Cell Biology:The Cell Membrane

Lesson 1 – Structure and Function of the Cell Membrane(Inquiry into Life pg. 67-69)

Today’s Objectives Analyze the structure and function of

the cell membrane, including: Apply knowledge of organic molecules such as

phospholipids, proteins, glycoproteins, glycolipids, carbohydrates, and cholesterol, to explain the structure and function of the fluid-mosaic membrane model

Identify the hydrophobic and hydrophilic regions of the phospholipid bilayer

Explain why the cell membrane is described as “selectively permeable”

Cell (Plasma) Membrane Function The cell membrane regulates the entrance

and exit of molecules into and out of the cell

Cell Membrane Function Impermeable – nothing can

pass through Permeable – most things

can pass through Semi-permeable – smaller

molecules pass through but not larger molecules

Selectively permeable – only certain small molecules and certain large molecules can pass through

Cell membranes are selectively permeable Also called differentially

permeable

Cell Membrane Structure Early Theory:

Proteins were sandwiched between 2 layers of phospholipid molecules

Problem: Since the phospholipid tails are hydrophobic (hate water) this theory did not explain how water (or other polar molecules) can travel freely through membranes

Cell Membrane Structure Accepted Theory: Fluid-Mosaic Model

There is a double layer of phospholipids but the proteins are scattered through the membrane (mosaic)

The proteins float among the semi-fluid phospholipids

Polar molecules can travel through the pores in the proteins

Proteins in the Cell Membrane The proteins have polar and

non-polar regions, which accounts for their placement among the phospholipid bilayer Proteins imbedded in the

membrane are called integral proteins (can move freely)

Proteins partially imbedded in the membrane (on one side) are called peripheral proteins

Peripheral proteins help maintain membrane shape and are often held in place by cytoskeletal filaments

Peripheral Protein

Integral Protein

Proteins in the Cell Membrane Some of the proteins and phospholipids have

carbohydrate chains attached to them (always on outer side of the cell membrane) Protein + Carbohydrate chain = glycoprotein Protein + Carbohydrate chain = glycolipid

These carbohydrate chains function as cell identification markers

They help identify pathogens (germs) and tell the body to attack them

An example of this is when a body rejects an organ transplant

Glycoproteins and Glycolipids Think of the carbohydrate chains as

antennae such as on an insect: Just like the antennae, the carbohydrate chains

can detect things either harmful or beneficial to the cell

Types of Proteins in the Cell Membrane There are several different types of proteins in

the cell membrane, each with a different function Control what enters and exits the cell Detect harmful pathogens Function as enzymes that catalyze specific

reactions We will learn more about some of these

proteins in the next lesson, but for now, here is a quick introduction

Types of Proteins

1) Channel Protein Allow particular molecules or ions to cross the

plasma membrane

Types of Proteins

2) Carrier Protein Selectively interacts with a specific molecule or

ion so that it can cross the plasma membrane

Types of Proteins 3) Cell Recognition

Protein Cell recognition proteins

are glycoproteins Help the body

recognize when it is being invaded by pathogens so that the immune system can destroy them

Types of Proteins 4) Receptor Protein

Shaped in such a way that a specific molecule can bind to it

The binding of the molecule causes the protein to change its shape, bringing about a cellular response (reaction)

Types of Proteins 5) Enzymatic

Protein Catalyze specific

metabolic reactions