A Tour of the Cell: Structure and Function of Cellular Organelles: Cell Biology

Chapter 6: A Tour of the Cell

Introduction

This chapter provides an overview of cell structure and function, focusing on the differences between prokaryotic and eukaryotic cells, the organization and roles of cellular organelles, and the importance of compartmentalization in cell biology. Understanding these concepts is fundamental for studying cell biology and the specialized functions of organelles within animal and plant cells.

Concept 6.2: Prokaryotic vs. Eukaryotic Cells

Domains of Life

• Prokaryotic cells are found in two domains: Bacteria and Archaea.

• Eukaryotic cells are found in the domain Eukarya, which includes animals, plants, fungi, and protists.

Key Differences Between Prokaryotic and Eukaryotic Cells

• DNA Location: Prokaryotic cells have their DNA in a region called the nucleoid, which is not membrane-bound. Eukaryotic cells store their DNA within a membrane-bound nucleus.

• Organelles: Eukaryotic cells contain membrane-bound organelles; prokaryotic cells do not.

Structure and Function of Prokaryotic Cell Components

• Cell wall: Provides structural support and protection.

• Plasma membrane: Regulates the movement of substances into and out of the cell.

• Bacterial chromosome: Contains the genetic material (DNA).

• Nucleoid: Region where the bacterial chromosome is located.

• Ribosomes: Sites of protein synthesis.

• Flagella: Structures used for cell movement.

Surface Area to Volume Ratio

• As cell size increases, the surface area to volume ratio decreases.

• This affects the efficiency of material exchange with the environment.

Formula:

$\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

• Lower ratios can limit cell size due to reduced efficiency in nutrient uptake and waste removal.

Concept 6.3: The Nucleus, Chromosomes, and Ribosomes

Nucleus Structure and Function

• Nuclear envelope: Double membrane that surrounds the nucleus; consists of two lipid bilayers.

• Nuclear pores: Allow the passage of molecules such as mRNA and ribosomal subunits.

• Nuclear lamina: Network of protein filaments that provides structural support to the nucleus.

• Nuclear matrix: Framework within the nucleus that organizes genetic material.

Chromosomes

• Chromatin: DNA and associated proteins found in the nucleus.

• Chromatin condenses to form chromosomes during cell division.

Ribosomes

• Function: Ribosomes are the molecular machines that synthesize proteins by translating mRNA.

• Components: Each ribosome consists of a large and a small subunit, both made of rRNA and proteins.

Types of Ribosomes

Type of Ribosome	Location	Product
Free ribosomes	Cytosol	Proteins used within the cytosol
Bound ribosomes	Attached to the endoplasmic reticulum or nuclear envelope	Proteins destined for secretion, insertion into membranes, or for certain organelles

Concept 6.4: The Endomembrane System

Components of the Endomembrane System

• Nuclear envelope

• Endoplasmic reticulum (ER) (smooth and rough)

• Golgi apparatus

• Lysosomes

• Vacuoles

• Plasma membrane

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins, some of which are glycoproteins (proteins with carbohydrate groups attached).

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

Major Functions of the Smooth ER

• Synthesis of lipids

• Metabolism of carbohydrates

• Detoxification of drugs and poisons

Additional info: Smooth ER is abundant in liver cells due to its role in detoxification.

Major Functions of the Rough ER

• Protein synthesis for secretion or membrane insertion

• Modification of proteins (e.g., glycosylation)

Golgi Apparatus

• Consists of flattened membranous sacs called cisternae.

• Has a cis face (receiving side) and a trans face (shipping side).

• Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Lysosomes

• Definition: Membrane-bound organelles containing hydrolytic enzymes for intracellular digestion.

• pH: Lysosomes maintain an acidic pH (around 4.5-5.0) optimal for enzyme activity.

• Functions:

  • Digestion of engulfed particles (phagocytosis)

  • Recycling of cellular components (autophagy)

Example: Human macrophages use lysosomes to digest pathogens.

Tay-Sachs disease: Caused by a deficiency in a lysosomal enzyme, leading to accumulation of undigested material.

Vacuoles

• Food vacuoles: Formed by phagocytosis; contain ingested food particles.

• Contractile vacuoles: Pump excess water out of cells (common in freshwater protists).

• Central vacuole (plants): Stores water, ions, and nutrients; maintains turgor pressure.

Concept 6.5: Mitochondria and Chloroplasts

Endosymbiont Theory

• States that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence:

  • Both have double membranes.

  • Contain their own DNA and ribosomes.

  • Replicate independently within the cell.

Mitochondria Structure

• Outer membrane

• Inner membrane (highly folded into cristae)

• Intermembrane space

• Matrix

• Ribosomes

Chloroplast Structure

• Outer membrane

• Inner membrane

• Intermembrane space

• Thylakoids (stacked into grana)

• Stroma

Functions

• Mitochondria: Site of cellular respiration; converts chemical energy in food to ATP.

• Chloroplasts: Site of photosynthesis; converts solar energy to chemical energy in sugars.

Compartmentalization and Other Organelles

• Peroxisomes: Organelles that contain enzymes for oxidation reactions, breaking down fatty acids and detoxifying harmful substances.

• Compartmentalization allows for specialized environments and efficient metabolic processes within the cell.

Summary Table: Comparison of Animal and Plant Cell Organelles

Organelle	Animal Cell	Plant Cell
Nucleus	Present	Present
Mitochondria	Present	Present
Chloroplasts	Absent	Present
Central Vacuole	Absent	Present
Lysosomes	Present	Rare
Cell Wall	Absent	Present

Study Tip

When studying cell structure, use labeled diagrams to visualize organelle locations and functions. Consider not only the structure but also the specialized roles of each organelle in different cell types.