Movements Through Cell Membranes 3.3 Rima Smadi

Cell Membrane Plasma membrane Contains the cell’s content Regulates movement of substances in and out of the cell Communication Selectively permeable membrane Phospholipids bilayer

Cell Membrane

Movement Passive Mechanism: Does not requires energy Diffusion Facilitated Diffusion Osmosis Filtration Active Mechanism: Requires energy Active transport Endocytosis Exocytosis

Diffusion Process by which molecules or ions scatter or spread spontaneously from regions where they are in higher concentrations toward regions where they are in lower a concentration gradient. Equilibrium: uniformly distributed, concentration no longer changes. Diffusion only occurs if: Membrane is permeable to the substance A concentration gradient exists.

Facilitated Diffusion Diffusion of substances that cannot pass through the lipid bilayer, need help from membrane proteins. High to low concentration Water insoluble substances (ions,) cannot pass through the inner hydrophobic part of the membrane. Glucose and amino acids are water soluble, but are too large to pass through. Substance attaches to carrier protein, the protein changes shape to allow the substance to pass through. Then it returns to its original shape. For example, Insulin helps open the carrier proteins (change their shape) to let cells take in glucose.

Osmosis Diffusion but only with water molecules. High to low concentration. In a solution: Solute: what is dissolved. (ex. Sugar) Solvent: what is doing the dissolving. (ex. Water) When solute cannot diffuse, then the solvent diffuse to the area of higher solute concentration. Osmotic pressure: the ability of osmosis to generate enough pressure to lift a volume of water. Water diffuses to an area of higher osmotic pressure.

Osmosis Isotonic: any solution that has the same osmotic pressure as body fluids. Hypertonic: solutions with a higher osmotic pressure than body fluids. If cell is in this solution, water moves out of cell (shrinks). Hypotonic: solution with lower osmotic pressure than body fluids. If cell is in this solution, water moves into the cell (swell up).

Filtration Forces molecules through membrane. Hydrostatic pressure: created by the weight of due to gravity, forces water molecules through to the other side. Occurs in blood vesicles from the pumping of the heart and in kidneys to cleanse the blood. Edema: condition in which excessive water is built up in tissue, because of failed filtration.

Active Transport Process that moves particles through membranes from low to high concentration. Equilibrium is never reached. Uses specific carrier proteins in the membrane also called pumps. Uses energy in the form of ATP (Adenosine triphophate) 40% of cell’s energy is used in active transport. Carrier molecules: proteins with binding sites that combine with substances and transport them. Release of energy changes the shape of the protein and transports the substance. Molecules include: sugar, amino acids, sodium, potassium, calcium, and hydrogen ions.

Endocytosis and Exocytose Use energy to move substances in and out without having to cross the membrane. Endocytosis: large substances that can’t enter a cell by any of the previous processes are taken within a vesicle formed from a section of the cell membrane. Exocytosis: a substance that is stored in a vesicle is secreted by merging the vesicle with the cell membrane and releasing the substance.

Endocytosis Endocytosis happens in 3 ways: Pinocytosis: (cell drinking) droplets of liquid are taken in by membrane when it indents. The open ends seal off, producing a small vesicle . The vesicle’s membrane then breaks down and the substance is released into the cytoplasm. Phagocytosis : (cell eating) same as pinocytosis but cell takes in solids. The substance usually attaches to receptors on the membrane then it indents, and detaches to become a vesicle. Lysosomes attach to the newly formed vesicle and break down the content. Ex. White blood cells (phagocytes) that take in solids like bacteria or harmful chemicals Receptor-mediated endocytosis: moves very specific kinds of particles into the cell. Where proteins from within the cell become receptors on the membrane waiting for specific molecules outside the cell (ligands).