Transcript H. Bio Cell Membrane

Chapter 5
Homeostasis &
The Plasma Membrane
The Plasma Membrane

It’s all about balance!

Failure to adjust….death

Cells maintain balance by controlling
materials entering/leaving

Concentrations outside the cell change

Plasma membrane must be able to
maintain balance

Selective Permeability-property that
allows some materials to pass through
while keeping others out
Membrane Structure

Made of two layers made up of sheets of
lipid molecules-lipid bilayer
Membrane Structure

Proteins are embedded in the bilayer
 Think of raisins in raisin bread
Fluid Mosaic Model

The phospholipid bilayer acts more like a
liquid than a solid.
The Phospholipid Bilayer

Most lipids have two fatty acids
attached to glycerol

The third fatty acid is replaced by a
phosphate group-phospholipid

Phospholipids-polar, water-soluble head
+ long, nonpolar, insoluble tail

Align to form double layer with “heads”
on outside and “tails” on the inside
Diffusion

Cells maintain homeostasis by controlling
the movement of substances across the
membrane

Cells must use NRG to transport some
substances…..Ex- Active Transport!

Other substances move across the
membrane with no NRG required
Diffusion

Diffusion is the net movement of
particles from an area of higher
concentration to an area of lower
concentration.

Goal of diffusion=Dynamic Equilibrium
 Continuous movement with no
concentration change

Concentration gradient-difference in
concentration across space
Dynamic Equilibrium
Diffusion!!
Osmosis-Diffusion of Water

Osmosis is the diffusion of water molecules
through a selectively permeable membrane
 Movement from high to low concentration
 Type of passive transport!

Isotonic solution-concentration of dissolved
substances equal to concentration inside the
cell…..NO NET MOVEMENT!
Isotonic Solution
Hypotonic vs Hypertonic

Hypotonic-solution where concentration
of dissolved substances is lower than
inside the cell
 Water moves into cell…swelling!
 Increases cell pressure-Turgor Pressure

Hypertonic-solution where concentration
of dissolved substances is higher than
inside the cell
 Water moves out of cell…Plasmolysis
Dealing with Osmosis

Cells must compensate for the water that
enters or exits the cell.

Animal Cells- Must maintain isotonic
environment

Plant Cells- Prefer a hypotonic environment
Plants vs. Animals
Protists

Have contractile vacuoles that actively
remove water from the cell.
Crossing the Membrane

Diffusion through ion channels:
 Ion Channels- transport proteins with polar
pores that ions can pass through.
 Pore is the thickness of the membrane
 Ion does not have touch the nonpolar interior of
the bilayer.
Ion Channel
Facilitated Diffusion

Carrier proteins allow needed substances to
move through membrane by binding,
carrying, and releasing substances.

This movement is called facilitated diffusion
Facilitated Diffusion

Carrier proteins change shape using
chemical energy to move particle through
the membrane

Once particle is passed, original protein
shape is restored
Facilitated Diffusion
Active Transport

In order to move particles from a lower
concentration to higher concentration a cell
must use energy…Active Transport

Proton pumps- specialized proteins for pushing
different substances against the concentration
gradient.

Need ATP for energy!!!!!
Active Transport
Sodium-Potassium Pump

Four Steps to the Pump:
 1. Three Na ions attach to the pump.
 2. Pump changes shape, transporting the ions
across the membrane.
 3. Two K ions outside bind to the pump
 4. Two K ions are transported are released
inside the cell.
Transport of Large Particles

Endocytosis-process in which cell surrounds
and takes in material from its environment
 Engulfed and enclosed by portion of cell
membrane

Exocytosis-expelling wastes from interior to
exterior environment
Endocytosis Types
Exocytosis