Tagged: energy

Respiration: A* Understanding for iGCSE Biology 2.33 2.34 2.35

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.

f-d-58fb95c61caa798de5a938e01f65970733c8f3fe943a17282b88533e+IMAGE_THUMB_POSTCARD+IMAGE_THUMB_POSTCARD.1

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.

breathing

Please leave a comment below if you find this post helpful or ask me about anything that isn’t clear….

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Feedback on Zondle Biology revision challenge part 2

The questions in this revision test were more challenging than last time.  I hope that players found them interesting and useful.

The plant transport questions at the start were well answered overall.  Osmosis is the only way water can ever cross a cell membrane and although active transport does occur in the root hair cells (pumping mineral ions such as nitrates into the cell against the concentration gradient), water cannot be directly pumped against its concentration gradient using energy from respiration.

The cloning questions were difficult but I think the low scores here were perhaps more to do with problems with my school wifi than with your abilities to answer them!  Micropropagation is the way that you learned when a small part of a plant is cut out, sterilised, washed and then added to a culture medium that triggers cell differentiation.  You probably did this experiment with explants from a cauliflower.  The aim was to produce whole new plants from these small explants.  This technique could not work with animals simply because animal’s bodies contain many more types of tissue and have a more complex internal architecture that requires a much more sophisticated genetic programme of development.

I want to talk about a few questions in the latter stages of the test that were not well answered.  I am sure there is plenty you can all learn from these.

The first was the one that asked you what was meant by a “diploid cell”.  More than half of you thought that  diploid meant having 46 chromosomes.  This is almost a trick question because of course in humans, diploid cells will have 46 chromosomes.  But diploid can be applied to any cell that has chromosomes found in homologous pairs.  The number 23 is only important to humans as for our species it is the number of homologous pairs of chromosomes found in our diploid cells.  Different species have differing numbers of pairs of chromosomes, some less than the number in humans but in many species they have more.

The second big idea question was the true or false question on whether energy is recycled in the ecosystem like carbon atoms.  It is vital you understand that there is absolutely no recycling of energy ever in an ecosystem.  Energy enters in the form of light energy being trapped by plants in photosynthesis and all this energy ultimately ends up as heat energy in the atmosphere.  To find out the details of how it gets there, please read the relevant sections on my blog.  Try the tag energy from the Tag cloud on the right of the screen.

There was one question in the quiz which not a single player answered correctly and it is the one about which type of cells produce antibodies.  Antibodies are made from a cell called a plasma cell.  Plasma cells secrete antibodies in large numbers to combat an infection.  Plasma cells are descended from B lymphocytes that have been activated by the presence of antigen.  This clonal selection theory is one of the most complicated bits in iGCSE Biology so make sure you have looked carefully at it.  The final question was about active v passive immunity.  This is not specifically mentioned in the specification so perhaps is a bit mean to include but if you can understand it properly, you understand how immunity works.  Passive immunity is the name for when antibodies are transferred, perhaps across the placenta for a foetus or in an injection as an adult.  Antibodies are made of protein and so do not exist for long in the blood – after a month or two they will all have been broken down and cleared from the blood.  So passive immunity cannot give long-lasting protection.  Active immunity is when memory cells are produced via a clonal selection response.  These memory cells can survive for an entire lifetime and so do provide long lasting protection.

By far the biggest thing you can learn from this quiz however was about virus structure.  I asked you whether “viruses are made from a different kind of cell not found in animals or plants – true or false.”  Almost everyone went for false but remember this can’t be correct:  viruses are definitely not made of cells!  They are much simpler than even the simplest cell and just consist of a protein coat with some genetic material (DNA or RNA) inside.  No cell membrane, no cytoplasm, no metabolism – just two chemicals associated into one simple particle.

Anyway I hope you enjoyed the quiz – look out for the next one on my Twitter feed and please use the comment facility on this blog to get in touch if you have any questions or want more explanations.

Feedback on Zondle Biology revision challenge part 2

The questions in this revision test were more challenging than last time.  I hope that players found them interesting and useful.

The plant transport questions at the start were well answered overall.  Osmosis is the only way water can ever cross a cell membrane and although active transport does occur in the root hair cells (pumping mineral ions such as nitrates into the cell against the concentration gradient), water cannot be directly pumped against its concentration gradient using energy from respiration.

The cloning questions were difficult but I think the low scores here were perhaps more to do with problems with my school wifi than with your abilities to answer them!  Micropropagation is the way that you learned when a small part of a plant is cut out, sterilised, washed and then added to a culture medium that triggers cell differentiation.  You probably did this experiment with explants from a cauliflower.  The aim was to produce whole new plants from these small explants.  This technique could not work with animals simply because animal’s bodies contain many more types of tissue and have a more complex internal architecture that requires a much more sophisticated genetic programme of development.

I want to talk about a few questions in the latter stages of the test that were not well answered.  I am sure there is plenty you can all learn from these.

The first was the one that asked you what was meant by a “diploid cell”.  More than half of you thought that  diploid meant having 46 chromosomes.  This is almost a trick question because of course in humans, diploid cells will have 46 chromosomes.  But diploid can be applied to any cell that has chromosomes found in homologous pairs.  The number 23 is only important to humans as for our species it is the number of homologous pairs of chromosomes found in our diploid cells.  Different species have differing numbers of pairs of chromosomes, some less than the number in humans but in many species they have more.

The second big idea question was the true or false question on whether energy is recycled in the ecosystem like carbon atoms.  It is vital you understand that there is absolutely no recycling of energy ever in an ecosystem.  Energy enters in the form of light energy being trapped by plants in photosynthesis and all this energy ultimately ends up as heat energy in the atmosphere.  To find out the details of how it gets there, please read the relevant sections on my blog.  Try the tag energy from the Tag cloud on the right of the screen.

There was one question in the quiz which not a single player answered correctly and it is the one about which type of cells produce antibodies.  Antibodies are made from a cell called a plasma cell.  Plasma cells secrete antibodies in large numbers to combat an infection.  Plasma cells are descended from B lymphocytes that have been activated by the presence of antigen.  This clonal selection theory is one of the most complicated bits in iGCSE Biology so make sure you have looked carefully at it.  The final question was about active v passive immunity.  This is not specifically mentioned in the specification so perhaps is a bit mean to include but if you can understand it properly, you understand how immunity works.  Passive immunity is the name for when antibodies are transferred, perhaps across the placenta for a foetus or in an injection as an adult.  Antibodies are made of protein and so do not exist for long in the blood – after a month or two they will all have been broken down and cleared from the blood.  So passive immunity cannot give long-lasting protection.  Active immunity is when memory cells are produced via a clonal selection response.  These memory cells can survive for an entire lifetime and so do provide long lasting protection.

By far the biggest thing you can learn from this quiz however was about virus structure.  I asked you whether “viruses are made from a different kind of cell not found in animals or plants – true or false.”  Almost everyone went for false but remember this can’t be correct:  viruses are definitely not made of cells!  They are much simpler than even the simplest cell and just consist of a protein coat with some genetic material (DNA or RNA) inside.  No cell membrane, no cytoplasm, no metabolism – just two chemicals associated into one simple particle.

Anyway I hope you enjoyed the quiz – look out for the next one on my Twitter feed and please use the comment facility on this blog to get in touch if you have any questions or want more explanations.

How is energy lost between one trophic level and the next? 4.4 4.5 4.6 4.7

Image The diagram above shows how energy moves up the food chain through feeding.  Remember that if you are asked what the arrows represent in a food chain, there is only one possible correct response.  “Arrows in a food chain show the flow of energy from one trophic level to the next”

The big idea here is that not all the energy in one trophic level can ever pass to the next.  The specification suggests that only 10% of the energy is transferred from one level to the next (but in fact the percentage varies between 0.1% to around 15%)

ImagekCal is a unit of energy and the pyramid shows that only 10% of the energy in one level is found in the next.

So there is a big question here – where does all the other 90% of the energy in one level end up?

There are a whole load of different ways energy is lost.  Consider the transfer of energy between mice and owls.

  • The mice use up energy in the process of respiration.  The glucose molecules that mice oxidise to provide the energy to move around are not available to an owl if the mouse is eaten.
  • Not all mice are eaten by owls or other predators. Many die of disease, starvation and exposure and a few might even live long enough to die peacefully in their sleep.  All these “dead mice” will have energy in their bodies that cannot pass up a food chain but instead passes to decomposers.
  • Even the mice that are eaten by owls are not eaten in their entirety.  The owl might only eat the energy-rich parts of a mouse and regurgitate out the bones and fur.  So some  energy is lost as not all the mouse is eaten and digested by the owl.
  • There will be parts of the mouse that even when swallowed and digested are not accessible.  Owl faeces will contain some molecules from the mice eaten that contain energy.   This energy is perhaps found in molecules that the owl digestive system cannot digest.  The energy present in the owl faeces is lost to the food chain and like the example above will pass to decomposers.

This energy adds up to around 90% of the energy in any trophic level.  Ultimately though where does it all go?  All the energy in all the organisms in an ecosystem has the same fate:  it ends up as heat that is dissipated into the system.  Energy can only enter an ecosystem in one way (as sunlight trapped in the process of photosynthesis in producers) and in the end, it all ends up leaving the system as heat energy.  This heat energy is a waste product of respiration.