Ch.4: mvmt of molecules across cell membranes with 100% correct answers already graded A+ 2025-2026

what is diffusion?

passive movement of molecules from one location to another as a result of their RANDOM thermal motion

diffusion direction

net flux always proceeds from higher to lower concentrations

magnitude of diffusion

-concentration gradient

-factors that influence this are

1. temperature

2. mass of molecule

3. surface area

4. medium (denser medium = slower net flux)

Fick's law for diffusion rate =

Ds A dC/dX

rate of diffusion depends on

-size of concentration gradient (dC/dX)

-size (mass) of molecule used to calculate the diffusion coefficient (Ds)

-diffusion area (A)

what happens to diffusion rates when a membrane is present?

the rate will decrease because the membrane acts as a barrier and affects diffusion coefficients (Ds)

what is the major factor that limits diffusion across a membrane?

the hydrophobic interior of its lipid bilayer

which nonpolar molecules diffuse rapidly through the lipid portion of membranes?

oxygen, carbon dioxide, fatty acids, & steroid hormones

what type of molecules do not diffuse readily through membranes?

polar and hydrophilic

what are channels?

pores that are integral membrane proteins

what uses specific protein channels to diffuse into and out of cells?

the ions Na+, K+, Cl-, & Ca2+

diffusion through ion channels

-when ions diffuse through a specific pore in a protein molecule to cross the cell membrane

-specificity depends on pore size of channel, charge, & binding sites

what is channel gating?

-process of opening and closing ion channels

-allows for regulation & controlled movement of ions

-passing through depends on how often and how long the channel is open

types of gated channels

-ligand gated

-voltage gated

-mechanically gated

ligand gated ion channel

binding ligands

voltage gated ion channel

changes in membrane potential

mechanically gated ion channel

mechanical stress

transporters

-specific for their ligand

-do not move as many molecules as channels do bc of binding and conformational shifts

-can be saturated when it reaches a maximum flux of molecules

mediated-transport systems

-Solutes are either too large and/or charged cannot get into the cell; needs help

-protein transporters (carriers) bring into & out of cells

-conformation changes in transmembrane protein

rate of transport depends on

-solute concentration

-affinity

-rate of conformational change

active transport

-uses ATP to move molecules against the concentration gradient

-transporters called pumps

-can become saturated

-defined by use of 2 types of energy sources

1. direct use of ATP in primary active transport

2. use of eletrochemical gradient across a membrane to drive the process in secondary active transport

primary active transport proteins found in most cells

Na+/K+-ATPase

Ca2+-ATPase

H+-ATPase

H+/K+-ATPase

Na+/K+-ATPase pump

-primary active-transport protein that hydrolyzes ATP and releases energy used to transport sodium ions out of cell and potassium ions in

-found in every cell & helps establish & maintain membrane potential of cell

Why is the Na+/K+-ATPase pump considered an active transporter?

-Na+ is a major extracellular ion

-K+ is a major intracellular ion

secondary active transport

-uses pre-existing electrochemical gradient across a plasma membrane as energy source to transport the ion coupled to transporter

-transporters have 2 binding sites:

1. an ion (Na+)

2. transported molecule (glucose)

-ion binds cause conformational change to open another binding site

cotransporters

aka symporters; move molecules in the same direction

countertransporters

aka antiporters; move molecules in opposite directions

osmosis

-net diffusion of water across a membrane

-facilitated by aquaporins which form channels for diffusion of water

what is an aquaporin?

Channel proteins that facilitate H2O diffusion; more of these allows for more permeability

isotonic solution

have same concentration of nonpenetrating solutes as normal extracellular fluid

hypotonic solutions

have a lower concentration of nonpenetrating solutes as normal extracellular fluid

hypertonic solutions

have a higher concentration of nonpenetrating solutes as normal extracellular fluid

endocytosis

movement of molecules into the cell via vesicles

what are the general types of endocytosis that may occur in a cell?

1. fluid endocytosis (pinocytosis)

2. phagocytosis (eating bacteria)

3. receptor-mediated endocytosis

exocytosis

movement of molecules out of the cell via vesicles

what are the functions of exocytosis for cells?

1. replace portions of plasma membrane that endocytosis has removed

2. adds new membrane components to membrane

3. provides a route by which membrane impermeable molecules synthesized by the cell can be secreted into extracellular fluid

epithelial transport

-molecules entering or leaving the body

-molecules moving between compartments

-molecules must cross a layer(s) of polarized epithelial cells

epithelial transport movement can occur

through cells (transcellular transport) or around cells (paracellular transport)

paracellular transport

-movement around cells

-minimized by tight junctions of epithelium

-regulated by changing tightness of junctions by claudins

what are claudins?

junctional proteins that form holes or pores to allow water, small ions, or small uncharged molecules to pass by paracellular transport

transcellular transport

-has to cross 2 membranes so it is a 2 step process:

1. "uphill" step; requires energy

2."downhill" step; passive and requires no energy

-regulated by adding/removing transporters on cell membrane

transcellular transport of glucose

-glucose transport from intestinal lumen or kidney tubule to extracellualr fluid

-involved 3 transport systems

1. sodium-glucose cotransporter

2. GLUT transporter

3. sodium-potassium ATPase pump (Na+/K+ ATPase pump)

sodium-glucose cotransporter

-secondary active transport

-brings glucose into cell against its gradient using sodium concentration gradient & kinetic energy

GLUT transporter

-facilitated diffusion

-transfers glucose to ECF by facilitated diffusion

sodium-potassium ATPase pump

-active transport

-pumps sodium out of cell to keep intracellular fluid sodium concentrations low

transcytosis

-vessicle transport

-large molecules that can't cross epithelium have to be transported this way

-molecule is brought into cell by receptor-mediated enodcytosis to form vesicle

-vesicular transport allows vesicles to cross the cell with the help from microtubules of cytoskeleton

-vesicle contents release into interstitial fluid by exocytosis

membrane potential: electricity

- Principles of electricity:

1. Human body is electrically neutral

- The net amount of electrical charge is zero

- For every (+) charge there is a (-) charge

2. Opposite charges (+ and -) are attracted to each other

3. Like charges (+ and +; - and -) repel each other

4. Separating (+) and (-) charges requires energy (ATP)

5. When separate, (+) and (-) charges can freely move toward each other through a conductor (water)

membrane potential

-defined as separation of electrical charges that exists across plasma membrane

-provides electrical force that influences movement of ions across membrane

-electrochemical gradient

what is a main contributor to the resting membrane potential?

K+

K+ ions and their contribution to the resting membrane potential

-membrane is more permeable to this ion

-K+ leaks out of cell down concentration gradient

-excess -ve charge buildup in cell bc they can't cross membrane and electrical gradient is formed

-ve charges attract K+ ion sback into cell down electrical gradient

-net movement of K+ stops and membrane potential at the electrical gradient that opposes the chemical gradient is known as equilibrium potential

equilibrium potential (E)

the membrane potential at which chemical and electrical forces are balanced for a single ion

resting

steady state electrical gradient seen in all living cells where cells are more negatively charged inside than outside

potential

electrical and chemical gradient created by active transporters in the membrane & represents a stored or potential energy