Cell Components

Introduction to Cell Structure and Function

The cell is the fundamental unit of structure and function in all living organisms. Cells are classified as either prokaryotic or eukaryotic, each with distinct structural features and functions. Understanding cell components is essential for grasping the complexity of biological systems.

• Prokaryotic cells: Found in Bacteria and Archaea; lack a nucleus and membrane-bound organelles.

• Eukaryotic cells: Found in protists, fungi, animals, and plants; possess a nucleus and membrane-bound organelles.

• Basic features of all cells: Plasma membrane, cytosol, chromosomes, and ribosomes.

Comparing Prokaryotic and Eukaryotic Cells

Prokaryotic and eukaryotic cells differ in their internal organization and complexity.

• Prokaryotic cells:

  • No nucleus; DNA located in the nucleoid region.

  • No membrane-bound organelles.

  • Cytoplasm bound by the plasma membrane.

• Eukaryotic cells:

  • DNA enclosed in a nucleus with a double membrane.

  • Membrane-bound organelles present.

  • Cytoplasm between plasma membrane and nucleus.

  • Generally larger than prokaryotic cells.

• Surface area to volume ratio: As cells increase in size, their volume grows faster than their surface area, affecting metabolic efficiency.

The Membrane

Plasma Membrane Structure and Function

The plasma membrane is a selective barrier that regulates the passage of substances into and out of the cell. It is composed of a phospholipid bilayer with embedded proteins and other lipids.

• Allows passage of oxygen, nutrients, and waste.

• Maintains cell integrity and supports cellular communication.

Cell Components

Eukaryotic Cell Organization

Eukaryotic cells contain internal membranes that compartmentalize functions, allowing for specialized environments and processes within the cell.

• Organelles: Specialized structures with distinct functions.

• Membrane composition: Double layer of phospholipids and other lipids.

• Plant and animal cells: Share most organelles, with some differences.

The Nucleus: Information Central

The nucleus is the control center of the cell, housing most of the genetic material and coordinating gene expression.

• Nuclear envelope: Double membrane separating nucleus from cytoplasm.

• Nuclear pores: Regulate entry and exit of molecules.

• Nuclear lamina: Protein network maintaining nuclear shape.

• Chromatin: DNA-protein complex; condenses into chromosomes during cell division.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis.

Ribosomes: Protein Factories

Ribosomes are responsible for protein synthesis, using genetic instructions from the nucleus.

• Structure: Composed of rRNA and protein.

• Locations:

  • Free ribosomes: In cytosol.

  • Bound ribosomes: Attached to endoplasmic reticulum or nuclear envelope.

The Endomembrane System

Components and Functions

The endomembrane system regulates protein traffic and performs metabolic functions. It includes several interconnected organelles.

• Nuclear envelope

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Lysosomes

• Vacuoles

• Plasma membrane

Endoplasmic Reticulum (ER)

• Smooth ER:

  • Synthesizes lipids.

  • Detoxifies drugs and poisons.

  • Stores calcium ions.

• Rough ER:

  • Has bound ribosomes; synthesizes glycoproteins.

  • Distributes transport vesicles.

  • Produces membranes for the cell.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and other products for transport.

• Structure: Flattened sacs called cisternae.

• Functions:

  • Modifies ER products.

  • Manufactures macromolecules.

  • Sorts and packages materials into vesicles.

Lysosomes

Lysosomes are digestive compartments containing hydrolytic enzymes.

• Digest macromolecules in acidic environments.

• Formed by rough ER and Golgi apparatus.

• Phagocytosis: Engulfing and digesting other cells.

• Autophagy: Recycling cell's own organelles and macromolecules.

Vacuoles

Vacuoles are large vesicles with diverse functions.

• Food vacuoles: Formed by phagocytosis.

• Contractile vacuoles: Pump excess water out of cells.

• Central vacuole: Stores inorganic ions and supports plant cell growth.

Energy and Metabolism

Mitochondria and Chloroplasts

Mitochondria and chloroplasts are organelles that convert energy from one form to another.

• Mitochondria: Site of cellular respiration; generates ATP using oxygen.

• Chloroplasts: Site of photosynthesis; found in plants and algae.

• Peroxisomes: Oxidative organelles; detoxify substances and break down fatty acids.

Endosymbiont Theory

This theory explains the evolutionary origins of mitochondria and chloroplasts.

• Suggests eukaryotes engulfed prokaryotic cells, forming endosymbionts.

• Evidence: Double membranes, free ribosomes, circular DNA, independent reproduction.

Mitochondria Structure

• Smooth outer membrane; inner membrane folded into cristae.

• Compartments: Intermembrane space and mitochondrial matrix.

• Cristae increase surface area for ATP synthesis.

Chloroplast Structure

• Contains chlorophyll and enzymes for photosynthesis.

• Structure: Thylakoids (stacked as granum), stroma (internal fluid).

• Part of plastid family in plants.

Peroxisomes

• Bounded by a single membrane.

• Enzymes transfer hydrogen to oxygen, forming hydrogen peroxide.

• Functions: Break down fatty acids, detoxify alcohol, convert fatty acids to sugars in seeds.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of fibers that organizes cell structures and activities, providing support and facilitating movement.

• Microtubules: Thickest fibers; shape cell, guide organelle movement, separate chromosomes.

• Microfilaments (Actin filaments): Thinnest fibers; support cell shape, involved in motility.

• Intermediate filaments: Middle diameter; permanent fixtures, support shape, anchor organelles.

Roles in Support and Motility

• Interact with motor proteins for cell movement.

• Vesicles travel along cytoskeletal tracks.

Microtubules

• Constructed from tubulin dimers.

• Functions: Cell shape, organelle movement, chromosome separation.

• Centrosomes and centrioles organize microtubules in animal cells.

Cilia and Flagella

• Microtubule-containing extensions for movement.

• Cilia: Numerous, short; Flagella: Few, long.

• Structure: Nine doublets in a ring, two singles in center, anchored by basal body, moved by dynein motor protein.

Microfilaments (Actin Filaments)

• Twisted double chain of actin subunits.

• Support cell shape, form cortex, core of microvilli.

• Involved in motility with myosin; enable crawling and cytoplasmic streaming.

Intermediate Filaments

• More permanent than other fibers.

• Support cell shape, fix organelles in place.

Extracellular Components and Cell Connections

Cell Walls of Plants

• Distinguish plant cells from animal cells.

• Made of cellulose, polysaccharides, and protein.

• Protect, maintain shape, prevent excessive water uptake.

• Layers: Primary cell wall, middle lamella (pectins), secondary cell wall (in some cells).

Extracellular Matrix (ECM) of Animal Cells

• Composed of glycoproteins (collagen, proteoglycans, fibronectin).

• Fibronectin binds to integrins in plasma membrane.

• Regulates cell behavior, gene activity, and mechanical signaling.

Cell Junctions

• Plasmodesmata: Channels connecting plant cells; allow passage of water, solutes, proteins, RNA.

• Tight junctions: Prevent leakage between animal cells.

• Desmosomes: Anchor cells together.

• Gap junctions: Provide cytoplasmic channels for communication.

Coordination of Cellular Activities

Integration of Cell Components

Cellular functions are coordinated among various components, such as the cytoskeleton, lysosomes, and plasma membrane, to achieve complex tasks like immune responses.

Summary Table: Cytoskeleton Structure and Function

Additional info:

• Cell size is limited by metabolic requirements and surface area to volume ratio.

• Endosymbiont theory is supported by structural and genetic similarities between mitochondria, chloroplasts, and bacteria.