Components of the membrane

Membrane Structure Practical

Functions of membrane components

Question: Other than as carrier proteins state two functions of membrane bound proteins (2). Receptors Enzymes Structural (attached to microtubules) Click here to hide answers Click to reveal answers

Membrane Permeability practical Learning Objectives Outline the effect of changing temperature on membrane structure and permeability Collection of quantitative data: Success Criteria Use a colorimeter to investigate the effect of temperature on membrane permeability. Produce a table which fulfils quantitative assessment criteria.

Cholesterol in cell membranes Cholesterol is a type of lipid with the molecular formula C 27 H 46 O. Cholesterol is also important in keeping membranes stable at normal body temperature – without it, cells would burst open. Cholesterol is very important in controlling membrane fluidity. The more cholesterol, the less fluid – and the less permeable – the membrane.

Proteins in membranes Proteins typically make up 45% by mass of a cell membrane, but this can vary from 25% to 75% depending on the cell type. integral protein Peripheral (or extrinsic) proteins are confined to the inner or outer surface of the membrane. Integral (or intrinsic, or transmembrane) proteins span the whole width of the membrane. peripheral protein Many proteins are glycoproteins – proteins with attached carbohydrate chains. carbohydrate chain

Integral proteins Many integral proteins are carrier molecules or channels. These help transport substances, such as ions, sugars and amino acids, that cannot diffuse across the membrane but are still vital to a cell’s functioning. Other integral proteins are receptors for hormones and neurotransmitters, or enzymes for catalyzing reactions.

Extrinsic proteins Extrinsic (or Peripheral) proteins may be free on the membrane surface or bound to an intrinsic (or integral) protein. Extrinsic proteins on the extracellular side of the membrane act as receptors for hormones or neurotransmitters, or are involved in cell recognition. Many are glycoproteins. Extrinsic proteins on the cytosolic side of the membrane are involved in cell signalling or chemical reactions. They can dissociate from the membrane and move into the cytoplasm.

Practical task Effect of Temperature on membrane permeability.

Practical Activity: Factors affecting membrane permeability See practical sheets : PAG 5 Colorimetry

Guidance for results write up Stick in protocol all raw data presented in a single table with columns correctly labelled and with units Independent variable in first column all volumes recorded to an accuracy of no more than 1 decimal place all units written correctly and in column headings only

Exam Practice Figure 4 shows part of a cell-surface membrane. (a) Describe two functions of the structure made from the parts labelled X. (2 marks) (b) Give one function of the molecule labelled Y. (1 mark)

Answers 4 (a) two of the following: selectively permeable; allows cell to maintain different concentrations either side; gives flexibility/fluidity; 2 (b) (surface/extrinsic protein) for cell recognition/binding to hormones/identification; 1 Make sure that you only give two answers rather than a long list – note two in bold! The examiner will not choose the correct answers from your list. You must decide which you want to give. You will not be given extra marks and it wastes time.

How temperature affects cell membranes  Temp below 0 – phospholipids don’t have much energy so can’t move much. Channel/carrier proteins may denature increasing permeability. Ice crystals may form and pierce membrane, making it highly permeable when it thaws. As temp increases phospholipids move more due to more kinetic energy, phospholipids and other components makes the membrane leaky, which allows substances that would normally not do so to enter or leave the cell – increases permeability. Above 45 o C – phospholipids bilayer starts to melt, water inside cell expands putting pressure on membrane, Proteins denature – increase permeability.