Cell Structure and Function: Foundations of Life

Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. Understanding cell structure and function is essential for studying biology, as it provides insight into how life operates at the molecular and cellular levels.

• Definition: A cell is the smallest unit of life that can carry out all life processes.

• Key Features: Cells are enclosed by a membrane, contain genetic material (DNA), and have internal structures that perform specific functions.

• Types of Cells: There are two main types: prokaryotic and eukaryotic cells.

• Example: Bacteria are prokaryotic cells, while plant and animal cells are eukaryotic.

Abiogenesis and the Origin of Cells

Abiogenesis: How Life Began

Abiogenesis refers to the process by which life arose naturally from non-living matter on early Earth. This concept is central to understanding the origin of cells.

• Monomers: Simple organic molecules (e.g., amino acids, nucleotides) formed the 'alphabet of life.'

• Polymers and Biomolecules: Monomers joined to form polymers (proteins, nucleic acids, carbohydrates, lipids).

• Membrane Enclosure: Biomolecules became enclosed by membranes, forming protocells.

• Catalysis: Chemical reactions, often catalyzed by metals or minerals, facilitated the formation of complex molecules.

• RNA World Hypothesis: Suggests that self-replicating RNA molecules were among the first catalysts and genetic materials.

• Lipid World Hypothesis: Proposes that lipid structures played a key role in early cell formation.

• Experimental Evidence: Directed evolution experiments show that RNA molecules can evolve catalytic activity.

• Example: Miller-Urey experiment demonstrated that amino acids could form under prebiotic conditions.

Cell Theory

Principles of Cell Theory

Cell theory is a foundational concept in biology, describing the properties and significance of cells.

• All living organisms are composed of one or more cells.

• The cell is the basic unit of structure and function in living things.

• All cells arise from the division of pre-existing cells.

• Genetic information and heredity occur within cells.

• Surface Area to Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting cell size for efficient exchange of materials.

• Example: Robert Hooke first described cells in 1665 using a microscope.

Microscopy and Cell Visualization

Techniques for Observing Cells

Microscopes and stains are essential tools for studying cell structure and function.

• Microscopes: Increase magnification and resolution to view small cellular structures.

• Stains: Bind to specific cell components (e.g., DNA, proteins) to enhance contrast and visualization.

• Example: Fluorescent stains can highlight DNA or specific organelles in cells.

Basic Components of Cells

Major Biomolecules

Cells are composed of four major classes of biomolecules, each with distinct functions.

• Proteins: Serve as enzymes, structural components, and signaling molecules.

• Nucleic Acids: DNA and RNA store and transmit genetic information.

• Carbohydrates: Provide energy and structural support.

• Lipids: Form membranes and store energy.

• Example: Phospholipids are key components of cell membranes.

Prokaryotic vs. Eukaryotic Cells

Comparison of Cell Types

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

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No (DNA in nucleoid)	Yes (membrane-bound)
Organelles	No membrane-bound organelles	Many membrane-bound organelles
Cell Wall	Present (peptidoglycan in bacteria)	Present in plants, fungi (cellulose/chitin)
Size	Smaller (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

• Example: Escherichia coli is a prokaryote; human skin cells are eukaryotes.

Prokaryotic Cell Structure

Key Features of Prokaryotes

Prokaryotic cells have a simple structure but are highly efficient and diverse.

• Nucleoid: Region containing circular DNA.

• Cell Wall: Provides shape and protection; contains peptidoglycan in bacteria.

• Plasma Membrane: Controls entry and exit of substances.

• Ribosomes: Sites of protein synthesis (smaller than eukaryotic ribosomes).

• Additional info: Some prokaryotes have flagella for movement and pili for attachment.

Eukaryotic Cell Structure

Key Organelles and Their Functions

Eukaryotic cells contain a variety of membrane-bound organelles, each with specialized functions.

• Nucleus: Contains DNA, surrounded by a double membrane (nuclear envelope) with pores.

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Studded with ribosomes; synthesizes and modifies proteins.

  • Smooth ER (SER): Lacks ribosomes; synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.

• Lysosomes: Contain hydrolytic enzymes for digestion and waste removal (mainly in animal cells).

• Vacuoles: Store water, nutrients, and waste; large central vacuole in plant cells maintains turgor pressure.

• Mitochondria: Produce ATP through cellular respiration; contain their own DNA.

• Chloroplasts: Site of photosynthesis in plants and algae; contain their own DNA.

• Plasma Membrane: Phospholipid bilayer that regulates transport and communication.

• Cytoskeleton: Network of protein filaments (microtubules, intermediate filaments, actin filaments) that maintain cell shape, enable movement, and organize organelles.

• Cell Wall: Present in plants, fungi, and some protists; provides structural support.

• Example: Animal cells have lysosomes; plant cells have chloroplasts and a large central vacuole.

Endomembrane System

Components and Functions

The endomembrane system is a group of interconnected organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Includes: Nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Protein Synthesis and Transport: Proteins made in the RER are sent to the Golgi for modification and sorting, then transported to their destinations via vesicles.

• Lipid Synthesis: SER synthesizes lipids and carbohydrates.

• Degradation: Lysosomes break down macromolecules and recycle cellular components.

• Example: Insulin is synthesized in the RER, processed in the Golgi, and secreted by vesicles.

Endosymbiont Theory

Origin of Mitochondria and Chloroplasts

The endosymbiont theory explains how mitochondria and chloroplasts originated from free-living bacteria that were engulfed by ancestral eukaryotic cells.

• Evidence: Both organelles have double membranes, their own circular DNA, and ribosomes similar to bacteria.

• Genetic Similarity: Genes in mitochondria and chloroplasts are closely related to those in certain bacteria (proteobacteria and cyanobacteria, respectively).

• Function: Mitochondria perform cellular respiration; chloroplasts carry out photosynthesis.

• Example: Mitochondria are present in almost all eukaryotic cells; chloroplasts are found in plants and algae.

Cytoskeleton

Structure and Function

The cytoskeleton is a dynamic network of protein filaments that provides structural support, enables movement, and organizes cellular components.

• Microtubules: Hollow tubes made of tubulin; form the mitotic spindle, maintain cell shape, and serve as tracks for organelle movement.

• Actin Filaments (Microfilaments): Thin filaments involved in cell movement, muscle contraction, and cytokinesis.

• Intermediate Filaments: Provide mechanical strength and anchor organelles; more stable than microtubules and actin filaments.

• Prokaryotic Cytoskeleton: Prokaryotes also possess cytoskeletal proteins (e.g., FtsZ, MreB) that help maintain cell shape and division.

• Example: Actin filaments enable white blood cells to move toward infection sites.

Summary Table: Eukaryotic Cell Organelles and Functions

Organelle	Main Function	Present In
Nucleus	Stores genetic material (DNA)	All eukaryotes
Rough ER	Protein synthesis and modification	All eukaryotes
Smooth ER	Lipid synthesis, detoxification	All eukaryotes
Golgi Apparatus	Protein and lipid modification, sorting, packaging	All eukaryotes
Lysosome	Digestion and waste removal	Animals, some protists
Vacuole	Storage, turgor pressure	Plants, fungi, some protists
Mitochondrion	ATP production (cellular respiration)	All eukaryotes
Chloroplast	Photosynthesis	Plants, algae
Plasma Membrane	Selective barrier, communication	All cells
Cell Wall	Structural support	Plants, fungi, some protists, bacteria