Final answer:
The two types of forces that accomplish transport across a cell membrane are passive transport, which requires no energy and moves substances down their concentration gradient, and active transport, which requires energy to move substances against their concentration gradient.
Step-by-step explanation:
The two general types of force that accomplish transport of substances across a cell membrane are passive transport and active transport. Passive transport is a process that does not require energy from the cell and moves substances from an area of higher concentration to an area of lower concentration. This includes simple diffusion, facilitated diffusion (via transport proteins), and osmosis. Active transport, on the other hand, requires energy, often in the form of ATP, to move substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. This process relies on transport proteins that act as pumps, and it allows the cell to maintain internal concentrations of small solutes that differ from those in its environment.