Technically, empty space is consumable. Empty space is represented in the display by white space. Food is represented in the display by red apples. Moving through a portal transports an agent to some other location in the world. The new location may be far away from the portal on the same screen, or it may be on a completely different screen.
Portals are one-way.
Some portals will only work if the agent is holding its key or has performed some other action prior to trying to pass through the portal. The destination reached through a portal may change depending on what the agent is carrying or previous actions of the agent. Portals appear in the display as doors. These can be pushed out of the way unless they are up against an immovable object. They appear in the display as blue blocks.