Sunday, November 30, 2008

amoeboid movement

Amoeboid movement is one example of how the microfilaments (the cell’s muscles) function. It is the same kind of movement that is involved in phagocytosis or “cell eating” which is the subject of my post last Nov. 19.

Biology – Online dictionary defines it as “A crawling-like type of movement in which the cell forms temporary cytoplasmic projections called pseudopodia (false feet) towards the front of the cell”.

Aside from the Amoeba, other cells that exhibit amoeboid movement are: neutrophils and macrophages (our professional phagocytes, remember?), monocytes (another kind of white blood cell), Kupffer cell of the liver, as well as cancer cells. Yes, cancer cells. This is the way by which cancer cells metastasize or spread to other parts of the body.

Saturday, November 29, 2008

table for cytoskeleton


Above is the corrected table for the cytoskeleton, yesterday's post.

Friday, November 28, 2008

cell bones and muscles

The cell has its own bones and muscles called the cytoskeleton. This cytoskeleton has 3 major components: microfilaments, intermediate filaments and microtubules.

These major components are usually associated with other proteins. The association enables them to do several functions like formation of scaffolding inside the cell, ciliary or flagellar movement on the cell surface and internal cell movements like chromosomal movement during mitosis.

A summary of the functions of the cytoskeleton is shown in the table below.

Microfilaments Intermediate Filaments Microtubules
Muscle contraction Support and tensile strength Cell motility
(cilia and flagella)
Amoeboid movement Maintenance of cell shape Chromosome movement
Cell locomotion Formation of nuclear lamina Movement of
and scaffolding organelles
Cytoplasmic streaming Strengthening of nerve cell axons Determination of cell shape
Cell division (cytokinesis) Keeping muscle fibers in Maintenance of cell
register shape
Maintenance of cell shape


I will talk about each of these functions in a future post.

Thursday, November 27, 2008

The cell’s post office

Yes, the cell has a post office. It’s called the Golgi complex or Golgi apparatus after Camilo Golgi, the scientist who identified it.

So what does the Golgi complex do? Well, just like any post office, it receives packages, sorts them, checks their labels and delivers them to the proper destination. It also sends back packages that are defective. However, it is more than the ordinary post office. Why? Because aside from doing all those functions mentioned above, it can also modify the packages, insert more identifying markers, compact and repackage them, and even add some fancy gift tags and ribbons.

The packages I am talking about here are of course the products coming from the ER. If you remember from a previous post (Nov 23, rough ER), I mentioned that the Golgi together with the ER are involved in “membrane trafficking” wherein vesicles move between them and the other parts of the cell.

Yes, the Golgi also sends back to the ER packages that contain defective proteins. Aside from that, it can also insert identifying markers like mannose-6-phosphate, the marker for proteins destined to be part of the lysosome.

The Golgi is also involved in the synthesis of proteoglycans. As the name implies, these are molecules that contain protein and sugars. These molecules are very important components of the cell’s immediate surrounding or the extracellular matrix. Furthermore, the Golgi also adds sulfates and phosphates to various cell products. Sulfation and phosphorylation are important for signaling and sorting. All these add-ons are the fancy gift tags and ribbons that I mentioned.

So as you can see, the Golgi complex is the cell’s super post office.

Oh, and by the way, just like a post office, the Golgi also has a receiving window (its cis face) and a releasing window (its trans face).

Wednesday, November 26, 2008

display, tell and kiss

A cell usually has many things on display at its surface. Most of these are receptors. Having receptors is a way by which cells communicate with other cells (see Nov 4 post “cells talk”). However, there are some cells that are “professional display artists.” These cells “make a living” by displaying bits and pieces of foreign antigen on there cell surface. I’m talking here about the “antigen presenting cells” or APCs.

APCs “eat” foreign antigens and process these in their lysosomes (see Nov 19 post “phagocytosis”). Then they display bits and pieces of these foreign antigens on their cell surface so they can attract circulating T lymphocytes. It’s their way of telling the T lymphocytes that a foreign substance is present in the body. When the two meet, they then “kiss” through the displayed antigen and the T lymphocyte receptor. After the “kiss” the T lymphocyte becomes activated and then sets the immune response rolling. The continuation of this story will be the subject of another post.

So in the cellular world, display, tell and kiss (not display, kiss and tell) is the way to go to get things moving.

Tuesday, November 25, 2008

plasmolysis

I am again in my “correcting misconception” mood today. I encounter so many of them in several biology textbooks and I always feel shivers ran up and down my spine every time I see them. I know that textbooks are the major source of information for most of our teachers so I don’t understand how such books (full of misconceptions) can even be approved as text. I also don’t understand how some authors can write some out of this world information.

Today my topic is about plasmolysis. I was shocked to read this in one textbook - “Plasmolysis explains why your skin wrinkles” What?

Please, plasmolysis does not cause wrinkling of the skin!!!

Plasmolysis occurs in plant and bacterial cells, not in human cells. When plant cells are placed in extremely hypertonic environment, water moves out of the cells by osmosis. This causes the plant vacuoles to shrink and the cell membrane detaches from the cell wall. This is plasmolysis.

Animal cells do not have cell walls so there is no such thing as detachment of cell membrane if cells are exposed to hypertonic environment. So plasmolysis does not occur in animal cells.

Red blood cells shrink or undergo crenation if placed in hypertonic solution in laboratory condition. Under laboratory condition also, blood cells bloat and even burst if placed in hypotonic solution. However, in natural condition these do not happen because our body has a system for regulating water loss and gain such that our individual cells are protected from shrinking or bursting.

Monday, November 24, 2008

cells need chaperones

Yes, you read that right – cells need chaperones! Cellular chaperones are of course different from the social chaperones that we are familiar with. However, to a certain extent, both kinds of chaperones actually perform similar functions. While a social chaperone ensures that the concerned parties (usually unmarried ladies or gents) behave properly during social occasions, the cellular chaperones also make sure that certain molecules in the cell (especially proteins and nucleic acids) behave properly.

So far, four instances have been discovered wherein chaperones are needed by the cell, but the list may still increase.

The first instance is in the association of the genetic material with histone proteins. When DNA, the genetic material associates with histone proteins as they fold together to form the chromosomes, chaperones make sure that folding is correct.

Second, most chaperones are actually heat shock proteins. Huh? What are those? Well, in the presence of heat and stress, proteins usually tend to misfold and aggregate. The heat shock proteins or chaperones try to prevent this misfolding because protein function is dependent on their proper folding. Once they are misfolded they lose their ability to function properly. One example of a disorder that can happen when proteins are misfolded is mad cow’s disease.

The third instance when chaperones are needed is during the formation of new proteins. As the polypeptide chains exit from the ribosomes during their synthesis, chaperones guide them to fold properly into fully functional proteins.

Lastly, chaperones guide polypeptides as they are transported across the membranes of the mitochondria and ER. The chaperones make sure that the polypeptide chains pass through the membrane properly before they fold over.

As you can see cellular chaperones just like their social counterparts also ensure proper behavior of their molecular protégés.

Sunday, November 23, 2008

rough ER

The presence of ribosomes attached to the cytoplasmic side of this membranous network gives it a rough appearance, thus the name. Its main function is protein synthesis.

Initially, the rough ER does not have any ribosomes yet. Synthesis actually starts in ribosomes that are freely floating in the cytoplasm. However, once the initial sequence of amino acids called the signal peptide, are formed, the ribosomes then attach to the ER and continue with the synthesis. Since different kinds of proteins are being synthesized regularly, a continuous stream of ribosomes attach to the ER thus making them practically integral parts of the ER.

Protein synthesis is a very systematic and closely monitored process. Proteins which are destined for use within the cell are processed separately from proteins that are “exported” out of the cell. Markers are added along the way as the synthesis of proteins progresses. The markers ensure that proteins reach their proper destinations.

The rough ER works closely with the Golgi apparatus. Between the two of them, there is a regular flow of proteins in vesicles. There is even a whole process of vesicular movement between the ER, the Golgi, and the cell membrane that is called “membrane trafficking”. Yes, there is traffic flow in cells, but unlike the chaotic traffic in our society, this cellular traffic is very well coordinated and there are no traffic snarls along the way. Maybe our traffic coordinators could learn a thing or two by observing how cells do their trafficking.

Saturday, November 22, 2008

cell design 101.4, nerve cell



Every nerve cell is as unique as one’s fingerprints. But all nerve cells have something in common. Each one is excitable. Each one can change from a serene, hu hum cell to a fiery, pulsating one. How is this possible?

Well, we are of course familiar with the usual picture of a nerve cell, right? The picture is usually that of a star-like cell, with some cell processes sticking out. These cell processes even differ into two kinds; one is short and highly branching (dendrites), while the other is long and usually non-branching (axon). However, the latter usually terminates in small knob-like endings called axon terminals. So above is a diagram of a nerve cell.

As mentioned earlier, every nerve cell is unique, so this is not really how every nerve cell looks. However, it fairly represents one group of nerve cells called motor neurons.

So back to the question, how can a nerve cell change from a serene to a fiery cell?

Well, nerve cells have a unique way of distributing their channel proteins. Along their cell body membranes, the channel proteins (both leaky and gated) are distributed about almost evenly. So small batches of ions constantly move in or out through leaky channels, while gated channels open or close depending on the presence of some disturbance or stimulus. Then of course, ion pumps like the Na-K-ATPase pumps constantly pull back potassium ions (K+) that sneak out and bale out sodium ions (Na+) that sneak in. But, it’s a different story where the axon of the neuron starts. In this area, the neuron’s membrane is fully studded with Na and K gated channels. So, when the disturbance or stimulus is strong enough, it can set a whole bunch of gated channels open or close. The action is much like a floodgate opening or closing, it is sudden and it creates instant action. Na ions rush inwards while K ions rush outwards. Thus, the nerve cell is usually described as “firing” when it is in this state.

All this action takes place in milliseconds and the nerve cell once more goes back to its resting state, wherein only a few Na and K ions move in and out of the cell until the next disturbance or stimulus comes.

So the way things are arranged in space and time can spell the difference between a humdrum life and an exciting one. However, just like the neuron, we need both kinds of arrangement – one kind makes us grounded, the other kind makes us reach greater heights. Exciting, isn't it?

Friday, November 21, 2008

smooth ER

In the cellular world “ER” does not mean “emergency room”. Rather, it means “endoplasmic reticulum,” and there are two kinds of them: smooth ER and rough ER.

Our story for the day is about the “smooth ER.” The other kind will be the subject of another post.

Liver cells, steroid hormone-secreting cells, and muscle cells especially abound with smooth ER. In these cells the smooth ER performs different functions.

In liver cells, the smooth ER performs two major functions: detoxification and glucose metabolism especially conversion of glycogen to glucose. People who regularly use drugs have liver cells with very extensive smooth ER. The enzymes of the smooth ER convert the drugs into water soluble compounds that can then be easily eliminated from the body. The detoxifying effect is the reason why drug users need higher and higher doses of drugs.

In steroid hormone-secreting cells, the smooth ER is involved in the synthesis of steroid hormones like estrogen, testosterone and progesterone. While in muscle cells, they serve as depot for Calcium ions.

Changes in Calcium ion levels inside cells is an important signal for activities like muscle contraction and exocytosis. It is thus important to keep low the levels of Calcium ions in the cytoplasm.

In muscle cells, the smooth ER has a special name - “sarcoplasmic reticulum”.

Thursday, November 20, 2008

cell design 101.4


Question – “How would you design a cell that has very limited space but needs lots of free surface for interaction?”
Answer – Add frills to its free surface. And that’s exactly how intestinal cells are designed.
Intestinal cells are involved in absorption so they need lots of surface where absorption takes place. However, they must also be attached closely and strongly with other intestinal cells. They cannot allow food substances to pass between them. Whatever food we eat, these have to pass through, not between the intestinal cells. Not only that, these cells must also be anchored close to blood vessels so that whatever food they absorb, these can be transferred right away to the blood and then distributed to the rest of our body. So imagine that – the intestinal cell design has so many constraints: anchored at one end, attached to the sides and large free surface area.

Well, good news, - our intestinal cells are designed exactly that way, as shown in the illustration above.

Wednesday, November 19, 2008

phagocytosis

Phagocytosis or “cell eating” is a way of getting large matter inside the cell. During this process, the cells form “pseudopods” or special folds of their cell membrane and entrap or enclose food particles, small organisms, and other particulate matter. The folds then fuse and form a vacuole that pinches off from the membrane. Once inside the cytoplasm, the vacuole is now called a phagosome and it fuses with a lysosome. Lysosomal enzymes then digest the particle and release the digested material to the cytoplasm where it is used by the cell for various purposes. Phagocytosis is how the Amoeba obtains its food from the environment.

In animals, some cells act as “professional” phagocytes. Among these are the neutrophils and macrophages. These cells act like roving guards. They move around in the body eating or phagocytizing any particulate material that they encounter. The particulate material can include foreign invaders, dead or damaged cells, and cellular debris. So in these cells, phagocytosis is more of a clean up process rather than an eating process.

In the bone, a special phagocyte, the osteoclast, “eats” old cartilage and bone tissue and partners with the osteoblast, a “bone builder”, in bone formation, reconstruction and repair.

While the “eating habits” of our phagocytes are meant to protect us, sometimes, some enterprising microorganism can take advantage of this activity. Mycobacterium tuberculosis, the tuberculosis-causing bacterium for example, is one such enterprising organism. Once taken inside the cell through phagocytosis, the bacterium secretes an enzyme that prevents fusion of the phagosome with the lysosome. Thus, lysosomal enzymes cannot digest the bacterium and it then can multiply inside the cell and infect other cells too.

So, cells just like organisms should also watch what they are eating or suffer the consequences.

Tuesday, November 18, 2008

active transport

Cells keep various molecules at specific concentrations. For example, Potassium ions (K+) are kept high inside cells but Sodium ions (Na+) are kept low. Whereas outside of cells, concentration of K+ is low while that of Na+ is high. There is thus a concentration gradient created across the cell membrane. How does the cell maintain this concentration difference? Through active transport, that’s how.

Active transport moves substances against their concentration gradients and thus requires the expenditure of metabolic energy, usually ATP. The transporters are called pumps so there is a Na+/K+ pump for example. This pump transports Na+ out of the cell and transports K+ back into the cell. This way, the concentration difference across the cell membrane is maintained.

What is the importance of active transport in biological systems? Well, for one, it is responsible for enabling us to absorb more food from our intestines. If there is no active transport, then most of the food that we eat will be wasted. If absorption only depends on diffusion, then once the concentration of food in and out of the intestinal cells becomes the same, then absorption will stop. But as we have experienced, we can have 2nd helpings or even 3rds of some of our favorite food. So we have active transport to thank (blame?)for that.

Second, active transport is responsible for selective reabsorption of substances in the kidneys. There is a Na pump in kidney cells that pumps back Na into the intercellular space. Through osmosis, water then naturally follows the solutes. This is how we are able to reabsorb some water.

Third, cells have proton pumps that pump hydrogen ions during energy generation. These pumps are also used to maintain intracellular pH.

Active transport is also used by plants in absorbing minerals from the soil. While animal cells use Calcium pumps to maintain intracellular Calcium (Ca++) levels. After every muscle contraction for example, Ca ions (Ca++) are pumped back into storage so that muscles can relax. When a person dies, the cells lack ATP and active transport of Ca++ cannot occur. Thus, the muscle cells remain contracted and the dead person exhibits rigor mortis.

Monday, November 17, 2008

intercellular communication

How do heart muscle cells synchronize their beat? How do the cells lining our windpipe synchronize the movement of their cilia? The answer my friend is – through rapid intercellular communication.

Neighboring cells maintain rapid communication lines with each other so they can act as a single unit and not as uncoordinated separate units. In animal cells the rapid communication lines (actually pipes) are called gap junctions while in plant cells they are called plasmodesmata.

Gap junctions are pipe-like structures that connect adjacent animal cells. Unlike a pipe however which is made up of one rounded piece, gap junctions consist of 6 pieces that can change orientation and therefore open or close the opening of the pipe.

In the heart for example, only one part, the pacemaker, needs to receive the activation information but the whole heart responds as an integrated beating unit. This is possible through the opening of gap junctions between adjacent cells which then rapidly spread the information received. The same process allows for the synchronized beating of the cilia in respiratory cells as they move substances from one end of the windpipe to the other end. The peristaltic movement of the digestive tube is also synchronized through the same rapid communication across gap junctions of smooth muscle cells.

Because of their cell walls, plant cells cannot move in any synchronized manner. However, through their plasmodesmata which are small channels not covered by cell walls, the cytoplasm of adjacent cells can communicate with each other and move substances between them. This way, parts of the plant can function as one metabolic unit.

Sunday, November 16, 2008

plant vacuoles

While vacuoles in animal cells are small and temporary, plant vacuoles are big and practically permanent. The vacuoles in plant cells usually occupy about 30% of the cell’s volume but can sometimes become as much as 90%. Why is this so? Well, plant vacuoles have many functions.

First, it can act a storage area of many different things ranging from water, minerals, enzymes, ions, and even toxic substances. The toxin helps protect the plant from being eaten by predators. If we cut a plant, the contents of the vacuoles, generally called cell sap, spill out.

Vacuoles also store pigments. These pigments are responsible for giving us the beautiful colors of flowers and fruits.

The second and even more important function of vacuoles in plant cells is to maintain turgor pressure against the cell wall. Turgor pressure makes the plant firm and rigid.

Vacuoles are surrounded by a special membrane called tonoplast. The tonoplast is selectively permeable and has pumps that actively pump ions in order to maintain the water content of the vacuoles. If you remember, I mentioned before that water moves by osmosis. It always moves to were the solutes are. So if the tonoplast for example pumps in potassium ions, then water follows the ions and moves inside the vacuole. If there is enough water available, then turgor pressure is easily maintained by the plant through this pumping action. The plant then remains firm and rigid.

If there is not enough water however, the vacuole shrinks and moves away from the cell wall. Turgor pressure then decreases. When this happens, the cells plasmolyse and the plant wilts. So vacuoles really help maintain the structural integrity of plant cells.

Oh by the way, some of the other materials stored in plant vacuoles are: opium, rubber, and garlic flavoring.

Saturday, November 15, 2008

cells have a detoxification center

Aside from having a recycling center, cells also have a detoxification center. The center is the peroxisome, another organelle found in eukaryotic cells.

Peroxisomes are so named because they use molecular oxygen for carrying out their function. Just like their cousins, the lysosomes, they can also recycle substances. For example, they breakdown fatty acid molecules into small chunks of carbon atoms resulting in the formation of acetyl CoA. Acetyl CoA is then recycled for use in biosynthetic reactions.

An important and specific function of peroxisomes however is – detoxification. If you are an alcoholic drinker for example, you can expect the peroxisomes of your liver and kidney cells to be working overtime in converting the toxic alcohol to non-toxic substances.

Peroxisomes also remove another toxic substance, hydrogen peroxide, from our system. Hydrogen peroxide is a by product of many reactions so it can readily build up in the cell. To prevent this build up, peroxisomes convert the hydrogen peroxide into water and oxygen.

Abnormality or malfunction of the peroxisome results in severe abnormalities in the brain, liver, and kidneys. Individuals with the abnormality immediately die soon after birth.

Friday, November 14, 2008

cells recycle

Let’s leave the cell membrane for a while and go inside the cell. Since environmental issues are so popular these days, let’s focus on the original recycling center found inside cells.

Eukaryotic cells have an organelle called the lysosome. This organelle contains hydrolytic enzymes that convert food particles, phagocytosed bacteria or viruses, and old, worn-out cell parts into smaller pieces that are then used, recovered, reused, and recycled by the cell into other functional parts of the cell. Lysosomes can even recover through the process of endocytosis, membranes that are once part of vesicles and receptors.

Lysosomes are so efficient in their function, they will “eat” anything they encounter, break it into pieces, and recycle them. There is practically zero waste inside cells because of lysosomal activity.

Sometimes however, lysosomes can malfunction because of lack of some enzymes. When this happens, substances can accumulate inside the cell and interfere with normal cell function. The cell then gets sick and of course the individual gets sick. Tay Sachs disease and Pompe’s disease are examples of diseases caused by malfunction of lysosomes.

In Tay Sachs disease, fatty proteins accumulate in the brain and affect a baby’s sight, hearing, movement, and mental development. In Pompe’s disease on the other hand, glycogen accumulate inside muscle cells and cause weakness of muscles including muscles of the heart and the respiratory system. The individual may then die of heart failure and/or respiratory failure.

All these for not being able to recycle because the cell's recycling center malfunctioned.

Thursday, November 13, 2008

ion channels

Ions are charged molecules and cannot easily pass through the cell membrane. As we know, cell membranes are basically made up of a double layer of phospholipids. By their nature, ions cannot therefore pass through these membranes. That’s why ions can only get in and out of cells through ion channels.

These channels are protein molecules inserted through the membranes. They have an inner core that is hydrophilic through which the ions can pass and an outer hydrophobic region that interacts with the phospholipid membrane.

There are two major kinds of ion channels: leaky channels and gated channels. As the name implies, leaky channels are open all the time and ions can leak through if there is a concentration difference across the membrane. However, there are only few leaky channels. The gated channels are more numerous and different factors can swing open or close their gates.

For example, some gated channels swing open or close depending on changes in voltage across the cell membrane. These voltage-gated channels usually participate in conducting electrical signals.

Other gated channels swing or close when specific molecules bind to receptors associated with them. They are therefore called ligand-gated channels.

There are also volume-gated channels that swing or close when there are changes in the volume in and around cells. And mechanical-gated channels like those associated with touch receptors, swing or close when there is mechanical change around them.

Opening and closing of these gated channels usually produce specific physiological changes in the cell. Changes in ion concentrations inside cells are signals for specific physiological activity. Examples of physiological activity are: nerve impulse conduction, beating of the heart, activation of visual receptors, touch receptors and other sensory receptors and many more.

Many disorders are associated with ion channel malfunction. Many toxins produce their effct by blocking ion channels. For example, the reason why red tide can poison people is due to blockage in sodium channels.

Wednesday, November 12, 2008

diffusion

I almost flipped when I encountered this sentence in a biology textbook – “The diffusion of sodium is done against a concentration gradient.” Huh? How can that happen? That is simply illogical.
Diffusion is passive transport. It is always along the concentration gradient, never against it. Diffusion is directly related to the difference in concentrations between two areas. This difference in concentration is called the concentration gradient. Molecules will always move along this gradient, that is, from where the molecules have greater concentration to where they are less concentrated. This movement is called diffusion. So if sodium moves by diffusion, then it’s moving down this concentration gradient, not against it. Molecules can only move against their concentration gradient by active transport.

Tuesday, November 11, 2008

osmosis

One of the most common misconceptions I usually encounter in biology textbooks is about osmosis. The following is a direct quotation from one of these textbooks. “Osmosis is the diffusion of water through a selectively permeable membrane from a greater concentration of water molecules to a lesser concentration of water molecules.”

Do you see anything wrong with that statement? I hope you do. It’s the phrase “concentration of water molecules.” Water is not concentrated, it’s the solution that is concentrated. A solution may contain more water molecules or less water molecules. The former is a diluted solution while the latter is concentrated. So it’s the solution that is either concentrated or diluted, not the water.

During osmosis, water moves through a differentially permeable membrane towards an area that has more solute molecules. Wherever the solutes are, that is where water will move to. Thus, for solutions that are separated by a differentially permeable membrane, water will always move from where there is more water molecules (the diluted solution) to where there is less water molecules (the concentrated solution). Please remember, water is not concentrated!

Monday, November 10, 2008

Cells touch

Touch is a very important form of communication. We touch somebody when we want to show love or sorrow, joy or approval. We touch babies and they smile. Research shows that babies who do not receive a loving touch usually develop abnormally. In the same manner, cells that do not receive the touch of other cells do not develop at all into specialized, functional cells. Why?

Remember we started as a single, fertilized cell? Well after fertilization, this single cell divided and divided until there were so many small cells that are literally clones – no different from each other. However, after they reached a certain number, they started communicating with each other through touch (through receptors on their membranes) and exchange of molecules. When the cells were dividing and dividing, there was not much communication going on. However, when they stopped dividing and started communicating with each other, something beautiful happened. The cells began to develop their own identities (in the language of developmental biology, this is called differentiation).

Some of the cells later started moving together until they reached a certain destination. There they establish their territory and develop into an intricately designed organ like the heart or the brain. Others formed the stomach or the lungs and all our other beautifully formed organs(in the language of developmental biology, again, this is called organogenesis).

It is amazing how a touch can change a clone-like cell to a beautiful beating heart cell or a fiery nerve cell.

Sunday, November 9, 2008

cell design 101.3

After incorporating two major variations (internal membrane system and cell wall) to the basic cell design, all further variations are what I call “icing on the cake.”

So what can we consider as “icing on the cake” variations? Well, in animal cells, further variations include changes in size, shape, structure, etc. Some cells stretched out, flattened, became spherical, developed processes, merged, branched, and exhibited other changes. With every change emerge a new cell. Thus we have so many different kinds of animal cells like: muscle cell, nerve cell, lung cell, red blood cell, liver cell, bone cell, egg cell, sperm cell, etc. In the human body for example, there are more than 200 different cell types.

What about plant cells, what kind of “icing on the cake” variations occurred? Well, because of their cell wall, plants cells cannot change shape or stretch out or flatten. So how do plant cells vary? Well, their variations are mostly in their cell walls. Some have very thick or very thin cell walls, while others have added new substances, or even spaces to their cell walls. Thus, we recognize plant cells as: collenchyma, parenchyma, or sclerenchyma. Their main differences are simply in their cell wall. Of course plant cells also differ from animal cells by having chloroplasts and a large vacuole in their cytoplasm.

Saturday, November 8, 2008

cells can be cultured

No, a cultured cell does not go to concerts or view art works or read classical pieces. Rather, a cultured cell is one that is kept in a controlled condition in the laboratory.

Yes, cells are found not only inside the body of an organism but can also be found outside. They can be removed from the organism’s body and cultured in the laboratory. Here they are kept alive for long periods of time where they can even grow and produce more cells.

In fact, the first ever group of cultured cells, the so called HeLa cells, have survived their owner. Henrietta Lacks died a long time ago but her cells (actually taken from her tumor) are still living today in various laboratories. These cells are now being used for various medical researches in cancer, drug tests, cosmetic tests, and many others. For example, Jonas Salk used HeLa cells when he developed and tested the polio vaccine.

Friday, November 7, 2008

water world

One of the very first lessons in Biology that we usually discuss is the biochemistry of cells. We then note that cells consist mainly of water plus biomolecules and some minerals. If I ask my students the question “why is water the most abundant component of cells?” – One answer that usually comes up is - “because water is a universal solvent.”

Huh? I simply cringe every time I get this answer. Sometimes I even get ballistic! Ugh! Double Ugh! How can one say water is a universal solvent??? Why is this mentioned in Biology books and even in Chemistry books? Water is not a universal solvent! It is just a very good solvent but not a universal solvent. If it is a universal solvent, then we will not even exist. There will be no cells, no plants, no animals, no mountains, rocks or stones. Everything will be dissolved in water. Everything will be in solution. Everything will be…water. In fact the only thing that there will be is…water!

Thursday, November 6, 2008

cells commit suicide

Some cells die because of cell injury but other cells die because they commit suicide. Yes, they do! And why would cells do that? There are various reasons.

One reason is, to remove severely damaged cells and prevent them from being duplicated. Cells generally repair some damages that may develop in them. If the damage is too extensive however, they receive signals to commit suicide rather than pass on their defect to the next generation of cells.

Another reason is to maintain the number of cells in the body. We cannot have too many or too little number of cells. So “old, worn-out cells” that have reached their “expiry date” are told to commit suicide. They are then replaced by new cells through the process of mitosis. In the cell world, there is no such thing as extension of retirement. Any extension is always detrimental to the whole organism. Some cancerous cells for example ignore this “expiry date” so the organism develops cancer.

Still another reason is “body sculpting” during development. In the formation of our fingers and toes for example, the starting form is a whole solid mass. During development, the “in-between” cells in the mass commit suicide and this leads to the formation of separate fingers and toes. Sometimes some cells may not commit suicide so the separation of fingers can be incomplete. Thus, we sometimes see people with partially fused fingers or toes.

The disappearance of the tadpole’s tail as it is transformed to an adult frog is an example of “body sculpting” produced by cell suicide.

There are other reasons for cell suicide but those three are the most important ones.

By the way, the scientific term for cell suicide is – “apoptosis” or programmed cell death.

Wednesday, November 5, 2008

cell design 101.2

After eukaryotic cells were formed, one other variation to the cell design was added. This variation is enough to differentiate plants from animals. I am talking about the addition of the cell wall in plant cells.

If one takes a closer look at all plant and animal cells, one will discover that this, the cell wall, is one of only two major differences between these cells. The other difference is the presence of chloroplasts in plant cells. But this will be the subject of a future post. In this post, I would like to focus on the cell wall first.

Imagine that, a major difference between our own cells and those of a rose plant’s for example is simply the presence of cells walls in the rose cells. Amazing! But just this single difference is enough to give us nerve cells that reach out and muscle cells that shorten and elongate. Plant cells cannot have cells that reach out or shorten and elongate, the rigid cell wall prevents that from happening. That’s the reason why plants stay rooted and rigid in one place while we can walk, jump and do cartwheels.

Tuesday, November 4, 2008

Cells talk

Do you know that cells talk non-stop? Huh? Yes they do. This is accomplished through receptors that are found mostly on their cell membrane. Some receptors however are found in their cytoplasm and in their nucleus.

Cells keep a variety of receptors on their membrane. These receptors respond to specific information produced by other cells and their surroundings. For example, neurons send information (neurotransmitters) to other cells adjacent to them while endocrine glands send information (hormones) to other cells that are far from them.

Anyway, these information molecules bind with specific receptors on the membranes of their target cells. Once binding occurs, a series of signals are set in motion and the target cell responds. The response depends on the information received. If the information for example is a neurotransmitter, and the target cell is another neuron, then this target neuron may be excited or inhibited. If on the other hand the information is a hormone like insulin, then the target cell starts absorbing glucose and using this for its metabolic needs.

Thus, a cell maintains a diversity of receptors so they can receive constant information from other cells and from their surroundings. They have to keep on talking with these other cells and their surroundings. The talk is essential for their survival.

Do you know that if cells can’t talk, they will die? This will be the subject of a future post.

Monday, November 3, 2008

cell design 101.1

As our cell story unfolds, observe that after putting together the basic requirements in cell design, a major variation is incorporated. This major variation is the formation of an internal membrane system that encloses the genetic material and several of the biosynthetic machinery molecules. What is/are the result(s) of this variation? Well, we can now recognize two large groups of cells – the prokaryotic cells and the eukaryotic cells. The eukaryotic cells are those with the internal membrane system while no such structure is present in prokaryotic cells. Because of this variation, the enclosed genetic material is now called the nucleus; while the enclosed biosynthetic machinery molecules are now identified as organelles. Isn’t that beautiful? Just one major variation, and a whole new world, the world of eukaryotic cells, with their nucleus and various organelles unfolds.

Sunday, November 2, 2008

Is the virus a cell?

This question becomes easy to answer once the basic requirements of cell design are known. One just needs to check if the 3 basic requirements: cell membrane, genetic material and biosynthetic machinery are all present in the virus. If one does that, then one will discover that the 3rd requirement, the biosynthetic machinery, is missing in viruses. There you have it; the virus cannot be a cell because it lacks the 3rd basic requirement for a cell. This is the reason why viruses have to infect other cells; they need to use the biosynthetic machinery of these cells in order to replicate their own genetic material and form membranes. That’s the only way to form more viruses.

Saturday, November 1, 2008

My favorite part of the cell

My favorite part of the cell is the cell membrane. Why? Because so many interesting things happen on and in the cell membrane.

Just think - what is the first thing that happens when a sperm meets an egg during fertilization? - cell membrane interaction of course. What is involved when a neuron gets excited and fires a nerve impulse? - cell membrane transport changes, that's what happens. When the environment gets freezing cold, what do cells do? - put on "kinky" cell membranes.

I could go on and on about cell membranes but I think you get what I mean. There is so much going on and in cell membranes. I can actually use up one whole class period just talking about cell membranes but this is not a class but a blog. So this is where I stop today. But I hope I got you thinking about cell membranes.
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