chapter 1: lipids, membrane structure and functions! Flashcards
where is triglycerides mainly found?
- found mainly as oil in plants and in adipose tissues of animal cells
what is the structure of triglycerides?
- 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
what is the difference between saturated and unsaturated fatty acids?
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)
relating the molecular structure and function of triglycerides
structure: large, uncharged/ non polar
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
what is the formation of triglycerides?
- 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
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
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
relating the molecular structure and function of triglycerides
structure: C-H bonds are non-polar and triglyceride molecules are hydrophobic
**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
structure: lower in molecular weight than water per unit volume
property: less dense than water
function: gives bouyancy to aquatic animals such as whales and seals
what is the source of phospholipids?
- cell surface membrane and membranes of several organelles such as chloroplasts, mitochondria and endoplasmic reticulum
what is the structure of phospholipids?
- 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
basic structure of the cell surface membrane (phospholipids)
what is the component and structure of the cell surface membrane?
- 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
basic structure of the cell surface membrane (phospholipids)
what is membrane fluidity?
- 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
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
- 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
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
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
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
this prevents close packing of phospholipid molecules at lower temperatures, increasing membrane fluidity
what are the roles of cholesterol?
- 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 - 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 - cholesterol helps to hold peripheral proteins in place by forming high-density lipid rafts capable of anchoring the protein
what are the two categories of membrane proteins?
- integral/ intrinstic membrane proteins
- peripheral/ extrinsic membrane protein
what are integral/ intristic membrane proteins?
- 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
what are peripheral/ extrinsic membrane proteins?
- 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
what are some proteins and lipids with short carbohydrate chains?
- 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
what is the transport of molecules across membrane important for?
- 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
what are the 5 specific way that a substance can be transported across the cell surface membrane?
- simple diffusion (passive)
- osmosis (passive)
- facilitated diffusion (passive)
- active transport (active)
- bulk transport (active)
transport across the cell surface membrane
what occurs during simple diffusion in the cell surface membrane
- 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