Category: IGCSE Biology posts
Male Reproductive Systems: Grade 9 Understanding for IGCSE Biology 3.8
I am slightly wary about writing about the male and female reproductive systems. Not because I get embarrassed with this topic (5 terms of human dissection at medical school removed any squeamishness about body parts….) But rather that I worry that the school’s internet filters might start blocking my website if the wrong words appear. But you don’t know until you try, so here goes…..
Male Reproductive System
I will start with the male reproductive system as males are simpler than females in many, many ways… The male reproductive system has three functions:
- to produce the male gametes, sperm cells, at a prodigious rate
- to make the male sex hormone testosterone
- to act as a delivery system to ensure sperm cells are carried into the female reproductive tract in conditions that will allow them to fertilise an egg
The first two functions listed above happen in the testis. There are cells in the testis that secrete the hormone testosterone into the blood from puberty onwards. Testosterone switches on secondary sexual characteristics in the male (body hair growth, muscle development, change in pitch of voice etc.) after puberty. The main part of the testis is made up of very long coiled tubules called seminiferous tubules in which the sperm cells are made.
Humans have over a hundred meters of seminiferous tubules in total in both testes and this allows sperm cells to made at a very fast rate. Even though it takes around 75 days to make an individual sperm cell, the testes make them at a rate of around 85 million sperm cells per day. The epididymis is found next to the testis in the scrotum and is a coiled tube in which sperm cells continue to develop and mature. Sperm are stored here too in readiness for ejaculation.
Everyone knows that in humans the testes are found outside the body cavity in order to keep them cool. Sperm production happens at a maximal rate 3 degrees below core body temperature and having testes outside the body keeps them at this temperature.
The vas deferens is a tube lined with smooth muscle that carries sperm cells away from the testis for ejaculation. As you can see it loops around the back of the bladder, before joining up with the urethra just below the bladder. The urethra is the tube that carries urine away from the bladder but can also carry semen once the vas deferens has joined with it.
There are three accessory glands in the male system (prostate gland, seminal vesicle and the Cowpers‘ glands) These glands produce the fluid that when mixed with the sperm cells is called semen. Semen contains a sugar fructose to provide energy for the sperm cells to swim. It is slightly alkaline to neutralise the acidity in the vagina and also contains mucus to make the fluid easy to move along the tubes.
The sperm cells only acquire the ability to swim when in the epididymis and only become totally mature and able to fertilise the egg right next to the egg cell in the female tract.
The penis is an organ that contains erectile tissue that can fill with blood to allow the penis to fit into the female vagina for ejaculation.
Alveoli and Gas Exchange: Grade 9 Understanding for IGCSE Biology 2.46 2.48
The primary function of the lungs is to allow gas exchange to occur. Oxygen gas can diffuse into the blood from the air in the lungs. Oxygen of course is needed for the process of aerobic respiration that is happening in every cell all the time. Aerobic respiration produces carbon dioxide as a waste product. Carbon dioxide diffuses out the blood in the lungs into the air in the lungs. Hence the name gas exchange – one gas (oxygen) diffuses in, another (carbon dioxide) diffuses out.
This diagram above shows the bronchial tree – the branching network of tubes that carry air into the lungs. The trachea at the top branches into the right and left bronchi, then each in turn branch into smaller bronchi and finally into the smallest tubes called bronchioles. Bronchioles carry air into a cluster of tiny airsacs called alveoli (not ravioli as AZB told his F division today…)
Diffusion is the passive movement of molecules of a liquid or gas from a high concentration to a low concentration. So the first question is what ensures that there is an appropriate concentration gradient for each gas to diffuse?
In order to understand this, you have to remember that the blood going to the lungs is deoxygenated. The right ventricle pumps deoxygenated blood to the lungs in the pulmonary arteries. The tiny alveoli are then covered with capillaries and these join together to form the pulmonary veins. The pulmonary veins carry the oxygenated blood back to the left atrium of the heart. So the blood coming to the lungs will have a low oxygen concentration but a high carbon dioxide concentration.
How are the structure of alveoli adapted for efficient gas exchange?
- The alveoli in total provide a large surface area for the diffusion of oxygen and carbon dioxide. The total surface area of the alveoli in humans is approximately 90 m2 – the equivalent of two tennis courts…..
- The walls of the alveoli are very thin. The alveolus is lined with a single layer of cells, and of course the capillaries are also only one cell thick. So the distance for the diffusion of oxygen and carbon dioxide is very small (hence the rate of diffusion is very fast)
- The alveoli have a rich blood supply. Alveoli are lined by many capillaries.
- The surface of the alveolus is moist. Gas exchange surfaces are always moist as oxygen and carbon dioxide will diffuse more rapidly if they are dissolved in water.
- Alveoli also contain a cell that secretes surfactant. This molecule reduces the surface tension in the film of water that lines the alveolus, allowing air to move in and out more smoothly.
Chromosomes video – a short PMG summary of some important ideas for GCSE
This is my first attempt at making a short summary video as a follow up to lessons. I apologise for the poor sound in places – I hope the content makes sense. Please tweet or leave a reply with any questions.
Chromosomes: Grade 9 Understanding for IGCSE Biology 3.15 3.32
I hope everyone reading this blog knows the definition of a gene. It is one of the few things in the iGCSE course that it is worth learning by heart.
“A gene is a sequence of a DNA molecule that codes for a single protein“.
In human cells, every nucleus contains about 23,000 genes. Remember there is about 1.5m of DNA inside each nucleus. For most of the life-cycle of the cell, this DNA is in a tangled web called chromatin. Chromatin is DNA molecules loosely associated with some scaffolding proteins. The scaffolding proteins are shown in the second level down of this excellent diagram as “beads on a string”.
But this tangled web of DNA in chromatin poses a problem for the nucleus. For the cell to divide by mitosis, it is essential that the nucleus replicates into two identical nuclei, one for each new cell. The DNA molecules in the nucleus will make a copy of themselves by semi-conservative replication but how then can you ensure that each daughter nucleus gets exactly one copy of each DNA molecule if they are all tangled up….? This is where chromosomes come in!
Each chromosome is a physical structure formed by supercoiling of the DNA round the scaffold proteins. The DNA coils, then folds back on itself, then coils again until each DNA molecule is so tightly coiled up that a visible chromosome appears in the nucleus. Chromosomes only become visible just before mitosis starts as for the rest of the time, the DNA is much more loosely coiled and so cannot be seen.
This also explains why each chromosome always looks X shaped. When chromosomes become visible the DNA has already replicated, so one chromosome is now made of two identical sister chromatids joined at a region called the centromere.
So the picture on the left shows a chromosome made as a single structure comprising one DNA molecule wrapped around the scaffold proteins. Then DNA replication occurs (in the S phase of the cell cycle) and now each chromosome is made of two identical chromatids joined at the centromere. Then the two chromatids are separated in mitosis and the chromosome returns to the structure it had at the start.
How many chromosomes are there in human cells?
The key idea here is that chromosomes are found in pairs in all body cells apart from gametes, These pairs of chromosomes (called homologous pairs) have exactly the same genes in the same locations on the chromosome. They are inherited one from each parent so one member of each pair will come from your father, one from your mother.
Different species have different numbers of pairs of chromosomes. For humans you should know that we have 23 pairs of chromosomes in the nucleus of every body cell (making a total of 46). Cells with chromosomes found in pairs are called diploid cells. Every cell in the body is diploid apart from the gametes. Gametes only have one member of each homologous pair and are called haploid cells.
Which of the following cells are diploid, which are haploid?
- Zygote
- Skin cell
- Sperm cell
- Liver cell
- Pollen grain
- Egg cell
If you are not sure, ask me by leaving a comment below….
Finally for this post, chromosomes determine the sex of a human. You can see in the picture above that the 23 pairs of chromosomes can be divided into pairs 1 to 22 – these are called autosomes and play no role in determining your sex. But the 23rd pair of chromosomes are called the sex chromosomes. Males have one large X chromosome and one tiny Y chromosome as their 23rd pair whereas females have two large X chromosomes.
Gametes are haploid so only have one member of each pair. So when a man makes sperm cells (by meiosis) 50% of his sperm cells will contain his X chromosome, 50% his Y chromosome. A woman’s egg cell will always contain one X chromosome. (Why is this?) So I hope you can see that at the moment of fertilisation, the babies sex is determined depending on whether it is a Y-containing sperm cell that happens to fertilise the egg or an X-chromosome containing sperm… If the former, the baby is male, if the latter female.
I might explain this more fully in a post some other time….
Final thing for this post. If you have got to the end of this and understand everything in the text above, you are in a tiny minority of school students. Well done! This is a tricky topic and if you really understand chromosomes, you stand a chance of understanding cell division and genetics.
“Cloning a Mammoth” 5.19B 5.20B
This is a great 3 min video from the new “Hay Levels” Youtube channel. Interesting links to IGCSE Biology sections on animal cloning.
Respiration: Grade 9 Understanding for IGCSE Biology 2.34 2.36 2.37 2.38
I can’t believe that it is over year since I started posting about iGCSE Biology misconceptions and yet I have never written about Respiration. If there is one topic that students misunderstand more than any other (apart perhaps from genetics), this must be it…. So I am going to try to explain in a straightforward way what respiration is and why it is so important for life.
Life requires energy. Living cells are constantly doing things that use up energy: pumping molecules across their cell membranes, moving organelles around the cell, cell division, nerve cells sending electrical impulses around the body, muscle fibres contracting etc. etc. In every case, this energy comes from a metabolic process called Respiration. It is a series of chemical reactions, catalysed by enzymes and in some way, it happens in all cells.
So let’s start with a good definition. [Examiners are simple souls and often start questions with the classic “What is Respiration?”]
Respiration is a series of chemical reactions that happens inside cells in which food molecules (for example glucose) are oxidised to release energy for the cell.
My definition has to be a little vague because although glucose is found in all the equations for respiration, other food molecules can certainly be respired. And oxygen is only used in aerobic respiration. Many organisms can only respire without oxygen (anaerobic respiration) and some, such as humans can switch between aerobic and anaerobic depending on the conditions.
Aerobic Respiration happens for the most part in tiny organelles in the cytoplasm called Mitochondria. The diagram above shows the structure of a mitochondrion (I wouldn’t worry about learning it but perhaps you should be able to recognise the characteristically folded inner membrane?)
What are the differences between aerobic and anaerobic respiration in humans?
Well we have mentioned two already and there are others…..:
- Aerobic respiration requires oxygen, anaerobic does not.
- Aerobic respiration takes place in mitochondria, anaerobic only occurs in the cytoplasm.
- Aerobic respiration produces much more energy per glucose molecule than anaerobic – it is a more complete oxidation of the glucose, so much more energy is released.
- Anaerobic respiration produces lactic acid as a waste product (in humans) whereas in aerobic, carbon dioxide and water are the products
The summary equations for the processes are different as well.
Aerobic respiration:
word equation Glucose + Oxygen ——> Carbon Dioxide + Water
balanced chemical equation C6H12O6 + 6O2 ——> 6CO2 + 6H20
Anaerobic respiration in humans:
Glucose —–> Lactic Acid
Anaerobic respiration in Yeast (a single celled fungus):
Glucose —–> Ethanol and Carbon Dioxide
A couple of final points to note:
Anaerobic respiration in muscle cells does not produce carbon dioxide as a waste product (see the equation above…) Lactic acid is the only waste product. But lactic acid will accumulate in muscles and stop the muscle functioning properly so after a period of intense activity, lactic acid needs to be removed. How does this happen?
Lactic acid moves from the muscle in the blood and is transported to the liver. In the liver, the lactic acid is metabolised in an aerobic pathway that uses oxygen. This is why sprinters will always be breathing fast after the race, even when they are standing still. Their body needs extra oxygen to oxidise the lactic acid they have produced during the race. This extra oxygen is termed an oxygen debt and is the oxygen needed in the liver to fully oxidise lactic acid to carbon dioxide and water.
Finally, respiration is not the same as breathing. Our American cousins sometimes muddle these processes up but in this one case, the British way is much better…. Use the term ventilation for breathing – moving air in and out of the lungs – and reserve respiration for the chemical reactions that happen inside the cells to release energy.
Please leave a comment below if you find this post helpful or ask me about anything that isn’t clear….
Characteristics of Life: Grade 9 Understanding for IGCSE Biology 1.1
The iGCSE specification says that all living organisms share the following basic characteristics and then lists 8 bullet points. This seems unnecessarily unhelpful because every student in the whole word learns MRS GREN for the 7 characteristics of life…
Make sure you understand the exact meaning of each of the following terms:
- Movement
- Respiration
- Sensitivity
- Growth
- Reproduction
- Excretion
- Nutrition
Not all organisms Move from place to place of course and lots of things move that are not alive. So that doesn’t make me think that this is a good way to start the whole study of Biology. It is true that all living things, without exception, Respire. “Respiration is a series of chemical reactions that happens inside cells in which food molecules are oxidised to release energy for the cell” – good definition that…. Sensitivity means the ability to detect and respond to changes in the environment. Mammals do this through their nervous and hormonal systems, plants through plant growth substances such as auxin. Growth either involves a cell getting larger or in multicellular organisms, the two processes of cell division and cell specialisation. All living things have the potential to Reproduce, to create new individuals of their species. Excretion is the removal of waste molecules (e.g. carbon dioxide, urea) that have been made inside cells. Nutrition means either obtaining food molecules by eating another organism or if you are a plant, and I guess none of you are, by making your own food molecules through photosynthesis.
The people who wrote the specification have added “they control their internal conditions” to the list. This is actually a better characteristic of life than many above as it is a universal feature of all life. The term for this process is Homeostasis – the ability to regulate and control the internal environment.
It is a shame that two of the best ways to decide whether something is alive have been left off the list. All living things on earth are made of cells. Some organisms are unicellular (Paramecium for example) but many are made of many cells. And all living organisms have the molecule DNA as their genetic material. If you get a question on this in the exam, it’s probably better to talk about the 8 characteristics of life the examiner likes… That’s exams for you!
New organisation of the IGCSE Biology posts on my blog
It has become clear to me that the majority of iGCSE students reading the blog are not those I teach. This is great to be honest and exactly what I wanted. So it seemed unhelpful to organise the blog around the teaching groups at my school so I have had a go at re-organising it in what I hope will be a more useful way.
Categories and Tags
I have sorted all my iGCSE Biology blog posts into 5 categories that relate to how the EdExcel iGCSE Biology specification is organised. I have also “tagged” each post with the specification points that it covers as well as the key words in the post. I hope you all find this more helpful.
Blog posts on the way
I have plans for more blog posts to ensure a comprehensive coverage of the entire iGCSE specification. So you should expect to be able to read soon about
- The Characteristics of Life
- Respiration
- Alveoli and Gas Exchange
- Chromosomes
- Human Reproductive Systems
There are a few more beyond this as well…..
Please do contact me via the blog if you have requests for posts or indeed with any questions you have. The best part of the blog is when students from all over the world contact me via the “Leave a Reply” box at the bottom of each post. I try to reply to all queries as soon as I can.
Keep working hard and enjoy your Biology.
DNA is Unstable! Luckily your Cells can handle that.
I like this site – well written summary of the work done to win the most recent Nobel prize on how DNA repairs.
Another Nobel Prize story?! DAMN RIGHT! This time it’s the prize for chemistry, and Tomas Lindahl, Paul Modrich, and Aziz Sancar will collectively bask in the glory for their outstanding work in studying the mechanisms of DNA repair. Given the billions of cell divisions that have occurred in your body between conception and you today, the DNA that is copied each time remains surprisingly similar to the original that was created in the fertilized egg that you once were. Why is that strange? Well from a chemical perspective that should be impossible, with all chemical processes being subject to random errors from time to time. Along with that, DNA is subjected to damaging radiation and highly reactive substances on a daily basis. This should have led to chemical chaos long before you even became a foetus! Now, I would hope that’s not the case for you, so how do our cells prevent this…
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PMG Biology 