5.1.1 Study Notes Plasma Membrane

A. Components of the Plasma Membrane

• a phospholipid bilayer with embedded proteins and cholesterol

  • Phospholipids are amphipathic molecules

    • having both hydrophilic and hydrophobic regions

      • nonpolar tails (hydrophobic) are directed inward

      • polar heads (hydrophilic) are directed outward to face both extracellular and intracellular fluid

• Proteins in the plasma membrane :

  • can be peripheral membrane proteins (on one side of the membrane)

  • integral membrane proteins(embedded in the plasma membrane)

    • usually found in the membrane and are held in place by the cytoskeleton and the extracellular matrix (ECM)

      • ECM are only found in animals and their functions include supporting the plasma membrane and communicating between cells

B. Fluid-Mosaic Model

• describes the plasma membrane

  • The fluid component refers to the phospholipids bilayer of the plasma membrane

    • allows cells to be pliable

    • affected by cholesterol molecules in the plasma membrane

  • The mosaic component refers to the protein content in the plasma membrane

    • prevent movement in the fluid phospholipid bilayer

C. Carbohydrate Chains

• Glycolipids have a structure similar to phospholipids except the hydrophilic head is a variety of sugar

  • they are protective and assist in various functions

• Glycoproteins have an attached carbohydrate chain of sugar that projects externally.

  • In animal cells, the glycocalyx is a “sugar coat” of carbohydrate chains and has several functions:

    • Cells are unique in that they have highly varied carbohydrate chains (a “fingerprint”).

    • The immune system recognizes foreign tissues that have inappropriate carbohydrate chains.

    • Carbohydrate chains are the basis for A, B, and O blood groups in humans.

D. The Functions of the Proteins

• Channel proteins 

  • allow a particular molecule to cross membrane freely (e.g., Cl- channels)

• Carrier proteins  / Carrier proteins

  • selectively interact with a specific molecule so it can cross the plasma membrane (e.g., Na+ K+ pump)

• Cell recognition proteins 

  • glycoproteins that allow the body’s immune system to distinguish between foreign invaders and body cells

• Receptor proteins 

  • are shaped so a specific molecule (e.g., hormone) can bind to it

• Enzymatic proteins 

  • carry out specific metabolic reactions

• Junction proteins 

  • join animal cells so tissues can function

E. Permeability of the Plasma Membrane  

• plasma membrane is selectively permeable

• only certain molecules can pass through. 

  • Small non-polar lipid molecules (alcohol, oxygen) pass through the membrane freely

  • Small polar molecules (carbon dioxide, water) move “down” a concentration gradient, i.e., from high to low concentration

  • Cells have channel proteins called aquaporins that allow water to cross a plasma membrane more quickly than expected

  • Ions and charged molecules cannot readily pass through the hydrophobic component of the bilayer and usually combine with carrier proteins

• Both passive and active mechanisms move molecules across membrane

  • Passive transport 

    • moves molecules across membrane without expenditure of energy

    • includes diffusion and facilitated transport

  • Active transport 

    • requires a carrier protein and uses energy (ATP) to move molecules across a plasma membrane

    • includes exocytosis, endocytosis, and pinocytosis

      • substances enter or exit a cell through bulk transport

E. How Do Cells Talk to One Another?

Cell Signaling

• •  Molecules, or chemical messengers, “talk” to other cells and may change cells, tissues, or organs

  • These cells do not respond to all molecules. They require binding to a receptor protein

  • Once the signaling molecule is bound to a receptor, the signal follows through a transduction pathway

  • The cell’s response to the transduction pathway can change the shape or movement of the cell, alter the metabolism or function of the cell, or alter the gene expression and amount of a cell protein