Chapter 4.  How Cells Work, part one of two

Much of a cell's work is accomplished by means of chemical reactions aided by enzymes.  Some of the chemicals acted upon by enzymes must enter the cell prior to reaching enzymes and some of the metabolic products produced by the action of enzymes must exit the cell and/or the cell organelle in which they are produced. This entry and exiting of cells and the organelles within, requires that chemicals cross membrane barriers. Membranes not only surround the cell (the plasma membrane), but they also extend throughout the cytoplasm and form the structure of many cellular organelles including the membranes of chloroplasts, mitochondria, central vacuoles, vesicles, Golgi bodies, and endoplasmic reticulum.  Just as chemicals must cross the plasma membrane to enter or exit the cytoplasm, chemicals also pass across the membranes of organelles as they enter of exit organelles.  Membranes, to a large extent, control what chemicals go into and out of both the cell as a whole and the organelles within.  Thus, the chemical composition of dissolved solutes within the cell and its organelles is very different from the chemical composition of the fluid outside the cell or cell's organelles. 

I.  How Substances Move In & Out of Cells (see sections 4.4 through 4.7).

A. Diffusion - a common way many substances move into and out of cells.  Diffusion may be defined as the passive movement of molecules from an area where they are more highly concentrated to an area where they are less concentrated. The movement occurs due to the inherit random motion of molecules.  Molecules within a solution are free to move about and become randomly dispersed as a result.  Cells produce certain molecules that remain dissolved within they cytoplasm resulting in an increased concentration of these dissolved molecules within the cytoplasm compared to body fluids outside the cell (e.g. carbon dioxide).  Since fluid is continuous, though chemically distinct, between cytoplasm and that outside of the cell,  certain of these molecules (see below) will freely disperse (diffuse) out of the cell as a result of random motion.  Likewise, for dissolved chemicals a cell consumes, the lowered concentration within the cell leads to a spontaneous uptake of certain chemicals by the same process.

ex. CO₂, O₂, alcohol & H₂O all readily diffuse across membrane surfaces; The importance of this is that cells are in some sense powerless to control the movements of these molecules; they will move along their concentration gradient, i.e., from high to low concentration.  If O₂ is more highly concentrated outside of cell then it will move into the cell by simple diffusion.  Concerning alcohol, your cells “don’t know when to say when.”

            Diffusion of water into or out of the cell = Osmosis, given that water is so important, we will consider osmosis in more detail in lab.

New Light on Cell walls of plants and fungi – Plants don’t have a kidney to regulate water levels in body fluids, but their design keeps them from swelling and bursting thanks to a bounding cell wall that maintains turgor pressure under normal, well-watered conditions.

B.  Passive transport - diffusion through transport proteins for which the phospholipid bilayer is a barrier, ex. glucose, fig. 4.13.

C.  Active Transport – transport proteins expend energy (ATP) to move molecules from low concentration to high concentration, ex. calcium, fig. 4.14.

In active transport molecules are moved into or out of cell against their concentration gradient, i.e. from low to high concentration and therefore this is opposite of diffusion, is not spontaneous, and requires cellular energy [ATP].  Active transport occurs where membrane proteins act as pumps, pumping  Na⁺, K⁺, Cl^-, ions pumped across nerve cell membranes that maintains an electrical potential.  These ions are the so-called electrolytes. 

D.  Endocytosis & Exocytosis (sec. 4.7)– vesicles forming at the plasma membrane bring material in; vesicles fusing with plasma membrane dump material out.  Ex. Phagocytosis = when white blood cells engulf bacteria.  Ex. Exocytosis = pancreas cells secrete insulin to body fluids.

GO BACK TO BI 101 HOME PAGE