HL IB Biology Carbohydrates & Lipids PDF

Head to savemyexams.com for more awesome resources HL IB Biology Your notes Carbohydrates & Lipids Contents Properties of Carbon Macromolecules Carbohydrates: Definit...

Head to savemyexams.com for more awesome resources HL IB Biology Your notes Carbohydrates & Lipids Contents Properties of Carbon Macromolecules Carbohydrates: Definition, Functions & Examples Role of Glycoproteins Lipids Fatty Acids Phospholipids Page 1 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Properties of Carbon Your notes Chemical Properties of Carbon Carbon forms covalent bonds A covalent bond forms when a pair of electrons are shared between two atoms A single covalent bond is represented by a short straight line between the two atoms, e.g. H-H Electrons are shared between atoms to generate strong bonds within compounds Electrons are shared in a covalent bond Carbon in biological molecules Carbon is present in all of the four major categories of biological molecules; this is why life on Earth is often described as "carbon based" Carbon is present in: Carbohydrates Lipids Proteins Nucleic acids Carbon has four electrons in its outer shell, meaning that each atom can form four covalent bonds Carbon can therefore be a component of large, stable molecules Page 2 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Carbon forms millions of different covalently-bonded compounds, mainly with hydrogen and oxygen Carbon atoms can arrange themselves to form a huge variety of chemical compounds; it can: Your notes Bond to other carbon atoms, or other atoms such as hydrogen, nitrogen, oxygen and sulfur Form molecules with long branched chains such as glycogen Form long straight chain molecules such as cellulose Form molecules containing cyclic single rings such as the pyrimidines (thymine, uracil and cytosine) Form molecules with multiple rings, including starches and the purines (adenine and guanine) Produce a tetrahedral structure which allows the formation of varied carbon compounds which have different 3-D shapes and hence, different biological properties Carbon atoms can form up to four single covalent bonds or a combination of double and single bonds, e.g. Carbon dioxide contains two double bonds Methane contains four single covalent bonds Covalent bonding in carbon-containing molecules diagram Carbon atoms can form either single or double bonds in a variety of molecules. Carbon dioxide (left) contains double bonds, while methane (right) contains single bonds. Double and triple bonds can form with an adjacent carbon atom, allowing unsaturated compounds to form Carbon atoms can also form part of many different functional groups that give organic compounds their individual properties, e.g. Hydroxyl groups Carboxyl groups Amino groups Phosphate groups Page 3 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Functional groups diagram Your notes Functional groups include hydroxyl (OH), amino (NH2), carboxyl (COOH), and phosphate (H2PO4) groups NOS: Scientific conventions are based on international agreement The professional scientific community is global, meaning that scientists from all over the world may work on the same research, and need to be able to communicate clearly with each other Scientific conventions are thereby agreed upon and used internationally SI (which stands for système international) unit prefixes is one example kilo = 103 centi = 10-2 Page 4 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources milli = 10-3 micro = 10-6 nano = 10-9 Your notes Page 5 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Macromolecules Your notes Formation of Macromolecules Carbon compounds can be large molecules made from many small, repeating subunits Monomers are the smaller units from which larger molecules are made Polymers are molecules made from a large number of monomers joined together in a chain The process by which monomers join to form polymers is polymerisation Macromolecules are very large molecules They contain 1000 or more atoms and so have a high molecular mass Polymers can be macromolecules, however, not all macromolecules are polymers; polymers must consist of many repeating subunits E.g. lipids are not polymers, as they do not consist of repeating monomers Key biological macromolecules table Macromolecule Monomer Carbohydrates (polysaccharides) Monosaccharides Lipids Fatty acids, glycerol, phosphate groups Proteins (polypeptides) Amino acids Nucleic acids Nucleotides Formation of macromolecules Macromolecules are formed during condensation reactions A condensation reaction occurs when molecules combine together, forming covalent bonds and resulting in polymers (polymerisation) or macromolecules Water is removed as part of the reaction Examples of condensation reactions Polysaccharides Page 6 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Polysaccharides are formed when two hydroxyl (OH) groups on different monosaccharides interact to form a strong covalent bond called a glycosidic bond Glycosidic bond formation diagram Your notes Polypeptides Polypeptides are formed by condensation reactions Two amino acid monomers interact to form a strong covalent bond called a peptide bond Peptide bond formation diagram Page 7 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Nucleic acids Separate nucleotides are joined together via condensation reactions to form a phosphodiester bond These condensation reactions occur between the phosphate group of one nucleotide and the pentose sugar of the next nucleotide It is called a phosphodiester bond because it consists of a phosphate group and two ester bonds Phosphodiester bond formation diagram Page 8 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Page 9 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Digestion of Polymers Macromolecules often need to be broken down into their monomers, e.g. this happens in Your notes digestion The reaction that allows this to occur is a hydrolysis reaction Hydrolysis means ‘lyse’ (to break) and ‘hydro’ (with water) In the hydrolysis of macromolecules, covalent bonds are broken when water is added The -O and -OH from the water molecule are used to form the functional groups of the products Examples of hydrolysis reactions include: The hydrolysis of glycosidic bonds in poly- or disaccharides to produce monosaccharides The hydrolysis of peptide bonds in polypeptides to produce amino acids Hydrolysis of ester bonds in triglycerides to produce three fatty acids and glycerol Hydrolysis of a disaccharide diagram Page 10 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Page 11 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Carbohydrates: Definition, Functions & Examples Your notes Monosaccharides The monomers of carbohydrates are monosaccharides Two monosaccharides can join to form a disaccharide Many monosaccharides join to form a polysaccharide Monosaccharides can join together via condensation reactions The new chemical bond that forms between two monosaccharides is known as a glycosidic bond Monosaccharides have the general formula C n H2n O n Where 'n' is the number of carbon atoms in the molecule Note that this formula only applies to monosaccharides Monosaccharide properties include: Colourless crystalline molecules Soluble in water There are different types of monosaccharide formed from molecules with varying numbers of carbon atoms, for example: Triose molecules contain 3 carbon atoms, e.g. glyceraldehyde Pentose molecules contain 5 carbon atoms, e.g. ribose Hexose molecules contain 6 carbon atoms, e.g. glucose Ribose and glucose structure diagrams Page 12 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Pentose sugars, such as ribose (top), can be recognised by their five-point carbon rings and hexose sugars, such as glucose (bottom) by their six-point carbon rings Glucose The most well-known carbohydrate monomer is glucose Glucose has the molecular formula C 6 H12 O 6 Glucose is the most common monosaccharide and is of central importance to most forms of life Glucose is the main substrate used in respiration, releasing energy for the production of ATP Glucose is produced during photosynthesis Glucose exists in two structurally different forms, alpha (α)glucose and beta (β) glucose, these structures are known as the isomers of glucose This structural variety results in different functions between carbohydrates This seemingly minor example of isomerism has far-reaching consequences on the functions of the polymers Glucose structure diagrams Page 13 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes The straight chain structure of glucose can form rings of alpha glucose. Glucose also forms rings of beta glucose. Page 14 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Different polysaccharides are formed from the two isomers of glucose Starch and glycogen are made from molecules of alpha glucose Cellulose is made from molecules of beta glucose Your notes Properties of glucose Glucose has several properties that are essential to its function in living organisms Stable structure due to the presence of covalent bonds which are strong and hard to break Soluble in water due to its polar nature Easily transportable due to its water solubility A source of chemical energy when its covalent bonds are broken Exam Tip You should be able to recognise ring structures of hexose and pentose monosaccharides, and use glucose as an example of a hexose monosaccharide Page 15 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Polysaccharides: Energy Storage The function of carbohydrates Your notes Carbohydrates function as essential energy storage molecules and as structural molecules Starch and glycogen are effective storage polysaccharides because they are: Compact Large quantities can be stored in a small space Insoluble This is essential because soluble molecules will dissolve in cell cytoplasm, lowering the water potential and causing water to move into cells If too much water enters an animal cell it will burst Cellulose is a structural polysaccharide because it is: Strong and durable Insoluble and slightly elastic Chemically inert; few organisms possess enz ymes that can hydrolyse it Polysaccharide function diagram The different structures of starch, glycogen and cellulose allow each polysaccharide to perform different functions Starch Starch is the storage polysaccharide of plants Starch is stored as granules in chloroplasts Page 16 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources It is made of alpha glucose monomers Starch is constructed from two different polysaccharides: Amylose (10 - 30 % of starch) Your notes Unbranched helix-shaped chain with 1,4 glycosidic bonds between α-glucose molecules The helix shape enables it to be more compact and thus it is more resistant to digestion Amylopectin (70 - 90 % of starch) Contains 1,4 glycosidic bonds between α-glucose molecules as well as 1, 6 glycosidic bonds, creating a branched molecule The branches result in many terminal glucose molecules that can be easily hydrolysed for use during cellular respiration, or added to for storage Amylose and amylopectin structure diagrams Page 17 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Amylose (top) and amylopectin (bottom); the two polysaccharides that form starch in plant cells Glycogen Glycogen is the storage polysaccharide of animals and fungi The monomer of glycogen is alpha glucose, joined by 1,4- and 1,6 glycosidic bonds Glycogen is more branched than amylopectin, providing more free ends where glucose molecules can be removed by hydrolysis This means that glycogen can be broken down quickly, supplying the higher metabolic needs of animal cells Page 18 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Liver and muscles cells contain glycogen as visible granules, enabling high rates of cellular respiration Your notes Glycogen structure diagram Glycogen is a highly branched storage molecule present in animals and fungi Page 19 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources The Structure of Cellulose Cellulose is a structural carbohydrate found in the cell walls of plants Your notes Molecules of cellulose are straight and unbranched Cellulose is a polymer of β-glucose monomers β-glucose differs very slightly in structure to α-glucose; the hydroxyl group on carbon 1 sits above the carbon ring in β-glucose, whereas it sits below the ring in α-glucose It means that in order to form a glycosidic bond with a molecule of β-glucose, every alternate molecule of β-glucose in the chain must invert itself, or flip upside down Beta glucose in cellulose diagram Every other molecule of beta glucose needs to flip upside down in order for glycosidic bonds to form in cellulose The alternating pattern of the monomers in cellulose allows hydrogen bonding to occur between strands of β-glucose monomers, adding strength to the polymer Hydrogen bonds link several molecules of cellulose to form microfibrils Hydrogen bonding in cellulose diagram Page 20 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Cellulose molecules are linked by hydrogen bonds Cellulose function diagram Page 21 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Cellulose molecules are joined by hydrogen bonds to form microfibrils; this gives cellulose its structural strength Page 22 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Polysaccharide structure summary table Your notes Starch Feature Glycogen Cellulose Amylose Amylopectin Monomer α-glucose α-glucose α-glucose β-glucose Yes Yes (approximately (approximately Branches No No every 20 every 10 monomers) monomers) Helix shape Yes No No No Glycosidic 1, 4 1, 4 and 1, 6 1, 4 and 1, 6 1, 4 bonds Present in cell Plant Plant Animal Plant type Page 23 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Role of Glycoproteins Your notes Role of Glycoproteins Carbohydrates and polypeptides can combine, via covalent bonds, to make structures called glycoproteins These are classed as proteins Glycoproteins, along with another group of molecules called glycolipids, form part of the structure of cell surface membranes They act as receptor molecules in processes such as Cell recognition and identification Receptors for cell signalling molecules such as hormones and neurotransmitters Endocytosis Cell adhesion and stabilisation Glycoproteins and ABO blood types Glycoproteins can act as antigens which can identify cells as either "self" or "non-self" Cells that are recognised as non-self will trigger an immune response within the organism A person's blood type is determined by the glycoprotein antigens on the surface of their red blood cells Blood type A individuals have type A glycoprotein antigens Blood type B individuals have type B glycoprotein antigens Blood type AB individuals have both types of glycoprotein antigens Blood type O individuals have neither The presence of antibodies within an individual can create an interaction with the glycoproteins if blood of the wrong type enters their body E.g. a person with Type A antigens on their red blood cells will have antibodies in their blood against type B antigens This can cause fatal issues during blood transfusions if the incorrect blood type is given, as the antibodies cause the incorrect antigens (from the transfused blood) to clump together, blocking blood vessels Blood Types and their Antigens and Antibodies Table Blood type Blood type A Blood type B Blood type O AB Red blood cell Type A Type B Type A & B None surface antigens Antibodies Anti-B Anti-A None Anti-B & anti-A Page 24 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources present in plasma Blood groups that may be used for A&O B&O A, B, AB, O O Your notes transfusion Page 25 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Lipids Your notes Lipids: Hydrophobic Properties Examples of lipids in living organisms are Fats Oils Waxes Steroids Lipid macromolecules contain carbon, hydrogen and oxygen atoms Basic lipid structure diagram Lipid molecules are composed of a glycerol molecule and fatty acid hydrocarbon chains Lipid solubility The structure of lipids affects their solubility Lipids contain hydrocarbon molecules which contain many non-polar covalent bonds The non-polar nature of lipid molecules means that lipids are insoluble in water or other polar solvents In living organisms, lipid solubility can be improved by combining lipid molecules with other molecules, e.g. Page 26 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Glycolipids Lipoproteins Your notes Page 27 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Formation of Triglycerides & Phospholipids Formation of triglycerides Your notes Some lipids are categorised as triglycerides Three fatty acids join to one glycerol molecule to form a triglyceride Fatty acids contain hydrocarbon chains that can be either saturated or unsaturated Saturated fatty acids contain only single carbon-carbon bonds Unsaturated fatty acids contain one or more double bonds Triglycerides are formed by a process known as esterification An ester bond forms when the hydroxyl (-OH) group of a glycerol molecule bonds with the carboxyl group (-COOH) of a fatty acid The formation of an ester bond is a condensation reaction For each ester bond formed a water molecule is released Therefore for one triglyceride to form, three water molecules are released Formation of a triglyceride diagram Page 28 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Page 29 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes A triglyceride forms from a glycerol molecule and three fatty acid molecules by the process of esterification Formation of phospholipids Phospholipids are also formed from glycerol and fatty acids Unlike triglycerides, phospholipids contain only two fatty acids bonded to a glycerol molecule, as the third has been replaced by a phosphate ion (PO 4 3-) As the phosphate is polar, it is soluble in water, or hydrophilic The fatty acid ‘tails’ are non-polar and therefore insoluble in water, or hydrophobic Phospholipids are said to be amphipathic, meaning that they have both hydrophobic and hydrophilic regions As a result of having hydrophobic and hydrophilic parts, phospholipid molecules can form monolayers or bilayers when placed in water Structure of a phospholipid diagram Page 30 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Phospholipids are the major components of cell surface membranes. They have fatty acid tails that are hydrophobic and a phosphate head that is hydrophilic, attached to a glycerol molecule. Page 31 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Properties of Triglycerides Lipids as an energy store Your notes The hydrolysis of triglycerides releases glycerol and fatty acids, which can form useful respiratory substrates Lipids are energy-dense in comparison to carbohydrates due to their high number of C-H bonds They contain 2× more energy per gram than most carbohydrates Lipids are insoluble so are not transported around the body easily and remain in their storage cells When lipids are respired a lot of water is produced compared to the respiration of carbohydrates This is called metabolic water and can be used as a dietary water source when drinking water is unavailable A camel's hump is not filled with water, but is a lipid-rich storage organ that yields metabolic water for the camel in its dry desert habitat A bird's egg also makes use of lipid-rich yolk to provide energy and metabolic water to the growing chick All these features make lipids ideal for long term energy storage Storage of lipids In animals, lipids are stored in adipose tissue Subcutaneous fats are stored below the skin Visceral fats are stored around the major internal organs Fat is stored in adipose cells, which are specialised to contain large globules of fat Adipose cells shrink when the fat is respired to generate metabolic energy Adipose tissue can be used as a thermal insulator in animals that live in particularly cold environments Seals and walruses are endotherms and have thick adipose tissue called blubber which helps trap heat generated by respiration In many plants, seeds have evolved to store fats to provide energy for a growing seedling plant Olives, sunflowers, nuts, coconuts and oilseed rape are good examples of crops whose oils are harvested for edible oil production by humans Page 32 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Fatty Acids Your notes Fatty Acids Both triglycerides and phospholipids contain glycerol with molecules known as fatty acids attached These fatty acids have long hydrocarbon ‘tails’ Hydrocarbons are molecules that contain hydrogen and carbon Fatty acids occur in two forms: Saturated fatty acids Unsaturated fatty acids Unsaturated fatty acids can be monounsaturated or polyunsaturated Saturated fatty acids In saturated fatty acids the bonds between the carbon atoms in the hydrocarbon tail are all single bonds The fatty acid is said to be ‘saturated’ with hydrogen This means that each carbon atom in the hydrocarbon tail (except for the final carbon atom) is bonded to two hydrogen atoms Saturated fatty acids are straight molecules, meaning that lipid molecules containing them are able to pack tightly together This increases their melting point and causes them to be solid at room temperature Saturated fatty acids are often used as storage molecules in animals for this reason, e.g. the fats in meat and butter Saturated fatty acid diagram Page 33 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Saturated fatty acids contain only single carbon-carbon bonds Unsaturated fatty acids Your notes In unsaturated fatty acids the bonds between the carbon atoms in the hydrocarbon tail are not all single bonds The fatty acid is said to be ‘unsaturated’ because the hydrocarbon tail does not contain the maximum number of hydrogen atoms possible; each carbon atom in a carbon-carbon double bond can only bond to one hydrogen atom instead of two These double bonds can cause the hydrocarbon tail of unsaturated fatty acids to kink, or bend, meaning they are not as straight as saturated fatty acids Unsaturated fatty acids cannot pack as tightly together as saturated fatty acids, so fats containing unsaturated fatty acids are often liquids at room temperature Unsaturated fatty acids contain at least one carbon-carbon double bond A fatty acid with one C=C double bond is known as monounsaturated fatty acid Lipids that contain monounsaturated fatty acids have a lower melting point than saturated fatty acids, meaning that they form liquid oils; some animals and plants store energy in the form of oils In some unsaturated fatty acids, there are many carbon-carbon double bonds; these are known as polyunsaturated fatty acids Lipids containing polyunsaturated fats also have a low melting point, so form oils that are used for energy storage in plants Mono- & polyunsaturated fatty acid diagrams Page 34 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Monounsaturated fatty acids (top) contain only one carbon-carbon double bond, while polyunsaturated fatty acids (bottom) contain more than one Exam Tip You should be able to recognise from a diagram whether a fatty acid is saturated, monounsaturated or polyunsaturated (look for any carbon-carbon double bonds)! Page 35 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Phospholipids Your notes Formation of Phospholipid Bilayers Phospholipids form the basic structure of the cell membrane Cell membranes are phospholipid bilayers Membranes are formed when a hydrophilic phosphate head bonding with two hydrophobic hydrocarbon (fatty acid) tails Phospholipids have a hydrophobic and a hydrophilic region The phosphate head of a phospholipid is polar, so is hydrophilic and therefore soluble in water The fatty acid tail of a phospholipid is nonpolar, so is hydrophobic and therefore insoluble in water Molecules with both polar/hydrophilic and non-polar/hydrophobic regions are said to be amphipathic Phospholipid structure diagram Page 36 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Your notes Phospholipids contain polar heads and non-polar tails, so are said to be amphipathic When placed in water, the hydrophilic phosphate heads of phospholipids orient themselves towards the water and the hydrophobic hydrocarbon tails orient themselves away from the water, causing them to form a phospholipid monolayer Phospholipid monolayer diagram Phospholipids can form monolayers on the surface of water Page 37 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources When phospholipids are mixed with water, two-layered structures known as phospholipid bilayers can form; this is the basic structure of the cell membrane Your notes Phospholipid bilayer diagram A phospholipid bilayer is composed of two layers of phospholipids; their hydrophobic tails face inwards and hydrophilic heads face outwards The amphipathic nature of phospholipids means that the phospholipid bilayer acts as a barrier to most water-soluble substances The non-polar fatty acid tails prevent polar molecules or ions from passing between them across the membrane This means that water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in Page 38 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to savemyexams.com for more awesome resources Passage Through Phospholipid Bilayers Small, nonpolar molecules, such as O 2 and CO 2 , are soluble in the lipid bilayer and can therefore Your notes easily cross cell membranes to be utilised by the cell They do not need proteins for transport and can diffuse across quickly Other larger, non-polar molecules can also enter the cell across the lipid bilayer, e.g. steroid hormones Steroid hormones contain cholesterol, a type of lipid The hydrocarbon region of cholesterol is non-polar, allowing it to cross lipid bilyars Cholesterol structure diagram Cholesterol has hydrophobic and hydrophilic regions Oestradiol and testosterone are two examples of steroid hormones formed from cholesterol They are produced by gonadal tissues in the reproductive organs Due to their lipid structure they can cross the lipid bilayer and can readily travel into and out of cells and nuclei Inside the nucleus these hormones alter and direct the process of transcription Page 39 of 39 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers