The phospholipid bilayer is a good barrier around cells, especially to water soluble molecules. However, for the cell to survive some materials need to be able to enter and leave the cell.
There are 5 basic mechanisms:
Passage Through The Phospholipid Bilayer
What molecules can pass through the phospholipids bilayer?
Small non polar (O2)
Large non polar (lipids)
Small polar (water and CO2)
What molecules cannot pass freely through the bilayer?
Large polar (starch, glucose, and proteins)
Ions (Na+, K+)
Diffusion of liquids
Diffusion through a membrane Cell membrane Inside cell Outside cell
Diffusion through a membrane Cell membrane Inside cell Outside cell diffusion
Diffusion through a membrane Cell membrane Inside cell Outside cell EQUILIBRIUM
Diffusion is the net movement of molecules (or ions) from a region of their high concentration to a region of their lower concentration.
molecules move down a concentration gradient .
Molecules have kinetic energy , which makes them move about randomly.
As a result of diffusion, molecules reach an equilibrium where they are evenly spread out.
Equilibrium = no net movement of molecules from either side.
DIFFUSION refers to the movement (or non) of the SOLUTE (glucose, starch, gases across the membrane!
Molecules that diffuse through cell membranes!
‘ The diffusion of water from an area of high concentration of water molecules to an area of low concentration of water across a semi permeable membrane.
Osmosis Cell membrane partially permeable. Inside cell Outside cell VERY High conc. of water molecules. VERY Low conc. of water molecules. Sugar molecule DILUTE SOLUTION CONCENTRATED SOLUTION
Osmosis Cell membrane partially permeable. Inside cell Outside cell High conc. of water molecules. Lower water potential. Low conc. of water molecules. High water potential. OSMOSIS
Osmosis Cell membrane partially permeable. Inside cell Outside cell OSMOSIS EQUILIBRIUM. Equal water concentration on each side. Equal water potential has been reached. There is no net movement of water
Movement of water in response to the amount of solute on either side of a membrane.
The Rule -
Water will always move toward the side of the membrane with the most solute!
Tonicity comes -in 3 types:
Hypertonic – the side the membrane or part of the solution with the greater amount of solute
Hypotonic – side/part with the lower amount of solute
Isotonic – equal solute content
? ? Which side is hypotonic and which side is hypertonic? A B Draw THIS U-TUBE as it will appear after osmosis takes place.
Which way will the water move?
A solution is made of a solute and solvent.
OSMOSIS: solvent is WATER.
Solute is what is dissolved in water.
THE DIRECTION OF WATER’S CONCENTRATION GRADIENT IS ALWAYS OPPOSITE TO THE DIRECTION OF THE SOLUTE’S CONCENTRATION GRADIENT .
LOW SOLUTE CONCENTRATION HIGH SOLUTE CONCENTRATION HYPOTONIC HYPERTONIC HIGH WATER CONCENTRATION LOW WATER CONCENTRATION LOW OSMOTIC POTENTIAL HIGH OSMOTIC POTENTIAL DILUTE CONCENTRATED
Remember: A hypertonic solution is always relative to a hypotonic solution
TONICITY IN CELLS
Water Movement in the Cell
This is an Osmosis tutorial…for the smart board….
Hypotonic vs. Hypertonic Cells placed in a:
animal cell will gain water – LYSE
Plant cell will swell and become TURGID
animal cell will lose water – shrivel
Plant will lose water – plant cell membrane will detach from cell wall - plasmolysis
Do Water Molecules Stop Moving in Isotonic Conditions ?
They continue to diffuse, however there is no net movement.
Equal does not mean STOP
Large polar molecules such as glucose and amino acids , cannot diffuse across the phospholipid bilayer. Also ions such as Na + or Cl - cannot pass.
These molecules pass through protein channels instead. Diffusion through these channels is called FACILITATED DIFFUSION.
Movement of molecules is still PASSIVE just like ordinary diffusion, the only difference is, the molecules go through a protein channel instead of passing between the phospholipids.
Facilitated Diffusion Animation
Facilitated Diffusion through a membrane Cell membrane Inside cell Outside cell Protein channel
Facilitated Diffusion through a membrane Cell membrane Inside cell Outside cell Protein channel diffusion
Facilitated Diffusion through a membrane Cell membrane Inside cell Outside cell Protein channel diffusion EQUILIBRIUM
Facilitated Diffusion: Molecules will randomly move through the opening like pore, by diffusion. This requires no energy, it is a PASSIVE process . Molecules move from an area of high concentration to an area of low conc.
Cells expend energy for active transport .
Active transport involves the aid of a transport protein in moving a solute up a concentration gradient (from an area of low concentration to an area of high concentration).
Energy is required to help the protein to move the solute molecule.
What molecular form does this energy usually have?
ATP – usable cellular energy!
Exocytosis and Endocytosis transport large molecules.
In exocytosis , membrane-bound vesicles (tiny membrane-bounded sacs) containing large molecules fuse with the plasma membrane and release their contents outside the cell.
In endocytosis , the plasma membrane surrounds materials outside the cell, closes around the materials, and forms membrane-bound vesicles to contain the materials.
Two important types of endocytosis are
phagocytosis (“cell eating”) and
pinocytosis (“cell drinking”).
The “engulfing” of material by the cell
The “cell drinking” of material by the cell
Phagocytosis takes in solid particles. Pinocytosis takes in liquids.
Active transport is used to:
Generate charge gradients. Ex. In the mitochondrion, hydrogen ion pumps, pump hydrogen ions into the “intermembrane” space of the organelle as part of making ATP.
2. Concentrate ions , minerals and nutrients inside the cell that are in low concentration outside.
3. Move sodium and potassium ions in and out of the cell. VERY important for nerve function.