Chapter 4: Cells and Organelles – Structure and Function in Eukaryotic Cells

Chapter Objectives

Learning Goals

• Identify structures found in both prokaryotic and eukaryotic cells.

• Describe the endosymbiotic theory.

• Define the function of major prokaryotic and eukaryotic cell components.

• Outline the evolution of eukaryotic cells and multicellular organisms.

The Eukaryotic Cell in Overview: Structure and Function

General Features of Eukaryotic Cells

• Eukaryotic cells are structurally complex, as seen in typical animal and plant cells.

• Key components include a plasma membrane, nucleus, membrane-bounded organelles, and cytosol interlaced by a cytoskeleton.

• Plant and fungal cells possess a rigid cell wall surrounded by an extracellular matrix.

The Plasma Membrane

Defining Cell Boundaries and Retaining Contents

• The plasma membrane surrounds every cell, ensuring cellular contents are retained.

• Composed of lipids (notably phospholipids) and membrane proteins, organized into a bilayer.

Amphipathic Membrane Components

• Each phospholipid molecule has two hydrophobic "tails" and a hydrophilic "head", making it amphipathic.

• The lipid bilayer forms with hydrophilic heads facing outward and hydrophobic tails inward.

• Membrane proteins are also amphipathic; some are glycoproteins (proteins with attached polysaccharides).

Proteins in the Plasma Membrane

• Enzymes: Catalyze reactions associated with membranes (e.g., cell wall synthesis).

• Anchors: Serve as structural supports for the cytoskeleton.

• Transport proteins: Move substances across the membrane.

• Receptors: Detect external signals and trigger cellular processes; these and transport proteins are transmembrane proteins.

The Nucleus

Information Center of the Eukaryotic Cell

• The nucleus is the most prominent structure in eukaryotic cells.

• Contains DNA and is surrounded by the nuclear envelope (inner and outer membranes).

• The envelope has pores lined with a nuclear pore complex for transport.

Chromosomes and Nucleoli

• Chromosome number is species-specific.

• Chromosomes are visible during mitosis; during interphase, they are dispersed as chromatin.

• Nucleoli (singular: nucleolus) are present and involved in ribosome synthesis.

Mitochondria and Chloroplasts: Cellular Energy Production

Mitochondria

• Mitochondria are the site of aerobic respiration in all eukaryotic cells.

• Comparable in size to bacteria; hundreds per cell, each with inner and outer mitochondrial membranes.

Mitochondrial Structure

• The inner membrane encloses the matrix, a semifluid material.

• The matrix contains small circular DNA and ribosomes for protein synthesis.

Mitochondrial Function

• Contains enzymes and intermediates for oxidation of sugars and ATP generation.

• Many enzymes are located on cristae (membrane infoldings).

• Cells with high energy needs (e.g., sperm, muscle cells) have many mitochondria.

Chloroplasts

• Chloroplasts are the site of photosynthesis in plants and algae.

• Large and numerous in green plant cells; surrounded by inner and outer membranes.

• Contain thylakoids (flattened sacs) interconnected by stroma thylakoids and stacked into grana.

Chloroplast Function

• Photosynthesis uses solar energy and $CO_2$ to produce sugars and organic compounds.

• Solar energy-dependent reactions occur on thylakoid membranes.

• Reduction of $CO_2$ to sugar occurs in the stroma (semifluid interior).

• Chloroplasts have their own ribosomes and circular DNA.

Plastids

• Chloroplasts are one of several types of plastids in plant cells.

• Chromoplasts: Pigment-containing plastids for coloration of flowers/fruits.

• Amyloplasts: Specialized for starch storage.

Endosymbiotic Theory: Origin of Mitochondria and Chloroplasts

Semiautonomous Organelles

• Mitochondria and chloroplasts are semiautonomous and share similarities with bacteria.

• Both have double membranes; inner membrane contains bacterial-type lipids.

• Both possess circular DNA without histones, similar rRNA sequences, ribosome size, and protein synthesis factors to bacteria.

The Endosymbiont Theory

• Suggests mitochondria and chloroplasts originated from ancient bacteria.

• Ancient bacteria entered protoeukaryotes and formed a symbiotic relationship via phagocytosis without digestion.

Alteration of Earth's Atmosphere

• Early atmosphere lacked oxygen, limiting cells to anaerobic processes.

• Photosynthetic cells evolved, producing $O_2$ as a by-product, altering the atmosphere.

• Accumulation of $O_2$ enabled evolution of aerobic cells, which extract energy more efficiently from glucose.

Mitochondria and Chloroplasts Evolution

• Mitochondria likely evolved when anaerobic protoeukaryotes ingested aerobic bacteria, which provided energy and gradually lost independence.

• Chloroplasts evolved when eukaryotic cells (with mitochondria) ingested photosynthetic bacteria, leading to mutual benefit and loss of unnecessary functions.

Outside-In vs. Inside-Out Theories of Eukaryotic Origins

• Outside-In Theory: Nuclear envelope formed by invagination of plasma membrane; cells engulfed bacteria/cyanobacteria by phagocytosis.

• Inside-Out Theory: Archaeal cells formed protrusions to interact with and engulf bacteria, leading to mitochondria and primitive endoplasmic reticulum; nuclear membrane originated from plasma membrane.

• Three-Membrane Problem: Theories predict three membranes for mitochondria, but only two are observed.

The Endomembrane System

Protein Synthesis and Trafficking

• Synthesizes proteins for organelles, membranes, or secretion.

• Proteins are packaged and directed via small membrane-bound vesicles.

Endoplasmic Reticulum (ER)

• Network of membranes in the cytoplasm; consists of tubular membranes and cisternae (flattened sacs).

• Internal space is the lumen; ER is continuous with other cell membranes.

Rough ER

• Rough ER is studded with ribosomes on the cytoplasmic side.

• Ribosomes synthesize polypeptides that accumulate in the membrane or are transported to the lumen.

• Free ribosomes are not associated with the ER.

Smooth ER

• Smooth ER has no role in protein synthesis.

• Involved in synthesis of lipids and steroids (e.g., cholesterol).

• Responsible for inactivating and detoxifying harmful substances.

• Sarcoplasmic reticulum (a type of smooth ER) is critical for muscle contraction.

The Golgi Apparatus

• Consists of a stack of flattened vesicles (cisternae).

• Processes and packages secretory proteins; synthesizes complex polysaccharides.

• Accepts transition vesicles from the ER.

Golgi Complex as a Processing Station

• Modifies and processes vesicle contents from the ER.

• Secretory and membrane proteins are mainly glycosylated (addition of short-chain carbohydrates), starting in the ER and completed in the Golgi.

• Processed substances are transported to other cell locations via vesicles budding from the Golgi.

Summary Table: Key Eukaryotic Cell Structures and Functions

Structure	Main Function	Key Features
Plasma Membrane	Defines boundaries, retains contents	Lipid bilayer, membrane proteins, amphipathic nature
Nucleus	Information storage, DNA replication/transcription	Nuclear envelope, pores, chromatin, nucleolus
Mitochondria	Aerobic respiration, ATP production	Double membrane, matrix, cristae, circular DNA
Chloroplasts	Photosynthesis	Double membrane, thylakoids, stroma, circular DNA
Endoplasmic Reticulum	Protein/lipid synthesis, detoxification	Rough (ribosomes), smooth (lipid synthesis)
Golgi Apparatus	Processing, packaging, glycosylation	Cisternae, transition vesicles

Example: Endosymbiotic Theory in Modern Biology

• Endosymbiosis is observed today in protists and invertebrates hosting photosynthetic bacteria or algae as endosymbionts.

Additional info: This summary expands on the provided slides with definitions, examples, and a comparative table for clarity and completeness.