Cell Biology Chapter 8 Flashcards
What is osmolarity?
Osmolarity is the total solute concentrations inside versus outside of the cell; Hypotonic-If the solute concentration is lower outside the cell; hypertonic-If the solute concentration is higher outside the cell; isotonic-will shrink and shrivel if moved to a hypertonic solution and will swell and perhaps burst (or lyse) if placed in a very hypotonic solution
Know the different mechanisms are involved in moving solutes across membranes.
Simple diffusion - most solutes cannot cross the membrane this way; Transport proteins assist most solutes across membranes; These integral membrane proteins recognize with great specificity the substances to be transported; Facilitated diffusion (or passive transport) – Transport proteins can move solutes against the concentration gradient: From LOW to HIGH; Active transport - Energy forms used: As that released by the hydrolysis of ATP; By the simultaneous transport of another solute down an energy gradient;
Which ones need energy and their relation to the concentration gradient. The different types within each mechanism
Which ones need energy and their relation to the concentration gradient, the different types within each mechanisms
The movement of a molecule that has no net charge is determined by its concentration gradient; Simple diffusion and facilitated diffusion involve movement “down” the concentration gradient; Active transport involves endergonic movement “up” the concentration gradient ; The Electrochemical Potential; The movement of an ion is determined by its electrochemical potential: the combined effect of its concentration gradient; the charge gradient across the membrane; The active transport of ions across a membrane creates a charge gradient, or membrane potential across the membrane; Transport proteins are large, integral membrane proteins with multiple transmembrane segments; Channel proteins form hydrophilic channels through the membrane to provide a passage route for solutes; Carrier proteins (transporters or permeases) bind solute molecules on one side of a membrane, undergo a conformation change, and release the solute on the other side of the membrane
Membrane Potential, membrane charges in and out the cell
Different concentrations of ions between the inside and the outside of cell is maintained by plasma membrane; A small difference in voltage between the inside (negative charge) and outside (positive charge) of a cell that normally occurs
K+ and Na+ diffusion and pump mechanisms
Most of membrane potential (~80%) is caused by the diffusion (passive) of K+ and Na+ down the concentration gradient; The pump (active) itself makes only a small direct contribution (~ 20%); The membrane potential is maintained by active transport of potassium ions inward and sodium ions outward; Active transport of K ->IN & Na -> OUT
Glucose facilitated transport
The glucose transporter is a uniport carrier for glucose; The anion exchange protein is an antiport anion carrier for Cl– and HCO3–; Both are found in the plasma membrane of red blood cells; d-glucose collides with and binds to GLUT1 in the T1 conformation; GLUT1 shifts to the T2 conformation; The conformational change causes the release of glucose; GLUT1 returns to the T1 conformation; A carrier protein can facilitate transport in either direction; The direction of transport is dictated by the relative solute concentrations outside and inside the cell; Glucose concentration is kept low inside most animal cells; Glucose is immediately phosphorylation upon entry into the cell; Once phosphorylated, glucose cannot bind the carrier protein any longer and is effectively locked into the cell
Red Blood cells: CO2, HCO3, Cl, O2; in cells and lungs
The anion exchange protein (also called the chloride-bicarbonate exchanger) facilitates reciprocal exchange of Cl– and HCO3– ions only; Exchange will stop if either anion is absent; The “Ping-Pong” Mechanism- The anion exchange protein is thought to alternate between two conformational states; In the first state, the protein binds a chloride ion on one side of the membrane, which causes a change to the second state; In the second state, the chloride is moved across the membrane and released; The release of chloride causes the protein to bind bicarbonate; The binding of bicarbonate causes a shift back to the first conformation; In this conformation, bicarbonate is moved out of the cell, allowing the carrier to bind chloride again; In tissues, waste CO2 diffuses into the red blood cells, where it is converted to HCO3– by the enzyme carbonic anhydrase; As the concentration of bicarbonate rises, it moves out of the cell, coupled with uptake of Cl– to prevent a net charge imbalance; In the lungs, the entire process is reversed
Na+/ glucose symporter
The Na+/glucose symporter is an example of indirect active transport; Uptake of Glucose via Sodium Symport Requires Energy driven by the Sodium/Potassium Pump; Not direct energy!
1. Two external Na+ ions bind their sites on the symporter, which is open to the exterior
2. This allows one molecule of glucose to bind
3. A conformational change in the protein exposes the glucose and Na+ inside the cell
4. The two Na+ ions dissociate in response to the low internal Na+ concentration
5. This locks the symporter in its inward-facing conformation until the glucose dissociates
6. The loss of glucose frees the symporter to return to the outward-facing conformation
Neuron Communication
Neurons use electrical and chemical signals to communicate; Ions use ions channels to enter or exit the neuron; Voltage-gated ion channels - Ion channels that change their structure in response to voltage changes; Membrane potential
The difference in total charge between the inside & outside of the cell; Voltage-gated ion channels open in response to changes in membrane voltage; After activation ->They become inactivated for a brief period and will no longer open in response to a signal; The Na+/ K+ pump brings 2 K+ions into the cell while removing 3 Na+ions per ATP consumed
As more cations are expelled from the cell than taken in, the inside of the cell remains negatively charged relative to the extracellular fluid
Action Potential
The formation of an action potential can be divided into five steps: (1) A stimulus from a sensory cell or another neuron causes the target cell to depolarize toward the threshold potential. (2) If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes. (3) At the peak action potential, K+ channels open and K+ begins to leave the cell. At the same time, Na+ channels close. (4) The membrane becomes hyperpolarized as K+ ions continue to leave the cell. The hyperpolarized membrane is in a refractory period and cannot fire. (5) The K+ channels close and the Na+/K+ transporter restores the resting potential. Key: Resting potential: K+ leak channels open, Na+ channels closed, -70mV. Depolarization: Na+ channels open, Na+ influx, +40mV. Repolarization: Na+ channels inactivate. K+ channels open, -80mV then -70mV.
Transmission of Action Potential
1) Action Potential reaches end of axon
2) Calcium gates open leading to an influx of Ca2+
3) Vesicles filled with acetylcholine [AcCh] (a type of neurotransmitter) signaled for exocytosis
4) AcCh binds AcCh receptors on receiving cell’s membrane.
5) Depolarization (Na+ gates open) leading to another action potential
Simple Diffusion
Simple diffusion - the unassisted net movement of a solute from high to lower concentration; Typically, this is only possible for: Gases, Nonpolar molecules, Small polar molecules such as water, glycerol, or ethanol; Diffusion Always Moves Solutes Toward Equilibrium; No need of energy for movement!
Facilitated Diffusion
In facilitated diffusion transport proteins are used to move solutes as dictated by its concentration gradient with NO need of energy; The role of the transport proteins is just to provide a path through the lipid bilayer, allowing the “downhill” movement of a polar or charged solute; It follows the concentration gradient as simple diffusion, but because of the molecule characteristics it needs a protein to facilitate the transport across (diffusion); Transport or carriers - Undergo conformational change; only open to one side of membrane at a time; Channel - Conformational change to open/close; open to both sides at once; Transport proteins are large, integral membrane proteins with multiple transmembrane segments; Carrier proteins (transporters or permeases) bind solute molecules on one side of a membrane, undergo a conformation change, and release the solute on the other side of the membrane
Active Transport
Active transport is used to move solutes up a concentration gradient, away from equilibrium; Active transport uses energy, usually through ATP hydrolysis; Active transport performs 3 important cellular functions: Uptake of essential nutrients, Removal of wastes; Maintenance of nonequilibrium concentrations of certain ions; Active transport allows the creation and maintenance of an internal cellular environment that differs greatly from the surrounding environment; Pumps are membrane proteins involved in active transport; They are called pumps because energy is required to move substances against their concentration gradients; Active transport differs from diffusion (both simple and facilitated) in the direction of transport; Diffusion is nondirectional with respect to the membrane and proceeds as directed by the concentrations of the transported substances; Active transport has an intrinsic directionality; The Coupling of Active Transport to an Energy Source May Be Direct or Indirect; Active transport mechanisms can be divided based: on the sources of energy; whether or not two solutes are transported at the same time; Active transport is categorized as: Direct, Indirect
