Cellular Level of Organization: Structure and Function in Anatomy & Physiology

Chapter 3: The Cellular Level of Organization

Overview of Cell Structure

The cell is the fundamental unit of life in both animals and plants. All physiological processes occur within cells, making them the smallest unit capable of performing all vital functions. Cells are surrounded by extracellular fluid, specifically called interstitial fluid, and maintain their own internal environment through homeostasis.

• Cells compartmentalize functions to increase efficiency and specialization.

• Each cell is isolated from its environment by a membrane.

• Cells are the building blocks for tissues and organs.

Structure of a Representative Cell

A typical animal cell contains various organelles, each with specialized functions. The cytoplasm includes all internal components except the nucleus.

• Nucleus: Control center containing DNA.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Endoplasmic Reticulum (ER): Synthesizes proteins (RER) and lipids/carbohydrates (SER).

• Mitochondria: Site of ATP production.

• Lysosomes: Digestive organelles for waste removal.

• Peroxisomes: Break down fatty acids and detoxify chemicals.

• Ribosomes: Synthesize proteins.

• Proteasomes: Degrade damaged or misfolded proteins.

• Cytoskeleton: Provides structural support and facilitates movement.

• Centrioles: Organize microtubules during cell division.

• Microvilli: Increase surface area for absorption.

• Cell membrane: Encloses the cell and regulates interactions with the environment.

Cell Membrane

The cell membrane (plasma membrane) is a dynamic structure composed primarily of phospholipids, cholesterol, and membrane proteins. It serves as a barrier and interface between the cell and its surroundings.

• Structural support: Maintains cell shape and integrity.

• Physical isolation: Separates intracellular and extracellular environments.

• Creation of gradients: Allows cells to establish concentration differences.

• Regulation of exchange: Controls movement of substances in and out.

• Sensing the environment: Contains receptors for signal detection.

Phospholipids

Phospholipids are the main component of cell membranes. They are amphipathic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

• Structure: Consist of a diglyceride backbone attached to a phosphate group (hydrophilic head) and two fatty acid chains (hydrophobic tails).

• Function: Form bilayers that create a selective barrier for the cell.

• Arrangement in water: Can form micelles, liposomes, or bilayers depending on conditions.

Membrane Lipids and Structures

Phospholipids and glycolipids self-assemble in water to minimize energy, forming structures such as micelles, liposomes, and bilayers.

• Micelles: Spherical structures with hydrophilic heads facing outward.

• Liposomes: Spherical vesicles with a bilayer membrane.

• Bilayer: Two layers of phospholipids with hydrophobic tails facing inward and hydrophilic heads facing outward.

Fluid Mosaic Model

The fluid mosaic model describes the cell membrane as a flexible, dynamic structure with proteins and lipids moving laterally within the bilayer.

• Cholesterol: Stabilizes membrane fluidity.

• Integral proteins: Span the membrane and are involved in transport and signaling.

• Peripheral proteins: Attached to the membrane surface, involved in signaling and structural support.

Types of Membrane Proteins

• Anchoring proteins: Attach the membrane to other structures.

• Recognition proteins: Identify the cell to other cells.

• Receptor proteins: Bind specific molecules (ligands) for signaling.

• Carrier proteins: Transport substances across the membrane, often requiring energy.

• Channel proteins: Allow passive movement of water and ions.

Membrane Carbohydrates

Carbohydrates on the cell surface form the glycocalyx, which is important for cell recognition, protection, and adhesion.

• Glycolipids: Carbohydrates attached to lipids.

• Glycoproteins: Proteins with small carbohydrate groups.

• Proteoglycans: Proteins with large carbohydrate groups.

Types of Organelles

Organelles are specialized structures within the cell, classified as either membranous or non-membranous.

• Non-membranous organelles: Not surrounded by membranes; include cytoskeleton, microvilli, centrioles, cilia, ribosomes, proteasomes.

• Membranous organelles: Surrounded by lipid membranes; include ER, Golgi apparatus, lysosomes, peroxisomes, mitochondria, vesicles.

The Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, shape, and facilitates movement within the cell.

• Microfilaments (actin): Form networks under the plasma membrane.

• Intermediate filaments (e.g., keratin): Provide mechanical strength.

• Microtubules (tubulin): Support organelle movement and cell division.

• Thick filaments (myosin, actin): Involved in muscle contraction.

Microvilli

Microvilli are finger-like projections of the plasma membrane that increase surface area for absorption, especially in cells lining the intestine.

• Contain a core of microfilaments attached to a terminal web.

Centrioles and Cilia

• Centrioles: Organize microtubules during cell division (mitotic spindle).

• Cilia: Hair-like structures that move fluids across the cell surface; anchored by a basal body and contain microtubules.

Ribosomes

Ribosomes are the site of protein synthesis. They can be free in the cytoplasm or attached to the rough ER.

• Composed of protein and ribosomal RNA (rRNA).

• Free ribosomes synthesize cytoplasmic proteins; ER-bound ribosomes synthesize secreted and membrane proteins.

Endoplasmic Reticulum (ER)

• Rough ER (RER): Contains ribosomes; synthesizes and packages proteins.

• Smooth ER (SER): Involved in lipid and carbohydrate synthesis and detoxification.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Involved in protein glycosylation and membrane renewal.

• Forms lysosomes and secretory vesicles.

Proteasomes

Proteasomes degrade damaged or misfolded proteins tagged with ubiquitin.

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes. They break down endocytosed material, damaged organelles, and can digest the entire cell if necessary.

• Primary lysosomes: Inactive, contain enzymes.

• Secondary lysosomes: Activated upon fusion with target material.

• Involved in processes such as autophagy and exocytosis.

Vesicular Trafficking

Cells transport materials via vesicles in processes such as exocytosis (export) and endocytosis (import).

• Proteins and lipids are synthesized in the ER, processed in the Golgi, and delivered to their destinations.

• Misfolded proteins are sent to proteasomes for degradation.

Mitochondria

Mitochondria are the powerhouses of the cell, producing ATP through aerobic respiration.

• Glycolysis occurs in the cytoplasm: $\text{Glucose} \rightarrow \text{Pyruvate}$

• Krebs cycle (TCA) occurs in mitochondria: $\text{Pyruvate} \rightarrow \text{CO}_2$ (aerobic)

• Mitochondria produce 95% of cellular ATP.

The Nucleus

The nucleus is the control center of the cell, storing genetic information and coordinating cellular activities.

• Surrounded by a double membrane with nuclear pores.

• Contains DNA (chromatin) and histone proteins.

• Produces messenger RNA (mRNA) and ribosomal RNA (rRNA) in the nucleolus.

Gene Activation and Protein Synthesis

Protein synthesis involves transcription and translation, using the genetic code stored in DNA.

• Transcription: DNA is copied into mRNA in the nucleus.

• Translation: mRNA is decoded by ribosomes to assemble amino acids into proteins.

• Triplet code: Three nucleotides (codon) specify each amino acid.

Genetic Code Example

• DNA: AGCT

• RNA: AGCU

• Base pairing: A:T (or A:U in RNA), G:C

• Codon example: AUG UUU UGU

• Amino acid sequence: PHE-CYS-ALA

Summary Table: Major Cell Organelles and Functions

Organelle	Main Function
Nucleus	Stores genetic material, controls cell activities
Ribosome	Protein synthesis
Endoplasmic Reticulum (RER/SER)	Protein and lipid synthesis
Golgi Apparatus	Modification, sorting, packaging of proteins/lipids
Mitochondria	ATP production
Lysosome	Digestion of cellular waste
Peroxisome	Detoxification, fatty acid breakdown
Proteasome	Protein degradation
Cytoskeleton	Structural support, movement
Centrioles	Cell division, microtubule organization
Microvilli	Increase surface area

Additional info: Some details, such as the specific steps of vesicular trafficking and the full process of lysosome activation, were inferred and expanded for academic completeness.