Compartmentation: Cells and Tissues Functional Compartments of the Body 3 The Lumens of Some Organs Are Outside the Body Functionally, the Body Has Three Fluid Compartments Biological Membranes The Cell Membrane Separates Cell from Environment Membranes Are Mostly Lipid and Protein Membrane Lipids Create a Hydrophobic Barrier Membrane Proteins May Be Loosely or Tightly Bound to the Membrane Membrane Carbohydrates Attach to Both Lipids and Proteins Intracellular Compartments Cells Are Divided into Compartments The Cytoplasm Includes Cytosol, Inclusions, Fibers, and Organelles Inclusions Are in Direct Contact with the Cytosol Cytoplasmic Protein Fibers Come in Three Sizes Microtubules Form Centrioles, Cilia, and Flagella The Cytoskeleton Is a Changeable Scaff old Motor Proteins Create Movement Organelles Create Compartments for Specialized Functions The Nucleus Is the Cell’s Control Center Tissues of the Body Extracellular Matrix Has Many Functions Cell Junctions Hold Cells Together to Form Tissues Epithelia Provide Protection and Regulate Exchange Connective Tissues Provide Support and Barriers Muscle and Neural Tissues Are Excitable Cells are organisms, Tissue Remodeling and entire animals Apoptosis Is a Tidy Form of Cell Death and plants are Stem Cells Can Create New Specialized Cells aggregates of these Organs organisms. Focus on … The Skin —Theodor Schwann, 1839 Background Basics Compartmentation Extracellular fl uid Hydrophobic molecules Proteins p H Covalent and noncova- lent interactions Pancreas cell 66 Compartmentation: Cells and Tissues hat makes a compartment? We may think of some- example is the intracellular compartment called the lysosome , thing totally enclosed, like a room or a box with a with an internal pH of 5 . Th is pH is so acidic that if the lyso- W lid. But not all compartments are totally enclosed . some ruptures, it severely damages or kills the cell that contains think of the modular cubicles that make up many modern it. workplaces. And not all functional compartments have walls . Th e disadvantage to compartments is that barriers between think of a giant hotel lobby divided into conversational group- them can make it diffi cult to move needed materials from one ings by careful placement of rugs and furniture. Biological com- compartment to another. Living organisms overcome this prob- partments come with the same type of anatomic variability, lem with specialized mechanisms that transport selected sub- ranging from totally enclosed structures such as cells to func- stances across membranes. tional compartments without visible walls. In this chapter we explore the theme of compartmentation The first living compartment was probably a simple cell by fi rst looking at the various compartments that subdivide the whose intracellular fl uid was separated from the external envi- human body, from body cavities to the subcellular compart- 3 ronment by a wall made of phospholipids and proteins—the cell ments called organelles. We then examine how groups of cells membrane. Cells are the basic functional unit of living organ- with similar functions unite to form the tissues and organs of the isms, and an individual cell can carry out all the processes of life. body. Continuing the theme of molecular interactions, we also As cells evolved, they acquired intracellular compartments look at how diff erent molecules and fi bers in cells and tissues separated from the intracellular fl uid by membranes. Over time, give rise to their mechanical properties : their shape, strength, groups of single-celled organisms began to cooperate and spe- fl exibility, and the connections that hold tissues together. cialize their functions, eventually giving rise to multicellular organisms. As multicellular organisms evolved to become larger and more complex, their bodies became divided into various Functional Compartments functional compartments. of the Body Compartments are both an advantage and a disadvantage for organisms. On the advantage side, compartments separate The human body is a complex compartment separated from biochemical processes that might otherwise confl ict with one the outside world by layers of cells. Anatomically, the body is another. For example, protein synthesis takes place in one sub- divided into three major body cavities: the cranial cavity (com- cellular compartment while protein degradation is taking place monly referred to as the skull ), the thoracic cavity (also called the in another. Barriers between compartments, whether inside a thorax ), and the abdominopelvic cavity ( Fig. 3.1 a). Th e cavities cell or inside a body, allow the contents of one compartment to are separated from one another by bones and tissues, and they diff er from the contents of adjacent compartments. An extreme are lined with tissue membranes . The cranial cavity { cranium, skull} contains the brain, our primary control center. The thoracic cavity is bounded RUNNING PROBLEM by the spine and ribs on top and sides, with the muscular diaphragm forming the floor. The thorax contains the heart, Pap Tests Save Lives which is enclosed in a membranous pericardial sac { peri -, around + cardium , heart}, and the two lungs, enclosed in sepa- Dr. George Papanicolaou has saved the lives of millions of rate pleural sacs . women by popularizing the Pap test, a screening method that detects the early signs of cervical cancer. In the past 50 Th e abdomen and pelvis form one continuous cavity, the years, deaths from cervical cancer have dropped dramatically abdominopelvic cavity . A tissue lining called the peritoneum in countries that routinely use the Pap test. In contrast, lines the abdomen and surrounds the organs within it (stomach, cervical cancer is a leading cause of death in regions where intestines, liver, pancreas, gallbladder, and spleen). Th e kidneys Pap test screening is not routine, such as Africa and Central lie outside the abdominal cavity, between the peritoneum and America. If detected early, cervical cancer is one of the the muscles and bones of the back, just above waist level. Th e most treatable forms of cancer. Today, Jan Melton, who pelvis contains reproductive organs, the urinary bladder, and had an abnormal Pap test several months ago, returns to the terminal portion of the large intestine. Dr. Baird, her family physician, for a repeat test. The results may determine whether she should undergo treatment for cervical cancer. The Lumens of Some Organs Are Outside the Body Th e hollow organs, such as heart, lungs, blood vessels, and in- testines, create another set of compartments within the body. Th e interior of any hollow organ is called its lumen { lumin, win- dow}. A lumen may be wholly or partially fi lled with air or fl uid. 67 Fig. 3.1 ESSENTIALS Levels of Organization: Body Compartments BODY COMPARTMENTS (a) ANATOMICAL: The Body Cavities (b) FUNCTIONAL: Body Fluid Compartments POSTERIOR ANTERIOR Extracellular fluid (ECF) Cells (intracellular Cranial cavity lies outside the cells. fluid, ICF) Blood Interstitial plasma fluid Fat cell: is the surrounds 50–150 μm extracellular most cells. fluid inside blood vessels. Pleural Ovum: μ sac 100 m Thoracic Pericardial cavity sac Red blood cell: 7.5 μm Diaphragm Abdominal White blood cavity cell: 15 μm Abdominopelvic cavity Smooth muscle Pelvic cell: 15–200 μm long cavity Cells subdivide into intracellular compartments (see Fig. 3.4). (c) Compartments Are Separated by Membranes Pericardial Tissue membranes Phospholipid bilayers membrane have many cells. create cell membranes. Cell Heart Loose connective tissue The pericardial sac is Seen magnified, the pericardial Each cell of the The cell membrane a tissue that surrounds membrane is a layer of flattened pericardial membrane is a phospholipid the heart. cells supported by connective has a cell membrane bilayer. tissue. surrounding it. 68 Compartmentation: Cells and Tissues For example, the lumens of blood vessels are fi lled with the fl uid Biological Membranes we call blood. For some organs, the lumen is essentially an extension Th e word membrane { membrana, a skin} has two meanings in bi- of the external environment, and material in the lumen is not ology. Before the invention of microscopes in the sixteenth cen- truly part of the body’s internal environment until it crosses tury, a membrane always described a tissue that lined a cavity or the wall of the organ. For example, we think of our digestive separated two compartments. Even today, we speak of mucous tract as being “inside” our body, but in reality its lumen is part membranes in the mouth and vagina, the peritoneal membrane of the body’s external environment . An analogy would be the that lines the inside of the abdomen, the pleural membrane that hole through a bead. Th e hole passes through the bead but is not covers the surface of the lungs, and the pericardial membrane actually inside the bead. that surrounds the heart. Th ese visible membranes are tissues: An interesting illustration of this distinction between the thin, translucent layers of cells. internal environment and the external environment in a lumen Once scientists observed cells with a microscope, the na- 3 involves the bacterium Escherichia coli. Th is organism normally ture of the barrier between a cell’s intracellular fl uid and its ex- lives and reproduces inside the large intestine, an internalized ternal environment became a matter of great interest. By the compartment whose lumen is continuous with the external en- 1890s, scientists had concluded that the outer surface of cells, vironment. When E. coli is residing in this location, it does not the cell membrane, was a thin layer of lipids that separated harm the host. However, if the intestinal wall is punctured by the aqueous fl uids of the interior and outside environment. We disease or accident and E. coli enters the body’s internal envi- now know that cell membranes consist of microscopic double ronment, a serious infection can result. layers ( bilayers ) of phospholipids with protein molecules in- serted in them. Th us, the word membrane may apply either to a tissue or to Functionally, the Body Has a phospholipid-protein boundary layer ( Fig.