Fertilisers is the term used for “chemicals or natural substances added to soil to promote the growth of plants”.
Key point: in spite of what it says on this packet of fertiliser, fertilisers are not food for plants. (Just adding this photo to the post makes me feel slightly sick inside: how could MiracleGro be so happy to confuse generations of people who visit garden centres…..?)
Plants are autotrophic: they make their own food molecules in the amazing process of photosynthesis. Plants use carbon dioxide from the air plus water from their roots to produce a whole range of organic molecules powered by the energy from sunlight.
But remember that in order to make amino acids, proteins and DNA plants will also need a source of nitrogen atoms. Carbon dioxide and water do not contain any nitrogen atoms and yet nitrogen is needed for building amino acids, proteins and DNA.
Where do plants get this nitrogen from?
Well the key idea is that they do not take it from the air. Nitrogen gas in the air is very un-reactive and cannot be fixed in the plant. But the soil contains nitrate ions and plants can absorb these by active transport in their root hair cells. Nitrate ions are transported up the plant in the xylem and can be used to make amino acids etc. in the leaf cells.
Nitrates are not the only mineral ions taken up by plants in their roots. Plants absorb phosphate (for making DNA), magnesium (for making chlorophyll), potassium (for a wider variety of cellular processes) amongst many others.
So fertilisers are a way of replenishing the concentration of these essential minerals in the soil. More fertiliser, more minerals, faster plant growth as more proteins/DNA etc. can be made in the leaves…. Simples!
The commonest type of inorganic (chemical) fertiliser are called NPK fertilisers. (Nitrogen, Phosphate, Potassium). These can be bought in handy 50kg sacks (see picture above), stored and then spread easily over fields.
Farmers can also use manure which is an organic fertiliser. Here are some advantages/disadvantages of organic fertilisers in case you are interested…. It is smelly, bulky and difficult to store.
You will have studied the Biological Molecules section in some detail I would imagine, perhaps in more detail than is absolutely required for the specification. This post is meant to help you focus your understanding onto those points that are most likely to be tested in iGCSE questions. Here goes…
You do need to understand some chemistry for this topic to make sense. In particular you need to understand what is meant by the following terms:
My personal definitions would be as follows:
Atom: the smallest particle that retains the chemical properties of the element – a structure made up of protons, neutrons and electrons
Molecule: a particle made of two or more atoms chemically bonded together – may contain just one type of atom or several
Element: a substance in which all the atoms are the same
Compound: a substance containing more than one type of element
Back to safer ground…..
Living organisms are made from a fairly small group of molecules. The commonest molecule in every organism is water and in humans water makes up about 70% of the mass. But if you were to remove all water, leaving behind just the dry mass, the most common molecules could be grouped into proteins, lipids, carbohydrates and nucleic acids (e.g DNA)
Carbohydrates contain just three elements – carbon, hydrogen and oxygen
Lipids (fats and oils) contain three elements – carbon, hydrogen and oxygen
Proteins contain four or five elements – carbon, hydrogen, oxygen, nitrogen and sometimes sulphur
Big idea: many of the molecules that living things are made from are examples of polymers. A polymer is a large molecule made up of a long chain of repeating subunits (called monomers)
Carbohydrates are grouped into three main types:
Simple sugars like glucose or fructose – these are called monosaccharides.
Some sugars like sucrose are made of two simple sugars joined together – these are called disaccharides
Some carbohydrates are macromolecules (polymers) made of many hundreds of sugar residues joined together – these are called polysaccharides.
You can see from the diagram above that there are three important polysaccharides in living organisms. All three are polymers of the sugar glucose but the arrangement of the glucose residues is different. Cellulose is the main constituent of plant cell walls. Starch is a storage polysaccharide found in plants and Glycogen is a similar storage molecule found in liver and muscle tissue in animals.
Glucose is detected using a Benedict’s Test. Heat the solution with Benedict’s,reagent to 90 degrees for 5 minutes. A positive test for glucose is a brick red colour.
Starch is tested for using iodine solution (in potassium iodide) Iodine solution turns blue-black in the presence of starch.
Proteins are also polymers but this time the individual monomer is not a sugar but a molecule called an amino acid.
This protein is then folded up into a complex 3D shape using a whole load of weak bonds that can easily be broken at high temperatures. This is why enzymes, made of protein, denature at high temperatures.
There are 20 different amino acids that could be incorporated into a protein so there are an almost limitless variety of different proteins that can be made.
Lipids are a group of water-repelling molecules that again contain C,H and O atoms. They used to be separated into fats and oils depending in whether they are a solid (fat) or liquid (oil) at room temperature. Many lipids are a type of molecule called a triglyceride and this is made of a single molecule of glycerol attached to three fatty acid tails.