Cell Structure: Theory, Size, Microscopy

• Chapter describes key facts about cell structure
• It details key concepts in cell biology
• Highlights cell theory, cell size constraints, and microscopy methods

Cell Theory

• Cells were initially seen in 1665 using a microscope by Robert Hooke
• Early cell studies were by Matthias Schleiden (1838) and Theodor Schwann (1839)
• Schleiden and Schwann together proposed the cell theory
• All organisms consist of one or more cells
• Cells represent the smallest living entities
• Cells arise only from pre-existing cells
• All present-day cells trace back to an unbroken line of descent from the earliest cells

Cell Size Limitations

• Diffusion efficiency constrains cell size, with cells being small
• Four factors influence diffusion rate: available surface area, temperature, concentration gradients, and distance

Surface Area-to-Volume Ratio

• Organisms with numerous small cells have advantages over those with few large cells
• Cell volume escalates more quickly than surface area as size increases
• Cell shape adaptations, such as neurons being long and narrow, bypass size limitations

Types of Microscopes

• Most cells are not visible without a microscope
• The dimensions of most cells are less than 50 µm
• Clarity is determined by magnification and resolution
• Resolution is the minimum distance to distinguish two points as separate
• Visible light microscopes provide 500x more resolution than the eye can see
• Unaided eyes cannot resolve objects closer than 100 µm

Light Microscopy

• Light microscopes use magnifying lenses with visible light as a source
• They can resolve structures that are 200 nm apart
• The resolution is limited by the properties of light itself

Electron Microscopy

• Electron microscopes use electron beams to visualize small structures
• They resolve structures as small as 0.2 nm
• Transmission electron microscopes (TEM) pass electron beams through samples
• Scanning electron microscopes (SEM) scan the surface of specimens with electron beams

Specialized Light Microscopy Techniques

• Unstained samples examined under a standard light microscope
• Bright-field microscopy uses transmitted light to view stained specimens
• Dark-field microscopy enhances contrast by illuminating the sample from the side
• Phase contrast microscopy enhances contrast based on differences in refractive indexes
• Differential interference microscopy manipulates light paths to enhance contrast
• Fluorescence microscopy uses fluorescent dyes to visualize specific cell structures

Basic Structural Similarities

• Include a nucleoid or nucleus where DNA is located
• The cytoplasm contains a semifluid matrix of organelles and cytosol
• Ribosomes synthesize proteins
• Plasma membrane is a phospholipid bilayer surrounding the cell

Prokaryotic Cells

• Prokaryotic cells are relatively simple
• The two domains of prokaryotes are Archaea and Bacteria
• They lack a membrane-bound nucleus; the DNA resides in a nucleoid region
• Prokaryotic cells have a rigid cell wall outside the plasma membrane
• Prokaryotes have ribosomes, but very few other organelles

Prokaryotic Organelles

• There are no organelles common to all prokaryotes
• Some prokaryotes contain specialized organelles with specific functions such as magnetosomes
• Some prokaryotes contain infoldings of the plasma membrane that aggregate reactions

Cytoskeleton in Prokaryotes

• Prokaryotes contain molecules similar to actin and tubulin
• The cytoskeleton, however affects cell shape mainly through influencing cell wall deposition
• Cell shape is determined by the rigidity of the cell wall

Bacteria

• Bacteria encased by a cell wall made of peptidoglycan
• Cell walls are different from plants, fungi, and protists, and maintain shape, prevent water uptake
• Bacteria’s response to antibiotics is based on cell wall structure
• Gram staining differentiates bacteria by cell wall structure

Archaea

• Archaea cell walls lack peptidoglycan found in bacteria
• Exhibit membrane lipid structure that differs from bacteria (using saturated hydrocarbons attach to glycerol at both ends)

Prokaryotic Flagella

• Prokaryotes may have one or more flagella for locomotion
• Flagella propel via rotary motion, powered by proton or sodium ion gradients

Eukaryotic Cells

• Defining features include membrane-bound nucleus and organelles facilitating compartmentalization of functions
• The cytoskeleton provides structure and support in eukaryotic cells

Animal vs Plant Cells Similarities

• Both have plasma membrane boundary
• Eukaryotes include membrane-bound organelles and an endomembrane system absent from prokaryotes
• A cell wall outside the plasma membrane is only in plants
• Chloroplasts and specialized internal vacuoles are only in plants

The Nucleus

• Location for genetic information and multiple linear chromosomes
• Surrounded by a double-membrane nuclear envelope with pores for transport
• Nucleolus is the place of ribosomal RNA synthesis
• DNA is organized into chromatin, chromosomes complexed with proteins

Ribosomes

• The site of protein synthesis machinery
• Ribosomes are present in all cell types across all three domains
• Made of ribosomal RNA (rRNA) and protein complex
• Ribosomal function requires messenger RNA (mRNA) and transfer RNA (tRNA)
• Ribosomes are freely suspended in cytoplasm or bound to internal membranes

Endomembrane System

• A system of interconnected membranes that divide the cell into functional compartments
• Unique to eukaryotes; absent in prokaryotes

Endoplasmic Reticulum (ER)

• Rough ER (RER) is studded with ribosomes and synthesizes secreted and membrane proteins
• Smooth ER (SER) is involved in lipid synthesis, calcium storage, and detoxification

Golgi Apparatus

• It consists of flattened, interconnected stacks (cisternae) that package and distribute molecules
• Has cis and trans faces for receiving and shipping
• Transport vesicles shuttle molecules to their destinations

Lysosomes

• Membrane-bound vesicles containing digestive enzymes
• Arise from the Golgi apparatus
• Break down macromolecules and recycle old organelles through phagocytosis

Microbodies (Peroxisomes)

• These are vesicles containing enzymes for oxidizing fatty acids
• They produce hydrogen peroxide as a byproduct, which is broken down by catalase

Vacuoles

• Membrane-bound structures mainly in plants that store materials
• Central vacuoles maintain cell turgor in plants
• Storage types store nutrients
• Contractile types regulate water balance in protists and fungi

Mitochondria

• Found in eukaryotic cells
• Bounded by outer and inner membranes
• The inner membrane folds into cristae
• Contains a matrix
• Proteins for oxidative metabolism are embedded in the inner membrane
• It has its own DNA

Chloroplasts

• Organelles for photosynthesis that are found only in plant cells and some eukaryotes
• Enclosed by two membranes, contain chlorophyll
• Thylakoids are internal membranous sacs that form grana (stacks)
• Contains its own DNA

Endosymbiosis Theory

• Eukaryotic organelles evolved from symbiotic relationships between cells
• A host cell engulfed a prokaryote, leading to the formation of mitochondria and chloroplasts
• Both mitochondria and chloroplasts share characteristics with prokaryotic cells

Cytoskeleton

• A network of protein fibers in eukaryotic cells
• Supports cell shape and fixes organelle locations
• It’s a dynamic system
• It constantly forms and disassembles

Microfilaments

• These are made of two intertwined chains of actin and are movement related

Microtubules

• Largest cytoskeletal elements
• These are dimers of α- and β-tubulin subunits
• These facilitate movement

Intermediate Filaments

• They are mid-sized in comparison of actin filaments and microtubules
• These are very stable

Centrosomes

• Organizing centers of microtubules situated near the nucleus which occur in pairs in animal cells and protists
• They not in fungi and higher plant cells
• They nucleate microtubule assembly

Cell Movement

• Movement of actin filaments, microtubules, or both helps cells move
• Cells crawl using actin microfilaments
• Eukaryotic flagella and cilia exhibit a 9 + 2 microtubule arrangement

Cilia vs Flagella

• Flagella have many different structures and functions
• Cilia is small and more numerous

Eukaryotic Cell Walls

• Eukaryotic cells in fungi, plants, and certain protists have cell walls
• Plants and protists make it of cellulose, whereas fungi use chitin
• Plant cells can have both primary and secondary walls

Extracellular Matrix (ECM)

• Animal cells have an extracellular matrix made of secreted glycoproteins
• Collagen forms the structural component, attaching cells to cell-surface
• The protein integrin enables cell communication

Cell-to-Cell Interactions

• Surface proteins function as cell identity markers
• Cells make contact, "read" each other, and react
• Glycolipids represent tissue-specific cell surface markers
• MHC proteins allows immune system to recognize "self" versus "nonself" cells

Animal Cell Junctions

• Adhesive: They mechanically link cells
• Septate/Tight: They seal cell sheets to prevent leakage
• Communicating: They permit passage of chemical or electrical signals

Plant Tissue Junctions

• Only plasmodesmata join plant cells
• Plasmodesmata form specialized openings in-between cell walls
• Function similar to gap junctions in animal cells

Cell Fractionation

• This studies the function parts of the cell
• It involves breaking cells and fractionating by centrifugation
• Centrifugal force creates pellet and supernatant layers for analysis