IB HL Biology notes cell membranes

7/18/2019 IB HL Biology notes cell membranes

http://slidepdf.com/reader/full/ib-hl-biology-notes-cell-membranes 1/12

Biology HL2.4

Membranes

~ 1 ~



Features of membrane structure

PhospholipidsThe phospholipid bilayer is the backbone of the

membrane in that it the membrane is held together bythe phospholipids (though not in a rigid structure). Thisis because the hydrophobic and hydrophilic regions ofphospholipids in the bilayer of the plasma membranecause the phospholipids to always align as a bilayer(forming a semi-permeable bilayer) if there is waterpresent and there is a large number of phospholipidmolecules. As the tails of the fatty acids do notstrongly attract each other, the membrane is fluid andflexible. This also allows processes called endocytosisand exocytosis to take place.

Diffusion across the phospholipid bilayer is onlypossible for small, uncharged molecules, e.g.substances like O2, water, CO2, urea, ethanol. Largeor charged molecules cannot cross the lipid bilayer, e.g. Na+, K+, Cl-, glucose.

CholesterolCholesterol is a lipidmolecule that helps toregu la te membrane

fluidity and is importantfor membrane stability.Because membranefluidity changes with thetemperature, cholesterolmolecules are necessarybecause they a l lowef fec t ive membranefunction at a wider range

of temperatures. For example, at low temperatures, it interferes with solidification of thephospholipid molecules, and so keeps the structure more fluid. Plant cells do not have

cholesterol molecules so they depend on saturated and unsaturated fatty acids to maintainproper membrane fluidity.

Membrane proteinsMembrane proteins have many functions. These functions include:

• hormone binding sites • immobilized enzymes • cell-to-cell communication • channels for passive transport • pumps for active transport • cell adhesion

There are, therefore, many different types of protein found on the plasma membrane to carryout this diverse range of functions.

Biology HL2.4

Membranes

~ 2 ~

Fig. 2: A model of a phospholipid

Non-polar hydrophobic fatty acid(hydrocarbon) tail – when packed closelytogether, they form a semi-permeable

bilayer

Polar hydrophilic

phosphorylated alcohol head

Fatty acid

Fatty acid

Gl

ycerol

Phosphorylated alcohol

Fig. 3: Structure of a phospholipid



Peripheral proteins: GlycoproteinsGlycoproteins are an example of peripheral proteins. They are proteins with a chain ofcarbohydrates (glycocalyx) protruding from the surface. They are involved in cell signalling, cell-to-cell recognition and in binding cells together. This is because the glycocalyx acts as a

receptor on cells which can bind to hormones, drugs, and other cells.

Integral proteinsThere are many types of integral protein, but the main two types are the channel protein and thecarrier protein.

Channel protein: A protein with a specific shape that compliments the shape of a certainsubstance to be transported across the plasma membrane. This type ofprotein carries out facilitated diffusion of molecules by providing acontinuous gateway for the passive transport of its substrate(s), often ions,in which case it is called an ion channel.

Carrier protein: A protein with a specific shape that compliments the shape of a certainsubstance to be transported across the plasma membrane. Carrierproteins can carry put facilitated diffusion or active transport. This is done byproviding a binding site in the membrane that can be accessed from withinor outside the cell but only during alternating movements of the carrierprotein. This non-continuous gateway allows for active (co-transport) orpassive transport.

In terms of the active transport mechanism, this is done by anATP-powered pump, where the protein employs the energy of ATP

hydrolysis to move substrates, often ions, against the concentrationgradient or an electric field.

Transport across membranes

Diffusion is the passive movement of particles from a region of high concentration to a region oflow concentration. Osmosis is the passive movement of water molecules, across a partially permeablemembrane, from a region of lower solute concentration to a region of higher soluteconcentration.

Osmosis and diffusion are passive processes that occur along a concentration gradient,which means that they do not require any energy expenditure. Whether a substance can pass across a membrane passively is influenced by two majorfactors: size and charge. Substances that are small in size and non-charged can move acrossmembranes with relative ease. Substances that are large in size, charged or both, cannot: theyrely on membrane transport proteins in order to be able to cross the plasma membrane. Small, uncharged molecules (e.g. oxygen, carbon dioxide, glycerol, urea) can betransported across the cell membrane (between the phospholipids) by diffusion. They are smallenough to pass through the transient gaps between the phospholipid molecules (Fig. 4). For larger molecules and ions, they can be transported across the plasma membrane by

facilitated diffusion or active transport, but they rely on membrane transport proteins to cross theplasma membrane. Some larger molecules can pass through channels in specific integralproteins in the plasma membrane (Fig. 5).

Biology HL2.4

Membranes

~ 3 ~



Passive transport

Biology HL2.4

Membranes

~ 4 ~

Concentration gradient

High concentration

Low concentration

Fig. 5: Simple diffusion of molecules acrossphospholipid bilayer via channels in an integral

protein in the plasma membrane

Extracellular space

Cytoplasm

Fig. 4: Simple diffusion of small molecules across phospholipidbilayer via transient gaps between phospholipid molecules

Phospholipid

Water molecule

Carbon dioxide

Oxygen



Facilitated diffusion

Facilitated diffusion is a particular type of diffusion involving a membrane with specific carrierproteins that are capable of combining with the substance in order to aid in its movement. The

carrier changes shape in order to accomplish this task but this does not require energy. It isworth noting that the rate of facilitated diffusion will level off when total saturation of availablecarriers occurs.

In facilitated diffusion, the substrate molecule binds to a carrier (integral) protein in theplasma membrane, where the protein’s binding site is initially exposed outside the bilayer. Asthe protein can exist in two different conformational states, it then inverts. This means that thebinding site is then exposed inside the cell, so the substrate is released (Fig. 6).

Examples of molecules that are transported through facilitated diffusion include glucose,fructose, non-fat soluble vitamins, urea and ions that the semi-permeable phospholipid bilayerwould normally prevent from diffusing.

Uniport: Where one solute is simply transported from one side of the membrane to theother.

Symport: Where the transfer of one solute depends on the simultaneous or sequentialtransport of a second solute in the same direction.

Antiport: Where the transfer of one solute depends on the simultaneous or sequentialtransport of a second solute in the opposite direction.

Biology HL2.4

Membranes

~ 5 ~

Extracellular space

Cytoplasm

“pong” “ping”

Molecule diffuses away from protein

Molecule fits into integral protein

Fig. 6: Facilitated diffusion of molecules acrossphospholipid bilayer via carrier proteins



Summary of passive transport

Active transport

Active transport is the uptake of substance(s) against the concentration gradient. This requiresATP (adenosine triphosphate), the form in which usable cellular energy comes and will involve

membrane proteins – in this case an ATP-powered pump that employs the energy of ATPhydrolysis to move substrates (often ions) against a concentration gradient or an electric field.An example of this is the sodium-potassium-ATP-ase process establishes the most importantgradients across the animal plasma membrane. The sodium gradient is utilised to effectivelyabsorb glucose from food in the small intestine (see Fig. 12).

Mechanism for active transport1

1. On the inside of the cell, a sodium ion binds to an integral protein (ATP pump) on the cellmembrane.

2. This triggers the breakdown of ATP into ADP and the energy which is released is used tophosphorylate the protein which forms the pump.

3. The protein changes shape and the sodium ion is transported to the outside of the cell.

4. The changed shape of the protein now allows potassium ions on the outside of the cell tobind to the protein.

5. This triggers dephosphorylation of the protein (the phosphate group is removed).

6. This changes the shape of the protein again and the potassium is transported to the inside

of the cell.

Biology HL2.4

Membranes

~ 6 ~

1 In the Na+ –K+ATP–ase pump, it is not (strictly speaking) an exchange of one sodium ion for one potassium ion.Rather, it is an exchange of two sodium ions for three potassium ions.

Type of transport Description

Simple diffusion

Substances other than water move between phospholipid moleculesor through proteins which possess channels. The rate of diffusion isproportional to the concentration gradient (but the rate is alsoinfluenced by the size of the particles, the distance over whichdiffusion must occur and the temperature

Facilitated diffusion

Non-channel protein carriers change shape to allow movement ofsubstances other than water. The rate of facilitated diffusion isproportional to the concentration gradient and to the number of carrier

proteins that are available.

OsmosisOnly water moves through the membrane using aquaporins which areproteins with specialised channels for water movement.



Biology HL2.4

Membranes

~ 7 ~

Extracellular space

Fig. 8: A sodium ion then binds to the ATP pump on thecell membrane.

Cytoplasm

Sodium ion

Extracellular space

Fig. 7: On the inside of the cell, an ATP molecule binds toan integral protein (ATP pump) on the cell membrane.

Cytoplasm

Phospholipidbilayer

Integral protein(ATP pump)

ATP molecule



Biology HL2.4

Membranes

~ 8 ~

Fig. 9: This triggers the breakdown of ATP into ADP and theenergy which is released is used to phosphorylate the proteinwhich forms the pump. The protein changes shape and thesodium ion is transported to the outside of the cell.

Extracellular space

Cytoplasm

ADP molecule

Extracellularspace

Cytoplasm

Potassium ion

Fig. 10: The changed shape of the protein now allows potassiumions on the outside of the cell to bind to the protein.



(N.B.: There is a tendency for sodium ions to diffuse back into the cell and potassium ions backout of the cell. Since the membrane is more permeable to potassium than sodium, more ions

leave the cell than enter. This reduces the tendency of water to enter the cell by osmosis thusthe sodium-potassium pump is a method of controlling the volume of the cell.)

In this active transport cycle (Fig. 7 – 11), the changes in the conformation (“ping-pong”) of theintegral proteins are analogous to those of integral proteins responsible for facilitated diffusion.

The sodium-potassium-ATP-ase pumps are an example of an antiport. They are particularlyimportant in animal cells in osmoregulation where they reset the water potential of the cell byexporting Na+ ions (and thus water), thereby preventing lysis of the cell.

They are also important in the creation of potential differences across membranes which arecrucial in the transmission of nerve impulses.

Biology HL2.4

Membranes

~ 9 ~

Extracellular space

Cytoplasm

Fig. 11: This triggers dephosphorylation of the protein (the phosphategroup is removed). This changes the shape of the protein again andthe potassium is transported to the inside of the cell. The events arecyclic, so further transport of sodium and potassium ions can occur,going back to Fig. 7.



Furthermore, sodium-potassium-ATP-ase pumps are important in animal cells in that they drivethe active transport of glucose2 (via a symport) by generating and maintaining the Na+ gradient

(Fig. 12

). It is for this reason that the mechanism for co-transporting glucose and sodium ions islikely to be very close to the mechanism responsible for the antiport transfer mechanism ofsodium and potassium ions3.

Endocytosis and Exocytosis

Endocytosis and exocytosis are processes that allow larger molecules to move across theplasma membrane. Endocytosis allows macromolecules to enter the cell, while exocytosisallows molecules to leave. Both processes depend on the fluidity of the plasma membrane. Theplasma membrane is fluid in consistency because the phospholipid molecules are not closelypacked together due to the rather “loose” connections between the fatty acid tails. It is alsoimportant to remember why the membrane is stable: the hydrophilic and hydrophobic propertiesof the different regions of the phospholipid molecules cause them to form a stable bilayer in anaqueous environment.

Biology HL2.4

Membranes

~ 10 ~

2 Glucose can also be transported via facilitated diffusion, though this depends on the concentration gradient

between the cytoplasm and the extracellular space.3 Other types of active transport pumps exist, for example Ca2+ pumps in the ER of muscle cells which areimportant in muscle contraction and H+ (proton) pumps in mitochondria and chloroplasts which are important inATP synthesis.

Na+

Na+

K+Glucose

Cytoplasm

Extracellularspace

Fig. 12: Diagram showing the glucose symport aided by theNa+ gradient across the plasma membrane generated andmaintained by the Na+ –K+ATP–ase pump



Endocytosis occurs when a portion of the plasma membrane is pinched off to enclosemacromolecules or other particulates. This pinching off involves a change in the shape of themembrane. The result is the formation of a vesicle that then enters the cytoplasm of the cell.

The ends of the membrane reattach because of the hydrophobic and hydrophilic properties ofthe phospholipids and the presence of water. This could not occur if it were not for the fluidnature of the plasma membrane.

Examples of endocytosis include phagocytosis – the intake of large particulate matter – andpinocytosis – the intake of extracellular fluids.

Exocytosis is essentially the reverse of endocytosis – where large molecules are released fromthe cell. The fluidity of the plasma membrane and the hydrophobic and hydrophilic properties ofits molecules are just as important as in endocytosis. Exocytosis usually begins in theribosomes of the rough endoplasmic reticulum (ER) and progresses through a series of foursteps until the substance is secreted to the outside environment of the cell.

1. Proteins are manufactured / synthesised / produced (for export) by the ribosomes of therough endoplasmic reticulum. They then enter the lumen of the endoplasmic reticulum.

2. A vesicle containing the proteins is pinched off. The proteins exit the ER and enter the cis(receiving) side / face of the Golgi apparatus (Fig. 13).

3. The Golgi apparatus packages and processes the proteins, and modifies them. Themodified proteins bud off (come off) the Golgi apparatus, and exits inside a vesicle on thetrans side of the Golgi apparatus, moving towards the plasma membrane.

4. The vesicle with the modified proteins moves to and fuses with the plasma membrane.This results in the secretion of the contents from the cell.

This is exocytosis, and it is a process that requires energy. An example of exocytosis is synaptictransmissions between neurons.

Biology HL2.4

Membranes

~ 11 ~

Fig. 13: Golgi apparatus

Trans side / face (shipping)

(Generally more curved)

Cis side / face (receiving)



Appendix:

To explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain thestructure of cell membranes, for example after exocytosis, in which the membrane is split(temporarily), we must first consider what it means to be hydrophilic and hydrophobic.

Hydrophilic molecules are attracted to water; hydrophobic molecules are repelled by water.Phospholipids contain both hydrophilic areas and hydrophobic areas.

The phosphate heads are polar and are therefore hydrophilic. The hydrocarbon tails are notpolar, so are hydrophobic.

Membranes are usually in aqueous environments – they are surrounded by water molecules.This means that the heads face towards water, whereas the tails are positioned away fromwater.

This forms a spontaneous, self-assembling (and therefore self-reassembling) bilayer, which ishydrophilic on the outside, and hydrophobic on the inside4.

Biology HL2.4

Membranes

~ 12 ~

4 Membrane proteins will be arranged in accordance with this structure.

Date post:	10-Jan-2016
Category:	Documents
Upload:	ayushfm
View:	13 times
Download:	1 times

IB HL Biology notes cell membranes

Documents