The Origin of the Eukaryotic Cell

The eukaryotic cell differs in many ways from the prokary-otic cell. How did it originate? Given the nature of evolutionary processes, the differences cannot all have arisen simultaneously. We think we can make some reasonable inferences about the most important events, bearing in mind that the global environment underwent an enormous change—from anaerobic to aerobic—during the course of these events. As you read this chapter, keep in mind that the steps we suggest are just that: reasonable inferences. This version of the story is one of a few under current consideration. We present it as a framework for thinking about this challenging problem, not as a set of facts.

The modern eukaryotic cell arose in several steps

The essential steps in the origin of the eukaryotic cell include:

► The origin of a flexible cell surface

► The origin of a cytoskeleton

► The origin of a nuclear envelope

► The appearance of digestive vesicles

► The endosymbiotic acquisition of certain organelles what A flexible cell surface allows. Many ancient fossil prokaryotes look like rods, and we presume that they, like most present-day prokaryotic cells, had firm cell walls. The first step toward the eukaryotic condition may have been the loss of the cell wall by an ancestral prokaryotic cell. This may not seem like an obvious first step, but consider the possibilities open to a flexible cell without a wall.

First, think of cell size. As a cell grows larger, its surface area-to-volume ratio decreases (see Figure 4.3). Unless the surface area can be increased, the cell volume will reach an upper limit. If the surface is flexible, it can fold inward and elaborate itself, creating more surface area for gas and nutrient exchange (Figure 28.2). With a surface flexible enough to

Infolding of the plasma membrane adds surface area without increasing the cell's volume.

Plasma membrane

Infolding of the plasma membrane adds surface area without increasing the cell's volume.

Plasma membrane

Infolding Plasma Membrane
28.2 Membrane Infolding The loss of the rigid prokaryotic cell wall allowed the plasma membrane to fold inward and create more surface area.

allow infolding, the cell can exchange materials with its environment rapidly enough to sustain a larger volume and more rapid metabolism. Further, a flexible surface can pinch off bits of the environment, bringing them into the cell by endocytosis (Figure 28.3).

The chromosome of a bacterial cell is attached to a site on its plasma membrane. If that region of the plasma membrane were to fold into the cell, the first step would be taken toward the evolution of a nucleus, the key feature of the eukaryotic cell. What are some other likely early changes?

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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