3 2 Crossing the Plasma Membrane A Introduction

3. 2 Crossing the Plasma Membrane

A. Introduction • The plasma membrane is selectively (semi) permeable – only certain ions and molecules can cross through • Passive processes – does NOT require cellular energy (ATP) • Active processes – DOES require cellular energy

B. Simple Diffusion • Movement of atoms or molecules from an area of higher concentration to an area of lower concentration (down a concentration gradient) • Movement of atoms or molecules occur until they are equally distributed • No cellular energy is required (passive) • Small lipid-soluble molecules will diffuse through the lipid portion of the membrane; includes blood gases

Diffusion

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C. Osmosis • Diffusion of water across a plasma membrane • Occurs when there is an unequal distribution of water on either side of a selectively permeable membrane; water passes freely but solutes do not • Osmotic pressure – force exerted on a selectively permeable membrane

Osmosis, cont 4. Tonicity – concentration of solute versus the concentration of water a. Isotonic – equal concentration of solutes (dissolved substances) and solvent (water) inside and outside cell; cell shape is maintained b. Hypotonic – higher concentration of water (lower concentration of solutes) outside cell; water moves into cell causing it to swell and eventually lyse c. Hypertonic – lower concentration of water (higher concentration of solutes) outside cell; water moves out of cell causing it to shrink or crenate

Tonicity

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D. Filtration • Filtration is the movement of liquid from high pressure to low pressure • Blood pressure creates the hydrostatic pressure in capillaries where filtration occurs

E. Transport by carriers • Facilitated diffusion a. Solutes are transported by means of a protein carrier b. Movement from area of higher concentration to area of lower concentration c. No energy required - passive

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Transport by carriers, cont 2. Active transport a. Solutes move up their concentration gradient (low concentration area to high concentration area) b. Requires a protein carrier (often called pumps) c. Requires the use of cellular energy d. The sodium/potassium pump (Na+/K+ pump) is an important pump that all cells have

Active Transport

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F. Endocytosis and Exocytosis • In endocytosis (bring into the cell) the plasma membrane envelopes a substance and forms an intracellular vesicle a. Phagocytosis (“cell eating”) – cell ingests solid particles b. Pinocytosis (“cell drinking”) – cell consumes solutions • In exocytosis (takes out of the cell) a vesicle fuses with the plasma membrane as secretion occurs

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Crossing the Plasma Membrane