Cell Homeostasis & Transport. Ch. 4

Transcription

1 Cell Homeostasis & Transport Ch. 4 1

2 Cell Membrane the border between the cell s innards and the outside world. Carbohydrate Phospholipid 2

3 Prior Knowledge Homeostasis is the maintenance of a constant internal environment. Potential energy is stored energy, whereas kinetic energy is the energy of motion. Lipids are part of the phospholipid, whereas proteins play integral roles in cellular communication (markers, receptors, channels). Cell membrane is selectively permeable, meaning it has the ability to regulate which materials enter or exit the cell (e.g. toxins or necessary molecules) in order to maintain homeostasis. 3

4 Passive Transport The process that enables material to move across a cell membrane in the direction of the concentration gradient without requiring energy. Four types of passive transport: Diffusion Osmosis Facilitated Diffusion Ion Channels 4

5 Concentration Gradient All four types of passive transport mechanisms operate with the concentration gradient. Concentration gradient = a change in the concentration of a substance across a space. Concentration gradient video clip 5

6 Diffusion The process by which molecules of a substance move from areas of high concentration of that substance to areas of lower concentration Occurs until equilibrium is reached. Equilibrium is a state in which no net change occurs (concentration of the substances on both sides of the membrane are equal). Click here for video Click here for video 6

7 Diffusion, continued Selectively permeable membranes allow certain molecules to pass via diffusion 7

8 Osmosis Diffusion of water molecules through a selectively permeable membrane from an area of high concentration to an area of lower concentration Will always move towards a higher concentration of solute since there is less water concentration there Osmosis video clip 8

9 Osmosis, continued Osmotic pressure a measure of the solute concentration of a solution Solute a substance dissolved in a solution (usually a solid), such as glucose or salt Solvent in a solution, the substance in which the solute is dissolved (liquid), such as water 9

10 Osmotic Pressure, continued We use the following terms to describe the osmotic pressures of solutions: Isotonic a solution whose concentration equals that inside a cell Hypertonic a solution whose solute concentration is higher than that inside a cell Hypotonic A solution whose solute concentration is lower than that inside a cell 10

11 Problems with Osmosis: Single-celled freshwater organisms (e.g. Paramecium) live in a hypotonic environment Must use contractile vacuole to pump out excess water Some multicellular organisms pump solutes out to dilute the solute inside the cytosol Plant s cell walls prevent from expanding turgor pressure 11

12 Plasmolysis Occurs when cells lose water in hypertonic solutions. Happens to plants when they are excessively fertilized or are in water with excess minerals. Observed through wilting. 12

13 Cytolysis Cells cannot always compensate for excess water intake and just burst cytolysis (cyto = cell, lyse = burst) 13

14 Facilitated Diffusion The transport of molecules across a membrane along with the concentration by a carrier protein. Similar to diffusion, except uses a carrier protein (goes with the c.g. & no energy) a. Carrier protein binds to molecule b. Carrier protein changes shape Carrier protein & substrate must be specific c. Molecule released on other side 14

15 Ion Channels Ion Channels Membrane proteins that provide a passageway across the cell membrane through which ions (charged atoms or molecules) can diffuse. Ions are polar (charged) and can t pass by nonpolar lipids Na +, K +, Ca 2+, and Cl - require specific ion channels Stimuli open/close gated channels: Cell membrane stretching Electric signals Chemicals in the cytosol or external environment 15

16 Remember. Marker proteins (with CHO tag) identify the cell s type Phospholipids make up the cell membrane Hydrophilic head Channel proteins allow larger/charged molecules to pass through the membrane Hydrophobic tail Receptor proteins receive information from outside the cell and transmit it inside 16

17 Active Transport Often cells must move materials against or up the concentration gradient (from low to high concentration). This requires energy and is known as active transport. Cells might need to stockpile glucose for fuel or ions in order to create a charge difference across a membrane. Active transport = energy-requiring process that enables material to move across the cell membrane against a c.g. 17

18 ATP = cellular energy ATP = Adenosine Triphosphate Ribose, adenine, triphosphate group Energy is stored in the highenergy bonds between the phosphate groups Energy is released/used when 3 rd phosphate is cleaved off and is donated to another molecule Energy from ATP drives many cellular reactions 18

19 Cell Membrane Pumps Type of active transport in which carrier proteins move molecules across the cell membrane against their c.g. Similar to carrier proteins in facilitated diffusion specific to certain substrates Different because they require energy to operate against the c.g. Enables cells to concentrate molecules (e.g. glucose or ions) 19

20 Sodium-Potassium Pump Type of cell membrane pump (active) Na + -K + pump establishes a membrane potential = difference in charge across a membrane which allows nerve impulses to travel along nerve cells (action potentials) Pumps Na + ions out and K + in Requires ATP (cellular energy) 20

21 Na + -K + Pump, continued a. 3 Na + ions bind to carrier protein b. ATP donates one phosphate group to the carrier protein giving it energy c. Phosphate binding causes carrier protein to change shape and release the 3 Na + ions outside the cell d. This allows 2 K + ions to bind to the carrier protein e. Phosphate group releases from carrier protein causing it to change shape again f. The 2 K + ions are released into cell s interior All of this generates a net negative charge inside the cell and a positive 21 charge outside the cell!!!

22 Endocytosis Used to take large materials into a cell by means of infoldings of the cell membrane Phagocytosis engulfing solids Pinocytosis engulfing liquids Cell membrane folds in & forms a small pouch Pouch pinches off from the cell membrane Vesicle enters the cell Remember the Amoeba! Phagocytes are cells in animals that purposely ingest bacteria & viruses and then fuse with a lysosome to use its digestive enzymes. 22

23 Exocytosis Large molecules are removed from the cell by the fusion of a vesicle with the cell membrane and then the material being expelled from the cell May be used to release the proteins synthesized by ribosomes (Golgi apparatus creates the vesicle) Nervous system releases neurotransmitters to initiate nerve impulses Endocrine system releases hormones 23