Chapter 6: A Tour of the Cell

Introduction to Cells

Cells are the fundamental units of structure and function in all living organisms. Understanding cell structure and function is essential for studying biology, as all life processes occur within cells. Eukaryotic cells, in particular, have complex internal organization that allows them to perform the functions of life efficiently.

Microscopy and Cell Study

Microscopy: Tools for Visualizing Cells

Microscopes are essential tools for studying cells, which are typically too small to be seen with the naked eye. Different types of microscopy provide varying levels of magnification, resolution, and contrast, allowing scientists to observe cellular structures in detail.

• Magnification: The ratio of an object's image size to its real size.

• Resolution: The clarity of the image, or the minimum distance between two distinguishable points.

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

Light microscopes can magnify up to about 1,000 times, but their resolution is limited. Electron microscopes, including scanning (SEM) and transmission (TEM) types, provide much higher resolution, allowing visualization of subcellular structures. Recent advances include fluorescent labeling, confocal microscopy, and cryo-electron microscopy, which preserve specimens at low temperatures for detailed study.

Cell Fractionation

Cell fractionation is a technique used to separate cellular components for individual study. This process involves homogenizing cells and using centrifugation to separate organelles based on size and density. It allows scientists to link cell structure with function through biochemical analysis.

Prokaryotic vs. Eukaryotic Cells

Basic Features of All Cells

• Plasma membrane

• Cytosol (semifluid substance)

• Chromosomes (carry genes)

• Ribosomes (synthesize proteins)

Prokaryotic Cells

Prokaryotic cells (domains Bacteria and Archaea) lack a nucleus and membrane-bound organelles. Their DNA is located in a region called the nucleoid, and the cytoplasm is bound by the plasma membrane.

Eukaryotic Cells

Eukaryotic cells (protists, fungi, animals, plants) have DNA enclosed in a nucleus and possess membrane-bound organelles. They are generally larger than prokaryotic cells, and their cytoplasm is the region between the plasma membrane and the nucleus.

The surface area-to-volume ratio is critical for cell function, as it affects the ability to exchange materials with the environment. As cells increase in size, their volume grows faster than their surface area, limiting efficient exchange.

The Nucleus and Ribosomes

The Nucleus: Information Central

The nucleus contains most of the cell's genetic material and is surrounded by a double-membrane nuclear envelope. Nuclear pores regulate the entry and exit of molecules, and the nuclear lamina provides structural support. DNA is organized into chromosomes, which are composed of chromatin (DNA and proteins). The nucleolus within the nucleus is the site of ribosomal RNA (rRNA) synthesis.

Ribosomes: Protein Factories

Ribosomes are complexes of rRNA and protein that synthesize proteins. They can be free in the cytosol or bound to the endoplasmic reticulum (ER) or nuclear envelope. Free ribosomes typically synthesize proteins for use within the cell, while bound ribosomes make proteins for export or for membranes.

The Endomembrane System

Components and Functions

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

Endoplasmic Reticulum (ER)

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

• Rough ER: Studded with ribosomes, synthesizes glycoproteins, distributes transport vesicles, and produces membranes.

Golgi Apparatus

The Golgi apparatus consists of flattened sacs (cisternae) and functions in modifying, sorting, and packaging products from the ER for secretion or delivery to other organelles.

Lysosomes

Lysosomes are membranous sacs containing hydrolytic enzymes for digesting macromolecules. They are involved in phagocytosis (engulfing food particles) and autophagy (recycling the cell's own components).

Vacuoles

Vacuoles are large vesicles with diverse functions. Food vacuoles form by phagocytosis, contractile vacuoles expel excess water, and central vacuoles in plants store ions and contribute to cell growth.

Endomembrane System Overview

Mitochondria, Chloroplasts, and Peroxisomes

Mitochondria: Cellular Respiration

Mitochondria are the sites of cellular respiration, converting oxygen and nutrients into ATP. They have a double membrane, with the inner membrane folded into cristae to increase surface area. The mitochondrial matrix contains enzymes for metabolic processes.

Chloroplasts: Photosynthesis

Chloroplasts are found in plants and algae and are the sites of photosynthesis. They contain thylakoids (stacked into grana) and stroma (internal fluid). Chloroplasts are a type of plastid.

Endosymbiont Theory

Mitochondria and chloroplasts are thought to have originated from prokaryotic cells engulfed by ancestral eukaryotes. Evidence includes their double membranes, own DNA, and ribosomes.

Peroxisomes

Peroxisomes are single-membrane organelles containing enzymes that transfer hydrogen from substrates to oxygen, producing hydrogen peroxide. They break down fatty acids and detoxify harmful substances.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of fibers that organizes cell structure and activities. It consists of microtubules, microfilaments (actin filaments), and intermediate filaments.

Roles in Support and Motility

The cytoskeleton maintains cell shape, anchors organelles, and enables cell movement through interactions with motor proteins. Vesicles and organelles move along cytoskeletal tracks.

Microtubules

Microtubules are hollow rods made of tubulin dimers. They shape the cell, guide organelle movement, and separate chromosomes during cell division. In animal cells, microtubules grow from the centrosome, which contains centrioles.

Cilia and Flagella

Cilia and flagella are microtubule-containing extensions that enable cell movement. They have a "9+2" arrangement of microtubules and are anchored by a basal body. Dynein motor proteins drive their movement.

Microfilaments (Actin Filaments)

Microfilaments are thin rods of actin that support cell shape and are involved in cell movement. They form the cortex under the plasma membrane and are essential for muscle contraction, cell crawling, and cytoplasmic streaming.

Intermediate Filaments

Intermediate filaments are more permanent structures that support cell shape and anchor organelles. They are larger than microfilaments but smaller than microtubules.

Extracellular Components and Cell Junctions

Plant Cell Walls

Plant cell walls provide structural support, maintain cell shape, and prevent excessive water uptake. They are composed of cellulose fibers and may have multiple layers: primary cell wall, middle lamella, and secondary cell wall.

Extracellular Matrix (ECM) of Animal Cells

The ECM is a network of glycoproteins (collagen, proteoglycans, fibronectin) that supports animal cells and facilitates communication through integrins. The ECM influences cell behavior and gene activity.

Cell Junctions

• Plasmodesmata: Channels connecting plant cells, allowing passage of water and small molecules.

• Tight junctions: Prevent leakage of extracellular fluid between animal cells.

• Desmosomes: Anchor cells together into strong sheets.

• Gap junctions: Provide cytoplasmic channels for communication between animal cells.

Integration of Cellular Components

No single component of the cell works in isolation. Cellular activities are coordinated through the interaction of organelles, the cytoskeleton, and the plasma membrane, enabling complex processes such as phagocytosis and intracellular transport.

Summary Table: Structure and Function of the Cytoskeleton

Additional info: This summary integrates key concepts from Chapter 6 of Campbell Biology, focusing on cell structure, function, and the methods used to study cells. It is designed to provide a comprehensive overview suitable for exam preparation in a General Biology course.