Membrane transport: The set of transport proteins in the plasma membrane, or in the membrane of an intracellular organelle, determines exactly what solutes can pass into and out of that cell or organelle. Each type of membrane therefore has its own characteristic set of transport proteins.

Each type of transport protein transports a particular type of molecules – selective set of solute are transported in or out

Two major classes of membrane transmembrane proteins

Carrier proteins bind a solute on one side and deliver it to the other side through a change in shape.

Cells can also transport macromolecules across the membrane

Channel proteins form tiny hydrophilic pores in the membrane and the specific molecules pass through by diffusion from high to low concentration. Most are ion channels

The ion concentrations inside a cell are very different from those outside

Because ions are electrically charged, their movements can create powerful electric forces across the membrane. Important in nerve cells, muscle cells, and in the mitochondria, for ATP synthesis using the electron transport chain.

Ion transport across cell membranes is of central importance in biology. Cells maintain an internal ion composition very different from that in the fluid around them and these differences

are crucial for the cell’s survival and function.

Animal cells pump Na+ out. If the pumping fails, water flows in by osmosis and causes the cell to swell and burst.

The positive and negative charges must be balanced by an almost exactly equal quantity both inside and outside of the cell

Differential ion concentrations crucial.

nucleic acids, proteins, etc.

Carrier proteins are required for the transport of almost all small organic molecules across the cell membranes. Each carrier is highly selective, often transporting just one type of molecule.

ADP

Each cell, each organelle has particular transport proteins.

The membrane transport proteins studied have polypeptide chains that traverse the lipid bilayer multiple times, forming a continuous protein-lined pathway allowing selected small hydrophilic molecules to cross without coming into contact with the hydrophobic lipid bilayer.

A basic difference between carrier proteins and channel proteins is the way they discriminate between solutes. Channel proteins mainly go on size and electric charge. Carrier proteins actually bind their molecules it transfers, and then changes conformation

Passive transport – no energy needed, with the concentration gradient

Active transport - energy needed, against the concentration gradient

Passive transport with a carrier protein = facilitated transport

Passive transport of glucose: carrier exists in at least to states (shapes). Example: liver cell after a large meal – lots of glucose outside in the extracellular fluid.

Opposite in liver when blood glucose becomes low. Liver cells breakdown glycogen. Glucose levels are higher inside the cell now. Passive transport moves glucose outside the cell.

Electrically charged molecules diffuse according to their concentration gradient and according to their charge – electrochemical gradient.

Membranes typically have a voltage difference across them. Negative inside.

steep gradient

Na+ is at a higher concentration outside of the cell so it tends to enter the cell if given the chance.

K+ is present at a higher concentration inside the cell – therefore there is little movement

inside

outside

Active transport moves solutes against their electrochemical gradients.

againstwith

Animal cells use the Energy of ATP hydrolysis to pump Na+ out of the cell to maintain the electrochemical gradient. This gradient is then used to pump other molecules into or out of the cell against their elecrochemical gradient.

operates ceaselesslyNa+-K+ ATPase or Na+-K+ pump

other ATP driven pumps create electrochemical gradients of H+ ions. Next chapter

Na+-K+ ATPase or Na+-K+ pump

Animal cells use the Na+ gradient to take up nutrients actively.

The Na+-K+ pump helps maintain the osmotic balance of animal cells.

• Cytosolic Ca+2 concentrations are kept low by Ca+2 pumps.

• Influx of Ca+2 is tightly regulated, since Ca+2 binds molecules (enzymes) and alters their activities (activation or inhibition).

• Influx of Ca+2 through Ca+2 channels is often used as a signal to trigger other intracellular events (muscle contraction).

• The cell maintains a low concentration, so that signaling via increases is kept sensitive.

Ion channels are ion selective and gated. Tthey show ion selectivity depending on the diameter and shape of the ion channel and on the distribution of charged amino acids in its lining.. Most ion channels are gated: they can switch between an open and a closed state by a change in conformation, which is regulated by conditions inside and outside the cell.

Ion channels are important in signaling in neurons

If the plasma membrane of animal cells was made permeable to Na+ and K+, the Na+-K+ pump would:A. be completely inhibited.B. begin to pump Na+ in both directions.C. begin synthesizing ATP instead of hydrolyzing it.D. continue to pump ions and to hydrolyze ATP but the energy of hydrolysiswould be wasted, as it would generate heat rather than ion gradients.E. continue to pump ions but would not hydrolyze ATP.

Ca2+ pumps in the plasma membrane and endoplasmic reticulum are important for:A. maintaining osmotic balance.B. preventing Ca2+ from altering the behavior of molecules in the cytosol.C. providing enzymes in the endoplasmic reticulum with Ca2+ ions that are necessaryfor their catalytic activity.D. maintaining a negative membrane potential.E. helping cells import K+.