Review of Key Concepts
Concepts of Cellular Structure (p. 94)
- Cytology is the study of cellular structure and function.
- All human structure and function is the result of cellular activity.
- Cell shapes are described as squamous, polygonal, stellate, cuboidal, columnar, spheroid, ovoid, discoid, fusiform, and fibrous.
- Most human cells are 10 to 15 ^m in diameter. Cell size is limited in part by the ratio of surface area to volume.
- A cell is enclosed in a plasma membrane and contains usually one nucleus.
- The cytoplasm is everything between the plasma membrane and nucleus. It consists of a clear fluid, the cytosol or intracellular fluid (ICF), and embedded organelles and other structures. Fluid external to the cell is extracellular fluid (ECF).
The Cell Surface (p. 98)
- The plasma membrane is made of lipid and protein.
- The most abundant lipid molecules in the membrane are phospholipids, which form a bilayer with their hydrophobic heads facing the ICF and ECF. Other membrane lipids include cholesterol and glycolipids.
- Membrane proteins are called integral proteins if they are embedded in the lipid bilayer and extend all the way through it, and peripheral proteins if they only cling to the intracellular face of the lipid bilayer.
- Membrane proteins serve as receptors, second-messenger systems, enzymes, channels, carriers, molecular motors, cell-identity markers, and cell-adhesion molecules.
- Channel proteins are called gates if they can open and close. Gates are called ligand-regulated, voltage-regulated, or mechanically regulated depending on whether they open and close in response to chemicals, voltage changes across the membrane, or mechanical stress.
- Second-messenger systems are systems for generating an internal cellular signal in response to an external one. One of the best-known examples results in the formation of a second messenger, cyclic AMP (cAMP), within the cell when certain extracellular signaling molecules bind to a membrane receptor.
- All cells are covered with a glycocalyx, a layer of carbohydrate molecules bound to membrane lipids and proteins. The glycocalyx functions in immunity and other forms of protection, cell adhesion, fertilization, and embryonic development, among other roles.
- Microvilli are tiny surface extensions of the plasma membrane that increase a cell's surface area. They are especially well developed on absorptive cells, as in the kidney and small intestine.
- Cilia are longer, hairlike surface extensions with a central axoneme, composed of a 9 + 2 arrangement of microtubules. Some cilia are stationary and sensory in function, and some are motile and propel substances across epithelial surfaces.
- A flagellum is a long, solitary, whiplike extension of the cell surface. The only functional flagellum in humans is the sperm tail.
Membrane Transport (p. 106)
- The plasma membrane is selectively permeable—it allows some substances to pass through it but prevents others from entering or leaving a cell. There are several methods of passage through a plasma membrane.
- Filtration is the movement of fluid through a membrane under a physical force such as blood pressure, while the membrane holds back relatively large particles.
- Simple diffusion is the spontaneous net movement of particles from a place of high concentration to a place of low concentration, such as respiratory gases moving between the pulmonary air sacs and the blood. The speed of diffusion depends on temperature, molecular weight, concentration differences, and the surface area and permeability of the membrane.
- Osmosis is the diffusion of water through a selectively permeable membrane from the more watery to the less watery side. Channel proteins called aquaporins allow passage of water through plasma membranes.
- The speed of osmosis depends on the relative concentrations, on the two sides of a membrane, of solute molecules that cannot penetrate the membrane. Osmotic pressure, the physical force that would be required
Saladin: Anatomy & I 3. Cellular Form and I Text I © The McGraw-Hill
Physiology: The Unity of Function Companies, 2003 Form and Function, Third Edition
126 Part One Organization of the Body to stop osmosis, is proportional to the concentration of nonpermeating solutes on the side to which water is moving.
- An osmole is one mole of dissolved particles in a solution. Osmolarity is the number of osmoles of solute per liter of solution. The osmolarity of body fluids is usually expressed in milliosmoles per liter (mOsm/L).
- Tonicity is the ability of a solution to affect the fluid volume and pressure in a cell. A solution is hypotonic, isotonic, or hypertonic to a cell if it contains, respectively, a lower, equal, or greater concentration of nonpermeating solutes than the cell cytoplasm does. Cells swell and burst in hypotonic solutions and shrivel in hypertonic solutions.
- Carrier-mediated transport employs membrane proteins to move solutes through a membrane. A given carrier is usually specific for a particular solute.
- Membrane carriers can become saturated with solute molecules and then unable to work any faster. The maximum rate of transport is the transport maximum (Tm).
- A uniport is a carrier that transports only one solute at a time; a symport carries two or more solutes through the membrane in the same direction (a process called cotransport); and an antiport carries two or more solutes in opposite directions (a process called countertransport).
- Facilitated diffusion is a form of carrier-mediated transport that moves solutes through a membrane down a concentration gradient, without an expenditure of ATP.
- Active transport is a form of carrier-mediated transport that moves solutes through a membrane up (against) a concentration gradient, with the expenditure of ATP.
- The Na+-K+ pump is an antiport that moves Na+ out of a cell and K+ into it. It serves for control of cell volume, secondary active transport, heat production, and maintenance of an electrical membrane potential.
- Vesicular transport is the movement of substances in bulk through a membrane in membrane-enclosed vesicles.
- Endocytosis is any form of vesicular transport that brings material into a cell, including phagocytosis, pinocytosis, and receptor-mediated endocytosis.
- Exocytosis is a form of vesicular transport that discharges material from a cell. It functions in the release of cell products and in replacement of plasma membrane removed by endocytosis.
The Cytoplasm (p. 115)
- The cytoplasm is composed of a clear gelatinous cytosol in which are embedded organelles, the cytoskeleton, and inclusions (table 3.4).
- Organelles are internal structures in the cytoplasm that carry out specialized tasks for a cell.
- Membranous organelles are enclosed in one or two layers of unit membrane similar to the plasma membrane. These include the nucleus, endoplasmic reticulum (which has rough and smooth portions), ribosomes, the Golgi complex, lysosomes, peroxisomes, and mitochondria. The centrioles and ribosomes are nonmembranous organelles.
- The cytoskeleton is a supportive framework of protein filaments and tubules in a cell. It gives a cell its shape, organizes the cytoplasmic contents, and functions in movements of cell contents and the cell as a whole. It is composed of microfilaments of the protein actin; intermediate filaments of keratin or other proteins; and cylindrical microtubules of the protein tubulin.
- Inclusions are either stored cellular products such as glycogen, pigments, and fat, or foreign bodies such as bacteria, viruses, and dust. Inclusions are not vital to cell survival.
Was this article helpful?
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.
Get My Free Ebook