1.3: Membrane Structure

Explain why phospholipids form bilayers in water.

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

Describe why the understanding of cell membrane structure has changed over time.​

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. Nature of Science: Falsification of theories with one theory being superseded by another-evidence falsified the Davson-Danielli model.

Contrast the two types of transport proteins: pumps and channels.​

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

Describe the function of cholesterol molecules in the cell membrane.​

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 Understanding: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

Describe the structural placement of cholesterol within the cell membrane.​

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. Understanding: Cholesterol is a component of animal cell membranes.

Identify the structure of cholesterol in molecular diagrams.

Cholesterol is a lipid that can be distinguished by its characteristic four-ring structure. Understanding: Cholesterol is a component of animal cell membranes.

Describe the observations and conclusions drawn by Davson and Danielli in discovering the structure of cell membranes.​

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. Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.

State the primary function of the cell membrane.

The cell membrane is semi-permeable and controls the movement of substances into and out of the cell. Understanding: Membrane proteins are diverse in terms of structure, position in the membranes and function.

Define amphipathic

A molecule that contains both hydrophilic and hydrophobic regions. i.e. a phospholipid Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Outline the amphipathic properties of phospholipids.

Amphipathic means there are both hydrophilic and hydrophobic regions in a single molecule. Phospholipids have a hydrophilic head region and hydrophobic tails. Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

List functions of membrane bound proteins.

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

Describe conclusions about cell membrane structure drawn from cell fusion experiments.

Cell fusion experiments showed that protein molecules can move from place to place within the cell membrane; there is fluidity. Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Describe conclusions about cell membrane structure drawn from freeze-etched electron micrograph images of the cell membrane.

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. Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Draw a simplified diagram of the structure of the phospholipid, including a phosphate-glycerol head and two fatty acid tails.

Head = phosphate and glycerol Tails = fatty acids Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Describe conclusions about cell membrane structure drawn from improvements in techniques for determining the structure of membrane proteins.

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. Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

Explain the purpose of developing models in science.

Models are conceptual representations used to explain and predict phenomena. Nature of Science: Using models as representations of the real world-there are alternative models of membrane structures.

Define hydrophobic

Nonpolar molecules (or regions of molecules) to which water will not attract. "Water fearing." Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Contrast the structure of integral and peripheral proteins.

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

Draw and label the structure of a cell membrane.

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 Skill: Drawing of the fluid mosaic model.

Define hydrophilic

Polar and/or charged molecules (or regions of molecules) to which water can attract. "Water loving." Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

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