Inside the Cell: Structure and Function of Cellular Components

Study Guide - Smart Notes

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UNIT 2: Chapter 7 – Inside the Cell

Overview

This chapter explores the internal structures of cells, emphasizing how life's properties emerge from the collaboration of these components. The study of cell structure and function is fundamental to understanding biology at the molecular and organismal levels.

Prokaryotic and Eukaryotic Cell Structures

Prokaryotic Cells

Prokaryotic cells are simpler and lack membrane-bound organelles. They include bacteria and archaea.

Chromosome: Usually a single, circular DNA molecule located in the nucleoid region (not membrane-bound).
Plasmids: Small, circular DNA molecules that carry additional genes.
Ribosomes: Macromolecular machines made of RNA and protein; site of protein synthesis.
Photosynthetic Membranes: Some prokaryotes have internal membranes for photosynthesis, developed from folds of the plasma membrane.
Cytoskeleton: Protein fibers that help maintain cell shape and assist in cell division.
Plasma Membrane: Phospholipid bilayer with embedded proteins; selectively permeable barrier.
Cell Wall: Tough, fibrous layer (often peptidoglycan) providing structural support and protection.
External Structures: Flagella (for movement) and fimbriae (for attachment).

Example: Escherichia coli (E. coli) is a well-studied prokaryote with all these features.

Eukaryotic Cells

Eukaryotic cells are more complex, with membrane-bound organelles. They include plants, animals, fungi, and protists.

Nucleus: Double-membrane organelle containing chromosomes; site of DNA storage and processing.
Ribosomes: Free in cytosol or bound to endoplasmic reticulum; synthesize proteins.
Endoplasmic Reticulum (ER): Network of membranes; rough ER (with ribosomes) synthesizes proteins, smooth ER synthesizes lipids and detoxifies molecules.
Golgi Apparatus: Stacked, flat membranous sacs (cisternae); processes, sorts, and ships proteins and lipids.
Lysosomes: Contain hydrolytic enzymes for digestion and recycling (mainly in animal cells).
Vacuoles: Large, membrane-bound structures in plants and fungi; store water, ions, and other substances.
Peroxisomes: Organelles for oxidation reactions; detoxify harmful substances.
Mitochondria: Double-membrane organelles; site of ATP production via cellular respiration.
Chloroplasts: Found in plants and algae; site of photosynthesis.
Cytoskeleton: Network of protein fibers (actin filaments, intermediate filaments, microtubules) for cell shape, movement, and organization.
Cell Wall: Found in plants, fungi, and some protists; provides structural support.

Example: Animal cells have mitochondria and lysosomes, while plant cells have chloroplasts and large central vacuoles.

Compartmentalization and Benefits of Organelles

Advantages of Membrane-Bound Organelles

Organelles allow eukaryotic cells to compartmentalize functions, increasing efficiency and separating incompatible chemical reactions.

Separation of reactions: For example, lysosomes digest macromolecules without harming other cellular components.
Efficiency: Enzymes and substrates are concentrated, speeding up reactions.

Nucleus: Structure and Function

Nuclear Envelope and Transport

The nucleus is surrounded by a double membrane (nuclear envelope) with nuclear pores for transport.

Nucleolus: Region where ribosomal RNA is synthesized and ribosome subunits are assembled.
Nuclear Pores: Allow selective transport of molecules (e.g., mRNA, ribosomal subunits) between nucleus and cytoplasm.
Nuclear Localization Signal (NLS): Sequence that directs proteins into the nucleus.

Endomembrane System

Components and Functions

The endomembrane system includes the ER, Golgi apparatus, and lysosomes, coordinating protein and lipid synthesis, processing, and transport.

Rough ER: Synthesizes proteins for secretion or membrane insertion.
Smooth ER: Synthesizes lipids, detoxifies chemicals, stores calcium.
Golgi Apparatus: Modifies, sorts, and ships proteins and lipids.
Lysosomes: Digest and recycle cellular materials.

Signal Hypothesis: Proteins destined for secretion have a signal sequence that directs them to the ER.

Protein Sorting and Vesicle Transport

Proteins are tagged for specific destinations (e.g., lysosome, plasma membrane).
Transported in vesicles that bud from one organelle and fuse with another.

Material Recycling in the Cell

Lysosomal Pathways

Receptor-mediated endocytosis: Uptake of specific molecules via receptors.
Phagocytosis: Engulfment of large particles or cells.
Autophagy: Digestion of damaged organelles and cytoplasmic components.

Cytoskeleton: Structure and Dynamics

Types of Cytoskeletal Fibers

Actin Filaments (Microfilaments): Thin fibers involved in cell movement and shape; interact with myosin for muscle contraction and cytokinesis.
Intermediate Filaments: Provide structural support; include nuclear lamins.
Microtubules: Hollow tubes of tubulin; serve as tracks for organelle and vesicle movement, form spindle fibers during cell division.

Motor Proteins and Movement

Kinesin: Motor protein that "walks" along microtubules, transporting vesicles.
Dynein: Motor protein involved in cilia and flagella movement.

Cellular Appendages

Flagella: Long, whip-like structures for movement; differ in structure between prokaryotes and eukaryotes.
Cilia: Short, hair-like projections for movement or fluid transport.

Endosymbiosis Theory

Origin of Mitochondria and Chloroplasts

The endosymbiosis theory proposes that mitochondria and chloroplasts originated as free-living bacteria engulfed by ancestral eukaryotic cells.

Both organelles contain their own DNA and ribosomes.
They replicate independently of the cell cycle.

Structure-Function Relationships in Cells

Specialization of Organelles

Cells have organelles tailored to their function (e.g., muscle cells have many mitochondria for ATP production).
Plant cells have chloroplasts for photosynthesis and large vacuoles for storage.

Summary Table: Prokaryotic vs. Eukaryotic Cell Structures

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present (membrane-bound)
Organelles	Absent (few exceptions)	Present (ER, Golgi, mitochondria, etc.)
DNA Location	Nucleoid region	Nucleus
Cell Wall	Usually present (peptidoglycan)	Present in plants, fungi, some protists (cellulose, chitin)
Size	Smaller (1–10 μm)	Larger (10–100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Key Equations

Surface Area to Volume Ratio:
ATP Synthesis (Simplified):

Additional info:

Some context and terminology were inferred from standard biology textbooks to clarify fragmented notes.
Scientific names and examples were added for completeness.