Cell Division

This section is the last of the topic "Patterns in Nature".  Cell division is very important for organisms.  Certain cells quite literally wear out, others are damaged by the environment and others can be damaged or killed by some type of trauma.

In addition, organism growth is a result of cell division, not cell growth.  Remember what happens to cells when they get larger?

And finally, not all cells will divide for your entire life.  For example, nerve cells tend to stop dividing completely by about the age of twelve in humans.  This is why injuries involving nerves, the spine or the brain are often lifelong.

The type of cell division we are looking at in some detail is mitosis.  There are 2 important stages in cell division:

1. Division of the DNA
2. Division of the cytoplasm.

Starting with the DNA, when we see the nucleus, the DNA is essentially unwound so that the cell can use it for instructions for various cell processes and products.  It is a bit like the wool in the left hand picture here.  If we were to divide this up, chances are there would be large amounts of DNA breakage which will result in the cells being not viable.  To prevent this, the cell winds up the DNA (the process is called condensation) so that they form the visible chromosomes.  In doing so, they arrange the DNA into structures like similar to those on the right, which is much easier to divide into 2 cells.

Let's run through the stages of mitosis.

1. Interphase - This is the longest phase and is the stage where DNA is copied and the cell increases in size.  Another feature of anaphase is mitochodria and chloroplasts divide independently of the cells at this stage.
2. Prophase - The nucleus begins to condense into visible strands of DNA or chromosomes
3. Metaphase - The chromosomes align themselves centre of the cell along the "equator"
4. Anaphase - The chromosomes pull apart forming chromatids
5. Telophase - The DNA in each daughter cell begins to unwind forming a new nucleus.

However, division of the nucleus is only one part of cell division.  The cytoplasm mus also be equally divided.  If its not, then the one of the daughter cells is likely to be non-viable, because there is not enough cytoplasm containing nutrients and organelles for the cell to keep functioning. If that happens, why divide in the first place?

To ensure cytokinesis (division of the cytoplasm) takes place effectively, the cell forms a band of contractile proteins around the equator of the cell.  These pinch the cell into two fairly equal parts.

Now you may have read that I said the mitochondria and the chloroplasts divide during interphase.  This because they have their own DNA regulating their growth.  A possible reason for this is discussed in the next topic called Life on Earth. It is sufficient to know at this stage that not only does the nucleus contain DNA, but so do these organelles.

So where does cell division occur?  In flowering plants it occurs in the bud at the end of the stem (called either the terminal bud or apical meristem).  Loss if this but causes plants to shoot at lower buds.  This is why gardeners will prune plants to encourage bushiness in the lower parts.

Grasses somewhat unusually grow at the base of the stem.  This adaptation allows them to survive being grazed by herbivores and fire.

Stem and branch thickening occurs in a layer of tissue between the xylem and phloem of the cell called the cambium layer.  More on this in the HSC course.

 Roots divide in a zone just behind the root cap.  From there they elongate.  Between these 2 processes, the roots are able to penetrate into the soil.

Zones of cell division in animals vary.  In humans (and probably mammals in general), one of the most active areas of cell division is the bone marrow which produces blood cells.  On average, a human turns over about 120 million red blood cells a day.  Having said that the blood stem cells have the potential to become not only red cells but platelets and a variety of white cells.  That is part of the differentiation process, something mentioned earlier.

However, the stem cells need to divide before differentiating.  Mitosis is the solution to this.

Removing Nitrogenous Wastes

Nitrogenous wastes are produced when the amine group of amino acids are removed.  This process is called deamination and take place in the liver.  An example is below.

One of the problems is the toxicity of ammonia.  It needs to be either quickly converted or removed from the body.  Examples of how ammonia is treated by various animals is shown below 

As you can see, most fish don't bother converting the ammonia.  They just excrete it straight into the environment via the gills.  Sharks convert it to urea and use it to regulate their internal fluid levels (not needed for this course).  However, land based and egg laying animals are faced with the challenge of not poisoning themselves while retaining water.  As a result while benefits are gained, it comes at a cost in energy.

Mammals and amphibians use the urea molecule as their solution.  It is about 100 times less toxic than ammonia, and very water soluble.  As a result removal of urea can occur with far less fluid.  In mammals the main organ of excretion is the kidney.

The fluid from the blood is squeezed into part of the nephron called the Bowman's capsule.  From there, it travels along the tubule.  By a combination of active transport, osmosis and the selectively permeable nature of membranes, things that the body needs are returned to the blood and waste salts and urea are removed via the urine stream.

Birds and reptiles also have kidneys.  However the material they excrete is uric acid.  Uric acid has a 2 key characteristics.

1. It has very low toxicity
2. It is very insoluble. 

Uric acid causes a great deal of pain to gout sufferers because of its insolubility.  But it means that if you are developing in an egg for a few weeks, the material will solidify before it can poison you.  In addition, it means that very little water is needed to get rid of it as it can be fed into the fecal pellets and voided with that waste.  Pictured below are the wastes of lizard and bird. The uric acid is the white stuff.

Insects on land generally lay eggs and so use uric acid as a means disposing of nitrogen containing wastes.  It is collected by a series of tubules in their bodies called Malpighian tubes.  The uric acid is concentrated and is then fed into the digestive tract where it can be excreted as part of the feces. 

Gas exchange 2 - Countercurrent flow and Plants

Fish gas exchange

Because the amount of oxygen in water is about 1/20th compared to air, fish need a highly efficient means of oxygen extraction. To this end they combine high SA to vol with something called counter-current gas exchange.  

Counter current flow means that the blood and the oxygen flow in opposite directions to each other (shown on the left hand side of the figure below).  If the blood and water flowed in the same direction, it would be called concurrent flow. 

The real benefit of counter current flow is the ability of the blood to always have a slightly lower concentration than the water flowing over it.  This means a concentration gradient is established for longer and more oxygen is removed from the water to the blood for the fish.  If blood and water flowed in the same direction, then the concentrations would be equal at some stage and no further gas exchange could take place. 





Plant Gas exchange

In the leaf, gas exchange structures are the stomates.  These not only allow O2 out and CO2 in,  but also allow minerals to be transported up the plant as transpiration acts as the driving force for the movement of water up the plant.

Stomates in plants that live in warmer drier climates like Australia tend to open in the morning, close down in the middle of the day, and then reopen in the afternoon.  This way, water loss is reduced in the warmest part of the day, but light intensity is not compromised. Stomates are generally on the underside of the leaves away from direct sunlight which also helps with water conservation.

Lenticels are corky areas of loosely packed cells that are located on branches, stems, and the trunks of plants.  They are able to let oxygen into these parts of the plants that cannot perform photosynthesis to be used in respiration and allow CO2 to escape.  

When it comes to the roots of plants, most absorb oxygen through from air spaces within the soil. This is why over-watered plants die, because the roots literally drown.  However, some plants live in areas where air can never get into the soil to allow them to efficiently exchange oxygen and CO2.  The best example of this are mangroves.  To compensate for this they have snorkel like extensions on their roots called pnematophores.