subject guide notes

B2.1.1—Lipid bilayers as the basis of cell membranes

membranes are composed of:

• lipids (phospholipids, glycolipids & sterols)

• proteins

• small amounts of carbs in the form of glycoproteins & glycolipids

*remember phospholipids are amphipathic

• this forms a lipid bilayer

B2.1.2—Lipid bilayers as barriers

the hydrocarbon tails that extend inwards, are important for determining the permeability of the membrane

• since the interior is non-polar & hydrophobic, then non-polar, hydrophobic & lipid-soluble substances (such as steroids) can pass through the bilayer

• but ions cannot pass through

• uncharged polar molecules (like glucose) are typically hydrophilic

  • the membrane is impermeable to them

• small, uncharged molecules can pass through

  • so, polar & non-polar molecules can enter & exit cell

so basically, permeability depends on the molecule’s size & if its hydrophilic or hydrophobic

B2.1.3—Simple diffusion across membranes

example of diffusion is movement of non-polar substances (such as O2 & CO2) which are important in gas exchange

oxygen diffuses down its concentration gradient

• will diffuse from the alveoli (high concentration) into the erythrocytes/RBCs in the capillaries (low concentration)

  • oxygen then carried to tissues

  • in tissues, O2 diffuses from erythrocytes (high concentration) to metabolically active cells (low concentration)

carbon dioxide also diffuses down its concentration gradient

• diffuses from tissues (high concentration) into bloodstream (low concentration)

• in lungs, diffuses out of blood (high concentration) into alveoli (low concentration)

*remember, due to the hydrophobic nature of the lipid bilayer, only non-polar substances (like O2 or CO2) or very small polar substances (like H2O & alcohol) can diffuse across membranes

B2.1.4—Integral and peripheral proteins in membranes

2 types - integral proteins & peripheral proteins

integral proteins - located in (embedded) into the lipid bilayer

• are amphipathic

  • hydrophobic regions interact with hydrophobic interior of lipid bilayer

  • hydrophilic regions interact with the hydrophilic heads

• most are transmembrane - extend across the membrane

• others are only found on side of bilayer

peripheral proteins - typically found on surface of membrane

• are hydrophilic (dont hv any hydrophobic regions)

function of membrane proteins

• transport proteins - facilitate movement of molecules in & out of cell

  • includes channel proteins & carrier proteins

• recognition - aid in cell-to-cell recognition

  • important, as it helps the immune system identify pathogens

• receptors - functions as receptors for chemical signals & are binding sites for molecules like hormones

• enzymes - show enzymatic activity & catalyse reactions

• can also aid in cell adhesion to other cells or environment

B2.1.5—Movement of water molecules across membranes by osmosis and the role of aquaporins

water moves by osmosis from an area of low solute concentration (high water concentration) to an area of high solute concentration (but low water concentration)

osmosis continues until equilibrium is achieved (the solute concentration is same on both sides of membrane)

remember - membrane is impermeable to polar solutes

• so water can pass through the membrane, just not that easily

aquaporins (type of channel protein) helps water move through the membrane

• composed of 4 monomeric subunits & each subunit contains a water channel

• they’re lined with specific hydrophilic side chains, allowing water to flow through but not ions

• are bidirectional (meaning water can enter & exit the cell)

• volume of water needed to be transported determines number of aquaporins

  • kidney cells hv more amounts of aquaporins as they need to reabsorb higher amounts of water

B2.1.6—Channel proteins for facilitated diffusion

transport proteins (channel proteins & carrier proteins) are important for facilitate diffusion, as they allow molecules (that are blocked by the cell membrane) to move down their concentration gradient

one example of channel proteins are ion channels

• allow ions, like sodium & potassium, to move

• are highly selective

• different channels are needed for different ions (so, not just one channel allows the movement of all ions)

• they’re also gated

  • so when they’re open, the ions can enter the pore & move down their concentration gradient

  • when they’re closed, there’s no movement of ions

selectivity of ion channels is due to:

• the binding sites of the hydrophilic amino side chains that line the channels, are very ion-specific

• the size of the pore acts as a filter

most open & close in response to certain stimuli, such as:

• changes in voltage across the membrane (voltage-gated channels?)

• binding of small molecules to channel proteins (ligand-gated channels?)

• mechanical forces, such as pressure

B2.1.7—Pump proteins for active transport

active transport is involved when molecules are transported against their concentration gradient

requires energy, so it’s coupled with energy-releasing/exergonic reaction (like breakdown of ATP)

pump proteins use energy from ATP to transport these molecules against their concentration gradient

B2.1.8—Selectivity in membrane permeability

permeability by simple diffusion isn’t selective

• meaning the permeability of the membrane depends on the size & hydrophilic or hydrophobic properties of the molecules

• as long as a molecule fits the criteria, it can pass through membrane, regardless of its toxicity

but facilitated diffusion & active transport allow selectivity permeability of membrane

B.2.1.9—Structure and function of glycoproteins and glycolipids

glycolipids - the product of covalent bonding between carbs & lipds

• carbohydrate part is polar = extends into extracellular environment

• lipid part is non-polar = embedded in membrane

• based on their structure, can be classified into:

  • glycoglycerolipids or glycerol-based lipids

  • glycosphingolipids or derivatives of sphingosine

• form hydrogen bonds with H2O molecules around cell = contribute to membrane stability

glycoproteins - is formed when there is covalent bonding between the oligosaccharides (short carbohydrate chains) & protein molecules

• the carb molecules of glycoproteins often stick out into extracellular environment

function of glycolipids & glycoproteins

cell recognition - glycolipids & glycoproteins function as markers on cells surface, helping cells recognize each other

• also help immune system cells to identify pathogens/foreign bodies

cell adhesion - both help cells attach to other cells & form tissues

• CAMS (cell-adhesion molecules) are cell-surface glycoproteins that play an important role in cell adhesion

cell signalling - function as receptors for enzymes & other molecules, aiding in cell signalling (receiving & transmitting chemical signals)

glycocalyx - is formed by the carb groups of glycolipids & glycoproteins

• helps in cell signalling, cell adhesion & cell-to-cell recognition

B2.1.10—Fluid mosaic model of membrane structure

hv to be able to draw a a two-dimensional representation of the model and include peripheral and integral proteins, glycoproteins, phospholipids and cholesterol. They should also be able to indicate hydrophobic and hydrophilic regions.

B2.1.11—Relationships between fatty acid composition of lipid bilayers and their fluidity

the hydrocarbon tails of the phospholipid can be composed of saturated or unsaturated fatty acids

• saturated fatty acids = higher melting point = provide stability to membrane (especially at high temps)

  • this is cuz they can fit together more tightly, making membrane more rigid

• unsaturated fatty acids = low melting point = ensures fluidity of membrane

  • this is cuz of the kinks, which prevent the fatty acids from tightly packing together

saturated fatty acids also freeze more easily than unsaturated fatty acids

• at low temps, phospholipid molecules come closer together

  • this decreases the fluidity of membrane

• but the kinks prevent the phospholipids from coming too close together (they maintain spaces between the phospholipids)

  • this maintains the fluidity

cold-blooded organisms (such as frogs) adapt to low temps by increasing the proportion of unsaturated fatty acids in their phospholipid molecules

• this maintains fluidity

also, when an animal hibernates, their body temp decreases

• so the proportion of unsaturated fatty acids increases

B2.1.12—Cholesterol and membrane fluidity in animal cells

cholesterol is located in both parts of the phospholipid bilayer for the animal cell

at low temps, the cholesterol prevents the fatty acid chains of the phospholipids, from coming too closely together

• this helps maintain membrane fluiditiy

at high temps, the cholesterol stabilizes the membrane, reducing fluidity

decreases permeability of membrane to ions & molecules

B2.1.13—Membrane fluidity and the fusion and formation of vesicles

Endocytosis is a process where particles are moved into cell

• the cell membrane slowly invaginates the particles, then pinches to form a vesicle containing these particles

  • can be phagocytosis (ingesting large, solid molecules)

    • ex: seen in WBC’s

      • pseudopodia slowly surround the particles

      • pseudopodia eventually meets & engulfs the particle, forming a vesicle called a phagosome

      • the phagosome now fuses with a lysosome

        • digestive enzymes of lysosome digests particle, releasing nutrients

      • the particle is now digested within the phagosome

  • or pinocytosis (ingesting liquid)

    • forms smaller vesicles

Exocytosis - a process where particles are secreted out of cell

• material to be removed in enclosed in vesicles

• the vesicles then fuse with plasma membrane, releasing the particles

• one example: glycolipids produced by endoplasmic reticulum & modified in Golgi apparatus

  • vesicles created by Golgi apparatus fuse with plasma membrane, releasing the contents to the outside

in exocytosis, as vesicles fuse with plasma membrane, lipids & proteins are added

in endocytosis, the reverse happens during invagination - idk wat this means

B2.1.14—Gated ion channels in neurons

examples of voltage-gated channels: voltage-gated sodium channels & voltage-gated potassium channels

• a stimulus causes the voltage-gated sodium channels to open first

• sodium ions enter the neuron

  • this causes interior of neuron to become more positively charged than exterior (depolarisation)

  • this creates an action potential, which travels down the neuron

• voltage-gated sodium channels close & voltage-gated potassium channels open

  • potassium ions diffuse out of neuron, making interior of neuron less positive (repolarisation)

• eventually these channels close

• resting membrane potential now established

*note the voltage needs to reach a threshold value in order for the gated channels to open or close

• also, voltage-gated channels experience channel inactivation - inactivation particle blocks channel pore

example of neurotransmitter-gated ion channel: nicotinic acetylcholine receptors (nAchR)

• binding of acetylcholine molecules = conformational change in channel

• allows sodium ions to enter cell

  • results in depolarisation (interior of cell becomes more positive)

• enzyme cholinesterase then breaks down acetylcholine, leading to ion channels to close

• resting membrane potential restored when voltage-gated potassium channels open, allowing potassium ions to leave cell

B2.1.15—Sodium–potassium pumps as an example of exchange transporters

energy released from the hydrolysis of ATP is used to drive the movement of sodium & potassium ions against their concentration gradient

how it works

initially, the pump is open to interior of cell

• allows 3 NA+ to bind to its 3 binding sites

binding of sodium triggers hydrolysis of ATP

• ATP is hydrolysed into ADP & a phosphate group

the phosphate group attaches to the pump, causing it to undergo a conformational change

• pump opens to exterior, allowing NA+ to flow out

at same time, 2 K+ attach to their binding sites

• causes phosphate group to detach from pump

pump undergoes conformational change again to regain its original form

• opens to interior of cell

this pump then helps establish & maintain voltage across membrane

• is important for re-establishing membrane potential

B2.1.16—Sodium-dependent glucose cotransporters as an example of indirect active transport

sodium ions bind to binding sites on outer surface of cotransporter

simultaneously, a molecule of glucose also binds to its binding site on cotransporter

results in conformational change that transports sodium ions & glucose molecule to interior of cell

• so sodium moves down its concentration gradient (area of high concentration (exterior) to an area of low concentration (interior))

• but this movement allows the glucose to move against its concentration gradient (area of low concentration (exterior) to area of high concentration (interior))

B2.1.17—Adhesion of cells to form tissues

cell-adhesion molecules (CAMs) are glycoproteins that’re important for organizing binding of cells to other cells, or to extracellular matrix

different forms of CAMs are utilized for different types of cell-cell junction

main types of cell junctions:

• adhesive junctions - present in epithelial cells & cardiac cells

  • often called anchoring junctions cuz they organize cell-cell adhesion in tissues to ensure structural stability & allow cells to withstand mechanical stress

• tight junctions - are in epithelial cells

  • form tight seal between 2 neighboring cells

    • this limits unregulated movement of molecules across barrier

• gap junctions - present in multiple types of cells

  • are intracellular channels that physically connect neighboring cells for movement of molecules

  • aid in cell-cell transfer of small molecules