lipids, membrane structure and functions!

Question 1

where is triglycerides mainly found?

Answer 1

• found mainly as oil in plants and in adipose tissues of animal cells

Question 2

what is the structure of triglycerides?

Answer 2

• triglycerides consists of two kinds of organic molecules: fats and oils

glycerol
1.an alcohol with 3 carbons ( C3H8O3)
> each carbon bears a hydroxyl group
2. soluble due to the presence of polar-OH groups
> which can form bonds with H2O molecules

fatty acids
1. made up of hydrocarbon chain and a carboxylic acid group
2. general formula is R-COOH , where R=H, CH3, (-CH2-)nCH3
3. hydrocarbon chain is made up of only carbon and hydrogen atoms
> the chain is non-polar so fatty acids are usually hydrophobic and insoluble
4. as the length of the hydrocarbon chain increases, triglyceride becomes increasingly hydrophobic and insoluble

• fatty acids can be saturated or unsaturated

Question 3

what is the difference between saturated and unsaturated fatty acids?

Answer 3

saturated: - does not contain carbon-carbon double bond
- have hydrocarbon chains that have the maximum possible number of hydrogen atoms
unsaturated: contains carbon-carbon double bonds

saturated:- straight chain allows close packing, and maximum hydrophobic interactions among fatty acid tails
- remain solid at room temperature ( most animal fats like butter)
unsaturated: - kinks due to C=C bonds in unsaturated fatty acids prevent close packing of fatty acids, leading to less hydrophobic interactions
- remain liquid at room temperature (fish oils and plant oils)

Question 4

relating the molecular structure and function of triglycerides

structure: large, uncharged/ non polar

Answer 4

property: large, prevented from diffusing out of cells
- insoluble in water
function: can be stored at high concentrations as droplets and does not affect the water potential of cells
> making it a suitable long term energy store, especially for hibernating animals

Question 5

what is the formation of triglycerides?

Answer 5

• 3 fatty acid molecules are joined to 1 glycerol to from 1 triglyceride molecule
• each fatty acid molecule is joined to glycerol by an ester bond which is formed during condensation reaction between a hydroxyl group (-OH) of the glycerol and the carboxylic acid group (-COOH) of a fatty acid
• 3 water molecules are released: 1 water molecule per bond formed
• triglycerides are hydrophobic and insoluble in water due to presence of non-polar fatty acid chains
  > the cannot form hydrogen bonds with water

Question 6

relating the molecular structure and function of triglycerides

structure: have a higher proportion of hydrogen and carbon to oxygen atoms per unit mass than carbohydrates

Answer 6

property: one gram of triglyceride (38kJ/g) yields about twice as much energy than one gram of carbohydrates (17kJ/g) when oxidised
function: large amount of energy is released to make ATP, hence an efficient energy store

property: for an equivalent amount of energy stored, triglyceride has about hald the mass of carbohydrates
function: light weight energy source
- useful for locomotion of animals which requires mass to be kept to a minimum
- usefule in seeds that are dispersed by wind or insects in which having a small mass is a necessity

Question 7

relating the molecular structure and function of triglycerides

structure: C-H bonds are non-polar and triglyceride molecules are hydrophobic

Answer 7

**property **: chemically inert in cells
**function **: serve as good thermal insulations
- artic mammals living in the cold climate have thick layers of fat beneath the skin called blubber, which forms effective insulator
- mammals have specialised cells for storing fat under their skin; cells are grouped together to form adipose tissues

function: act as electrical insulators
> allowing rapid transmission of electrical impulses along myelinated neurons

function: serve as protective layer around delicate and vital organs like heart and kidneys
> absorbs shock and cushions from impact and physical damage

Question 8

structure: lower in molecular weight than water per unit volume

Answer 8

property: less dense than water
function: gives bouyancy to aquatic animals such as whales and seals

Question 10

what is the source of phospholipids?

Answer 10

• cell surface membrane and membranes of several organelles such as chloroplasts, mitochondria and endoplasmic reticulum

Question 11

what is the structure of phospholipids?

Answer 11

• made up of one glycerol, two fatty acids and one phosphate group (PO43- )
• fatty acids are linked to glycerol via ester bonds, while the phosphate group is linked to the glycerol via the phosphoester bond
• the ester and phosphoester bonds are formed via condensation, where water molecules are lost in the process
• the charged phosphate group forms the hydrophilic end while the non-polar fatty acid tails form the hydrophobic region of the molecule
• since the molecule has both hydrophobic and hydrophilic regions, phospholipid is an
• AMPHIPATHIC MOLECULE

Question 12

basic structure of the cell surface membrane (phospholipids)

what is the component and structure of the cell surface membrane?

Answer 12

• the main component of the cell membrane is the phospholipid
  > whose physical properties account for the formation of the sheet-like structure of membranes
  > a membrane is commonly referred to as a phospholipic bilayer
• the structure of the bilayer is maintained by multiple interactions between neighbouring fatty acid tails and between charged phosphate heads of the phospholipids respectively
• the non-polar fatty acid/hydrocarbon tails of the phospholipids are hydrophobic and face inwards, creating a hydrophobic core of the membrane
• the close packing of these hydrocarbon tails is stabilised by hydrophobic interactions between them
• degree of saturation of fatty acid tails determine the extent of hydrophobic interaction and hence membrane fluidity
• the charged phosphate heads are hydrophilic and face outwards to the aqueous environment on either side of the membrane
• ionic and hydrogen bonds stabilise the interaction of the phospholipid heads with one another and with water respectively

Question 13

basic structure of the cell surface membrane (phospholipids)

what is membrane fluidity?

Answer 13

• membrane fluidity arises from the movement of both phospholipids and proteins
• hydrophobic interactions allow most membrane lipids and proteins to move laterally within the plane of the membrane
• lateral movement of phospholipids occurs frequently while flip-flopping is rare

the fluidity of membranes depends on two factos:
- degree of saturation of fatty acid tails of phospholipids
- presence of cholesterol

• the greater the proportion of saturated fatty acid tails in the phosphilipid bilayer, the less fluid the membrane is, and vice versa

Question 14

relating the structure of phospholipid to its function in the membrane

structure:
amphipathic: phosphate group is hydrophilic and fatty acid tails are hydrophobic

in an aqueous environment, phospholipid molecules self-assemble themselves into a bilayer:
- the hydrophilic phosphate heads face outwards and form hydrogen bonds with aqueous environment on either side
- the hydrophobic tails face inwards and interact with one another via hydrophobic interactions, forming the hydrophobic core

Answer 14

• the phospholipid bilayer forms the cell surface membrane that functions as a partially permeable barrier to separate contents of the cell from external environment
• hydrophobic region allows regulation of transport across membrane
• only small non-polar, hydrophobic substances can pass through the hydrophobic core
• allows compartmentalisation so that reactions can take place at optimal condition within the organelles
• impermeable to charged particles
  > allows formation of ion concentration gradient

Question 15

relating the structure of phospholipid to its function in the membrane

structure: phospholipids are held by weak hydrophobic interactions between the hydrocarbon tails, and hence can move laterally

Answer 15

contributes to membrane fluidity
- fluidity allows formation of transient gaps for simple diffusion of small, non-polar molecules
- fluidity allows the membrane to reseal itself if it is disrupted

Question 16

relating the structure of phospholipid to its function in the membrane

structure: some fatty acids of phospholipids are unsaturated, with one or more double bonds
> introduce kinks in fatty acid tails

Answer 16

this prevents close packing of phospholipid molecules at lower temperatures, increasing membrane fluidity

Question 17

what are the roles of cholesterol?

Answer 17

1. cholesterol regulates membrane fluidity
   - at high temperatures, the kinetic energy of phospholipids increases, thus causing them to vibrate more rapidly
   - when phospholipids ‘bump’ ino cholesterol, some of their kinetic energy is lost
   - the bulky nature of cholesterol ( due to its four inter-connected carbon rings
   - restricts the phospholipid movement
   - at low temperatures, cholesterol acts as spacers
   - helps to separate phospholipids so that they do not pack closely together
   - HENCE, membrane solidity is provented
2. cholesterol lowers membrane permeability by increasing packing of phospholipids ( cholesterol can fit into spacees between phospholipids)
   - preventing small water-soluble molecules from too freely diffusing across the membrane
3. cholesterol helps to hold peripheral proteins in place by forming high-density lipid rafts capable of anchoring the protein

Question 18

what are the two categories of membrane proteins?

Answer 18

1. integral/ intrinstic membrane proteins
2. peripheral/ extrinsic membrane protein

Question 19

what are integral/ intristic membrane proteins?

Answer 19

• majority of integral membrane proteins completely span across the phospholip bilayer
  >these proteins are known as trnasmembrane proteins
• there are some integral proteins which are only embedded into part of the bilayer ( not transmembrane)
• integral membrane protein molecules must posess different domains/ regions to interact with the membrane
• the region of the protein that contact the fatty acid tails consist of amino acid residues with non-polar R groups on the exterior surface
  > forming hydrophobic interactions with the hydrophobic fatty acid tails of the phospholipids
• the protein region that contacts the phosphate heads or face the aqueous environment on either side of the membrane consist of amino acid residues with charged/ polar R-groups on the exterior surface
  > forming ionic and hydrogen bonds with the phosphate heads and water molecules repectively
• some transmembrane proteins are also held in place by attachments to microfilaments of cell’s cytoskeleton

Question 20

what are peripheral/ extrinsic membrane proteins?

Answer 20

• peripheral proteins are not embedded in the bilayer and are usually loosely bound to the membrane surface by ionic and hydrogen bonds formed between phosphate heads and water molecules and charged/ polar R groups of amino acid residues on the surface of the protein
• they can be found on either side of membrane

Question 21

what are some proteins and lipids with short carbohydrate chains?

Answer 21

• some proteins and lipids have short branching carbohydrate chains covalently attached
  > forming glycoproteins and glycolipids respectively
• carbohydrate attachment occurs via a process knowns as glycosylation
• the carbohydrate chains of glycoproteins and glycolipids are important for cell-cell communication, adhesion and recognition

Question 22

what is the transport of molecules across membrane important for?

Answer 22

• maintain a suitable pH and ionic concentration within the cell for enzyme activity
• generate electrochemical (ionic) gradients that are required for nervous and muscular activity
• uptake nutrients such as glucose and amino acids
• excrete substances such as ammonia or urea
• secrete hormones, enzymes or antibodies

Question 23

what are the 5 specific way that a substance can be transported across the cell surface membrane?

Answer 23

• simple diffusion (passive)
• osmosis (passive)
• facilitated diffusion (passive)
• active transport (active)
• bulk transport (active)

Question 24

transport across the cell surface membrane

what occurs during simple diffusion in the cell surface membrane

Answer 24

• diffusion is the movement of substances from a region of high concentration to a region of low concentration, down a concentration gradient
  > it occurs until there is no net movement of particles at dynamic equilibrium
• small, non-polar and hydrophobic molecules (eg. hydrocarbonds, oxygen, carbon dioxide and steroid hormones) can pass through the hydrophobic core of cell membranes directly without the assistance of transport proteins
• this process is passive
  > does not require any energy ( from ATP hydrolysis) and happens spontaneously

Question 25

transport across the cell surface membrane

what happens during osmosis in a cell surface membrane?

Answer 25

• osmosis is the net movemnt of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane with no use of energy, ATP (passive)
• osmosis can occur bi-directionally

while the hydrophobic core of the membrane impeded the direct passage of ions and polar molecules which are hydrophilic, water (a very small polar molecule) is able to pass through
> due to formation of small transient pores in the membrane when phospholipids move laterally

Question 26

transport across the cell surface membrane

what happens during facilitated diffusion in a cell surface membrane?

Answer 26

• it is the movement of specific subtances from a region of high concentation to a region of low concentration with no use of energy (passive)
  > via the help of specific transmembrane transport proteins
• hydrophilic substances cannot pass through the hydrophobic core of the phospholipid bilayer. these include:
  1. polar molecules: too large to fit between phospholipids
  2. charged ions (e.g Na+, K+), hence hydrophilic, thus repelled y the hydrophobic core of the phospholipid bilayer in the membrane
• specific transmembrane transport proteins facilitate the diffusion of these solutes across the membrane
  > channel proteins and carrier proteins
• these transport proteins have a hydrophilic interior to facilitate binding and/ or assist in movement of solutes
• both are specific for the solute that they transport
  > only a particular charged ion or polar molecule can diffuse across the membrane through the channel or carrier protein
  > eg. Na+ ion channels allow passage of Na+ ions only and glucose carrier proteins only translocate glucose across the membrane
• direction of facilitated diffusion is determined by the difference of concentration on both sides of the membrane
  > solutes can be transporter out or into the celll
  > transport is bidirectional

Question 27

transport across the cell surface membrane - facilitated diffusion

what is the function of channel proteins?

Answer 27

• channel proteins provide an aqueous pore that extend across the lipid bilayer
• when open, these pores allow specific soltues ( usually inorganic ions of appropriate size and charge) to pass through them
• these channels can be gated
  > which means they require a stimulus to cause them to open or close
  > gated ion channels are important in conduction of nerve impulses or transduction of signals within cells

Question 28

transport across the cell surface membrane - facilitated diffusion

what is the function of carrier proteins?

Answer 28

• they have a binding site with a 3D conformation that is complementaru to the solule that it transports
• they bind the specific solute and undergo a series of confromational changes to translocate the bound solute across the membrane

Question 29

transport across the cell surface membrane - facilitated diffusion

why can some water molecules diffuse directly through a phospholipid bilayer?

Answer 29

• some water molecules can diffuse directly through the bilayer
  > because even though water molecules are polar, they are small enough to pass throguh the gaps of the phospholipid
• specialised channel proteins known as aquaporins allow for more water molecules to cross the cell surface membrane
  > aquaphorins thus play a role in water regulation and transport

Question 30

transport across the cell surface membrane

what occurs during active transport across the cell surface membrane?

Answer 30

active transport is the transport molecules or ions across a membrane against a concentration gradient using energy from ATP hydrolysis
- requires transmembrane transport proteins: carrier proteins or protein pumps
- unlike facilitated diffusion, these transport proteins require energy to transport substances against the concentration gradient

**active transport allows:
**1. accumulation of metabolites in the cell
2.the maintenance of concentration gradients of ions and small molecules across a membrane

examples include
1. sodium-potassium pump located on the plasma membrane of all cells
> sodium potassium pumps actively transport 3Na+ ions out of the cell and 2 K+ into the cell against their respective concentration gradients
2. proton pump located on the inner membrane of mitochondrion and the thylakoid membrane in a chloroplast

Question 31

what is bulk transport?

Answer 31

• bulk transport is the movement of macromolecules or large quantities of substances into or out of the cell via vesicles and require energy from ATP hydrolysis
• bulk transport usually involves microtubules ( a kind of cytoskeletal proteins which also include intermediate dilaments and microfilaments) and/ or motor proteins
• two mechanisms of bulk transport are exocytosis and endocytosis

Question 32

bulk transport

what is exocytosis?

Answer 32

• exocytosis is the bulk transport of materials out of the cell

process of exocytosis
1. secretory vesicles containing transported substances move along microtubules (of the cytoskeleton) in the cytoplasm to the cell surface membrane
2. the movements of vesicles along the microtubules require energy supplied by ATP hydrolysis
3. when the vesicles and cell surface membrane come into contact, the two membranes fuse
4. contents of the vesicles are released out of the cell

function:
1. secretion of useful secretory products:
- B-cells in pancreas manufacture the hormone insulin and secrete it into the bloodstream by exocytosis
- proteins and certain carbohydrates from the golgi vesicles are delivered to the outside of cell via exocytosis when plant cells are synthesising cell walls
2. excretion of waste materials

Question 33

bulk transport

what is endocytosis? and what are the different types of it?

Answer 33

• buld intake of macromolecules and particulate substances into cells

general description of endocytosis:
1. when substances come into contact with the cell surface membrane, a small portion of the plasma membrane first invaginates (folds inwards) to form a cavity containing the substance
2. the substance will progressively be enclosed by the membrane
3. the membrane then pinches off to form and endocytic vesicle within the cell

3 types of endocytosis:
- phagocytosis
- pinocytosis
- receptor-mediated endocytosis

Question 34

different types of endocytosis

what occurs during phagocytosis?

Answer 34

• phagocytosis occurs when a cell uses large endocytic vesicles to ingest large particles such as microorganisms and dead cells

**process of phagocytosis
**1. when the particles bind to the cell surface membrane, the membrane form pseudopodia (single: pseudopodium) to engulf the particle
> pseudopodia are outward extensions of the cell membrane
> this process requires energy from ATP hydrolysis

2. the tips of the pseudopodia fuse and pinch off to form a membrane- bound vescile inside the cell known as phagocytic vacuole
   - the phagocytic vacuole then fuses with a lsosome containing hydrolytic enzymes
   - the hydrolytic enzymes in the lysosome digest the contents in the phagocytic vacuole into soluble products
   - useful products are released into the cytoplalsm while unwanted products or indigestible substances will be released out of the cells via exocytosis

Question 35

different types of endocytosis

what occurs during pinocytosis?

Answer 35

• fluid and solutes are taken into the cell via pinocytosis
  > it is not the fluid itself that is needed by the cell but the molecules which are dissolved in it
  > eg. the uptake of nutrients from the surrounding follicle cells by a human egg cell

process of pinocytosis
1. the cell membrane invaginates and pinches off to form a vesicle inside the cell containing the substances
2. movement of the vesicles in the cell cytoplasm requires energy from ATP hydrolysis

Question 36

what are the functions of the cell surface membrane?

Answer 36

1. compartmentalisation
2. transprot
3. enyzmatic activity
4. interaction with the external environment and surrounding cells

Question 37

different types of endocytosis

what occurs during receptor-mediated endocytosis?

Answer 37

• enables the cells to regulate transport and acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid

process of receptor mediated endocytosis:
1. specific substances to be taken into cells
eg. macromolecules bind to complementary binding sites of transmembrane receptor proteins on the plasma membrane
2. these receptor proteins are ususally clustered in regions of the cell surface membrane called coated pits
> these pits are coated with a protein knwon as clathrin
3. the coated pits deepen with help of coat proteins through invagination of the membrane and pinches off to form a vescicle cointaining the substances- receptor complex
- movemnt of vescicles within the cell require energy from ATP hydrolysis
- after the ligand molecules are released from the vesicle, the receptors are recycled with the cell surface membrane
> eg. uptake of cholesterol by cells

Question 38

functions of the cell surface membrane

how does the cell surface membrane carry out compartmentalisation?

Answer 38

• to compartmentalise the cell, both from its external environment and within the cell
• it separates the cytoplasm from the external environment, thus maintaining a constant environment inside the cell
• compartmentalisation also provides many different local environments that facilitate biochemical pathways
  eg. the mitochondrion requires certain enzymes to produce ATP whereas the nucleus require the other enzymes to synthesise DNA
• these two organelles are using two different sets of enzymes that must be separated so that metabolic reactions can take place without interference from other enzymes, thus increasing efficiency
  eg. the pH within the lysosome or mitochondrion can be maintained at an acidic pH which would otherwise be detrimental to the processes occuring in other parts of the cell

Question 39

functions of the cell surface membrane

how does the cell surface membrane facilitate transportation?

Answer 39

• biological membranes are partially permeable and allow only specific substances to pass through
  > different substances also move through the membrane at different rates
• presence of the hydrophobic core, specific transport proteins and receptors facilitate this regulation

Question 40

functions of the cell surface membrane

what enzymatic activities occur in the cell surface membrane?

Answer 40

• membranes may contain integral proteins with enzymatic functions embedded within them
  eg. adenylyl cyclase, which catalyse the synthesis of cyclic adenosine monophosphate (CAMP) from ATP
• several membrane-bound enzymes may be organised together to carry out sequential reactions of a metabolic pathway
  >eg. oxidative phosphorylation via the electron transport chain in cellular respiration occurs on the inner mitochondrial membrane

Question 41

why is it called a fluid mosaic model?