A cell requires energy to make the molecules it needs for activities such as growth, reproduction, responsiveness, and movement. Energy is harvested from fuel molecules in the mitochondria (found in all eukaryotic cells) and from sunlight in the chloroplasts of plant cells. In contrast, energy transformations in prokaryotic cells are associated with enzymes attached to the inner surface of the cell membrane or to extensions of the cell membrane that protrude into the cytoplasm.

MITOCHONDRIA In eukaryotic cells, the breakdown of energy-rich molecules such as glucose begins in the cytoplasm. The molecules that result from this partial degradation enter the mitochondria (singular mitochondrion), whose primary function is to harvest the chemical energy of those molecules in a form that the cell can use, namely the energy-rich molecule ATP (adenosine triphosphate) (see Key Concept 8.2). The production of ATP in the mitochondria, using fuel molecules and molecular oxygen (O₂), is called cellular respiration.

Typical mitochondria are somewhat less than 1.5 μm in diameter and are 2–8 µm in length—about the size of many bacteria. They can reproduce and divide independently of the central nucleus. The number of mitochondria per cell ranges from one large organelle in some unicellular protists to a few hundred thousand in large egg cells. An average human liver cell contains more than 1,000 mitochondria. Cells that are active in movement and growth require the most chemical energy, and these tend to have the most mitochondria per unit of volume.

Mitochondria have two membranes. The outer membrane is smooth and protective, and it offers little resistance to the movement of substances into and out of the organelle. Immediately inside the outer membrane is an inner membrane, which folds inward in many places and thus has a surface area much greater than that of the outer membrane (Figure 5.11). The folds tend to be quite regular, giving rise to shelflike structures called cristae. The inner membrane exerts much more control over what enters and leaves the space it encloses than does the outer membrane. Embedded in the inner mitochondrial membrane are many large protein complexes that participate in cellular respiration.

Question

Cells with high energy requirements, such as muscle cells, would have a lot of mitochondria.

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The space enclosed by the inner membrane is referred to as the mitochondrial matrix. In addition to many enzymes, the matrix contains ribosomes and DNA that are used to make some of the proteins needed for cellular respiration. As we will discuss later in this chapter, it is likely that this DNA is the remnant of a larger, complete chromosome from a prokaryote that may have been the mitochondrion’s progenitor. In Chapter 9 we will discuss how the different parts of the mitochondrion work together in cellular respiration.

PLASTIDS One class of organelles—the plastids—is present only in the cells of plants and certain protists. Like mitochondria, plastids can divide independently of the cell nucleus and probably evolved from independent prokaryotes. There are several types of plastids, with different functions.

Just as melanosomes are a compartment for pigment in animal cells (see the chapter opening), so are chloroplasts, which contain the green pigment chlorophyll and are the sites of photosynthesis (Figure 5.12). In photosynthesis, light energy is converted into the chemical energy of bonds between atoms. The molecules formed by photosynthesis provide food for the photosynthetic organism and for other organisms that eat it. Directly or indirectly, photosynthesis is the energy source for most of the living world.

Like a mitochondrion, a chloroplast is surrounded by two membranes. In addition, there is a series of internal membranes whose structure and arrangement vary from one group of photosynthetic organisms to another. Here we concentrate on the chloroplasts of the flowering plants.

The internal membranes of chloroplasts look like stacks of flat, hollow pita bread. Each stack is called a granum (plural grana) and the pita bread–like compartments are called thylakoids (see Figure 5.12). Thylakoid lipids are distinctive: only 10 percent are phospholipids, whereas the rest are galactose-substituted diglycerides and sulfolipids. Because of the abundance of chloroplasts, these are the most abundant lipids in the biosphere.

In addition to lipids and proteins, the membranes of the thylakoids contain chlorophyll and other pigments that harvest light energy for photosynthesis (we will see how they do this in Key Concept 10.2). The thylakoids of one granum may be connected to those of other grana, making the interior of the chloroplast a highly developed network of membranes, much like the ER.

The fluid in which the grana are suspended is called the stroma. Like the mitochondrial matrix, the chloroplast stroma contains ribosomes and DNA, which are used to synthesize some, but not all, of the proteins that make up the chloroplast.

Other types of plastids, such as chromoplasts and leucoplasts, have functions different from those of chloroplasts. Chromoplasts make and store red, yellow, and orange pigments, especially in flowers and fruits—again, just as the melanocytes in the opening story store pigment.

Leucoplasts are storage organelles that do not contain pigments. An amyloplast is a leucoplast that stores starch.

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