Download BIO 1: CHAPTER 4 – CELLS AND THEIR ENVIRONMENT Passive

BIO 1: CHAPTER 4 – CELLS AND THEIR ENVIRONMENT
I. Passive transport: movement of substances across the cell membrane that does not require energy
A. Concentration Gradient– a difference in concentration of a substance across a space (membrane)
1. Requires an area of high and low concentration
2. When things move from high conc. to low conc. we say they move down the concentration
gradient
B. Equilibrium – condition where concentration is equal over space (or on each side of membrane)
C. Players:
1. Solute = the substance dissolved
2. Solvent = the substance dissolving (usually water)
3. Solution = solute + solvent
D. Three types:
1. Diffusion - a general term for random, rapid movement of a substance across a space
a. Movement is random based on kinetic energy in the molecules
b. Moves from High concentration to Low concentration
c. If allowed to continue, it can reach equilibrium
2. Osmosis – diffusion of WATER through a semi-permeable membrane
a. Ions are dissolved in water inside and outside of cell
b. This sets up the concentration gradient
c. Three scenarios:
i. Hypotonic = lesser; smaller amount of solute in the solution
1. water moves into cell and the cell bursts
2. any situation that involves PURE water will cause water to move into cell
ii. Hypertonic = more; greater; larger amount of solute in the solution
1. water moves out of cell causes the cell to dehydrate and shrink
2. Foods high in salt cause this to happen to our cells – any situation where there is a high
salt concentration will cause cells to lose water
iii. Isotonic = equal; same
1. no net movement of water
2. equilibrium is established
3. an equal # of water molecules enter the cell as leave the cell – the cell is normal
d. Unicellular eukaryotes (protists) have contractile vacuoles to gather excess water & pump it
back out of the cell to maintain proper water levels
e. Organisms with a cell wall exhibit turgor pressure because of osmosis – the pressure the cell
membrane exerts on the cell wall as a result of an increase of water inside the cell
3. Facilitated Diffusion– a type of passive transport down the concentration gradient where helpers
allow the non-polar tails of the lipid bilayer to be bypassed
a. Transport/carrier proteins- move amino acids, sugars, or other large into or out of cell
b. Ion channel– a transport protein with a polar pore through which ions can pass
c. Pores – proteins that allow small polar molecules, like water, to cross the membrane
II. Active Transport: transport across the cell membrane against the concentration gradient (low to high) that
requires energy (ATP)
A. Can use specific carrier proteins to pump substances from low to high; therefore called membrane
pumps
1. Sodium- Potassium Pump
a. Cell uses ATP to move 3 Na+ out and 2 K+ in
b. Important for several reasons
i. Prevents Na+ from accumulating in cells and causing a hypotonic situation where the cell
could burst open
ii. Maintains the right concentration gradient of Na+ and K+ which helps set up the right
environment for moving other things
c. Ion concentrations are important for proper muscle contraction and nerve impulses
B. Movement in Vesicles – needed for big substances like proteins and polysaccharides that can’t fit
through a carrier protein/channel
1. Endocytosis – movement of a substance into the cell
a. Cell membrane forms a pouch around the substance by using pseudopodia
b. Pouch closes up and pinches off on the inside of cell
c. Sometimes lysosomes add digestive enzymes to help break down the material inside the vesicle
d. Pinocytosis – the way the cell takes in liquids; a special type of endocytosis - vesicles pinched
inward; “drinking”
e. Phagocytosis – the way the way the cell takes in solids; a type of endocytosis – a vacuole
extends off of the cell engulfs the solid and brings it back into the cell; “eating”
2. Exocytosis – movement out of the cell by vesicle
a. Vesicle fuses with cell membrane & releases content to outside
b. Used to export proteins processed by Golgi body
c. Nerve cells & glands release proteins this way too