bonding withing glucose??

this might be a silly question but if the C1 in glucose is already bonded to H, OH, and C2, how can it form a double bond to the O atom? wouldnt this make 5 bonds? i thought carbon only formed 4

Glucose has both a (pair of) ring form(s) and an open chain form.
In the open chain form, you have a C=O bond on C1. In the ring form(s), it does not - if you do A level chemistry, you’ll see in the second year that there is a type of reaction called nucleophilic addition which occurs when the open chain form isomerises to the ring form(s). Though I am sure this is beyond the syllabus and when I did AS biology (with Edexcel B), there was no need to be aware of the open chain form of glucose.