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.