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Biology Cell Class Project

Semipermeable Membranes

A-Semipermeable Membranes
C- Cells and Viruses
D- Molecular Biology
E- E.R. and Golgo Appartus
F- Photosynthesis
G- Mitochondria
H- Macromolecules
I- Chemiosmotic gradients
J- Shape and Forms of Eukaryotic Cells

Students know cells are enclosed within semipermeable membrances that regulate their interaction with theri surroundings.



A membrane is the thin layer that forms the outer boundary of a living cell or of an internal cell compartment. The outer membrane of the cell is called the plasma membrane and the membranes that enclose over some of the cell's compartments are called organelles.

Semipermeable Membranes
A type of membrane that allows some materials to pass through it but not all.


This picture shows 3 different types of semipermeable membreanes. In all 3 examples there all blue dots and water in them. All the cases show that water is permeable to the membrane. Water can diffuse from a higher concentration to a lower concentration but there are three results depending if the water diffuses or not.  The three pictures show the three results that can happen. In A it shows that the net movement will be the same on both sides of the membrane making o difference in water changes on either side. B shows that the water diffused from left to right showing that the level of water rised on the right. C shows that the water diffused to the left allowing the blue dots to have more room. 


Diffusion is the movement of a substance from an area of greater concentration to an area of its lesser concentration. One example of diffusion is when you put food coloring in water. Instead of the dye staying in one spot it moves throughout the water changing the whole cup of water a different color. This is diffusion since it was in one general spot but then diffused to other parts.

Osmosis is the diffusion of water through a selectively permeable membrane. It is when water moves from an area of high concentration  to an area of low concentration. 


At first in A we have the water solution on one side and water with a solute on the other. Water will move across the selectively permaeble membrane from where it is more concentrated to where the concentration of water is less. In other words, the water will move more to the other side since the solute is taking up most of the space and water is less concentrated there. In picture B, you see the results of the osmosis and that on the left side is has a lesser percentile of solute since water was added.  

Isotonic, Hypertonic, and Hypotonic

When a cell encounters a water solution there are three different outcomes that happens to the cell.
Isotonic- The concentration of solute in the solution can be equal to the concentration of solute in the cells.  This means that the cell stays the same.  Iso also means normal so the cell is normal when comes into contact with the water solution. For a red blood cell in this condition, it looks normal and and in a plant cell it also looks normal with no defaults in the cells.
Hypotonic- The concentration of solute in the solution can be greater than the concentration of solute in the cells.  In this situation, water leaves the cell by osmosis and makes the cell to shrivel up. When a red blood cell goes through this process the cell may continue to swell until it bursts while a plant cell swell beyound their normal size and the pressure keeps on increasing.
Hypertonic- The concentration of solute in the solution can be less than the concentration of solute in the cellsCells in this condition experience osmosis because water moves through the plasma membrane and into the cell causing the cell to build in pressure internally.  Plant cells in hypertonic conditions, lose pressure as the plasma membrane shrinks away from the cell wall.  Red blood cells in this condition shrival up and loses its water content.

The Job of the Membrane
The job of the membrane is to allow a steady supply of nutrients to come into the cell no matter what the external conditions are.  It is a flexible boundary between the cell and its environment. The plasma membrane is composed of a phospholipid bilayer. A phospholipod consists of a glycerol backbone, two fatty acid chains, and a phosphate group. The inside of the bilayer consists of the fatty acid tails while the heads of the phosolipids are the external parts.  Phosolipds heads are water soluable while the internal fatty tails are water-insoluable. 


This picture of the phospholipid bilayer shows how the outer layer of a membrane is water soluable while the inside is not. It also displays where the "heads" and "tails" of phospholipids are.

The middle of the plasma membrane is a group of fatty acids. The acids are non-polar and items that are polar have difficulty passing through the membrane.  Membrane proteins allow a special passage for these polar items in order to recieve the nutrients from the outside without having the fatty acids becoming a problem. 

The Importance of Transport Proteins

A transport protein is a type of protein that help regulate what molecules can enter and leave the cell.  They move needed nutrients or proteins through the membrane. This means is helps gives the cell the type of nutrients it needs in order for it to survive and it helps throw away the unneeded items such as waste. 

Different Types of Transports

Active Transport- A cell uses its energy in order to have their molecules move through a selectively permeable membrane against their concentration gradients.  Active transport uses special types of pumps to move the gradients.  2 types of active transports are endocytosis and exocytosis.
Passive Transport- When materials move into cells without using energy.  A type of passive transport is diffusion because the cell doesn't use any of its cellular energy in order to get materials inside of it.  Another type of passive transport is called facilitated diffusion.