chapter 5: homeostasis and transport
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Chapter 5:Chapter 5:Homeostasis and Homeostasis and
TransportTransport
Section 5-1:Passive Transport
• Cell membrane helps organisms maintain homeostasis by controlling what enters/exits cell
• Some substance can cross cell membrane with out using energy from cell
• PASSIVE TRANSPORT
Diffusion
• Simplest
• movement of substances from area of high concentration to area of low concentration
• Driven by kinetic energy that molecules posses
• kinetic energy = molecules constantly in motion
• Move randomly in straight line until they hit an object Move in straight line in concentration gradient because to molecule to bounce off of
Concentration Gradient• Difference in concentration of molecules
across a space
Equilibrium
• When the concentration of molecules of a substance is the same throughout a space
• Even at equilibrium the random motions of molecules still occur
• Random motions on one direction balance out those of the other direction
Diffusion Across Membrane
• If molecules can diffusion across cell membrane it will move from high concentration to low concentration
• Ability of molecule to move across membrane depends on:– size– type of molecule– chemical nature of membrane
• Membrane = phospholipids
• THUS any substance that can dissolve in lipids will pass through membrane
• Also VERY small molecule can fit through pores even if they are not soluble in lipids
REMEMBER
• Chapter 2: Solution = solute dissolved in solvent
• CELLS: Solutes = organic and inorganic moleculesSolvents= water
Osmosis
• Process by which water molecules diffuse across cell membrane from high concentration to low concentration
• Does not cost cell energy to occur
Direction of Osmosis
• Net direction of osmosis depends on relative concentration of solutes on either side of membrane
Name Solutes concentration
Movement of water
Effect on cell
Hypotonic More solute in cytosol; lower concentration outside cell
Water moves INTO cell
Cell enlarges
Hypertonic Less solute in cytosol; higher concentration outside cell
Water moves OUT of cell
Cell shrinks
Isotonic Same concentration in/out
Water moves in/out at equal rates
Cell remains same size
Put it together...• If a solution is hypotonic then the cytosol is hypertonic
• If solution is hypertonic then cytosol is hypotonic
Cell Environments
• Isotonic environment cells are fine(cells of vertebrate animals on land or sea animals)
• Cells in hypotonic environment cells must rid themselves of excess water (unicellular organisms in FRESH water)
How do cells deal with osmosis?
1- Contractile vacuoles
2- Solute Pumps
3- Cell Walls
Contractile Vacuoles
• How some unicellular organisms rid themselves of excess water
• Organelle which removes water
• How?Collect excess water and then contract, pumping water out of cell
**REQUIRES ENERGY**
Solute Pumps• Mostly in multicellular
organisms
• Pump solutes OUT of cytosol
• Bring cytosol solute concentration closer to that of the solution
• water less likely to move into cell
Cell Walls
• Plant roots swell as they absorb water
• Swelling stops when cell membrane is pressed against cell wall
• Cells walls able to withstand pressure of water
• Turgor pressure: pressure that water molecules exert against cell wall
Plasmolysis• In hypertonic
environment water leaves cells
• Cells shrink away from cells walls
• Turgor pressure is lost
• Plants wilt
Cytolysis• Bursting of cells
• Cells which lack contractile vacuoles, solute pumps, and cells walls
• Water continuously moves into cells
Facilitated Diffusion• Process used for
molecules that cannot diffuse rapidly through cell membranes
• Why not?– Not soluble in lipids– Too large to pass
through pores
Carrier Proteins• Assist in diffusion of insoluble/large
molecules across membrane
• STILL PASSIVE!
• Molecules moved from high concentration to low concentration
How carrier proteins work…
1. Carrier proteins binds to molecules it transports
2. Carrier protein changes shape
3. Protein shields molecules from hydrophobic area of membrane
4. Molecule transported through membrane
5. Molecule released into/out of cell
6. Protein returns to original shape
Transport of Glucose• Facilitated
diffusion
• Glucose molecules too large
• Cells need for energy
• Carrier proteins are specific for certain molecules
Diffusion through Ion Channels• Passive transport involving membrane
proteins
• Ions: Na+, K+, Ca2+, and Cl-
• Each type of ion has specific ion channel
• Some ion channels always open
• Others have “gates” that open to allows OR close
• Gates may open due to three kinds of stimuli– Stretching of cell membrane– Electrical signals– Chemicals in cytosol or
external environment
5-2: Active Transport
• Sometimes cells must move materials against concentration gradient [low] [high]
• Active transport: requires cell to use energy
Cell Membrane Pumps• Some carrier proteins also help active
transport
• Same “Bind-Change shape-Transport-Return shape” process
Sodium-Potassium Pump• Transport Na + and K +
• Most animals cells must have high Na + outside of cells and low K + inside cell
Process
1. Three Na + ions bind to sodium-potassium pump WHILE carrier protein split phosphate group from ATP molecule
2. Phosphate group binds to carrier protein and changes carrier protein shape
3. Now has shape need to bind two K + ions
4. When K + ions bind, phosphate group releases and carrier protein changes shape again
5. K + ions released into cell
6. Ready to do it again!
Summary of Sodium-potassium Transport…
• 3 Na + ions now outside
• 2 K + ions now inside
• Usually about 450 Na + ions and 300 K + PER SECOND!
• Movement of 3:2 Na + :K + ions create electrical gradient
• Outside :positively charged
• Inside :negatively charged
• IMPORTANCE: conduction of electrical impulses along nerve cells
For things too large…• Macromolecules
• Food particles
• Types:– Endocytosis
(endo in/enter)– Exocytosis
(exo out/exit)
Endocytosis
• Cells ingest external fluid, macromolecules, and large particles (including other cells)
• Pouch formed Materials enclosed by portion of cell
• Vesicle: membrane bound organelle consisting of pinched off pouch from cell membrane
• Some vesicles fuse with lysosomes to digest materials
Type of Endocytosis
Based on kind of material taken in:
• Pinocytosis
• Phagocytosis
Pinocytosis
• transport of solutes or fluids
Phagocytosis• movement of large
particles or whole cells– how some organisms
feed– how animal cells ingest
bacteria and viruses
• Phagocytes: cells that allow lysosomes to fuse with vesicles that contain harmful things
Exocytosis• Reverse of endocytosis
• Vesicle fuse with cell membrane and release content’s into cell environment
• Cell may use to release proteins
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