Figure 3.23 Facilitated diffusion down a concentration gradient through a membrane protein.
Active transport occurs when cells move molecules across their membraneagainstconcentration gradients (Figure
3.24). A major difference between passive and active transport is that active transport requires adenosine triphosphate
(ATP) or other forms of energy to move molecules “uphill.” Therefore, active transport structures are often called
“pumps.”
Figure 3.24 Active transport against a concentration gradient via a membrane pump that requires energy
Group translocation also transports substances into bacterial cells. In this case, as a molecule moves into a cell
against its concentration gradient, it is chemically modified so that it does not require transport against an unfavorable
concentration gradient. A common example of this is the bacterial phosphotransferase system, a series of carriers that
phosphorylates (i.e., adds phosphate ions to) glucose or other sugars upon entry into cells. Since the phosphorylation
of sugars is required during the early stages of sugar metabolism, the phosphotransferase system is considered to be
an energy neutral system.
Photosynthetic Membrane Structures
Some prokaryotic cells, namely cyanobacteria and photosynthetic bacteria, have membrane structures that enable
them to perform photosynthesis. These structures consist of an infolding of the plasma membrane that encloses
photosynthetic pigments such as greenchlorophyllsand bacteriochlorophylls. In cyanobacteria, these membrane
structures are called thylakoids; in photosynthetic bacteria, they are called chromatophores, lamellae, or chlorosomes.
104 Chapter 3 | The Cell
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