- WELCOME TO LIPIDS PART THREE.
IN THIS VERY BRIEF TUTORIAL, WE'LL GIVE A BROAD OVERVIEW
OF HOW SUBSTANCES MOVE THROUGH OR ACROSS THE CELL MEMBRANE.
LET'S GET STARTED. ALL RIGHT.
SO THE FIRST THING IS, JUST LINGUISTICALLY,
DON'T GET HUNG UP ON THE WORDS.
SOME PEOPLE TALK ABOUT GOING ACROSS THE MEMBRANE,
SOME PEOPLE SAY THROUGH.
YOU KNOW, EITHER WAY.
THE MAIN THING TO THINK ABOUT IS THAT IN THE LIPID BILAYER,
WHILE THE POLAR HEADS ARE ON THE EXTERIOR,
IT'S THE NONPOLAR TAILS
THAT CREATE THE CARRIER OF THE MEMBRANE.
SO WHEN WE THINK ABOUT CELL MEMBRANES,
WE THINK NONPOLAR.
SO WHENEVER WE WANT TO, LIKE, YOU KNOW,
PREDICT HOW A SUBSTANCE WILL TRANSPORT THROUGH THE MEMBRANE,
WE THINK ABOUT WHAT WE KNOW ABOUT INNER MOLECULAR FORCES
AND LIKE DISSOLVES LIKE.
SO SUBSTANCES THAT ARE NONPOLAR ARE GOING TO TRAVEL EASILY
THROUGH THE CELL MEMBRANE AND SUBSTANCES ARE--
THAT ARE POLAR WON'T.
BECAUSE WITHIN OUR BODY FLUIDS,
WE HAVE IONS AND SMALL COVALENT MOLECULES AND LARGER MOLECULES,
THERE'S A VARIETY OF MECHANISMS
THAT CAN USE FOR GETTING SUBSTANCES
THROUGH OUR MEMBRANES.
THEY ARE CLASSIFIED BY THE ENERGY REQUIRED.
SO HERE, WE CAN SEE THE PASSIVE TRANSPORT. RIGHT?
SO PASSIVE TRANSPORT IS WHERE NO ENERGY IS NEEDED.
THE SUBSTANCES CAN GET THROUGH
WITHOUT ADDITIONAL ENERGY. RIGHT?
SO NO ENERGY NEEDED FOR TRANSPORT.
ALL RIGHT? SO HERE,
WE COULD HAVE SIMPLE DIFFUSION.
IT COULD BE DIRECTLY THROUGH THE MEMBRANE
OR THROUGH A CHANNEL PROTEIN.
WE DO HAVE FACILITATED DIFFUSION.
AND THEN THERE ARE CARRIER PROTEINS
THAT HELP WITH ALL OF THAT.
AND THEN FINALLY, WE HAVE ACTIVE TRANSPORT.
AND SO ACTIVE TRANSPORT, AS YOU CAN SEE RIGHT HERE,
IS ENERGY IS NEEDED FOR TRANSPORT THROUGH THE MEMBRANE.
SO THE TWO BIG DISTINCTIONS ARE PASSIVE AND ACTIVE TRANSPORT.
AND SO NOW, WE'LL LOOK AT THOSE A LITTLE MORE CLOSELY.
BEFORE WE MOVE ON THOUGH,
I JUST WANTED TO NOTE THE IMPORTANCE
OF THE ROLE OF THE VARIOUS MEMBRANE PROTEINS.
SO WE DIDN'T TALK ABOUT MEMBRANE PROTEINS A LOT
IN OUR PROTEIN CHAPTER,
BUT WE'RE--
BECAUSE WE WERE REALLY WAITING UNTIL WE GOT TO LIPIDS
AND CELL MEMBRANES BECAUSE THEY WORK TOGETHER AS A TEAM.
ALL RIGHTY.
SO NOW, LET'S JUST TAKE A LITTLE BIT OF WHAT WE KNOW ABOUT,
YOU KNOW, SUBSTANCES BEING SOLUBLE AND MOVING.
SO LET'S LOOK HERE. RIGHT?
SO REMEMBER PASSIVE TRANSPORT,
NO ENERGIES NEEDED FOR TRANSPORT.
IT HAPPENS BASICALLY BY DIFFUSION. RIGHT?
WE KNOW THAT SUBSTANCES WANT TO GO FROM A HIGH CONCENTRATION
TO A LOW CONCENTRATION.
SO SIMPLE DIFFUSION IS LITERALLY THAT THE--
WE CAN SEE IN OUR DIAGRAM. RIGHT?
WE DON'T EVEN NEED A MEMBRANE PROTEIN.
THE SUBSTANCES CAN GO DIRECTLY THROUGH THE MEMBRANE
ALL BY THEMSELVES. RIGHT?
SO SIMPLE DIFFUSION MEANS PASS DIRECTLY THROUGH THE MEMBRANE.
ALL OTHER FORMS OF TRANSPORT, AS WE CAN SEE,
REQUIRE SOME SORT OF MEMBRANE PROTEIN.
SO WHAT KIND OF SUBSTANCES CAN JUST GO STRAIGHT THROUGH?
WELL, RIGHT, THEY'RE GOING TO BE HYDROPHOBIC, RIGHT,
WHICH IS ANOTHER WAY TO SAY NONPOLAR.
AND SO THIS WOULD BE, FOR EXAMPLE,
LIKE OXYGEN AND CARBON DIOXIDE AND THE STEROIDS,
WHICH WE KNOW SHOW UP AS A VARIETY OF HORMONES
IN DIFFERENT SUBSTANCES. OKAY.
WELL, WE'LL GET TO THAT.
SO STAY TUNED FOR CHOLESTEROL.
AND THEN THE FACILITATED DIFFUSION, RIGHT,
SO KEEP IN MIND THAT'S ALSO PASSIVE TRANSPORT. RIGHT?
SO NO ADDITIONAL ENERGY IS REQUIRED BUT IT NEEDS TO BE
FACILITATED BECAUSE THE SUBSTANCES ARE HYDROPHILIC.
SO THEY'RE NOT ATTRACTED TO THE NONPOLAR CELL MEMBRANE.
SO WHAT HAPPENS HERE IS THE MOLECULES BIND
TO THE MEMBRANE PROTEIN,
WHICH THEN--
IN THE CASE OF THE FACILITATED TRANSPORT, RIGHT,
IT CHANGES SHAPE TO ALLOW THE MOLECULE
TO BE RELEASED ON THE OTHER SIDE
OR IT COULD BE JUST A DIRECT CHANNEL.
SO THAT WOULD BE THE SIMPLEST FORM.
OKAY. SO WHAT KIND OF SUBSTANCES ARE WE GOING TO--
ARE GOING TO REQUIRE FACILITATED DIFFUSION
TO GET THROUGH THE CELL MEMBRANE?
SO THIS IS GOING TO BE SMALL IONS
AND SMALL--
EXCUSE ME.
SMALL POLAR COVALENT COMPOUNDS. RIGHT?
SO THESE ARE BOTH PASSIVE FORMS OF TRANSPORT.
AND THEN BRIEFLY, WE'LL LOOK AT ACTIVE TRANSPORT.
AND WHAT WE WILL SEE THERE,
RIGHT, IS NOW WE HAVE TO HAVE ENERGY SUPPLIED.
AND THE REASON FOR THIS IS BECAUSE THE SUBSTANCES ARE
MOVING AGAINST THE GRADIENT.
WE'RE GOING TO TRY TO GO FROM LOW CONCENTRATION TO HIGH.
SO THAT REQUIRES ENERGY.
IT'S LIKE SWIMMING UPSTREAM.
ALL RIGHTY.
SO IN MOST CASES, YOU'LL SEE, RIGHT,
THAT IT'S ENERGY FROM ATP,
SO WE SEE THAT RIGHT HERE.
RIGHT? SO THERE'S OUR ATP.
AND VERY COMMON IS THE MOVEMENT OF THE SODIUM AND POTASSIUM IONS
ACROSS THE MEMBRANE. RIGHT?
SO INSIDE THE CELL,
WE HAVE A HIGH POTASSIUM ION CONCENTRATION,
AND OUTSIDE THE CELL,
WE HAVE A LOW POTASSIUM CONCENTRATION.
HOWEVER, IF WE'D LOOK AT THE ARROWS HERE,
WE SEE WE'RE MOVING POTASSIUM INTO THE CELL.
THE POTASSIUM IONS ARE TRYING TO MOVE IN.
SO THAT REQUIRES ATP ENERGY.
LIKEWISE, IF WE LOOK INSIDE THE CELL,
WE SEE THAT THERE IS A LOW SODIUM ION CONCENTRATION.
AND OUTSIDE THE CELL,
WE HAVE A HIGH SODIUM ION CONCENTRATION.
SO WE CAN SIMULTANEOUSLY GET THE SODIUM IONS
OUT OF THE CELL GOING AGAINST THIS CONCENTRATION GRADIENT
WHILE THE POTASSIUM COMES IN.
ALL RIGHTY.
SO THAT WRAPS UP OUR OVERVIEW
OF HOW SUBSTANCES MOVE THROUGH THE CELL MEMBRANE.
THE MAIN THING TO KEEP MIND IS LIKE DISSOLVES LIKE,
AND THAT WE CONSIDER CELL MEMBRANES NONPOLAR.
AND THEN THE DIFFERENT--
WHETHER WE NEED PASSIVE TRANSPORT OR ACTIVE TRANSPORT
IN THE ROLE OF THE MEMBRANE PROTEINS.
PLEASE TAKE SOME TIME NOW TO WORK HOMEWORK PROBLEMS
TO REINFORCE YOUR UNDERSTANDING.