Chapter 6: A Tour of the Cell

Microscopy and Cell Study

Microscopy is essential for studying cells, allowing scientists to observe cellular structures and functions. Different types of microscopes provide varying levels of magnification and resolution.

• Light Microscope: Uses visible light to magnify specimens up to 1000X. Suitable for viewing living cells.

• Scanning Electron Microscope (SEM): Uses electron beams to scan cell surfaces, providing high-resolution images of external structures.

• Transmission Electron Microscope (TEM): Electron beams pass through specimens, revealing internal cell structures at high resolution.

• Cell Fractionation: Technique using centrifugation to separate cellular components, enabling study of organelle functions.

Example: SEM is used to study the surface of a cell, while TEM is used to examine mitochondria or chloroplasts in detail.

Cell Theory and Basic Cell Features

The cell theory, developed by Schleiden and Schwann, states that all organisms are composed of cells, cells arise from pre-existing cells, and cells are the basic units of structure and function.

• Plasma Membrane: Selective barrier composed of a phospholipid bilayer, proteins, and cholesterol.

• Cytosol: Jelly-like fluid inside cells where organelles are suspended.

• Chromosomes: Carry genetic information (DNA).

• Ribosomes: Sites of protein synthesis.

Formula: Surface area to volume ratio: , (for a cube). As cell size increases, volume grows faster than surface area, limiting cell size.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structure and complexity.

• Prokaryotic Cells: Domains Bacteria and Archaea. No nucleus; DNA in nucleoid. No membrane-bound organelles. Cell wall, capsule/slime layer, flagella, and fimbriae may be present.

• Eukaryotic Cells: Domain Eukarya (protists, plants, fungi, animals). Have a nucleus and membrane-bound organelles. Cytoplasm includes organelles suspended in cytosol.

Plant vs. Animal Cells:

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

• Animal Cells: Have centrioles, lysosomes, extracellular matrix (ECM).

The Nucleus and Ribosomes

The nucleus houses genetic material and coordinates cellular activities. Ribosomes synthesize proteins based on instructions from DNA.

• Nucleus: Surrounded by nuclear envelope with pores. Contains nucleolus (site of ribosome synthesis).

• Chromatin: DNA and associated proteins; condenses into chromosomes during cell division.

• Ribosomes: Made of ribosomal RNA (rRNA) and protein. Found free in cytosol or bound to rough ER/nuclear envelope.

Example: Ribosomes on rough ER synthesize proteins for secretion; free ribosomes synthesize proteins for use within the cell.

Endomembrane System

The endomembrane system regulates protein traffic and performs metabolic functions. It includes interconnected organelles that communicate via vesicles.

• Nuclear Envelope

• Endoplasmic Reticulum (ER): Network of tubules and sacs (cisternae). Rough ER has ribosomes; synthesizes secretory proteins and glycoproteins. Smooth ER lacks ribosomes; synthesizes lipids, detoxifies drugs, stores calcium.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport. Receives vesicles on cis face, ships on trans face.

• Lysosomes: Membranous sacs with hydrolytic enzymes; digest macromolecules, recycle organelles (autophagy). Unique to animal cells.

• Vacuoles: Large sacs for storage and transport. Central vacuole in plants stores water, nutrients, and waste.

• Plasma Membrane: Boundary of the cell; involved in endocytosis and exocytosis.

Protein Pathway Example: Protein synthesized at ribosome on rough ER → transported in vesicle to Golgi → modified and packaged → transported in vesicle to plasma membrane → secreted by exocytosis.

Energy Organelles: Mitochondria, Chloroplasts, and Peroxisomes

Mitochondria and chloroplasts convert energy for cellular use and are considered semi-autonomous due to their unique features.

• Mitochondria: Site of cellular respiration; found in both plant and animal cells. Double membrane; inner membrane forms cristae. Matrix contains enzymes for ATP synthesis.

• Chloroplasts: Site of photosynthesis; found in plants and algae. Double membrane; contains thylakoids (stacked as grana), stroma, and chlorophyll.

• Peroxisomes: Single-membrane organelles; perform oxidation reactions, break down fatty acids, detoxify substances. Produce hydrogen peroxide, converted to water by peroxidases.

Endosymbiont Theory: Mitochondria and chloroplasts originated from prokaryotic cells engulfed by ancestral eukaryotes. Both have their own DNA, ribosomes, and replicate independently.

Cytoskeleton

The cytoskeleton is a dynamic network of protein fibers that organizes cell structure, movement, and transport.

• Microtubules: Hollow tubes; maintain cell shape, guide organelle movement, separate chromosomes during division. Form cilia and flagella.

• Microfilaments (Actin Filaments): Solid rods; maintain/change cell shape, muscle contraction, cell motility.

• Intermediate Filaments: Rope-like; provide structural support, anchor organelles.

• Centrosome: Microtubule organizing center; contains centrioles in animal cells.

• Centrioles: Cylindrical structures; involved in cell division.

Flagella vs. Cilia:

• Flagella: Longer, fewer; undulating motion.

• Cilia: Shorter, numerous; back-and-forth motion.

• Basal Body: Anchors cilium/flagellum to cell.

Cellular Surfaces and Junctions

Cells interact with their environment and neighboring cells through extracellular structures and junctions.

• Cell Wall: Found in plants, fungi, prokaryotes; made of cellulose in plants. Provides protection, shape, prevents excess water uptake.

• Plasmodesmata: Channels between plant cells for communication.

• Extracellular Matrix (ECM): In animal cells; composed of glycoproteins (collagen, proteoglycans, fibronectin). Supports, protects, and regulates cell behavior.

• Cell Junctions:

  • Tight Junctions: Seal cells together, prevent leakage.

  • Desmosomes: Anchor cells together in sheets.

  • Gap Junctions: Allow communication via cytoplasmic channels.

Example: Gap junctions in animal cells are analogous to plasmodesmata in plant cells.

Summary Table: Plant vs. Animal Cell Structures

Key Terms and Definitions

• Hydrolysis: Chemical breakdown of a compound due to reaction with water.

• Glycoprotein: Protein with carbohydrate attached; important in cell recognition.

• Phagocytosis: Cellular process of engulfing particles.

• Contractile Vacuole: Pumps excess water out of cells (in protists).

• Chromatin: DNA-protein complex in nucleus.

• Cisternae: Membranous folds in ER and Golgi.

• Stroma: Fluid inside chloroplast.

• Thylakoid: Membranous sac in chloroplast; site of photosynthesis.

• Granum: Stack of thylakoids.

• Plastids: Group of plant organelles (includes chloroplasts, amyloplasts, chromoplasts).

Integration of Cell Structures

Cellular function depends on the integration of organelles and structures. For example, macrophages use cytoskeleton, lysosomes, and plasma membrane to engulf and digest bacteria.

Additional info: Some details, such as the specific figures referenced, were inferred from standard biology textbooks. The summary table was constructed to clarify plant vs. animal cell differences. All key terms from the provided list were defined or explained in context.