HL IB Biology: Carbohydrates & Lipids

Properties of Carbon

• Carbon forms covalent bonds, which involve sharing electrons between atoms to generate strong bonds within compounds.
• Carbon has four electrons in its outer shell, meaning it can form four covalent bonds.
• Carbon can form millions of different covalently-bonded compounds, mainly with hydrogen, oxygen, and other elements.

Carbon in Biological Molecules

• Carbon is present in all four major categories of biological molecules: carbohydrates, lipids, proteins, and nucleic acids.
• Carbon is a key component of large, stable molecules due to its ability to form four covalent bonds.

Formation of Macromolecules

• Macromolecules are formed through the process of polymerization, where monomers join together to form a large molecule.
• Macromolecules are very large, containing 1000 or more atoms, and have a high molecular mass.
• Examples of macromolecules include carbohydrates, proteins, and nucleic acids.

Carbohydrates: Definition, Functions, and Examples

• Monosaccharides are the monomers of carbohydrates, and they can join together to form disaccharides or polysaccharides.
• Monosaccharides have the general formula CnH2nOn, where 'n' is the number of carbon atoms in the molecule.
• Examples of monosaccharides include glucose, ribose, and glyceraldehyde.
• Glucose is a key monosaccharide that serves as a source of energy for cells and is produced during photosynthesis.
• Glucose exists in two structurally different forms: alpha (α) glucose and beta (β) glucose, which are isomers of glucose.

Polysaccharides: Energy Storage

• Polysaccharides, such as starch and glycogen, function as energy storage molecules in cells.
• Starch is the storage polysaccharide of plants, while glycogen is the storage polysaccharide of animals and fungi.
• Starch is composed of two polysaccharides: amylose and amylopectin, which have different structures and functions.
• Amylose is an unbranched helix-shaped chain, while amylopectin is a branched molecule with many terminal glucose molecules.
• Glycogen is a more branched polysaccharide than amylopectin, providing more free ends for glucose molecules to be removed by hydrolysis.

Digestion of Polymers

• Macromolecules need to be broken down into their monomers through hydrolysis reactions, which involve the addition of water to break covalent bonds.

• Hydrolysis reactions are used to break down polysaccharides, proteins, and nucleic acids into their respective monomers.

• Examples of hydrolysis reactions include the breakdown of glycosidic bonds in polysaccharides, peptide bonds in proteins, and ester bonds in triglycerides.### Cellulose Structure

• Cellulose is a structural carbohydrate found in plant cell walls.

• Molecules of cellulose are straight and unbranched.

• Cellulose is a polymer of β-glucose monomers.

• β-glucose differs from α-glucose in the position of the hydroxyl group on carbon 1.

• To form glycosidic bonds, every alternate molecule of β-glucose in the chain must invert itself.

Cellulose Function

• The alternating pattern of β-glucose monomers in cellulose allows for hydrogen bonding between strands.
• Hydrogen bonds link several molecules of cellulose to form microfibrils.
• Cellulose molecules are linked by hydrogen bonds, giving cellulose its structural strength.

Polysaccharide Structure

• A comparison of starch, glycogen, and cellulose:
  • Starch: composed of α-glucose, found in plants, has a helix shape, and has branches.
  • Glycogen: composed of α-glucose, found in animals, has a helix shape, and has branches.
  • Cellulose: composed of β-glucose, found in plants, has no helix shape, and has no branches.

Role of Glycoproteins

• Glycoproteins are formed when carbohydrates and polypeptides combine via covalent bonds.
• Glycoproteins are classified as proteins.
• Glycoproteins are involved in cell recognition, cell signaling, endocytosis, and cell adhesion.
• Glycoproteins can act as antigens, identifying cells as "self" or "non-self".

Blood Types

• Glycoproteins on the surface of red blood cells determine an individual's blood type.
• A person's blood type is determined by the presence or absence of antigens on their red blood cells.
• The presence of antibodies in an individual's blood can interact with glycoproteins, causing an immune response.

Lipids

• Lipids are examples of hydrophobic molecules found in living organisms.
• Lipid molecules contain carbon, hydrogen, and oxygen atoms.
• Lipids are insoluble in water due to their non-polar nature.

Formation of Triglycerides and Phospholipids

• Triglycerides are formed when three fatty acids join to a glycerol molecule.
• The process of triglyceride formation is called esterification.
• Phospholipids are formed when a glycerol molecule and two fatty acids combine, with a phosphate ion replacing the third fatty acid.
• Phospholipids are amphipathic, having both hydrophobic and hydrophilic regions.

Phospholipids

• Phospholipids are the major components of cell surface membranes.
• Phospholipids have a hydrophobic fatty acid tail and a hydrophilic phosphate head.
• Phospholipids can form monolayers or bilayers in water.

Properties of Triglycerides

• Lipids are energy-dense molecules, containing 2x more energy per gram than carbohydrates.
• Lipids are insoluble and are stored in adipose tissue.
• When lipids are respired, they produce metabolic water.
• Lipids are ideal for long-term energy storage.

Fatty Acids

• Fatty acids are molecules with long hydrocarbon chains.
• Fatty acids occur in two forms: saturated and unsaturated.
• Saturated fatty acids have single bonds between carbon atoms, while unsaturated fatty acids have double bonds.
• Unsaturated fatty acids can be monounsaturated or polyunsaturated.

Formation of Phospholipid Bilayers

• Phospholipids form the basic structure of the cell membrane.

• Phospholipid bilayers are formed when hydrophilic phosphate heads bond with hydrophobic fatty acid tails.

• Phospholipids are amphipathic, having both polar and non-polar regions.### Phospholipids and Cell Membrane

• Phospholipids have a polar head and a non-polar tail, making them 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, forming a phospholipid monolayer.

• Phospholipids can form monolayers on the surface of water.

Phospholipid Bilayer

• Phospholipids can form a two-layered structure called a phospholipid bilayer when mixed with water, which is the basic structure of the cell membrane.
• A phospholipid bilayer is composed of two layers of phospholipids, with their hydrophobic tails facing inwards and hydrophilic heads facing outwards.
• The amphipathic nature of phospholipids means that the phospholipid bilayer acts as a barrier to most water-soluble substances.

Function of Phospholipid Bilayer

• The non-polar fatty acid tails prevent polar molecules or ions from passing between them across the membrane.
• Water-soluble molecules such as sugars, amino acids, and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in.

Passage Through Phospholipid Bilayers

• Small, non-polar molecules like O2 and CO2 can easily cross cell membranes to be utilized by the cell.
• They do not need proteins for transport and can diffuse across quickly.
• 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, which allows them to cross lipid bilayers.
• Examples of steroid hormones include oestradiol and testosterone, which are produced by gonadal tissues in the reproductive organs.
• These hormones can cross the lipid bilayer and travel into and out of cells and nuclei, where they alter and direct the process of transcription.

Revision notes for HL IB Biology students covering carbohydrates, lipids, macromolecules, and more. Study properties of carbon, glycoproteins, fatty acids, and phospholipids.