Cell Membrane
The human body consists of 75 trillion cells that look and act very different. Nerve cells look and act different than skin cells. Differences in cell shape make different functions possible. (Nerve vs. Skin) Once again the concept of structure and function.
Example of a Typical Cell
A cell consists of three main parts: the nucleus, the cytoplasm and the cell membrane. The cytoplasm is the fluid that the organelles are contained in. The fluidity of the cytoplasm allows for movement of organelles, cell products and other cellular transport. Organelles are specialized cells that perform a specialized function.
Cell Membrane is the barrier that separates the outside from the inside. The cell membrane communicates with neighboring cells and tissue because of complex adaptations on the outside of the cell membrane. It controls what can pass through the membrane and protects the inside contents from the outside world. The cell membrane is referred to as selectively permeable because it allows some things to pass through, controlling when and how much. It excludes other items completely.
Questions:
Would you want harmful bacteria to pass through the cellular membrane? _______________________
Would you want to control the flow of nutrients? ___________________________________________
Would you want your nucleus to travel outside the cell membrane? ____________________________
Cell Membrane Structure
The cell membrane is very thin and composed of two layers. Each layer is made of phospholipids, which contain a phosphate head and two fatty acid tails each. The phospholipid is on the outside. The fatty acid tails point to the inside. The two layers create a zone of phospholipids. The membrane is referred to as a phospholipid bilayer. This structure creates its very unique control function.
The phospholipids are charged molecules, so they chemically interact well with water and other charged molecules. The fatty acid tails do not like to be around water and charged molecules (hydrophilic). They prefer to interact with other fatty acid molecules (hydrophobic).
Hydrophilic – “water loving” – Chemically interacts with charged molecules.
Hydrophobic – “water hating” – Does not interact with polar molecules, prefers lipid molecules.
The cell membrane is like a sandwich – polar molecules on the outside and hydrophobic on the inside. By having the hydrophobic zone on the inside, it controls and limits what can pass through the membrane. Anything that goes through the membrane has to interact with the hydrophobic zone. This prevents charged molecules from passing through the membrane. These molecules have to pass through the cell’s transport proteins, which are regulated by the membrane. Gases and steroid hormones can pass through the phospholipid bilayer. Small cholesterol molecules (hydrophobic molecules) are in the lipid bilayer to control fluidity of the membrane and further prevent polar molecules from passing through.
Questions:
When you place oil in water, what happens? __________________________________________________________________________
Does the oil prefer to stay by other oil molecules? ___________________________________________________________________________
Is this an example of the hydrophobic molecules wanting to interact with other hydrophobic? __________________
Do hydrophobic oil molecules want to avoid hydrophilic water molecules? ______________________
If you next added charged sugar molecules, would they go to the hydrophobic or hydrophilic region? _______________________________________________________________________________________________________________________________________
Fluid Mosaic is a term used to describe the membrane, moving and fluid phospholipids with proteins embedded and fixed in the membrane.
Cell membrane proteins provide a variety of functions. Some proteins are floating on the top of the membrane and are mainly for communication with other cells, hormones, and body signals. The proteins that do not travel all the way through the cell membrane are called peripheral proteins.
What does peripheral mean? _________________________________________________________
Transmembrane proteins pass all the way through the phospholipid bilayer. These proteins can be used to transport molecules from the outside of the cell to the inside. The transmembrane proteins also act as structural anchors to provide support to the cellular framework.
Questions:
Why are proteins that travel all the way through the phospholipid bilayer needed for transporting molecules from one side of the membrane to the other? ______________________________________
Can polar molecules cross the cell membrane? _____________________________________________________________________________
What if the polar molecules were traveling in a tube and did not interact with the lip layer? _____________________________
Is it an advantage for the cell to have free flowing of molecules through the tube? Y or N?
Why? _____________________________________________________________________________
If tubes only allowed specific molecules through, would that help the cell regulate and control flow? Y or N? Would this be more protective?