Membrane Bound Organelles

The presence of membrane-bound organelles is an important feature that sets eukaryotic cells apart from their prokaryotic counterparts.

The Nucleus

The predominant distinguishing feature of the eukaryotic cell is the nucleus, which contains the DNA. Enclosing this structure are two concentric lipid bilayer membranes: the inner membrane and the outer membrane. These membranes make up the nuclear envelope. Spanning the membranes are complex protein structures that form nuclear pores, allowing large molecules such as ribosomal subunits and proteins to be transported into and out of the nucleus (figure 3.53). The nucleolus is a region within the nucleus where ribosomal RNAs are synthesized. These RNAs, along with ribosomal proteins that are synthesized in the cytoplasm and then transported into the nucle-olus, are then assembled into ribosomal subunits. These then

3.13 Membrane-Bound Organelles 75

pass through the nuclear pores into the cytoplasm, where the subunits combine to form 80S ribosomes.

The nucleus contains multiple chromosomes, each one encoding different genetic information. Unlike the situation in most prokaryotic cells, double-stranded chromosomal DNA is linear. To add structure and order to the long DNA molecule, it is packed by winding it around positively charged proteins called histones. These bind tightly to the negatively charged DNA molecule. One packing unit, called a nucleosome, consists of a complex of histones around which the linear DNA wraps twice (figure 3.54). The complex of DNA and proteins that together form the chromosomes is called chromatin.

Events that take place in the nucleus during cell division distinguish eukaryotes from prokaryotes. In eukaryotic cells, after DNA is replicated, chromosomes go through a nuclear division process called mitosis, which ensures the daughter cells receive the same number of chromosomes as the original parent. Through mitosis, a cell that is diploid, or has two copies of each chromosome, will generate two diploid daughter cells. A different process, meiosis, generates haploid daughter cells.

Mitochondria and Chloroplasts

Mitochondria and chloroplasts are organelles that function as powerhouses generating ATP, the universal form of energy used by all cells. Mitochondria are found in nearly all eukaryotic cells, whereas chloroplasts are found exclusively in plants and algae. These harvest the energy of sunlight to synthesize organic compounds—a process called photosynthesis. ■ photosynthesis, p. 156

Mitochondria and chloroplasts generate ATP using mechanisms analogous to those occurring in the cytoplasmic membrane of prokaryotes; this is not surprising considering that they evolved from intracellular bacteria (see Perspective 3.1 ). Contained within their extensive internal membranes are the components of the electron transport chain. Recall that these transfer electrons and,

Nucleus Freeze Fracture
Figure 3.53 Nucleus Organelle that contains the DNA. (a) Diagrammatic representation. (b) Electron micrograph of a yeast cell (Geotrichum candidium) by freeze-fracture technique.

Chapter 3 Microscopy and Cell Structure

PERSPECTIVE 3.1 The Origins of Mitochondria and Chloroplasts

Mitochondria and chloroplasts bear such a striking similarity to prokaryotic cells that it is no wonder scientists have speculated for many decades that these organelles evolved from bacteria.The endosymbiont theory states that the ancestors of mitochondria and chloroplasts were bacteria residing within other cells in a mutually beneficial partnership.The intracellular bacterium in such a partnership is called an endosymbiont. As time went on, each partner became indispensable to the other, and the endosymbiont eventually lost key features such as a cell wall and the ability to replicate independently.

Several early observations have supported the endosymbiont theory. Mitochondria and chloroplasts, unlike other eukaryotic organelles, both carry some of the genetic information necessary for their function.These include genes for some of the ribosomal proteins and ribosomal RNAs that make up their 70S ribosomes.These ribosomes contrast with the typical 80S ribosomes that characterize eukaryotic cells and, in fact, are equivalent to the prokaryotic 70S ribosomes. Interestingly, cellular DNA encodes some of the components that make up these ribosomes. Another characteristic that supports the model that mitochondria and chloroplasts were once intracellular bacteria is the double membrane that surrounds these organelles. Present-day endosymbionts retain their cytoplasmic membranes and live within membrane-bound compartments in their eukaryotic host cell.

Evidence in favor of the endosymbiont theory continues to accumulate. Recent technology enables scientists to readily determine the precise order, or sequence, of nucleotides that make up DNA.This allows comparison of the nucleotide sequences of organelle DNA with genomes of different bacteria. It has become apparent that some mitochondrial DNA sequences bear a striking resemblance to DNA sequences of members of a group of obligate intracellular parasites, the rickettsias.These are quite likely relatives of modern-day mitochondria.

A tremendous effort is now under way to determine the nucleotide sequence of mitochondria from a wide variety of eukaryotes, including plants, animals, and protists. While the size of mitochondrial DNA varies a great deal among these different eukaryotic organisms, common sequence themes are emerging.Today, researchers are no longer discussing "if" but "when" these organelles evolved from intracellular prokaryotes.

in the process, eject protons to generate proton motive force. Proteins embedded within the membranes then use proton motive force to synthesize ATP. What sets mitochondria apart from chloroplasts is the source of the electrons transferred by the electron transport chain.

Mitochondria and chloroplasts do have several other similarities. They have two membranes: an outer and an inner mem

Nucleosome

Figure 3.54 Chromatin (a) Chromatin fibers; the threads as well as the "beads"on the threads consist of DNA wrapped around histones. (b) Diagram of a nucleosome, the fundamental packing unit of eukaryotic DNA.

Nucleosome

Figure 3.54 Chromatin (a) Chromatin fibers; the threads as well as the "beads"on the threads consist of DNA wrapped around histones. (b) Diagram of a nucleosome, the fundamental packing unit of eukaryotic DNA.

brane. Within the region enclosed by the inner membrane, called the matrix in mitochondria and the stroma in chloroplasts, they have DNA, ribosomes, and other molecules necessary for protein synthesis. Notably, their ribosomes are 70S rather than the 80S ribosome found in the cytoplasm of eukaryotic cells.

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Responses

  • estella
    Why are organelles important?
    2 years ago
  • sebastian mutka
    What is the name two membrane bound organelles?
    1 year ago

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