Ch. 12 Transport Across Cell Membrane

The plasma membrane is involved in cell communication, the import and export of molecules, and cell growth.
  1. ==Receptor proteins== in the plasma membrane act as ==sensors== that enable the cell to receive information about changes in its environment and respond to them.
  2. If a cell is to survive and grow, nutrients must pass inward across the plasma membrane, and waste products must pass out. The ==highly selective channels and pumps (protein molecules)== allow for specific substances to be imported and others to be exported.
  3. The flexibility of the membrane and its capacity for expansion allow cell growth and cell movement.
Specialized membrane transport proteins are responsible for transferring small water-soluble molecules across the cell membrane
  • ==Protein-free artificial lipid bilayers== are ==impermeable== to most water-soluble molecules except a few solutes such as ==CO2, O2, and H2O.==
  • Cell membranes are permeable to many water-soluble molecules through specialized membrane transport proteins
  • Each type of transport protein transfers a ==particular type of molecule==, causing a ==selective set of solutes== to end up inside the membrane-enclosed compartment
  • Cells must import nutrients, eliminate metabolic waste products, and regulate the intracellular concentrations of a variety of ions.
The difference in ion composition between a cell’s interior and its environment
  • ==inorganic ions such as Na+, K+, Ca2+, Mg2+, Cl-, and H+== (protons) are the most plentiful of all the solutes in a cell’s environment.
  • The movement of ions across the cell membrane plays an essential role in many biological processes, including the activity of nerve cells and ATP production by all cells
  • ==Na+ is the most plentiful positively charged ion (cation) outside the cell, while K+ is the most plentiful inside==
  • ==The high Na+ outside is balanced mainly by extracellular high Cl-, which high K+ inside is balanced by a variety of negatively charged intracellular ions (anions)==
The ==rate== at which a molecule diffuses across a synthetic lipid bilayer depends on its ==size== and ==solubility.==
  • In general, the smaller the molecule, the more soluble it is in ==oil==, the more rapidly it will diffuse across the membrane
  • ==Small hydrophobic molecules== (O2, CO2, and N2) and ==small uncharged polar molecules== rapidly cross the bilayer.
  • Hydrophobic interior of the lipid bilayer creates a barrier to the passage of ==ions== and larger ==unchanged polar molecules==
  • Synthetic bilayers are a billion times more permeable to H2O than they are even to such small ions as Na+ or K+.
Small molecules and ions can enter the cell through a transporter or a channel
  • Each protein provides a highly ==selective passageway== across the membrane for a particular class of molecules, such as ions, sugars, or amino acids,
  • Membrane transport proteins can be divided into 2 main classes: ==transporters and channels==
  • A ==transporter== is a membrane protein that undergoes a series of ==conformational changes== to transfer small water-soluble molecules across the lipid bilayer
  • A ==channel== is a ==hydrophilic pore== across the lipid bilayer, with walls made of protein, through which specific ions or small molecules can diffuse.
  • ==Channels transfer molecules at a much faster rate than transporters==
  • Ion channels can exist in either ==an open or a closed conformation== and they transport only in the open conformation.
Transport proteins: Transporter and Channel Protein
  • ==Transporter==: undergoes conformational changes to transfer specific molecules across the membrane
  • ==Channel== has a hydrophilic pore, through which specific molecules can diffuse.
Protein Transport
  • ==Passive Transport:== no energy is required
    • if a molecule moves from an area of high concentration to an area of low concentration, then the molecule is moving ==down== its concentration gradient
    • ==Simple diffusion==: no transport protein required
    • ==Facilitated diffusion==: transport proteins required
  • ==Active transport:== requires energy
    • if a molecule moves from an area of low concentration to an area of high concentration, then the molecule is moving ==up== its concentration gradient.
Both ==concentration gradients== and ==electrical forces== drive passive transport.
  • The electrochemical gradient is determined by 2 factors:
  1. The concentration gradient of an ion
  2. The distribution of positive and negative charges on either side of the membrane is called the membrane potential.
3 Types of Active Transport
  1. ==Coupled Transport==- the movement of one molecule down its concentration gradient is coupled to the movement of another molecule up its concentration gradient.
  2. ==ATP-driven pump==- the movement of one molecule up its concentration gradient is coupled to the ==hydrolysis of ATP to ADP==
  3. ==Light-driven pump== (most are in bacteria)- the movement of one molecule up its concentration gradient is coupled to the ==light energy.==
The Na+-K+ pump plays a key role in membrane transport in animals cells
  • Why does a cell need to control the concentration of Na+ and K+ across its membrane?
  1. It is important for a cell to maintain a high concentration of Na+ ion on the extracellular side of the membrane, ==as the movement of Na+ into the cell is coupled to the movement of other molecules across the membrane==
  2. It is also important for a cell to maintain a high concentration of K+ on the cytoplasmic side of the membrane for the proper functioning of the nervous system.
The Na+-K+ pump transports ions in a cyclic manner

==ATPase==: enzyme that converts ATP To ADP

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Another ion transporter: the Ca2+ pump
  • intracellular ==signaling often involves an increase of intracellular (cytosolic) Ca2+==, therefore, the intracellular concentration of Ca2+ must be kept ==low==
Coupled Transport vs Uniport

Transport proteins perform 2 different types of couple transport:

  1. ==Symport==: the 2 molecules are transported in the same direction across the membrane
  2. ==Antiport==: the 2 molecules are transported in the opposite direction across the membrane

   These vary from the ==uniport==, in which only one molecule is transferred across the membrane.

The glucose-Na+ symport protein uses the electrochemical Na+ gradient to drive the import of glucose
  • the high extracellular concentration of Na+ can be used for symport, such as glucose/Na+ pump