Cell Structure - BIOL 1406

Chapter 4: Cell Structure

BIOL 1406 - General Biology I

Spring 2025

Lecture Outline

  • 4.1 Cell Theory

  • 4.2 Prokaryotic Cells

  • 4.3 Eukaryotic Cells

  • 4.4 The Endomembrane System

  • 4.5 Mitochondria and Chloroplasts: Cellular Generators

  • 4.6 The Cytoskeleton

  • 4.7 Extracellular Structures and Cell Movement

  • 4.8 Cell-to-Cell Interactions

Cells

  • Diversity:

    • Cells exhibit extreme diversity.

    • Nearly all require a microscope for visibility.

    • Each cell type in the body has specialized functions.

Historical Background on Cells

  • First Observations:

    • Cells were first observed using a microscope in 1665 by Robert Hooke.

  • Foundational Studies:

    • Early studies of cells were conducted by:

    • Mathias Schleiden (1838)

    • Theodor Schwann (1839)

    • Schleiden and Schwann proposed the Cell Theory.

Microscopy and Cell Size

  • Visibility:

    • Most cells are less than 50 μm in diameter, making them invisible to the naked eye.

  • Resolution:

    • Definition: Minimum distance at which two points can be distinguished as separate.

    • The unaided human eye can resolve objects only when they are 100 μm apart.

Microscopes

  • Light Microscope:

    • Limited by the properties of light, able to resolve structures 200 nm apart.

  • Electron Microscope:

    • Invented in the 1930s and uses a beam of electrons to resolve structures that are 0.2 nm apart.

Size Scale of Biological Structures

  • Size Ranges:

    • 0.1 nm - Atoms

    • 1 nm - Amino acids

    • 10 nm - Proteins

    • 100 nm - Most bacteria

    • 1 μm - Human red blood cell

    • 10 μm - Frog egg

    • 100 μm - Mouse cell

    • 1 mm - Human egg

    • 1 cm - Chicken egg

    • Up to 10 m - Blue whale.

Why Are Cells So Small?

  • Cells are limited in size due to the need for adequate surface areas to facilitate the entry and exit of materials:

    • Surface-area-to-volume ratio: Smaller cells have a larger ratio, facilitating exchanges.

  • Factors Affecting Diffusion Rate:

    • Surface area available

    • Temperature

    • Concentration gradient

    • Distance.

Examples of Adaptations

  • Microvilli in the small intestine increase surface area for nutrient absorption.

  • Erythrocytes (red blood cells) are biconcave, enhancing surface area.

  • Some cells, like neurons, can elongate and narrow to maintain function despite size constraints.

Cell Theory

  1. All organisms are composed of cells.

  2. Cells are the smallest living things.

  3. Cells arise only from pre-existing cells—cells today represent a continuous line of descent from the first living cells.

Basic Structural Similarities Among Cells

  1. Nucleoid or Nucleus:

    • Location of DNA.

  2. Cytoplasm:

    • Semifluid matrix consisting of organelles and cytosol.

  3. Ribosomes:

    • Structures that synthesize proteins.

  4. Plasma Membrane:

    • Phospholipid bilayer that encloses the cell.

Prokaryotic Cells

  • General Characteristics:

    • Simplest of organisms, classified into two domains: Archaea and Bacteria.

    • Lack a membrane-bound nucleus; DNA is present in the nucleoid.

    • Have a cell wall located outside the plasma membrane.

    • Contain ribosomes with no organelles universal to all prokaryotes.

Organelles in Prokaryotes

  • Specific organelles like magnetosomes and infolded plasma membranes exist for specialized functions.

Bacterial Microcompartments (BMCs)

  • Structures that isolate specific metabolic processes, ranging from 40 to 400 nm in size.

  • Functional but structurally analogous to eukaryotic organelles.

Cell Walls in Prokaryotes

  • Most bacterial cells possess strong cell walls composed of peptidoglycan.

  • Functions include:

    • Protecting the cell

    • Maintaining shape

    • Preventing excessive water uptake or loss.

  • Sensitivity to Antibiotics:

    • Depend on cell wall structure.

Archaean Cell Walls

  • Lack peptidoglycan and have diverse components.

  • Membrane lipid structure distinguishes archaea from bacteria.

    • These lipids may contain saturated hydrocarbons.

Flagella in Prokaryotes

  • Present in some prokaryotes, used for locomotion and exhibit rotary motion.

Shapes of Bacteria

  • Basic Forms:

    • Coccus: Round

    • Bacillus: Rod-shaped

    • Spirillum: Spiral-shaped

  • **Arrangement Types:

    • Diplo- (Pairs)

    • Strepto- (Chains)

    • Staphyl- (Clusters)**

Eukaryotic Cells

  • Possess membrane-bound nuclei and are more complex than prokaryotic cells.

  • Characterized by compartmentalization achieved through membrane-bound organelles and an endomembrane system.

  • Contain a cytoskeleton to support cellular structure.

Comparison of Animal and Plant Cells

  • Both types have plasma membranes and similar organelles, but plant cells contain:

    • A cell wall outside the plasma membrane.

    • Chloroplasts for photosynthesis.

    • Specialized vacuoles.

Structural Components of Animal and Plant Cells

Animal Cell Structures:

  • Cytoskeleton, Actin filament, Microtubule, Intermediate filament, Centrioles, Cytoplasm, Lysosome, Nucleus, Nuclear envelope, Nucleolus, Nuclear pore, Ribosomes, Rough and Smooth endoplasmic reticulum, Peroxisome, Mitochondrion, Microvilli, Golgi apparatus.

Plant Cell Structures:

  • Golgi apparatus, Chloroplast, Rough and Smooth endoplasmic reticulum, Ribosome, Nucleus, Nucleolus, Nuclear envelope, Nuclear pore, Cell wall, Plasma membrane, Central vacuole, Plasmodesmata, Cytoskeleton.

Key Differences between Prokaryotic and Eukaryotic Cells

  • Prokaryotes: Lack membrane-bound organelles and primarily possess a single circular chromosome.

  • Eukaryotes: Have multiple linear chromosomes and various organelles like mitochondria and the Golgi apparatus.

Nucleus in Eukaryotes

  • Repository of genetic information, most has a single nucleus.

  • Nucleolus: Site for ribosomal RNA synthesis.

  • Nuclear Envelope: Composed of two phospholipid bilayers with nuclear pores controlling movement in and out.

  • DNA Organization: DNA organized into linear chromosomes and chromatin (DNA plus proteins).

Ribosomes

  • Machinery for protein synthesis found across all cell types.

  • Composed of ribosomal RNA (rRNA) and proteins, includes messenger RNA (mRNA) and transfer RNA (tRNA).

  • Ribosomes can be free in cytoplasm or associated with internal membranes.

The Endomembrane System

  • A series of membranes dividing the cell into compartments for various functions, marking a key distinction between eukaryotes and prokaryotes.

Endoplasmic Reticulum (ER)

  • Rough ER (RER):

    • Studded with ribosomes, involved in synthesizing proteins for secretion and organelle distribution.

  • Smooth ER (SER):

    • Fewer ribosomes, involved in lipid synthesis, calcium storage, and toxin processing.

    • The ratio of RER to SER varies depending on cell function.

Golgi Apparatus

  • Composed of flattened stacks of membranes, involved in packaging and distribution of cellular molecules.

  • Along with transport vesicles, it processes proteins from the rough ER for delivery to specific destinations.

Protein Transport Process:

  1. Ribosome synthesizes protein, which is packed into vesicles and shipped to Golgi.

  2. Proteins are modified and packaged within the Golgi apparatus.

  3. Proteins are released into the extracellular environment by secretion from vesicles.

Lysosomes

  • Membrane-bound digestive vesicles formed from the Golgi apparatus, containing enzymes for breaking down macromolecules.

  • Functions include recycling organelles or digesting foreign material taken in by the cell.

Microbodies

  • Enzyme-bearing vesicles including peroxisomes, which oxidize fatty acids and convert by-products (like hydrogen peroxide) into harmless substances.

Vacuoles

  • Membrane-bound structures often found in plant cells with various functions:

    • Central vacuoles for storage.

    • Contractile vacuoles in some fungi and protists.

Mitochondria

  • Present in all eukaryotic cells, bounded by double membranes:

    • Outer membrane

    • Inner membrane formed into folds (cristae).

    • Matrix contains enzymes for oxidative metabolism, and mitochondria have their own DNA.

Chloroplasts

  • Organelles in plant cells responsible for photosynthesis, surrounded by double membranes.

  • Contain thylakoids (membranous sacs) where photosynthesis occurs, which are organized into grana. Chloroplasts also contain their own DNA.

Endosymbiosis Theory

  • Proposes that eukaryotic organelles like mitochondria and chloroplasts arose from ancient symbiotic relationships between free-living prokaryotes engulfed by ancestral eukaryotic cells.

Cytoskeleton

  • A network of protein fibers supporting cell shape, organization, and intracellular transport. It is a dynamic system constantly forming and disassembling.

Three Types of Cytoskeletal Fibers:

  1. Microfilaments (Actin Filaments):

    • Composed of two actin chains; involved in movements like contraction and cell crawling.

  2. Microtubules:

    • The largest cytoskeletal elements, made of dimers of α- and β-tubulin; assist in cell movement and intracellular transport.

  3. Intermediate Filaments:

    • Stable fibers of a size between actin filaments and microtubules; maintain cell structure and stability.

Cell Movement

  • Movement facilitated by actin filaments and microtubules in cells:

    • Some cells utilize actin microfilaments for crawling.

    • Eukaryotic flagella and cilia typically exhibit a 9 + 2 arrangement of microtubules, consisting of 9 outer pairs surrounding 2 central microtubules; cilia are shorter and more numerous than flagella.

Eukaryotic Cell Walls

  • Found in plants, fungi, and some protists:

    • Chemically and structurally distinct from prokaryotic cell walls.

    • Plant and protist walls composed of cellulose.

    • Fungal cell walls composed of chitin.

    • Plant cells may have primary and sometimes secondary cell walls.

Extracellular Matrix (ECM) in Animal Cells

  • Unlike cells with walls, animal cells secrete a complex mixture of glycoproteins into the surrounding space:

    • Collagen forms a protective layer around cell surfaces.

    • Integrins anchor ECM to the cytoskeleton, influencing cell behavior.

Comparison Table

Table 4.3: A Comparison of Prokaryotic, Animal, and Plant Cells

  • Exterior Structures:

    • Prokaryotes: Cell wall present (protein-polysaccharide); Cell membrane present; Flagella may be present.

    • Animal cells: Cell wall absent; Cell membrane present; Flagella may be present.

    • Plant cells: Cell wall present (cellulose); Cell membrane present; Flagella absent (except in sperm).

  • Interior Structures:

    • Prokaryotes: No endoplasmic reticulum, ribosomes absent, Golgi apparatus absent, nucleus absent.

    • Animal cells: Endoplasmic reticulum usually present, ribosomes present, Golgi apparatus present, nucleus present, mitochondria present.

    • Plant cells: Endoplasmic reticulum usually present, ribosomes present, Golgi apparatus present, nucleus present, mitochondria present, chloroplasts present, large vacuoles typically present.

Cell-to-Cell Interactions

  • Surface proteins on cells give identity and facilitate communication between cells:

    • Cells can recognize each other and react based on surface markers, like glycolipids for tissue specificity.

    • MHC proteins help the immune system distinguish between self and nonself cells.

Types of Cell Connections:

  1. Adhesive Junctions:

    • Mechanically link cytoskeletons of neighboring cells or cells to ECM (includes adherens junctions, desmosomes, hemidesmosomes).

  2. Tight Junctions:

    • Connect plasma membranes of adjacent cells to form impermeable barriers.

  3. Communicating Junctions:

    • Allow chemical or electrical signals to pass directly between adjacent cells (gap junctions in animals, plasmodesmata in plants).

Plasmodesmata

  • Specialized openings in plant cell walls that connect the cytoplasm of adjoining cells.

  • Function similarly to gap junctions in animal cells, allowing for intercellular communication.