Chapter 06: A Tour of the Cell

Key Questions and Concepts

This chapter explores the structure and function of cells, the fundamental units of life. It covers microscopy techniques, cell fractionation, the differences between prokaryotic and eukaryotic cells, organelle functions, the endomembrane system, the endosymbiotic theory, the cytoskeleton, and cell junctions.

• Microscopy: Types, purposes, and limits of resolution

• Cell Fractionation: How cells are separated into components

• Cell Structure: Features of prokaryotic and eukaryotic cells

• Organelle Functions: Identification and roles of cell parts

• Endosymbiotic Theory: Origins of mitochondria and chloroplasts

• Cytoskeleton: Types of fibers and their functions

• Cell Junctions: Types and their roles in multicellular organisms

Microscopy and Cell Study

Types of Microscopy

Microscopes are essential tools for visualizing cells, which are typically too small to be seen with the naked eye.

• Light Microscope (LM): Uses visible light passed through a specimen and glass lenses to magnify images. Suitable for viewing living cells and tissues.

• Electron Microscopes (EM): Use beams of electrons for much higher resolution.

  • Scanning Electron Microscope (SEM): Focuses electrons onto the surface, producing 3D images of cell surfaces.

  • Transmission Electron Microscope (TEM): Passes electrons through thin sections of specimens, revealing internal structures.

Key Parameters of Microscopy:

• Magnification: Ratio of image size to real size.

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

• Contrast: Visible differences in brightness between parts of the sample.

Cell Fractionation

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

• Cells are broken apart and spun in a centrifuge.

• Heavier components (nuclei, mitochondria) settle at the bottom; lighter components (ribosomes, membranes) remain in the supernatant.

• This allows researchers to study the function of specific organelles.

Cell Structure and Function

Basic Features of All Cells

• Plasma Membrane: Selectively permeable barrier controlling entry and exit of substances.

• Cytosol: Semifluid substance within the cell.

• Chromosomes: Carry genetic information (DNA).

• Ribosomes: Sites of protein synthesis.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells:

  • No nucleus; DNA is in an unbound region called the nucleoid.

  • No membrane-bound organelles.

  • Cytoplasm is bound by the plasma membrane.

• Eukaryotic Cells:

  • DNA is contained within a nucleus, bounded by a double membrane.

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

  • Cytoplasm is the region between the plasma membrane and nucleus.

Surface Area to Volume Ratio: As a cell increases in size, its volume grows faster than its surface area, limiting cell size due to the need for efficient exchange of materials.

Structure of the Plasma Membrane

• Composed of a phospholipid bilayer with embedded proteins.

• Hydrophilic (water-attracting) heads face outward; hydrophobic (water-repelling) tails face inward.

• Controls the movement of substances in and out of the cell.

Internal Organization of Eukaryotic Cells

Nucleus and Ribosomes

• Nucleus: Contains most of the cell's DNA, organized as chromosomes. Surrounded by a double membrane (nuclear envelope) with nuclear pores.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis.

• Ribosomes: Complexes of rRNA and protein; build proteins in the cytosol (free ribosomes) or on the rough ER/nuclear envelope (bound ribosomes).

The Endomembrane System

• Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Components are either continuous or connected via vesicles.

Endoplasmic Reticulum (ER)

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs/poisons, stores calcium ions.

• Rough ER: Studded with ribosomes; synthesizes proteins (especially glycoproteins), distributes transport vesicles, and is a membrane factory for the cell.

Golgi Apparatus

• Consists of flattened membranous sacs (cisternae).

• Modifies, sorts, and packages products from the ER for secretion or delivery to other organelles.

Lysosomes

• Membranous sacs containing hydrolytic enzymes for digesting macromolecules.

• Carry out phagocytosis (engulfing food particles) and autophagy (recycling cell components).

Vacuoles

• Large vesicles derived from the ER and Golgi apparatus.

• Types include food vacuoles (formed by phagocytosis), contractile vacuoles (pump excess water out), and central vacuoles (in plants, store ions and aid growth).

Energy Conversion Organelles

Mitochondria

• Sites of cellular respiration; convert chemical energy in food to ATP.

• Have a double membrane; inner membrane is folded into cristae, creating intermembrane space and mitochondrial matrix.

Chloroplasts

• Found in plants and algae; sites of photosynthesis.

• Contain chlorophyll, thylakoids (stacked into grana), and stroma (internal fluid).

Peroxisomes

• Oxidative organelles that detoxify harmful substances by transferring hydrogen to oxygen, forming hydrogen peroxide ().

• Break down fatty acids and detoxify alcohol in the liver.

Endosymbiotic Theory

• Suggests mitochondria and chloroplasts originated as prokaryotic cells engulfed by an ancestral eukaryote.

• Evidence: Both have double membranes, their own DNA and ribosomes, and reproduce independently within the cell.

The Cytoskeleton

The cytoskeleton is a network of fibers that provides structural support, cell shape, and motility.

Centrosomes and Centrioles

• Centrosome: Microtubule-organizing center in animal cells.

• Centrioles: Pair of structures within the centrosome, each composed of nine triplets of microtubules.

Cilia and Flagella

• Structures for cell movement; composed of microtubules in a 9+2 arrangement.

• Anchored by a basal body; movement driven by the motor protein dynein.

Extracellular Components and Cell Junctions

Plant Cell Walls

• Extracellular structure providing protection, shape, and prevention of excessive water uptake.

• Composed mainly of cellulose.

• Layers: Primary cell wall (thin/flexible), middle lamella (pectin-rich), and sometimes a secondary cell wall (added strength).

Extracellular Matrix (ECM) of Animal Cells

• Network of glycoproteins (collagen, proteoglycans, fibronectin) outside the plasma membrane.

• ECM proteins bind to integrins (receptor proteins) in the plasma membrane, facilitating communication and structural support.

Cell Junctions

Summary Table: Prokaryotic vs. Eukaryotic Cells

Example: The human intestinal lining contains cells with microvilli (supported by actin microfilaments) to increase surface area for absorption.

Additional info: Some details, such as the specific arrangement of cytoskeletal fibers and the full process of cell fractionation, have been expanded for clarity and completeness.