Where Do Cells Obtain Their Energy?
Figure 1: For photosynthetic cells, the main energy source is the sun.
For photosynthetic cells, the main energy source is the sun.
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).
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.
How Do Cells Turn Nutrients into Usable Energy?
Figure 3: The release of energy from sugar
Compare the stepwise oxidation (left) with the direct burning of sugar (right). Through a series if small steps, free energy is released from sugar and stored in carrier molecules in the cell (ATP and NADH, not shown). On the right, the direct burning of sugar requires a larger activation energy. In this reaction, the same total free energy is released as in stepwise oxidation, but none is stored in carrier molecules, so most of it will be lost as heat (free energy). This direct burning is therefore very inefficient, as it does not harness energy for later use.
Figure 4: Enzymes allow activation energies to be lowered.
Enzymes lower the activation energy necessary to transform a reactant into a product. On the left is a reaction that is not catalyzed by an enzyme (red), and on the right is one that is (green). In the enzyme-catalyzed reaction, an enzyme will bind to a reactant and facilitate its transformation into a product. Consequently, an enzyme-catalyzed reaction pathway has a smaller energy barrier (activation energy) to overcome before the reaction can proceed.
What Specific Pathways Do Cells Use?
Figure 5: An ATP molecule
ATP consists of an adenosine base (blue), a ribose sugar (pink) and a phosphate chain. The high-energy phosphate bond in this phosphate chain is the key to ATP's energy storage potential.
The particular energy pathway that a cell employs depends in large part on whether that cell is a eukaryote or a prokaryote. Eukaryotic cells use three major processes to transform the energy held in the chemical bonds of food molecules into more readily usable forms — often energy-rich carrier molecules. Adenosine 5'-triphosphate, or ATP, is the most abundant energy carrier molecule in cells. This molecule is made of a nitrogen base (adenine), a ribose sugar, and three phosphate groups. The word adenosine refers to the adenine plus the ribose sugar. The bond between the second and third phosphates is a high-energy bond (Figure 5).
Figure 6: Metabolism in a eukaryotic cell: Glycolysis, the citric acid cycle, and oxidative phosphorylation
Glycolysis takes place in the cytoplasm. Within the mitochondrion, the citric acid cycle occurs in the mitochondrial matrix, and oxidative metabolism occurs at the internal folded mitochondrial membranes (cristae).
How Do Cells Keep Energy in Reserve?
Figure 7: Examples of energy storage within cells.
A) In this cross section of a rat kidney cell, the cytoplasm is filled with glycogen granules, shown here labeled with a black dye, and spread throughout the cell (G), surrounding the nucleus (N). B) In this cross-section of a plant cell, starch granules (st) are present inside a chloroplast, near the thylakoid membranes (striped pattern). C) In this amoeba, a single celled organism, there is both starch storage compartments (S), lipid storage (L) inside the cell, near the nucleus (N). Scale bar in B and C = 1µm.
Cells need energy to accomplish the tasks of life. Beginning with energy sources obtained from their environment in the form of sunlight and organic food molecules, eukaryotic cells make energy-rich molecules like ATP and NADH via energy pathways including photosynthesis, glycolysis, the citric acid cycle, and oxidative phosphorylation. Any excess energy is then stored in larger, energy-rich molecules such as polysaccharides (starch and glycogen) and lipids.