Carbohydrates, Lipids and Membranes

Carbohydrates

Monosaccharides

Aldose

  • carbonyl group at the end of the carbon chain

Function

Plant cells- Osmotic pressure exerted accounts for:
a. turgidity in plant cells, provide support for plants to stand upright to get sunlight
b. elongation of young plant cells during growth
c. transportation of water from cell to cell; opening and closing the stomata

  1. Act as respiratory substrates
  2. Raw materials for synthesis of other carbohydrates, proteins, lipids and nucleic acid

Properties

b. Physical Properties:
i. soluble in water, low molecular weight, sweet taste
ii. crystalline in solid state
iii. readily soluble in water
-> exert osmotic pressure

a.Chemical properties
i. Reducing properties

  • eg. Benedict's reagent, Fehling's reagent
  • contain aldehylde groups that are oxidised to carboxylic acids
  • for oxidation to occur, cyclic form first ring-open to give reactive aldehyde

Structure
(eg. Glucose)

a. Ring Formation:

  • occurs between the carbon containing the carbonyl group and one of the carbons with a hydroxyl group
  • 𝛼 and 𝛽 forms of glucose
  • 𝛽 more stable than 𝛼

Ketose

  • carbonyl group in middle of carbon chain

Polysaccharide

Disaccharides

b. Some disaccharides are reducing sugars
eg. Maltose and Lactose

  • have unsubstituted anomeric carbons.
  • end of free anomeric carbon = reducing end
  • other end = non-reducing end

-> Sucrose is NOT a reducing sugar

  • both anomeric carbon are substituted -> no free hydroxyl group
  • substituted anmoeric carbon cannot be converted to the aldehyde configuration -> cannot participate in oxidation-reduction reactions (charateristics of reducing sugar)

Starch

Cellulose

Glycogen

a. Amylose

Structure:

  • highly branched molecule of 𝛼(1-->6) branches

Function:

  • used for structural support in cell walls of plants and many algae

b. Amylopectin

Structure:

  1. polysaccharide comprising of 𝛼-glucose
  2. composed of linear chains of D-glucose in 𝛼 (1-->4) linkages
  3. helical structure / helical spiral / unbranched helix

Properties:

  • poorly soluble in water
  • high temp -> hydrogen bonds broken -> increase solubility
  • react with iodine to give a blue colour

Function:

  • used for energy storage in plant cells (eg. potatoes)

Structure

  • highly branched chain of glucose unit
  • linear linkage of 𝛼 (1-->4)
  • branched linkage of 𝛼 (1-->6)
  • branched helices

Properties:

  • poorly soluble in water
  • more soluble than amylose
    -> more branch -> more hydroxyl group that interact with water -> more soluble
  • react with iodine to give a red-violet colour

Properties:
i. more solvated with water molecules

  • more easily hydrolysed than starch
  • meet a higher energy needs of metabolically active tissues (eg. liver and muscles) than starch

Function:

  • used for energy storage in animal cells (eg. liver and muscles)

Structure + Properties:

  1. linear homopolymer of 𝛽-glucose unit, 𝛽(1-->4) glycosidic bond
  • flip orientation of glucose unit allows multiple hydrogen bonds to form between adjacent, parallel strands of cellulose
  1. Intrachain hydrogen bonds
  • allows many strands to be pack closely -> lesser interaction with water molecule -> less likely squeeze enzyme to hydrolyse the bonds
  1. Staggered sheets of chain
  • give strength and stability to wall
  • become extremely resistant to hydrolysis
  1. Intermolecular hydrogen bond -> microfibrils -> macrofibrils -> cellulose fibrils -> form meshwork -> distribute stress in all directions -> give rise to great tensile strength that provide support and mechanical strength + prevent bursting in high w.p solution

a. Formation of glycosidic bond

  • Formed: condensation reactions joining two monosaccharides together
  • Break: hydrolysis of glycosidic bond

Lipids

Fatty acids

Glycerol

Membrane

Triglycerides

Phospholipid

Structure:
i. alcohol with 3 carbons, each with hydroxyl group(-OH)


Properties:
i. soluble in water
ii. hygroscopic (ability to hold and attract water)

Structure:
i. long hydrocarbon chain + terminal carboxyl group

Properties:
i. Saturated

  • all carbon bonds are single
  • extremely flexible due to free rotation of carbon-carbon bonds

ii. Unsaturated

  • one or more double bonds in the hydrocarbon chain
  • double bonds are nearly always in cis configuration -> cause bend / "kink" in fatty acid chain

a. Formation of ester bond

  • Formed: condensation reactions between glycerol and 3 fatty acids
  • Break: hydrolysis of ester linkages

Structure:

  • glycerol linked to a phosphate group and 2 hydrocarbon chains
  • phosphate group may be bonded to a small organic molecule that is charged or polar

Properties:
i. non-polar
ii. insoluble in water but are soluble in organic solvents (eg. chloroform, methanol, ether, benzene
iii. less dense than water -> float

Functions:

  1. Serves as energy source and energy storage
  • highly reduced carbons
    -> yield large amts of energy in oxidative reactions of metabolism
  • aggregate in highly anhydrous forms
  1. Provides metabolic water for cellular activites
  • oxidation of triglycerides releases carbon dioxide and water

3. Good thermal Insulation

  • body fat for insulation
  • oil prevents excessive evaporation of water

4. Buoyancy


5. Protect organs from shock and injury

Properties:
i. amphipathic

  • head group: hydrophilic (outer surface)
  • hydrocarbon tail: hydrophobic (inner surface)

ii. can self assemble into aggregates that shield their hydrophobic tails

Functions:

  1. Act as a barrier to separate the contents of the cell from the surrounding
  2. Allow for internal compartmentalisation
  3. Determine the surface charge of a membrane
  4. Restrict movement of hydrophilic molecules across the surface

"Fluid Mosaic" Model:

  • Phospholipids able to move laterally and rotationally (fluid)
  • contains a myriad of proteins, cholesterol and phospholipids floating with the phospholipid bilayer (mosaic)

Proteins

a. Peripheral proteins

  • outside of the membrane
  • globular proteins that interact with the membrane mainly through electrostatic and hydrogen-bonding interactions with integral proteins

b. Integral proteins
i. embedded in the membrane


ii. amphipathic
hydrophilic domains:

  • give certain regions of the protein hydrophilic character

  • interact with water and stick out into the aqueous environment



    hydrophobic domains:

  • give other regions hydrophobic character
  • interact with fatty acids in the interior of the phospholipid bilayer

Roles

Glycoprotein and Glycolipids

a. Transport:
i. provide a hydrophilic channel across the membrane
ii. shuttle the substance from one side to another by changing shape

b. Enzymatic activity
i. organised as a team to carry out sequential steps of a metabolic pathway

c. Signal Transduction:
-> membrane protein have binding site with a specific shape that fits shape of chemical messenger (eg. hormone)
-> signalling molecule cause protein to change shape -> relay message inside cell

d. Cell-cell recognition:

  • some glycoproteins serve as identification tags that are recognised by membrane of other cells

e. Intercellular joining

  • membrane proteins of adjacent cells hook together

f. Attachment to the cytoskeleton and extracellular matrix and extracellular matrix (ECM)

  • maintain cell shape and stabilise location of certain membrane proteins

Glycolipids:

  • consist of a carbohydrate covalently bonded to a lipid
  • serve as a recognition signal for interactions between the cell

Glycoprotein:

  • one or more short carbohydrate chains covalently bonded to a protein

Cholesterol

Functions:

Roles:

  1. mammalian cell growth
  • require cholesterol for cell growth synthesise cholesterol to satisfy molecular requirement
  1. reduce passive permeability of lipid bilayers to solutes
  • due to restriction of movement of hydrocarbon tails of phospholipids near the head groups
  1. reduces membrane fluidity at moderate temperature
  • reducing phospolipid movement
  • low temperature -> hinders solidfication, disrupt the regular packing of phospholipids

a. As a barrier

  • separates internal environment of the cell from the external environment -> maintain constant environment to allow the cell to function efficiently

b. As site for mulit-enzyme reaction

  • anchor enzymes in common biochemical pathway in sequential manner -> allow reactions to proceed more efficiently

c. Regulate transport of materials

  • membranes are selectively permeable due to protein carriers and protein channels
  • regulate the movement of substances across the membrane -> allow cell to function effifciently

d. Cell-cell communication

  • interact with specific molecules corresponding to external stimuli
  • generate a signal cascade that will stimulate or inhibit internal activities of cell in response to the stimuli

e. Cell-cell recognition

  • interactions between glycolipids and glycoproteins on membrane allow for one cell to bind to another cell
  • play a role in regulating cell growth through contact inhibition
  • if lost -> uncontrolled cell growth -> cancer

Transport Across membrane

d. Active transport

  • transport particles across a selectively permeable membrane against their concentration gradient
  • requires energy that is released from hydrolysis of ATP
  • involves carrier proteins
    eg. sodium-potassium pump
    -> breaks down a molecule of ATP to ADP and a free phosphate ion

c. Facilitated diffusion

  • particles are transported across the membrane down their concentration gradient through transport proteins
  • does not require energy

Transport proteins:
i. Channel Proteins

  • form a hydrophilic "tube" across the membrane
    eg. aquaporin

ii. Carrier Proteins

  • undergo conformational change, exposing binding site
    eg. glucose transporter

b. Osmosis

  • net movement of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential

e. Endocytosis & Exocytosis

  • bulk transport
  • used to transport particles that are too large to pass trhough the membrane
  • require energy

a. Simple diffusion

  • particles pass through the phospholipid bilayer of the membrane down a concentration gradient

2. Pinocytosis:

  • cell continually "gulps" droplets of extracellular fluid into tiny vesicles, formed by infolding of plasma membrane

3. Receptor-Mediated Endocytosis

  • specialised type of pinocytosis
  • enables cell to acquire bulk quantities of specific substances
  • proteins embedded in membrane exposed to extracellular fluid -> specific solute bind to the site -> receptor proteins then cluster in coated pits -> form a vesicle containing the bound molecules

1. Phagocytosis:

  • cell engulfs a particle
    by extending pseudopodia around it and package it within a membranous sac -> food vacuole -> digested by lysosomes
  1. Cell engulf large particle
  2. Cell membrane extend around large particle, form pseudopodia.
  3. Pseudopodia seals off to form phagosome
  4. Phagosome fuse with lysosome -> phagolysosome
  5. Particles are broken down by hydrolytic enzyme from lysosome

Eg. Uptake of LDL

  1. LDL receptor recognises and binds to LDL particle
  2. Induces membrane to invaginate and pinch off from the cell membrane -> form endosome
  3. Endosome fuse with lysosome
  4. Acidic pH causes conformational change in LDL receptor -> release LDL receptor
  5. Proteins and lipids of LDL are broken down into their constituent parts by enzyme in lysosome
  6. LDL receptor recycled to the cell surface