Free online app that contains a library of pre-made cells, proteins, membrane shapes, organs, lab equipment, etc. that you can drag-and-drop so you don't have to spend time drawing each element of the figure out yourself. Saves a lot of time for creating schematic figures, and the icons are all created by scientific illustrators so they're both beautiful and accurate! And it's free for educational use.
Inkscape is free to use for any purpose (open source app). Even the interface and usability almost the same as illustrator which can be run on multiple platform.
For graphical abstracts, I recommend you the top software, PowerPoint, Inkscape, and Adobe Illustrators. You can also improve your graphical abstract by using other free online software:
You can use Inkscape. It is a free and open-source vector graphics editor for GNU/Linux, Windows and macOS X. It offers a rich set of features and is widely used for both artistic and technical illustrations such as cartoons, clip art, logos, typography, diagramming and flowcharting. It uses vector graphics to allow for sharp printouts and renderings at unlimited resolution and is not bound to a fixed number of pixels like raster graphics.
Go to: https://inkscape.org/
For editing Images and getting the desired output from Inkscape use