Biology Chapter 6

A Tour of the Cell

Introduction to Cell Biology

Cells are the fundamental units of life, and their internal organization enables them to perform essential biological functions. Biologists use microscopes and biochemical techniques to study cells, which are typically too small to be seen by the naked eye. Understanding cell structure and function is crucial for comprehending all biological processes.

Microscopy and Cell Study

Microscopes are essential tools for visualizing cells. In a light microscope (LM), visible light passes through a specimen and glass lenses, which refract the light to magnify the image. This allows scientists to observe cell structures and their organization.

Comparing Prokaryotic and Eukaryotic Cells

Basic Features of All Cells

• Plasma membrane: Selective barrier regulating passage of substances.

• Cytosol: Semifluid substance within the cell.

• Chromosomes: Carry genetic information (genes).

• Ribosomes: Sites of protein synthesis.

Prokaryotic Cells

Prokaryotic cells lack a nucleus and membrane-bound organelles. Their DNA is located in an unbound region called the nucleoid, and their cytoplasm is enclosed by the plasma membrane.

• No nucleus

• No membrane-bound organelles

• DNA in nucleoid

• Cytoplasm bound by plasma membrane

Eukaryotic Cells

Eukaryotic cells have a nucleus surrounded by a double membrane and contain membrane-bound organelles. Their cytoplasm is the region between the plasma membrane and nucleus, and they are generally larger than prokaryotic cells.

• DNA in nucleus (bounded by double membrane)

• Membrane-bound organelles

• Cytoplasm between plasma membrane and nucleus

• Larger size compared to prokaryotes

Cell Size and Surface Area-to-Volume Ratio

Metabolic requirements set upper limits on cell size. The plasma membrane must allow sufficient passage of oxygen, nutrients, and waste to service the cell's volume. As a cell increases in size, its volume grows faster than its surface area, making the surface area-to-volume ratio critical for cell function.

A Panoramic View of the Eukaryotic Cell

Internal Membranes and Compartmentalization

Eukaryotic cells have internal membranes that divide the cell into compartments called organelles. These compartments provide distinct environments for different cellular processes, allowing incompatible reactions to occur simultaneously. The basic structure of biological membranes is a double layer of phospholipids and other lipids.

• Plant and animal cells share most organelles

• Compartmentalization increases efficiency

The Nucleus and Ribosomes

The Nucleus: Information Central

The nucleus contains most of the cell's genes and is usually the most prominent organelle. It is enclosed by a double membrane called the nuclear envelope, which separates it from the cytoplasm. Nuclear pores regulate the entry and exit of molecules.

• DNA organized into chromosomes

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis

Ribosomes: Protein Factories

Ribosomes are complexes of rRNA and protein that build proteins in two locations:

• Free ribosomes: In the cytosol

• Bound ribosomes: On the endoplasmic reticulum or nuclear envelope

The Endomembrane System

Components and Functions

The endomembrane system regulates protein traffic and performs metabolic functions. It includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane. These components are either continuous or connected by vesicles.

• Endoplasmic Reticulum (ER): Biosynthetic factory; continuous with nuclear envelope

• Smooth ER: Synthesizes lipids, detoxifies drugs, stores calcium ions

• Rough ER: Has ribosomes; synthesizes glycoproteins, distributes transport vesicles, produces membranes

• Golgi Apparatus: Modifies, sorts, and packages products from ER

• Lysosomes: Digestive compartments with hydrolytic enzymes

• Vacuoles: Maintenance compartments; central vacuole in plants stores ions and supports growth

Mitochondria, Chloroplasts, and Peroxisomes

Mitochondria: Chemical Energy Conversion

Mitochondria are the sites of cellular respiration, generating ATP by using oxygen. They have a smooth outer membrane and a highly folded inner membrane (cristae), creating compartments for metabolic reactions.

• Intermembrane space

• Mitochondrial matrix (site of metabolic steps)

• Cristae increase surface area for ATP synthesis

Chloroplasts: Capture of Light Energy

Chloroplasts are found in plants and algae and are the sites of photosynthesis. They contain chlorophyll and have a structure including thylakoids (stacked as granum) and stroma (internal fluid).

• Thylakoids: Membranous sacs

• Granum: Stack of thylakoids

• Stroma: Internal fluid

Peroxisomes: Oxidation

Peroxisomes are specialized metabolic compartments bounded by a single membrane. They contain enzymes that transfer hydrogen atoms to oxygen, forming hydrogen peroxide, which is then converted to water.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of fibers that organizes cell structures and activities, anchoring organelles and providing support and motility. It consists of three types of molecular structures:

• Microtubules: Thickest; shape cell, guide organelle movement, separate chromosomes

• Microfilaments: Smallest; made of actin

• Intermediate filaments: Medium diameter

Centrosomes and Centrioles

Centrosomes and centrioles organize cell division in animal cells. The centrosome contains a pair of centrioles, which are composed of microtubules arranged in a specific pattern.

Cilia and Flagella

Flagella are long, whip-like extensions that propel cells through their environment. Cilia are shorter and more numerous, moving fluid across the cell surface or moving the cell itself.

Extracellular Components and Cell Connections

Cell Walls of Plants

The cell wall is an extracellular structure distinguishing plant cells from animal cells. It protects the cell, maintains its shape, and prevents excessive water uptake. Plant cell walls are made of cellulose fibers embedded in polysaccharides and protein.

Extracellular Matrix (ECM) of Animal Cells

Animal cells lack cell walls but have an elaborate extracellular matrix (ECM) composed of glycoproteins and other molecules. The ECM regulates cell behavior and gene activity through integrins and mechanical signaling.

Cell Junctions

Cells in tissues, organs, or organ systems adhere, interact, and communicate through direct physical contact. Three types of cell junctions are common in animal tissues:

• Tight junctions: Prevent leakage of extracellular fluid

• Desmosomes: Fasten cells together into strong sheets

• Gap junctions: Provide cytoplasmic channels for communication

Plasmodesmata in Plant Cells

Plasmodesmata are channels connecting plant cells, allowing water, solutes, proteins, and RNA to pass from cell to cell.

Summary Table: Comparison of Cell Types and Structures

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No	Yes (double membrane)
Membrane-bound organelles	No	Yes
Cell size	Small	Larger
Cell wall	Yes (most)	Plants (yes), Animals (no)
DNA location	Nucleoid	Nucleus

Additional info: The notes expand on brief points from the original slides, providing definitions, examples, and context for each cell structure and function. Images included are directly relevant to the explanation of cell compartmentalization and centrosome structure.