As Biology Cell Membranes

8/3/2019 As Biology Cell Membranes

http://slidepdf.com/reader/full/as-biology-cell-membranes 1/24

AS Biology. Foundation. Cellmembranes and Transport 1

AS BiologyFOUNDATION

Chapter 4

CELL MEMBRANES

andTRANSPORT




The Cell




Learning Objectives1. Describe the fluid mosaic model of membrane structure and explain the

underlying reasons for this structure.2. Outline the roles of phospholipids, cholesterol, glycolipids, proteins and

glycoproteins in membranes.

3. Outline the roles of the plasma membrane, and the roles of membranes within

cells.

4. Describe and explain how molecules can get in and out of cells (cross cell

membranes) by the processes of diffusion, facilitated diffusion, osmosis,

active transport, endocytosis and exocytosis.

5. Describe the effects on animal and plant cells of immersion in solutions of

different water potential.

6. Describe the features of the gaseous exchange surface of mammalian lung.

7. Describe the features of root hairs that enable the uptake of ions by active

transport.




Key words you should know

Phospholipids Solution Pinocytosis

Polar Solute Micropinocytosis

Hydrophilic Solvent Exocytosis

Hydrophobic Partially permeable Gaseous exchange

Micelles Water potential Alveoli

Phospholipid bilayer Solute Potential Root hair

Fluid mosaic model Pressure Potential Surface area

Glycoproteins Turgid Epidermis

Glycolipids Plasmolysis Passive transport Cholesterol Plasmolysed

Proteins Incipient plasmolysis

Transport proteins Active transport

Enzymes Carrier protein

Receptor molecules Bulk transport

Diffusion Endocytosis

Concentration gradient Phagocytosis

Facilitated diffusion Phagocytes

Osmosis Phagocytic vacuoles




Cell membrane

All living things are surrounded by a membrane.

A cell membrane is also known as plasma membrane.

Controls exchange of materials such as nutrients and

waste between cells and their environment.Has other important functions for example to enablecells to receive hormones.

To understand the function of anything in biology, you

must study the structure first!




Cell Membranes from OpposingNeurons (TEM x436,740).

} cell membrane

7nm wide

Cell membrane {

Nerve cell

Nerve cell

Gap between cells




HYDROPHILIC heads(water liking)-Attracted to thewater

called POLAR

HYDROPHOBIC tails

(water fearing)-Not attracted to thewater

called NON-POLAR

Cell membranes are made ofPHOSPHOLIPIDs

A Phospholipid




Phospholipids are important structural components of cellmembranes. Phospholipids are modified so that a phosphate group (PO4-) replaces one of the three fatty acids normally found on a

lipid. The addition of this group makes a polar "head" and twononpolar "tails".

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossPQ.html

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookglossPQ.html




A phospholipidHYDROPHILIC HEADAt the other end of the phospholipid is a

phosphate group and several doublebonded oxygens. The atoms at this endof the molecule are not shared equally.This end of the molecule has a chargeand is attracted to water. It is POLAR

HYDROPHOBIC TAILSThe two long chains coming off of the

bottom of this molecule are made up ofcarbon and hydrogen. Because both ofthese elements share their electronsevenly these chains have no charge. Theyare NON POLAR. Molecules with nocharge are not attracted to water; as aresult water molecules tend to pushthem out of the way as they areattracted to each other. This causesmolecules with no charge not to dissolvein water. 3D model of a

Phospholipid




A Phospholipid Bilayer

Phospholipids can form:

BILAYERS

-2 layers ofphospholipids with

hydrophobic tailsprotected inside by thehydrophilic heads.

The PHOSPHOLIPIDBILAYER is the basicstructure of membranes.







Structure of the cell membranePhospholipidsCell membranes are made mainly of

phospholipids. They have:

HYDROPHILIC heads (water liking)-Attracted to the water POLARHYDROPHOBIC tails (water fearing)-Not attracted to the water NON-

POLAR

Phospholipids can form BILAYERS-2 layers of phospholipids withhydrophobic tails protectedinside by the hydrophilicheads.

The PHOSPHOLIPID BILAYER isthe basic structure of membranes.




Diagram representing the cell membraneRemember the membrane is 7nm wide




Fluid mosaic model

Cell membranes also contain proteins within the phospholipid bilayer.This ‘model’ for the structure of the membrane is called the:

FLUID MOSAIC MODELFLUID- because individual phospholipids and proteins can move around

freely within the layer, like it’s a liquid.MOSAIC- because of the pattern produced by the scattered protein

molecules when the membrane is viewed from above.




Diagram of a cell membrane




TEM of freeze-fractured cellmembrane.

The fracture occurs

between the twophospholipid layers.

You can clearly see theexposed proteins

sticking out of thetwo layers.

Individual phospholipidsare too small to see.




Cell Membranes from OpposingNeurons (TEM x436,740).

} Phospholipid Bilayer

7nm wide




Features of the fluid mosaic model

Double layer – BILAYER of phospholipids which can move about by………………………… in their own ……………………….

Phospholipid tails point inwards forming a ……. ………. ………………………… interior.

The phospholipid heads point outwards facing the aqueous (water containing)medium surrounding the membrane. Some phospholipids fatty acid tails are ……………………….. – straight so fit

together tightly. Some are ………………………… – bent so fit together …………………….The more unsaturated tails there are the more ……………… the membranebecomes. The lower the temp, the ……………….fluid.

Most protein molecules …………………. like icebergs in the layers, some are fixedto ………………………. inside the cell and don’t float.

Some proteins are embedded in the outer layer, some in the inner layer andsome ………………… the two layers. Hydrophobic and Hyrdophilic parts of theprotein molecules sit next to the …………………………… and ………………………….. portionsof the ……………………………… of the membrane. This ensures the proteins stay inthe membrane.

The membrane is ……nm thick on average. Some phospholipids have carbohydrates attached to them called-

……………………….. Some of the proteins have carbohydrates attached to them called–

…………………… The membrane also contains molecules of ………………………………..




Features of the fluid mosaic model

Double layer – BILAYER of phospholipids which can move about by DIFFUSIONin their own MONOLAYER

Phospholipid tails point inwards forming a NON-POLAR HYDROPHOBIC interior.The phospholipid heads point outwards facing the aqueous (water containing)medium surrounding the membrane.

Some phospholipids fatty acid tails are SATURATED – straight so fit togethertightly. Some are UNSATURATED – bent so fit together loosely. The moreunsaturated tails there are the more ‘fluid’ the membrane becomes. The lower

the temp, the less fluid. Most protein molecules float like icebergs in the layers, some are fixed to

structures inside the cell and don’t float. Some proteins are embedded in the outer layer, some in the inner layer and

some span the two layers. Hydrophobic and Hyrdophilic parts of the proteinmolecules sit next to the Hydrophobic and Hydrophilic portions of the

phospholids of the membrane. This ensures the proteins stay in the membrane. The membrane is 7nm thick on average. Some phospholipids have carbohydrates attached to them – GLYCOLIPIDS Some of the proteins have carbohydrates attached to them – GLYCOPROTEINS The membrane also contains molecules of CHOLESTEROL




Roles of components of cell membranes

Using the following headings produce a table on A4 to summarise roles of t hedifferent types of molecules found in the cell membrane. Try not to copy, pick

out the relevant information and write it in note form You may want to usebullet points, different coloured pens etc basically whatever helps you to

remember them.There will be a short test on this next lesson! Use pages 53-54.

Component Function

1. Phospholipids

2. Cholesterol

3. Proteins

4. Glycolipids and Glycoproteins




Summary Cell membranes have a basic structure composed of a PHOSPHOLIPID BILAYER.

Phospholipds have HYDROPHOBIC (non-polar) tails and HYDROPHILIC (polar) heads.

The best model of the cell membrane is called the FLUID MOSAIC MODEL The average thickness of the membrane is 7nm.

The fatty acid tails of phospholipids can be SATURATED (straight) or UNSATURATED

(bent)

Proteins can float or be fixed and also have hydrophobic and hydrophilic portions.

Some proteins and phospholipids have carbohydrates attached to them to form

GLYCOPROTEINS AND GLYCOLIPIDS.

Phospholipids form the bilayer, act as barrier to most water soluble substances

Cholesterol regulates the fluidity of the membrane, gives mechanical stability and help to

prevent ions from passing through the membrane.

Proteins act as transport proteins to act as channels for substances to move into or out of

the cell. Some act as membrane enzymes and some have important roles in membranes of

organelles.

Glycolipids and Glycoproteins help to stabilise membrane structure, some act as receptor

molecules eg for hormones and neurotransmitters or as antigens for other cells to

recognise them.




Movement of selected moleculesacross the cell membrane




Moving amoeba




This powerpoint was kindly donated to

www.worldofteaching.com

http://www.worldofteaching.com is home to over a

thousand powerpoints submitted by teachers. This is a

completely free site and requires no registration. Please

visit and I hope it will help in your teaching.

http://www.worldofteaching.com/




Documents

As Biology Cell Membranes