Functional Morphology of the Cell

Learning Objectives

• Identify major cell organelles

• List the major functions of organelles

• Predict potential changes in organelle/cell structure

• Explain dependence of overall cellular function on organelles/cell structure

• Relate cell structure to everyday contexts

• By the end of this lecture, students will be able to achieve these objectives.

Major Features of Eukaryotic Cells

• Centrosome: Serves as the main microtubule-organizing center (MTOC).

• Cytoskeleton: Provides structural support and facilitates movement.

• Peroxisome: Involved in lipid metabolism and chemical detoxification.

• Chromatin (DNA): Packages DNA into a small volume to fit into the nucleus and protect the structure sequence.

• Extracellular matrix: Provides structural and biochemical support to surrounding cells.

• Nuclear pore and nuclear envelope: Regulates transport of molecules between the nucleus and cytoplasm.

• Vesicles: Small membrane-bound sacs that transport and store substances.

• Ribosomes: Site of biological protein synthesis (translation).

• Cytosol: The fluid portion of the cytoplasm where many metabolic reactions occur.

• Lysosome: Contains digestive enzymes to break down waste materials.

• Mitochondrion: The "powerhouse" of the cell, generating ATP.

• Nucleolus: Site of ribosomal RNA ($rRNA$) synthesis and ribosome assembly.

• Dimensions, e.g., $5\ \mu m$ (example scale)

• Endoplasmic Reticulum (ER): Major site for protein and lipid synthesis.

Structure of the Nucleus

Overview

• Outer Membrane

• Inner Membrane

• Perinuclear Space

• Cytosol Relationship: Smooth ER is adjacent to the nuclear envelope.

Key Components

• Nuclear Pore Complex: Facilitates the exchange of components between the nucleus and cytoplasm.

• Nuclear Lamina: Structural framework within the nuclear envelope.

• Chromatin: Stores genetic material; composed of DNA.

Details on Nuclear Structure

• Outer Membrane and Inner Membrane have distinct components and roles.

• The nuclear envelope separates the nucleus from the cytoplasm, maintaining a controlled environment.

Function of Nuclear Envelope

• Provides structural framework to the nucleus.

• Acts as a barrier: Keeps the genome separate from the cytoplasm.

• Regulates passage of molecules via nuclear pores: Controls the entry and exit of proteins and $RNA$.

• Ribosomes can be found associated with the outer membrane.

• Perinuclear space is connected with the lumen of the ER.

• Inner membrane proteins are specific to nuclear functions.

Nuclear Lamina

Structure

• Lamin Proteins: Form coiled-coil dimers that assemble into polymers.

• Polymers align to form filaments, providing structural integrity to the nucleus.

Function

• Provides support to the structure of the nucleus.

• Mammalian cells have four types of lamins: A, B1, B2, and C.

• Interact with inner nuclear membrane proteins (e.g., Emerin, LBR) and chromatin components ($H2A$, $H2B$) to organize DNA and maintain nuclear shape.

When Nuclear Structure Fails: Laminopathies

• Hutchinson–Gilford Progeria Syndrome (HGPS):

  • Caused by defective splicing of the $LMNA$ exon 11, leading to the production of progerin.

  • Progerin disrupts lamin filament assembly and nuclear mechanics, resulting in premature aging symptoms.

Ribosome Structure and Function

• Structure: Composed of large and small subunits made of ribosomal $RNA\ (rRNA)$ and proteins.

• Function: The cell’s machinery for protein synthesis (translation).

  • Essential across all three domains: Bacteria, Archaea, Eukarya.

  • Requires messenger $RNA\ (mRNA)$ and transfer $RNA\ (tRNA)$.

  • Can be found free in the cytoplasm or attached to organelle membranes (e.g., RER).

Endoplasmic Reticulum (ER)

Rough Endoplasmic Reticulum (RER)

• Structure: Studded with ribosomes.

• Function: Site of protein synthesis for secretion, lysosomes, or the plasma membrane.

Smooth Endoplasmic Reticulum (SER)

• Structure: No ribosomes attached.

• Function:

  • Synthesis of lipids (including phospholipids and steroids).

  • Storage of calcium ions (critical for cell signaling).

  • Detoxification of drugs and poisons.

Mitochondrial Structure and Function

• Structure: Inner/outer membranes, intermembrane space, matrix, and cristae. Contains its own DNA.

• Function:

  • ATP production via oxidative phosphorylation.

  • Apoptosis regulation: Plays a key role in programmed cell death.

  • Metabolic pathways: Site of the $TCA$ Cycle and fatty acid oxidation.

  • ROS Generation: Produces reactive oxygen species for signaling.

  • Cardiolipin: Inner membrane phospholipid that stabilizes ATP production complexes.

Golgi Apparatus

• Structure: Composed of stacked membranous sacs with cis (receiving) and trans (shipping) faces.

• Function:

  • Packaging and distribution of molecules.

  • Processing station: Modifies proteins and lipids from the ER (e.g., glycosylation).

  • Vesicle sorting: Prepares cargo for transport to final destinations.

Lysosome and Peroxisome Functions

• Lysosome: Digestion of waste, old organelles, and pathogens via hydrolytic enzymes and phagocytosis.

• Peroxisome:

  • Performs $\beta$-oxidation of fatty acids.

  • Detoxification: Uses enzymes like catalase to break down hydrogen peroxide ($H<em>{2}O</em>{2}$).

  • Relies on protein import via $PTS1/PTS2$ signals.

Cytoskeleton & Centrosomes

• Microtubules/Actin/Intermediate Filaments: Maintain shape, facilitate crawling movement, and compose flagella/cilia ($9+2$ arrangement).

• Centrosomes (MTOC):

  • Nucleates microtubules for the cytoskeleton.

  • Forms the mitotic spindle for proper chromosome segregation during division.

  • Establishes cell polarity and guides intracellular transport.