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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Chemical Elements in Life
Living organisms are made up of chemicals! Everything from the tiniest bacteria to the largest whale contains the same basic chemical elements, just arranged in different ways. In this session, we'll explore the building blocks that make up all living things and understand why certain elements are essential for life.
Key Definitions:
Living organisms are primarily made up of just six elements: carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P) and sulfur (S). These are sometimes remembered using the acronym CHNOPS. These elements form the basis of all biological molecules including proteins, carbohydrates, lipids and nucleic acids.
These elements weren't chosen at random! They have specific properties that make them perfect for building living things. Carbon can form four bonds, making it ideal for creating complex molecules. Hydrogen and oxygen can form water, which is essential for life processes. Nitrogen is crucial for proteins and DNA. Phosphorus is vital for energy transfer and DNA structure, while sulfur helps proteins fold correctly.
Carbon is often called the backbone of life and for good reason! It makes up about 18-20% of the human body by mass and is found in every single organic molecule. What makes carbon special is its ability to:
This versatility allows carbon to form the complex molecules needed for life, from simple sugars to DNA. Without carbon, life as we know it wouldn't exist!
The most abundant element in the universe makes up about 10% of your body mass. It's crucial for water formation and is found in almost all biological molecules. Hydrogen bonds (weak attractions between molecules) are vital for DNA structure and protein folding.
Making up about 65% of your body mass, oxygen is the most abundant element in humans. It's essential for cellular respiration, where it helps convert food into energy. Oxygen is also a key component of water, which makes up most of your body.
About 3% of your body is nitrogen, which is a crucial component of proteins and nucleic acids (DNA and RNA). Nitrogen is found in amino acids, the building blocks of proteins and in the nitrogenous bases that form the genetic code.
Phosphorus makes up about 1% of your body weight but plays crucial roles:
Sulfur accounts for only 0.25% of your body weight but is vital for:
While CHNOPS elements make up about 99% of living organisms, other elements are needed in tiny amounts. These "trace elements" are essential for normal function despite their small quantities.
Essential for haemoglobin in red blood cells, which transports oxygen around the body. Iron deficiency leads to anaemia, causing fatigue and weakness.
Vital for strong bones and teeth, muscle contraction, nerve signalling and blood clotting. Makes up about 1.5% of body mass.
Required for thyroid hormones that regulate metabolism. Deficiency can cause goitre (swollen thyroid) and developmental issues.
Before iodised salt was introduced, people living far from the sea often suffered from goitre - an enlargement of the thyroid gland caused by iodine deficiency. Seafood is naturally rich in iodine, so coastal communities rarely experienced this problem. In the early 20th century, many inland regions in the UK had high rates of goitre. The introduction of iodised table salt in the 1920s dramatically reduced these cases, demonstrating how even tiny amounts of an element can have huge impacts on health.
Scientists can identify which elements are present in biological samples using various chemical tests. Here are some common ones you might perform in your IGCSE practical work:
When organic compounds containing carbon are heated strongly, they often char (turn black). This is because carbon remains while other elements are driven off. More specifically, heating organic matter with concentrated sulfuric acid causes dehydration, leaving behind carbon.
What you'd observe: Black residue indicates carbon is present.
When organic compounds are heated in a dry test tube, water often condenses on the cooler parts of the tube. This water forms from the hydrogen and oxygen in the sample.
What you'd observe: Colourless liquid (water) condensing on the cooler parts of the test tube.
These elements require more specific tests:
The elements we've discussed don't exist on their own in living things - they combine to form biological molecules. Here's how the main elements contribute to key biological molecules:
Made primarily of carbon, hydrogen and oxygen, usually in a ratio of 1:2:1 (CH₂O). Examples include glucose (C₆H₁₂O₆), which provides energy and cellulose, which gives plants structural support.
Contain carbon, hydrogen, oxygen, nitrogen and often sulfur. Proteins are made of amino acids linked together and perform countless functions from enzymes to structural components.
Mostly carbon and hydrogen with some oxygen. Lipids include fats, oils and waxes and are important for energy storage, insulation and cell membranes.
Contain carbon, hydrogen, oxygen, nitrogen and phosphorus. DNA and RNA store and transmit genetic information that determines all your characteristics.
Understanding the chemical elements in living organisms helps us appreciate how similar all life on Earth really is. From bacteria to blue whales, we're all made of the same basic building blocks - just arranged in different ways!
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