Cell Transport Mechanisms and Homeostasis

Cell Cytoplasm is full of many different Substances.

Mostly WATER as the Solvent making movement and reactivity of the various molecules possible.

≈ 80% Water!

Lots of dissolved Ions – but different concentrations than in the Blood.

Sugars and Proteins and other molecules.

An IV represents Balance with the Cells of your Blood. Not the same Stuff, but equal amounts!

5% Sugar (Dextrose or Ringer’s Lactate) or 0.9% Salt = Cell.

If the Balance is

wrong, your blood cells will

not survive or function.

Pre-Made!

Precisely Balanced!

Cells!

5%

Cell Transport Mechanisms and Homeostasis

OUTSIDE = Extracellular Fluid.

Contains all sorts of things that cells need and that the Organism Provides. Your Digestive Tract is Choosy, but not perfectly Picky!

INSIDE = Cytoplasm.

THE RIGHT STUFF!

Contains a Precisely Balanced Solution of all of the RIGHT STUFF! This is Homeostasis! Must be able to Transport the RIGHT STUFF IN!

The Cell NEEDS Specific Things:

The right Ions, as many Amino Acids as Possible, Glucose, “Its” Proteins, etc…

Other things are Kicked OUT.

Selective Permeability allows Cells to Choose and Move the RIGHT STUFF –

CONTROLLED!!

RR

RR

r

Cell Transport Mechanisms and Homeostasis

Transport Proteins can do their job because they are specific in SHAPE! Only Proteins can

do this because they are so Variable!

? ??

But then the Chain?

But Remember!!

VARIABLE in SHAPE!!

Opening COULD BE: OR OTHERS!

Hydrophobic Core

Inside Membrane!

Hydrophilic Edges

Outside Membrane!

H20

Water is the Solvent!

Dry Solutes cannot move – they are

bound together as a Solid.

Water diffuses into the Solute (High to low conc.)

and dissolves it!

H20

H20

Water IS the SOLVENT and breaks-up the SOLUTES from each other.

Once the SOLUTES are separate, they WILL SCATTER until

completely distributed as a Solution.

If there is no Restriction to the Movement of the Solutes (Remember – Water will move by itself!),

they will form a SOLUTION!

Solutes Stick Together!!

BUT! Solvents Stick to Solutes!

DIFFUSION leads to Equal Concentrations Everywhere!

NORMAL CONDITION!

Cells will HAVE TO Choose and Separate what is in the Cytoplasm. This Membrane is NOT doing

that here, but Cell’s Membrane will be SELECTIVELY PERMEABLE!

INSIDE ≠ OUTSIDE

Cells DO NOT Really WANT This! Must have the RIGHT STUFF in the

RIGHT Place!

Cell Transport Mechanisms and Homeostasis

DIFFUSION is the most basic form of molecular movement. In Diffusion, substances scatter as much as possible. As they

collide and scatter, they will spread out.

Begin with packed Solutes = High Concentration

Once D

iffusi

on has

pro

ceed

ed, t

he so

lute

s hav

e bee

n

disru

pted a

nd sca

ttere

d as

much

as

possib

le w

ithin

thei

r

spac

e. D

iffusi

on mak

es s

ubstan

ces

spre

ad o

ut com

plete

ly

and is

GOIN

G TO H

APPEN UNLESS IT

IS S

TOPPED!!

End with randomly separated Solutes =

Lowest Concentration =

SOLUTION!!

Always Moving!

Diffusion WILL MAKE

ALL SOLUTIONS THE SAME!!

Cell Transport Mechanisms and HomeostasisBUT REMEMBER! The Cell is surrounded by a Plasma

Membrane that IS NOT just Open to Anything/Everything. The Plasma Membrane Controls Access to the Cell!

Polar “Heads” line-up with Aqueous Surface Inside and

Outside. Hydrophilic = Loves Water!

Fatty Acid “Tails” are Non-Polar and avoid

Water Inside the Membrane = Fat

Sandwich!

Hydrophobic = Fears Water.

Most Substances CANNOT pass through the membrane – would need to be Polar AND

NonPolar -so the Bilayer is a real boundary!

But they CAN Pass through “their”

Transport Protein IF IT IS OPEN!

Cell Transport Mechanisms and Homeostasis

SIMPLE DIFFUSION

The simplest process of molecular Transport.

Always High conc. To low conc. Always moves to make balance on both sides. Diffusion is trying to force everything to be a Solution. That is NOT always what the Cell wants!

High Concentration

Outside

Low Concentration

Inside.

Concentration Gradient

High to low.

IF THIS Happened ALL of the TIME:

Cytoplasm would be the SAME as the

Outside of the Cell.

It is NOT, so there have to be ways to restrict movement of various

molecules.

AND THERE ARE!!

Cell Transport Mechanisms and Homeostasis

Facilitated Diffusion: Passive Transport through a Selectively Permeable Membrane.

Only a few substances can move through a Plasma Membrane through Simple Diffusion. Everything else will require a Transport Protein that forms an openable and closeable Hole through the Lipids.

Naturally moving H L, No Energy required!

Naturally moving L H, No Energy required!

The Transport Protein must

have an opening that “suits” the

substance. Shape, Charge, Size, etc… all

must be Right!

Since each substance requires a different

Transport Protein – There are LOTS in the

membrane of every Cell!!

Can Move Anything IF IT

FITS!!

Cell Transport Mechanisms and Homeostasis

Active Transport

Cells have very Specific Needs and sometimes need to “Pack” molecules in and out of the membrane.

That would be moving AGAINST the Concentration Gradient and is impossible in Diffusion. To move things from low to High concentration requires Energy = ATP!

ATP

ADP

Wants Out!

Why?! But?!

Wants In?! Why Not?!

But, The Cell NEEDS these molecules and must “Pump” them IN! That is “Work”!

Work requires?

The substance will bind with “its” Transport

Protein, but waits for?

Once “Energized”, the substance is “pumped”

against the Concentration Gradient to make one side MORE

Concentrated! Anti-Diffusion!

This is h

ow the Cell “

Get’s” what it

NEEDS!!

Diffusio

n makes everything the sa

me, but

Active Transport can Force High

Concentrations to Happen!!

Cell Transport Mechanisms and Homeostasis

OSMOSIS: The reality of molecular Transport. Everything is Aqueous!!

Most Molecules CANNOT pass through the Cell Membrane directly. However, Water is a small molecule and can “leak” through the various Transport Proteins and CANNOT be completely Stopped! Water is Always Moving!!

This can be a Problem

because?

High Conc. of the RIGHT STUFF Inside! Wants OUT (!), But cannot exit. It was moved inside by the cell.

Low Conc. Outside, but it

cannot penetrate the Membrane.

Not Equal.

But Water is everywhere as a solvent. It is always sticking to the various Solutes of the Environment. They have to Polar

if they are Dissolved!!

In Osmosis, water moves to make both

sides Equal!

The Solutes (Hearts) cannot

move (NOW!), but Water can always

penetrate the Membrane!

Cell Transport Mechanisms and Homeostasis

Endocytosis

The previous forms of Membrane Transport moved one molecule at a time. Sometimes Cells want to engulf Large Amounts of a substance – often Food Particles.

Endo = Inside, Cyto = Cell. This is often referred to as Bulk Transport. Phopholipid Bilayers will merge and separate if the Cytoskeleton attaches and pulls on them.

In Animals, this is a captured Food

Vacuole.FOOD

Membrane and Food pinched into Cell.

Transported on to be

Digested/Used!

Cell Transport Mechanisms and Homeostasis

ExoCytosis

What goes in – Must come Out! After it has been used and Processed by the Cell! Or Made for Export!

Exo = Outside, Cyto = Cell. This form of Bulk Transport removes materials from the Cell. After the Food Vacuole has been “used”, the waste that is still in the “container” can be expelled.

Cell Transport Mechanisms and Homeostasis

Cells HAVE to be in “Balance” because of Osmosis. The Water molecules cannot be completely stopped, so the Cytoplasm WILL BE in equal concentration with the Extracellular Fluid in Animal Cells.

However, the inside and Outside will be VERY DIFFERENT because of Membrane Transport.

Extracellular FluidExtracellular Fluid

CytoplasmCytoplasm

Some substances are

Blocked = Kept Out!

Some Substances are let/carried In = Selective!

Many Substances are Kept/Trapped Inside!

Waste/Used Substances

are expelled = Exocytosis!

Selective

Permeability

Cell Transport Mechanisms and Homeostasis

Animal Cells are Delicate and require very precise Transport Processes to stay in Homeostasis. No Extra Support.

Other Cells Possess a external Cell Wall that helps allow them to survive changing external conditions.

WATER!

But this is OK because the Vacuole holds the water

and keeps it separate from the Cytoplasm!

Why is it Important to keep the Water OUT of

the Cytoplasm?!