Carbohydrates, Lipids, Membranes, and Cell Structure: General Biology Study Notes

Chapter 5: Carbohydrates

Chemical Nature and Diversity of Monosaccharides

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the formula (CH2O)n. Monosaccharides are the simplest carbohydrates and serve as building blocks for more complex sugars.

• Monosaccharides differ in the arrangement of their atoms, the number of carbon atoms, and the location of their carbonyl group (aldose vs. ketose).

• Examples: Glucose, fructose, galactose.

• Isomerism allows for diversity in structure and function.

Linkages and Comparison with Other Macromolecules

• Monosaccharides are linked by glycosidic bonds to form disaccharides and polysaccharides.

• Carbohydrates can be compared to other macromolecules such as proteins (linked by peptide bonds), nucleic acids (phosphodiester bonds), and lipids (ester bonds).

Structure and Function of Major Polysaccharides

Polysaccharides are long chains of monosaccharide units. Their structure determines their function in cells.

Monomer	Linkage(s) Present	Overall Structure	Overall Function
Starch	α(1→4) and α(1→6) glycosidic bonds	Helical, branched (amylopectin) or unbranched (amylose)	Energy storage in plants
Glycogen	α(1→4) and more frequent α(1→6) branches	Highly branched, compact	Energy storage in animals
Cellulose	β(1→4) glycosidic bonds	Straight, unbranched chains forming fibers	Structural support in plant cell walls
Chitin	β(1→4) bonds with N-acetylglucosamine	Linear, forms tough sheets	Structural support in fungal cell walls and exoskeletons of arthropods
Peptidoglycan	β(1→4) bonds with peptide cross-links	Mesh-like layer	Structural support in bacterial cell walls

• Major functions of carbohydrates: energy storage, structural support, cell recognition, and signaling.

Chapter 6: Lipids and Membranes

Characteristics of Lipids

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids. They are not polymers but are grouped based on their insolubility in water.

• Fats (triglycerides): composed of glycerol and three fatty acids; function in energy storage.

• Phospholipids: consist of glycerol, two fatty acids, and a phosphate group; form the basis of cell membranes.

• Steroids: characterized by a four-ring structure; cholesterol is a key example, important for membrane fluidity.

Phospholipid Bilayer and Membrane Structure

• Phospholipids arrange themselves into bilayers, with hydrophilic heads facing outward and hydrophobic tails inward.

• This structure forms the fundamental architecture of biological membranes.

Membrane Permeability

• Membrane permeability is influenced by fatty acid saturation, cholesterol content, and temperature.

• Unsaturated fatty acids increase fluidity and permeability; saturated fatty acids decrease it.

• Cholesterol stabilizes membrane fluidity across temperature changes.

Transport Across Membranes

• Passive transport: movement of substances down their concentration gradient without energy input (includes simple diffusion and facilitated diffusion).

• Osmosis: diffusion of water across a selectively permeable membrane.

• Active transport: movement of substances against their concentration gradient, requiring energy (usually ATP) and protein pumps.

• Difference between hypertonic, hypotonic, and isotonic solutions:

  • Hypertonic: higher solute concentration outside the cell; cell loses water.

  • Hypotonic: lower solute concentration outside; cell gains water.

  • Isotonic: equal solute concentration; no net water movement.

Protein Carriers and Transport

• Different proteins facilitate passive (channels, carriers) and active (pumps) transport.

• Active transport is distinct from passive transport as it requires energy and moves substances against their gradient.

Chapter 7: Cell Structure and Organelles

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotes: lack a nucleus and membrane-bound organelles; DNA is in the nucleoid region (e.g., bacteria, archaea).

• Eukaryotes: have a nucleus and membrane-bound organelles (e.g., plants, animals, fungi, protists).

Cell Organelles and Their Functions

• Nucleus: stores genetic material.

• Mitochondria: site of cellular respiration and ATP production.

• Chloroplasts: site of photosynthesis in plants and algae.

• Endoplasmic reticulum (ER): protein and lipid synthesis.

• Golgi apparatus: modification and sorting of proteins and lipids.

• Lysosomes: digestion and waste processing.

• Endomembrane system: network of membranes involved in transport within the cell.

Chapter 8: Energy, Enzymes, and Metabolism

Energy and Chemical Reactions

• Free energy (G): energy available to do work in a system.

• Exergonic reactions: release energy; $\Delta G < 0$; spontaneous.

• Endergonic reactions: require energy input; $\Delta G > 0$; nonspontaneous.

• Entropy (S): measure of disorder.

• Enthalpy (H): total energy of a system.

• Relationship: $\Delta G = \Delta H - T\Delta S$

ATP and Energetic Coupling

• ATP (adenosine triphosphate): main energy currency of the cell.

• Energetic coupling: the use of energy released from exergonic reactions (like ATP hydrolysis) to drive endergonic reactions.

Enzyme Function and Regulation

• Enzymes lower the activation energy of reactions, increasing reaction rates.

• Regulation includes:

  • Competitive inhibition: inhibitor binds to the active site.

  • Allosteric regulation: inhibitor or activator binds to a site other than the active site, changing enzyme activity.

  • Feedback inhibition: end product of a pathway inhibits an earlier step.

Graphical Analysis of Reactions

• Reaction progress diagrams show free energy changes, activation energy, and the effect of enzymes.

• At equilibrium, $\Delta G = 0$.

Practice Questions and Applications

• Interpretation of graphs relating to membrane permeability and hydrocarbon tail length.

• Analysis of reaction diagrams to identify exergonic/endergonic reactions, activation energy, and spontaneity.

• Comparison of structural features of polysaccharides (cellulose, chitin, peptidoglycan).

• Understanding glycosidic linkages and their analogy to protein bonds.

• Factors affecting membrane permeability (e.g., double bonds, temperature, cholesterol, hydrocarbon chain length).

• Osmosis and tonicity in plant cells (e.g., celery stalks in different solutions).

• Matching types of membrane transport (simple diffusion, facilitated diffusion, active transport) to their characteristics.

• Identifying four things all cells have: plasma membrane, cytoplasm, DNA, and ribosomes.

• Explanation of energetic coupling and its role in driving nonspontaneous reactions.

Additional info: Some explanations and table entries were expanded for clarity and completeness based on standard General Biology content.