18. Key Concept What are the main functions of the cell membrane and the cell wall? The cell membrane regulates what enters and leaves the cell and also provides protection and support. The main function of the cell wall is to provide support and protection for the cell.
19. Key Concept What happens during diffusion? Because diffusion depends upon random particle movements, substances diffuse across membranes without requiring the cell to use energy.
20. Key Concept What is osmosis? It is the diffusion of water through a selectively permeable membrane.