Grade 12 Biology Study Notes

Proteins

• Proteins are large, complex molecules composed of amino acids.
• They are essential for the structure, function, and regulation of the body’s tissues and organs.
• Examples: enzymes, antibodies, hemoglobin, hormones.

Monomer of a Protein: Amino Acid

• Proteins are polymers of amino acids.
• Each amino acid has:
  • A central (alpha) carbon
  • A hydrogen atom
  • An amino group ($\text{--NH}_2$)
  • A carboxyl group ($\text{--COOH}$)
  • An R group (side chain) that varies with each amino acid

Amino Acids Held Together

• Amino acids are linked by peptide bonds through dehydration synthesis (water is removed).
• Bonds form between the carboxyl group of one amino acid and the amino group of another, creating a polypeptide chain.

Differences in R Groups and Their Purpose

• The R group (side chain) is the unique part of each amino acid.
• R groups determine if an amino acid is polar, nonpolar, acidic, or basic.
• These properties affect how proteins fold, how they interact with other molecules, and their biological function.

Levels of Protein Structure

1. Primary: The specific sequence of amino acids.
2. Secondary: Local folding into alpha helices or beta-pleated sheets, held by hydrogen bonds.
3. Tertiary: The 3D shape of the entire polypeptide due to interactions between R groups (hydrophobic interactions, disulfide bridges, etc.).
4. Quaternary: The association of multiple polypeptide chains into one functional protein (e.g., hemoglobin).

Protein Denaturation

• Denaturation is when a protein loses its shape due to changes in pH, temperature, or chemicals.
• This disrupts its function because the shape determines how it works.
• Once denatured, many proteins cannot return to their original form.

Functions of Proteins in the Body

• Enzymes: Speed up chemical reactions (e.g., catalase, amylase)
• Structural: Support body structure (e.g., collagen in skin and bone)
• Transport: Carry substances (e.g., hemoglobin transports oxygen)
• Hormones: Regulate body processes (e.g., insulin controls blood glucose)
• Antibodies: Fight infections as part of the immune system
• Movement: Enable muscle contraction (e.g., actin and myosin)

Lipids

Function of Lipids

• Lipids are nonpolar macromolecules with several important roles:
  • Energy storage (more than twice the energy of carbohydrates)
  • Insulation and protection (fat cushions organs and maintains body heat)
  • Structure (main component of cell membranes)
  • Communication (hormones like steroids)
  • Waterproofing (e.g., waxes on leaves)

Triglycerides

• A triglyceride is made from:
  • 1 glycerol molecule
  • 3 fatty acids
• Formed by dehydration synthesis, releasing 3 water molecules.
• Used for long-term energy storage in fat cells (adipose tissue).

Saturated vs. Unsaturated Fatty Acids

Phospholipids

• Made from:
  • 1 glycerol
  • 2 fatty acids
  • 1 phosphate group
• Found in cell membranes (phospholipid bilayer).

Structure and Polarity

• Phosphate head: Polar and hydrophilic (water-loving)
• Fatty acid tails: Nonpolar and hydrophobic (water-fearing)

Cell Membrane Formation

• In water, phospholipids arrange into a bilayer:
  • Heads face outward toward water (inside & outside cell)
  • Tails face inward away from water
• This structure forms the selectively permeable membrane around all cells.

Steroids

• Lipids with a structure of four fused carbon rings
• Do not contain fatty acids or glycerol
• Nonpolar and hydrophobic

Functions & Examples

• Hormones: e.g., testosterone, estrogen, cortisol
• Cholesterol: maintains membrane fluidity and is a building block for steroid hormones
• Help regulate metabolism, salt balance, reproduction, and stress response

Waxes

• Lipids made from long fatty acid chains joined to alcohols or carbon rings
• Extremely hydrophobic

Functions & Examples

• Protective coatings in plants and animals:
  • Cuticle on leaves: prevents water loss
  • Earwax (cerumen): protects the ear canal
  • Beeswax: used to build honeycombs

Summary Table

Enzymes

What Are Enzymes?

• Enzymes are biological catalysts—proteins that speed up chemical reactions without being used up or permanently changed.
• They are highly specific to the reactions they catalyze.

How Enzymes Affect Activation Energy and Reaction Rate

• Every chemical reaction needs a certain amount of energy to start, called activation energy.
• Enzymes lower the activation energy, making it easier and faster for reactions to occur.
• As a result, they increase the rate of reaction, often by millions of times.

Substrate and Active Site

• The substrate is the specific molecule that the enzyme acts on.
• The active site is the region on the enzyme where the substrate binds.
• The shape and chemical environment of the active site are precisely suited to the substrate.

Lock and Key Model

• The lock and key model suggests that the enzyme's active site has a fixed shape that exactly matches the shape of the substrate—like a key fitting into a specific lock.
• This model explains enzyme specificity, but not flexibility.

Induced Fit Model

• More accurate than the "lock and key" model.
• The induced fit model shows that the enzyme changes shape slightly when the substrate binds, creating a tighter fit.
• This model explains enzyme flexibility and precision better.

Optimal pH and Temperature

• Enzymes have optimal conditions where they work best:
  • Optimal temperature (e.g., 37^\circ \text{C} for human enzymes).
  • Optimal pH (e.g., pH 2 for pepsin in the stomach; pH 7 for most body enzymes).
• Too high or too low temperatures or pH can denature the enzyme (change its shape), causing it to lose function.

Allosteric Regulation

• Allosteric sites are regions on the enzyme other than the active site.
• Molecules can bind to these sites and change the enzyme's shape, regulating its activity.

Allosteric Activation

• An activator molecule binds to the allosteric site, changing the enzyme’s shape so the active site becomes functional.
• This enhances enzyme activity.

Allosteric Inhibition

• An inhibitor molecule binds to the allosteric site and distorts the active site, so the substrate can’t bind.
• This decreases or stops enzyme activity.

Feedback Inhibition

• A type of allosteric inhibition used for self-regulation in cells.
• When the end product of a metabolic pathway builds up, it binds to an enzyme earlier in the pathway and shuts it off.
• This prevents overproduction and helps maintain balance (homeostasis).

Summary

Carbohydrates

What Are Carbohydrates?

• Carbohydrates are organic macromolecules made of carbon (C), hydrogen (H), and oxygen (O), usually in a 1:2:1 ratio.
• Main roles:
  • Primary energy source for cells
  • Structural support in plants and animals
  • Stored as short- or long-term energy reserves

Monosaccharides (Simple Sugars)

• Monomers (single units) of carbohydrates
• Basic formula: C6H{12}O_6 (for hexose sugars)
• Soluble in water and easily absorbed

Types:

• Glucose: Main energy source in blood (produced during photosynthesis, used in cellular respiration)
• Fructose: Found in fruits and honey; sweetest natural sugar
• Galactose: Found in dairy products; part of lactose

Basic Structure:

• Ring or straight-chain form
• Contains a carbon backbone, multiple hydroxyl ($\text{--OH}$) groups, and a carbonyl group ($\text{C=O}$)

Disaccharides

• Formed when two monosaccharides join through a dehydration synthesis reaction (loss of water)
• Bond formed is called a glycosidic linkage

Examples:

• Maltose = Glucose + Glucose
• Sucrose = Glucose + Fructose
• Lactose = Glucose + Galactose

Basic Structure:

• Two sugar rings joined by an oxygen bridge (glycosidic bond)
• Can be broken down by hydrolysis (adding water)

Polysaccharides

• Complex carbohydrates made of many glucose monomers
• Function in energy storage or structure

Starch

• Found in plants (e.g., potatoes, grains)
• Used for energy storage
• Made of two components:
  • Amylose: Unbranched chain, forms a helix, slower to digest
  • Amylopectin: Branched chain, digests more quickly

Glycogen

• Storage form of glucose in animals (liver & muscles)
• Highly branched → fast release of energy

Glucose

• Found in blood, fruits, honey, and as part of many larger carbohydrates
• Basic structure: 6-carbon ring (hexose), multiple –OH groups
• Functions:
  • Used in cellular respiration to produce ATP
  • Monomer for starch, cellulose, glycogen

Cellulose

• Found in plant cell walls
• Structure: Long chains of beta-glucose with beta-1,4-glycosidic bonds
• Straight, rigid structure with many hydrogen bonds between chains (forms strong microfibrils)

Why Humans Can’t Digest It:

• We lack cellulase, the enzyme needed to break beta-1,4 bonds
• Acts as fiber, aiding digestion but not providing energy

Chitin

• Found in:
  • Exoskeletons of insects and crustaceans
  • Cell walls of fungi
• Similar to cellulose, but has nitrogen-containing side chains (N- acetylglucosamine)
• Strong, flexible, and biodegradable

Summary Table