ocular anatomy quiz 2

what are the 3 processes used to transport substances through the cell membrane? do they require energy?

diffusion (no energy needed)

osmosis (no energy needed)

active transport (energy required)

what is simple diffusion? does it involve other molecules?

the movement of molecules along their concentration gradient without the direct involvement of any other molecules

true or false. simple diffusion can involve either the spreading of a material through a medium or the transport of a particle across a membrane.

true

in most biological systems, diffusion occurs across a semi-permeable membrane made of a _______. the membrane has pores and openings to allow the passage of specific molecules.

lipid bilayer

what is required for facilitated diffusion to occur?

the presence of another molecule (the facilitator)

what type of transport is necessary for the movement of large or polar molecules across the hydrophobic lipid bilayer?

facilitated diffusion

why is facilitated diffusion necessary for the biochemical processes of every cell?

because there is communication between various subcellular organelles

while gases and small molecules like methane or water can diffuse freely across a plasma membrane, larger charged molecules like carbohydrates or nucleic acids need the help of __________ forming pores or channels.

transmembrane proteins

Since they are relatively large openings in the plasma membrane, these integral membrane proteins also have high specificity. For instance, the channel protein that transports potassium ions has a much ____ affinity for that ion than a very similar sodium ion, with nearly the same size and charge.

higher

Osmosis is the spontaneous movement of solvent molecules, typically water, through a semipermeable membrane from an area of ____ solute concentration to an area of _____ solute concentration, aiming to equalize the concentrations on both sides of the membrane.

lower, higher

which process is essential in biological systems for the absorption and retention of water and nutrients?

osmosis

what is the primary difference between diffusion and osmosis?

In diffusion, any type of particle, whether it be a gas molecule, ion, or solute, can move from an area of higher concentration to an area of lower concentration. Diffusion does not always require a membrane, and it can occur in gases, liquids, and solids.

In contrast, osmosis involves only water molecules, which acts as the solvent, and always requires a selectively permeable membrane. This membrane permits water passage while blocking larger solute molecules. While diffusion equalizes particle concentration throughout a space, osmosis equalizes water potential across a membrane by moving water.

what are the two requirements for osmosis that make it different from diffusion?

it only involves water molecules and it requires a selectively permeable membrane

what are the 3 mechanisms of active transport?

ion pumps, exocytosis, endocytosis

how do ion pumps work? give an example.

the movement of ions across the cell membrane against their concentration gradient. example: Na/K pump, which transports sodium ions out and potassium ions into the cell.

how does exocytosis work?

expels substances from the cell. vesicles fuse with the plasma membrane to release their contents outside the cell.

how does endocytosis work? explain the two types.

brings substances into the cell.

phagocytosis: engulfs solid particles.

pinocytosis: engulfs liquid substances.

what is active transport?

the movement of ions or molecules against their concentration gradient. it REQUIRES ENERGY, typically in the form of ATP.

true or false. the differences in extracellular and intracellular fluids are extremely important for cells to survive.

true

the composition of drug preparations, such as intravenous fluids, is often stated in ____ rather than mEq/L. The molarity refers to the number of dissolved particles, and does not account for the number of available charges. For that reason, for an element such as Na⁺, which has a valence of one, 1 mmol/L = 1 mEq/L,whereas for a divalent element (i.e.an element having a valence of two) such as Mg²⁺ or Ca²⁺, 1 mmol/L = 2 mEq/L.

mmol/L

for Ca2+ (valence of 2), 1mmol/L = ______

2 mEq/L

While mole describes the physical number of atoms, mol is a unit to describe the ______. If we have one mole of NaCl (58.45g) dissolved in one litre of water, this gives us a concentration of one mol. Similarly, if we only dissolve 5.845g of NaCl in one liter than this has a concentration of 0.1 mol (or 100 mmol, a mmol is a thousandth of a mol).

concentration

The IV bag will often say 0.9% w/v, this means that for every liter of water there is 9 g of NaCl dissolved in that water. If one mole is 58.45g, then 9g is 0.154 moles (0.154 x N molecules). Now that we dissolve this into water, it becomes a concentration - 0.154 moles in 1 liter is a ___ mmol solution.

154 mmol

is the lipid bilayer miscible with water?

no. it is a barrier against the movement of water and water-soluble substances. however, lipid soluble substances can pass through by diffusion.

can lipids pass through the lipid bilayer?

yes! they pass through by diffusion.

what are the two types of membrane proteins used for transport?

channel proteins (watery spaces) and carrier proteins (binds with substance to be transported)

true or false. diffusion requires kinetic energy.

true

true or false. active transport requires a source of energy (ATP) other than kinetic energy. (does not require kinetic energy).

true

true or false. carrier proteins are highly selective for molecules and ions.

true

true or false. carrier proteins can be used in either diffusion or active transport.

true. they can be used in facilitated diffusion and active transport.

which type of active transport uses ATP or other high energy phosphate compounds?

primary active transport

which type of active transport uses a counter-transport system due to the ion gradient?

secondary active transport

which category of diffusion is through protein channels?

simple diffusion

what are the two types of protein channels (used in simple diffusion)?

selective permeability protein channels and gated protein channels

what are the different types of gated protein channels that can be used in simple diffusion (besides selective permeability protein channels)?

voltage gating and chemical/ligand gating

what is another term for facilitated diffusion?

carrier-mediated diffusion

random molecular movement through intermolecular spaces in the membrane

simple diffusion

continual motion of molecules among one another in liquids or gases

diffusion

Random molecular movement of substances (molecules or ions), molecule by molecule, either through intermolecular spaces in membranes or in combination with carrier proteins

diffusion

what is responsible for diffusion?

the kinetic energy of molecules that are in random motion

true or false. in diffusion, ions will diffuse in the same manner as a larger molecule.

true

in simple diffusion, __________ moves through a membrane opening.

molecular kinetic energy

true or false. in simple diffusion, there is no need to bind to a carrier protein (pores are involved).

true

what determines the rate of simple diffusion?

the amount of substance available and the number and sizes of the opening in the membrane

where does simple diffusion specifically occur in the membrane?

via interstices of the lipid bilayer or via water channels that penetrate through transport proteins

facilitated diffusion requires interaction of a ______ with molecules or ions.

carrier protein

in facilitated diffusion, a carrier protein ______ to molecules or ions and shuttles them through the membrane.

chemically binds

in simple diffusion that occurs through interstices of the lipid bilayer, what kinds of substances can pass through? give examples.

lipid-soluble substances (O2, N2, CO2, and alcohol)

in simple diffusion that occurs through interstices of the lipid bilayer, the rate of diffusion is ________ to the lipid solubility of the lipid-soluble substances.

directly proportional

which substances are lipid-soluble and can diffuse through membrane protein channels during simple diffusion?

O2, N2, CO2, and alcohol

in simple diffusion that occurs through interstices of the lipid bilayer, ____ enters the interior of a cell as if the membrane does not exist. what is the advantage of this?

O2. the advantage is that large amount of O2 could be delivered to the interior of cells.

in simple diffusion that occurs through water channels (use large transport proteins) that penetrate all the way through the lipid bilayer, what kinds of substances can pass through? give an example.

lipid-insoluble substances. for example, water is highly insoluble but can easily enter the cell through channels made by protein molecules in the membrane.

Total #s of water molecules that move through a red cell membrane each second is 100X more than the volume of red cells. what type of diffusion is this demonstrating?

simple diffusion through water channels that penetrate all the way through the lipid bilayer

Lipid insoluble molecules that are water-soluble (if small enough), can pass through protein pore channels just like water. If large, their penetration falls off rapidly and they cannot get through as well. Example: urea molecule diameter is only 20% greater than H₂O, yet it penetrates 1000X less than water. what type of diffusion is this demonstrating?

simple diffusion through water channels that penetrate all the way through the lipid bilayer

in diffusion, pores are composed of integral cell membrane proteins that form ______ through the membrane and are always open.

open tubes

which aspect of protein pores provides selectivity that permits only certain molecules to pass through?

the diameter of the pore and its electrical charges

which protein pores permit the rapid passage of water through membranes while excluding other molecules?

aquaporin or water channels

at least ______ different types of aquaporins have been found in different cells of the human body. (relates to diffusion through protein pores)

13

how do aquaporins allow water molecules to diffuse?

Aquaporins have a narrow pore to permit water molecules to diffuse through the membrane in a single file. The pore is too narrow to allow passage of any hydrated ions.

what is this depicting?

2D structure of aquaporin 1 (AQP1) depicting the six transmembrane alpha-helices and the five inter helical loop regions A-E.

protein channels are distinguished by two characteristics. what are they?

1) they are selectively permeable to specific ions and molecules

2) channels that can be opened or closed by gates that are regulated by electrical signals (voltage gated) or chemicals that bind to channels (ligand gated)

some protein channels are a gate-like extension of transport protein molecules. these gates can close or open or lift away depending upon _________________.

conformational change in the shape of the protein structure

which type of protein channel gating is the basic mechanism for eliciting action potentials in nerves that are responsible for nerve signals?

voltage gating

which type of protein channel gating is the mechanism used for the transmission of nerve signals from one nerve cell to another?

chemical (ligand) gating

in voltage gating, molecular conformation of the gate responds to the _______ across the cell membrane. examples: Na+ and K+ channels.

electrical potential

(voltage gating) when strong negative charges are inside the membrane, it causes the outside Na+ gate to _____. when the inside of the membrane loses its negative charges, the gates opens for Na+ to pass.

remain tightly closed

(voltage gating) when do K+ channels open? (the K+ gate is intracellular)

when inside the cell membrane becomes positively charged

in chemical (ligand) gating, some protein channel gates are open upon the binding of a chemical substance (a ligand) to the protein, causing a ___________ that opens or closes the gate. example: acetylcholine channel

conformational change in the channel protein

(chemical/ligand gating) Acetylcholine opens the gate of the channels through a negatively charged pore of 0.65 nm in diameter, which allows uncharged or positively charged molecules (smaller than the pore diameter) to pass through. why is this important?

this gating is important for transmission of nerve signals from one nerve cell to another and from a nerve cell to muscle to cause muscle contraction

the potassium K+ channel permits the passage of K+ ______ more readily than Na+ across the cell membrane.

1000x

is the selectivity of the potassium K+ channel solely based on the diameter of the ion?

no, because the K+ ion is slightly larger than the Na+ ion

what is the main reason for the selectivity of the potassium K+ channel?

the X-ray crystallographic structure shows that K+ channels are tetrameric structures consisting of four identical protein subunits surrounding a central pore. at the top of the channel pore, there are pore loops that form a narrow selectivity filter. Lining the selectivity filter are carbonyl oxygens (actually carboxyl groups).

how do K+ ions pass through the highly selective, narrow selectivity filter?

when hydrated K+ ions enter the selectivity filter, the K+ ions interact with the carbonyl oxygen and shed most of their bound water molecules (of the K+ ions). this permits the dehydrated K+ ions to pass through the channel.

how are Na+ ions excluded from the K+ selectivity filter of K+ channels?

the carbonyl oxygen is too far apart to enable it to closely interact with smaller Na+ ions. therefore, smaller Na+ ions are excluded by the filter, preventing passage through the pore.

in sodium channels, the alpa-subunit has _______, labelled I through IV (1-4), each containing six membrane-spanning segments, labelled S1 through S6.

four repeat domains

in sodium channels, which segment acts as the channel’s voltage sensor?

the highly conserved S4 segment

what is responsible for the sensitivity of the Na+ sodium channel (in regards to the S4 segment/sensor)?

positive amino acids located at every third position

when stimulated by a change in transmembrane voltage, how does the S4 segment allow the channel to become permeable for ions?

it moves toward the extracellular side of the cell membrane

in the Na+ sodium channel, the ions are conducted through a pore, which can be broken down into two regions. what are they?

the external portion (more extracellular) and the inner portion (more cytoplasmic)

how is the external portion of the Na+ sodium channel ion pore formed?

the “P-loops” (the region between S5 and S6) of the four domains. this region is the narrowest part of the pore and is responsible for its ion selectivity.

how is the inner portion of the Na+ sodium channel ion pore formed?

by combined S5 and S6 segments of the four domains. the region linking domains III and IV (3 and 4) is also important for channel function. this region shuts off the channel after prolonged activation, inactivating it.

which part of the inner portion of the Na+ sodium channel ion pore is responsible for shutting off the channel after prolonged activation?

the region linking domains III and IV (3 and 4)

what is used to record ion current-flow through protein channel(s)?

patch clamp physiology

what are the basic steps of patch clamp physiology?

1) micropipette

2) contact the outside of the cell membrane

3) apply suction to pull the membrane to the tip of the pipette (gigaOhm seal)

4) this creates a seal (the edge of pipette touches membrane)

5) this results in a minute membrane patch at the tip of the pipette through which electrical flow could be recorded

is patch clamp technology all-or-none? explain

yes! the gate will rapidly snap open or closed. there is a tight range of voltage that controls the opening and closing of the channel.

explain the postulated mechanism of regulation of facilitated diffusion

1) a molecule enters the pore and becomes bound to a receptor inside the carrier protein

2) a conformational or chemical change occurs in the carrier protein

3) the pore opens to the opposite side of the membrane

4) since the binding force of the receptor is weak, the thermal motion of the attached molecule causes it to break away and be released

in the postulated mechanism of regulation of facilitated diffusion, what is the rate-limiting step?

the rate of transportation of a molecule cannot be greater than the rate that the carrier protein can undergo conformational change between its two states

how does facilitated diffusion differ from simple diffusion (in terms of rate)?

in facilitated diffusion, the rate of diffusion reaches a maximum (called Vmax) as the concentration of substance increases

why is facilitated diffusion limited by Vmax?

the rate at which a molecule can be transported by facilitated diffusion can never be greater than the rate at which the carrier protein can undergo conformational change between two states.

what are the most important substances that pass through the membrane by facilitated diffusion?

glucose and amino acids. this is a principal mechanism by which insulin controls glucose use in the body.

the net rate of diffusion of a substance in the desired direction depends on 3 things. what are they?

• concentration difference (net rate INWARD is proportional to the concentration of molecules on the OUTSIDE. and vice versa.)

• membrane electrical potential difference (electrical charges of ions causes them to move through membranes even though no concentration difference exists to cause movement.)

• pressure difference (higher pressure on one side of the membrane means that higher amount of energy are available to cause net movement of molecules from higher to lower pressure.)

concentration effect

Rate of diffusion inward or outward is proportional to concentration of substance inside and outside.

electrical potential effect

If an electrical potential is applied across the membrane, the electrical charges of the ions cause them to move even if no concentration difference exists to cause movement.

pressure effect

Like a piston develops high pressure so too in the cell high pressure can occur on one side of the pore causing more molecules to strike the pore on that side resulting in more molecules diffusing to the other side.

osmosis

the process of net movement of water caused by a concentration difference of water across the membrane, causing cells to either swell or shrink

osmotic pressure

the exact amount of pressure required to stop the osmosis of water

what determines osmotic pressure?

the number of particles per unit volume (molar concentration) of the fluid. NOT by the mass of the particle.

why is osmotic pressure determined by # of particles (molar concentration) instead of mass?

each particle, regardless of its mass, exerts on average the same amount of pressure against the membrane

in regards to osmotic pressure, larger particles with greater mass will move at ____ velocity, and smaller particles move at higher velocity (v).

lower

the average kinetic energy of a particle is determined by _____

k=mv²/2