Chapter 12: Transport across cell membranes - Very impermeable to all charged molecules including
inorganic ions due to their strong attraction to water
- Vast majority of substances cannot diffuse in and out of cell so they
are transported by membrane transport proteins The Ion Concentrations Inside a Cell Are Very Different from Those Outside
- Cells membranes control traffic of inorganic ions and small water-
- Because they are impermeable to inorganic ions, intracellular ionic
soluble molecules. They can also selectively transfer
conc is very different than its surrounding environment
macromolecules such as proteins
- This is crucial for cell survival and function
- Two main classes of transfer proteins: Transporters and channels
- Na+, K+, Ca2+, Cl–, and H+ (protons) are the most important ions
- Transporters: shift small organic molecules or inorganic ions from
- Mostly important for producing ATP and nerve cell communication
one side of the membrane to the other by changing shapes
- Na+ is the most abundant positive ion outside the cell
- Channels: form tiny hydrophilic pores across the membrane
- K+ is the most abundant positive ion inside the cell
through substances can pass by diffusion
- Quantity of positive charge inside must be balanced equally by
- Most channels are ion channels because they only allow passage for
negative charge outside
electrically charged ions
- High conc of Na+ outside the cell is balanced by extracellular Cl-
Principles of Transmembrane Transport - And K+ conc inside is balanced by a variety of negative organic and
inorganic ions including nucleic acids and proteins
- Hydrophobic interior of lipid bilayer creates a barrier to most
hydrophilic molecules and all ions Differences in the Concentration of Inorganic Ions Across a Cell Membrane
- But many hydrophilic substances such as inorganic ions, sugars, Create a Membrane Potential
amino acids, nucleotides diffuse the lipid bilayer very slowly
- Tiny excess of positive or negative charge concentrated in the
- Facilitated transport is a process by which hydrophilic substances
neighbourhood of the plasma membrane do occur and these
are quickly transported through specialized membrane proteins
electrical imbalances create voltage difference across the
Lipid Bilayers Are Impermeable to Ions and Most Uncharged Polar membrane called membrane potential
Molecules - Resting membrane potential is the unstimulated state of
membrane when the cations and anions across membrane is
- Any molecule can simply diffuse through a lipid bilayer but it takes
perfectly balanced
forever depending on the size and solubility property
- Resting membrane potential is not zero
- Small, hydrophobic, and nonpolar substance diffuse quickly and
- In animals it is between -20 mV to -200 mV
easily across a membrane
- Resting membrane potential is negative because the interior of the
- But many substances that are needed by the cells are polar and
cell is more negative
water soluble and thus these solutes cannot pass the bilayer
- This membrane potential allows cells to power the transport of
without the aid of transport proteins
certain metabolites and provides those cells that are excitable with a
- Permeable to small nonpolar molecules such as oxygen and
means to communicate with their neighbors.
carbon dioxide dissolve readily in bilayer and diffuse them
- Uncharged polar molecules like water and ethanol are also
able to diffuse easily compared to larger uncharged
molecules
, Cells Contain Two Classes of Membrane Transport Proteins: Transporters - In addition to membrane potential, concentration gradient also
and Channels plays a role in their movement
- This net driving force is called the electrochemical gradient
- Most membrane transport proteins have chains that
- Some ions will have the voltage and concentration gradient working
transverse/multipass the membrane many times; they are called
in the same direction
multipass transmembrane proteins
- Example: Na+ is positively charged so membrane potential will pull
- Two main classes of transmembrane proteins: transporters and
it into the cell and it is also concentrated outside the cell so it will
channel proteins
move in down its conc gradient
- Channel proteins are selective towards size and electric charge; ions
- Thus Na+ has a steep electrochemical gradient
and molecule with appropriate charge and size can pass through an
- Some ions will have the voltage and concentration gradient working
open channel
in opposite directions
- Transporter proteins are selective in the way that they only
- Example: K+ is positive so the membrane potential will pull it into
transport ion or molecules that fit the binding sites
the cell but it is concentrated more inside the cell so it tends to
- They bind their solutes with great specificity
move out down its conc gradient
Solutes Cross Membranes by Either Passive or Active Transport - Thus K+ has a small electrochemical gradient so there is a small net
movement of K+
- Direction of ion/molecule flow depend on the relative
concentrations of the solute on either side of the membrane Water Moves Passively Across Cell Membranes Down Its Concentration
- Passive pathway: spontaneous movement from concentrated region Gradient—a Process Called Osmosis
to low concentrated region; this is passive transport
- Water molecules can directly diffuse through lipid bilayer because
- No energy is required for passive transport
they are small and uncharged but slowly
- All channel proteins and some transporter proteins carry passive
- Some plasma membrane has aquaporins, specialized channel
transport
proteins for the facilitation of water
- Active pathway: movement of ion/molecule through special
- Cells contain a high conc of solutes
proteins called pumps against their concentration gradient
- There’s more solute inside than the outside the cell – osmolarity
- Energy needed for active transport comes from the hydrolysis of
- So, water generally moves into cells down its conc gradient
ATP
- Osmosis – movement of water from a region of low solute conc to a
Both the Concentration Gradient and Membrane Potential Influence the region of high solute conc
Passive Transport of Charged Solutes - Too much entry of water can cause the cell to swell
- Different cells have different mechanisms to deal with excess water
- For uncharged molecules, direction of passive transport depends entry
entirely on its concentration gradient - Most animal cells have gel like cytoplasm that can resist swelling
- But for electrically charged molecules there are additional criteria - Water protozoans eliminate water using contractile vacuoles that
- Membrane potentials apply force to any charged molecule discharge water to the exterior
- Membrane potential tends to pull positive molecules into the cell - Plant cell prevent swelling using their tough cell walls that can
(negative cytosol) and drive away negative charged molecules out of tolerate large osmotic difference across their plasma membrane
the cell
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