Cell Structure Review (Chapter 4)
CHAPTER 4 Cell Structure
4.1 Studying Cells
Definition of a Cell:
A cell is the smallest fundamental unit of structure and function in a living thing.
Organisms can be unicellular (like bacteria, consisting of a single cell) or multicellular (like humans, forming a complex structure from various specialized cells).
Cell Specialization:
Different types of cells exist, each highly specialized for specific functions within a multicellular organism.
Examples of specialized cells:
Epithelial cells: Form protective barriers and linings in surfaces and cavities, often involved in secretion or absorption.
Bone cells (osteocytes): Provide structural support, protection, and calcium storage, embedded in a mineralized matrix.
Immune cells (e.g., lymphocytes, macrophages): Identify and fight infections and foreign invaders through various defense mechanisms.
Blood cells (e.g., erythrocytes, leukocytes): Transport oxygen, carbon dioxide, nutrients, and waste products, and play roles in immune responses.
Common Characteristics of Cells:
Despite their diversity, all cells share fundamental characteristics, including a plasma membrane, cytoplasm, genetic material (DNA), and ribosomes.
Chapter Outline:
4.1 Studying Cells
4.2 Prokaryotic Cells
4.3 Eukaryotic Cells
4.4 The Endomembrane System and Proteins
4.5 The Cytoskeleton
4.6 Connections between Cells and Cellular Activities
4.2 Prokaryotic Cells
Definition of Prokaryotic Cells:
Classified as simple, mostly unicellular organisms, primarily found in the domains Bacteria and Archaea.
They are characterized by the absence of a membrane-bound nucleus and other membrane-bound organelles.
Common Components of All Cells:
Plasma membrane: The outer covering that separates the cell’s interior (cytoplasm) from its external environment, regulating the passage of substances.
Cytoplasm: The jelly-like substance (cytosol) filling the cell, where many metabolic reactions occur.
DNA: The genetic material of the cell, organized into chromosomes, containing the instructions for cell function and replication.
Ribosomes: Molecular machines responsible for protein synthesis (translation) based on genetic instructions.
Differences Between Prokaryotic and Eukaryotic Cells:
Prokaryotic DNA is typically a single circular chromosome located in a specialized region called the nucleoid, without an enclosing membrane; eukaryotic DNA is found in multiple linear chromosomes within a membrane-bound nucleus.
Size of Prokaryotic Cells:
Typically very small, between 0.1 to 5.0 \mum in diameter, which is about one-tenth the diameter of eukaryotic cells.
This small size confers advantages for the efficient diffusion of substances (nutrients, waste) within the cell, as surface area to volume ratio is optimized.
Structural Features of Prokaryotic Cells:
Many prokaryotes have a peptidoglycan cell wall that provides protection, maintains cell shape, and prevents dehydration.
Some possess flagella (long, whip-like appendages) for locomotion through liquid environments.
Pili (shorter, hair-like structures) are used for attachment to surfaces and other cells, including facilitating gene transfer between bacteria through conjugation.
Fimbriae are even shorter bristles that help bacteria adhere to host cells and other surfaces.
4.3 Eukaryotic Cells
Key Characteristics of Eukaryotic Cells:
Membrane-bound nucleus: Houses the cell's genetic material (DNA) and controls cell activities.
Numerous membrane-bound organelles: Specialized compartments that perform distinct functions, allowing for compartmentalization of cellular processes and increased efficiency.
Rod-shaped chromosomes: Linear DNA molecules tightly coiled around proteins to form structures visible during cell division.
Role of the Plasma Membrane:
A phospholipid bilayer embedded with various proteins and carbohydrates, forming a selectively permeable barrier.
It controls the passage of materials (nutrients, ions, waste) in and out of the cell, maintains cellular homeostasis, and plays roles in cell-cell recognition and signaling.
Major Eukaryotic Organelles:
Nucleus:
The most prominent organelle, containing chromatin (a complex of DNA and proteins).
The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and assembled with proteins to form ribosomes.
Enclosed by a nuclear envelope, a double membrane perforated by nuclear pores that regulate the transport of molecules between the nucleus and cytoplasm.
Ribosomes:
Sites of protein synthesis.
Found in two locations: free ribosomes in the cytosol, typically synthesizing proteins that function within the cytosol; or attached ribosomes to the rough endoplasmic reticulum, synthesizing proteins destined for secretion, insertion into membranes, or delivery to certain organelles.
Mitochondria:
Often called the "powerhouses" of the cell, responsible for ATP (adenosine triphosphate) production through cellular respiration.
They have a double membrane system: a smooth outer membrane and a highly folded inner membrane (cristae) that increases surface area for ATP synthesis.
Uniquely, mitochondria have their own circular DNA and ribosomes, supporting the endosymbiotic theory.
Peroxisomes:
Small, spherical organelles that carry out oxidation reactions, breaking down fatty acids and detoxifying harmful substances (like alcohol) in liver cells.
They produce hydrogen peroxide (\text{H}2\text{O}2) as a byproduct, which is then converted into water and oxygen by the enzyme catalase.
Golgi Apparatus (or Golgi Complex):
A stack of flattened membrane-bound sacs called cisternae.
It receives vesicles containing proteins and lipids from the ER at its cis face.
It then further modifies, sorts, and packages these molecules into new secretory or transport vesicles at its trans face for delivery to their final destinations.
Plays a key role in the glycosylation of proteins and lipids.
Vesicles and Vacuoles:
Vesicles: Small, membrane-bound sacs involved in storage and transport of substances within the cell.
Vacuoles: Larger sacs, especially prominent in plant cells, where a large central vacuole stores water, nutrients, and waste, and contributes to turgor pressure against the cell wall, providing structural support.
Cytoskeleton:
A network of protein filaments and tubules throughout the cytoplasm.
Maintains cell shape, anchors organelles, and enables various forms of cellular movement.
Consists of:
Microfilaments (actin filaments): The thinnest, involved in muscle contraction, cell division (cytokinesis), and maintaining cell shape.
Intermediate filaments: Provide structural support, bear tension, and help anchor organelles, forming a strong framework.
Microtubules: The thickest hollow tubes, serve as tracks for motor proteins (e.g., dynein, kinesin) to move vesicles, form the core of cilia and flagella, and organize chromosomes during cell division (spindle fibers).
Specialized Structures:
Centrosomes and Centrioles: (primarily in animal cells) Centrosomes are microtubule-organizing centers; centrioles within them are involved in cell division and the formation of cilia and flagella basal bodies.
Cell Walls: (in plant cells) A rigid outer layer made primarily of cellulose, providing structural support, protection, and preventing excessive water uptake.
Chloroplasts: (in plant cells and algae) Sites of photosynthesis, converting light energy into chemical energy.
Large Central Vacuoles: Unique to plant cells, as described above.
4.4 The Endomembrane System and Proteins
Definition of the Endomembrane System:
A group of membranes and organelles within eukaryotic cells that work together to modify, package, and transport proteins and lipids. These components are either directly connected or communicate via the transfer of vesicles.
Includes: the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles, and it interacts directly with the plasma membrane.
Endoplasmic Reticulum (ER):
Rough ER (RER):
Studded with ribosomes on its cytoplasmic surface, giving it a "rough" appearance.
Primarily involved in the synthesis and modification of proteins destined for secretion, insertion into membranes, or delivery to other organelles (like lysosomes or the Golgi).
Also synthesizes phospholipids for membranes and aids in protein folding and quality control (with the help of chaperone proteins).
Smooth ER (SER):
Lacks ribosomes, hence its "smooth" appearance.
Involved in diverse metabolic processes, including the synthesis of carbohydrates, lipids (e.g., steroids, phospholipids), and detoxification of drugs and poisons (especially in liver cells).
Also stores calcium ions, which are crucial for muscle contraction and other cellular responses.
Golgi Apparatus:
Receives transport vesicles containing newly synthesized proteins and lipids from the ER at its cis face.
It then further modifies, sorts, and packages these molecules into new secretory or transport vesicles at its trans face for delivery to their final destinations.
Plays a key role in the glycosylation of proteins and lipids.
Lysosomes:
Often called the cell's "recycling centers" or "digestive organelles."
Membrane-bound sacs containing powerful hydrolytic enzymes that break down waste materials, cellular debris, worn-out organelles (autophagy), and foreign invaders (phagocytosis).
They are crucial for cellular waste disposal and defense.
4.5 The Cytoskeleton
Components of the Cytoskeleton:
Microfilaments (Actin Filaments):
Composed of actin protein, they are the narrowest components of the cytoskeleton, with a diameter of about 7 nm.
They function in cellular movements (e.g., muscle contraction via interaction with myosin, amoeboid movement), maintaining cell shape, forming cellular extensions like microvilli, and pinching the cell during division (cytokinesis).
Intermediate Filaments:
Intermediate in size (8-12 nm in diameter) and more permanent than microfilaments or microtubules.
Composed of various proteins (e.g., keratin, vimentin), they provide structural support, withstand tension, and help anchor organelles within the cell.
They form a strong, durable framework that reinforces cell shape.
Microtubules:
Hollow cylinders made of tubulin protein, with a diameter of about 25 nm.
They serve as tracks along which motor proteins (dynein and kinesin) move vesicles and other organelles, provide structural support, form the spindle fibers that separate chromosomes during cell division, and are the main components of cilia and flagella.
Cilia and Flagella:
Both are specialized cellular appendages responsible for movement, built from microtubules arranged in a characteristic "9+2" pattern (nine doublets of microtubules surrounding a central pair).
Flagella: Long, whip-like structures typically few in number, used for propelling the entire cell through a fluid medium (e.g., sperm cells).
Cilia: Shorter, hair-like structures, usually numerous, that can either move substances along the cell surface (e.g., in the trachea) or be used for cellular locomotion.
4.6 Connections between Cells and Cellular Activities
Extracellular Matrix (ECM):
A complex network of macromolecules secreted by cells, forming a structural and biochemical support system outside of the cells in animal tissues.
The ECM is primarily made up of proteins such as collagen (for strength), elastin (for elasticity), and fibronectin (for cell adhesion), along with glycoproteins and proteoglycans (for hydration).
It provides structural support, regulates cell behavior (e.g., adhesion, migration, differentiation), and facilitates communication between cells.
Types of Intercellular Junctions:
Plasmodesmata (in plant cells):
Channels that directly connect the cytoplasm of adjacent plant cells through their cell walls.
They allow for the rapid transport of water, small solutes, and even some larger molecules (proteins, RNA) between cells, facilitating intercellular communication and nutrient distribution.
Tight Junctions (in animal cells):
Formed by integral membrane proteins (e.g., claudins and occludins) that create a water-tight seal between adjacent cells, effectively preventing the passage of fluids and solutes between them.
Crucial in tissues where leakage must be prevented, such as the epithelial lining of the intestine or the blood-brain barrier.
Desmosomes (in animal cells):
Function like "spot welds" or rivets, joining cells at localized points.
They anchor cells together by connecting their intermediate filaments (e.g., keratin filaments) from the cytoplasm to the plasma membrane, providing strong adhesion and resisting mechanical stress in tissues like muscle or skin.
Gap Junctions (in animal cells):
Channels formed by protein complexes called connexons that create direct cytoplasmic connections between adjacent cells.
They allow for the rapid passage of ions, small molecules, and electrical impulses, facilitating chemical and electrical communication between cells, particularly in cardiac muscle and neural tissues.
Conclusion on Cell Communication:
Intercellular junctions and the ECM are vital for coordinating activities and responses in tissues and organs, enabling multicellular organisms to function as integrated units.
Key Terms
Prokaryotic:
Simple, predominantly unicellular organisms that lack a membrane-bound nucleus and other membrane-bound organelles. Example: Bacteria and Archaea.
Eukaryotic:
Cells characterized by the presence of membrane-bound organelles, including a true nucleus that houses the genetic material. Examples: Animal cells, plant cells, fungi, protists.
Microscopy:
A set of techniques and tools (like light microscopes and electron microscopes) utilized for visualizing cells and their components, which are too small to be seen with the naked eye.
Cell Theory:
A fundamental concept in biology stating that all living things are composed of one or more cells, that the cell is the basic unit of life, and that all new cells arise from existing cells.
Chapter Summary
Cells form the fundamental unit of life, facilitating various functions through specialized structures and organizational complexities. The