Cell Transport

The movement of molecules into and out of a cell

How is a phospholipid constructed?

Interactions of phospholipid molecules with waterThe basis of phospholipid bi-layers

The phospholipid bilayer is the fundamental structure of the membrane

The cell membrane contains both hydrophilic and hydrophobic regions

The Cell Membrane

• The cell membrane is composed of 2 layers of phospholipids

• Also called the phospholipid bilayer or plasma membrane

• The Polar heads face the outside of the membrane where water exists

• The Nonpolar tails face each other in the inside of the bilayer creating a hydrophobic region

Membranes are primarily composed of phospholipids and proteins

Properties of Membrane

Membranes have the important function of regulating the movement of substances into or out of cells or organelles

The Fluid-Mosaic Model of a membrane

Two types of proteins in membranes – peripheral proteins and integral proteins

peripheral protein integral protein

The Passage of Molecules across the Plasma Membrane

Hydrophobic molecules and non-polar molecules (such as O2) cross the membrane with ease.

Hydrophilic molecules cannot, however, slip through the extensive hydrophobic core of the membrane.

The transport of hydrophilic compounds requires the help of the integral proteins of the membrane.

Diffusion -- a spontaneous process in which particles move from an area of high concentration to an area of low concentration across a concentration gradient

When two different solutes are diffusing at the same time, they will each follow their own concentration gradients from high to low.

The net diffusion occurs from a region of high concentration toward a region of low concentration.

High concentration Low concentration

What happens if we remove the barrier?

Diffusion – fills the space – it is spontaneous, no work is done

High concentration Low concentration

Diffusion Summarized

Some molecules can simply pass through

Example: small molecules

No energy required to pass: passive transport

Move from high low concentration: diffusion

Diffusion will occur until equilibrium is reached

OsmosisOsmosis – the diffusion of water across a selectively-permeable membrane

higher solute concentration

Solute is defined as polar or charged molecule or ion

Water is attracted to solutes!

OsmosisOsmosis – the diffusion of water across a selectively-permeable membrane

water level rises

Osmosis Summarized

Water can simply pass through Special type of diffusion: osmosis No energy required: passive transport

Tonicity – the ability of a solution to cause a cell to loose or gain water

Cellwith higher

soluteconcentration

Water with low solute concentration

hypotonic

H2O

Hypotonic solution – contains fewer solutes than cell (Cell will enlarge when placed in this solution!)

Tonicity – the ability of a solution to cause a cell to loose or gain water

Cell(with solutes)

Water with high solute concentration

hypertonic

H2O

Hypertonic solution – contains more solutes than cell (Cell will shrink when placed in this solution!)

Tonicity – the ability of a solution to cause a cell to loose or gain water

Cell(with solutes)

Water with equal solute concentration

isotonic

H2O

Isotonic solution – contains equal solutes as cell (Cell will retain size when placed in this solution!)

Tonicity in animal cells

Tonicity in plant cells

Turgid(normal)

Flaccid(slight water stress)

Plasmolyzed(severe water stress)

Facilitated Diffusion

No ATP energy required

Molecules follow their concentration gradients

They are just “helped” across the hydrophobic core of the membrane by a transport protein

Facilitated Diffusion

Net rate of diffusion from high concentration toward low concentration

channel

gate

Facilitated Diffusion Summarized

Some molecules are too large to pass normally Example: monosaccharides Carrier proteins assist: facilitated diffusion No energy required: passive transport

Facilitated Diffusion Summarized

Ions have a charge: cannot normally pass Examples: Na+, Ca+, Cl-

Can pass through ion channel No energy required: passive transport

Active Transport

Molecules go against their concentration gradients

They are “helped” across the hydrophobic core of the membrane by a transport protein, but only when energy is expended.

Work is done by the cell! Requires ATP (Energy)!!

Active Transport Summarized

Molecules often move from low high concentration Such movement requires energy: active transport Carrier protein used as cell membrane “pump” Example: Na+ and K+ pump

Review the Differences

Bulk Transport

• EndocytosisEndocytosis – movement of large molecules into the cell– Phagocytosis – movement of foods (cell

eating)– Pinocytosis – movement of fluids (cell

drinking)

• ExocytosisExocytosis – movement of large molecules out of the cell