1.3: Membrane Structure

*Contrast the two types of transport proteins: pumps and channels.​* Understanding: Membrane proteins are diverse in terms of structure, position in the membranes and function.

*Channel proteins* are used for passive transport of molecules as they move across the bilayer from higher to lower concentration. *Pump proteins* are used for active transport of molecules as they move across the bilayer from lower to higher concentration.

*Contrast the structure of integral and peripheral proteins.* Understanding: Membrane proteins are diverse in terms of structure, position in the membranes and function.

*Peripheral proteins* sit on one side of the surface of the cell membrane. *Integral proteins* are embedded in the hydrophobic middle of the bilayer. Some integral proteins are "transmembrane" meaning they span both sides of the bilayer.

*List functions of membrane bound proteins.* Understanding: Membrane proteins are diverse in terms of structure, position in the membranes and function.

1. *Receptor proteins* receive extracellular signals. 2. *Transport proteins* move ions and molecules across the bilayer. 3. *Enzymes* catalyze reactions. 4. *Adhesion proteins* anchor the cell to other cells. 5. *Recognition proteins* identify the cell type in a multicellular organism.

*Define amphipathic.* Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

A molecule that contains both hydrophilic and hydrophobic regions. i.e. a phospholipid

*Outline the amphipathic properties of phospholipids.* Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Amphipathic means there are both hydrophilic and hydrophobic regions in a single molecule. Phospholipids have a hydrophilic head region and hydrophobic tails.

*Describe why the understanding of cell membrane structure has changed over time.​* Nature of Science: Falsification of theories with one theory being superseded by another-evidence falsified the Davson-Danielli model.

As tools and technology advance, our understanding of biological structures and functions improves. Techniques such as freeze-fracture, cell fusion, fluorescent tagging and protein purification have enabled scientists to gain a more accurate understanding of the structure of the cell membrane.

*Describe conclusions about cell membrane structure drawn from cell fusion experiments.* Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Cell fusion experiments showed that protein molecules can move from place to place within the cell membrane; there is fluidity.

*Describe conclusions about cell membrane structure drawn from freeze-etched electron micrograph images of the cell membrane.* Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Cells are rapidly frozen and then fractured. Fracture occurs along lines of weakness, including between the two layers of phospholipids. Freeze-etched cell membranes provided evidence that the membrane was a bilayer with embedded proteins.

*Describe the function of cholesterol molecules in the cell membrane.​* Understanding: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

Cholesterol acts as a regulator of membrane fluidity. At *high temperatures* if stabilizes the membrane and reduces melting. At *low temperatures* is prevents stiffening of the membrane. Membrane fluidity influences how permeable the structure is to solutes. Too fluid (higher temps) = too permeable Too stiff (lower temps) = not permeable enough

*Describe the structural placement of cholesterol within the cell membrane.​* Understanding: Cholesterol is a component of animal cell membranes.

Cholesterol fits between phospholipids in the cell membrane, with its hydroxyl (-OH) group by the heads and the hydrophobic rings by the fatty acid tails.

*Identify the structure of cholesterol in molecular diagrams.* Understanding: Cholesterol is a component of animal cell membranes.

Cholesterol is a lipid that can be distinguished by its characteristic four-ring structure.

*Describe the observations and conclusions drawn by Davson and Danielli in discovering the structure of cell membranes.​* Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.

Davson and Danielli proposed the "protein-lipid sandwich" model of the cell membrane, in which a phospholipid bilayer was embedded between two layers of proteins.

*Draw a simplified diagram of the structure of the phospholipid, including a phosphate-glycerol head and two fatty acid tails.* Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Head = phosphate and glycerol Tails = fatty acids

*Describe conclusions about cell membrane structure drawn from improvements in techniques for determining the structure of membrane proteins.* Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Improvements in tools and techniques allows scientists to extract membrane proteins and determine their chemical and physical properties. The membrane proteins were found to be varied in shape and size. Additionally, some proteins had hydrophobic regions. These findings did not match the model proposed by Davson and Danielli, in which proteins were uniform in shape and hydrophilic.

*Explain the purpose of developing models in science.* Nature of Science: Using models as representations of the real world-there are alternative models of membrane structures

Models are conceptual representations used to *explain and predict* phenomena.

*Define hydrophobic.* Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Nonpolar molecules (or regions of molecules) to which water will not attract. "Water fearing."

*Draw and label the structure of a cell membrane.* Skill: Drawing of the fluid mosaic model.

Phospholipid bilayer shown with heads facing in opposite directions Phospholipids with labelled hydrophilic/phosphate head and hydrophobic/hydrocarbon tail Peripheral protein, shown as globular structure at the surface of the membrane Integral protein shown as embedded globular structure Glycoprotein shown as embedded globular structure with protruding carbohydrate (shown as a branching, antenna-like structure) Channel protein shown with a pore passing through it Cholesterol shown in between adjacent phospholipids

*Define hydrophilic.* Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Polar and/or charged molecules (or regions of molecules) to which water can attract. "Water loving."

*Compare the Davson-Danielli model of membrane structure with the Singer-Nicolson model.​* Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Singer and Nicolson proposed a membrane model that incorporated evidence about membrane proteins that did not comply with the Davson-Danielli model. Rather than having proteins on the surface of the phospholipids, Singer-Nicolson proposed a model in which proteins were embedded within and through the membrane, called the fluid mosaic model.

*State the primary function of the cell membrane.* Understanding: Membrane proteins are diverse in terms of structure, position in the membranes and function.

The cell membrane is semi-permeable and controls the movement of substances into and out of the cell.

*Explain why phospholipids form bilayers in water.* Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

There is water both outside and inside the cell. Phospholipids will arrange themselves in a bilayer so that the hydrophilic head associates with water and the hydrophobic tails face each other, away from the water.