Cell Membrane Study Guide

U1.3.1: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules (Oxford Biology Course Companion page 26).

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

2. Define hydrophilic and hydrophobic.Hydrophilic: molecule that is attracted to water

Hydrophobic: molecule that is repelled by water

3. Define amphipathic and outline the amphipathic properties of phospholipids.A molecule that has both hydrophilic and hydrophobic parts.Phospholipid has a hydrophilic head and hydrophobic tails

4. Explain why phospholipids form bilayers in water, with reference to hydrophilic phosphate heads and two hydrophobic hydrocarbon tails.

The hydrophilic heads are attracted to water while the hydrophobic tails are repelled by water. The bilayer alignment allows the tails to be removed from water while the heads are exposed to extracellular fluid/cytosol

NOS 1.3.1: Using models as representations of the real world-there are alternative models of membrane structures (Oxford Biology Course Companion page 26).

5. Explain what models are and their purposes in science.Models are conceptual representations used to explain and predict phenomena

Physical models Computer models Mathematic models

6. Describe the observations and conclusions drawn by Gorter and Grendel in discovering the structure of cell membranes.Gorter and Grendel (1925) investigated the surface area of membranes. They noticed that the surface area of an intact red blood cell was half of the surface area of the lipids when spread on a surface.

They concluded that the cell membrane is made of a bilayer of lipids with head groups facing the inside and outside of the cell and the tails of each layer facing inward towards each other.

S1.3.2: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model (Oxford Biology Course Companion page 28).

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

In 1935 Davson and Danielli proposed the “protein-lipid sandwich” model of the cell membrane. In electron micrographs, they observed two dark parallel lines with a light region in between.

Since proteins appear dark and lipids appear light in micrographs, Davson and Danielli proposed that the phospholipid bilayer was embedded between two layers of proteins.

S1.3.3: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model (Oxford Biology Course Companion page 29).

8. 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 the center of membranes.

Globular structures throughout the membrane were interpreted as transmembrane proteins. Transmembrane proteins were not accounted for by the Davson-Danielli model of the cell membrane.

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

This experiment showed that protein molecules can move from place to place on the cell membrane.

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

Improvements in tools and techniques allowed 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 were hydrophobic (or partially hydrophobic). These findings did not match the model proposed by Davson and Danielli, in which proteins would be relatively uniform in shape and hydrophilic in nature.

11. 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-Danielle 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 membranes, called the Fluid-Mosaic Model.

NOS1.3.2: Falsification of theories with one theory being superseded by another-evidence falsified the Davson-Danielli model (Oxford Biology Course Companion page 27).

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

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

S1.3.1: Drawing of the fluid mosaic model (Oxford Biology Course Companion page 31).13. Draw and label the structure of membranes. Include:

- Phospholipid bilayer- Integral proteins shown spanning the membrane- Peripheral proteins on membrane surface- Protein channels with a pore- Glycoproteins with a carbohydrate side chain- Cholesterol between phospholipids in the hydrophobic region- An indication of thickness (10nm)

U1.3.2: Membrane proteins are diverse in terms of structure, position in the membranes and function (Oxford Biology Course Companion page 30).

14. State the primary function of the cell membrane.

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

15. Contrast the structure of integral and peripheral proteins.

Peripheral proteins sit on the surface or have small sections that dip into the bilayer.

Integral proteins have large sections embedded in the hydrophobic middle of the membrane. Some integral proteins are “transmembrane” meaning they cross the membrane.

16. List at least four functions (with example) of membrane bound proteins.

1. Receptor proteins communicate signals between the cells internal and external environments (i.e. hormone receptors)

2. Transport proteins move ions and molecules across the membrane (i.e. aquaporin transports water)

3. Enzymes catalyze reactions (i.e. ATP synthase)4. Adhesion molecules anchor the cell to other cells (i.e. adherin)5. Recognition proteins identify the cell type (i.e. major histocompatibility complex

proteins)

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

Channel proteins are used for passive transport of molecules, often shaped like pores/tunnels. Move from high to low concentrations.

Pump proteins are used for active transport of molecules. Move from low to high concentrations.

U1.3.3: Cholesterol is a component of animal cell membranes (Oxford Biology Course Companion page 32).

18. Identify the structure of cholesterol in molecular diagrams.

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

19. 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.

A1.3.1: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes (Oxford Biology Course Companion page 33).

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

Cholesterol acts as a regulator of membrane fluidity (which is the viscosity of the cell membrane).

At high temperatures, it stabilizes the membrane and raises the melting point. At low temperatures, it prevents phospholipids from packing too close together

which would lead to stiffening.

The membrane fluidity effects how permeable the structure is to solutes Too fluid → too much permeability Too stiff → not enough permeability