Many substances have both kinds of bonds. The most obvious ones are the acids. Sulphuric acid contains atoms of hydrogen, sulphur and oxygen. Covalent bonds hold the atoms of oxygen and sulphur together. However you should know that acids are substances that release hydrogen ions in solution, so sulphuric acid must have electrovalent bonds. These hold positively charged hydrogen ions (cations) onto negatively charged sulphate ions (anions). Again, you can have a look at this on my ions page to refresh your memory.
In your biology lessons you will learn about enzymes. Enzymes are biological catalysts. They are affected by temperature and pH. They must have the correct 3D shape to do their jobs (the lock and key model), and will not work if their shapes are changed by pH or temperature. Enzymes are protein molecules. They contain atoms of carbon, hydrogen, oxygen, nitrogen and sometimes sulphur and even phosphorus. These are all chemically combined by covalent bonds into a long chain. However, some of the hydrogen atoms are attracted to atoms of oxygen or nitrogen by a much weaker kind of bond called a “hydrogen bond”.
Hydrogen bonds are caused by attractions between the positively charged nucleus of a hydrogen atom and a pair of electrons in the outer shell of another atom, either oxygen or nitrogen. These are very weak bonds and easily affected by temperature. Hydrogen bonds help to hold an enzyme into its proper 3D shape, and this will change if the enzyme is heated or cooled.
Proteins are also held into their correct shapes by weak electrical attractions between positive and negative charges on some of the atoms. Some of the amino acids in a protein contain an extra amino group that can gain a hydrogen ion becoming positively charged. Other amino acids have an extra acid group that can lose a hydrogen ion becoming negatively charged. Attractions between these extra positive and negative regions of a protein molecule also help to hold it into its correct shape. These attractions are upset if the pH of the solution is changed.