Fertilisers: Grade 9 understanding for IGCSE Biology 5.3
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.
Deforestation: Grade 9 Understanding for IGCSE Biology 4.18B
Deforestation means the cutting down of mature forest and woodland for non-forestry purposes. No new trees are planted and so the total area of forest decreases. Humans have cut down forests for many reasons and have built their economies on exploiting natural resources. Forests today are cleared to provide wood for logging, to provide land for building homes, for subsistence farmers as well as for commercial growing of crops and cattle farming.
Deforestation happens all the world. (It is worth noting that the only reason there are no European countries on the list below is because all our forests were cleared very effectively some time ago….)
What are the biological consequences of deforestation?
Wherever deforestation occurs, the biological consequences are the same:
- Atmospheric Gases
- Soil Erosion
- Disturbance to the Water Cycle
- Loss of Biodiversity (inexplicably omitted by the people who wrote the specification
Growing trees have a net uptake of carbon dioxide and a net loss of oxygen due to photosynthesis. Carbon dioxide is a gas that acts as a pollutant in our atmosphere because it is a greenhouse gas. Carbon dioxide concentrations have been rising over the past century and this is leading to permanent and potentially damaging alterations to the earth’s climate system – a process called climate change. Oxygen is the gas that almost all organisms require for their respiration.
In many tropical regions, forests protect the sometimes violent tropical storms from hitting the ground. When forest cover is removed, rainfall hits the soil much harder and this can lead to loss of topsoil in a process called soil erosion. As the water runs through the soil, it will dissolve minerals as it goes, thus leaving the soil that is left denuded of essential minerals for plant growth. This leaching of minerals makes it difficult to use the land cleared for agriculture and so more forest is cleared.
Deforestation also disrupts the water cycle. Trees move large volumes of water a year from the soil into the atmosphere in a process called transpiration. So when trees are lost, less water evaporates from the soil, more water is lost in run-off and so rainfall can be reduced.
The diagram below shows a before and after explanation of how the water cycle is disrupted. (Evapotranspiration is a term for the total water evaporated from a piece of land, combining evaporation directly from the ground and transpiration lost from plants)
There is a final problem with deforestation although the examiners have omitted it from the specification for some inexplicable reason. Forests provide a habitat for a wide variety of animal and plant species. So when forests are lost, species become extinct. This loss of biodiversity is a final terrible consequence of deforestation. 80% of known species live in tropical rainforest so the fact that in the last 50 years, over half of this area has been cleared is a major concern. The rate of loss of rainforest is around 140,000 square kilometres a year although in some parts of the world, the rate of loss is slowing.
Deforestation is a complex issue and a GCSE revision blog like this is not the place to go into the interesting political and cultural details. I would direct you to the WWF site for more information and indeed some ideas as to what we can do to help.
Mineral ions in Plants: Grade 9 Understanding for IGCSE Biology 2.22
This posts addresses one of the commonest misconceptions you encounter as a biology teacher and it concerns a mistaken belief about the function of the roots of a plant.
The roots anchor the plant in the ground and so prevent it toppling over due to wind. But their main function is to do with the absorption of materials from the soil into the cells of the plant. The question is what exactly is taken up in the roots?
Well most people remember that water is absorbed in the roots by osmosis. The best candidates will remember the microscopic root hair cells in the root that massively increase the surface area for the uptake of water. This absorbed water is transported into the xylem tissue in the centre of the root and then moved up the plant to the leaves by transpiration pull.
Roots also absorb mineral ions from the soil by active transport. Active transport is the process where energy from respiration in the cell is used to pump material across the cell membrane against the concentration gradient. Mineral ions absorbed included nitrate ions (needed to make amino acids and proteins), magnesium ions (needed to make chlorophyll) and phosphate ions (needed to make DNA)
So where is the common misconception? This all seems sensible and fairly straightforward. Roots absorb water by osmosis and mineral ions by active transport.
Whenever root function is tested in exams, many candidates get in a pickle as they confuse mineral ions (nitrate, phosphate, magnesium, potassium) with food molecules. Plants do NOT absorb food molecules through their roots. There are very few food molecules such as glucose, amino acids, and lipids in soil. If there were, more animals would eat soil as a source of nutrition…… Plants do not need to absorb food molecules of course: the big idea you learn is that plants can make their own food molecules in the leaves in the process of photosynthesis.
So in your exam, if you ever find yourself writing anything that suggests that plants take in food through their roots, stop, take a deep breath, cross it all out and count yourself lucky you have prevented yourself from one horror answer at least!