Chapter 7: Cell Biology

Cells are the fundamental units of life. This chapter explores the intricate structures of cells, the biological molecules that sustain them, and the mechanisms that regulate their internal environment.

7.1 Cell Structure and Function

The cell is the smallest unit of a living thing. Organisms are classified as either prokaryotic (lacking a nucleus) or eukaryotic (containing a membrane-bound nucleus and organelles).

Key Organelles and Functions

Nucleus: Houses DNA; the “control center.”
Mitochondria: Site of ATP (energy) production via cellular respiration.
Ribosomes: Protein synthesis.
Endoplasmic Reticulum (ER): Rough ER (protein modification) and Smooth ER (lipid synthesis).
Golgi Apparatus: Sorting, tagging, and packaging proteins/lipids.

Figure 7.1: Eukaryotic cells contain specialized compartments called organelles. Plant cells are distinguished by a cell wall, chloroplasts, and a large central vacuole.

7.2 Carbohydrates

Carbohydrates are energy-providing macromolecules with the general formula \((CH_2O)_n\).

Monosaccharides: Simple sugars like glucose, galactose, and fructose (isomers with formula \(C_6H_{12}O_6\)).
Disaccharides: Formed via dehydration reactions (e.g., Sucrose = Glucose + Fructose).
Polysaccharides: Long chains used for storage (Starch in plants, Glycogen in animals) or structure (Cellulose in plant cell walls).

Figure 7.2: The ring structure of Alpha-D-Glucose, the primary energy source for cells.

7.3 Lipids and Proteins

These molecules form the structural and functional “machinery” of the cell.

Lipids

Lipids are hydrophobic and include triglycerides (fats), phospholipids (membrane components), and sterols (like cholesterol). - Saturated fats: No double bonds; solid at room temperature. - Unsaturated fats: Contain double bonds (“kinks”); liquid at room temperature.

Proteins

Proteins are polymers of amino acids. Their function is determined by four levels of structure: 1. Primary: Sequence of amino acids. 2. Secondary: Local folding (Alpha-helices and Beta-pleated sheets). 3. Tertiary: Overall 3D shape. 4. Quaternary: Multiple polypeptide subunits.

Figure 7.3: Levels of Protein Structure

7.4 Water

Water’s unique properties stem from its polarity and ability to form hydrogen bonds.

Cohesion and Adhesion: Water molecules stick to each other and to surfaces.
High Specific Heat: Helps organisms maintain stable internal temperatures.
Universal Solvent: Polar nature allows it to dissolve ionic and polar substances.

Figure 7.4: Partial charges (δ+ on H, δ- on O) allow water molecules to attract one another.

7.5 Enzymes

Enzymes are biological catalysts that speed up reactions by lowering the activation energy (\(E_a\)).

Kinetics and Inhibition

Michaelis-Menten Kinetics: Graphs reaction rate (\(V\)) against substrate concentration (\([S]\)).
Vmax: Maximum reaction velocity.
Km: Substrate concentration at half \(V_{max}\).
Competitive Inhibition: Inhibitor binds to the active site.
Non-competitive Inhibition: Inhibitor binds to an allosteric site, changing the enzyme’s shape.

Figure 7.5: Enzyme Kinetics Graph

7.6 Plasma Membrane

The plasma membrane is a selectively permeable barrier described by the Fluid Mosaic Model.

Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.
Cholesterol: Regulates membrane fluidity.
Proteins: Integral (transmembrane) and peripheral proteins assist in transport and signaling.
Carbohydrates: Attached to lipids (glycolipids) or proteins (glycoproteins) for cell recognition.

Figure 7.6: The mosaic of proteins and lipids allows the membrane to be dynamic and functional. ```