Cell Structure and Function: Cytoplasm and Organelles

Cells: The Living Units

Introduction

Cells are the fundamental units of life in all living organisms. They carry out essential processes that sustain life, and their structure is closely related to their function. This section focuses on the cytoplasm and the various organelles found within eukaryotic cells.

Cytoplasm

Definition and Components

The cytoplasm is the cellular material located between the plasma membrane and the nucleus. It serves as the site for most cellular activities and contains several key components:

• Cytosol: A gel-like solution composed mainly of water and dissolved solutes such as proteins, salts, and sugars. It provides a medium for chemical reactions.

• Inclusions: Insoluble molecules that vary with cell type, including glycogen granules, pigments, lipid droplets, vacuoles, and crystals. These serve as storage or have specialized functions.

• Organelles: Specialized structures that perform distinct metabolic functions within the cell. Organelles can be membranous (surrounded by membranes) or nonmembranous (not surrounded by membranes).

Cytoplasmic Organelles

Classification

Organelles are classified based on the presence or absence of surrounding membranes:

• Membranous Organelles: Mitochondria, Endoplasmic Reticulum (ER), Golgi Apparatus, Peroxisomes, Lysosomes

• Nonmembranous Organelles: Ribosomes, Cytoskeleton, Centrioles

Membranes allow compartmentalization, which is crucial for efficient cellular functioning.

Mitochondria

Structure and Function

Mitochondria are often called the "power plants" of the cell because they produce most of the cell's energy molecules, adenosine triphosphate (ATP), via aerobic (oxygen-requiring) cellular respiration.

• Enclosed by double membranes; the inner membrane has many folds called cristae.

• Cristae are embedded with membrane proteins that play a key role in cellular respiration.

• Mitochondria contain their own DNA, RNA, and ribosomes, resembling bacteria and capable of dividing by fission.

Example:

Muscle cells have a high number of mitochondria to meet their energy demands.

Ribosomes

Structure and Function

Ribosomes are nonmembranous organelles responsible for protein synthesis. They are composed of protein and ribosomal RNA (rRNA).

• Free ribosomes: Float freely in the cytosol; synthesize soluble proteins that function within the cytosol or other organelles.

• Membrane-bound ribosomes: Attached to the membrane of the endoplasmic reticulum (ER); synthesize proteins destined for membranes, lysosomes, or export from the cell.

Endoplasmic Reticulum (ER)

Structure

The endoplasmic reticulum (ER) is a network of parallel, interconnected cisterns—flattened membranous tubes that enclose fluid-filled interiors. It is continuous with the outer nuclear membrane and exists in two varieties:

• Rough ER: Studded with ribosomes; site of synthesis for proteins to be secreted from the cell, incorporated into plasma membranes, or sent to organelles.

• Smooth ER: Lacks ribosomes; involved in lipid metabolism, synthesis of cholesterol and steroid hormones, absorption and transport of fats, detoxification of chemicals, conversion of glycogen to glucose, and storage/release of calcium (especially in muscle cells as the sarcoplasmic reticulum).

Golgi Apparatus

Structure and Function

The Golgi apparatus consists of stacked and flattened membranous sacs. It modifies, concentrates, and packages proteins and lipids received from the rough ER.

• Transport vesicles from ER fuse with the cis (inner) face of the Golgi.

• Proteins and lipids are further modified, tagged, sorted, and packaged.

• The Golgi directs the final products to one of three pathways:

Pathway	Description
Pathway A	Secretory vesicles containing proteins fuse with the plasma membrane and release contents via exocytosis.
Pathway B	Vesicles containing lipids or transmembrane proteins fuse with plasma or organelle membranes, inserting contents directly.
Pathway C	Lysosomes containing digestive enzymes remain in the cell until needed.

Peroxisomes

Structure and Function

Peroxisomes are membranous sacs containing powerful detoxifying substances that neutralize toxins.

• Contain enzymes such as oxidase (converts toxins to hydrogen peroxide, H2O2) and catalase (converts H2O2 to water).

• Play a role in the breakdown and synthesis of fatty acids.

Lysosomes

Structure and Function

Lysosomes are spherical membranous bags containing digestive enzymes (acid hydrolases).

• Isolate potentially harmful intracellular digestion from the rest of the cell.

• Digest ingested bacteria, viruses, and toxins.

• Degrade nonfunctional organelles.

• Break down and release glycogen and calcium from bone.

• Can cause autolysis (cell self-digestion) if released into the cytoplasm.

Example: Lysosomal Storage Disease

Tay-Sachs disease is caused by a deficiency in a lysosomal enzyme needed to break down glycolipids in brain cells, leading to nervous system dysfunction.

Endomembrane System

Components and Functions

The endomembrane system includes the ER, Golgi apparatus, secretory vesicles, lysosomes, nuclear envelope, and plasma membrane. These structures work together to:

• Produce, degrade, store, and export biological molecules.

• Degrade potentially harmful substances.

Cytoskeleton

Structure and Types

The cytoskeleton is an elaborate network of rods throughout the cytosol, providing structural support and facilitating movement of cell components. It consists of:

• Microfilaments: Thinnest elements, composed of actin; involved in cell shape, motility, and endocytosis/exocytosis.

• Intermediate filaments: Ropelike fibers, provide tensile strength; attach to desmosomes and act as internal guy-wires.

• Microtubules: Largest elements, hollow tubes made of tubulin; determine cell shape, organelle distribution, and serve as tracks for motor proteins.

Motor Proteins

Motor proteins use microtubules as tracks to move organelles and substances around the cell, powered by ATP.

Centrosome and Centrioles

Structure and Function

The centrosome is a microtubule organizing center near the nucleus, consisting of a granular matrix and a pair of barrel-shaped centrioles. Centrioles form the basis of cilia and flagella and play a role in cell division.

Cellular Extensions

Cilia, Flagella, and Microvilli

• Cilia: Whiplike, motile extensions on cell surfaces; move substances across the cell surface in a coordinated manner.

• Flagella: Longer extensions that propel the entire cell (e.g., sperm tail).

• Microvilli: Minute, fingerlike extensions of the plasma membrane that increase surface area for absorption; contain a core of actin microfilaments for structural support.

Structural Patterns

• Cilia and flagella have a "9 + 2" pattern of microtubules (nine doublets surrounding a central pair).

• Centrioles have a "9 + 0" pattern (nine triplets, no central pair).

Nucleus

Structure and Function

The nucleus is the largest organelle, containing the genetic library for synthesis of nearly all cellular proteins. It responds to signals that dictate protein synthesis. Most cells are uninucleate, but some are multinucleate (e.g., skeletal muscle), and red blood cells are anucleate.

Main Structures

• Nuclear Envelope: Double-membrane barrier enclosing the nucleoplasm; outer layer is continuous with rough ER and studded with ribosomes; inner layer (nuclear lamina) maintains nuclear shape; nuclear pores regulate transport.

• Nucleoli: Dark-staining spherical bodies involved in rRNA synthesis and ribosome subunit assembly; usually one or two per cell.

• Chromatin: Composed of DNA (30%), histone proteins (60%), and RNA (10%); arranged in nucleosomes (DNA wrapped around histones); chemical modifications regulate gene expression. Chromosomes are condensed chromatin, protecting DNA during cell division.

Additional info:

• ATP production in mitochondria can be summarized by the equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP$

• Protein synthesis by ribosomes involves translation of mRNA into polypeptide chains.