Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, forming the basis for all living organisms. The study of cells and their components is essential for understanding biological processes.

• Cell: The basic unit of life, capable of performing all necessary functions for survival.

• Cell Theory: States that all living things are made up of cells, cells are the basic units of structure and function in living things, and new cells are produced from existing cells.

• Key Contributors: Hooke (coined "cell"), Van Leuwenhoek (first observed living cells), Schleiden (all plants are made of cells), Schwann (all animals are made of cells), Virchow (cells come from other cells).

Types of Cells

Cells are classified into two main types based on their internal structure: prokaryotes and eukaryotes.

• Prokaryote: Organisms lacking a nucleus and membrane-bound organelles. Examples: bacteria, archaea.

• Eukaryote: Cells containing a nucleus and membrane-bound organelles. Examples: protists, fungi, plants, animals.

• Nucleoid Region: Area within a prokaryotic cell where DNA is found.

• Capsule: Outermost sticky layer for prokaryotic cells, used for attachment.

• Pili: Short hairlike structures aiding in attachment.

• Flagella: Tail-like projections for movement, made of microtubules.

Microscopy and Cell Visualization

Microscopes are essential tools for studying cell structure.

• Light Microscope: Uses light to magnify specimens up to 1000X.

• Transmission Electron Microscope (TEM): Uses electrons to view internal cell structures at up to a million times magnification.

• Scanning Electron Microscope (SEM): Uses electrons to create 3D images of cell surfaces.

Cell Size and Surface Area

The efficiency of cellular transport depends on the ratio of surface area to volume.

• Volume > Surface Area: Cell transport becomes inefficient, threatening cell survival.

• Surface Area > Volume: Efficient transport of materials.

Cell Membrane and Its Structure

Cell Membrane Function and Structure

The cell membrane regulates the entry and exit of substances, maintaining cellular homeostasis.

• Phosphate Head: Hydrophilic and polar.

• Fatty Acid Tails: Hydrophobic and nonpolar.

• Fluid Mosaic Model: Describes the cell membrane as a flexible structure composed of various molecules.

Organelles and Their Functions

Endomembrane System

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

• Nuclear Envelope: Surrounds the nucleus.

• Endoplasmic Reticulum (ER): Includes rough and smooth ER.

• Golgi Apparatus: Modifies, sorts, and packages products from the ER.

• Lysosomes: Contain digestive enzymes.

• Vacuoles: Store food, nutrients, water, or waste.

• Plasma Membrane: Regulates transport.

Nucleus and Nucleolus

The nucleus is the control center of the cell, containing genetic material. The nucleolus is involved in ribosome synthesis.

• Nucleus: Contains DNA and controls cellular activities.

• Nucleolus: Synthesizes ribosomes.

Endoplasmic Reticulum (ER)

The ER is divided into rough and smooth types, each with distinct functions.

• Rough ER: Synthesizes secretory and membrane proteins.

• Smooth ER: Synthesizes lipids, breaks down glycogen, and processes toxins.

Ribosomes

Ribosomes are responsible for protein synthesis.

• Free Ribosomes: Synthesize proteins for use in the cytosol.

• Bound Ribosomes: Attached to the ER, synthesize proteins for secretion or membrane insertion.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for delivery.

• Function: Processes and ships products from the ER.

Lysosomes

Lysosomes contain hydrolytic enzymes for breaking down food and damaged cell parts.

• Function: Digestion and recycling of cellular materials.

Vesicles and Vacuoles

Vesicles transport materials within the cell, while vacuoles store substances.

• Vesicle: Transports materials.

• Vacuole: Stores food, nutrients, water, or waste.

• Contractile Vacuole: Pumps out excess water.

• Central Vacuole: In plant cells, stores large amounts of water.

Peroxisomes

Peroxisomes break down lipids and toxins using hydrogen peroxide.

• Function: Detoxification and lipid metabolism.

Mitochondria

Mitochondria are the site of cellular respiration, producing ATP.

• Function: Energy production through cellular respiration.

• Cellular Respiration Equation:

Chloroplasts

Chloroplasts are the site of photosynthesis in plant cells.

• Function: Converts light energy into chemical energy (sugar).

• Photosynthesis Equation:

• Chlorophyll: Green pigment in thylakoids that absorbs light energy.

Theory of Endosymbiosis

This theory proposes that mitochondria and chloroplasts originated as prokaryotes that formed symbiotic relationships with ancestral eukaryotic cells.

Cell Wall

The cell wall provides protection and support for plant cells and is made of cellulose.

• Function: Structural support and protection.

Plasmodesmata

Plasmodesmata are plant cell junctions that allow sharing of water, nutrients, and communication between cells.

Cytoskeleton and Cell Junctions

Cytoskeleton

The cytoskeleton is a network of proteins that provides shape, organization, and transport within the cell.

• Microtubules: Thick, hollow tubes made of tubulin; act as tracks for motor proteins and form centrioles, cilia, and flagella.

• Intermediate Filaments: Made of keratin; reinforce cell shape and anchor organelles.

• Microfilaments: Thin, solid rods made of actin; support cell shape and are involved in motility.

Cilia and Flagella

Cilia are hair-like structures involved in movement, while flagella are tail-like projections that help move cells.

Centrioles

Centrioles are microtubule structures involved in cell division in animal cells.

Extracellular Matrix (ECM)

The ECM provides support, movement, and regulation for cells.

• Collagen: Strong glycoprotein abundant in the ECM.

• Integrin: Transmembrane protein connecting cells to the ECM.

Cell Junctions

Cell junctions facilitate communication and adhesion between cells.

• Tight Junction: Prevents leakage of extracellular fluid.

• Anchoring Junction: Fastens cells together into sheets.

• Gap Junction: Channels that allow communication between animal cells.

Summary Table: Major Eukaryotic Cell Organelles

Additional info: Expanded explanations and context were added to ensure completeness and academic quality for exam preparation.

The Cell: Historical Foundations and Cell TheoryHistorical Figures in Cell BiologyThe study of cells began with the invention and use of microscopes, leading to foundational discoveries in biology.Robert Hooke (1665): Used a microscope to view cork and coined the term "cell."Anton van Leeuwenhoek (1674): Improved lenses and observed living cells and microorganisms.Matthias Schleiden (1838): Concluded all plants are made of cells.Theodor Schwann (1839): Concluded all animals are made of cells.Rudolph Virchow (1855): Stated all cells arise from pre-existing cells.The Cell TheoryCell theory is a fundamental concept in biology, stating:All living things are composed of one or more cells.Cells are the basic units of structure and function in organisms.Cells are produced only from pre-existing cells.Microscopy: Tools for Studying CellsMagnification and ResolutionMicroscopes are essential for visualizing cells and their structures. Magnification is the ratio of an object's image size to its real size, while resolution is the clarity of the image, defined as the minimum distance two points can be distinguished as separate.Types of MicroscopesLight Microscopes (LM): Use light to view specimens, magnify up to ~1000X, and can observe living cells.Electron Microscopes (EM): Use electron beams, magnify up to ~1,000,000X, and reveal internal structures but only for dead cells.TEM (Transmission Electron Microscope): Views internal cell structures.SEM (Scanning Electron Microscope): Produces 3D images of cell surfaces.Pros and Cons of MicroscopesEM Advantages: High magnification and resolution; reveals organelles.EM Disadvantages: Only for dead cells; expensive and large.LM Advantages: Can study live cells; affordable and portable.LM Disadvantages: Lower magnification and resolution.Parts of the MicroscopeEyepiece, pointer, body tube, revolving nose piece, stage, stage clips, diaphragm, lamp, arm, fine/coarse focus, base, objectives (scanning, low, high, oil immersion).Microscope Use and CareCarry with two hands; keep away from desk edge; clean with lens paper.Start with lowest power objective; use coarse then fine focus; adjust diaphragm.Never use coarse focus on high power; avoid 100X objective unless trained.Cell Structure and FunctionLimits Governing Cell SizeCell size is limited by transport efficiency. As cells grow, volume increases faster than surface area, reducing efficiency in nutrient and waste exchange.Small cells have a higher surface area-to-volume ratio, making them more efficient.Larger organisms have more cells, not larger cells.Plasma (Cell) MembraneThe plasma membrane is a phospholipid bilayer with embedded proteins, forming a selective barrier for the cell.Encloses cell contents.Regulates entry and exit of substances.Fluid-mosaic model: dynamic structure with proteins and cholesterol.Cell CharacteristicsAll cells have a plasma membrane, cytosol, DNA (chromosomes), and ribosomes.Cytoplasm includes cytosol and organelles; contains nutrients and enzymes.Cell Types: Prokaryote vs. EukaryoteCells are classified as prokaryotic or eukaryotic based on internal structure.Prokaryotic Cells: No nuclear membrane, no membrane-bound organelles, small (1-10 µm), found in bacteria and archaea.Eukaryotic Cells: Membrane-bound nucleus and organelles, larger (2-1000 µm), found in protists, fungi, plants, and animals.Prokaryotic Cell StructureNucleoid region (DNA), ribosomes, plasma membrane, cell wall, capsule, pili, flagella.Ribosomes differ from eukaryotes and are targets for antibiotics.Eukaryotic Cell StructureEukaryotic cells are more complex, with compartmentalized organelles for specialized functions.Genetic control: Nucleus, ribosomesManufacturing, distribution, breakdown: ER, Golgi, lysosomes, vacuoles, peroxisomesEnergy processing: Mitochondria, chloroplastsStructural support, movement, communication: Cytoskeleton, plasma membrane, cell wallPlant vs. Animal CellsPlant Cells: Cell wall, chloroplasts, large central vacuole, plasmodesmata, no lysosomes or centrioles.Animal Cells: No cell wall or chloroplasts, small vacuoles, lysosomes, centrioles.Eukaryotic Organelles and Endomembrane SystemNucleusThe nucleus is the control center, containing DNA (chromatin) and enclosed by a nuclear envelope. The nucleolus synthesizes ribosome subunits.Endoplasmic Reticulum (ER)Smooth ER: No ribosomes; synthesizes lipids, breaks down glycogen, processes toxins.Rough ER: Covered with ribosomes; synthesizes proteins for export or membrane insertion.RibosomesSite of protein synthesis; subunits made in nucleolus.Free ribosomes synthesize cytosolic proteins; bound ribosomes synthesize membrane/exported proteins.Golgi ApparatusStacks of flattened sacs that modify, sort, and package ER products into vesicles for export or membrane incorporation.LysosomesMembranous sacs containing digestive enzymes; fuse with food vacuoles or damaged organelles for intracellular digestion and recycling.Defects in lysosomal enzymes cause diseases (e.g., Pompe’s, Tay-Sachs).VacuolesFood vacuoles: Store nutrients.Contractile vacuoles: Expel excess water.Central vacuole (plants): Storage and water balance.PeroxisomesMetabolic compartments with enzymes to break down fatty acids and detoxify compounds; generate and degrade hydrogen peroxide ( 2H_2O_2 \rightarrow 2H_2O + O_2 ).Energy-Converting OrganellesMitochondriaSite of cellular respiration; double membrane with highly folded inner membrane (cristae); produces ATP from sugars.ChloroplastsSite of photosynthesis; contains chlorophyll; compartments include stroma, thylakoids, and grana.Endosymbiosis TheoryMitochondria and chloroplasts evolved from prokaryotes living within larger cells, evidenced by their double membranes, own DNA, ribosomes, and independent reproduction.Cell Walls and Surface StructuresPlant Cell WallsRigid, cellulose-based walls provide support and protection; plasmodesmata allow communication and transport between cells.Cytoskeleton and Cell SurfacesCytoskeletonNetwork of protein fibers organizing cell structure and movement.Microtubules: Hollow, thickest; shape cell, tracks for motor proteins, form centrioles, cilia, flagella.Intermediate filaments: Reinforce cell shape, anchor organelles.Microfilaments: Solid rods, thinnest; support shape, involved in motility.Cilia and FlagellaStructures for movement; cilia are short and numerous, flagella are longer and fewer. Both are composed of microtubules.CentriolesCylinders of microtubules in animal cells, involved in cell division.Extracellular Matrix (ECM)Animal cells have an ECM composed mainly of glycoproteins (especially collagen), providing support, movement, and regulation. ECM connects to the cytoskeleton via integrins.Cell Junctions in Animal TissuesTight junctions: Prevent leakage of extracellular fluid.Anchoring junctions: Fasten cells together into sheets.Gap junctions: Channels for communication between cells.Summary of Key ConceptsMicroscopes are essential for understanding cell structure and function.Cell theory defines the cell as the basic unit of life.Cell size is limited by surface area-to-volume ratio.Cell membranes are dynamic, selective barriers.Prokaryotic and eukaryotic cells differ in complexity and compartmentalization.Organelles perform specialized functions in eukaryotic cells.Plant and animal cells have distinct structural features.Mitochondria and chloroplasts likely evolved by endosymbiosis.Cytoskeleton and ECM provide internal and external support.Cell junctions enable communication and structural integrity in tissues.