Cell Membrane Transport

What aspects play a role in permeability?

1. specific transport proteins

2. Discriminating barrier of lipid bilayer = selective permeability

Who gets in?

small, nonpolar, hydrophobic molecules (hydrocarbons, CO2, O2)
use passive transport (no energy)

Who needs help?

polar, hydrophilic molecules
some use passive but need special channel/protein
use active transport (energy)

What is passive transport?

movement of substances across membrane with no energy
molecules move from high to low (down concentration gradient)W

What are the 3 mechanisms of passive transport?

1. simple diffusion

2. facilitated diffusion

3. osmosis

What is simple diffusion?

passive, no energy
small, non-polar, hydrophobic
down concentration
dynamic equilibrium = as many molecules cross membrane in one direction as other

What is facilitated diffusion?

passive, no energy
require transport protein
molecules move down gradient
small, polar, ion (charged)

What transport proteins are required in facilitated diffusion?

channel proteins and carrier proteins

What are channel proteins in facilitated diffusion?

S: top, bottom and inner core composed of hydrophillc AA → attract ions &/or polar
some opan all the time
others gated
examples:
aquaporins → specific to H2O
muscle cells gated channels allowing muscle contraction when opened

What are carrier proteins in facilitated diffusion?

all are specific to single substance
bind to substance, change shape and carry it across membrane
allow movement in either direction as gradient changes
examples
glucose transport proteins (GLUTS)

What is osmosis?

passive (no energy)
require transport protein
down gradient
water

diffusion of water across membrane
occur when solute cannot pass through membrane

What is tonicity?

extracellular solution can change volume of cell by affecting osmosis

What is hypertonicity?

extracellular fluid higher osmolarity than cytosol
water leaves cell

What is isotonicity?

extracellular fluid and cyotsol same osmolarity
water doesn’t move

What is hypotonicity?

extracellular fluid lower osmolarity than cytosol
water enters cell

What is osmoregulation?

cell walls (plants, fungi, bacteria and some protists) prefer hypotonic extracellular solutions
creates turgor pressure that is critical to organismal growth and functions
hypertonic solutions cause plasmolysis → plasma membrane detaches from cell well (shriveled plants)

What is active transport?

anytime ion or molecule transported through membrane protein
against concentration or electrochemical gradient
requires energy

What are the three types of active transport?

primary → ATP provides energy
secondary → electrochemical gradient provides energy
bulk transport → large or many molecules at once

What is primary active transport?

moves ion or molecule up gradient from ATP
examples: Na+, K+ pump → 3 Na out, 2 K+ in using 1 ATP

What is secondary active transport?

uses electrochemical gradient created by primary transport to move substances against concentration gradient

What is bulk transport?

molecules/particles too large to pass through transport protein
requires energy

What are the two mechanisms of bulk transport?

endocytosis → going into cell
exocytosis → leaving cells

What are the 3 types of endocytosis?

• phagocytosis

• pinocytosis

• receptor mediated endocytosis

What is phagocytosis?

• cellular eating

• cell membrane surrounds particle and engulfs it

What is pinocytosis?

• cellular drinking

• membrane invaginates, surrounds small volume of fluid and pinches off

What is receptor-mediated endocytosis?

• uptake of specific substance is targeted by binding to receptors on external surface of membrane

What is exocytosis?

• vesicles containing substances fuse with plasma membrane

• contents release to exterior of cell