1.3: Membrane Structure

Question 1

1.3.U1 Draw a simplified diagram of the structure of the phospholipid

Answer 1

Question 2

1.3.U2 State the primary function of the cell membrane

Answer 2

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

Question 3

1.3.U2 Contrast the structure of integral and peripheral proteins

Answer 3

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.

Question 4

1.3.U2 List at least four functions (w/ examples) of membrane bound proteins

Answer 4

1. Receptor proteins communicate signals between the cells internal and external environments (ie. hormone receptor)
2. Transport proteins move ions and molecules across the membrane (ie. aquaporin transports water)
3. Enzymes catalyse reactions (ie. ATP synthase)
4. Adhesion molecules anchor the cell to other cells (ie. adherin)
5. Recognition proteins identify the cell type (ie. major histo-compatibility complex proteins)

Question 5

1.3.U2 Contrast the two types of transport proteins

Answer 5

Channel proteins are used for passive transport of molecules, often shaped like pores/tunnels. Molecules move from [high] to [low].

Pump proteins are used for active transport of molecules. Molecules move from [low] to [high].

Question 6

1.3.U3 Identify the structure of cholesterol in molecular diagrams

Answer 6

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

Question 7

1.3.U3 Describe the structural placement of cholesterol within the cell membrane

Answer 7

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

Question 8

1.3.A1 Describe the function of cholesterol molecules in the cell membrane

Answer 8

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

• at high temperatures it stabilises the membrane and raises the melting point.
• at low temperatures it prevents phospholipids from packing together 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

Question 9

1.3.S1 Draw the structure of the cell membrane

Answer 9

Question 10

1.3.S2 Describe the observations and conclusions drawn by Davson and Danielli in discovering the structure of cell membranes

Answer 10

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.

Question 11

1.3.S3 Describe conclusions about cell membrane structure drawn from freeze-etched electron micrograph images of the cell membrane

Answer 11

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.

Question 12

1.3.S3 Describe conclusions about cell membrane structure drawn from cell fusion experiments

Answer 12

When a human cell and a mouse cell, each with cell membrane proteins tagged with different fluorescent antibodies, were fused together, the membrane proteins initially did not mix. However, after 40 minutes the proteins had mixed. This experiment showed that protein molecules can move from place to place on the cell membrane.

Question 13

1.3.S3 Describe conclusions drawn about cell membrane structure from improvements in techniques for determining the structure of membrane proteins

Answer 13

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 hydrophillic in nature.

Question 14

1.3.S3 Compare the Davson-Danielli model of membrane structure with Singer-Nicolson model

Answer 14

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.

Question 15

1.3.NOS1 Explain what models are and their purposes in science

Answer 15

Models are conceptual representatives used to explain and predict phenomena

• physical models
• computer models
• mathematic models

Question 16

1.3.NOS1 Describe the observations and deductions drawn by Gorter and Grendel in discovering the structure of cell membranes

Answer 16

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 water 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.

Question 17

1.3.NOS2 Describe why the understanding of cell membrane structure has changed over time

Answer 17

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.