IB Biology SL Topic B1.1

How is a covalent bond different to an ionic bond and a hydrogen bond?

• Covalent: Shared pairs of electrons between non-metal atoms (strongest intradomain chemical bond).

• Ionic: Electrostatic attraction between oppositely charged ions formed by electron transfer.

• Hydrogen: Intermolecular attraction between a partially positive hydrogen atom (bonded to N, O, or F) and an electronegative atom on another molecule (or another part of the same large molecule).

How many covalent bonds can carbon form in total?

Carbon can form four (4) covalent bonds in total.

What is the difference between a single and double covalent bond?

Single Bond: One pair of electrons shared between two atoms (C–C)

Double Bond: Two pairs of electrons shared between two atoms (C=C)

State two reasons for why life is based on carbon compounds.

Carbon can form up to four stable covalent bonds, allowing complex 3D structures (branched, ringed, linear).

Carbon readily bonds with itself and other essential elements (H, O, N, P, S), providing huge molecular diversity.

Name three disaccharides and their monosaccharides.

Three Disaccharides and Their Monosaccharides:

• Maltose: alpha glucose + alpha glucose

• Sucrose: alpha glucose = fructose

• Lactose: glucose + galactose

State the difference between a pentose and a hexose, and give an example of each.

• Pentose: 5-carbon sugar. Example: Ribose (or Deoxyribose).

• Hexose: 6-carbon sugar. Example: Glucose (or Fructose, Galactose).

State four properties of glucose that make it a good molecule to be used by life.

• Soluble in water (easy transport in blood/sap).

• Chemically stable under physiological conditions.

• High energy content per molecule relative to small size.

• Small size allows easy movement across membranes via transport proteins.

What is the name of the reaction for forming polymers from monomers?

Condensation reaction (dehydration synthesis).

What is the name of the reaction for breaking down polymers into monomers?

Hydrolysis reaction.

Name the polysaccharide used for storage in animals and what it is composed of.

Glycogen, composed of alpha glucose units.

Name the polysaccharide used for storage in plants and what it is composed of.

Starch (composed of amylose and amylopectin), built from alpha glucose units.

What is the name of the bond used for forming long chains in amylose, amylopectin, and glycogen?

Main Chain Bond in Amylose, Amylopectin, & Glycogen: 1,4 alpha - glycosidic bonds.

What type of bond is used to start a new branching chain in amylopectin and glycogen?

Branching Chain Bond: 1, 6 alpha - glycosidic bonds.

State a similarity and difference between the two types of starch.

(Amylose vs. Amylopectin):

• Similarity: Both are polymers made of $\alpha\text{-glucose}$ used for energy storage in plants.

• Difference: Amylose is an unbranched, helical chain ($1,4\text{-links}$ only), while amylopectin is branched ($1,4\text{-links}$ with $1,6\text{-branch points}$).

State the relationship between the extent of branching in polysaccharides and the rate at which they can by hydrolysed to their monomers.

Branching Extent vs. Hydrolysis Rate: Higher branching provides more terminal glucose ends available for enzymatic attack, resulting in a faster rate of hydrolysis.

State how the branching structure of glycogen relates to its function.

Glycogen is highly branched (1,6 links every 8–12 glucose units), allowing rapid release of glucose monomers to satisfy high metabolic energy demands in animals.

State three reasons why organisms have evolved to store carbohydrates in polysaccharides and not as monosaccharides.

• Osmotically inactive: Does not alter the osmotic potential of the cell (prevents lysis or swelling).

• Insoluble: Keeps energy reserves localized and easily stored without dissolving away.

• Compact: Highly branched or coiled structures pack large amounts of energy into a small space.

Where is cellulose found in life?

Found in plant cell walls

State three ways that cellulose is different to amylose.

• Monomer: Cellulose uses beta glucose, Amylose uses alpha glucose

• Shape: Cellulose forms straight, unbranched chains; Amylose forms a coiled helix.

• Orientation: In cellulose, alternating beta glucose monomers are inverted 180^o relative to neighbors.

State how the structure of cellulose makes it ideal for forming fibres with other cellulose chains.

Straight, linear chains lie parallel to each other, allowing extensive cross-linking hydrogen bonds between adjacent chains to group into rigid microfibrils.

What is the name of the bond used for forming long chains in cellulose?

Cellulose Main Bond: 1, 4, beta glycosidic bonds.

State a functional role of carbohydrates (aside from cellulose or energy storage) and give an example.

Cell recognition / Cell signaling: Example: Glycoproteins on cell surface membranes (e.g., ABO blood group antigens).

State four examples of lipids.

Triglycerides, Phospholipids, Steroids (e.g., cholesterol), Waxes.

State two things that lipids have in common.

• Non-polar and hydrophobic (insoluble in water).

• Mainly composed of carbon, hydrogen, and oxygen (with significantly lower proportion of oxygen relative to carbs).

Draw a saturated fatty acid.

How many double bonds do these fatty acids have: saturated, monounsaturated, polyunsaturated.

• Saturated: 0 double bonds.

• Monounsaturated: 1 double bond.

• Polyunsaturated: 2 or more double bonds.

What are the components of a triglyceride?

Components of a Triglyceride: 1 glycerol molecule bonded to 3 fatty acid chains.

What are the components of a phospholipid?

1 glycerol, 2 fatty acid chains, and 1 phosphate group.

How are fatty acids bonded to glycerol?

Linked via ester bonds (formed by condensation reactions).

What is the relationship between the number of double bonds and the viscosity of a fat?

Increasing the number of double bonds (kinks in tails) reduces packing density, resulting in lower viscosity (more fluid).

What is the relationship between the number of double bonds and the melting point?

Increasing the number of double bonds lowers the melting point (making unsaturated fats liquid at room temp).

State the major difference in composition in fats and oils.

• Fats: Higher proportion of saturated fatty acids (solid at room temperature).

• Oils: Higher proportion of unsaturated/polyunsaturated fatty acids (liquid at room temperature).

What is the difference in energy content (per gram) in fats and carbs?

Lipids contain roughly 2 times the energy density of carbohydrates

State two advantages of organisms storing energy as triglycerides and why.

• High energy density: Stores twice as much energy per gram, reducing mass carried by the organism.

• Insoluble in water: Does not affect cell osmotic balance and provides thermal insulation.

Why do many mammals store triglycerides in adipose tissue at the surface of the body?

Provides thermal insulation against heat loss and serves as a protective cushion for internal body structures.

Why do endotherms store fats high in saturated fatty acids?

Saturated fats remain solid at physiological core temperatures providing stable structural support and long-term insulation reserves.

their high, stable internal body temperatures keep these fats liquid enough for metabolic use, while allowing the molecules to pack tightly together for maximum energy storage

Explain the advantages of organisms storing energy as carbohydrates.

• Rapidly accessible: Easily hydrolysed into glucose for rapid ATP synthesis in respiration.

• Soluble monomers: Glucose easily dissolves in blood plasma for efficient transport to tissues.

• Aerobic & Anaerobic energy: Carbohydrates can yield ATP with or without oxygen (lipids require oxygen).

What molecule is used in animals for long-term storage and where is it stored?

Triglycerides stored in adipose tissue (adipocytes).

What molecule is used in animals for short-term storage and where is it stored?

Glycogen stored in the liver and skeletal muscle tissue.

Where do plants store energy as fats, and why?

Stored in seeds / cotyledons because fats provide high energy in a lightweight package, aiding seed dispersal and providing energy for germination before photosynthesis begins.

Where do plants store energy as carbohydrates?

Stored as starch in chloroplasts, roots, tubers (e.g., potatoes), and seeds.

What does amphipathic mean?

A molecule possessing both hydrophilic (water-attracting/polar) and hydrophobic (water-repelling/non-polar) regions.

State why a phospholipid is amphipathic?

They have a hydrophilic phosphate head (polar) and two hydrophobic fatty acid tails (non-polar).

How do phospholipids arrange themselves when placed in water?

Hydrophilic heads face outward toward the aqueous environment, while hydrophobic tails face inward, shielded from water.

What structure emerges when phospholipids are placed in water?

A phospholipid bilayer (or micelle/liposome).

While molecules will be able to pass through phospholipid bilayers? Give examples.

• Passes through: Small, non-polar / uncharged molecules (e.g., O_2, CO₂, steroid hormones, H₂O in small amounts).

• Blocked: Large, polar, or charged ions (e.g., Na^+, K⁺, Cl^-, glucose) which require protein channels or transporters.