A Tour of the Cell: Structure and Function of Animal Cells

Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, analogous to atoms in chemistry. All living organisms are composed of cells, which carry out essential biological processes.

• Definition: A cell is the smallest unit of life that can carry out all vital physiological functions.

• Key Features: All cells are surrounded by a plasma membrane, contain cytoplasm, genetic material (DNA), and ribosomes.

• Example: Human body cells, such as muscle cells and nerve cells, perform specialized functions.

Cell Size and Its Limitations

Why Are Cells Small?

Cell size is limited by the need to efficiently exchange materials with the environment and maintain homeostasis.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting the rate of material exchange.

• Metabolic Needs: Larger cells have greater metabolic demands but relatively less surface area for exchange.

• Formula: $\text{Surface Area} \propto r^2$ $\text{Volume} \propto r^3$ As radius increases, volume grows faster than surface area.

• Microvilli: Some cells have microvilli to increase surface area for absorption.

Additional info: Cells remain small to maximize efficiency in nutrient uptake and waste removal.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two major types based on their internal structure.

• Prokaryotic Cells: Lack a nucleus and most organelles; found in Bacteria and Archaea.

• Eukaryotic Cells: Have a nucleus and various membrane-bound organelles; found in animals, plants, fungi, and protists.

• Size Comparison: Prokaryotic cells are generally smaller (0.1–5 μm) than eukaryotic cells (8–100 μm).

Common Features of All Cells

Universal Cell Components

Despite differences, all cells share certain structural features.

• Plasma Membrane: Separates cell contents from the external environment.

• Cytoplasm: Jellylike substance (cytosol) containing organelles.

• DNA: Genetic material that directs cellular activities.

• Ribosomes: Sites of protein synthesis.

Structure of Animal Cells

Major Components and Their Functions

Animal cells contain specialized organelles that perform distinct functions.

• Plasma Membrane: Double layer of phospholipids with embedded proteins; selectively permeable to maintain homeostasis and facilitate signaling and adhesion.

• Cytoplasm & Cytosol: Region between the nucleus and plasma membrane; cytosol is the fluid portion containing organelles.

• Nucleus: Stores DNA; surrounded by a double nuclear envelope with pores for molecular exchange; contains nucleolus for ribosome synthesis.

• Ribosomes: Composed of rRNA and protein; free ribosomes synthesize intracellular proteins, while bound ribosomes (on rough ER) synthesize secretory and membrane proteins.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER (RER) has ribosomes and synthesizes proteins, smooth ER (SER) lacks ribosomes and synthesizes lipids, detoxifies drugs, and stores calcium.

• Golgi Apparatus: Series of flattened sacs; modifies, sorts, and packages proteins and lipids for transport.

• Transport Vesicles: Membrane-bound sacs that move molecules between organelles.

• Lysosomes: Specialized vesicles containing digestive enzymes for intracellular digestion.

• Peroxisomes: Contain enzymes for detoxification and breakdown of fatty acids; convert hydrogen peroxide to water and oxygen.

Plasma Membrane Structure

Phospholipid Bilayer and Proteins

The plasma membrane is a dynamic structure crucial for cell integrity and communication.

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Embedded within the bilayer; function as channels, receptors, and enzymes.

• Selective Permeability: Allows certain substances to pass more easily than others.

Endomembrane System

Components and Functions

The endomembrane system is a network of organelles involved in synthesis, modification, and transport of cellular materials.

• Includes: Nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Function: Coordinates protein and lipid synthesis, sorting, and transport.

• Flow of Substances: Proteins synthesized in ER → modified in Golgi → transported via vesicles to destinations (membrane, lysosome, secretion).

Organelle Functions Table

Summary of Major Animal Cell Organelles

Organelle	Main Function	Key Features
Plasma Membrane	Selective barrier, communication	Phospholipid bilayer, proteins
Nucleus	Stores genetic material	Double membrane, nucleolus
Ribosomes	Protein synthesis	Free or bound to ER
Rough ER	Synthesizes secretory proteins	Ribosome-studded
Smooth ER	Lipid synthesis, detoxification	No ribosomes
Golgi Apparatus	Modifies, sorts, ships proteins/lipids	Flattened sacs (cisternae)
Transport Vesicles	Move molecules between organelles	Membrane-bound sacs
Lysosomes	Intracellular digestion	Hydrolytic enzymes
Peroxisomes	Detoxification, fatty acid breakdown	Oxidative enzymes

Free vs. Bound Ribosomes

Differences in Location and Function

• Free Ribosomes: Float in cytosol; synthesize proteins for use within the cell.

• Bound Ribosomes: Attached to rough ER; synthesize proteins for secretion, membrane insertion, or use in organelles.

Flow of Substances Through the Endomembrane System

Protein Traffic and Membrane Flow

• Step 1: Proteins are synthesized by ribosomes on the rough ER.

• Step 2: Proteins are packaged into transport vesicles and sent to the Golgi apparatus.

• Step 3: Golgi apparatus modifies, sorts, and packages proteins for their final destination.

• Step 4: Proteins are delivered to the plasma membrane, lysosomes, or secreted outside the cell.

Additional info: This coordinated system ensures proper cellular function and response to environmental changes.