There are two major ways that molecules can be moved across a membrane, and the distinction has to do with whether or not cell energy is used. Passive mechanisms like diffusion use no energy, while active transport requires energy to get done.

Your cells perform three main types of work: chemical work, mechanical work, and transport work (Figure 7-10). An example of chemical work is building large molecules such as proteins.

(adenosine triphosphate) the main energy source that cells use for most of their work. it drives out three types of cellular work, called , chemical work, mechanical work, and transpot work. it also takes in energy in from food, and turns it into energy for working cells.

ATP-Driven Work In a cell, this work takes three main forms — chemical work, mechanical work, and transport work. Glutamine synthesis is an example of how ATP hydrolysis is used to perform chemical work by driving unfavorable chemical processes that would not occur spontaneously.

The most common example of transport in cells is performed by adenosine triphosphate, or ATP, which both transports energy inside the cell and acts as a unit of energy for certain metabolic processes. The ATP molecule transports three oxygen-phosphate groups, which are used in chemical reactions around the cell.

(When molecules move in this way, they are said to move down their concentration gradient.) Three common types of passive transport include simple diffusion, osmosis, and facilitated diffusion. Simple Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration.

The simplest forms of transport across a membrane are passive. Passive transport does not require the cell to expend any energy and involves a substance diffusing down its concentration gradient across a membrane.