How do organisms keep things How do organisms keep things ““just rightjust right””??

How is the plasma membrane How is the plasma membrane like the border between two like the border between two

countries? countries?

Just like countries Just like countries regulate who get in regulate who get in and who goes out, our and who goes out, our cells must do the same cells must do the same thing?thing?

Get an iPad from the cart Get an iPad from the cart and a copy of theand a copy of the

““Cell Membrane Web Quest” Cell Membrane Web Quest”

Use the Use the information in the information in the

web quest and web quest and the web links the web links (they are also (they are also located at the located at the

bottom of todays bottom of todays biology page) to biology page) to

complete the complete the activity in your activity in your

biology notebook. biology notebook.

Cholesterol

Carbohydrate chains

CytoplasmMembrane proteins

The plasma The plasma membrane helps the membrane helps the cell maintain cell maintain homeostasis by homeostasis by regulating what goes regulating what goes in and out of the cell in and out of the cell

You can think of the plasma membrane like a “plastic bag with tiny wholes” because . . .

The bag holds all of the cell pieces and fluids inside the cell and keeps any nasty things outside the cell. The holes are there to let some things move in and out of the cell.

The cell membrane is not a solid structure. It is made of millions of smaller molecules that create a flexible and porous container.

Phospholipids

Proteins

Proteins and phospholipids make up most of the membrane structure.

The fluid mosaic model shows you that phospholipid molecules are shaped with a head and a tail region.

The head section of the molecule likes water (hydrophilic) while the tail does not (hydrophobic).

Because the tails want to avoid water, they tend to stick to each other and let the heads face the watery (aqueous) areas inside and outside of the cell. The two surfaces of molecules create the lipid bilayer.

Click the membrane to view a YouTube video of the fluid mosaic model

You will find thousands (millions?) of proteins throughout the cell membrane. Some are just on the inside of the cell and some on the outside. A special few cross the cell membrane. Each type of protein has a specific purpose. There are also embedded proteins in the other membranes for cell organelles.

Membrane proteins

There are two types of proteins in the cell membrane -- peripheral proteins and integral proteins. Integral membrane proteins are permanently connected to the cell membrane. They have large sections embedded in the hydrophobic (middle) layer of the membrane.

Peripheral proteins are not bonded as strongly to the membrane. They may just sit on the surface of the membrane, anchored with a few hydrogen (H) bonds.

Integral proteins are the hard workers of the cell membrane. Some integral proteins cross the membrane and act as pathways for ions and molecules.

Substances such as oxygen, carbon dioxide and steroids easily diffuse across the cells membrane . . .

. . . because the cell membrane is mostly lipids, only lipid soluble molecules (things that can dissolve in lipids) are able to easily pass through

Glucose, amino acids, ions and water need the help of various proteins for transport across the membrane.

Because they are soluble (dissolve) in water, not lipids.