Cell Structure: Principles, Types, and Functions

Overview

This study guide covers the fundamental aspects of cell structure, including the limitations to cell size, differences between prokaryotic and eukaryotic cells, the endomembrane and endosymbiont theories, organelle functions, cytoskeletal elements, cell walls, cellular junctions, and the relationship between the extracellular matrix and the cytoskeleton. These concepts are essential for understanding the organization and function of living organisms at the cellular level.

Limitations to Cell Size: Surface Area to Volume Ratio

Why Are Cells So Small?

• Surface Area to Volume Ratio (SA:V): As a cell increases in size, its volume grows faster than its surface area. This limits the efficiency of material exchange (nutrients, gases, waste) across the plasma membrane.

• Mathematical Relationship: For a sphere, surface area = , volume = . As radius increases, volume increases more rapidly than surface area.

• Functional Limitation: Cells must maintain a high SA:V ratio to efficiently exchange materials with their environment. Larger cells would not be able to move substances in and out quickly enough to support life.

Example: Microvilli in intestinal cells increase surface area to maximize absorption.

Microscopy: Tools for Studying Cells

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

• Resolution: The clarity of the image; the ability to distinguish two close points as separate.

• Contrast: The difference in brightness between parts of the image, enhancing visibility of structures.

Types of Microscopy:

Types of Cells: Prokaryotes vs. Eukaryotes

Fundamental Differences

• Prokaryotes: "Pro" = before, "karyote" = nucleus. Lack a membrane-bound nucleus and organelles. DNA is located in a nucleoid region.

• Eukaryotes: "Eu" = true, "karyote" = nucleus. Have a membrane-bound nucleus and various organelles.

Structure of Prokaryotic Cells

• Nucleoid: Region containing circular DNA, not surrounded by a membrane.

• Cell Wall: Composed of peptidoglycan in bacteria; provides structural support.

• Capsule: Protective outer layer made of polysaccharides and proteins (in some bacteria).

• Ribosomes: Sites of protein synthesis, free in cytoplasm.

• Plasmids: Small, circular DNA molecules carrying nonessential genes (e.g., antibiotic resistance).

Structure of Eukaryotic Cells

• Nucleus: Contains linear DNA organized into chromosomes; surrounded by a double membrane (nuclear envelope).

• Organelles: Membrane-bound structures with specialized functions (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) providing structure and facilitating movement.

• Cell Wall: Present in plants (cellulose) and fungi (chitin), absent in animals.

Origin of Eukaryotic Cells

Endomembrane Theory

• Proposes that the endomembrane system (nuclear envelope, ER, Golgi, etc.) originated from infoldings of the plasma membrane in an ancestral prokaryote.

• These infoldings increased surface area and eventually separated from the plasma membrane, forming internal compartments.

Endosymbiont Theory

• Suggests that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by an ancestral eukaryotic cell.

• Evidence includes double membranes, their own circular DNA, and similarities to certain bacteria.

Cellular Organelles and Their Functions

Nucleus

• Function: Stores genetic material (DNA); site of transcription (RNA synthesis).

• Nuclear Envelope: Double membrane with nuclear pores for molecular transport.

• Nucleolus: Dense region where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Chromatin: DNA-protein complex; can be euchromatin (active, loosely packed) or heterochromatin (inactive, tightly packed).

Ribosomes

• Function: Protein synthesis; composed of rRNA and proteins.

• Location: Free in cytoplasm or bound to rough ER.

Endomembrane System

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

• Function: Synthesis, modification, and transport of proteins and lipids.

Mitochondria and Chloroplasts

• Mitochondria: Site of cellular respiration and ATP production.

• Chloroplasts: Site of photosynthesis in plants and algae.

Cytoskeleton: Structure and Function

• Microtubules: Hollow tubes of tubulin; maintain cell shape, facilitate intracellular transport, and form spindle fibers during cell division.

• Microfilaments (Actin Filaments): Thin strands of actin; involved in cell movement and muscle contraction.

• Intermediate Filaments: Rope-like fibers; provide mechanical strength and maintain cell integrity.

Cell Walls

• Bacteria: Peptidoglycan cell wall provides rigidity and protection.

• Fungi: Chitin-based cell wall.

• Plants: Cellulose-based cell wall; provides structural support and protection.

Cellular Junctions

• Tight Junctions: Seal adjacent cells to prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together, providing mechanical stability.

• Gap Junctions: Allow direct communication between animal cells via cytoplasmic channels.

• Plasmodesmata: Channels between plant cells for transport and communication.

Extracellular Matrix (ECM) and Cytoskeleton Linkage

• ECM: Network of proteins (e.g., collagen, fibronectin) and polysaccharides outside animal cells.

• Function: Provides structural support, regulates cell behavior, and connects to the cytoskeleton via integrin proteins.

Summary Table: Prokaryotic vs. Eukaryotic Cell Features

Additional info: Some details about the structure and function of organelles, as well as the endomembrane and endosymbiont theories, have been expanded for academic completeness.