Inside the Cell: Structure and Function of Cells (BIOL 1115, Ch. 7)

Types of Cells

Prokaryotic Cells

Prokaryotic cells are among the simplest forms of life and are characterized by the absence of a nucleus and membrane-bound organelles. They are found in the domains Bacteria and Archaea.

• Genetic Material: DNA is located in a region called the nucleoid, not enclosed by a membrane.

• Plasmids: Small, circular DNA molecules that carry extra genetic information.

• Cell Wall: Most have a cell wall containing peptidoglycan (in bacteria), providing structural support.

• Ribosomes: Present for protein synthesis, but smaller than those in eukaryotes.

• Organelles: Generally lack membrane-bound organelles, but may have simple structures for specific functions (e.g., magnetite crystals for orientation).

Example: Escherichia coli is a common prokaryotic bacterium found in the human gut.

Eukaryotic Cells

Eukaryotic cells are more complex and are found in the domain Eukarya, which includes plants, animals, fungi, and protists. They possess a true nucleus and various membrane-bound organelles.

• Nucleus: Contains the cell's genetic material, surrounded by a nuclear envelope.

• Organelles: Includes mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and others.

• Cell Size: Generally larger than prokaryotic cells.

• Compartmentalization: Organelles create specialized environments for different cellular processes.

• Cell Wall: Present in plants and fungi, but not in animal cells. Plant cell walls contain cellulose.

Example: Homo sapiens (human) cells are eukaryotic.

Unifying Features of Cells

Basic Structural and Functional Units

All cells share certain fundamental features, regardless of their type.

• Plasma Membrane: A semi-fluid boundary that separates the cell from its environment and regulates the movement of substances.

• Cytoplasm: The jelly-like substance inside the cell, containing organelles and molecules.

• Genetic Material: DNA carries hereditary information.

• Ribosomes: Sites of protein synthesis.

Additional info: The plasma membrane is composed primarily of a phospholipid bilayer with embedded proteins.

Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Cell Size	Smaller (1-10 μm)	Larger (10-100 μm)
DNA Structure	Circular	Linear, within nucleus
Cell Wall	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi), absent in animals

Domains of Life

Bacteria, Archaea, and Eukarya

Life is classified into three domains based on genetic and structural differences.

• Bacteria: Single-celled prokaryotes with peptidoglycan cell walls.

• Archaea: Single-celled prokaryotes with unique membrane lipids and cell walls, often found in extreme environments.

• Eukarya: Includes all eukaryotic organisms (plants, animals, fungi, protists).

Additional info: Archaea are more closely related to eukaryotes than to bacteria at the molecular level.

Cell Surface and Membranes

Cell Wall Structure

The cell wall provides structural support and protection. In bacteria, the cell wall contains peptidoglycan, while in plants, it contains cellulose.

• Gram-positive Bacteria: Thick peptidoglycan layer.

• Gram-negative Bacteria: Thin peptidoglycan layer and an outer membrane.

• Plant Cell Wall: Composed mainly of cellulose, a polysaccharide.

Organelles of Eukaryotic Cells

Endomembrane System

The endomembrane system is a network of membranes within eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Nuclear Envelope: Double membrane surrounding the nucleus.

• Endoplasmic Reticulum (ER): Network of tubules and sacs; divided into rough (RER) and smooth (SER) types.

• Golgi Apparatus: Stack of flattened sacs that modify, sort, and package proteins and lipids.

• Vesicles: Small membrane-bound sacs for transport.

• Lysosomes: Contain enzymes for digestion and recycling.

• Plasma Membrane: Boundary of the cell, involved in transport and communication.

Nucleus

The nucleus is the control center of the cell, containing most of the genetic material.

• Nuclear Envelope: Double membrane with pores for transport.

• Nucleolus: Region where ribosomal RNA is synthesized and ribosome subunits are assembled.

• Chromatin: DNA packaged with proteins.

Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; site of protein synthesis for proteins destined for membranes or secretion.

• Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Protein Synthesis in RER: Proteins with a signal sequence are directed to the RER, where they are synthesized and processed.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids received from the ER.

• Cisternae: Flattened membrane sacs.

• Vesicular Transport: Proteins and lipids are transported in vesicles between the ER, Golgi, and other destinations.

Lysosomes

Lysosomes are specialized vesicles containing hydrolytic enzymes for breaking down macromolecules.

• Function: Digestion and recycling of cellular waste.

• Formation: Bud from the Golgi apparatus.

Mitochondria and Chloroplasts

Endosymbiosis Theory

Mitochondria and chloroplasts are believed to have originated from free-living prokaryotes through endosymbiosis.

• Mitochondria: Site of cellular respiration and ATP production; found in almost all eukaryotic cells.

• Chloroplasts: Site of photosynthesis; found in plants and algae.

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

Cytoskeleton of Eukaryotic Cells

Components of the Cytoskeleton

The cytoskeleton provides structural support, enables movement, and organizes cellular contents.

• Microtubules: Hollow tubes made of tubulin; involved in cell shape, organelle movement, and chromosome separation.

• Microfilaments: Solid rods made of actin; involved in cell shape, movement, and muscle contraction.

• Intermediate Filaments: Cable-like structures; provide mechanical strength.

Microtubules

• Structure: Composed of α- and β-tubulin dimers.

• Function: Serve as tracks for organelle transport, form the mitotic spindle, and are components of cilia and flagella.

• Assembly: Dynamic, with plus and minus ends; originate from the microtubule organizing center (MTOC), such as the centrosome.

Microfilaments

• Structure: Twisted double chain of actin subunits.

• Function: Maintain cell shape, enable cell movement (e.g., amoeboid movement), and serve as tracks for myosin motor proteins.

Intermediate Filaments

• Structure: Intermediate in size; made of various proteins (e.g., keratin).

• Function: Provide mechanical strength and support for the nucleus and other organelles.

Cilia and Flagella

Cilia and flagella are appendages that enable cell movement. Both have a "9+2" arrangement of microtubules.

• Cilia: Short, numerous, move fluid over cell surfaces.

• Flagella: Longer, usually singular, propel cells through liquid.

• Movement: Generated by dynein motor proteins causing microtubules to slide against each other, resulting in bending.

Motor Proteins and Cellular Motility

Kinesin, Dynein, and Myosin

Motor proteins move cellular cargo along cytoskeletal tracks using energy from ATP.

• Kinesin: Moves toward the plus end of microtubules (anterograde transport).

• Dynein: Moves toward the minus end of microtubules (retrograde transport); also powers cilia and flagella movement.

• Myosin: Moves along actin filaments; involved in muscle contraction and other forms of cell motility.

Summary Table: Cytoskeletal Elements

Element	Structure	Protein Subunit	Main Functions
Microtubules	Hollow tubes (25 nm)	α- and β-tubulin	Cell shape, organelle movement, cilia/flagella, mitosis
Microfilaments	Solid rods (7 nm)	Actin	Cell shape, movement, muscle contraction
Intermediate Filaments	Cable-like (10 nm)	Various (e.g., keratin)	Mechanical strength, nuclear support

Key Equations

• Surface Area to Volume Ratio: As cell size increases, the surface area to volume ratio decreases, affecting efficiency of transport and metabolism. $\text{Surface Area of a Cube} = 6a^2$ $\text{Volume of a Cube} = a^3$ $\text{Surface Area to Volume Ratio} = \frac{6a^2}{a^3} = \frac{6}{a}$

Additional info:

• Plant cells contain unique organelles such as chloroplasts and a large central vacuole.

• Vesicular transport includes endocytosis (bringing material into the cell) and exocytosis (releasing material outside the cell).

• Macromolecular recycling in cells occurs via lysosomes, proteasomes, and autophagy.