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The Digestive System – GCSE IGCSE 9-1 Biology – Science – Succeed In Your GCSE and IGCSE

The Digestive System – GCSE IGCSE 9-1 Biology – Science – Succeed In Your GCSE and IGCSE


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lessons. SucceedSchool.com, learn to succeed. Hey class I’m Mr. Thornton and
I’m going to help you Succeed in your GCSE and IGCSE this lesson The
Digestive System. This topic was requested by Asbah Afridi. If you’ve got
a topic you’d like me to cover then just leave a comment below. The digestive
system is actually a little funny in the 2016 specifications. If you’re studying
an IGCSE course you need to know about it, it’s as important as ever and I’ll be
covering everything you need in this video. If you’re studying a GCSE course
things are a little more complicated. The GCSE specifications expect you to know
about the features and functions of the digestive system but they don’t expect
you to study them at GCSE. They say you should have learned all about it at Key
Stage 3. They kind of want to have their cake and eat it because it looks to me
like they’ll use ideas about the digestive system to frame concepts like
enzyme action, fighting infections, homeostasis and transport in animals. I
think it’s probably worth you spending a few minutes refreshing your memory if
you’re rusty on it. I can see this turning into one of those sneaky exam
questions which everyone complains they didn’t know to prepare for. I’m going to
assume in this video that you’re familiar with enzymes. If you’re not
please click here to watch a video all about them. The digestive system is an
organ system, that is, a group of organs all working together to achieve a
particular goal, in this case digestion of nutrients from food and the excretion
of waste. It’s essentially a long tube, known as the alimentary canal, running
from the mouth to the anus with various organs along the way. You need to be
familiar with the basic functions of each organ and if you’re studying IGCSE
you may also need to be able to put them in order or identify them on a
diagram like this one. We begin in the mouth, where food enters the body. This is
known as ingestion. Food has to be ingested before we can digest it.
As I’m sure you’re aware we generally chew solid food before we swallow it,
breaking it down into smaller pieces. This is called mechanical digestion and
it increases the surface area of the food to increase the rate of reactions
in the digestive system. Please see my video on rate of reactions for more
details on this. Humans have a mixed set of teeth; some sharp for cutting and some
flat to grind food. I’ll cover teeth in more detail in a future lesson.
Another type of digestion also takes place in the mouth. Our saliva, produced
by the salivary glands, contains a type of carbohydrase enzyme called amylase,
which begins breaking down starchy molecules into glucose as soon as we
place food in our mouth. This is chemical digestion. Broadly speaking, when we
physically chop up food without changing the actual molecules it’s made of, this
is mechanical digestion. Performing a reaction on it to change the molecules
themselves, for example with enzymes or stomach acid, is chemical digestion. A
process called absorption also takes place to a small degree in the mouth.
This is where digested nutrients from food, such as glucose in this case, can be
absorbed into the bloodstream. It’s only possible for small molecules which can
dissolve in water, such as glucose dissolved in the water in our saliva, to
be absorbed. Absorption mainly happens in the small intestine though, so I’ll cover
it more when we get there, but I wanted to mention that it also
happens in the mouth because I’ve seen test and exam questions which have asked
about this in the past. Once we’ve chewed our food we swallow it and it passes down a
tube called the oesophagus to our stomach. The common spelling of the word in the
UK begins OES while the common spelling in the U.S. begins ES. Both
spellings are accepted. Waves of muscular contractions pass along the oesophagus to
help push food towards the stomach, so humans are even able to swallow food
while lying down or hanging upside down, Though obviously it’s easier and there’s less
risk of choking if we’re upright. Believe it or not even though we’ve not made it
to the stomach yet we’ve now seen most of the key concepts of the processes
taking place in the digestive system. Mechanical digestion, production of
substances like enzymes for chemical digestion, breaking down large pieces of
food into tiny soluble molecules for absorption into the bloodstream, and
peristalsis pushing everything along. Most of the rest of the journey along
the alimentary canal just repeats these themes so we can move pretty quickly
from here onwards. Following its journey down the oesophagus the food reaches the
stomach; a stretchy muscle lined sack containing digestive juices. These are a
mixture of substances like enzymes and acid. The acid is important for two
reasons. Firstly it performs some chemical
digestion and secondly because it has a low pH, around pH 2, it denatures enzymes
which work around neutral pH. This can kill a lot of the bacteria which might
be in food. Of course it also means enzymes present in the stomach such as
pepsin, which is a type of protease, have to work at low pH. This mixture of
digestive juices also contains water. Ultimately all the nutrients will need
to be broken down into small enough molecules to dissolve in the water, which
acts as a solvent in order for them to be absorbed. The stomach walls churn this
mixture for a while and then it passes out of the stomach via the duodenum. This
is the first part of the small intestine and it’s where bile from the gallbladder,
which is situated within the liver, mixes with those digestive juices to
neutralise the acid. It also has a second function in that it allows fats from
those digestive juices to dissolve in an emulsion. We can simulate that with this
mixture of oil and water. This is the oil layer on the top and this is the water
layer on the bottom. I’ve actually added some dye to the water layer so that it’s
easier to tell the difference between the two. Now as i shake up this mixture
of oil and water of course the oil and water mix but they
do start to separate out quite quickly. Hopefully you can already see that the two layers are beginning to separate out, with a lighter
coloured layer which is mainly oil on the top, a much darker colour layer which is
the water on the bottom, and this sort of intermediate section in the middle which
is where they are still mixed but they’re quite rapidly separating out.
Next what I’m going to do is add a little bit of bleach to my mixture. The
bleach has a high pH and so this is going to simulate our bile. It’s strongly
alkaline and I don’t actually need to add very much of it. This is just
household bleach by the way if you want to try this experiment yourself
at home. I’m just adding a couple of drops of it
there, and then I put the lid back on and I’m just going to give it a single shake
again. This time however although there is a
little bit of separation it’s happening much more slowly. The mixture is forming
more of an emulsion. This is where there are droplets of oil suspended in amongst
the water droplets and they are microscopic droplets. That’s why it still
got this cloudy appearance. Yes there’s still quite a lot of water down here at
the bottom but there is much more up here mixed in with the oil. The oil is
going nowhere near as clear. It’s worth noting at this point as well that this
cloudy appearance is as a result of the microscopic droplets of oil in here
refracting and reflecting all the light which hits it, in lots of different
directions. This is very very common of emulsion. This is a characteristic
appearance and it is something which does occasionally show up in the GCSE as
well. In addition to bile from the gallbladder, the pancreas also adds other
digestive chemicals in the duodenum such as more of the enzyme amylase which was
first added in the mouth. Remember this is because most of the amylase from the
mouth will have been denatured by the stomach acid. Next this mixture of food
and digestive juices passes into the main part of the small intestine. This is
technically in two sections; the jejunum and the ileum, but the differences
between them are pretty subtle and at GCSE and IGCSE level all you need to
know about is the basic function and structure of the ileum. Peristalsis again
moves the mix of semidigested food and digestive
juices along the ileum, with the digestive juices continuing to break
down the food until it’s in the form of single small molecules ready to be
absorbed by the body. The inner surface of the ilium is covered with long finger
like projections called villi. The singular noun is villus. These in turn
are covered in microvilli. This gives the ileum a huge surface area
in a very small volume, increasing the rate at which absorption of nutrients
can happen. Absorption remember is where small molecules which are dissolved in
water are able to pass from the intestine into the bloodstream, in this
case via the villi. Once these nutrient molecules have been absorbed into the
blood plasma, and blood plasma is just another water-based liquid, the nutrients
dissolve in, once they’ve been dissolved into there they are transported around
the body and then cells can absorb the nutrients and use them for growth and
repair in a process called assimilation. The majority of the water from the
mixture is absorbed in the small intestine too. Any substances which the
body was unable to digest or absorb now pass into the large intestine, the first
part of which is called the colon. The colon reabsorbs most of the remaining
water and various dissolved electrolytes from the mixture as it pushes the
remains of the food along with peristalsis. At the end of the colon is
another section of the large intestine called the rectum, where whatever’s left,
faeces to give it its technical name, waits until the next toilet break before
passing through the anus in a process called egestion. One final point to be
aware of is that what I’ve just described isn’t always exactly how
things work. A good example of this is diarrhoea which is when water isn’t
effectively reabsorbed, which causes watery faeces to be egested. This failure
to reabsorb water can make diarrhoea lead to rapid and potentially dangerous
dehydration if the sufferer doesn’t drink plenty of water. There can be many
causes of diarrhoea, but a historically important one is cholera. This is a
bacterial infection spread through drinking water which has been contaminated
with faeces. The bacterium produces a toxin which causes chloride ions to be
secreted in the small intestine. This causes water to move from the blood
back into the intestine by osmosis. This significantly raises the water content
of the faeces. Thankfully modern plumbing and water treatment make cholera a thing
of the past for most of us, so if you do have diarrhoea it’s probably something
else. All the same I feel I should mention that if you do have diarrhoea
then drink plenty of water and see a doctor if your symptoms persist. LIGHTNING ROUND!
We chew food in our mouth which is mechanical digestion, plus enzymes from the salivary
glands like amylase begin the chemical digestion. The food passes down the
oesophagus, helped along by peristalsis, into the stomach acid. The low pH there
denatures enzymes in bacteria and is also necessary for our own enzymes like
pepsin to work. From there bile from the gallbladder neutralizes the acid in the
duodenum and emulsifies fats, and more enzymes are added by the pancreas. Peristalsis again passes the mixture through the ileum, the main part of the
small intestine, where nutrients are absorbed into the bloodstream across the
ileum’s huge surface area, due to the villi and microvilli. The nutrients are
dissolved and transported in the blood plasma to cells where they’re
assimilated and the indigestible waste passes into the large intestine. The
colon reabsorbs water and electrolytes, the rectum stores the faeces until you
can make it to the toilet, and the faeces is then egested through the anus.
Cholera is a bacterial infection from dirty water which causes chloride ions
to increase in the small intestine, forcing water out of the blood plasma by
osmosis and causing dehydration and diarrhoea. I hope that video really helps
you. If it did it’d be great if you let me know in the comments.
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GCSEs and IGCSEs and thanks very much for watching.

3 Replies to “The Digestive System – GCSE IGCSE 9-1 Biology – Science – Succeed In Your GCSE and IGCSE”

  • Thank you for the consistent videos and keep it up! Weirdly, every single video talks about a topic which I need to revise 😀

  • This was SO HELPFUL!! I could never remember all this when I was studying before, but you have made it so much easier. Thanks so much!!!

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