Lesson video

This lesson is called digestion and enzymes and is from the unit, the human digestive system.

Hi there, my name's Mrs McCreedy and I'm here to guide you through today's lesson.

So let's take a look at what we're going to learn today.

In our lesson today we're going to describe the processes of mechanical and chemical digestion that break down food to release nutrients, and in particular we're going to have a look at the role of enzymes.

Now in our lesson today we're going to come across a number of keywords which are shown up on the screen.

If you'd like to make a note of them, then pause the video now and write them down or write them down with their definitions as well.

So in our lesson today on digestion and enzymes, we're going to firstly have a look at mechanical and chemical digestion.

Then we're going to consider how we get from large molecules to small ones, before finally looking at chemical digestion by enzymes.

So are you ready to get started? I am, let's go.

We need to remember that the food that we eat must be broken down into small pieces that are small enough to be absorbed by the body.

And it's this process of breaking large molecules down into small ones which is called digestion.

Now, food is broken down into those nutrients, into those really small particles called nutrients, which are then absorbed into the body and used for energy, growth, and repair.

So that's our background.

Now most of the food that we eat is insoluble, it does not dissolve in water.

For instance, let's take potato.

Does potato dissolve in water? Let's have a look at a video or try it in the classroom to see.

So you can see from the video that potato does not dissolve in water, it is insoluble in water.

And, in order to absorb potato into the blood, we must now digest it into small pieces that are soluble.

Now, potato contains carbohydrate, which can be broken down into sugar.

So, is sugar dissolvable in water? Well let's try that for ourselves.

So we can see from the video that sugar does dissolve in water, sugar is soluble, and that's really important.

We're trying to move molecules from being insoluble and turning them into soluble ones so that we can absorb them into the blood.

So once food has been digested, the nutrients are small and they are soluble in water.

And once they are small and soluble, they can then be absorbed into the blood.

We can see that in the diagram.

So let's have a quick check of understanding.

Food is digested so that it is soluble in water, is insoluble in water, or can be absorbed into the blood? Which option do you think is correct? I'll give you five seconds to think about it.

Okay, well done if you spotted both correct answers.

It is digested, food is digested so that it is soluble in water and can be absorbed into the blood.

Well done.

Now that process of digestion, breaking food down into small pieces, can be both mechanical or chemical.

And, mechanical digestion is the process of physically breaking down food using processes that act on the molecules themselves.

So this can include chewing, biting, tearing, ripping, crushing, churning, those sorts of things.

That's mechanical digestion, physically breaking down food by acting on the food directly.

And mechanical digestion happens in several places within the digestive system, including cutting and grinding food using teeth in the mouth and rolling it around using the tongue, and also churning our food, that means turning it over, in the stomach.

These are all examples of mechanical digestion where the process is acting physically on the food to break it down into smaller pieces.

Digestion can also be chemical and chemical digestion is the process of breaking down food using chemicals.

Now, these chemicals also include enzymes and chemicals and enzymes are present within saliva in the mouth, and in the stomach, and in the small intestine.

So all of these are examples of chemical digestion where we're using chemicals to break down food into small pieces.

So let's quickly check our understanding.

Complete the sentences with a word each.

An example of something digestion is chewing.

And saliva is an example of something digestion.

I'll give you five seconds to think about it.

Okay, let's check our work.

An example of mechanical digestion is chewing and saliva is an example of chemical digestion.

Did you get both of those blanks right? Well done if you did.

So what I'd like you to do now is to firstly, write definitions for mechanical and chemical digestion and include an example of each.

Then I would like you to complete the sentences using the words in purple on the screen.

So which word best fits into those spaces in the sentences? So pause the video, take your time, and come back to me when you're ready.

Okay, let's mark our work then.

So for the definitions for mechanical and chemical digestion you might have written that mechanical digestion is the process of breaking down food by moving and cutting it.

Teeth cut food into small pieces and the stomach churns food.

Whereas for chemical digestion you might've said that chemical digestion is the process of breaking down food using chemicals and saliva in the mouth is an example.

So just review your work against these examples and see if you've missed anything out or need to add anything extra and well done.

Secondly, I asked you to complete the sentences using some of these words.

So did you put the correct words in? The mouth contains saliva to chemically digest food.

And the teeth grind food to digest it mechanically.

Did you get both of those correct? Well done if you did.

Okay, let's move onto the second part of today's lesson where we're gonna look at how we get from large molecules to small ones.

So remembering that food contains a mixture of different chemicals including carbohydrates, proteins, fats, and fibre and water.

Now, carbohydrates, proteins, and fats are large, insoluble molecules and must be broken down into small molecules so that they can be absorbed into the blood and that process of breaking them down into small molecules is called digestion.

So we need to digest carbohydrates, proteins, and fats so that they can be absorbed.

Now, starch is an example of a carbohydrate and is found in foods like potatoes, pasta, rice, and bread.

So if you've ever peeled potatoes or seen the kind of white liquid that's left at the bottom of the saucepan when you've boiled pasta or rice, that oozy substance, that white, oozy substance is starch.

And starch is a large, complex carbohydrate and it must be digested down into small carbohydrates such as glucose, a type of sugar, before they can be absorbed.

So starch must be broken down into glucose molecules so that it can be absorbed in the process of digestion.

Proteins are found in meat and fish as well as nuts and beans and these are also large molecules.

They are large and complex and they also have to be broken down into smaller nutrients so that we can absorb them.

So large protein molecules must be broken down into amino acids before they can be absorbed.

So that process of digestion acting on protein, breaks protein down into amino acids and amino acids are small enough to be absorbed.

Finally, fats, also known as lipids, are really important for storage and used for energy in our body.

And again, these are large, complex molecules which must be broken down into two parts called fatty acids and glycerol before they can be absorbed.

So a lipid is digested into glycerol and fatty acids so that the glycerol and the fatty acids can then be absorbed.

And that again is the process of digestion of lipids.

So let's just check our understanding.

What are proteins broken down into? Are they broken down into carbohydrates, fatty acids and glycerol, or amino acids? I'll give you five seconds to think about it.

Okay, let's see what you put.

Did you say that proteins are broken down into amino acids? Well done if you did.

Okay, what I'd like you to do now is consider this conversation between Jun and Sofia.

So they're discussing how large molecules are broken down into smaller molecules, but they've made a few mistakes.

So what I would like you to do is to rewrite their sentences correctly, but keep the word in bold unchanged.

So Jun says carbohydrates are broken down into amino acids.

So keep that word amino acids the same.

Sofia says many amino acids are joined together to make lipids like starch.

So rewrite her sentence, keeping the word starch the same.

Pause the video, come back to me when you're ready.

Okay, let's check our work.

So, you could've corrected Jun to say that proteins are broken down into amino acids.

Whereas for Sofia, you should've corrected her to say that many sugars are joined together to make carbohydrates like starch.

So just review your work and make sure that you have corrected their statements correctly.

Well done.

Right, let's move onto the last part of our lesson today on chemical digestion by enzymes.

Enzymes are biological catalysts and a catalyst is a chemical that speeds up reactions.

And in the case of digestion, enzymes are involved in speeding up reactions that break down large molecules into smaller ones.

So, enzymes make the reactions that break carbohydrates down and proteins and lipids into their smaller components, it speeds up the reactions so that those reactions happen faster.

And you could consider this action to be a bit like a pair of scissors where we're taking a really large molecule and we're chopping bits off of it into smaller pieces, a bit like a pair of scissors.

However, enzymes do not look like scissors, they are large, biological molecules and complex ones at that.

And enzymes are involved in chemical digestion of food.

So you can see in the diagram there that an enzyme might be involved in breaking carbohydrates in food down into smaller pieces.

So this is what an enzyme is, it's a large, biological molecule, not a pair of scissors.

So enzymes break down or digest large food molecules into smaller ones.

For instance, enzymes that break down carbohydrates into sugars are an example of an enzyme involved in digestion and the enzyme will take the carbohydrate and break it down into small pieces.

Now you can see in this diagram that the enzyme remains unchanged in this process.

The enzyme has retained its same shape whereas the carbohydrate has changed from being two pieces linked, to two separate pieces, but the enzyme has remained the same.

And that means that once those sugar molecules have left the enzyme, the enzyme is free to digest another carbohydrate down into sugar.

And the enzyme can be reused again and again and again.

So let's quickly check our understanding.

What do enzymes do to the rate of a reaction? Do they speed up the rate of a reaction, slow down the rate of a reaction, or make no change to the rate of a reaction? I'll give you five seconds to think about it.

Okay, well done if you said that enzymes speed up the rate of a reaction.

Now, each enzyme is highly specific.

That means it can only catalyse one chemical reaction.

So the enzyme that catalyses the reaction for carbohydrates can only catalyse that reaction and most enzymes have this ending, ASE, ase.

So, enzymes that break down carbohydrates are called carbohydrases and you can see that ASE ending there.

Whereas enzymes that break down proteins are called proteases, again, ASE.

So what do you think the enzymes are called that break down lipids? Well, well done if you guessed something along the lines of lipases.

Enzymes that break down lipids are called lipases.

So let's summarise that before we move on.

Carbohydrates are broken down by carbohydrases into sugars.

Proteins are broken down by proteases into amino acids.

And lipids are digested by lipases into fatty acids and glycerol.

So, let's quickly check our understanding.

What is the name of the group of enzymes that break down proteins? Is it carbohydrases, proteases, or lipases? I'll give you five seconds to think about it.

Okay, well done if you said that proteases break down proteins.

And one other check.

In the following reaction, which part is the enzyme? Is it starch, amylase, or maltose? So well done if you spotted that amylase with the ASE ending over the top of the arrow is the enzyme.

Okay, what I'd like you to do now is three tasks.

Firstly, I'd like you to complete the gap fill to define an enzyme.

Then, I would like you to draw a diagram to show how enzymes work.

And finally, I'd like you to complete the table to show which enzyme works on which type of food.

So take your time, pause the video, and come back to me when you're ready.

Okay, let's check our work then.

So to complete the gap fill, you should've added the following words.

An enzyme is a catalyst which speeds up the rate of a reaction.

Then the diagram might have looked something like the one I've got on my screen now, but you might have drawn it out in a slightly different way.

As long as you've got that idea of going from a large molecule to a small one.

And finally, the table to complete which shows which enzyme works on which type of food.

So carbohydrates are digested by carbohydrases from starch to sugar.

Whereas proteins are acted upon by proteases and they turn proteins to amino acids.

And finally, lipids are digested by lipases from fats to fatty acids and glycerol.

So well done if you got all of that correct.

So in our lesson today we have seen that food is broken down by mechanical and chemical digestion, these two different processes that act on food and these processes turn insoluble food into soluble nutrients that are small enough to be absorbed into the blood.

We've also seen that enzymes are catalysts that speed up chemical reactions and they are really important in chemical digestion because they break large food molecules down into smaller ones.

We've also seen that enzymes have the ending ASE and include carbohydrases, proteases, and lipases.

So thank you very much for joining me today, I hope you have enjoyed the lesson and I hope to see you again soon, bye.