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Hello. I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the first in a six-part series all about the organisation of plants and animals, basically how all the bits of us work together.

Here we’re going to mainly focus on the human digestive system, but first let’s get you up to speed with how cells, tissues, organs and organ systems interact.

Make sure you’ve got a pen and paper handy to take some notes or draw a diagram. And remember, you can pause at any time, if you need to make some notes.

First things first, we’ve got the most basic building blocks of any organism – the cell. If you’re a living thing, you’re made up of cells.

If this sounds new to you, go and listen to our series on The Cell on ����ý Sounds.

A tissue is a group of similar cells, in the same place that work together to do a similar job or function. An example of this is muscle tissue, where lots of similar cells group together to make your muscles work.

So cells work together to form something greater: tissues.

But when lots of different tissues work together in the same place, they form organs. Things like the heart, lungs and stomach.

And then finally, organs work together to form organ systems. This is where a group of organs (now they’re not necessarily in the same place) but they all team up to do a function or a job.

For instance, your digestive system, it’s not just your stomach, even though you might think so. It’s a combination of your salivary glands, stomach, large intestine, small intestine, all working together to help you digest, break down, and absorb food.

So, the human digestive system. This is an organ system that makes sure that we get what we need from food and gets rid of any waste products. It’s the focus of this episode.

We’re going to look at the different organs involved in digestion and the special adaptations they have in order to do their function. So grab a pen and write this down.

1. Its first job is to break down food into different molecules. Food is made up of large and insoluble molecules, so organs involved in the digestion process have special tricks up their sleeve to break down food into smaller molecules.

2. absorbing the nutrients. They’re also involved in absorbing the nutrients we need from food, in an exchange system. It does this by having a very large surface area.

So let’s go on the journey of food moving through our bodies. You’re going to need to remember this.

Digestion starts in the mouth.

Have you ever noticed saliva appearing in your mouth when you eat? The salivary gland is an organ that releases saliva, which contains an enzyme called amylase, and amylase helps to break down carbohydrates. This is called chemical digestion.

We’re going to talk about enzymes in much more detail in the next episode, but right now you’re good.

So when we bite our food, our teeth are helping to break it down into smaller pieces, which increases the surface area of the food. This is called mechanical digestion. So that combination of mechanical and chemical digestion helps move things along.

So after swallowing the food, it travels down a long tube called the oesophagus, which carries food from the mouth to the stomach, and that’s where the next stage of digestion happens.

The stomach - it’s a big, hollow, muscular, J-shaped organ, that’s got lots of weapons to break down food.

It’s got strong, muscular stomach walls, and that churns up the food. So that’s mechanical digestion.

The stomach contains an enzyme called protease. That breaks down proteins so that is chemical digestion.

The stomach also contains hydrochloric acid, and that’s a strong acid. So it kills any bacteria in food and it lowers the pH of the environment, so that helps protease enzymes to work. So that’s chemical digestion.

After all of that has happened in the stomach, the food particles get passed along, into the small and large intestines, and that’s the next step of our digestion journey.

So let’s start with the small intestine, because that’s where the food goes next. The small intestine is made up of two sections, and they have weird names, the duodenum and the ileum.

The food passes first into the duodenum, that’s where lipids, that’s fats, are digested. Plus, there’s even more digestion of proteins and carbohydrates.

So the ileum is the next part of the small intestine – it’s a super special place of the digestive system. This is because, all of those pieces of food, this is where they finally get absorbed by the bloodstream and taken to the rest of the body for energy, growth and repair. We talked about this in the series on The Cell when we covered active transport.

So what makes the ileum so good at absorbing all of this? Let’s talk form and function.
 Imagine your ileum is like an empty toilet roll tube, and on the inside of this tube, there are millions of tiny finger-like projections that stick out and help grab the nutrients from your food as it passes down the tube.

These are called villi. Villi is the plural form to describe lots of them, an individual one is called a villus. In our series on The Cell, we spoke about how important surface area is to absorption. All of those villi increase the surface area quite a lot, lots of surface area means lots of absorption.   So let’s take a look at these villi

They have three adaptations that help with its main function of absorbing food.

1.  the surface layer of villus is only one cell thick - so there’s a thin wall for food to travel through

2. the villi have a good blood supply from a network of blood vessels and that helps carry the absorbed food away

3.  each villus is covered in its own little villi. Yes, the villi have villi, and those villi are called microvilli.

Your liver, pancreas, and gall bladder all help out here. Your liver produces bile, and that gets stored in the gall bladder, before being squirted into the small intestine, like soap in a car wash.

Remember, in the stomach there was a lot of hydrochloric acid to help enzymes there work and to kill bacteria. So we need bile to neutralise that acidity of the food, so digestive enzymes made in the pancreas can work.

Bile also helps with fat digestion. It makes these large fat globules smaller, so that increases the surface area for the enzymes to work on.

Anything left in the small intestine carries on the journey to the large intestine. This is where water is absorbed and a process called egestion begins. Egestion, not digestion, which is taking food in.

Egestion is a fancy way of saying that the undigested food leaves your body as faecal matter. Don’t say poo in your exam, you’re better than that. This faecal matter travels out through the rectum to the anus.

I’m Dr Alex Lathbridge and this is Bitesize Biology – subscribe on ����ý Sounds

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the second episode of a six-part series on the organisation of plants and animals.
Today we’re going to be talking about some proteins known as enzymes.

Organisms have loads of chemical reactions, they’re happening inside of us all the time. The thing about chemical reactions though, is that they’re not all fast.

Without enzymes, everything in an organism would happen too slowly. The chemical reactions just would take too long, and so the organism wouldn’t be able to function.

Enzymes speed up the rate of chemical reactions but aren’t themselves alive. You can think of them as helpful molecules. They make things happen and that means enzymes are a catalyst.

So, a good definition of enzymes would be: enzymes are biological catalysts which speed up chemical reactions.

Now enzymes don’t just help speed up the processes of breaking things down, you could call those breakdown enzymes.

Enzymes also help build things up, so you can call them synthesis enzymes. Synthesis, making things.

We’re going to talk about how they work, a model known as Lock and Key. And we’re going to go into a little bit more detail about how enzymes aid digestion.

Now something that we’ve talked about before is this idea of form and function, the two Fs.

It’s the idea that the shape of something directly impacts its ability to do its job. This is really, really, really important when it comes to enzymes, they are the perfect example of this.

Enzymes are not just a few molecules thrown together. No, these are large, complex, unique 3-Dimensional structures – they’re made up of long chains of proteins, all folded into squiggly shapes. This shape is what helps them to speed up chemical reactions, because on the enzyme, it creates a space known as an active site. This is a biological term that I’m going to help you understand.

I want you to look around right now for a couple of different small objects, that you can safely hold. I’m at my desk, with my dog Ollie sat on his bed next to me. So I’ve got an old football and a tennis ball. Picking up the tennis ball with my left hand, it naturally cups around it. And picking up the football with my right hand, it makes a far wider and looser grip.

Now – this is the most important bit – if I keep my hand shape the same but I switch the objects round, my left hand can’t pick up the football – its grip is perfect for a smaller tennis ball. And if I try to pick up the tennis ball with my right hand, it just falls out – it’s made to grip a larger object.

The hands there are two different enzymes and the two objects are two different substrates, the molecules that interact with the enzyme. And the pocket that your hand makes in the palm of your hand, to help pick up the object, that is the active site of the enzyme – which is complementary in shape (that's just a sciencey word of saying that the substrate and active site fit together). If you drop the object (or substrate), your hand (or the enzyme) isn’t destroyed.

The actual name for this is the Lock and Key Model. Because a lot like how the pattern of your keys fits perfectly into the shape of the locks in your door, allowing you to open that door, the same key won’t work with other doors.

Enzymes are brilliant, but are very sensitive to their shapes changing. If the large, 3-Dimensional structure of the enzyme changes, it means the shape of the active site changes, and that means it can’t bind to the substrate (the molecule that interacts with the enzyme) and so it won’t work.

There are 2 main ways this change can happen:

1. pH - the acidity or alkalinity of the environment. If this becomes too high (very alkaline) or too low (really acidic), the enzyme changes shape, as the bonds that hold the enzyme together are damaged. Remember, enzymes aren’t alive. So these bonds breaking means that they don’t die, they become denatured.

2. Temperature – if the temperature is too high, the bonds break and the enzyme denatures. But if the temperature is too low, they don’t denature but the molecules move a lot slower, so the rate at which the enzymes and substrates collide, it lowers, so that reduces the rate of reaction.

Now let’s look at enzymes in action.

The human digestive system couldn’t function without enzymes because they help break food down.

There are two types of digestion – chemical and mechanical. Now if you don’t remember that, go back and listen to the previous episode to help get you up to speed.

There are a couple of names of digestive enzymes you are going to need to remember. It’s alright because enzymes always end with ‘ase’ and a lot of these enzymes have similar names to the food that they break down, which makes them a lot easier to remember.

Carbohydrases are enzymes that break down carbohydrates into sugars.

Amylase is a type of carbohydrase. It breaks down starch, a carbohydrate, and turns it into simple sugars. Amylase is made in the salivary glands, the pancreas and the small intestine.

Next up we have proteases. Those break down proteins into amino acids. Proteins are really long chains so they can’t be easily digested, so protease is another enzyme that comes in and helps us do this. Proteases are in the stomach, the pancreas and small intestine.

And Lipases are enzymes that break down lipids, now those are fats and oils. It breaks it down into smaller molecules called glycerol and fatty acids. Lipases are found in two places, the pancreas and small intestine.

I’m Dr Alex Lathbridge and this is Bitesize Biology – you can listen to the whole series now on ����ý Sounds

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.This is the third episode of a six-part series on the organisation of plants and animals.
In this episode we’re going to talk about the circulatory system. We’re looking at the structure of the heart and how it works, and we’re finding out about the different types of blood vessels.  The circulatory system is the main transport system in humans. You can think of all the blood vessels as a big network of roads winding inside our body, and transporting food, oxygen, and waste to where it needs to go. It’s got valves. They’re kind of like road signs, they make sure everything goes in the right direction.

And key to all of this is one organ - the human heart.

The heart pumps blood around the body by contracting, this just means squeezing.

You can think of blood as the cars that speed around our roads, our blood vessels, transporting everything that our body needs. The heart constantly pumps the blood, keeping us alive.

The heart is made up of four chambers, with valves and important blood vessels transporting things in and out of it.

It’s key that you understand the structure of the heart, and how blood flows through it, so grab a pen and let’s draw a rough diagram so you know what each part of the heart is called and what it does.

And remember, this is important, for those diagrams, left and right refers to the side in a person’s body, not how its viewed on the page. You can go to the Bitesize website and take a look.

Draw a heart shape and divide it into four. These are the four chambers of the heart.

Alright let’s label them. The chambers in the top half of the heart are called the left atrium and the right atrium and the two chambers in the bottom half of the heart are called the left ventricle and the right ventricle.

On the right-hand side of our heart, the blood enters into the right atrium from a blood vessel called the vena cava.

On the left-hand side of heart, the blood enters into the left atrium from a blood vessel known as the pulmonary vein, so let's add that in.

The atria fill up with blood and then they contract, so the blood is pushed down into the bottom half of the heart – the ventricles.

Then the ventricles contract which pushes the blood up and out of the heart.

On the left-hand side, blood leaves the heart by the aorta and goes to the rest of the body.

And on the right-hand side, blood leaves the heart by the pulmonary artery and that goes to the lungs.

This is the only direction of travel in the heart, blood is prevented from travelling backwards by the valves. There is a barrier between the left side and the right side of the heart, so the blood doesn’t go between them.

A good way that I use to remember this, is by saying that the vessels that take blood into the heart are the veins. The vena cava and the pulmonary vein are veins, and the word “vein” ends in “in,” veins take blood in.

The other way, the blood goes away from the heart by the blood vessels that begin with “A”, the pulmonary artery and the aorta.

Time for a revelation, this going to blow your mind – the process is different on each side of the heart.  The heart actually pumps blood through two separate circuits, so the circulatory system is really a double system, so you're going to need to know what each side does.

The right-hand side of the heart looks after pulmonary circulation.

Pulmonary just means lungs. This side pumps blood to the lungs for gas exchange. So oxygen moves from the lungs, into the blood and then that blood is now oxygenated.

This oxygenated blood then returns back to the heart. So really, pulmonary circulation is just a loop between the heart and lungs.

The heart’s left-hand side looks after systemic circulation.

It pumps oxygenated blood to the rest of the body. So the blood carries oxygen to all the places in our body that it’s needed – which is everywhere! It then gives up the oxygen, or drops it off, at the cells.

The blood is again now without oxygen again, so it gets called deoxygenated and goes back to the heart, and the heart pumps it round to the lungs again to pick up more oxygen, and so on and so forth!

Now if that sounds confusing, what you can do is draw for me a number eight. In the middle of that number eight, that’s where the heart is. At the top are the lungs and at the bottom is the rest of the body. So that’s how it all comes together. At the top you’ve got pulmonary circulation and at the bottom you’ve got systemic circulation.

The wall of the left-hand side of the heart has much thicker muscle than the right-hand side, as it needs to pump blood much further around the whole body. The lungs and the heart are really close together, so it’s not such a big job for the right-hand side.

And the final thing we’re going to talk about at today is blood vessels – there are three different types of blood vessels and each are adapted for their function:

1. Arteries – these always carry blood away from the heart . Remember: “A for Arteries” and “A for away.” The heart pumps blood out at really high pressure, so they have to be very strong. Arteries are made of muscle and have thick, elastic walls. The middle space in arteries where blood flows is known as the lumen and its narrow.

2. Capillaries – these are tiny vessels where substances are exchanged. If we think of blood vessels as a road network, these would be the tiny driveways where cars park. They carry red blood cells close to the other cells in the body. Their special feature is that they have permeable walls, so that means they are really thin – they're only one cell thick, and that allows food and oxygen, and waste products like carbon dioxide, to diffuse in and out easily.

3. Veins – these carry blood back to the heart. The walls of veins are thin, they’re much less muscly than arteries, because blood going back to the heart from the rest of the body, is under a lot less pressure. The Lumen, the middle part of the tube, inside veins is very large. As blood is travelling slower when it reaches the veins, they need to valves inside to ensure the blood keeps moving. It would not be good for the blood to start flowing backwards!

I’m Dr Alex Lathbridge and this is Bitesize Biology – all episodes available on ����ý Sounds

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the fourth episode of a six-part series on the organisation of plants and animals.

In this episode we’re going to talk about disease. More specifically, we’re going to talk about two different types of disease, communicable diseases and non-communicable diseases.

And a quick heads up, in this episode we’re going to be talking about cancer and other illnesses.

Let’s start with cardiovascular diseases which affect the heart or blood vessels.

In the last episode, we talked about how the heart pumps blood to the lungs and around the body to transport oxygen and food. As our hearts are super important, it can leave us in a bit of a pickle when our heart or blood vessels are diseased.

The main cardiovascular disease you are going to need to remember is coronary heart disease. Grab a pen so you can write this down and we’ll get into it.

Coronary heart disease is a disease of the coronary arteries. Those are the ones that supply the heart cells themselves with oxygen. They get blocked by fatty materials and so blood flow gets restricted, and that results in less oxygen reaching the heart, which can lead to a heart attack.

Thankfully there are options to help manage coronary heart disease and they both begin with  “st,” which is good because they stop blood cells getting stuck.

First, stents. These are special tubes that can be inserted into coronary arteries and force them to stay open, therefore helping blood to pass through to the heart, lowering the risk of a heart attack.

However, there can be complications when stents are inserted during the operation and risk of infections from surgery.

Second are statins. These work to reduce cholesterol. You might have heard of cholesterol in things like butter. Too much of it in the blood can cause those fatty deposits to build up in the arteries. Statins are drugs that reduce cholesterol in blood, so fewer fatty deposits form.

Statins have some advantages like reducing the risk of coronary heart disease and strokes, which is what happens when your brain loses blood supply and can increase the amount of good cholesterol in your blood.

But there are disadvantages too. Statins are a long-term drug, so that doesn’t suit everyone. There may also be dangerous side effects such as liver damage.

A more major option is a whole heart transplant, sometimes using hearts from donors who have recently died. But when no donors are available, plastic or metal artificial hearts are available. These artificial hearts pump blood around the body.

Artificial hearts are less likely to be attacked by our immune system, but they do have disadvantages. Surgery can be dangerous and lead to infection, and because artificial hearts are mechanical, something could go wrong with them, and they’d need to be replaced.

Inside the heart, heart valves can also be replaced if they need to be. Again, these can be biological or mechanical. Replacement biological valves can be from other humans or mammals like pigs, whereas mechanical valves are man-made.

There is also something called a heart bypass, where blood vessels from elsewhere in the body are grafted onto the coronary artery.

How are you feeling today?  Because health is a measure of how well your physical and, or mental wellbeing, is.

Say you have a cold, you might feel under the weather for a few days, but usually you recover and feel better. More serious illnesses are diseases where medical treatment might be required.

There are two types of disease that you need to understand:

1. Communicable Diseases – these can spread between organisms like person to person, or between animals and people. Have you ever caught a cold ‘from’ someone? Same for things like measles, food poisoning and malaria.

2. Non-Communicable Diseases – these don’t spread between people or between people and animals. If untreated, they tend to get worse slowly and can last a long time. Some examples are diabetes, cancer and heart disease. These are non-infectious, you can’t catch cancer, heart disease, diabetes or other non-communicable diseases from another person or animal.

Not being funny but it’s quite literally in the name. When two or more people are chatting or sharing information in some way, they’re communicating. And when two or more people are sharing diseases, they’re communicable diseases.

You need to remember three factors other than serious disease that can cause health problems and they can all be linked:

1. Diet. It’s important to have a varied diet and eat lots of different things, so our body can function well.

2. Stress. Life can be stressful, everyday stress like exams, annoying family, stuff that you get exposed to online. No matter your age, too much stress can cause very real health problems. Prolonged stress can lead to high blood pressure for example.

3. Life circumstances. Our life’s circumstances can affect our health. Not everyone has the same access to medicines, doctors' appointments, information or can afford to buy healthy food or live a stress-free life. It’s often outside of their control. This is known as health inequality.

Now, there’s one serious non-communicable disease that is unfortunately quite prominent in our society, cancer.

Cancer isn’t one disease. There are lots of different cancers found in different parts of the body, like breast cancer, testicular cancer, lung cancer, and others. Some are a lot rarer than others. Cancers develop in the body by uncontrolled cellular division.

Our bodies need cell division to keep us ticking over, repairing and replacing cells, but if it's out of control, changes can occur to the cells and cause the formation of a tumour, which means a growth or lump of cells formed from uncontrolled cell division.

There are two types of tumour.

1, benign tumours. These are growths or lumps that are not cancerous. They tend to grow slowly, stay in one place, and look a little bit more smooth.

The second type are malignant tumours. These ones are the cancerous ones and are more worrying, because they can invade other parts of the organ or travel in the bloodstream and invade other tissues, that’s known as metastasis.

But remember, cell division is happening all the time, and the body has lots of ways to keep cells under control.

The best thing you can do is just be generally aware of how your body normally feels and if you discover anything that you think “oh, that’s a bit odd,” it’s worth letting a parent or guardian know.

I’m Dr Alex Lathbridge and this is Bitesize Biology – all episodes  available to download now on ����ý Sounds

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the fifth episode of a six-part series on the organisation of plants and animals.

Do you know how to be healthy? Do you make time for exercise or are you more of a video games and chill type? Life choices have a big impact on your health.

In the last episode I talked about the different types of disease – communicable and non-communicable. If you don’t remember that, go back to the last episode and listen again.

This episode is going to focus on the risk factors that can lead to getting ill with non-communicable diseases, things like cancer, cardiovascular disease and diabetes.

Risk factors are things that can increase the chances of someone developing a disease and therefore having poor health.

They can be related to lifestyle, stress or diet.

They can also be related to your environment, which means how our bodies interact with our surroundings, like chemicals in the air caused by air pollution from cars, buildings and what have you, or the harmful chemical substances that build up in our bodies.

Let’s take a look at some risk factors that are known to directly cause a disease. So that means there is actual scientific evidence demonstrating causation. So that’s a link between cause and effect, basically saying that this thing increases the risk of developing this disease. So grab a pen and let’s make some notes.

First up, obesity. So being dangerously overweight can cause Type 2 diabetes and cardiovascular disease.

Obesity leads to high blood pressure and the arteries get blocked by fatty deposits, which as we know from the previous episode, can cause cardiovascular disease. A high level of body fat also impacts the body’s ability to use a special hormone called insulin.

In Type 2 diabetes, the body’s cells respond less effectively to the hormone insulin and this disrupts how your body processes sugar. Type 2 diabetes can be controlled by diet and exercise but there is no cure for it. The risks of developing cardiovascular disease and Type 2 diabetes can be reduced by eating a balanced diet of whole foods, things that aren’t processed, and exercising regularly.

Next up is smoking. It’s well known that smoking tobacco causes heart and lung diseases, things like lung cancer.

Let's take a moment to focus on the lungs.

Our lungs do two jobs for gas exchange: oxygen in and carbon dioxide out. Breathe in, breathe out.

When you breathe in air, it travels down the trachea. The trachea then splits into two branches, one for each lung, these are known as the bronchi. And then within each lung, the bronchi split into smaller and smaller tubes called bronchioles. At the very end of these tubes are air sacs called alveoli and this is where gas exchange happens. We’ve got millions and millions of alveoli in our lungs. So, remember trachea to bronchi, bronchi to bronchioles, bronchioles to alveoli.

The alveoli are surrounded by a network of capillaries. They have a very good blood supply, very thin walls and a large surface area. Oxygen travels across a concentration gradient out of the alveoli in our lungs, and into the blood where there is a low concentration of oxygen.  At the same time, carbon dioxide diffuses out of the blood into the alveoli, to be breathed out of the body.   Alright so hopefully at this point you understand that you need your lungs in good working order.

There is no safe level of smoking and that includes passive smoking, being around people that are smoking. You can’t say “yeah, if you only smoke one a month, you’re good” or “but shisha filters all of the bad stuff out.” Take it from me, miss that entirely, it’s not worth it.

Next is excessive alcohol drinking. The liver processes alcohol and each time it does, some of the liver cells die. That’s fine because it can regenerate itself. Excessive drinking over many years can prevent regeneration, leading to serious and permanent damage to your liver. Not just that, it can also lead to brain shrinkage, memory problems and psychiatric issues later down the line.

Lifestyle choices are especially important for pregnant people, because they can affect the unborn baby growing inside of them. Smoking and drinking when pregnant is not a good idea, because it reduces the oxygen that the baby needs during development, and this can lead to a whole load of issues pre- and post-birth.

Just because a risk factor might exist in someone’s life, doesn’t mean that they’re going to get the disease or diseases associated with it. So just because someone doesn’t exercise very much doesn’t mean that they will definitely get heart disease.

This is the difference between causation and correlation.

Causation is the idea that something makes another thing happen. So with risk factors, chemicals that go into your body from smoking cause cancer.

But correlation is where there’s some kind of relationship, but it's not necessarily cause and effect.

Like this relationship between ice-cream and the weather: people eat ice cream when it’s hot, but that doesn’t mean that ice cream makes the sun shine. This is an example of correlation rather than causation, like some risk factors

Non-communicable diseases also have human and financial costs.

I have a brain condition called nocturnal epilepsy, meaning that if it’s not properly controlled, I can have seizures at night, and they can impact my sleep. But also, the medication has some annoying side effects. So the human cost is that it lowers my quality of life, it made school harder, and affects my mental and physical health today. A financial cost would be something like, I need to spend money to travel to the hospital for doctor’s appointments.

So human costs are the impacts that disease has on human beings. Non-communicable diseases like asthma, heart disease and cancer can impact life expectancy, quality of life and mental health. Not to mention the impact on people around them, wider society and globally.

Financial cost is more simple to understand. It can be personal, like the ability to earn money being reduced, or on a larger scale, like with the NHS, our National Health Service of doctors, nurses, scientists, ambulances and people who work in hospitals. It has to spend lots of money researching and treating these non-communicable diseases.

Let’s keep going with the risk factors but now, we’re going to focus on risk factors relating to cancer.

First up, obesity. This has links to different cancers; it increases the chances of being diagnosed with bowel and liver cancer. And, as obesity is about eating too much food and having a poor diet, the organs that are affected are those involved in the digestive system. A poor diet often involves lots of processed foods - crisps, chocolate bars and sausage rolls - and not enough unprocessed foods, like fruits, vegetables, wholegrains and nuts.

Next is smoking. Smoking has been known to cause lung cancer since the 1950s, but it's also thought that it can increase the chance of being diagnosed with other cancers, such as those in the mouth and stomach. You might have seen pictures in your science lesson of a lung that’s turned black due to the chemicals in cigarettes. It’s not a good look.

Next up is UV, or ultraviolet, radiation. This is the energy released naturally by the sun, and artificially by things like sunbeds. You can’t feel it. The sun is really good for the world, especially for plants, but too much exposure to UV radiation can damage the DNA in our skin cells, and if too much damage builds up it can lead to uncontrolled cellular division.

It’s not saying that getting sunburnt once means you’ll get skin cancer. But the more times you get sunburnt, the higher your risk. That’s a big reason why it’s important to use sunscreen, when you’re outside and exposed to sunlight, even on cool days. And me, my wife, my friends, my family all use sunscreen. Because although dark skin is naturally more protective than fair skin, all skin types are susceptible to damage from UV rays.  So another thing to think about when it comes to developing cancer, has nothing to do with lifestyle choices, its genetics. Sometimes you can inherit genes that have specific mutations that increase the likelihood of developing breast or ovarian cancer, for example. And some women choose to have breast tissue removed if they have a high risk of developing breast cancer.

I’m Dr Alex Lathbridge and this is Bitesize Biology – catch up on ����ý Sounds

Hello, I’m Dr Alex Lathbridge and this is Bitesize Biology.

This is the last episode of our six-part series on the organisation of plants and animals.
In this episode we’re going to talk about plants and how they are organised, the different tissue structures in leaves, and we’re going to revisit those two transport tubes in plants - the xylem and the phloem - and talk about what they do, transpiration and translocation.

So where to start? Well the first thing to know is that plants have a similar organisational structure to animals: cells, tissues, organs and organ systems (that’s episode one stuff, so go back and have a listen if you don’t remember.)

Plants’ organs are leaves, roots and stems. Yes, the leaf of a plant is actually one of its organs. The leaf is the main organ for photosynthesis, which is how plants make food, by absorbing energy from the sun and turning it into glucose, a sugar. Just like us humans, plant organ systems work together to transport different substances around the plant.

So the leaf is an organ. And we remember what an organ is? It’s a group of different types of tissues in the same place, all working together to complete a function.

So let’s imagine that we’re taking a microscope and zooming in on a little cross section of a leaf, to see all the different layers of tissue inside it. Each layer has adaptations for its functions: photosynthesis and gas exchange. Each layer is a bit like a big sponge cake, with different fillings.

Now you’ll need to remember the layers, so I really suggest grabbing a pen and writing it down, this always confused me, so it might help you to draw your own diagram.

The top layer of a leaf is the upper epidermal tissue. This is very thin and transparent, so it allows as much light in as possible for photosynthesis to happen.

The leaf’s epidermal tissue is coated in a waxy cuticle. And so that makes leaves look shiny and smooth, and this helps them to remain waterproof.

Then below the epidermis there’s the palisade mesophyll tissue. This layer is packed with lots of chloroplasts, where photosynthesis happens. (Now if you don’t remember what photosynthesis is, you can go back to the series on The Cell and listen to episode 6.) The palisade mesophyll tissue is right at the top of the leaf to absorb as much light as possible and increase the rate of photosynthesis.

Now if you need to remember this, palisade mesophyll tissue is filled with chloroplasts. And what are chloroplasts filled with? Chlorophyll.

Below the palisade mesophyll tissues are spongy mesophyll tissues. Now this layer of tissue has big gaps, or air spaces, which mean that gases can diffuse in and out really easily.

Running up and down the plant, so that’s through the roots, stems and leaves, are two special tubes: the xylem and the phloem (you can go back to the series on The Cell and episode three if you’ve forgotten about them.)

These are the plant’s plumbing system. They transport substances like nutrients, water and minerals through the whole plant. We’ll be looking at them in more detail in a little bit…

And finally, the lower epidermal layer. This has special holes in it to help with gas exchange and these holes are called stomata that open and close. This is where gases like carbon dioxide enter and oxygen leaves the leaf. The stomata open and close using guard cells

As the stomata open to let gases out, water gets lost by the process of transpiration. So closing the stomata helps to control water loss.   So let’s look at the xylem and phloem – the plant’s plumbing system.

Plants are equipped with two special tubes to carry water and nutrients to the rest of the plant. In our series on The Cell we looked at the features of the xylem and phloem that make them specially adapted to their functions (and you should listen to that series again if you need a refresher). Now we’re going to look at how substances move around the xylem and phloem.

The phloem’s main job is to move food, in the form of liquid sugar produced by photosynthesis in the leaves, known as sap, to different areas of the plant. Food is transported up and down the plant in a process called translocation. Literally transpiration moving from one location to another.

The xylem is a column of dead cells with ends eroded away to make tubes, where water and minerals can move up from the roots, through the xylem and to the leaves. This movement of water through the xylem is called.

Think of transpiration as one long column of water constantly being pulled up through the plant, all the way from root to leaf. When the water from inside a leaf evaporates into the air, this causes a shortage of water in the rest of the plant. So water then gets drawn up and through the xylem to replace this lost water. This means that more water gets drawn up from the root, so there are never gaps.

Think of this column of water as one long line of water molecules all linking arms, where they never let go. Transpiration is not driven by the roots, it’s not the roots pushing water up, it’s the other way round. Water evaporates out of the leaf, and the water column is pulled up towards the xylem.

You need to make sure that you get the spelling right, because these things are spelled weirdly:Xylem – x-y-l-e-m – xylem – that’s for water, transpiration

Phloem – p-h-l-o-e-m –  phloem - for food, and that’s translocation

Finally there a few factors that can affect how well transpiration works and we have to consider them:

1. Temperature. When the temperature is higher, molecules move faster, so water evaporates from the leaves quicker, increasing the rate of transpiration.

2. Humidity. Humidity is how much water vapour is in the air. When there is low humidity, it’s very dry, transpiration speeds up. Because the diffusion of water between the leaf and the air increases, because there’s a high concentration gradient – lots of water in the leaf, no water in the dry environment. So water gets sucked out.

3. Air movement. The movement of air around a plant can affect transpiration. If an environment has good airflow (a fun way of saying windy), the transpiration rate will increase. If there is very little wind, the transpiration rate will decrease because water vapour will stay surrounding the leaf, so there is a low concentration gradient between the leaf and the air.

4. Light intensity. The more light there is, the more photosynthesis will occur. During photosynthesis, stomata open and so more water diffuses out of the leaf.

I’m Dr Alex Lathbridge and this is Bitesize Biology – to hear more, search Bitesize Biology on ����ý Sounds