Membrane Transport

Reasons For Membrane TransportCells need membrane

transport to undergo cellular processes:

-- get water and nutrients into the cell

-- remove waste from the cell

-- communicate with other cells

-- maintain homeostasis

Crossing the Cell MembraneThe cell membrane is mostly phospholipids

Only non-polar (lipid soluble) substances can cross the membrane

-- oxygen molecules, carbon dioxide, other small non-polar molecules

-- occasionally, water molecules can find gaps

Large, polar molecules like water, glucose and ions need to cross through protein channels.

By regulating what goes in and out, cells maintain the proper concentrations for metabolism

We say that cell membranes are selectively permeable – choosing what to let in or out

The Concentration GradientConcentration – the amount of ions per unit volume of a substance

The concentration gradient is the comparison of the concentration of a substance inside the cell to the concentration of a substance outside the cell.

Substances usually move freely “down (with) the gradient”

Down (or with) the Gradient – moving from an area of high concentration to an area of low concentration

Against the Gradient – moving from an area of low concentration to an area of high concentration

-- to move against the gradient usually requires an input of energy and does not happen freely

Drawing of a Concentration Gradient

High concentration

Low Concentration

Movement “Down” or “with” the gradient

Movement “against” the gradient

Cell Membrane

DiffusionDiffusion – the movement of molecules down or with the concentration gradient

-- this is how non-polar substances move easily

Factors that affect diffusion rates:

-- size of the gradient

-- bigger gradient faster diffusion

-- temperature

-- higher temp faster diffusion

-- electric charge of the molecules

-- pressure

Diffusion Into a Cell

Transport MechanismsMolecules like glucose, water, etc. . . that cannot pass through the phospholipid bilayer need another way of entering and exiting the cell.

-- proteins act as transporters for these molecules

Transporters open and close when molecules bind

-- only open to one side of the membrane at a time

Transport can be either passive or active (some proteins do both and are called cotransporters)

Passive TransportPassive Transport – diffusion across a membrane through a transport protein

-- depends only on concentration or charge gradient

-- always goes “with” or “down” the gradient

-- also called “facilitated diffusion”

Active TransportActive Transport – the pumping of molecules against the gradient through proteins

-- process requires energy (from ATP)

Why would a cell do this?

-- need to maintain both a concentration and charge gradient for cells to function properly

-- muscle and nerve cells need an electric charge potential to contract

Examples of Active Transport Mechanisms

-- Calcium pump

-- Sodium/Potassium Pump

OsmosisOsmosis – the diffusion of water across a cell membrane

-- more water crosses the membrane than anything else

The diffusion gradient for water depends on the number of solute molecules on both sides of the cell membrane (called tonicity)

Hypotonic Solution – the solution outside the cell has a lower concentration of solutes than inside the cell

Hypertonic Solution – the solution outside the cell has a greater concentration of solutes than inside the cell

Isotonic Solution – the concentration of solution inside and outside the cell are equal

Solutions tend to flow from hypotonic to hypertonic fluids

A Cell in Hypotonic ConditionsThe concentration of solutes inside the cell is greater than

the concentration of solutes outside the cell

-- water rushes into the cell

-- cells expand and burst

Distilled Water

0% sucrose

2% sucrose

A Cell in Hypertonic ConditionsThe concentration of solutes outside the cell is greater

than the concentration of solutes inside the cell

-- water rushes out of the cell

-- the cell shrivels up

Solution of

10% sucrose

2%sucrose

A Cell in Isotonic ConditionsSince the concentrations both inside and outside the cell are equal, there is no net movement of water

-- cell remains the same

-- most body fluids are isotonic

-- cells also have built-in mechanisms to adjust to changes in tonicity

Solution of

2% sucrose

Solution of

2% sucrose

2%sucrose

2%sucrose

Avoiding Changes in Fluid PressureAnimal cells transport

solutes in and out of cells to prevent bursting and shrinking

-- water pressure builds up in the cell (called turgor pressure)

-- cell membrane pulls away from cell wall

-- eventually, turgor pressure reaches a point called the osmotic pressure, where the force of the pressure prevents further osmosis

Plant cells continue to expand because of their cell wall

Taking In Large MoleculesCells can only move small molecules through the membrane.

To move large molecules, cells use vesicles, which are small, membrane-bound sacs to move molecules to and from the cell surface

This process is called either exocytosis or endocytosis

ExocytosisIn exocytosis, the cell moves large molecules out of the cell

A vesicle containing material to leave the cell forms in the cytoplasm

The vesicle reaches the cell membrane and fuses with it

The contents of the vesicle are released outside the cell

EndocytosisIn endocytosis, cells take in large molecules

When large molecules are near the surface, the cell membrane balloons inward and traps the molecules

The membrane pinches off to form a vesicle inside the cell

PhagocytosisPhagocytosis is a specialized form of endocytosis, in which cells take in very large molecules

-- looks more like a whole section of cell forming around a molecule than the small vesicle of endocytosis

-- this is how amoebas (and other protists) get food, and how our white blood cells destroy viruses

-- actually means “cell eating”