The Plasma Membrane Transport Across the Membrane

Intro. To the Plasma Membrane  separates the living cell from its surroundings.  8 nm thick, controls traffic into and out of the cell.  selectively permeable, allowing some substances to cross more easily than others  Major macromolecules in membranes are lipids, proteins, and some carbohydrates  Made of a bilayer of phospholipids. With polar heads, hydrophilic, and non-polar tails, hydrophobic.

Intro Cont….  The membrane is represented as a fluid mosaic model, a fluid environment with a mosaic of proteins and carbs. embedded or attached that serve several functions.

Membrane Movement and Cholesterol  Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other.  Cholesterol is wedged between phospholipids molecules in the plasma membrane of animals cells. It restrains the movement of the phospholipids in warm temps. and maintains fluidity by preventing tight packing at cold temps.

Cells can Change their Membrane Composition  Cells can modify the lipid make-up of membranes to compensate for changes in fluidity caused by changing temperatures. Ex, winter wheat, increases the percentage of unsaturated phospholipids in the autumn. This lets them prevent their membranes from solidifying during winter.

A Mosaic of Proteins  Membranes are very complex and dynamic containing many different parts.  Proteins decide most of the membrane’s functions.  Contain lipids and carbohydrates also  The collection of molecules in the membrane vary from membrane to membrane  All of the structures in the membrane serve various functions like cell recognition proteins.

2 Types of Proteins  Peripheral proteins are not embedded in the lipid bilayer, they are loosely bounded to the surface.  Integral proteins penetrate, often completely spanning the membrane (a transmembrane protein).

Protein Functions

Carbohydrates in the Membrane  Used for cell to cell recognition, the ability of a cell to distinguish one type of neighboring cell from another. important in cell sorting and organization as tissues and organs in development.  Basis of immune response. Ex. WBC and T-cell response  Membrane carbohydrates are usually branched oligosaccharides with fewer than 15 sugar units  vary from species to species, individual to individual, and even from cell type to cell type within the same individual. Ex. ABO group

Crossing the Membrane  steady traffic of small molecules and ions moves across the plasma membrane in both directions Ex, sugars, amino acids, and other nutrients enter a muscle cell and waste products leave  membranes are selectively permeable so all this traffic is under some control. Esp. the large molecules.  Passage is controlled in part due to the hydrophobic core of the membrane. So other hydrophobic molecules cross easily while polar molecules and ions have difficulty.  Proteins assist and control the transport of ions and polar molecules.

Transport Proteins  ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span the membrane. Some transport proteins have a hydrophilic channel Others bind molecules and carry passengers across the membrane physically  Each transport protein is specific Ex. Gluclose transport in liver. Not fructose.

Passive Transport  Requires no energy from the cell  Substances pass by diffusion or osmosis.  Diffusion is the movement of substance from a high concentration to a low concentration.  This occurs due to kinetic theory. The movement of single particle is random but on a whole they move form high to low.  Osmosis is the movement of water from high to low.

Diffusion of 1 Solute

Passive Transport Cont..  substances will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient. NO ATP NEEDED  Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances. KEY: concentration gradient represents potential energy and drives diffusion. When there is a concentration gradient substances move when there is not the do not move.

Osmosis, Water Movement  Movement of water from high to low concentration.  3 types of solutions: solution with the higher concentration of solutes is hypertonic. solution with the lower concentration of solutes is hypotonic. Solutions with equal solute concentrations are isotonic  direction of osmosis is determined only by a difference in total solute concentration

Osmosis Examples

Osmosis Example: Cell Survival Depends on Osmoregulation

Facilitated Diffusion  Still diffusion so requires no energy or ATP  Many polar molecules and ions that are normally stopped by the lipid bilayer of the membrane diffuse passively with the help of transport proteins.  passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion.

Transport Proteins  much in common with enzymes. They can have specific binding sites for the solute. Transport proteins can become saturated when they are tmoving passengers as fast as they can. Transport proteins can be inhibited by molecules that resemble the normal “substrate.”

Channel Proteins  provide corridors allowing a specific molecule or ion to cross the membrane.  Allow for fast transport  water channel proteins, aquaprorins, make possible massive amounts of diffusion

Gated Channels  open or close depending on the presence or absence of a physical or chemical stimulus.  Ex. neurotransmitters bind to specific gated channels on the receiving neuron, these channels open.  This allows sodium ions into a nerve cell.  When the neurotransmitters are not present, the channels are closed.

Active Transport  Requires ATP, ENERGY.  move solutes against their concentration gradient, from the side where they are less concentrated to where they are more concentrated.  Vital for a cell to keep up its internal concentrations of small molecules that would normally diffuse away.  done by specific proteins embedded in the membranes.

Sodium-Potassium Pumps.  maintains the gradient of sodium (Na + ) and potassium ions (K + ) across the membrane.  Typically, animal cells have higher concentrations of K + and lower concentrations of Na + inside the cell sodium-potassium pump uses the energy of one ATP to pump three Na + ions out and two K + ions in against the gradient.

Sodium-Potassium Pump Action

Examples

Exocytosis and Endocytosis  Ways of getting large molecules in and out of the cell.  Phagocytosis is cell eating and involves solids.  Pinocytosis is cell drinking and involves liquids.

Receptor Mediated Endo. Very Specific