The Cell & Its Chemical Foundations

Overview of Cell Structure and Function

The cell is the fundamental unit of life, with its structure and chemical composition dictating its function. Understanding the chemistry of cells is essential for grasping how biological processes occur and how cells interact with their environment.

• Cell Theory: All living organisms are composed of cells, which arise from preexisting cells.

• Cell Diversity: Over 200 types of human cells exist, differing in size, shape, and function.

• Basic Cell Parts: Plasma membrane, cytoplasm, and nucleus.

Chemistry Review: Inorganic and Organic Molecules

Inorganic Compounds

Inorganic compounds, such as water, salts, and acids, are vital for cellular function. Water is the most abundant compound in cells, serving as a solvent and medium for chemical reactions.

• Water Molecule: Polar, with partial positive (δ+) and negative (δ−) charges, enabling hydrogen bonding.

• Ions in Solution: Salts dissociate into ions (e.g., Na+, Cl−) that are essential for electrical activity and osmotic balance.

Organic Compounds: Synthesis and Hydrolysis

Organic molecules contain carbon and are unique to living systems. They are synthesized and broken down by cells to provide energy and structural components.

• Major Organic Compounds: Carbohydrates, lipids, proteins, nucleic acids.

• Synthesis: Formation of larger molecules from smaller units (monomers).

• Hydrolysis: Breakdown of complex molecules into simpler units by addition of water.

Carbohydrates: Structure and Function

Monosaccharides, Disaccharides, and Polysaccharides

Carbohydrates are organic molecules that serve as energy sources and structural components. They are classified based on the number of sugar units.

• Monosaccharides: Simple sugars (e.g., glucose, fructose, galactose, ribose, deoxyribose).

• Disaccharides: Two monosaccharides linked together (e.g., sucrose, maltose, lactose).

• Polysaccharides: Long chains of monosaccharides (e.g., glycogen, starch).

Digestion and Absorption of Carbohydrates

Carbohydrates are digested in the small intestine, where enzymes break down polysaccharides into monosaccharides for absorption.

• Enzymatic Digestion: Polysaccharides and proteins are broken down by enzymes into monosaccharides and amino acids.

• Absorption: Monosaccharides and amino acids are absorbed into the bloodstream via active transport.

Lipids: Structure and Function

Phospholipids and Fats

Lipids are hydrophobic molecules that serve as energy storage, structural components of cell membranes, and signaling molecules.

• Phospholipids: Modified triglycerides with a polar head (hydrophilic) and nonpolar tails (hydrophobic). Essential for cell membrane structure.

• Saturated and Unsaturated Fats: Saturated fats have no double bonds, while unsaturated fats have one or more double bonds, causing bending in the molecule.

Proteins: Structure and Function

Levels of Protein Structure

Proteins are polymers of amino acids that perform a wide range of functions, including catalysis, structural support, and transport. Their function depends on their structure, which is organized into four levels.

• Primary Structure: Sequence of amino acids in a polypeptide chain.

• Secondary Structure: Formation of α-helices and β-sheets stabilized by hydrogen bonds.

• Tertiary Structure: Three-dimensional folding of the polypeptide.

• Quaternary Structure: Association of multiple polypeptide chains.

Nucleic Acids: DNA and RNA

Structure and Function

Nucleic acids store and transmit genetic information. DNA is the genetic material, while RNA is involved in protein synthesis.

• DNA: Double helix composed of nucleotides (adenine, thymine, cytosine, guanine) with a sugar-phosphate backbone.

• RNA: Single-stranded molecule involved in transcription and translation.

Cellular Organelles and Their Functions

Generalized Cell Structure

Cells contain various organelles, each with specialized functions necessary for cellular metabolism, growth, and division.

• Plasma Membrane: Flexible boundary that regulates entry and exit of substances.

• Cytoplasm: Contains cytosol, inclusions, and organelles.

• Nucleus: Control center containing DNA.

Mitochondria: The Powerhouse of the Cell

Mitochondria generate ATP through aerobic respiration and contain their own DNA and ribosomes.

• Structure: Double membrane with inner folds (cristae).

• Function: ATP production via oxidative phosphorylation.

Endoplasmic Reticulum (ER)

The ER is a network of membranous tubules involved in protein and lipid synthesis.

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion and membrane incorporation.

• Smooth ER: Lacks ribosomes; involved in lipid metabolism, detoxification, and calcium storage.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for transport within or outside the cell.

• Structure: Stacked, flattened membranous sacs.

• Function: "Traffic director" for cellular products.

Peroxisomes and Lysosomes

Peroxisomes detoxify harmful substances, while lysosomes digest cellular waste and foreign materials.

• Peroxisomes: Contain oxidase and catalase enzymes for detoxification.

• Lysosomes: Contain acid hydrolases for digestion of macromolecules.

Endomembrane System

The endomembrane system includes the ER, Golgi apparatus, lysosomes, vesicles, and membranes, working together to produce, modify, and transport biological molecules.

• Function: Coordination of synthesis, storage, and export of cellular products.

Cytoskeleton and Cellular Extensions

Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, facilitates movement, and organizes cell contents.

• Microfilaments: Actin filaments involved in cell movement and shape changes.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Tubulin filaments involved in intracellular transport and cell division.

Centrosome and Centrioles

The centrosome is the microtubule organizing center, containing centrioles that are essential for cell division and formation of cilia and flagella.

• Centrioles: Barrel-shaped organelles that form the basis of cilia and flagella.

Cilia and Flagella

Cilia and flagella are cellular extensions made of microtubules, responsible for movement of cells or substances across cell surfaces.

• Cilia: Short, numerous extensions that move substances (e.g., mucus) across cell surfaces.

• Flagella: Longer extensions that propel cells (e.g., sperm).

Nucleus: Genetic Control Center

Structure of the Nucleus

The nucleus is the largest organelle, containing the genetic material and controlling cellular activities.

• Nuclear Envelope: Double membrane with nuclear pores for transport.

• Nucleoli: Sites of ribosomal RNA synthesis and ribosome assembly.

• Chromatin: DNA-protein complex organized into nucleosomes; condenses into chromosomes during cell division.

Chromatin and Chromosomes

Chromatin is composed of DNA, histone proteins, and RNA. During cell division, chromatin condenses into chromosomes to protect genetic material.

• Nucleosomes: Fundamental units of chromatin, consisting of DNA wrapped around histones.

• Chromosomes: Condensed chromatin for cell division.

Summary Table: Major Cell Organelles and Their Functions