Quiz 2/Exam 2 Membrane Transport

proton pump in plant cells?

Another important transporter is the sucrose-proton (sucrose-H+) cotransporter in plant cells. As positively charged hydrogen ions are pumped out of the cell by the proton pump, the membrane potential changes. This new electrochemical gradient in which the voltage is more positive outside the cell than inside the cell drives the sucrose-H+ cotransporter. Protons are moved into the cell down the gradient helping to move sucrose into the cell. Sucrose and protons move into the cell together. As protons move back into the cell with sucrose, the proton pump is reactivated maintaining the electrochemical gradient across the plasma membrane.

concentration gradient

Areas of high and low concentration in the same general area. (the unequal concentration of solution)

channels

Channels are tunnel-like structures through which molecules or ions can pass

electrochemical gradient

Difference in concentration of ions across a membrane maintained by energy activated proton pump cotransporters. (like Na/K pump)

Compare and contrast diffusion and osmosis.

Diffusion and osmosis are two mechanisms by which the concentration of solute and solvents are maintained within a cell. Diffusion is the passive movement of molecules away from a source of origin down a concentration gradient. Osmosis, in contrast, is the movement of water across a barrier to equalize the concentration of solute on both sides of the barrier. It is a specialized form of diffusion.

Temperature and molecule size affect diffusion

Diffusion rates of molecules vary with temperature and molecule size. As temperature rises, molecules diffuse faster. However, the diffusion rate of smaller molecules increases more quickly than the diffusion rate of larger molecules

gated channels

In contrast, gated channels require an electrical or chemical stimulus to open. Whether open or gated, channels are highly selective. They allow the transport of only one substance or a small number of related substances.

facilitated diffusion

In facilitated diffusion, polar molecules and ions unable to cross the membrane on their own can diffuse with the help of transport proteins called channels. Facilitated diffusion via open channels requires no energy investment to accomplish the transfer, and therefore is a type of passive transport. Channels, tunnel-like structures through which polar molecules or ions can pass, are large protein molecules that span the membrane. Some channels are always open, allowing molecules and ions to move from high-concentration solutions to those of lower concentration.

osmoregulation example in human

In humans, osmoregulation depends on tight control of water levels by the kidneys. The kidneys regulate water and solute balance in the body so that cells are maintained in a relatively constant aqueous environment. Therefore, the kidneys ultimately control cell tonicity, which is a measure of the water potential of two solutions separated by a semipermeable membrane.

passive transport

In passive transport, substances diffuse across a plasma membrane without the need for cellular energy. Small polar molecules and hydrophobic molecules can move across the phospholipid bilayer, but most large polar molecules and charged molecules must find a hydrophilic channel to move through. This diffusion occurs through specified channels down a concentration gradient and is responsible for much of the traffic in and out of a cell.

Facilitated transport pumps can perform three types of transfers

In uniport, one molecule or ion type is transported either into or out of the cell down its concentration gradient. In symport, two or more different molecules or ions are moved in the same direction. In antiport, two different molecules or ions are moved in opposite directions. Symport and antiport are both co-transports.

Tonicity in plant vs animal cells

Isotonic- Animal cells typically need to be in an isotonic environment. In contrast, when in an isotonic solution with no net diffusion of water, plant cells become flaccid — a lack of pressure pushing the membrane against the cell wall causes the cell to become limp, an appearance you will recognize in a house plant that needs to be watered. hypotonic- Water diffuses into the cell, often causing animal cells to lyse, or burst.Plants are less vulnerable to the effects of hypotonicity because their cell walls prevent the cells from bursting. The slow, steady uptake of water in a hypotonic environment helps a plant cell maintain turgidity. In the turgid state, internal water pressure pushes the plasma membrane against the cell wall, much like the inner tube of a tire presses against the outer tire wall. Once a cell is turgid, internal hydrostatic pressure prevents more water from entering. Hypertonic- In a hypertonic solution water leaves the cell, causing it to shrink. Plant and animal cells respond similarly to hypertonic conditions.

tonicity

Measure of the water potential of two solutions separated by a semipermeable membrane.

diffusion

Movement of materials from areas of generally higher concentrations to areas of generally lower concentrations; requires no input of energy.

osmosis

Movement of water through a semipermeable, or selectively permeable, membrane.

Na+/K+ pump work?

Na+ from the cell cytoplasm binds to the pump. The binding stimulates phosphorylation of the pump by hydrolysis of ATP. Phosphorylation is the addition of a phosphate group to a protein. As the charges on the phosphate group repel other charges in the protein, the protein changes its shape, and Na+ is released outside the cell. K+ binds to the pump on the extracellular side. This triggers release of the phosphate group on the intracellular side of the pump, returning it to its original shape. K+ is released into the cell cytoplasm, and the affinity for Na+ is restored. The pump is ready to repeat the cycle.

what happens in osmosis of sugar and water with a semipermeable membrane

Osmosis occurs when free water molecules diffuse through a membrane. A membrane separates two solutions, one with a higher concentration of dissolved solute (sugar) than the other. Although the same volume of solution is present on both sides of the membrane, the higher concentration of solute on one side of the membrane binds more water molecules. Therefore, the free water molecules are out of equilibrium. They diffuse through the membrane to the side with the higher concentration of solute but lower concentration of free water. In doing so, the water itself is moving from high to low concentration. Ultimately, equilibrium is reached with the same concentration of solute on both sides, despite the different volumes of the two solutions. MORE IN DEPTH:When two solutions with different concentrations of sugar are separated by a selectively permeable membrane, free water molecules are able to pass through the membrane, whereas sugar molecules cannot cross because of their larger size . Hydrophilic or "water-loving" solutes bind to water molecules, preventing those water molecules from crossing the membrane. Only the water molecules free of solute molecules can diffuse down the concentration gradient across the membrane. In this case, osmosis occurs and free water molecules diffuse through the membrane until the solution concentration is equalized on both sides of the membrane. The pressure (force per unit area) needed to prevent water passing through a membrane to a more concentrated solution is called osmotic pressure.

sodium-potassium pump (Na+/K+ pump)

Present in animal cells that maintain a low sodium and high potassium ion concentration in the cell; involved in signal transduction in the organism. The carrier protein transports three sodium ions out of a cell and two potassium ions into the cell. Both ions are moved against their concentration gradient. One third of the cell's energy is used to keep the ions at their optimum concentrations. The Na+/K+ pump maintains the concentration and charge gradients that are critical in cells responsible for muscle contraction and transmitting nerve impulses

turgid

Sufficient pressure to make the cell swollen or firm as the result of water uptake (hypotonic). The cell wall is strong enough to prevent more expansion and further uptake of water.

Explain how membrane potentials are generated and maintained.

The Na+/K+ pump actively transports potassium and sodium ions into and out of the cell, respectively. This protein is found in all animal plasma membranes. The cell uses the Na+/K+ pump to generate a membrane potential. More positive ions are pumped out of the cell than into it. The difference creates electrochemical potential that is employed in regulating or triggering muscle contraction and transmitting nerve impulses. Membrane potentials can also be used to cotransport molecules, as in the sucrose-proton cotransporter.

Different types of tonicity

The exterior environment of the cell relative to the cell interior can be: Hypertonic: less water inside. A greater solute concentration outside than inside Hypotonic: more water inside. A lesser solute concentration outside than inside Isotonic: equal solute concentrations outside and inside.

Acetylcholine

The nicotinic acetylcholine (nACh) receptor forms a gated ion channel in the plasma membrane. When no external signal is present, the pore is closed. When acetylcholine molecules bind to the receptor, they trigger a conformational change (structural change) that opens the aqueous pore and allows ions to flow into the cell.

active transport

The process of active transport moves molecules against a concentration gradient across the plasma membrane, transporting them into or out of a cell. In contrast to passive transport, active transport requires energy, provided by the HYDROLYSIS of ATP.

osmoregulation

To maintain homeostasis, cells must control which substances pass through the membrane. In particular, the cell must regulate the concentration of solutes. Osmoregulation is the process by which living organisms control the internal concentration of water and solutes. In humans, osmoregulation depends on tight control of water levels by the kidneys. The kidneys regulate water and solute balance in the body so that cells are maintained in a relatively constant aqueous environment. Therefore, the kidneys ultimately control cell tonicity, which is a measure of the water potential of two solutions separated by a semipermeable membrane.

dynamic equilibrium

When equal concentrations are achieved on both sides of a membrane, there is no longer a concentration gradient, so the substance reaches dynamic equilibrium. There is no net movement across the membrane in dynamic equilibrium, but individual molecules or atoms continue to move randomly across the membrane.

aquaporins

aquaporins are a family of channels that regulate water moving in and out of the cell . These channels make the membrane more permeable to water and speed its transport.