton. To even up matters, I’ll talk about the microtubules today.
The microtubules consist mainly of the protein tubulin which has 2 phases, the alpha and beta tubulin. These tubulin molecules form a tube like structure that can elongate at one end and shorten at the other. This is a continuously occurring process so the microtubules and also the microfilaments are always in a state of dynamic instability. That means that nothing is permanent with the cell’s cytoskeleton.
Microtubules serve as scaffolding inside cells and act as “tracks” on which cells can move organelles, chromosomes, vesicles and other things inside. In other words, they act like bullet trains inside cells. Microtubules are also responsible for the movement of cilia and flagella. Imagine that, molecules that can act as scaffolding, train, and propeller at the same time! Yessiree, those are your microtubules.
In order to do their function however, microtubule need to associate with proteins like dynein and kinesin. These two serve as motors to power the movement of microtubules. If something goes wrong with these motors, then any of the movements mentioned above will not be possible. Sperm cells for example will be immotile if dynein is absent in their flagellum.
The microtubules consist mainly of the protein tubulin which has 2 phases, the alpha and beta tubulin. These tubulin molecules form a tube like structure that can elongate at one end and shorten at the other. This is a continuously occurring process so the microtubules and also the microfilaments are always in a state of dynamic instability. That means that nothing is permanent with the cell’s cytoskeleton.
Microtubules serve as scaffolding inside cells and act as “tracks” on which cells can move organelles, chromosomes, vesicles and other things inside. In other words, they act like bullet trains inside cells. Microtubules are also responsible for the movement of cilia and flagella. Imagine that, molecules that can act as scaffolding, train, and propeller at the same time! Yessiree, those are your microtubules.
In order to do their function however, microtubule need to associate with proteins like dynein and kinesin. These two serve as motors to power the movement of microtubules. If something goes wrong with these motors, then any of the movements mentioned above will not be possible. Sperm cells for example will be immotile if dynein is absent in their flagellum.
Monday, December 1, 2008
cytokinesis
Cytokinesis is another activity that is generated by the cytoskeleton, particularly the microfilament.
After the chromosomes of a cell separate during anaphase, the microfilaments together with their associated protein, myosin, create a contractile ring somewhere near the middle of a cell. This ring tightens like a purse string until finally the cell is divided into two. This division completes the final stage of mitosis wherein two new cells with the same chromosome number are formed.
Separation of chromosomes and cytokinesis have to be properly coordinated so that the chromosome number of each generation of cells remain the same. If the timing of these two processes is off, we can end up with cells that have abnormal chromosome number or cells that can develop into cancerous ones.
After the chromosomes of a cell separate during anaphase, the microfilaments together with their associated protein, myosin, create a contractile ring somewhere near the middle of a cell. This ring tightens like a purse string until finally the cell is divided into two. This division completes the final stage of mitosis wherein two new cells with the same chromosome number are formed.
Separation of chromosomes and cytokinesis have to be properly coordinated so that the chromosome number of each generation of cells remain the same. If the timing of these two processes is off, we can end up with cells that have abnormal chromosome number or cells that can develop into cancerous ones.
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