Campbell Biology in Focus: A Tour of the Cell

Chapter 4: A Tour of the Cell

Introduction

  • The diorama of cellular biology is complex, highlighting the roles various cellular structures play in maintaining life.

Concept 4.1: Biologists Use Microscopes and Biochemistry to Study Cells

  • Most cells are too small to be seen by the unaided eye.

Microscopy

  • Light Microscopy (LM)

    • Visible light is passed through a specimen and through glass lenses which refract (bend) the light, magnifying the image.

  • Important Parameters in Microscopy:

    • Magnification: Ratio of an object’s image size to its actual size.

    • Resolution: Measure of image clarity; the minimum distance between two distinguishable points.

    • Contrast: The difference in brightness between light and dark parts of the image.

Light Microscopy Advantages and Limitations

  • Magnification Limit: Light microscopes can effectively magnify to about 1,000 times the actual specimen size.

    • Uses staining or labeling techniques to enhance contrast.

    • Most organelles are too small for light microscopy to resolve.

Size Range of Cells

  • Size scales include:

    • Human height: ~1.7 m

    • Chicken egg: ~1 cm

    • Most plant/animal cells: 10 - 100 μm

    • Nucleus: ~10 μm

    • Most bacteria: 1 - 5 μm

    • Mitochondrion: ~1 μm

    • Viruses: 100 nm

    • Ribosomes: 10 nm

    • Proteins: 1 nm

    • Atoms: 0.1 nm

Electron Microscopy (EM)

  • Types of EMs:

    • Scanning Electron Microscopes (SEMs): Focus a beam of electrons onto the surface of a specimen to create three-dimensional images.

    • Transmission Electron Microscopes (TEMs): Focus a beam of electrons through a specimen to study internal structures.

Advances in Light Microscopy

  • Techniques like fluorescent markers improve detail visualization.

  • Confocal microscopy sharpens images of tissues.

  • Cryo-Electron Microscopy: Preserves specimens at very low temperatures for structural visualization.

Concept 4.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Types of Cells

  • Two main types:

    • Prokaryotic Cells: Found in domains Bacteria and Archaea.

    • Eukaryotic Cells: Found in protists, fungi, animals, and plants.

Common Features of Cells

  • Plasma membrane

  • Semifluid substance called cytosol

  • Chromosomes containing genes

  • Ribosomes that make proteins.

Comparisons Between Prokaryotic and Eukaryotic Cells

  • Eukaryotic Cells:

    • Contain a nucleus, organelles, larger than prokaryotic cells (10 - 100 μm).

    • DNA housed in a double membrane-bound nucleus.

  • Prokaryotic Cells:

    • No nucleus or membrane-bound organelles, DNA is in the nucleoid region, typically 1 - 5 μm in size.

Importance of Plasma Membrane

  • Selective barrier regulating the passage of substances across it; critical for surface area-to-volume ratio.

  • Scaling effects:

    • Surface area increases with the square of the dimension, volume increases with the cube.

Concept 4.3: The Eukaryotic Cell’s Genetic Instructions Are Housed in the Nucleus and Carried Out by the Ribosomes

Structure of the Nucleus

  • The most conspicuous organelle, responsible for housing most of a cell’s DNA.

  • Nuclear Envelope: Double membrane separating nucleus from cytoplasm.

  • Nuclear Pores: Regulate molecular passage.

Chromatin and Chromosomes

  • Chromatin: Composed of DNA and proteins.

    • Condenses into distinct chromosomes during cell division.

  • Nucleolus: Site of rRNA synthesis within the nucleus.

Ribosomes: Protein Factories

  • Composed of ribosomal RNA and protein, not considered organelles.

  • Ribosomes synthesize proteins at two locations:

    • Free Ribosomes: In the cytosol.

    • Bound Ribosomes: Attached to rough ER or nuclear envelope.

Concept 4.4: The Endomembrane System Regulates Protein Traffic and Performs Metabolic Functions

Components of the Endomembrane System

  • Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

  • Continuous or connected through vesicular transport.

Endoplasmic Reticulum (ER)

  • Accounts for over 50% of the total membrane in many eukaryotic cells.

  • Two types:

    • Smooth ER: Lacks ribosomes; involved in lipid synthesis, carbohydrate metabolism, detoxification, and calcium ion storage.

    • Rough ER: Studded with ribosomes; synthesizes glycoproteins and distributes transport vesicles.

Golgi Apparatus

  • Series of flattened membranous sacs (cisternae).

  • Modifies, stores, and packages ER products for transport.

  • Uses molecular identification tags like phosphate groups for sorting.

Lysosomes

  • Membranous sacs of hydrolytic enzymes for digestion.

  • Enzymes function optimally in an acidic environment.

  • Capable of phagocytosis to form food vacuoles; involved in autophagy for recycling organelles.

Vacuoles

  • Large vesicles derived from ER and Golgi apparatus; composition differs from cytosol.

  • Types include food vacuoles (from phagocytosis) and contractile vacuoles (in freshwater protists).

  • Central vacuoles in plants store ions and organic compounds.

Concept 4.5: Mitochondria and Chloroplasts Change Energy from One Form to Another

Mitochondria

  • Sites of cellular respiration generating ATP from sugars and fats.

  • Structure includes an outer smooth membrane and an inner folded membrane forming cristae; structurally distinct areas: intermembrane space and mitochondrial matrix.

Chloroplasts

  • Involved in photosynthesis, containing chlorophyll and enzymes.

  • Structure includes thylakoids (stacked in granums) and stroma, the internal fluid of chloroplasts.

Peroxisomes

  • Metabolic compartments bounded by a single membrane; remove hydrogen atoms from molecules producing hydrogen peroxide, which is converted to water.

Concept 4.6: The Cytoskeleton Is a Network of Fibers That Organizes Structures and Activities in the Cell

Overview of the Cytoskeleton

  • Network of fibers throughout the cytoplasm organizing structures and maintaining cell shape.

  • Interacts with motor proteins for cellular movement.

Types of Fibers in the Cytoskeleton

  • Microtubules: Thickest fibers; composed of tubulin dimers.

  • Microfilaments (Actin Filaments): Thinnest components; bears tension and aids cellular motility.

  • Intermediate Filaments: Provide structural support and are more permanent than other two classes.

Concept 4.7: Extracellular Components and Connections Between Cells Help Coordinate Cellular Activities

Cell Walls of Plants

  • Protect plant cells, maintain shape, and prevent excessive water uptake.

  • Composed of cellulose microfibrils and polysaccharides.

    • Layers include primary cell wall, middle lamella, and secondary cell wall (in wood).

Extracellular Matrix (ECM) in Animal Cells

  • Lacks cell walls; consists of glycoproteins connecting to integrins in the plasma membrane.

Cell Junctions

  • Plasmodesmata: Channels allowing material movement between plant cells.

  • Animal Cell Junctions: Include tight junctions (prevent fluid movement), desmosomes (anchoring cells), and gap junctions (channels for communication).

Concept 4.8: A Cell Is Greater Than the Sum of Its Parts

  • Cellular functions arise from the integrated activities of all cellular components, demonstrated in macrophages which exhibit cellular coordination to perform infection response functions.