Chapter 4 - Cell Membrane Transport

Movement across the plasma membrane

allows cells to eliminate wastes and obtain nutrients. these movements are greatly impacted by energy

Energy has a great impact in what?

the rate and direction that a molecule moves

Factors affecting the direction of transport

- passive processes

- active transport

- concentration gradient

- electrical driving forces

Passive processes

move molecules form areas of greater concentration to areas of lesser concentration. these processes are completely spontaneous. there is no use of energy in these processes

Examples of passive processes

simple diffusion and osmosis

Active transport

typically moves molecules from lesser to greater concentrations. due to this movement, there is a need for energy (typically in the form of ATP) to complete the movement of the specific molecules

Active transport is always mediated by _____ in the plasma membrane of a cell

proteins

Sometimes, energy from one molecule can be used to what?

can be used by another molecule to move across the membrane

Concentration gradient

occurs when a substance is present in different concentrations on either side of a membrane

What happens on the concentration gradient

molecules tend to move down a concentration gradient spontaneously, from a greater concentration (pressure) to a lower concentration (pressure).

For uncharged molecules, how does the concentration gradient work?

the concentration gradient is the primary driving factor that moves the molecules

Driving factor

a force that pushes molecules in particular direction

Chemical driving force

difference in energy due to a concentration gradient that causes a molecule to move from an area of high concentration to an area of low concentration

Electrical driving forces

in this process, there is a distribution of ion on either side of the plasma membrane (Vm)

Membrane potential (electrical potential)

the difference in charge on either side of the plasma membrane

What is the average membrane potential of human cells and what does it suggest?

-70mV; suggests that the inside of the cell is more negative than the outside

How a membrane potential impacts the movement of charged molecules

a positive charged ion has a strong electrical driving force which pushes the ion to enter the cell. the reverse it true for negatively charged particles

A larger Vm equates to what?

a larger electrical driving force

Electrochemical force

the total of all chemical and electrical driving forces that act on ions

Chemical and electrical forces impact on Na+

concentration gradient and electrical gradient

both of these forces are working in the same direction. since they are working in the same direction, then the solute/molecule will move in that direction. clearly, the total driving force for charged particles is the combination of the chemical gradient and electrical gradient across the membrane

Chemical and electrical forces impact on Na+ (concentration gradient)

sodium moves from greater to lesser concentration

Chemical and electrical forces impact on Na+ (electrical gradient)

Na+ is attached to the negative ions inside of the cell and it is repelled by the positive charges external to the cell

Calcium ions

respond in a very similar fashion to sodium ions

Chemical and Electrical forces impact on K+

Concentration gradient and electrical gradient

in this case, the two forces above are working in opposing directions. when this occurs, the LARGER force dictates the direction of molecular movement

Concentration gradient (K+)

K+ would tend to leave a cell since it is more abundant inside the cell when compared to outside the cell

Electrical gradient (K+)

K+ is attracted to the negative changes inside of the cell but is repelled by the positive charges on the outside of the cell

Possible results for K+

- when the membrane potential is equal to the chemical force for K+ movement, there is no net movement of K+

- when the membrane potential is less than the chemical force for K+ movement, there will be a net movement of K+ out of the cell

- when the membrane potential is greater than the chemical force for K+ movement, there will be a strong driving force for K+ to enter the cell

The rate of transport across a plasma membrane

defined as the number of molecules that cross per unit time (moles/sec)

The rate of molecular movement is equivalent to what?

flux

In a biological system, movement of molecules often occur?

in both directions

In a physiological setting, the net flux?

is what we will look at

Passive transport

does not require energy to occur

Types of passive transport

simple diffusion, facilitated diffusion, diffusion through a channel protein

Simple diffusion

the movement of molecules through the lipid portion of a plasma membrane. in this, molecules always move down their concentration gradient. this also moves molecules towards equalibrium

There is no external energy used to drive this process

simple diffusion

What is moved through simple diffusion?

numerous small molecules are moved through plasma membranes via simple diffusion. this includes fatty acids, steroidal based hormones, O2, CO2, and fat soluble vitamins

Membrane surface area

as a rule, the larger the internal wall of the plasma membrane, the faster the movement of molecules across the membrane

Example of membrane surface area

the epithelia lining the internal wall of the small intestines are highly folded to increase surface area

Membrane permeability

this can be influenced by a number of factors including: membrane thickness and the properties of the moving molecules (hydrophobic, hydrophilic, size, shape, charge

Frick's law

is used to illustrate the impact of permeability on net flux

Net flux = PA(deltaC)

P=permeability

A=membrane surface area

Delta C=concentration gradient

in this equation, flux increases as the P gets larger

Temperature

molecules have a higher thermal motion at higher temperatures so they tend to diffuse faster at this increased temperature

this effect is usually not important in human physiology since body temperature is reactively constant

Facilitated diffusion

Factors affecting rates of simple diffusion

- magnitude of the driving force

- membrane surface area

- membrane permeability

- temperature

Magnitude of the driving force

in general, as the driving force increases in magnitude, the rate of net flux will increase in a corresponding fashion

In simple diffusion, the net flux of a substance is directly proportional to the size of?

the concentration gradient or to the size of the elctrochemical gradient

If the concentration gradient becomes equal, then what happens to the net flux?

there is a net flux of zero. at this point there is still movement across the membrane, but at equal rates in both direction

Facilitated diffusion (mediated transport)

involves the use of plasma membrane proteins that move molecules from areas of greater concentration to areas of lesser concentration

Membrane proteins are referred to as?

carrier proteins

What happens to larger molecules (such as sugars) and ion in facilitated diffusion?

they are often carried through cell membranes

Factors that affect the rate of facilitated diffusion

- the transport rate of the protein transporter (this varies from one protein transporter to another)

- the number of protein transporters in the plasma membrane. an increase in the number of protein transporters increases the likelihood of that a solute will bind to the protein; thus, increasing the rate of molecular movement across the plasma membrane

- the magnitude of the concentration/electrochemical gradient

Process of glucose movement through a carrier protein

1. the protein moves glucose from greater to lesser concentration

2. glucose attaches to protein, causing a conformational change in the protein such that the binding site faces the inside of the cell

3. the glucose is released to the inside of the cell. with this the protein returns to its original shape and is ready to bind to another glucose molecule

4. in this process, there can actually be a "glucose saturation" associated with the proteins involved in facilitating diffusion

Diffusion through a channel protein

this involves movement through a membrane protein via a passageway or pore that extends through the Plasma Membrane. again, these proteins are typically specific for a certain molecule

Characteristics of diffusion through a channel protein

- these are often involved in the passage of ions through a plasma membrane. because of this, the proteins are referred to as ION CHANNELS. this process is a PASSIVE PROCESS

- ions can move through these channels in single file. therefore, multiple ions can move through the channel at one time

- molecules move through channel proteins based on a concentration gradient (greater to lesser concentration)

- these can work in both directions

Aquaporins

highly selective pores that allow for the passage of WATER across a membrane

Active transport

this refers to the movement of molecules against a concentration/electrochemical gradient (lesser to greater concentration). an input of energy is required for this process to occur

The transporters involved in active transport are typically?

proteins. these proteins are specific for certain molecules. plasma membranes usually have a set number of active transport proteins

Which direction do active transport proteins move?

many times, active transport proteins move molecules across a plasma membrane in BOTH DIRECTIONS

Active transport plays a key role in a number of physiological processes including what?

impulse formation, muscle contraction, absorption of nutrients in the G.I. tract, and kidney filtering

Two types of active transport

primary and secondary

Primary active transport

uses ATP or some other chemical energy source directly to transport substances through a plasma membrane

Proteins involved in primary active transport

referred to as pumps

What do pumps do in primary active transport?

these pumps can serve as enzymes as well. since these enzymes can catalyze ATP hydrolysis which releases energy to the pump, they are also referred to as "ATPases"

Examples of pumps

Na+/K+ pump, Ca2+ pump in the S.R. of muscle fibers

Na+ - K+ pump

this pump contains three sodium binding sites and two potassium binding sites. this specific protein functions by moving sodium out of a cell and potassium into the cell (against their electrochemical gradient)

Step of the Na+ - K+ pump

1) intracellular Na+ ion bind to the pump protein. this triggers phosphorylation of the pump by ATP

2) the energy received from the ATP by the pump induces a conformational change in the protein which leads to the release of Na+ ions into the extracellular fluid

3) next, K+ ions bind to the pump and trigger the release of the phosphate from the protein. loss of this phosphate allows the protein to return to its original shao which carries K+ into the intracellular fluid of the cell

End result of the Na+ - K+ pump

3 sodium ions moved out of the cell and 2 potassium ions moved into the cell per one hydrolyzed ATP molecule

Secondary active transport

in this transport mechanism, a transporter uses the energy stored in the electrochemical gradient of one molecule to drive the transport of a second molecule against its chemical/electrochemical gradient

2 types of secondary active transport

cotransport (symport) and countertransport (antiport)

Cotransport (symport)

two molecules move in the same direction

Example of cotransport (symport) transport

Na+ - Glucose linked transport

- in this process, Na+ moves down its chemical/electrochemical gradient through the plasma membrane. as this movement proceeds, the Na+ releases energy that drives the movement of glucose against its concentration gradient - in the same direction that Na+ is moving

Countertransport (antiport)

two molecules move in opposite directions

Examples of countertransport (antiport) transport

The Na+ - proton exchange

Na+ flows into a cell along its concentration gradient. as this occurs, energy is released that drives the movement of H+ out of the cell against its gradient

Osmosis

the diffusion of water across a semipermeable plasma membrane

How does water move in osmosis

from greater to lower concentrations or towards equilibrium

Osmosis is always...

PASSIVE

Why is osmosis important physiologically

since water flow in the human body influences: the volume and composition of body fluids, secretion of fluids from glands, filtering of blood in the kidneys

Osmolarity

the total solute concentration of a solution

Example of osmolarity

water moves from low solute to high solute. water is described as being the solvent

Iso-osmotic

intracellular fluid is approximately 300mOsm

if a cell is placed in a 300mOsm solution, there would be no net movement of water

Hyperosmotic

if a cell was placed in a 500mOsm solution, the osmolarity is higher than outside the cell

hypo-osmotic

if a cell was placed in a 100mOsm solution, the osmolarity is lower outside of the cell

Osmotic pressure

another term that describes a solution's total solute concentration. this term is often used to describe physiological foces related to osmosis

An increase in osmolarity means?

an increase in osmotic pressure

Tonicity

is a function of the concentration of nonpermeating solutes outside a cell relative to the concentration inside the cell. this determines the behavior of a cell placed in the solution

Isotonic

if a cell contains 300mOsm of impermeable solutes and is placed in a 300mOsm solution, there is no net movement of water molecules

Hypertonic

if a 300mOsm cell is placed in a 400mOsm solution, the water will diffuse out of the cell into the solution (from greater to lesser water concentartion)

Hypotonic

if the same cell is placed in a 100mOsm solution, then water will diffuse into the cell, causing the cell to swell

Transport of molecules in membrane-bound compartments

in this process, large molecules are moved through plasma membranes in small compartments known as VESICLES. this process requires an input of energy

Phagocytosis

"cell eating", in this process the plasma membrane extends around molecules to form a membrane bound vesicle. the vesicle is moved through the plasma membrane with the usage of energy

Endocytosis vs. Exocytosis

Endocytosis : taking in the vesicle

Exocytosis : vesicle leaving golgi apparatus on cell

Pinocytosis

"cell drinking", this is the uptake of extracellular fluid via a vesicle

Simple epithelium

forms a barrier between extracellular and intracellular environments (cells lining the small intestine)

Absorption

movement of molecules from extracellular areas to intracellular compartments

Secretion

movement of materials from intracellular to extracellular environments

Epithelial tissue always sits atop what?

a layer of connective tissue (usually areolar tissue)

Basement membrane

the layer between epithelial tissue and connective tissue

Apical surface

the free edge of the epithelium

Basolateral membrane

rests on the basement membrane

What allows for movement through simple epithelial tissue?

many molecular movement devices talked about earlier in the chapter (diffusion, osmosis, active transport)