Section 4: A Tour of the Cell

Introduction to the Cell

The cell is the fundamental unit of structure and function in all living organisms. Eukaryotic cells, which include those of plants, animals, fungi, and protists, are characterized by internal compartmentalization and complex organization. This section explores the main components and functions of eukaryotic cells.

• Cell Theory: All living things are composed of cells, and all cells arise from pre-existing cells.

• Types of Cells: Prokaryotic (bacteria and archaea) and eukaryotic (plants, animals, fungi, protists).

• Microscopy: Biologists use light and electron microscopes to study cell structure.

Panoramic View of the Eukaryotic Cell

Main Features of Eukaryotic Cells

Eukaryotic cells contain membrane-bound organelles that compartmentalize cellular functions. The two major types are plant and animal cells, which share many structures but also have unique features.

• Plasma Membrane: A selective barrier that regulates the passage of substances in and out of the cell.

• Cytoplasm: The region between the nucleus and the plasma membrane, containing organelles suspended in cytosol.

• Organelles: Specialized structures with distinct functions (e.g., nucleus, mitochondria, chloroplasts, endoplasmic reticulum).

• Cytoskeleton: A network of protein fibers that provides structural support and facilitates cell movement.

Comparison of Plant and Animal Cells

• Plant Cells: Have a cell wall, chloroplasts, and a large central vacuole.

• Animal Cells: Lack a cell wall and chloroplasts, but contain centrioles and lysosomes.

Key Concepts

• Microscopes and biochemistry are essential for studying cells.

• Eukaryotic cells have internal membranes that compartmentalize their functions.

• The genetic instructions of the cell are housed in the nucleus and carried out by ribosomes.

• The endomembrane system regulates protein traffic and performs metabolic functions.

• Mitochondria and chloroplasts are responsible for energy conversion.

• The cytoskeleton organizes structures and activities within the cell.

• A cell is greater than the sum of its parts, due to the integration of its components.

Learning Objectives

• Identify techniques of microscopy and cell fractionation.

• Compare and contrast prokaryotic and eukaryotic cells, as well as animal and plant cells.

• Describe the structure and function of the nucleus, chromosomes, and ribosomes.

• Identify the parts of the endomembrane system and their roles.

• Compare and contrast the structures and functions of mitochondria and chloroplasts.

• Describe the subunits and structures of the three types of cytoskeletal fibers and their functions.

• Explain how the parts of the cell work together.

Major Organelles and Their Functions

Nucleus: Information Central

The nucleus is the control center of the cell, containing most of the cell's genetic material in the form of DNA.

• Structure: Surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for molecular exchange.

• Function: Stores genetic information, coordinates cell activities such as growth and reproduction.

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

Ribosomes: Protein Factories

Ribosomes are complexes of rRNA and protein that synthesize polypeptides according to genetic instructions.

• Types: Free ribosomes (in cytosol) and bound ribosomes (attached to the endoplasmic reticulum or nuclear envelope).

• Function: Translate messenger RNA (mRNA) into proteins.

Endomembrane System

The endomembrane system is a group of membranes and organelles that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Function: Protein synthesis and transport, lipid metabolism, detoxification, and storage.

Endoplasmic Reticulum (ER)

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

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

Golgi Apparatus

• Structure: Stacks of flattened membranous sacs (cisternae).

• Function: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

Lysosomes

• Function: Contain hydrolytic enzymes for digestion of macromolecules, recycling of cellular components (autophagy), and defense against pathogens.

Vacuoles

• Types: Food vacuoles, contractile vacuoles (in protists), and central vacuole (in plants).

• Function: Storage, waste disposal, water balance, and cell growth.

Mitochondria and Chloroplasts: Energy Conversion

These organelles are responsible for energy transformation in eukaryotic cells.

• Mitochondria: Sites of cellular respiration; convert chemical energy from food into ATP using oxygen.

• Chloroplasts: Sites of photosynthesis in plants and algae; convert solar energy into chemical energy stored in glucose.

Endosymbiont Theory

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

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

Cytoskeleton

The cytoskeleton is a dynamic network of protein fibers that provides structural support, maintains cell shape, and facilitates movement.

• Types of Fibers: Microtubules, microfilaments, and intermediate filaments.

• Functions: Organize cell structure, anchor organelles, and enable intracellular transport and cell motility.

Surface Area-to-Volume Ratio

The surface area-to-volume ratio is a key factor in determining cell size and shape, affecting the efficiency of material exchange with the environment.

• Formula for a rectangular cell:

• As a cell increases in size, its volume grows faster than its surface area, limiting the rate of exchange.

• Elongated or flattened shapes increase surface area relative to volume, enhancing exchange efficiency.

Summary Table: Major Eukaryotic Organelles

Conclusion

Eukaryotic cells are highly organized structures with specialized organelles that work together to maintain life. Understanding their structure and function is fundamental to the study of biology.