A very large proportion of the GCSE Biology course is focused on the human body, teaching you about the structure and functions that help us to survive day in, day out.
This particular area of Biology, related to the digestive system, nutrition, and excretion, covers a wide spectrum of subtopics, however, the two subject matters that are the most directly linked with humans are those linked to diet and digestion. As such, this is what I will focus on in this educational blog aimed at GCSE level students.
To help you to follow the biological parts, processes and functions described below, we have provided a glossary of terms, put together by the experts at BBC Bitesize, which can be referred to whilst you read this text, or can be saved for future reference. Go to their website for tonnes more guides, tests and other useful references during your course.
Note: Having a list of key terminology can be very useful when you come to revise for your exam, as this topic is likely to come up in at least one question on your paper.
(Extracted From BBC Bitesize)
Acid: Corrosive substance which has a pH lower than 7. Acidity is caused by a high concentration of hydrogen ions.
Active Transport: The transport of molecules against their concentration gradient from a region of low concentration to a region of high concentration.
Alimentary Canal: The digestive tract which runs from mouth to anus.
Alkaline: Having a pH greater than 7.
Amino acid: The building blocks that make up a protein molecule.
Amylase: An enzyme that can break down starch into simple sugars.
Bile: A substance produced in the liver. It emulsifies fats to prepare them for digestion.
Calorimetry: Measuring the amount of heat given out or taken in by a process, such as the combustion of a fuel.
Catalyst: A substance that changes the rate of a chemical reaction without being changed by the reaction itself.
Cellulose: A carbohydrate. It forms the cell wall in plant cells.
Compound: A substance formed by the chemical union of two or more elements.
Cytoplasm: The living substance inside a cell (not including the nucleus).
Diffusion: The movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.
Digestion: The breakdown of large insoluble food molecules to smaller soluble ones.
Egestion: The process of passing out the remains of food that has not been digested, as faeces, through the anus.
Emulsify: To mix water with fats and oils to produce a cloudy mixture called an emulsion.
Enzyme: A protein which catalyses or speeds up a chemical reaction.
Fatty acids: Carboxylic acids with a long chain of carbon atoms (usually 4 to 22). Fatty acids react with glycerol to produce lipids (fats and oils).
Glucose: A simple sugar used by cells for respiration.
Glycerol: A soluble carbohydrate which is coverted into glucose by the liver.
Immunity: When a person’s body is not prone to a disease because they have a resistance to it.
Ion: Electronically charged particle, formed when an atom or molecule gains or loses electrons.
Large intestine: The lower part of the alimentary canal (gut) where absorption of water and production of faeces happens.
Lymph: The liquid which circulates within a mammal’s body transporting the products of fat digestion from the lacteals.
Maltase: The enzyme that converts maltose (a disaccharide) into glucose (a monosaccharide).
Maltose: The disaccharide made from two glucose molecules joined together.
Mass: The amount of matter an object contains. Mass is measured in kilograms (kg).
Microorganism: The name for a microbe. It is microscopic and is an organism, such as a virus or bacteria.
Neutralise: To be made neutral by removing any acidic or alkaline nature.
Oesophagus: The gullet, the tube that leads from the mouth to the stomach.
Peristalsis: Wave-like muscular contractions in the smooth wall of the gut which move food through the alimentary canal.
pH: Scale of acidity or alkalinity. A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is alkaline.
Respiration: The chemical change that takes place inside living cells, which uses glucose and oxygen to release the energy that organisms need to live. Carbon dioxide is a by-product of respiration.
Sedentary: Taking little or no physical activity as part of everyday living.
Starch: A type of carbohydrate. Plants can turn the glucose produced in photosynthesis into starch for storage, and turn it back into glucose when it is needed for respiration.
Stomach: Muscular organ in the digestive system that produces hydrochloric acid and protease enzymes.
Villi: Finger-like projections in the small intestine that provide a large surface area for the absorption of food.
A balanced diet is a term we use to describe a diet that includes all of the components needed to maintain a healthy body. Appropriate portions are recommended for optimum results, including well-balanced amounts of carbohydrates, protein, lipids, vitamins, minerals, water and dietary fibre.
Although a balanced diet looks similar for everyone, the amount of energy needed by individuals varies according to activity levels, age and other factors like ill-health and pregnancy.
Babies and children need less food than adults. Photo by deanwissing on Visualhunt
A baby needs much less energy than a teenager or an adult, but growing teenagers and young adults need approximately 17% more energy than those beyond retirement age.
There are seven vital nutrition groups that make up a balanced diet, which are:
Carbohydrates, i.e. pasta, potatoes, rice, sugar and vegetables, act as sources of energy. Proteins, however, which consist of beans, eggs, fish, meat, lentils, peas and soya, have the purpose of repairing and growing cells. Meanwhile, lipids (fats and oils) like butter, cheese, margarine, nuts and oils act as a source of energy as well as providing storage and insulation in the body.
Vitamins help to maintain a healthy body but are only required in small amounts.
Vitamin C, referred to scientifically as ascorbic acid, is what is needed by the body to help heal cuts and wounds and keep connective tissues healthy. These tissues in turn support the vital organs. One of the best sources of vitamin C are citrus fruits, such as oranges but it can also be found in leafy greens.
Oranges are a good source of Vitamin C. Photo on Visual Hunt
Vitamin D is made by the human body when skin is exposed to the sun, but additional sources of vitamin D can be found in oily fish, eggs and cereals. The main function of vitamin D is to maintain healthy teeth and bones. If vitamin D levels in the body are too low, this can lead to health problems like rickets or achy bones.
Vitamin A is linked to good vision (especially in dim light), healthy skin and a strong immune system. Vitamin A can be found in eggs, dairy productsand oily fish. People say that eating carrots help you see in the dark, which isn’t just an old wives’ tale. The body converts beta-carotene into vitamin A from carrots as well as mangoes and spinach.
Mineral ions, like vitamins, help to maintain a healthy body when ingested in small amounts.
Calcium is responsible for maintaining healthy teeth and bones, and is found primarily in milk, cheese, eggs and some green vegetables. Additional functions include helping blood clots to develop normally and controlling muscle contractions. If you don’t have enough calcium in your system, you could end up feeling weak, get muscle spasms and have complications when you bleed.
Iron, which is required by the body to produce haemoglobin (found in red blood cells), reduces your risk of developing anaemia. Anaemia causes you to become tired and weak because of a lack of oxygen being transported in the blood. You can keep your iron levels up by eating green vegetables, liver, red meats, dried fruits, beans and nuts.
Aside from vitamins and mineral ions, your body needs a certain amount of water and fibre to function properly. Water can be drunk on its own but is also present in lots of foods and drinks. Water is found in the cytoplasm of body cells and makes up a whopping two thirds of our body!
Dietary fibre, meanwhile, is found in whole grain foods like cereals, as well as fruit and vegetables. This is important to the body because it helps the walls of the intestine to move food along the gut during digestion. If you don’t have enough dietary fibre in your diet, you will notice signs of being constipated.
Most of your body’s cells are made up of protein, so it makes sense that we need to eat lots of protein to keep this count up. The protein we ingest makes new cells and helps to replace old or damaged cells. Made up of smaller amino acids that stick together, there are around 20 different types of amino acids found in protein-rich foods like fish, egg, chicken, beans and meat.
Although, as we have already discovered, children need a smaller amount of food than adults, they especially need a good level of protein while they are growing as a deficiency can result in illness and other problems.
We talk about fat as if it is something really bad for you but, in reality, fat is needed in your diet to give you energy. Taken in moderation, foods like butter, crisps, cheese and sausages, for example, contain high levels of energy and act as insulation for your body, depositing a store under the skin to cut down on the loss of heat.
There are two types of fat: saturated and unsaturated.
The first comes from animal-derived foods, whilst the latter is from plant-based produce. Too much of this unsaturated fat can lead to spikes in cholesterol (a fatty deposit that sits in the blood and can clog arteries) so maintaining a good balance of the types of fat we eat (saturated fats in turn reduce levels of cholesterol) is very important to avoid heart disease or heart attacks.
Finally, one source of nutrition that we take for granted is water. Did you know that our bodies are made up of roughly 65% water? This doesn’t just account for the drinking water we ingest, as water is taken in when we eat too.
Water is so important in the process of digestion because it allows necessary chemical reactions to take place, helps waste to be carried away from our bodies, enables our blood to transport substances which dissolve in the fluid and lets our body sweat in order to cool us down on hot days.
To think that some countries don’t have enough clean drinking water is a tragedy, so we mustn’t take the value of drinking water for granted. To ensure you drink enough fluid in a day, you could get a flask that contains markers on it and encourages you to drink recommended amounts.
When learning about diets, you will start to understand the different types of foods and their functions. For instance, you will be taught about carbohydrates, proteins and lipids and their functions in relation to the human body. You will then discover sources of vitamins and how deficiencies in these areas can affect us too.
Whilst encouraging you to seek out your specific energy requirements and how to maintain a healthy, balanced diet, you will additionally be taught how other influences can affect our diets including pregnancy, activity levels and illness.
When it comes to learning about our digestive system, you will explore how each of the food types, once ingested, are broken down and absorbed and how the chewed ball of food then moves through the body before being excreted. In addition, you will be told how certain foods impact on our teeth and cause decay.
Your teacher will be able to teach you about food and energy sources in more detail throughout the academic year but, in the meantime, here is a brief overview of why it is so important to eat healthily.
When you think about food, your mind probably jumps to your favourite meal or snack. But, if you put the pleasure of eating certain products aside and get your Biology hat on, you almost certainly know the main reasons why your body needs food to function properly. Energy, growth, and health are those key reasons.
Your body needs food to keep your muscles and organs strong. Just like you fill a car up with petrol, food is your body’s fuel and it can’t work without it.
Unlike a machine though, your body uses the fuel you ingest to make new cells and to allow your body to grow. The cells produced help to keep your body healthy, creating certain chemicals to rectify imbalances or to fight off infections.
As a student, it is likely that your parents buy the weekly shopping, but that doesn’t mean that you can’t have a say in what you eat! If you make sensible recommendations to your guardian about the meals and snacks you want for the week, they may well listen and it might inspire the whole family to sit down and think about their eating habits.
We all know that fruit and vegetables are good for us, but are we eating the recommended 5-a-day? If you are unsure what portions make up one of your five a day, then visit the NHS Eat Well pages for more information and examples.
In addition to fruit and veg, we must all eat starchy foods, milk and dairy products, as well as proteins. The best sources of proteins come from beans, pulses, fish, eggs and meat. Think of meals like a light chilli con carne, red fish with pasta or a lean turkey breast burger as examples of a balanced meal.
Oils and spreads can give us the fat we need but it is important to look for unsaturated fats (like olive, rapeseed or sunflower oils) to avoid developing problems with cholesterol.
If you are overweight, one of the first things you should cut out is unnecessary sugars, like sweets and chocolate. While it is okay to have treats like this in moderation, unhealthy foods like this can account for the majority of cases of obesity in the UK. According to the NHS, most British adults are classed as overweight or obese, do you want to follow this path?
You can change your attitude towards food by educating yourself about diet and nutrition, which is one of the primary goals of this Biology module. Furthermore, you can check how you weigh in on the BMI chart, and determine whether you need to reassess your food intake or if you are a healthy weight for your age and height.
Although small, your teeth play a big part in digesting food. The process of mechanical digestion is what happens when food is broken down into pieces in the mouth whilst you chew. As your teeth crush and cut food, saliva helps to form a ball of food.
Your teeth consist of incisors, which help to bite and cut, canines, which hold and cut, and molars, which are there to crush and chew food.
Teeth are essential for chewing and breaking down food. Photo on Visual Hunt
Once ingested and chewed inside the mouth, the balls of food go down the oesophagus and into the stomach. Muscles in the gut wall work together to squeeze food in contractions, in a process called peristalsis.
The gut consists of two types of muscles: circular muscles reduce the diameter of the gut as they contract and longitudinal ones reduce the length.
While the teeth and stomach drive mechanical digestion, enzymes break down nutrients in a process called chemical digestion. These enzymes work at different pH levels.
The stomach is a muscular organ, a bit like a sac, that can be located in our upper abdomen, roughly just below our ribs on the left side. It is a continuation of the oesophagus and serves as a connection between the oesophagus and the small intestine. Muscular sphincters separate these organs.
The stomach begins at the lower esophageal sphincter. Our stomachs are very muscly with multiple layers.
The first region of the stomach is called the cardi and is the layer closest to the oesophagus. After this, there is the fundus, which is the top-most arch of the stomach. This region is followed by the body of the stomach, the largest region. Finally, you have the pylorus region, which is closest to the exit into the small intestine; the two are separated by the pyloric sphincter.
Take a look at this labelled digestive system. Image sourced from http://freeencyclopedia.wikia.com/wiki/File:Digestive-system-diagram.jpg
After being in the stomach, food makes its way to the small intestine where food molecules get into the blood through the wall of the intestine. This is called absorption. Enzymes work best in alkaline conditions, so bile is produced by the liver, stored in the gallbladder and then released into the small intestine to help to emulsify fats and help the lipases to work more effectively.
As we’ve seen, enzymes play an important role in the chemical breakdown of food during digestion. But what exactly are these molecules and how do they do their job?
We already know that enzymes work at their best when at their optimum pH, so they rely heavily on the right conditions being set up for them in the stomach or gut.
There are three principal types of enzymes: Protease, Carbohydrase, and Lypase.
Protease enzymes are found in the stomach, small intestine and pancreas and their task is to digest proteins. Pepsin is a common type of protease and helps to break down the long chains of amino acid molecules found in proteins into smaller links called peptides and then into individual amino acids which are easier absorbed in the small intestine. You may be required to know the equation for this process which is:
proteins -protease-> amino acids
Carbohydrase enzymes, as you might expect, break down long starch molecules. They are secreted by the mouth, pancreas and small intestine. One type, named amylase, is found in our saliva and starts doing its work as we chew our food.
The first step is to break down the complex starch molecules into maltose molecules (taking them from polysaccharide to disaccharide) and then it gets broken down again into glucose.
The equation is as follows:
starch -amylase-> maltose -maltase-> glucose
Lastly, Lipase is found in the pancreas and the walls of the small intestine. Complex fats, called lipids, get broken down into soluble fatty acid and glycerol molecules as indicated in the word equation:
lipids -lipse-> fatty acids + glycerol
Below are the main enzymes found in the human digestive system and what they do.
|Digestive enzyme name||Where in the body it is produced||What reaction is catalysed|
|Protease||Stomach, small intestine and pancreas||Turns protein into amino acids|
|Carbohydrase||Saliva, pancreas and small intestine||Turns starch into glucose|
|Lipase||Pancreas and small intestine||Turns fats into fatty acid and glycerol|
Did you know that the longest part of the small intestine would measure between 2-4 metres long if laid down flat? The large surface area means that absorption happens quickly, helped by finger-like villi which further increase the area.
Different things happen to digested and undigested foods once they have passed through the gut; digested food molecules provide energy and build new proteins in a process called assimilation while indigestible substances reach the colon, the first part of the large intestine.
Once all of the remaining water has been absorbed, the bacteria and cells (undigested food) that are left become waste, also known as faeces. This semi-solid material is stored in the rectum, which makes up the final part of the large intestine and then gets passed out of the anus in a process called egestion.
The rectum provides temporary storage, and it is as the rectal walls expand due to collecting faeces that cause you to feel the need to go to the toilet, or defecate. Peristaltic waves then push the waste out of the rectum.
The anus then takes over and controls the expulsion of the faeces, in particular by the anal sphincter muscle. The internal and external muscles relax, allowing the waste to be passed and then by pulling the anus up over the exiting feaces.
Egestion, i.e. the process of passing semi-solid waste when you go to the toilet, is not be confused with excretion.
Excretion is the removal of waste products from the body via other organs, like the skin.
During your course, you will also learn more about urine and how this leaves the body in both males and females.
If all of the above feels a bit scientific and has left your mind a bit frazzled, take a look at the following fun facts and look at the digestive system in more relatable terms.
It’s fascinating what our bodies can do and how much work they put in just to keep us alive and well. It is no wonder we need to rest at night! But even while we sleep, our bodies continue to carry out their numerous and momentous tasks…
As part of your revision for Biology GCSE, you may wish to brush up on the following topics: