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Water and its importance in plants and animals

Properties and functions of water

I mean, water. What more is there to say? It's water for crying out loud. It does cool tricks.

Take for example alphabet soup.


In the beginning, it's just a dry a$$ powder, overly salty, overly hard, overly dry, totally inedible and all-round disappointing. But add a bit of hot water and BAM! you have yourself a totally delicious, mind-blowingly satisfying dish. 

Same with life. It can't just be earth. It needs water. It needs a solvent, a containment environment for its chemicals. Many of life's basic reactions like condensation and hydrolysis rely on water being present.




Water is to solvent as bear is to North Pole . It is polar. Water is a polar solvent. I could have just said it plainly but I had to insert a ludicrous arctic animal joke. So, water is polar. Because the oxygen in water has a negative charge relative to the hydrogens which bear a relative positive charge, ions such as those found in sodium chloride (NaCl) can bind respectively to the oxygen side or to the hydrogen side. Thus, the salt is soluble and dissolves.

The dipole nature of water enables hydrogen bonding which takes place between the hydrogen atoms of one water molecule and the oxygen atom of another.

As such, it is no wonder that water is the main component of cellular cytoplasm. It is present both inside and outside of cells and fulfils the role of transport medium in plants and animals. This includes the cell sap in plants which enables turgor pressure to prevent cells from collapsing onto themselves, and animal fluids such as plasma, serum, tissue fluid, lymph and urine.




Solutes in water include sugars and proteins, as well as electrolytes such as hydrogen ions (H+), potassium ions (K+), sodium ions (Na+), chloride ions (Cl-), hydrogencarbonate ions (HCO3-), magnesium ions (Mg2+) and others.

The exact composition of different fluids varies between individuals, over time, between species, etc. In order to investigate fluid composition, various methods exist. Sugars and proteins can be detected and measured in body fluids as well as plant extracts using reagent test strips and biosensors.

For example, glucose test strips can measure the glucose concentration in a small blood sample.

Basic legacy tests for the presence of reducing sugars and proteins are the Benedict  test and Biuret test. These involve adding a reagent to a test sample and observing a simple colour change.

Benedict's reagent is light blue. A reducing sugar such as glucose will turn it orange/brick red. A non-reducing sugar like sucrose will not change its colour.


You also need to know about the Biuret test for proteins. Biuret is also a pale blue solution which, when added to protein, will turn lilac. The sample to be tested could be taken from a certain drink for example.

Colorimetry is a more sophisticated method of assessing colour changes to identify the presence of specific molecules. It uses automated equipment such as colorimeters which pass a beam of split visible light through a liquid sample and measure how much light of that wavelength is absorbed by the sample.

Spectrophotometers are more useful and commonly used in labs; they pass a beam of light through a liquid sample and measure how much light of a specific wavelength is recorded on the other side i.e. the amount of light that has passed through the sample rather than been absorbed.

For example, 600 nm is the wavelength used to measure the amount of bacterial cells in a sample. 220 nm and 280 nm are the specific wavelengths used to measure the amount of protein and DNA in a sample.





Some spectrophotometers use test volumes of 1 ml while others, especially for measuring DNA and protein, can use as little as one tiny drop (2 μl) of the test sample.

The higher the value, the more opaque the sample i.e. the more target molecule is present in the sample.


Carbohydrates

Life in its ecosystems, species and organisms has been expressed, and continues to do so, in a great variety of ways.


Yet on the biochemical level, deep down into the tissue, cells and microscopic components that make up these living things, the basic building blocks are the same! A bit like how everything is just made up of atoms or quarks or just empty space really but it somehow ends up looking really interesting to us.

Some of these basic building blocks are called monomers ("single part", Greek) and join together to create bigger molecules called polymers ("many parts"? I don't know, I'm making this up).

Examples of monomers that we'll go into more depth in later topics include monosaccharides such as glucose, amino acids such as methionine, and nucleotides which make up DNA.

How can monomers create a polymer? The types of chemical reactions yielding bonds to enable this to happen are many. One of the key ones is called condensation. It is a joining reaction and releases water in the process.

Its reverse reaction uses up water and breaks up the bond. You might have come across it before - hydrolysis (breaking water? breaking with water?).


In this context the monomers are like standard lego pieces that can be joined any way to build bigger things with - notice the hydrogen (H) and hydroxyl (OH) groups on either end of each small molecule. That's where further molecules can join up and grow really large molecules.


Carbohydrates as well as proteins are polymers and contain only a few different types of atom. In the case of carbohydrates, the basic molecular units are called monosaccharides - these are the monomers. (mono = single; poly = multiple; saccharide = sugar)

They are organic compounds with potential for limitless diversity that underpins biological function. Organic molecules are based on carbon plus additional elements, notably hydrogen and oxygen.

α (alpha) glucose is the most important monosaccharide to learn, as you need to be able to draw it:


The points where the lines intersect each symbolise a carbon (C) atom. You need not show those. The figure above is taken from the specification itself, so take it as a good guide. So the monosaccharide alpha glucose (commonly, just glucose) somehow becomes a polysaccharide, This is achieved by condensation reactions, and the bonds formed are called glycosidic bonds




You should be able to draw this. The resulting molecule, maltose, is a disaccharide (two monomers). If you keep adding glucose molecules to the chain, you get... *drum roll please* ...starch. Starch is made up of multiple (very many indeed) monomers, so it is a polymer i.e. it is made of multiple monosaccharides, so it is a polysaccharide.

Potatoes anyone?

You also need to know about lactose.

Lactose is made of glucose and galactose.

It's easy enough to remember: lactose (MILK) is also made of galactose (galaxy - Milky Way).



A couple of important polysaccharides are starch and glycogen.


Starch

Basic unit: α glucose



Function: the main storage molecule in plants

Structure: starch is made of two compounds - amylose and amylopectin. Both are, of course, made of α glucose, but their overall shapes differ. Amylose is a spiral, while amylopectin has branches. Combined, they give starch the appearance of a tightly wound molecule like a brush.

Crucially, starch is an excellent storage compound, so must satisfy certain requirements. Its size must be relatively big so that it is not soluble. This prevents it from causing an osmotic effect in cells whereby water floods in. The molecule must be compact in order to take up little space rather than a lot. This is achieved by the branches and spirals within starch. Finally, the branches also contribute to the readiness of the glucose molecules of being "nipped off" and quickly usable. This is because only glucose molecules at the ends of starch can be used in that way.

There is a test for starch. Chop a potato (they are practically made of the stuff), add iodine which is yellow. The starch in the potato will turn it blue. This is a very simple test for starch - if the solution stays yellow it's negative, if it goes blue, it's positive.




colorimeter can be used to actually measure the different colour intensities that correlate with starch concentration. In an experiment, different starch concentrations can be measured by taking readings of reaction volume samples in cuvettes. An enzyme that breaks starch down, such as amylase, can be added to investigate the rate of starch breakdown. The absorbance obtained at time intervals e.g. every 5 minutes can then be plotted on a graph.


Glycogen

Basic unit: α glucose

Function: the main storage carbohydrate in mammals

Structure: the structure of glycogen is essentially the same as that of amylopectin i.e. branched structure. The difference is that glycogen is even further branched compared to amylopectin. This enables a quicker build-up and breakdown of glycogen, hence meeting the superior energy demand of animals as compared with plants.



The glycosidic bonds between glucose monomers are termed 1, 4 glycosidic bonds because they take place between a monomer's carbon number 1 and another monomer's carbon number 4.




At the point of branching off, a different bond must take place, otherwise the addition of further monomers would just result in a simple continuous chain. This is a 1, 6 glycosidic bond that takes place between the 1st carbon and the 6th carbon on two monomers.



Osmosis

Osmosis is the diffusion of water across a semi-permeable membrane. The "concentration" of water is referred to as water potential. So osmosis is the movement of water from a higher water potential to a lower water potential across a membrane.



For osmosis to occur it is essential that there is a semi-permeable membrane separating two environments with a different solute concentration. The solute must be unable to cross the membrane (molecules too big), but the water molecules are free to pass through and lead to an equilibrium. In the above image, the right side of the beaker has a higher water potential than the left side, so water moves in from right to left.

You must also learn the term isotonic. A solution is isotonic when it has the same water potential as another solution. An example is Ringer's solution which has the same water potential as blood plasma, so can be used to keep tissues alive.

The terms that describe the overall water potential (ψ) are solute potential (ψπ) and pressure potential (ψp), amongst other factors. Solute potential refers to the property determined by the concentration of solute dissolved in water, while pressure potential refers to the mechanical pressure that the water system is under. The sum of these gives the overall water potential.

ψ = ψπ + ψp

Knowing the values for these terms enables easy calculation of overall water potential.

Extreme shifts is cell water potential in animal and plant cells result in the plasma membrane being pulled away from the cell wall, or by the cell lysing.

In animal cells which do not have cell walls, this is called lysis and is caused by too high a water potential (drawing in excess water and leading to bursting). If the water potential is very low and solutes are drawn in while water leaves the cell, it causes it to shrink. This effect is called crenation.




Plant cells have cell walls, so these can offer some resistance to water pushing against the cell wall if the water potential runs high. Cell bursting is resisted by default due to the turgor conferred by the cell wall. In the opposite scenario of water leaving the cell, the plasma membrane starts pulling away from the cell wall. This is called incipient plasmolysis. When the plasma membrane has completely detached from the cell wall, plasmolysis is complete.

Practicals involving manipulating factors that affect osmosis can be carried out. Notable factors are solute concentration and the concentration gradient, alongside the semi-permeable membrane. For example, creating a very steep concentration gradient across a semi-permeable membrane such as that of cells can result in fast plasmolysis. The concentration gradient can be changed by using different solute concentrations either outside of the cell or inside the cell.




The classic potato pieces (short or long cylinders) experiment illustrates this. Tall cylinders can be immersed in water and then have their length compared to control cylinders (not immersed in water). Alternatively, shorter cylinders can be cut out of potatoes and immersed in water. Instead of measuring their length and comparing to control cylinders, they can be weighed.

In both experiments, the cylinders immersed in pure water will have taken up water and lengthened/become heavier, while those immersed in sugary water will have shrunk/become lighter.

Ok byeeeeeeeeeee

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