Cell Structure and Function: A Tour of the Cell

Cell Theory and the Origin of Cells

Major Lines of Evidence for Common Origin

All presently living cells share several fundamental characteristics, suggesting a common evolutionary origin.

• Universal Biochemical Similarities: All cells use DNA as genetic material, the same genetic code, and similar metabolic pathways.

• Cellular Structure: All cells are surrounded by a plasma membrane and contain ribosomes for protein synthesis.

• Reproduction: All cells arise from pre-existing cells through cell division.

• Conservation of Genes: Many genes are highly conserved across all domains of life.

• Energy Conversion: All cells use similar energy conversion reactions (e.g., glycolysis, ATP synthesis).

Surface Area to Volume Ratio

The surface area to volume ratio (SA:V) is a key factor limiting cell size and influencing cell function.

• Definition: The ratio of a cell's surface area (through which materials are exchanged) to its volume (which determines metabolic demand).

• Importance: As cells grow, volume increases faster than surface area, making it harder for the cell to efficiently exchange materials with its environment.

• Implication: Limits the maximum size of cells; larger cells may develop adaptations (e.g., microvilli) to increase surface area.

Effect of Cell Growth on SA:V Ratio

• As a cell grows larger, its volume increases more rapidly than its surface area, causing the SA:V ratio to decrease.

• This decrease limits the rate of diffusion and transport of materials, constraining cell size.

Microscopy and Cell Visualization

Light Microscopy (LM) vs. Electron Microscopy (EM)

Microscopes are essential tools for studying cells, with different types offering varying levels of resolution and magnification.

• Light Microscopy (LM):

  • Uses visible light to illuminate specimens.

  • Magnification up to ~1000x; resolution limited by the wavelength of light (~0.2 μm).

  • Suitable for viewing live cells and general cell structure.

• Electron Microscopy (EM):

  • Uses beams of electrons for much higher resolution (up to ~2 nm).

  • Two main types:

    • Transmission Electron Microscopy (TEM): Electrons pass through thin sections of specimens, revealing internal structures in high detail.

    • Scanning Electron Microscopy (SEM): Electrons scan the surface, producing detailed 3D images of cell surfaces.

  • Cannot be used for live cells; requires extensive sample preparation.

• Resolution: The ability to distinguish two points as separate; EM has much higher resolution than LM.

• Magnification: The degree to which the image size is increased; both LM and EM can magnify, but EM provides much greater detail.

Table: Comparison of LM, TEM, and SEM

Microscope	Source of Illumination	Resolution	Magnification	Best For
LM	Visible light	~0.2 μm	Up to 1000x	Live cells, general structure
TEM	Electrons	~2 nm	Up to 500,000x	Internal ultrastructure
SEM	Electrons	~10 nm	Up to 100,000x	Surface topology (3D images)

Cell Fractionation

Process and Purpose

Cell fractionation is a laboratory technique used to separate cellular components for study.

• Purpose: To isolate and analyze specific organelles or macromolecules.

• Steps:

  1. Lyse the cells (break open) using mechanical disruption or detergents.

  2. Homogenize the mixture to ensure even distribution of components.

  3. Centrifugation: Spin the homogenate at various speeds to separate components by size and density.

  4. Pellet: The heavier components that collect at the bottom of the tube after centrifugation.

  5. Supernatant: The liquid above the pellet, containing lighter components.

Repeated centrifugation at increasing speeds allows for sequential isolation of nuclei, mitochondria, membranes, and ribosomes.

Prokaryotic vs. Eukaryotic Cells

Size and Organization

• Prokaryotic Cells:

  • Smaller (typically 1–10 μm in diameter).

  • Lack membrane-bound organelles and a true nucleus.

  • DNA is located in a region called the nucleoid.

• Eukaryotic Cells:

  • Larger (typically 10–100 μm in diameter).

  • Contain membrane-bound organelles (e.g., nucleus, mitochondria, ER).

  • DNA is enclosed within a nuclear envelope.

Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Size	1–10 μm	10–100 μm
Nucleus	Absent	Present
Organelles	Absent	Present
DNA Location	Nucleoid	Nucleus
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Cellular Components and Organelles

Cytoplasm, Cytosol, Nucleoplasm, and Membranes

• Cytoplasm: The entire region between the plasma membrane and the nuclear envelope, containing organelles and cytosol.

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

• Nucleoplasm: The semi-fluid matrix inside the nucleus, containing chromatin and the nucleolus.

• Membranes: Define cell boundaries, compartmentalize functions, and regulate transport.

Major Organelles: Structure and Function

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

• Ribosomes: Sites of protein synthesis; composed of rRNA and proteins; found free in cytosol or bound to ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies chemicals, stores calcium ions.

• Golgi Apparatus: Stack of flattened membranes; modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules and worn-out organelles.

• Vacuoles: Large vesicles for storage and transport; prominent in plant cells (central vacuole).

• Peroxisomes: Contain enzymes for oxidation reactions, including breakdown of fatty acids and detoxification.

• Glyoxysomes: Specialized peroxisomes in plants; convert fats to sugars during seed germination.

• Mitochondria: Sites of aerobic respiration; double-membraned; generate ATP.

• Chloroplasts: Sites of photosynthesis in plants and algae; contain chlorophyll; double-membraned.

Animal vs. Plant vs. Fungal vs. Prokaryotic Cells

• Animal Cells: Lack cell walls, have centrioles, contain lysosomes.

• Plant Cells: Have cell walls, chloroplasts, central vacuole, and plasmodesmata.

• Fungal Cells: Have cell walls (chitin), vacuoles, and lack chloroplasts.

• Prokaryotic Cells: Lack membrane-bound organelles, have cell wall (peptidoglycan in bacteria).

Nucleus and Genetic Material

Nuclear Envelope, Pores, Chromatin, Chromosomes, Nucleolus

• Nuclear Envelope: Double membrane surrounding the nucleus; contains nuclear pores for regulated exchange.

• Nuclear Pores: Protein complexes that allow selective transport of molecules between nucleus and cytoplasm.

• Chromatin: DNA-protein complex; exists as diffuse fibers during interphase.

• Chromosomes: Condensed chromatin; visible during cell division; carry genetic information.

• Nucleolus: Dense region within the nucleus; site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Endomembrane System and Protein Trafficking

Pathways for Secreted and Plasma Membrane Proteins

• Proteins destined for secretion or the plasma membrane are synthesized on ribosomes bound to the rough ER.

• They are transported in vesicles to the Golgi apparatus for modification and sorting.

• Secreted proteins are packaged into vesicles that fuse with the plasma membrane, releasing contents outside the cell.

• Plasma membrane proteins are inserted into the membrane during vesicle fusion.

Cisternal Maturation Model of the Golgi

• Golgi cisternae progress from the cis to the trans face, carrying and modifying cargo as they mature.

• Enzymes and resident proteins are recycled backward via vesicles.

Other Organelles and Structures

Structure and Function

• Vesicles: Small membrane-bound sacs for transport within cells.

• Microbodies: General term for small, enzyme-containing organelles (e.g., peroxisomes, glyoxysomes).

• Lysosomes: Digestive organelles; degrade macromolecules and damaged organelles.

• Peroxisomes: Break down fatty acids, detoxify harmful substances, produce hydrogen peroxide.

• Glyoxysomes: Convert stored fats to sugars in plant seeds.

Mitochondria and Chloroplasts

Mitochondrion Structure and Function

• Double-membraned; outer membrane and highly folded inner membrane (cristae).

• Matrix: Internal space containing enzymes for the citric acid cycle.

• Site of aerobic respiration: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy}$

• Contains its own DNA and ribosomes; replicates independently.

Chloroplast Structure and Function

• Double-membraned; contains thylakoids (stacked into grana) and stroma.

• Site of photosynthesis: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Contains its own DNA and ribosomes.

Endosymbiont Theory

• Mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence:

  • Double membranes

  • Own circular DNA and ribosomes

  • Similar size to bacteria

  • Reproduce independently by binary fission

Cytoskeleton and Cell Movement

Functions of the Cytoskeleton

• Maintains cell shape

• Facilitates cell movement

• Anchors organelles

• Involved in intracellular transport and cell division

Main Types of Cytoskeletal Elements

• Microfilaments (Actin Filaments): Thin, flexible fibers; involved in cell movement and muscle contraction.

• Intermediate Filaments: Rope-like fibers; provide mechanical strength.

• Microtubules: Hollow tubes; maintain cell shape, form spindle fibers, serve as tracks for motor proteins.

Motor Proteins

• Proteins (e.g., kinesin, dynein, myosin) that move along cytoskeletal filaments, transporting vesicles and organelles.

Extracellular Structures and Cell Interactions

Animal Cell Glycocalyx and ECM Interaction

• Glycocalyx: Carbohydrate-rich layer on the cell surface; involved in protection, adhesion, and cell recognition.

• Extracellular Matrix (ECM): Network of proteins and polysaccharides outside animal cells; provides structural support.

• Key Components:

  • Collagen: Main structural protein in ECM.

  • Fibronectin: Glycoprotein that connects cells to the ECM.

  • Integrin: Transmembrane protein that links ECM to the cytoskeleton.

Example: Integrins bind fibronectin, which in turn binds collagen, anchoring the cell to the ECM and facilitating signal transduction.

Additional info: Some diagrams and drawings referenced in the original notes are not included here, but students are encouraged to review textbook figures for visual representations of cell types and organelles.