IB Biology2 Cells

2.4 Cell Membranes

Jason de Nys

All syllabus statements ©IBO 2007All images CC or public domain or link to original material.

http://www.flickr.com/photos/edsweeney/6346198056/

2.4.1 Draw and label a diagram to show the structure of membranes.

http://www.youtube.com/watch?v=w9VBHGNoFrY

2.4.2 Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes.

http://www.flickr.com/photos/zorin-denu/5385963280/

What happens when you put a drop

of oil inwater?

http://www.flickr.com/photos/zorin-denu/5385963280/

The Oil droplet stays together and makes a perfect circular shape.

The oil molecules areHydrophobic

Oil Molecules are non-polar and water molecules are polar. See 3.1.5

http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_4.svg

Phospholipid molecules have a

polar (charged) phosphate head and long non-polar lipid

tails

The head is attracted to water and the

tails are not.

*

*h not b!

http://commons.wikimedia.org/wiki/File:Micelle_scheme-en.svghttp://commons.wikimedia.org/wiki/File:Liposome_scheme-en.svg

When put into water, an emergent property is that phospholipids will self-organise to keep their heads

‘wet’ and their tails ‘dry’

micelle liposome

http://commons.wikimedia.org/wiki/File:Phospholipids_aqueous_solution_structures.svg

In this 3D representation you can see that a

phospholipid bilayer is one way that the tails

can be removed from the water.

Phospholipid molecules can flow past each other laterally but can’t move

vertically

But wait! there’s more!The plasma membrane is not just made of

phospholipids

http://commons.wikimedia.org/wiki/File:Cell_membrane_detailed_diagram_en.svg?uselang=en-gb

Proteins:Integral proteins are permanently embedded, many go all the way through and are polytopic (poly = many, topic = surface), integral proteins penetrating just one surface are monotopic.

Peripheral proteins usually have a temporary association with the membrane, they can be monotopic or attach to the surface

Some human examples on the next page

http://opm.phar.umich.edu/protein.php?pdbid=2wf1http://opm.phar.umich.edu/protein.php?pdbid=1afo

http://opm.phar.umich.edu/protein.php?pdbid=1hmt

Muscle fatty acid binding protein

Beta-secretase 1

Glycophorin A

Glycophorin (integral) carries sugar molecules into red blood cells

Muscle Fatty acid binding protein (peripheral) is

involved in the transport of fatty acids

Beta-secretase 1 (peripheral) has a role

in creating myelin sheaths on nerve cells

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These are just three examples, they are from different cell types. There are thousands that have been found so far and thousands more are added each year.Protein structure covered in AHL 7.5

Glycoproteins:Are proteins with an oligosaccaride (oligo = few, saccharide = sugar) chain attached.

They are important for cell recognition by the immune system and as hormone receptors

Cholesterol: (It’s not all bad!)It makes the phospholipids pack more tightly and regulates the fluidity and flexibility of the membrane. more later in 2.4.8

Bad analogy: imagine a room full of people wearing fluffy jumpers (sweaters). It is crowded but they can slip past each other easily enough. Now sprinkle the crowd with people wearing Velcro™ suits…

2.4.3 List the functions of membrane proteins

List: Give a sequence of names or other brief answers with no explanation

Proteins associated with membranes have many functions. Can you think of any ‘jobs’ that proteins could help cells do?

Let’s look at 6

http://www.flickr.com/photos/silveraquarius/1395277674/

Cell Adhesion Molecules: Enable cells to make tight connections to one another

They may play a part in the immune

response.

1

http://www.flickr.com/photos/gilderic/2728059828/

Channel Proteins: Allow or help ions and large molecules to pass through the membrane by diffusion

2

Protein Pumps move ions across the membrane to create and maintain concentration gradients.

They require energy to carry out this active transport

http://www.flickr.com/photos/erix/298857080/

3AT

P

Hormone Binding sites (hormone receptors) bind to specific hormones and start signalling processes to change the behaviour of the cell

e.g. insulin

http://www.flickr.com/photos/18735339@N00/2926816259/

4

Cell to cell communication:e.g. receptors for neurotransmitters at synapses

Nicotinic acetylcholine receptor, beta2 subunit

http://ifaketext.com/http://opm.phar.umich.edu/protein.php?pdbid=2ksr

Outside cell

Cytoplasm

Inside membrane

5

http://opm.phar.umich.edu/protein.php?pdbid=3d59

Outer surface of membrane

Plasma platelet activating factor acetylhydrolase

Enzymes on the surface of the cell

6

http://www.flickr.com/photos/dpup/3338948041/

What is

2.4.4 Define diffusion and osmosis

Diffusion, is the motion of all (liquid or gas) particles at temperatures above absolute zero. • The rate of this movement relates to temperature, viscosity of

the fluid and the size (mass) of the particles. • Diffusion explains the net flow of molecules from a region of

higher concentration to one of lower concentration.• The result of diffusion is a gradual mixing of material. In the

absence of other influences, the diffusion process will eventually result in complete mixing.

Diffusive equilibrium is reached when the concentrations of the diffusing substance in the two compartments becomes equal. (Wikipedia)

http://www.flickr.com/photos/stevon/3145375973/

Simulate!1. Grab your classmates or a group

of friends.2. Take one die each.3. Stand together in a clump; in a

room or outside.4. Everybody rolls their die in their

hand5. For a 1, step forward; for a 2,

step to the right; for a 3, step backwards and for a 4, step left

6. If you roll 5 or 6, or if you can’t move because a person or object is in the way, roll again

Do this for 3 minutes, you should have dispersed, some will have got further than others.

If you have 4 sided dice then all the better.

This simulation is known as a random walk.

How could you simulate an increase of heat

energy?

What is

osmosis?

http://www.flickr.com/photos/luchilu/399970490/

Osmosis may occur when there is a partially permeable membrane, such as a cell membrane. When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration (outside the cell) to one of high solute concentration (inside the cell) (Wikipedia)

Aquaporin is an integral protein that, as it’s name suggests, acts as a pore in the membrane that speeds the movement of water molecules

http://opm.phar.umich.edu/protein.php?pdbid=1sor

http://commons.wikimedia.org/wiki/File:Osmotic_pressure_on_blood_cells_diagram.svghttp://commons.wikimedia.org/wiki/File:Turgor_pressure_on_plant_cells_diagram.svg

The importance of osmotic control

Simple Diffusion

http://commons.wikimedia.org/wiki/File:Scheme_simple_diffusion_in_cell_membrane-en.svg

2.4.5 Explain passive transport across membranes in terms of simple diffusion and facilitated diffusion.

http://commons.wikimedia.org/wiki/File:Scheme_facilitated_diffusion_in_cell_membrane-en.svg

Facilitated Diffusion: Large and polar molecules can’t get across the membrane via simple diffusionTransmembrane (polytopic) proteins recognise a particular molecule and help it to move across the membrane. The direction it moves is dependent on the concentration gradient.

Watch the animation

2.4.6 Explain the role of protein pumps and ATP in active transport across membranes

Primary active transport requires ATP.Integral protein pumps use the energy from the hydrolysis of ATP to move ions or large molecules across the cell membrane.Molecules are moved against their concentration gradient

http://commons.wikimedia.org/wiki/File:Scheme_sodium-potassium_pump-en.svg

http://commons.wikimedia.org/wiki/File:Scheme_secundary_active_transport-en.svg

In secondary active transport, the required energy is derived from energy stored in the form of concentration differences in a second solute.

Typically, the concentration gradient of the second solute was created by primary active transport, and the diffusion of the second solute across the membrane drives secondary active transport.

2.4.7 Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus and plasma membrane.

Vesicles are small spheroidal packages that bud off of the RER and the Golgi apparatus

They carry proteins produced by ribosomes on the RER to the Golgi apparatus, where they are prepared for export from the cell via another vesicle

2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and reform during endocytosis and exocytosis.

Endocytosis: The taking in of external substances by an inward pouching of the plasma membrane, forming a vesicle

Exocytosis: The release of substances from a cell (secretion) when a vesicle joins with the cell plasma membrane.

Diagrams on following slides

http://commons.wikimedia.org/wiki/File:Exocytosis_types.svg

Regulated secretion is in response to a trigger e.g. the release of neurotransmitters

Constitutive secretion occurs continuously in cells, depending on their function

http://commons.wikimedia.org/wiki/File:Endocytosis_types.svg

“Cell eating” “Cell drinking”

As mentioned, phospholipids can move freely past each other laterally but very rarely do they move vertically or flip

The plasma membrane is embedded with proteins

Cholesterol molecules stiffen and stabilise the plasma membrane

It is because of the lateral movement and the presence of other molecules studding it’s surface that our understanding

of the plasma membrane is referred to as the

fluid mosaic model

Further information:

Most of the excellent biology images in this slideshow are by graphic designer Mariana Ruiz Villarreal (LadyofHats) who has graciously released them to the

public domain.

Three of the best sites for IB-specific Biology information. The top link takes you to the PPT by Stephen Taylor