<h2 class="section-header"><strong class="title">Proteins</strong></h2>
<h3 class="para">Molecular make-up</h3>
<p class="para">Proteins contain <a data-lesson="carbon-cycle" data-subject="biology" data-tutorial="ecology-overview" href="https://app.ulearngo.com/biology/ecology-overview/carbon-cycle">carbon</a> (C), hydrogen (H), oxygen (O), nitrogen (N) and may have other elements such as iron (Fe), phosphorous (P) and sulfur (S).</p>
<h3 class="para">Structural composition</h3>
<p class="para">Proteins are made of <em class="emphasis">amino acids</em>. There are 20 common amino acids from which all proteins in living organisms are made. Nine of them are considered <em class="emphasis">essential amino acids</em>, as they cannot by synthesised in the body from other compounds, and must be obtained from the diet. Amino acids are bonded together by <em class="emphasis">peptide bonds</em> to form <em class="emphasis">peptides</em>. A long peptide chain forms a protein, which folds into a very specific three-dimensional shape. This three-dimensional shape is completely determined by the identity and order of the amino acids in the peptide chain. We often refer to <em class="emphasis">four different levels </em>of protein structure (<a href="#attachment_98751">see figure below</a>):</p>
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<h3>Fact:</h3>
<p class="para">Because the sequence of amino acids determines the way that a protein folds, if you start with a certain peptide chain, you will always get the same three-dimensional structure!</p>
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<li class="item"><em class="emphasis">Primary structure</em>: This refers to the sequence of amino acids joined together by peptide bonds to form a polypeptide chain. Some proteins have fewer than a hundred amino acids, while others have several thousand.</li>
<li class="item"><em class="emphasis">Secondary structure</em>: This is the first level of three dimensional folding. It is driven completely by <i class="emphasis">hydrogen bonding</i>. Hydrogen bonding usually results in regions of the chain coiling and other regions forming sheets.</li>
<li class="item"><em class="emphasis">Tertiary structure</em>: This is the second level of three dimensional folding and is the overall final shape of the protein molecule. The secondary structures and unstructured regions of the chain further fold into a globular shape, driven by hydrophobic interactions (non-polar regions trying to escape the water in the cell environment) and electrostatic interactions (polar and charged regions wanting to interact with the water environment and each other).</li>
<li class="item"><em class="emphasis">Quaternary structure</em>: Some proteins are complex: two or more peptide chains fold into their tertiary structures, then these complete structures associate together by hydrophobic and electrostatic interactions to form the final protein.</li>
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<div class="wp-caption alignnone" id="attachment_98751" style="width: 382px"><a href="https://data.ulearngo.com/assets/2200034d-c865-40e5-9598-0c70790b2063"><img alt="protein-structure" class="size-full wp-image-98751" height="666" src="https://data.ulearngo.com/assets/2200034d-c865-40e5-9598-0c70790b2063" width="382"/></a><p class="wp-caption-text">Image credit: Siyavula</p></div>
<h3 class="para">Role in animals and plants</h3>
<p class="para">Proteins are important in several crucial biological functions. Proteins are found in hair, skin, bones, muscles, tendons, ligaments and other structures and perform key structural and mechanical functions. Proteins are also important in cell communication and in the immune system. Proteins can also act as an energy reserve when broken down through digestive processes. Certain proteins called <em class="emphasis">enzymes </em>are important in catalysing cellular <a data-lesson="endergonic-exergonic-reactions" data-subject="biology" data-tutorial="cell-metabolism" href="https://app.ulearngo.com/biology/cell-metabolism/endergonic-exergonic-reactions">reactions</a> that form part of metabolism.</p>
<p class="para">Proteins are essential to any diet. A lack of protein results in a disease called <em class="emphasis">kwashiorkor</em> (see figure below) or <em class="emphasis">marasmus</em> (see figure below). Marasmus is caused by a general nutritional deficiency (starvation), and kwashiorkor is caused by a deficiency in protein specifically.</p>
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<td><a href="https://data.ulearngo.com/assets/0e5fdba5-6e69-4f74-b9f8-e2895ff2a07a"><img alt="" class="size-full wp-image-98752 alignnone" height="348" src="https://data.ulearngo.com/assets/0e5fdba5-6e69-4f74-b9f8-e2895ff2a07a" width="229"/></a> </td>
<td><a href="https://data.ulearngo.com/assets/61f1ceb6-9fb9-439b-a8cc-7da72fb94e00"><img alt="" class="size-full wp-image-98753 alignnone" height="353" src="https://data.ulearngo.com/assets/61f1ceb6-9fb9-439b-a8cc-7da72fb94e00" width="244"/></a> </td>
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<h3 class="wp-caption alignnone" id="attachment_98743">Fact:</h3>
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<p class="para"><em class="emphasis">Meat or vegetables: which is a better source of protein?</em></p>
<p class="para">Both animal protein and protein from vegetables is good for health. But each type comes with other nutrients. So which 'package' of nutrients-meat or vegetables is better for health?</p>
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<li class="item">A 180 gram steak provides 40 g of protein BUT also provides 38 g of fat which is more than the Recommended Dietary Allowance</li>
<li class="item">The same amount of salmon gives 34 g of protein and 18 g of fat.</li>
<li class="item">A cup of cooked lentils has 18 g of protein and 1 g of fat.</li>
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<p class="para">Therefore when choosing protein-rich foods, pay attention to what comes along with protein. Vegetable sources of proteins such as beans, nuts and whole grain are excellent choices and they offer healthy fibre, <a data-lesson="vitamins" data-subject="biology" data-tutorial="chemistry-of-life-intro" href="https://app.ulearngo.com/biology/chemistry-of-life-intro/vitamins">vitamins</a> and minerals. The best animal protein sources are fish and poultry.</p>
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Proteins

Biology studies life and living organisms. Biology can be divided into different sections that cover the morphology, physiology, anatomy, behavior, and so on of living organisms.

Biology

Discover the fundamental role of chemistry in life, explore the importance of molecules such as water, minerals, and nutrients for plant growth, and learn about the essential organic compounds, such as carbohydrates, lipids, and proteins, as well as enzymes, nucleic acids, and vitamins, and the recommended dietary allowance.


Proteins

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