membranes and transport

structure of membranes

• all membranes are made up of phospholipid bilayer, with membrane proteins, glycoproteins, glycolipids and cholesterol molecules embedded or attached

• membranes include

  • cell surface membrane that surrounds the cell

  • membranes that surround some of the organelles → organelle envelope

• cell surface membranes are 7nm thick

• hydrophilic phosphate heads of phospholipids face outwards and make contact with the aqueous exterior and interior (cytoplasm) of the cell

• hydrophobic fatty acid chains of phospholipids face inwards and are sandwiched between hydrophilic phosphate heads

  • hydrophobic fatty acid chains are shielded from the aqueous environment

• some fatty acid chains are saturated while others are unsaturated

  • phospholipids with saturated fatty acids are more closely packed, while phospholipids with unsaturated fatty acids are less closely packed ⇒ contain one or more C=C double bonds causing kinks in tail which prevent packing of phospholipids, preventing membranes from freezing at low temperatures

  • greater number of unsaturated fatty acid chains, the more kinks there are to prevent the close packing of phospholipids, hence the more fluid the membrane is

fluid mosaic model

• commonly used to describe the arrangement of phospholipids and proteins in cell membranes

fluid: phospholipids and membrane proteins are always in constant motion unless they are anchored to the cytoskeleton or extracellular matrix

• phospholipids and membrane proteins move laterally in the plane of the membrane

• phospholipids are held together primarily by hydrophobic interactions between fatty acid chains

mosaic: membrane proteins are scattered in the sea of phospholipids

• e.g.: intrinsic and extrinsic proteins

why are membranes asymmetric in nature

• the outer and inner layers of the membrane differ in composition and function

  • different types or amount of membrane proteins, phospholipids and cholesterols between the bilayers

  • different membrane proteins, glycoproteins and glycolipids in the membrane

    • glycoproteins and glycolipids on the side of the cell surface membrane typically face the exterior of the cell → involved in cell-cell recognition, cell-cell communication or cell-cell adhesion

cholesterol

• are amphipathic in nature

  1. hydrophobic hydrocarbon skeleton of cholesterol interacts with the hydrophobic fatty acid chains of phospholipids via hydrophobic interactions

  2. hydrophilic -OH group of cholesterol interacts with the hydrophilic phosphate heads of phospholipids via hydrophilic interactions

• regulates membrane fluidity ⇒ fluidity of the membrane does not fluctuate too much even at extreme temperatures

  • at low temperatures:

    • phospholipids have lower kinetic energy and move lesser ⇒ membrane is less fluid

    • cholesterol increases membrane fluidity by preventing the close packing of phospholipids ⇒ prevents membranes from freezing

  • at high temperatures:

    • phospholipids have higher kinetic energy and move more vigorously ⇒ membrane is more fluid

    • cholesterol decreases membrane fluidity by interacting with the fatty acid chains of the phospholipid and glycolipids molecules

membrane proteins

• are scattered in the sea of phospholipids, forming a mosaic

1. intrinsic proteins

   • embedded in the membrane ⇒ not easily removed

   • have both hydrophilic and hydrophobic regions

     • non-polar amino acid residues of intrinsic proteins interact with hydrophobic fatty acid chains of phospholipids via hydrophobic interactions

     • hydrophilic (polar or charged) amino acid residues of intrinsic proteins interact with hydrophilic phosphate heads of phospholipids via hydrophilic interactions (hydrogen bonds, ionic bonds)

   • those that span the entire phospholipid bilayer ⇒ transmembrane proteins

2. extrinsic proteins

   • largely hydrophilic

   • attached loosely to the surface of membrane ⇒ easily removed

function of membrane proteins

1. transport proteins (channel or carrier)

   • transmembrane proteins with hydrophilic channel to shield polar molecules and charged ions from hydrophobic core of phospholipid bilayer ⇒ transport across the membrane

2. enzymes

   • active site exposed to substrates in cytosol for enzymatic reaction

3. receptors for cell signalling

   • binding site exposed to exterior of cell for ligands to bind

   • for cell signally: allows cell to detect and respond to external stimulus to trigger a cellular response within the cell

   • usually have a carbohydrate side chain

4. cell-cell recognition and cell-cell communication

   • glycoproteins bind to proteins/glycoproteins/glycolipids of other cells during cell-cell recognition ⇒ act as receptors involved in cell-cell recognition, and the cells can then determine if the other cell is the same or different from itself

5. cell-cell adhesion

   • glycoproteins/proteins bind to glycoproteins/proteins of adjacent/neighbouring cells in the correct orientation during cell-cell adhesion → important for the regulation of cell growth and division, and the formation of tissues

6. attachment to cytoskeleton

   • components of the cytoskeleton bind to membrane proteins to maintain cell shape and stabilise the location of certain membrane proteins

function of all membranes

1. form a hydrophobic boundary between the external environment and cytoplasm of the cell, and the cytoplasm of the cell and organelle

   • regulates movement of substances into and out of cell/organelle

   • membranes are partially permeable → only non-polar molecules can move across the membrane directly, whereas polar molecules and charged ions are unable to

2. provide compartmentalisation within cell/organelles due to hydrophobic boundary

   • ensures maintenance of constant internal environment within cell/organelle

     • e.g.: cells have a lower sodium ion concentration than outside of the cells

   • ensures maintenance of optimal conditions necessary for enzymatic processes (e.g. optimal pH, high concentration of reactants within cell/organelle)

     • e.g.: enzymes within lysosomes requires low pH to function

   • prevents intermediates of one pathway from interfering with another, so that several metabolic processes can take place simultaneously and independently

     • e.g.: krebs cycle occurs in the mitochondria while glycolysis occurs in the cytosol → occur simultaneously and independently without interfering with each other

3. allows attachment of enzymes/receptor proteins/proteins involved in maintaining cell shape

   • allows for more efficient reaction sequence

     • e.g.: enzymes/proteins embedded in the membrane are organised in a sequential order so that a series of coordinated metabolic reactions can take place

   • allows for efficient cell signalling pathway

     • receptors allow coordinated signalling pathway and signal transduction

   • maintains the shape of the cell/organelle

     • membrane proteins are attached to the cytoskeleton

4. membranes can be extensively folded to increase surface area, thus allowing more enzymes/proteins to be attached to increase the rate of reaction

function of only cell surface membrane

1. receive external stimulus

   • receptors are attached in the cell surface membrane detect and respond to changes in the external environment

2. cell-cell recognition, cell-cell communication, cell-cell adhesion with other/neighbouring cells

   • receptors embedded in the cell surface membrane bind to glycoproteins/glycolipids/proteins of neighbouring cells for cell-cell recognition, cell-cell communication, cell growth and division or formation of tissues

3. formation of finger-like extensions to increase surface area for absorption/exchange

   • cell surface membrane of some animal cells are extensively folded to increase rate of absorption of nutrients

4. formation of pseudopodia

   • cell surface membrane of phagocytes (macrophages, neutrophils) extends outwards to surround and engulf particles during phagocytosis

why are transport processes across membranes important?

1. entry of useful substances

2. secretion of useful substances

3. removal of waste/excretory products

4. maintenance of optimal conditions (pH and ionic concentrations) essential for normal functioning of the cell/optimal enzymatic activity

permeability of phospholipid bilayer

• membranes are partially permeable as they allow some substances to pass through but not others

  • hydrophobic fatty acid chains of phospholipid bilayer create a hydrophobic core which prevents polar molecules or charged ions from crossing the membrane directly → diffuse across the membrane via specific transport proteins embedded in the phospholipid bilayer

  • only non-polar molecules can diffuse directly across the membrane

simple diffusion

net movement of molecules from a region of higher concentration to a region of lower concentration down a concentration gradient

important features of diffusion

• will always occur when there is a concentration gradient

• does not require energy (via ATP hydrolysis) from the cell

  • the random spontaneous motion of particles until all particles are uniformly distributed (no net movement of particles) is due to the inherent kinetic energy of the particles

  • diffusion occurs readily in both living and non-living systems

• non-polar molecules can cross the hydrophobic core of the phospholipid bilayer via simple diffusion

  • no membrane proteins are required

factors affecting rate of diffusion

1. temperature/kinetic energy of diffusing particles increase → rate of diffusion increases

2. concentration gradient increases → rate of diffusion increases

3. surface area over which diffusion occurs increases → rate of diffusion increases

4. hydrophobicity of diffusing substance increases (more hydrophobic/non-polar/lipid soluble) → rate of diffusion increases

5. distance over which diffusion occurs increases → rate of diffusion decreases

6. size of diffusion particle increases → rate of diffusion decreases

facilitated diffusion

net movement of polar molecules or charged ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, via specific transport proteins

• no energy is required

• specific transport proteins are required

features of transport proteins

• transport proteins are transmembrane proteins embedded in the phospholipid bilayer

  • exterior surface of transport proteins is made of non-polar amino acid residues, which forms hydrophobic interactions with the hydrophobic fatty acid chains of the phospholipids, enabling the transport protein to be attached in the phospholipid bilayer

  • hydrophilic channel/interior lining of transport proteins is made of hydrophilic (polar/charged) amino acid residues, which shield polar molecules and charged ions from the hydrophobic core of the phospholipid bilayer, allowing them to diffuse across the bilayer

    • polar molecules: glucose, sucrose

    • charged ions: Na+, K+, Cl-, Ca2+

• transport proteins are specific to the molecules/ions being transported

• transport of molecules/ions across membranes via transport proteins can reach saturation point

  • only a number of each type of transport protein is present

  • when all transport proteins are utilised, rate of transport occurs at the maximum rate

channel proteins

• are specific as only molecules/ions of specific shape and charge can pass through

• are intrinsic/transmembrane proteins embedded in the membrane and they have hydrophilic channels

  • ion channels: channel proteins that transport ions

• can become saturated → when all the channel proteins are used up, transport is maximum

• can be inhibited when the inhibitor binds at the pore of the channel and prevents the diffusing molecule/ion from passing through

• some channel proteins have gates that open or close in response to a stimulus (chemical or electrical signal)

carrier proteins

• specific as only molecules/ions of specific shape and charge can pass through

  • shape of binding site of transport protein is complementary to the shape of the molecules/ions transported

• exist in two conformation → they change shape in order to transport the molecule/ion across the membrane

  • in one state, the binding site is exposed on one side of the membrane (e.g. extracellular side)

  • once the molecule/ion binds at the binding site, it results in a conformational change in the carrier protein (changing to the other state) which results in the molecule/ion being released to the other side of the membrane

• can become saturated → when all the carrier proteins are used up, transport is maximum

• can be inhibited when the inhibitor, similar in shape to the diffusing molecule or ion, binds and competes with the diffusing molecule/ion for the binding site of the carrier protein

osmosis

net movement of water molecules from a region of higher water potential to a region of lower water potential, down a water potential gradient

• water molecules can move across cellular membrane (cell surface membrane, tonoplast) or artificial membrane (visking tubing)

• no energy is required

• both direct passage and aquaporin-facilitated movement of water molecules

  • water is small, so despite being polar, a small number of water molecules can directly pass through the phospholipid bilayer

• water molecules move from a region of higher water potential to a region of lower water potential through a partially permeable membrane

cells in solutions with different water potential

1. cells placed in hypotonic solution with higher water potential

   • animal cells

     1. net movement of water molecules from solution (higher water potential) to cell (lower water potential) via osmosis

     2. volume of cell increases, cell expands

     3. bursts because there is no cellulose cell wall to prevent further uptake of water into the cell

   • plant cells

     1. net movement of water molecules from solution (higher water potential) to cell (lower water potential) via osmosis

     2. volume of cell increases, cell expands (increase in length) and is turgid

     3. does not bursts because the cellulose cell wall prevents further uptake of water into the cell

2. cells placed in isotonic solution with equal water potential

   • animal cell

     1. no net movement of water molecules as water potential of solution is the same as that of animal cell

     2. animal cell neither shrinks nor swells

   • plant cell

     1. no net movement of water molecules as water potential of solution is the same as that of plant cell

     2. plant cell is flaccid due to lack of turgor pressure ⇒ no net movement of water into cell

3. cells placed in hypertonic solution with lower water potential

   • animal cell

     1. net movement of water molecules from cell (higher water potential) to solution (lower water potential) via osmosis

     2. volume of animal cell decreases and animal cell shrinks

   • plant cell

     1. net movement of water molecules from cell (higher water potential) to solution (lower water potential) via osmosis

     2. volume of plant cell decreases and plant cell shrinks (plasmolysis) and is flaccid

• incipient plasmolysis: occurs when cell surface membrane starts to pull away from cellulose cell wall

• full plasmolysis: occurs when cell surface membrane is extensively pulled away from cellulose cell wall

active transport

movement of molecules or ions from a region of lower concentration to a region of higher concentration against a concentration gradient

• energy is required

• specific transport protein is required

important features of active transport

• energy is required → usually from hydrolysis of ATP

• phosphate group from ATP is transferred to the carrier protein, which induces conformational change in the carrier protein to transport the molecule/ion across

  • protein pumps: carrier proteins that actively pump molecules across membranes

  • ion pumps: ions are transported by protein pumps

significance of active transport

1. continually take up nutrients, even when their concentrations outside the cell is lower than inside the cell

2. continually remove unwanted substances (waste), even when their concentrations outside the cell is higher than inside the cell

3. maintain optimal internal concentration of molecules/ions inside the cell or organelle

bulk transport

• large (and hydrophilic) molecules (e.g. proteins and polysaccharides) are packaged in vesicles before they enter (endocytosis) or are released (exocytosis) from the cell

• process requires energy

endocytosis

• uptake of particles into the cell via formation of vesicles from the cell surface membrane

  • energy via hydrolysis of ATP is required

general process

1. cell surface membrane extends outwards, forming pseudopodia (extensions) to surround the particles OR the cell surface membrane invaginates (sinks inwards) to form a depression to surround the particles

2. particles enter the cell via endocytosis, forming an endocytic vesicle (or endosome)

types of endocytosis

• phagocytosis

• pinocytosis

• receptor-mediated endocytosis

phagocytosis

• uptake of solids/large insoluble particles

  • e.g.: uptake of antigens (e.g. bacteria) by phagocytes (e.g. macrophages, neutrophils), uptake of food particles by amoeba

• selective in update of particles

• process:

  1. the cell surface membrane extends outwards, forming pseudopodia (extensions) to surround the particles OR the cell surface membrane invaginates (sinks inwards) to form a depression to surround the particles

  2. the particle enters the cell via phagocytosis

  3. the phagocytic vesicle (phagosome) formed fuses with a lysosome to form phagolysosome

  4. hydrolysis enzymes in the lysosome (lysosomal enzymes) digest the particles

  5. useful substances are absorbed into the cytoplasm for use by the cell

pinocytosis

• uptake of liquids → including particles dissolved in it

• not selective in uptake of particles

receptor-mediated endocytosis

• uptake of specific particles: require binding of particles to binding site of specific receptors

  • e.g.: uptake of influenza virus by respiratory epithelial cells, uptake of antigen-antibody complexes by phagocytes

• shape of particle is complementary to shape of binding site of receptor

• process:

  1. particle binds to the binding site of specific receptors on the cell surface membrane

  2. cell surface membrane invaginates (sinks inwards) to form a depression to surround the particles

  3. particle enters the cell via receptor-mediated endocytosis

  4. phagocytic vesicle formed fuses with the lysosome to form phagolysosome

  5. hydrolytuc enzymes in the lysosome (lysosomal enzymes) digest the particles

  6. useful substances are absorbed into the cytoplasm for use by the cell

  7. most receptors are recycled back to the cell surface membrane for reuse

exocytosis

release of particles out of the cell via fusion of vesicle membrane with the cell surface membrane

• energy via hydrolysis of ATP is required

• e.g.: secretion of antibodies by plasma B cells into bloodstream, secretion of insulin by β-cells in islets of langerhans in the pancreas, secretion of extracellular digestive enzymes by pancreatic and intestinal cells

general process

1. secretory vesicles containing the proteins/secretory substances bud off from the trans-face of Golgi apparatus

2. and travel along the microtubules of cytoskeleton

3. membrane of secretory vesicles fuses with the cell surface membrane

4. contents of secretory vesicles are released out of the cell via exocytosis

similarities between simple diffusion and facilitated diffusion

1. net movement of molecules down a concentration gradient

2. energy is not required

differences between simple diffusion and facilitated diffusion

feature of comparison	simple diffusion	facilitated diffusion
particles transported	non-polar molecules	polar molecules and charged ions
mode of transport	molecules diffuse directly across phospholipid bilayer	molecules or ions transported via specific transport proteins

similarities between osmosis and facilitated diffusion

1. net movement of molecules/ions down a concentration gradient

2. energy is not required

3. occurs across a membrane

differences between osmosis and facilitated diffusion

feature of comparison	osmosis	facilitated diffusion
substances transported	water molecules only	polar molecules and charged ions
mode of transport	can move directly across phospholipid bilayer (slower rate) OR via aquaporins (faster rate)	molecules or ions transported via specific transport proteins with hydrophilic channels

similarities between active transport and facilitated diffusion

1. occurs across a membrane

2. require specific transport proteins

differences between active transport and facilitated diffusion

feature of comparison	active transport	facilitated diffusion
direction of transport	against concentration gradient	down concentration gradient
energy (e.g. from ATP hydrolysis)	required	not required
type of transport protein	carrier proteins	both channel and carrier proteins