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