General Biology: Cell Structure, Membrane Transport, Biochemistry, and Enzyme Function

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Cell Structure and Function

Cell Organelles and Their Functions

Cells contain specialized structures called organelles that perform distinct functions necessary for cellular life. Understanding the size and function of each organelle is fundamental in cell biology.

Nucleus: Contains genetic material (DNA) and controls cellular activities.
Mitochondria: Site of ATP (energy) production through cellular respiration.
Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.
Lysosomes: Contain digestive enzymes to break down waste materials and cellular debris.
Ribosomes: Sites of protein synthesis.
Plasma Membrane: Regulates entry and exit of substances.

Best unit: The 'best unit' for measuring organelles is typically the micrometer (μm).

Cell Parts and Their Functions

Cell membrane: Selectively permeable barrier.
Cytoplasm: Jelly-like fluid where organelles are suspended.
Cytoskeleton: Provides structural support and facilitates movement.

Cell Membrane Structure and Transport

Phospholipid Membrane Structure

The phospholipid bilayer forms the fundamental structure of the cell membrane. Each phospholipid molecule has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.

Hydrophilic heads: Face outward toward water inside and outside the cell.
Hydrophobic tails: Face inward, away from water.

Proteins, cholesterol, and carbohydrates are embedded within or attached to the bilayer, contributing to membrane function and fluidity.

Types of Membrane Transport

Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, osmosis, facilitated diffusion).
Active Transport: Movement of substances against their concentration gradient, requiring energy (ATP).

Cell Environments: Hypertonic, Hypotonic, and Isotonic

Cells can be exposed to different types of solutions:

Hypertonic: Higher solute concentration outside the cell; water moves out, cell shrinks.
Hypotonic: Lower solute concentration outside; water moves in, cell swells.
Isotonic: Equal solute concentration; no net water movement.

Membrane changes: In hypertonic solutions, cells lose water and may crenate; in hypotonic, they may lyse; in isotonic, they remain stable.

Biochemistry: Acids, Bases, Buffers, and Functional Groups

Strong and Weak Acids

Acids are substances that donate protons (H+) in solution. Strong acids dissociate completely, while weak acids only partially dissociate.

Example of strong acid: Hydrochloric acid (HCl)
Example of weak acid: Acetic acid (CH3COOH)

Buffers and Their Importance

Buffers are solutions that resist changes in pH when acids or bases are added. They are crucial for maintaining stable pH in living organisms.

Example: The bicarbonate buffer system in blood helps maintain pH around 7.4.

Conjugate Acid-Base Pairs

A conjugate acid-base pair consists of two species that differ by one proton (H+).

Example: Acetic acid (CH3COOH) and acetate ion (CH3COO-).

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical properties of those molecules.

Hydroxyl (-OH): Found in alcohols; polar.
Carboxyl (-COOH): Found in acids; acidic properties.
Amino (-NH2): Found in amino acids; basic properties.
Phosphate (-PO42-): Found in nucleic acids; energy transfer.
Sulfhydryl (-SH): Found in some amino acids; forms disulfide bonds.

These groups influence molecular reactivity, solubility, and interactions.

Macromolecules: Structure and Function

Types of Macromolecules

There are four major types of biological macromolecules:

Carbohydrates: Energy storage and structural support. Example: Glucose (C6H12O6), starch, cellulose.
Lipids: Long-term energy storage, insulation, and membrane structure. Example: Triglycerides, phospholipids.
Proteins: Catalysis (enzymes), structure, transport, signaling. Example: Hemoglobin, enzymes.
Nucleic Acids: Store and transmit genetic information. Example: DNA, RNA.

Structural Formulas and Isomers

Macromolecules can have different structural forms (isomers) with the same molecular formula but different arrangements.

Isomer: Molecules with the same chemical formula but different structures. Example: Glucose and fructose (both C6H12O6).

Carbohydrate Structure: Alpha and Beta Forms

Carbohydrates can exist in alpha and beta anomeric forms, especially in cyclic structures like glucose. These forms differ in the orientation of the hydroxyl group on the anomeric carbon.

Biochemical Reactions and Enzyme Function

Types of Biochemical Reactions

Catabolic reactions: Break down molecules and release energy.
Anabolic reactions: Build complex molecules from simpler ones, requiring energy.
Exchange reactions: Involve both synthesis and decomposition.

ΔG and Reaction Progress

ΔG (Gibbs free energy change) indicates whether a reaction is spontaneous.

If ΔG < 0, the reaction is spontaneous (exergonic).
If ΔG > 0, the reaction is non-spontaneous (endergonic).

ΔG is used to predict the direction and extent of chemical reactions.

Enzyme Function and Regulation

Enzymes are biological catalysts that lower the activation energy (Ea) of reactions, increasing reaction rates.

Competitive inhibition: Inhibitor competes with substrate for the active site.
Non-competitive inhibition: Inhibitor binds elsewhere, changing enzyme shape.
Allosteric regulation: Effector molecules bind to sites other than the active site, modulating activity.
Feedback inhibition: End product of a pathway inhibits an earlier step.

Enzyme Kinetics Equation

The rate of enzyme-catalyzed reactions can be described by the Michaelis-Menten equation:

Where v is the reaction rate, Vmax is the maximum rate, [S] is substrate concentration, and Km is the Michaelis constant.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2 across membrane
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump