Category: IGCSE Biology posts
DNA video – a great summary for IGCSE Biologists
Human Diet: Grade 9 Understanding for IGCSE Biology 2.24
All animals are heterotrophic. This means that they cannot make their own food molecules but need to get them from some external source. Humans get a variety of different food molecules from what they eat. Diet is a term for what an animal eats (and in a biological context has no associations with any attempt to lose weight or change body shape). A balanced diet is a combination of foods that provides the correct proportions of all the various food molecules for any particular individual at any particular stage of their life.
Components of a Balanced Diet
Carbohydrates
Carbohydrates are a family of molecules that includes sugars, starch and other polysaccharides. They contain C,H and O atoms only and their main function in the diet is to provide molecules that can be respired to release energy for cells. Carbohydrates are thus one of the main respiratory substrates in our diet. All sweet foods will contain sugars of course and starch-rich foods are vegetables like potatoes, pasta and rice. Starch is a polymer of glucose and so needs to be digested to glucose because it is too large a molecule to be absorbed in the small intestine.
Protein
Proteins are a family of macromolecules needed to build new cells and thus for growth. Like starch, Proteins are also polymers and thus get digested into their constituent monomers, in this case amino acids in the digestive system. Protein-rich foods include all meat and some pulses and beans. Proteins in the diet are needed to build muscle tissue, to form some components of cell membranes and to make all the enzymes that catalyse all the metabolic reactions in cells.
Lipids
Lipid is a general term for all fats and oils. Despite the popular misconception that fat is “bad” in our diet, in fact lipids are essential molecules in the diet. We need lipids as a respiratory substrate, for long term energy storage in adipose tissue under the skin and for the electrical insulation of nerve cells. Foods rich in lipids are red meat, many processed foods, and food containing olive oil or other vegetable oils.
Minerals
Humans need a wide variety of mineral ions in very low concentrations in our diet. The most important mineral in our diet is Calcium which is needed for making healthy teeth and bones. Iron is also needed in relatively high amounts as it is required to make the protein haemoglobin found in red blood cells. Mineral ions come from eating a wide variety of foods, but the main source of calcium is from milk and other dairy products. Iron is found in high concentrations in red meat. 
Vitamins
Rather like minerals, vitamins are needed in very small amounts in a diet but are absolutely crucial for the healthy functioning of the body. The diseases associated with a lack of a particular vitamin in the diet are called deficiency diseases. You need to know about three vitamins – A. C and D 
Vitamin A is a molecule called retinal found in carrots, red peppers and swede. It is needed for healthy growth and a functioning immune system. Vitamin A is also essential for normal vision since it is used to make the pigment found in rod cells in the retina. Vitamin A deficiency in the diet often causes poor vision, especially at night. Vitamin C is needed for the enzyme that produces the protein collagen in the body. It is found in all fruit especially citrus fruits. A lack of vitamin C causes the deficiency disease scurvy. Vitamin D is an unusual vitamin since it can be made in the skin using UV light. Vitamin D is needed in the small intestine to absorb mineral ions such as calcium, magnesium, etc. into the blood. A lack of vitamin D often results in a deficiency disease called rickets in which the bones malform. 
Fibre
Dietary Fibre is actually made up from the molecule cellulose. No mammal including humans possesses a cellulase enzyme and so when plant material passes through the intestines, dietary fibre is never digested. This means it passes into the large intestine where it helps prevent constipation. Foods rich in fibre included wholegrain bread, vegetables and some breakfast cereals.
Water
Water is the final component of a balanced diet. It is needed to replace water lost by sweating and in urine and acts as a solvent of course for all the metabolic reactions that happen in every cell.
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!
Cell Structure: Grade 9 Understanding for iGCSE Biology 2.2 2.3 2.4
All living organisms are made from cells. Indeed the cellular nature of life is one of the universal features shared by all life on earth. Some organisms are made from just one cell (unicellular organisms) while at some point around 1 billion years ago, cells starting clumping together and specialising to form multicellular organisms such as animals and plants.
What do all cells have in common?
All cells are surrounded by a cell membrane. The cell membrane is made from a mixture of proteins and a type of lipid called a phospholipid. The cell membrane serves many functions but perhaps the most significant is acting as a partially permeable barrier that can control which molecules can enter and leave the cell.
Inside the cell membrane there is a watery solution of chemicals called the cytoplasm. The cytoplasm is the site of many metabolic reactions in the cell because many enzymes are dissolved in the cytoplasm. The cytoplasm also contains many tiny nano machines for assembling proteins called ribosomes.
And that is about it for things all cells have in common. Prokaryote cells (bacteria) have a very different cell structure with no organelles but in this section you need to understand the simplified structure of two eukaryote cells: a typical animal (on the left below) and a typical plant cell (on the right).
Both animal and plant cells have a nucleus. This is the largest organelle and contains the DNA which is the genetic material. The DNA is found in long thread-like structures called chromosomes. The nucleus controls the division of the cell and also the various functions of the cell by regulating which proteins get made.
Animal and Plant cells both contain mitochondria which are the organelles associated with aerobic respiration. Mitochondria are recognisable in the cytoplasm of the cell as sausage-shaped organelles with a folded inner membrane (see diagram above).
Structures found only in Plant cells
1) All plant cells have a thick rigid cell wall made of the carbohydrate cellulose. The cell wall allows plant cells to become turgid since when the cell takes in water by osmosis, the rigid cell wall prevents the cell from bursting. The cell wall also acts as a transport pathway across plant tissues and can provide a barrier to some pathogens.
2) All plant cells have a large permanent central sap vacuole. This organelle is bounded by a membrane called the tonoplast and in many plant cells takes up the majority of the volume of the cell.
The sap vacuole provides a compartment in the cell into which excretory molecules can be moved to stop them poisoning the cytoplasm. It also plays a role in the water balance of plant cells since because of all the solute dissolved in it, the cell sap has a low water potential. This helps draw in water by osmosis from the cytoplasm and hence from outside the cell across the cell membrane.
3) Many but not all plant cells contain chloroplasts. These are organelles associated with the process of photosynthesis. Chloroplasts can be recognised in a light microscope image as small, green structures in the cell. The green pigment comes from the chlorophyll molecules that trap energy from sunlight. In an electron micrograph, chloroplasts are distinguished due to their stacks of membrane discs called grana.
Differences between plant and animal cells
Levels of Organisation: Grade 9 Understanding for IGCSE Biology 2.1
The Easter holiday is the most important time of year for this iGCSE Biology blog. With exams in early May, the next few weeks should be the time when students are working at their maximal rate. I intend to add one post a day such that by the middle of April, the entire EdExcel iGCSE Biology specification has been covered on this blog. This should then make it a useful resource for all GCSE Biology students to help them with their revision.
Today I will put up two posts that will look at two of the simplest topics in the specification: the first one will be Levels of Organisation and then Cell Structure (2.1, 2.2, 2.3 and 2.4)
Living things (or organisms to be precise) are complex entities. Even the simplest organism will be made up of millions of different molecules arranged in an organised and complex way. Human beings are organisms made up of about 10 trillion cells of roughly 210 different cell types all put together in a organised and systematic way. It makes it much easier to study such complexity if we have a system to break the complexity down into constituent parts. This is what scientists mean by levels of organisation.
So, starting with the smallest things that might be of interest to a biologist……
All matter on earth including the matter of living things is made of atoms (e.g. a carbon atom, an oxygen atom etc.). Atoms can combine together in a variety of ways to form molecules (a water molecule H2O, a carbon dioxide molecule CO2,) How atoms combine to form molecules is chemistry, and the levels of organisation smaller than an atom forms part of physics, so we won’t worry too much about them….
But molecules in an organism are interesting and worth studying – you learn about carbohydrates, lipids, proteins, DNA in your iGCSE course. These molecules can be grouped together to form structures inside cells called organelles. If you are being really precise with your terminology, an organelle is a membrane-bound compartment inside a eukaryotic cell (remember bacterial cells have no organelles at all). Examples of organelles are structures like the nucleus, chloroplasts, mitochondria and so on.
Cells are structures enclosed by a cell membrane that contain many different organelles. You have probably looked at a human cheek cell using a light microscope at some point in the past. In multicellular organisms, cells of the same type are often attached together to form a Tissue. A tissue is a group of similar cells often attached to each other that carry out the same function in an organism. (It is also a small disposable piece of rectangular fabric used for blowing your nose but that is something quite different….) Tissues are grouped together to form larger structures called Organs. For example, the lungs are an organ made up of a particular arrangement of epithelial tissues together with some blood and connective tissues. Organs can be grouped into Organ Systems based on their function such as the Digestive System (oesophagus, tongue, stomach, pancreas, liver, intestines etc.) An Organism such as you or I is made up of many organ systems (nervous system, cardiovascular system, digestive system, excretory system and so on….. You get the idea I’m sure!)
You can study levels of organisation bigger than the organism. This branch of biology is called Ecology – and indeed you should know the meaning of the terms population, community and ecosystem – but perhaps that is for another post……
Viruses: Grade 9 Understanding for IGCSE Biology 1.4
Viruses appear in your syllabus in a section entitled “Variety of Living Organisms”. This is rather unfortunate because of course viruses are not classified as living organisms at all. The reason they are not alive is simple: they are not made of cells and they are incapable of carrying out any metabolic reactions. Viruses are much smaller than any cell, even very small prokaryotic cells such as bacteria.
In the diagram above you can see in the top right of the picture part of a red blood cell. Red blood cells are one of the smaller cells in the human body. The bacterial cell at the bottom of the diagram E.coli is much smaller, and all the blue viruses are much much smaller still. [The units of length on this diagram are nanometers (nm) – a nanometer is 10-9 m]
All viruses are parasitic as they have to infect a living cell in order to reproduce.
What are viruses made of?
Well remember viruses are not made of cells. The individual virus particles are called virions and are simply made up of a protein coat (called a capsid) that encloses some genetic material. The genetic material in a virion can either be DNA or a similar molecule called RNA.
Viruses can infect all different kinds of living organisms. The virus on the left in the diagram above is called a bacteriophage and it infects bacterial cells. You can see the protein coat is arranged into a head, tail and fibres and the genetic material in a bacteriophage is DNA. The virus on the right is the Influenza virus that infects mammals and birds, causing the disease influenza or “flu”. Influenza virus is an RNA virus.
Many human diseases are caused by viral pathogens. Influenza is one (see above) and the other one mentioned in the syllabus is HIV – a virus that causes the diseases AIDS. HIV is also an RNA virus. [HIV stands for Human Immunodeficiency Virus and the disease AIDS is acquired immunodeficiency syndrome.]
Some viruses infect and cause disease in plants. Tobacco Mosaic Virus infects tobacco plants causing yellow leaves as chloroplasts are not formed correctly in the leaves.
This is a diagram of a Tobacco Mosaic Virus
So to summarise:
- Viruses are not made of cells but are made of tiny particles called virions
- Viruses are not alive
- Virus can cause disease in a whole variety of different organisms – animals, plants, bacteria etc.
Plant Tropisms: Grade 9 Understanding for IGCSE Biology 2.83 2.84 2.85
Plant Sensitivity
Sensitivity is a characteristic of all living organisms. It means the ability to detect and response to changes in the environment. When most students think of sensitivity they immediately think of receptors and the nervous system of mammals. In this post, I will summarise a few ways that plants are able to detect and respond to changes in their environment.
What stimuli are plants able to detect?
Stimulus Response
Light shoot grows towards light
Gravity root grows in the direction of gravity
Water roots will grow towards moisture
Touch some plants can grow towards touch
There are others such as day length, presence of certain chemicals and temperature too. But you can see that for all the stimuli in the table above, the response is by altering the pattern of growth. Animals often respond to stimuli with muscle contractions leading to movement. Plants have no equivalent of muscle tissue of course and so they respond through growth.
A tropism is a growth response in a plant in which the direction of the stimulus determines the direction of growth in the plant.
For example, positive phototropism would be the term used when a growing shoot will grow towards unidirectional sunlight. Growing roots will show negative phototropism when they grow away from light.
How is positive phototropism in a growing shoot brought about?
Plant growth is controlled by a family of chemicals called Plant Growth Substances (PGS) – in the past these were sometimes called plant hormones. The most important PGS and indeed the only one you need to know about for iGCSE is called Auxin. This chemical is made in the tip of the growing shoot and when it diffuses down a millimetre or two to the growing region, it can stimulate growth in two ways. Auxin causes cells in the shoot to divide by mitosis and also influences the cell wall of the plant cells allowing them to elongate. The net effect of this is to stimulate growth.
Auxin is actually a chemical called Indole Acetic Acid (IAA) and sometimes you will see it referred to by this name.
The detailed mechanism of positive phototropism in shoots is not well understood but we do know that if the shoot has brighter light on one side than the other, auxin will be moved towards the darker side of the shoot. This lateral redistribution of auxin allows the darker side to grow faster, so the shoot bends towards the light.
- Phototropism in plants is brought about by a chemical called AUXIN or IAA.
- Auxin is made in the tip of the growing shoot and diffuses down the stem towards a region of cell growth.
- If the shoot is growing in unidirectional light, the auxin will accumulate on the dark side of the shoot.
- Auxin stimulates mitosis in the growing region as well as causing individual cells to elongate.
- For this reason the darker side of the shoot will grow faster and so the shoot will bend towards the light.
Geotropism – a growth response to gravity
We know that the growing root and shoot are also able to detect gravity. A growing root will always grow downwards (positive geotropism) and the shoot upwards (negative geotropism). How are these responses brought about?
Well it’s our old friend auxin again…..
This diagram shows auxin moving downwards in a shoot under the influence of gravity. The lower side has a higher concentration of auxin and so grows faster. This results in negative geotropism in the shoot: it will grow in the opposite direction to gravity.
As far as I know, no-one really understands how the growing shoot and root are able to detect the direction of gravity. No-one really knows how the lateral redistribution of auxin due to light happens either…. There are still plenty of unknowns in this topic!
This final diagram shows positive geotropism in a growing root. It is positive geotropism because the root grows in the direction of gravity. If auxin accumulates on the lower side of the growing shoot and root (and it does) how does this work? Well it turns out that while auxin stimulates growth in the shoot, the same chemical inhibits growth in the root. So the diagrams above show auxin being produced at the tip (D) and accumulating on the lower side of the root (B). Auxin inhibits the growth of the root causing less growth on this side, so the root bends downwards.
Plant Tropisms – I can’t make myself do it….. 2.83 2.84 2.85
Is there a single topic more likely to strike fear into the heart of a GCSE Biology student? Everyone hates plant tropisms – students because it seems dull, and teachers because it seems like 19th century science and none of us can see for one second why it should appear in 21st century specifications….. I will write a post about auxins, phototropism and geotropism for your all but it will take me a day or two to build up to it….. As I said, everyone hates tropisms.
“How to build a human heart” video
Great video introduction to Nervous Systems: Grade 9 Understanding for IGCSE Biology 2.86
You all should really subscribe to the Crash Course YouTube channel. Some of their output is perhaps more suitable for A level students, but the scientific content is great. I really like the presentation too but it is quite American (I hope my readers in the US will understand….) so I am prepared for people to disagree….
Anyway here is the introduction video to Nervous Systems: great for my two Year 11 groups just now. Enjoy.
PMG Biology 