MODERN BOTANY - P. RAVEN - 1990

CHAPTER I. THE PLANT CELL

CHAPTER 4. MOVEMENT OF SUBSTANCES INTO AND OUT OF CELLS

Conclusions

Cell/30.html">The Plasma Membrane regulates the movement of substances into and out of The Cell, thereby enabling it to maintain Structural and functional integrity. This regulation depends on the interaction between the membrane and the molecules passing through it.

Water is one of the primary substances entering and leaving Cells, moving via bulk flow, diffusion, and osmosis. The direction of water movement is determined by water potential; that is, water moves from a region of higher water potential to one of lower water potential. Bulk flow refers to the Movement of water, for example, down a slope or under pressure.

Diffusion is driven by the random motion of molecules, resulting in a net movement along a concentration gradient. Carbon dioxide and oxygen are the two main substances that move into and out of cells by diffusing across the membrane. Diffusion is most efficient over short distances and across steep concentration gradients. The rate of intracellular movement is further enhanced by cytoplasmic streaming.

Osmosis is the movement of water across a membrane that is permeable to water but impermeable to dissolved solutes. Such a membrane is termed selectively permeable. In the absence of other forces, Osmotic water movement occurs from a region of lower solute concentration (a hypotonic environment with a higher water potential) to a region of higher solute concentration (a hypertonic environment with a lower water potential). Turgor pressure (firmness) in plant cells is a direct result of osmosis.

According to the fluid-mosaic model, cell membranes consist of a lipid bilayer in which Globular Proteins are embedded. Some of these proteins act as carriers, transporting molecules across the membrane. When carrier-mediated transport occurs down a concentration gradient, it is known as facilitated transport. Conversely, when transport requires an expenditure of energy, it is called Active Transport, which enables substances to move against a concentration gradient. One of the most prominent active transport systems in animal cells is the sodium-potassium pump, which maintains a low concentration of sodium ions and a high concentration of potassium ions within the Cytoplasm.

The controlled movement of substances into and out of the cell can also occur via Endocytosis and Exocytosis, which are Vesicular Transport processes. The endocytosis of solid particles is termed phagocytosis, whereas the uptake of dissolved Materials is called pinocytosis.

Metabolites and other substances can be exchanged between plant cells through plasmodesmata—fine cytoplasmic strands that interconnect the protoplasts of adjacent cells. This pathway of movement is referred to as symplastic transport.

Appendix. Imbibition

Water molecules possess a remarkable capacity for cohesion because their two ends carry opposite charges. This same polarity gives water an affinity for both positively and negatively charged surfaces. Many large biological molecules, such as Cellulose, become electrically charged upon Hydration and consequently attract water molecules. The adhesive property of water molecules thus accounts for another important biological phenomenon known as imbibition.

Imbibition (from the Latin imbibere, meaning "to drink")—is the uptake of water molecules by substances such as wood or gelatin, causing them to swell and increase in volume. The pressures generated by imbibition can be remarkably high. It is said that stones for the ancient Egyptian pyramids were quarried by driving wooden wedges into rock crevices and then wetting them with water. The Swelling wood exerted a force sufficient to split the stone slabs. In living plants, imbibition occurs notably in seeds, which can consequently undergo a manyfold increase in volume.