5.1Cell Membrane Structure
Prokaryotic and Eukaryotic Cells
Membranes found in cells are called biological membranes. Their basic structure consists of a lipid bilayer to which many proteins are bound. The cell membrane surrounding a cell is also a biological membrane. It is believed that primitive cells were created as a result of genetic material and its reproduction structures becoming surrounded by membranes. As shown in Figure. 5-1, bacteria (an example of prokaryotic cells) have a simple structure with just a plasma membrane. Conversely, animal and plant cells (examples of eukaryotic cells) are surrounded by a plasma membrane containing - as shown in Table 5-1 - organelles surrounded by double lipid bilayers consisting of inner and outer membranes (such as the nucleus, mitochondria and chloroplasts) and organelles surrounded by a single lipid bilayer (such as the plasma membrane, endoplasmic reticula, Golgi apparatuses and lysosomes).
Organelles of Eukaryotic Cells
The biological structure that separates the interior of a cell from its outer environment is called the plasma membrane. Bacteria and animal cells are separated from the outside by a lipid bilayer plasma membrane, whereas plant cells have a strong cell wall outside the plasma membrane (Fig. 5-1). The plasma membrane is characteristic in that many of the membrane proteins located on its outer surface are modified by sugar chains. The plasma membrane has fluidity, and parts of it continually diffuse into the cell. In this membrane (as discussed later in the chapter), many channels and transporters carry materials, and receptors pass information from the outer environment to the interior of the cell.
■Nucleus and Nuclear Envelope
Eukaryotic cells normally have one nucleus, which contains a genome - a complete set of hereditary information for an organism. In the nucleus, DNA replication and RNA transcription occur. Linear DNA and binding proteins (e.g., histones) form a complex (i.e., chromatin) in the nucleus. There are two types of nuclear chromatin: euchromatin - a lightly packed form under active transcription - and heterochromatin, a tightly packed form in which transcription is limited. During cell division, DNA in the nucleus becomes increasingly condensed until it forms rod-like structures called chromosomes that are then distributed to the two daughter cells. The nuclear envelope has many holes known as nuclear pores, which control the movement of materials across the envelope. As an example, mRNA generated by transcription passes through the nuclear pores out into the cytoplasm, where it is translated into proteins (see Chapter 3).
■Endoplasmic Reticula and Golgi Apparatuses
Endoplasmic reticula and Golgi apparatuses are involved in the synthesis and transport of secretory proteins and the constituents of membranes. Endoplasmic reticula synthesize and process proteins by sorting, adding sugar chains and providing other modifications. Such endoplasmic reticula have a ribosome-rich surface, and are called rough endoplasmic reticula. Those without attached ribosomes are known as smooth endoplasmic reticula.
Endoplasmic reticula are connected to the outer membrane of the nuclear envelope and form a mesh-like structure. Consisting of a stack of flattened membrane structures, Golgi apparatuses are located near endoplasmic reticula, adding sugar chains to membrane proteins and secretory proteins as well as sorting proteins. Transportation of lipids and membrane proteins between organelles is performed by small bag-like structures made of biological membranes called transport vesicles (see Fig. 5-10).
■Endosomes and Lysosomes
Endosomes and lysosomes play a role in the incorporation and digestion of extracellular materials. Part of the membrane invaginates and pinches off to form an endosome inside the cell. This process is called endocytosis. Endocytosed proteins and lipids are transported to organelles after being sorted within endosomes or digested within lysosomes. Plant vacuoles have functions similar to those of lysosomes, and regulate cell turgor. Lysosomes contain enzymes that digest nucleic acids, proteins and lipids, and the interior of lysosomes is kept acidic (pH 5) by proton pumps. Vesicular transport also plays an important role in the movement of lipids and membrane proteins between these organelles.
■Mitochondria, Chloroplasts and Peroxisomes
The inner mitochondrial membrane is compartmentalized into many cristae. The space it encloses is called a matrix (see Fig. 8-2 in Chapter 8), and contains DNA unique to mitochondria. Mitochondria are found in almost all eukaryotic cells, and perform oxidative phosphorylation via the electron transport chain to synthesize ATP.
Chloroplasts are flattened, disk-shaped organelles found in plants and algae, and play a role in photosynthesis. The material inside the inner membrane is called the stroma, which contains stacks of flattened, bag-like structures called thylakoids that perform photosynthesis. The stroma contains DNA unique to chloroplasts.
Peroxisomes contain many oxidases, and perform lipid oxidization and the metabolism of various materials. As an example, peroxisomes in plants synthesize carbohydrates from stored lipids.
Vesicular transport does not occur between mitochondria and chloroplasts or between peroxisomes and other organelles.