Cell Biology: Chapter 9

Membrane Transport

hydrophobic molecules

passively diffuse i.e. hydrocarbons, carbon dioxide, and oxygen

less permeable due to having lower kinetic energy and the small pore sizes in the membrane

larger molecules are

less permeable, but very small, polar uncharged (water) molecules can diffuse

polar molecules are hydrophilic which makes them

less permeable. molecules increase in size when surrounded by a coat of water (hydration shell)

charged molecules are hydrophilic which makes them

hydrophobic molecules, molecular size, polarity, and charge

important factors of membrane transport

passive transport, active transport, endocytosis (phagocytosis and pinocytosis), exocytosis

molecule movement and cells

types of passove transport

• diffusion

• osmosis

• facilitated diffusion

passive transport

movement of molecules through the membrane

energy; concentration gradient

In passive transport, no ___ is required, and movement is in response to a ___ ___

diffusion

movement from an area of high molecule concentration to an area of low concentration

the concentration is the same in all regions

diffusion continues until

small molecules and gases; for larger molecules and charged molecules it is not

passive transport is easy for

electrochemical gradient

concentration (chemical) graident and the mebrane potential (electrical gradient) across the membrane

concentration gradient and membrane potential across the membrane (the electrochemical gradient)

as charged particles, movement of ions across a membrane is influenced by both their

osmosis

special form of diffusion that involves the movement of water into and out of cell (down its concentration gradient)

hydrostatic pressure

osmotic property of cells; movement of water causes fluid mechanical pressure; pressure gradient across a semi-permeable membrane

Donnan Equilibrium

a situation where the presence of a large membrane-impermeable molecule with counterions creates a salt concentration gradient across a membrane that is permeable to small cations and anions.

solution

solvent + solute

Hypotonic

swollen; solutes in cell more than outside; outside solvent will flow into cell

Isotonic

normal; solutes equal inside and out of cell

Hypertonic

crenated; solutes greater outside cell; fluid will flow out of cell

extrusion

water is ejected through contractile vacuoles; some cells use in maintaining osmotic balance

isomotic regulation

keeping cells isotonic with their environment

• Water-dwelling organisms have developed ways to deal with salt

  (marine) or water (fresh water) overload

• Terrestrial animals circulate isotonic fluid throughout their

  bodies

Turgor Pressure

plant cells use this to push the cell membrane against the cell wall and keep the cell rigid

channels or pores

passive transport use

simple or passive diffusion

crossing the membrane

facilitated trnasport

assisted by membrane-floating proteins

Active transport pumps and carriers

• ATP is required

• enzymes and reactions may be required

Diffusion

spontaneous movement of particles from high concentration to low concentration

ΔG < 0

Diffusion

energy

Diffusion does not require ___

energy

particle random kinetic motion

stops

diffusion ___ when concentration on both sides equal

independent

diffusion of one compound is ___ of diffusion of other compunds

concentration difference

the driving force of diffusion of uncharged molecules into cell is

outside

ΔG < 0 when concentration ___ is greatest

negative

greater concentration difference more ___ ΔG

ΔG < 0

passive diffusion

ΔG > 0

active transport

ΔG

concentration difference + charge difference

ion diffusion

charge across membrane affects

membrane potential

voltage difference across membrane due to difference in positive and negative ions on different side of membrane

-60 mV

membrane potential

resting potential

a membrane potential of excitable cells that are at rest; a special case of membrane potential

organic ions

“-” inside cell

K+

high inside cell for electrical balance

voltage difference across membrane

membrane potential is measured as

K+

in mammals, ___ determines as main “+’ ion in cell

K+ leak channels

membrane contains

electrical gradient in

concentration gradient out

Diffusion directly through lipid bilayer

Factor Affecting Diffusion Through Membrane

more permeable

greater lipid soluability

smaller size more permeable

equal lipid solubility

less permeable

greater charge

lipid bilayer

ions cannot diffuse through

O2, CO2, and H2O

small, uncharge molecules that can diffuse through lipid bilayer

semipermeable

membrane is ___

• Free diffusion of waterFree diffusion of water

• Larger hydrophilic uncharged molecules such as sugars do not freely diffuse

• Direction of movement of each molecule determined by concentration of that molecule (ion)

• Molecules diffuse from high concentration to low

• Charged molecule (ion) diffusion depends on concentration gradient and membrane potential

channel proteins

discriminate based on size and charge

carrier proteins-transporters

• bind and change conformation opening on the other side of membrane

recognize molecular structure

facilitated diffusion

no energy required

glucose carrier

has two conformations

• randomly switches conformations

  • exposes binding site to outside or inside cell

facilitated diffusion: glucose entry

bidirectional; moves down concentration gradient

process of facilitated transport

• Protein binds with molecule

• Shape of protein changes

• Molecule moves across membrane

• No energy is used

saturation

rate independent of concentration

• carrier limited

carrier proteins and facilitated diffusion

help transport both ions and other solutes

concentration gradient

in carrier proteins and facilitated diffusion, the ___ ___ is required, but energy is not

selective

carrier proteins and facilitated diffusion is ___ for the molecule they transport

saturated

carrier proteins and facilitated diffusion can be

glucose phosphorylation

maintains concentration gradient

phosphorylated

glucose enters cell and is immediately

aqueous pore

membrane spanning proteins form an

gated

channels are

ion channels

allow the passage of ions

gated channels

open or close in response to stimulus (chemical or electrical)

3 conditions determine direction of ion channels

1. Relative concentration on either side of

membrane

2. Voltage differences across membrane

3. Gated channels: channel open or closed

voltage gated channel

• conformation dependent on membrane potential

  • electrical signal transmission in plants protozoans and animals

chemical gated channels

• ligand gated; conformation dependent upon binding ligand

• neurotransmitters

stress (mechanical) activated channel

• vibrations open auditory cell channels

• touch recptors