Answer:
matter
Explanation:
"In thermodynamics, a closed system can exchange energy (as heat or work) but not matter, with its surroundings"
<h2> What are proteins made of?</h2><h3>Proteins are made up of smaller building blocks called amino acids, joined together in chains. There are 20 different amino acids. Some proteins are just a few amino acids long, while others are made up of several thousands. These chains of amino acids fold up in complex ways, giving each protein a unique 3D shape. Some, like keratin, the hair protein, form long fibres. Others, like haemoglobin, the protein that carries oxygen around your body, are roughly spherical.</h3>
19. The atomic number indicates how many neutrons there are, and the atomic mass tells us how many protons and neutrons are present within the nucleus of an atom.
