Cellular Transport (2012) - Phoenix PDF

## Cellular transport/movement ### Why is it important? - To obtain food supply for energy and raw material for metabolism. - To excrete toxic/waste materials - To maintain pH and ionic concentration for enzyme activity - To generate ionic gradients across membranes essential for nervous and muscl...

## Cellular transport/movement ### Why is it important? - To obtain food supply for energy and raw material for metabolism. - To excrete toxic/waste materials - To maintain pH and ionic concentration for enzyme activity - To generate ionic gradients across membranes essential for nervous and muscle activity. ### Two types of transport processes across membranes: - **Passive:** Movement is down the concentration gradient. No expenditure of energy by the cell. - **Active:** Transport is against the concentration gradient. There is expenditure of energy by the cell. ### Examples of passive transport/movements: - Simple diffusion - Facilitated diffusion - Osmosis ### Diffusion: net movement of like molecules or ions down a concentration gradient. (From high concentration to low concentration ). - It transports non polar molecules that are small enough to pass through the lipid bilayer. E.g. CO2, O2, ether, lipid soluble Vitamin D etc. ### Speed of movement depends on: - **Concentration gradient** - **Temperature of solution** - **Diameter of molecules** - **Density of a solution** **Note:** As distance becomes greater diffusion become insufficient to transport things within the cell. It is useful within a cell or between two cells. Across a cell membrane, diffusion may take a whole hour. ### Osmosis - **Definition:** Diffusion of water across a selectively permeable membrane - Water uses **Aquaporins** channels for transport. - Osmosis depends on the number of solutes dissolved in the water. #### Terms to note: - **Osmotic pressure:** Pressure required to stop the osmotic movement of water into a solution. - **Osmotic potential:** Tendency of H2O to move into a medium by osmosis - **Water potential:** Potential energy of water. Symbol for water potential is Ψ (Psi) measured in kilo pascals. Pascal = kgm-1s-2 - Presence of solutes decreases water potential. ### Terms used to compare solute concentration of solutions - **Tonicity:** Relative concentration of two fluids - **Isotonic solutions:** Solutions with equal solute concentration - **Hypertonic/hypotonic solution:** A solution with higher/lower total solute concentration than the other solution. - Hypertonic solution has high osmotic pressure and low water potential - Hypotonic solution has vice versa. - **Isotonic ???** ### Note: - **Blood plasma (fluid component of blood) is isotonic to RBC cells.** - **Plant cells are hypertonic to the growing medium.** - **Marine species of Amoeba are isotonic to sea water.** - **Fresh water species of Amoeba/Paramecium are hypertonic.** They use a contractile vacuole to expel excess water. - **Hemolysis (blood splitting) occurs when cells are in hypotonic solution (solution has more water).** - **Crenation (shrinking) occurs when cells are in hypertonic solution.** - **Plasmolysis (shrinking of protoplasm from cell wall) occurs when plant cells are placed in hypertonic medium.** The image depicts a visual representation of how red blood cells behave in different solutions: - **Hypotonic:** Red blood cells are swollen and may burst due to water moving into the cell. - **Isotonic**: Red blood cells remain normal as the concentration is the same both inside and outside the cell. - **Hypertonic**: Red blood cells shrink as water moves out of the cell due to a higher concentration of solutes outside the cell. ### Facilitated Diffusion - Carrier/channel mediated diffusion. - The speed depends on the number of carriers/channels present. - **Channels** are water-filled pores that close and open (i.e., have a gating mechanism) - **Carriers** bind to a substance temporarily and undergo a conformational change during the transport process. - **Note:** Carriers are specific to substances - **Example of substances:** Ions of Na, K, Ca, Cl, molecules of amino acids, glucose, etc. ### Active Transport: Definition: Movement across a membrane against concentration gradient. - Occurs because some substances are required by cells in concentrations higher than that of their surroundings. - E.g. K ions are more in cells, but still transported inside the cells. - Requires metabolic energy - Depends on membrane protein carriers - Facilitates movements of ions of Na, Ca, K, glucose, amino acids. ### Three types of proteins used in active transport: - **Uniporters:** Transport one type of solute across a membrane; these are also involved in facilitated diffusion. - **Cotransporters:** Transport two ions or molecules. - **Symporters:** transport two solutes or ions in the same direction e.g. glucose and Na ions. - **Antiporters:** transports two molecules in opposite direction e.g. K and Na ions. ### E.g. of active transport: - Sodium and potassium pump (Na+ - K+ pump) - It transports these against their concentration gradients. - This is the case in neurons of animals and is necessary for impulse transport across nerves. - Only confined to vertebrates and invertebrates. ### Vesicle mediated/facilitated transport: - Transport of large particles of substances by vesicles - Involves macromolecules, liquids or whole organisms - **Two types:** - **Exocytosis:** Secretion from cells e.g. of enzymes, hormones, products of Golgi bodies. - **Endocytosis:** Taking in of materials - **3 Types of Endocytosis:** - Phagocytosis = cell eating, e.g. bacteria eaten by white blood cells, protists use it to feed - Pinocytosis = cell drinking, involves smaller vesicles - Receptor mediated endocytosis ### Receptor mediated endocytosis - Need receptors located on membranes at a particular site to bind to a specific substance in the environment outside the cell - Receptors are specific hence its more efficient - Receptors are located on the vesicle. - E.g. Low density lipoprotein (LDL) = the form in which cholesterol is transported in the blood. - LDL binds to receptors to transport the cholesterol.