Homeostasis and Plasma Membrane

Lectures 1-3

Homeostasis

process by which a stable internal environment is maintained

What are the homeostatic set points of cells?

O2 and CO2 concentration, blood glucose level, pH, temperature, osmolality

What is the optimal temperature for a cell?

37C

What is the optimal osmolality for a cell?

300 mOsm/L

What is the function of the plasma membrane?

Separate the inside and outside environment of the cell while selectively regulating the movement of substances in and out

Plasma

ECF within blood vessels

Interstitial fluid

ECF directly surrounding the cell

Which fluid have a larger volume in the body, ECF or ICF?

ICF

Why is it important to maintain ICF volume?

important for osmolality

Why must ICF and ECF be separated?

to retain important organelles and nutrients within the cell

need different chemical conc of ICF and ECF for cell function

How do ICF and ECF vary in key ion concentrations?

ICF → high K+, lower Na+, lower Cl-

ECF → higher Na+, slightly higher Cl-, lower K+

What is the plasma membrane composed of? How does this contribute to it’s function?

Phospholipids which have a polar, negatively charged head and non-polar hydrophobic tails

Forms a lipid bilayer with the polar heads faced outwards and inner hydrophobic lipid environment

What are the types of membrane-associated proteins?

integral proteins, transmembrane proteins, peripheral proteins

Integral proteins

proteins that are anchored in the plasma membrane (either pass through both sides of the membrane or are anchored in one side of the cell)

Transmembrane proteins

proteins that are anchored within the plasma membrane and pass through it; are also integral membranes

What is an example of a type of transmembrane protein?

Channel

Peripheral proteins

proteins that associate with the surface of the plasma membrane

What is a common characteristic of peripheral proteins?

hydrophilic structure

What are the 2 modes of transport that solutes use to pass through the plasma membrane?

passive/facilitated diffusion, active transport

Passive diffusion

diffusion of solute directly through plasma membrane without any assistance

Facilitated diffusion

diffusion of solute across membrane with the help of a protein

Active transport

movement of a solute across the plasma membrane with the requirement of energy

What are a few solutes that use passive diffusion?

hydrophobic molecules (i.e., O2, CO2, N2)

small uncharged polar molecules (i.e., urea, glycerol, EtOH)

water

Fick’s laws of diffusion

describes how molecules move from an area of high conc to an area of low conc

What factors influencing diffusion make up Fick’s law?

conc of gradient, surface area of plasma membrane, lipid solubility of substance, molecular weight of substance, distance of diffusion

Which types of diffusion does Fick’s law apply to?

passive and facilitated (at low conc)

How does water move through the plasma membrane?

osmosis and via aquaporins

Osmosis

transport of water across plasma membrane in relation to non-permeable solute

Hydrostatic pressure

the pressure exerted by a fluid at rest due to the force of gravity

Osmotic pressure

tendency for osmotic flow of water into a solution because of its relative concentration of non-permeable solutes and water molecules

Osmolarity

total concentration of all solute particles that are free in a solution

What is the unit of osmolarity?

milliosmoles/liter (mOSM/L)

What is a solution that exceeds 300 mOsm/L called?

hyperosmotic solution

What is a solution that is below 300 mOsm/L called?

hypoosmotic solution

Tonicity

effect a solution has on cell volume due to it’s non-permeable solute concentration

Hypotonic (in reference to a cell)

ECF has a lower concentration of non-permeable solutes that compared to in the ICF in a normal cell

Hypertonic

ECF has a higher concentration of non-permeable solutes that compared to in the ICF in a normal cell

Isotonic

ECF and ICF have the same concentration of non-permeable solutes in normal cell

What is the phenotype of hypotonic cells?

cell swells as there is a net flux of water into the cell

What is the phenotype of hypertonic cells?

cell shrink as there is a net flux of water out of the cell

Provide an example of how tonicity affects a cell type in the body (real-life example)

Patients are commonly provides 0.9% saline solution due to tonicity of red blood cells

Giving patients normal water would lead to hypotonicity and cell swelling

Giving patients super salty water would lead to hypertonicity and cell shrinkage

What are the main types of transmembrane proteins that facilitate transport across plasma membrane?

Carrier

Channel

Describe the structure and characteristics of channel proteins

Hydrophobic exterior

“Tunnel-like” interior lined with charged amino acids

Allow for fast transport

Transport only into or out of cell

Selective for specific type of ion or charge

What kind of molecules typically utilize channel proteins?

ions (due charged tunnel-like interior)

Saturable transport

process by which movement across the plasma membrane is limited by the number of channel proteins in the membrane, limiting available proteins that molecules can use to pass the membrane

What are the types of channel proteins?

Leak (non-gated)

Voltage-gated

Ligand-gated

Leak channels

channel proteins that are always open, allowing for a constant ion flux as long as there is an electrochemical gradient

Voltage-gated channel proteins

channel proteins that are opened or closed by the membrane voltage

Ligand-gated channel proteins

channel proteins that are opened or closed by ligand binding

What does the transport of molecules with channel proteins depend on?

electrochemical gradient

whether channel is open

Carrier protein

type of membrane-associated protein that helps solutes pass through plasma membrane

What occurs when a solute bind to a carrier protein?

changes in conformation allowing solute to pass through to other side of plasma membrane

What are some characteristics of carrier proteins related to their function?

changes in conformation when a solute binds

always open to one side of the plasma membrane

slower than channel proteins due to binding and conformational changes

saturable transport

driven by concentration gradient of at least one solute

What are two types of carrier proteins?

primary transporters

secondary transporters

Primary transporters

carrier proteins that utilize active transport (requires energy)

Secondary transporters

carrier proteins that utilize facilitated transport (solute’s concentration gradient)

What are the types of secondary transporters?

uniporters, symporters, antiporters

What is a common example of a primary transporter?

Na+-K+ pump

How does the Na+-K+ pump work?

1) Carrier protein is open

2) 3 Na+ bind to the carrier protein using ATP which results in a product of ADP and a phosphate group

3) The bound phosphate group causes a conformational change in the carrier protein which releases Na+ out of the cell

4) 2 K+ then bind the the carrier protein, causing the release of the phosphate group from the protein

5) This then reverts the structure of the protein back to its original conformation and K+ is released into the cell

What are the benefits of the Na+-K+ pump in the cell?

Generates a Na+ rich ECF which is used to drive active transport of other molecules and contributes to membrane potential (negative charge inside the cell)

Contributes to osmotic balance

Uniporter

secondary transporter carrier protein that moves 1 molecule in one direction

Symporter

secondary transporter carrier protein that moves 2 or more molecules in one direction

Antiporter

secondary transporter carrier protein that moves 2 or more molecules in opposite directions

What is an example of an important symporter in the body?

sodium and glucose symporter (SGLT)

How does the SGLT work?

1) Na-K+ pump creates ion gradient

2) Na+ binds to protein and glucose also enters with it from the ECF

3) Na+ and glucose are released into the ICF

What molecules are too large for transport via channels and carriers? How do they pass through the plasma membrane?

proteins, transmitters, bacteria

via vesicles

Vesicles

small, fluid-filled sacs enclosed by a lipid bilayer membrane

Endocytosis

process by which plasma membrane forms a vesicle to transport material into the cell

Exocytosis

process by which vesicles carrying molecules merge with the plasma membrane to transport molecules to the ECF