Cell Membrane Structure

How Is the Structure of a Membrane Related to Its Function? The Cell Membrane We want to figure out: How Is the Structure of a Membrane Related to Its Function?

Big Idea Membranes are “Fluid Mosaics” in which proteins move within layers of lipids The phospholipid bilayer Is the fluid portion of the membrane A mosaic of proteins is embedded in the membrane Membranes are dynamic, ever-changing structures

Big Idea Why called a FLUID? Phospholipid bilayer is flexible, allowing for cellular shape changes. Membrane lipids (and some proteins) can drift laterally within the membrane. Individual phospholipid molecules are not bonded to one another. Proteins drift more slowly than lipids. Some membrane proteins are tethered to the cytoskeleton and cannot move far. Watch this video!

Membrane proteins form a “Mosaic” Integral Proteins Span the entire membrane (transmembrane proteins are examples) Permanently attached to the cell membrane. Peripheral Proteins Not embedded- attached to one surface of the membrane. Attach only temporarily to the cell membrane. Sometimes attach to integral proteins.

Roles of Membrane Bound Proteins: Receptor Proteins Enzyme Proteins Adhesion Proteins Recognition Proteins Channel Proteins Pump Proteins

Functions of membrane proteins Receptor proteins each type binds to a specific molecule found outside the cell (like a hormone) which triggers a change inside the cell, or a cell action. Can be integral/peripheral

Functions of membrane proteins Receptor proteins

Functions of membrane proteins Receptor protein Example -Insulin Receptor Protein: Insulin is a hormone released by the pancreas when blood sugar levels are high. Insulin binds to the insulin receptor protein, which then causes the cell to open the typically closed glucose transport protein. This allows glucose to enter the cell from the blood.

Functions of membrane proteins Insulin Receptor Protein:

Functions of membrane proteins Enzyme proteins promote chemical reactions that synthesize or break apart biological molecules. Can be either integral or peripherial.

Functions of membrane proteins Enzyme protein Example -ATP Synthase: An important enzyme that provides energy for the cell by synthesizing adenosine triphosphate (ATP) Works by adding a phosphate onto adenosine diphosphate (ADP) to create ATP. Located in the membranes of chloroplasts and mitochondria (used in both photosynthesis & respiration) and the cell membranes of some prokaryotes.

Functions of membrane proteins ATP Synthase:

Functions of membrane proteins Adhesion proteins Anchors the cell membrane to the inner cytoskeleton or proteins outside the cell as well as to other cells. Can be integral or peripheral.

Functions of membrane proteins Adhesion protein example Cadherins: A type of transmembrane protein. In the presence of calcium- binds cells within tissues together.

Functions of membrane proteins Recognition proteins Serve as identification tags on the surface of a cell. Often times these are glycoproteins (proteins with an attached sugar molecule) Can be integral or peripheral.

Functions of membrane proteins Recognition protein example Major Histocompatibility Complex (MHC) Proteins: These are proteins on the surface of cells that belong to a particular individual. These MHC proteins interact with immune system cells to identify which cells belong to body and which cells are foreign.

Functions of membrane proteins Channel Proteins Serve as pores/tunnels for large or hydrophilic molecules to be passively (no energy required) transported into/out of the membrane All are integral

Functions of membrane proteins Channel protein example Glucose channel protein. Glucose is too large and too hydrophilic to diffuse naturally through the phospholipid bi- layer. Glucose needs a channel protein in order to do this. Notice in the picture glucose is moving from a high to low concentration, so no energy is required.

Functions of membrane proteins Pump Proteins Serve as pores/tunnels for large or hydrophilic molecules to be actively (energy required) transported into/out of the membrane All are integral

Functions of membrane proteins Pump protein example

Functions of membrane proteins Pump protein example Sodium/Potassium Pump Notice in this picture, both Na and K are moving against their concentration gradients (from low to high), which requires energy in the form of ATP.

Membranes must be fluid to work properly!!!!! Membranes must be fluid enough for the cell to move. Membranes need to be fluid enough for the required substances to move across them. Membranes if too fluid however, won’t be able to regulate effectively what substances move in and out (think of holes or rips occurring in the membrane).

In eukaryotes, a fat molecule called cholesterol modulates the membrane fluidity by making the membrane: Less fluid at warmer temperatures More fluid at lower temperatures

Cholesterol does this by binding adjacent phospholipids together. When temperatures rise, the cholesterol prevents rips in the membrane from occurring by holding the phospholipids together. When temperatures lower, the cholesterol prevents the membrane from freezing solid, by acting as a spacer between the phospholipid molecules.

Cholesterol tends to be found in high quantities in animal cells versus plant/fungi/bacteria. This is because animal cells don’t have a cell wall to maintain their structure, but still need to be flexible (fluid). Cold blooded animals (who can’t regulate body temperatures) tend to have more cholesterol than warm-blooded.