Cell Energy, Cell Functions | Learn Science At Scitable - Nature

Figure 1: For photosynthetic cells, the main energy source is the sun.For photosynthetic cells, the main energy source is the sun.© 2010 Nature Education All rights reserved. Cells, like humans, cannot generate energy without locating a source in their environment. However, whereas humans search for substances like fossil fuels to power their homes and businesses, cells seek their energy in the form of food molecules or sunlight. In fact, the Sun is the ultimate source of energy for almost all cells, because photosynthetic prokaryotes, algae, and plant cells harness solar energy and use it to make the complex organic food molecules that other cells rely on for the energy required to sustain growth, metabolism, and reproduction (Figure 1).

Cellular nutrients come in many forms, including sugars and fats. In order to provide a cell with energy, these molecules have to pass across the cell membrane, which functions as a barrier — but not an impassable one. Like the exterior walls of a house, the plasma membrane is semi-permeable. In much the same way that doors and windows allow necessities to enter the house, various proteins that span the cell membrane permit specific molecules into the cell, although they may require some energy input to accomplish this task (Figure 2).

Figure 2: Cells can incorporate nutrients by phagocytosis.This amoeba, a single-celled organism, acquires energy by engulfing nutrients in the form of a yeast cell (red). Through a process called phagocytosis, the amoeba encloses the yeast cell with its membrane and draws it inside. Specialized plasma membrane proteins in the amoeba (in green) are involved in this act of phagocytosis, and they are later recycled back into the amoeba after the nutrients are engulfed.© 2006 The Company of Biologists All rights reserved. Figure Detail