Cell Membrane

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From the early days of the microscope, the cell has been differentiated as having an outer boundary membrane (the cell or plasma membrane) containing a heterogeneous soup (cytoplasm) and a nucleus. As microscopy has improved, more details have emerged from the cellular morass to be seen as distinct objects. Through study of pathologic cells and experimentation, we have come to learn about the function of these objects called organelles. For instance, a pathologic cell deficient in the organelle called rough endoplasmic reticulum (RER) will demonstrate an inability to produce many proteins. Therefore, RER is involved with protein production.

Figure 2.1 shows a generic animal cell. The outer boundary, or cell membrane, forms a compartment that is biochemically distinct from

Nucleus

Plasma membrane

Rough endoplasmic reticulum

Bound ribosomes Free ribosomes

Nucleolus

Nuclear pore

Nuclear envelope

Peroxisome

Nucleus

Peroxisome

Secretory vesicle

Figure 2.1 Typical animal cell showing characteristic organelles and cellular inclusions. The arrangement of the intracellular features and the shape of the cell vary from cell to cell. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

Secretory vesicle

Lysosome Centrioles Endosome

Golgi apparatus

Smooth endo' plasmic reticulum

Mitochondrion Microfilaments

Microtubule

Figure 2.1 Typical animal cell showing characteristic organelles and cellular inclusions. The arrangement of the intracellular features and the shape of the cell vary from cell to cell. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

the outside environment. The chemical soup enclosed by the cell membrane is regulated by the membrane and its components allowing some substances into and out of the cell. This differential permeability is a hallmark of a cell membrane.

2.1.1 Phospholipid bilayer

When examining the cell membrane via ultramicroscopy (magnification greater than 1000X), details of the membrane structure can be seen (Fig. 2.2). The cell membrane is actually a bilayer composed of phos-pholipids. The structure and formation of phospholipids were introduced in Sec. 1.6.2. Different types of phospholipids are shown in Fig. 1.20. The amphipathic (having two natures) character of phospholipids helps to maintain a hydrophilic (polar or water soluble) and hydrophobic (nonpolar or non-water-soluble) orientation to the cell membrane. This feature of the membrane causes water and other polar (water-soluble) molecules and compounds to stay on one side of the membrane. The amphipathic nature of phospholipids also allows for self-assembly as described in Application Box 2.1. Some hydrophobic molecules (lipid-soluble) are able to pass through the membrane with ease. This character of the cell

Extracellular fluid

Extracellular fluid

Nonpolar regions

Figure 2.2 Cell membrane. Membranes are composed of a phospholipid bilayer and associated proteins. Proteins include embedded, or integral proteins, as well as peripheral proteins on a surface of the membrane. Membrane proteins serve a variety of purposes including cell communication and identification. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

Nonpolar regions

Figure 2.2 Cell membrane. Membranes are composed of a phospholipid bilayer and associated proteins. Proteins include embedded, or integral proteins, as well as peripheral proteins on a surface of the membrane. Membrane proteins serve a variety of purposes including cell communication and identification. (Reproduced with permission from McKinley and O'Loughlin, Human Anatomy, 1st ed. McGraw-Hill, New York, 2006.)

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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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