Lecture 4: Movements of Solutes & Water

Most of our body fluids are

water

Water makes up ____% of body mass in infants; _____% in adults;

 _____% in elderly; slightly higher percentage in men

70; 50-60; 45-50

Semipermeable plasma membranes

separate body water into separate compartments

Intracellular fluid (ICF)

Water contained in all of our cells (trillions)

 2/3^rd of total body water (28 L in our adult male)

Extracellular fluid (ECF)

  Water outside of cell

  1/3^rd of total body water (14 L)

  ECF divided into

Intravascular fluid (IVF): Water in blood vessels, 1/4^th of ECF  (7 L)

 Interstitial fluid (ISF): Water outside of cells and blood vessels, 3/4^th of ECF (21 L)

Different compartments have different

composition   of solutes

Remember plasma membranes are

semipermeable- some solutes cross easily, some slowly, and some are not capable of crossing plasma membrane

There is constant flow of_______ and ______ across   these membranes- seek equilibrium on both sides of membrane

solutes and water

In ECF, Na⁺ is ______ and K⁺ is _____

high; low

In ICF, Na⁺ is _____ and K⁺ is ______

low; high

Proteins are high in ____, low in _____, essentially absent in ISF

ICF; Plasma

Water crosses membranes and will

equilibrate (become equal   on each side) rapidly

Normal water concentration is determined by

OSMOLALITY of fluid compartment

Osmolality =

counting each different solute and adding them all up   in a fluid compartment

Normal is about_______

Pure water is

300 mOs/L

0 mOs/L

If the fluid on one side of membrane is 300 mOs/L, and fluid on other side is 200 mOs/L, water will flow across membrane from

low side (200) to high side (300) until they are equal

Solutes that cannot cross membranes will be

confined in one compartment, effecting concentrations of water and other solutes on both sides

Diffusion is

• a passive process

• does not require ATP

• ALWAYS net flow is from high concentration to low

• Steepness of gradient increases diffusion

• Increased temperature increases diffusion

• Limited by distance

In diffusion the net flux is always greater from regions of ______ concentrations to regions of _______concentrations

This is referred to as ______ diffusion/ or the substance has   moved “________.”

higher; lower

“downhill”

The greater the difference in concertation between any 2 regions, the ______ the magnitude of the net flux

greater

The concentration difference determines both the ________and the   _______ of the net flux

direction; magnitude

The magnitude depends on:

Temperature: the more elevated, the greater the speed of molecular movement and the faster the flux

Mass:  larger substances (proteins) mover more slowly than smaller molecules (glucose)

Surface Area: the greater the space available for diffusion the faster the net flux

Medium: molecules move through air faster than water/ and through water faster than through other matrices

Simple diffusion

• Molecules pass between phospholipid molecules

• Small and nonpolar solutes

• Includes: respiratory gases (O₂ and CO₂), some fatty   acids, ethanol, urea

• Not regulated by plasma membrane

• Movement dependent on concentration gradient

• Continues to move as long as gradient exists

Membranes act as

barriers that slow the diffusion of   molecules across their surfaces

The major factor limiting diffusion across a membrane is the

hydrophobic interior of its lipid bilayer

Oxygen, carbon dioxide, fatty acids, and steroid hormones are   examples of ________ that diffuse rapidly through the lipid portions of membranes

nonpolar molecules

Nonpolar molecules have large

permeability coefficients

Lipophilic (lipid-loving) substances move

through the PM easily

Lipophilic (lipid-loving) substances dissolve in the

nonpolar regions of the phospholipid   bilayer membrane made up of fatty acid chains

Polar molecules and hydrophilic (water-loving) do not

Organic molecules in various metabolic pathways are ionized or   polar (PO₄^3-), have low solubility in the bilayer, and are   retained within the cell/organelles

diffuse readily through the membranes

Polar molecules have smaller ________ because   _________________

permeability coefficients

they are insoluble in membrane lipid fatty acids

Facilitated diffusion

a passive process—requires no ATP

Transport process for small charged or polar solutes   requires assistance from plasma membrane proteins

Two types of facilitated diffusion:

1. Channel-mediated diffusion

2. Carrier-mediated diffusion

Facilitated diffusion is diffusion across a membrane with

Examples:

the help of a channel or carrier molecule

Glucose and amino acids

Facilitated diffusion, and simple diffusion, do not require

ATP

What is the difference between facilitated diffusion and simple diffusion?

Facilitated diffusion uses a transporter protein to move solute

Simple diffusion the movement is random from [high] to [low]

Both are “downhill” across a membrane

Net facilitated diffusion continues until

the concentrations of the solute on the two sides of the membrane are equal

GLUTs: 

glucose transporters allow cells to import the polar molecule   glucose where it undergoes cellular respiration to produce ATP,  CO₂, water, and heat

Continuous flux of _______ into cells

glucose

Insulin

must be present and bound to receptors on the GLUTs for glucose to get into cells (except for neurons)

Sodium-Potassium Pump- 

Occurs in every cell in body- 3 Na⁺ ions are moved from ICF à ECF and 2 K⁺ ions are moved from ECF à ICF- uses 1 ATP

This maintains the sodium/potassium levels in different compartments

The Na/K pump uses about

40% of ATP in the basal metabolic weight of the body each day

Water is

polar

Osmosis is facilitated by channel proteins called

aquaporins

Water channels:

the main channel for movement/diffusion of water in and out of the cell

Some cells are more _________ to water than others

permeable

Permeability can be altered in response to

various signals

Aquaporin expression and insertion into the membrane  

These are especially important in the

varies among cell types

epithelial cells lining certain kidney ducts

Active Transport

Uses energy (ATP) to move solute across membrane

Works against concentration gradient

Moves solute from low concentration   compartment to high concentration   compartment

Secondary Active Transport

Transport of some solutes are linked to the Na/K pump to move them across plasma membranes

Na+ extracellular concentration (mM)

145 mM

Na+ intracellular concentration (mM)

15 mM

K+ extracellular concentration (mM)

5 mM

Na+ intracellular concentration (mM)

150 mM

Isotonic solution

a solution that does not cause a change in cell volume; one that contains 300 mOsmol/L of nonpenetrating solutes, regardless of the concentration of membrane-penetrating solutes present

Hypertonic

Hypotonic

Isoosmotic

Hyperosmotic

A solution containing greater than 300 mOsmol/L of solutes, regardless of its composition of membrane-penetrating and nonpenetrating solutes

Hypoosmotic

A solution containing less than 300 mOsmol/L of solutes, regardless of its composition of membrane-penetrating and nonpenetrating solutes

Active process

Used to move large substances across cell   membranes- uses vesicles

Exocytosis-

Large substances secreted from cells

  EXo EXits

Endocytosis-

Reverse of exocytosis- moves large   substances into cells

  ENdo Enters

Phagocytosis-

cellular ‘eating’ pulls in large particle

a form of endocytosis

Most endocytic vesicles fuse with a series of intracellular vesicles known as

endosomes

Endosomes lie between

the plasma membrane and the Golgi apparatus.

Endosomes distribute

the contents of the vesicle and its membrane to various locations.

The Plasma Membrane establishes and maintains    an electrochemical gradient—

the resting membrane potential (RMP) - Electrical charge difference at plasma membrane

Essential for muscle and nerve cell function

Membrane potential—

potential energy of charge difference

Resting membrane potential (RMP)—

potential when a cell is at rest

Unequal distribution of ions/molecules across plasma membrane

•   More K⁺ in cytoplasm than in interstitial fluid

•   More Na⁺ in interstitial fluid than in cytoplasm

•   Due to Na⁺/K⁺ pumps

Unequal relative amounts of positive and negative charges:

More positive on outside than inside of cell

The direction and magnitude of ion fluxes across the membrane depends on both the ________ difference and the _______ difference (membrane potential) which together make up the _________ across a membrane

concentration; electrical; electrochemical gradient  (μ)