unit 5- Comparing Components of Cells

Discovery of Cells

• 1660’s: Robert Hooke builds a microscope.

• Observed cork, discovered small compartments named "cells" after the rooms occupied by monks.

• Noted that all organisms are composed of cells.

Cell Theory

• Fundamental Units of Life:

  • Cells are essential for life; organisms without cells are not alive.

  • Origin of life correlates with the origin of cells.

• Composition of Organisms:

  • All organisms are made of cells.

  • Studying cells relates directly to understanding life.

• Continuity of Life:

  • All cells originate from pre-existing cells.

  • Life is continuous, with all cells stemming from the first cell.

Cells Are Very Small

• Typical size range: 1-100 µm (micrometers).

• 1000 µm = 1 mm.

• Exceptions: Some algae cells can be seen without aid.

Why Are Cells So Tiny?

• Size Limitations:

  • Restricted by the surface area-to-volume ratio.

  • As objects grow, their volume increases faster than surface area.

  • Larger volumes require more chemical reactions, increasing demand for materials and waste elimination.

• Surface Area Constraints:

  • Cell membrane limits the efficiency of material absorption and waste disposal.

Use of Microscopes

• Key Factors:

  • Magnification: Image size.

  • Resolution: Image clarity.

• Limitations:

  • Light microscopes have restrictions.

  • Electron microscopes offer better resolution but can only view dead cells.

• Medical Relevance:

  • Pathology studies cells under the microscope to diagnose diseases like cancer.

The Plasma Membrane

• Cell Enclosure:

  • Encompasses the cell; similar across all cell types.

  • Made up of a phospholipid bilayer with embedded proteins.

• Functions:

  • Selectively permeable barrier controlling entry and exit of substances.

  • Hosts transport proteins (channels & carriers).

  • Facilitates communication and adhesion with other cells.

Prokaryotes and Eukaryotes

• Cell Classification:

  • All cells are categorized as either prokaryotic or eukaryotic.

• Prokaryotes:

  • Include bacteria and archaea; lack nucleus or organelles; smaller and simpler in structure.

• Eukaryotes:

  • Include animals, plants, fungi, protists; possess a nucleus and membrane-bound organelles.

  • Larger and more complex.

Tree of Life

• Major lineages:

  • Bacteria: diverse species, including Gram-positive, Gram-negative, and Cyanobacteria.

  • Archaea and Eukarya: connection to fundamental life forms and evolutionary biology.

A Closer Look at Prokaryotes

• Dominance in Numbers:

  • Form numerically superior organisms on Earth.

  • Prokaryotes show a wide range of species.

• Size and Structure:

  • Size: 1-10 µm.

  • Each cell is a singular organism, capable of forming chains or clusters.

Structures Found in All Prokaryotes

• Plasma Membrane:

  • Regulates the passage of substances; provides communication and adhesion.

• Cell Wall:

  • Provides structural support and shape; a target for many antibiotics.

• Nucleoid:

  • Region containing DNA.

• Cytoplasm:

  • Semi-fluid interior containing ribosomes for protein synthesis.

• Outer membrane:

  • Provides extra protective layer, influences treatment decisions.

• Peptidoglycan:

  • Component referring to bacterial cell wall; critical for structure and function.

• Capsule:

  • Slimy protective layer.

  • Prevents desiccation and aids in attachment to surfaces.

• Flagellum:

  • Tail-like structure aiding mobility.

  • Requires ATP for movement; some bacteria possess multiple flagella.

• Fimbriae & Pili:

  • Short hair-like structures assisting with surface adherence.

  • Role in biofilm formation and genetic exchange through sex pili.

Eukaryotic Cells

• Size and Characteristics:

  • Approximately 10 times larger than prokaryotes (10-100 µm).

  • Similar structures: plasma membrane, cytoplasm, and ribosomes.

  • Contain membrane-enclosed organelles, each with specific functions.

Ribosomes

• Composition:

  • Made of ribosomal RNA (rRNA) and over 50 proteins.

• Location and Function:

  • Can exist freely in the cytoplasm or bound to the endoplasmic reticulum.

  • Act as molecular factories for protein synthesis.

The Nucleus

• Characteristics:

  • Often the largest organelle; houses DNA.

  • Key functions: DNA replication and transcription.

  • Contains nucleolus, where ribosome assembly occurs.

• Structure:

  • Surrounded by a nuclear envelope consisting of two membranes with numerous pores.

  • Proteins need recognition sequences to enter.

  • Nuclear lamina maintains the nuclear shape.

  • Outer membrane connects with the endoplasmic reticulum.

• Organization:

  • Chromatin comprises DNA and proteins; condenses to form chromosomes.

  • Human cells have 46 chromosomes, encoding over 20,000 proteins.

The Endomembrane System

• Components:

  • An interconnected system involving various membrane-enclosed compartments (plasma membrane, nuclear envelope, ER, Golgi apparatus, lysosomes).

  • Vesicles facilitate transport within the system.

• Processes:

  • Rough Endoplasmic Reticulum (RER) synthesizes proteins and packages them in vesicles directed to the Golgi apparatus.

  • Smooth Endoplasmic Reticulum (SER) synthesizes lipids, and performs toxin modification.

  • Golgi apparatus modifies and sorts proteins and lipids, packaging them for the membrane or lysosomes.

The Endoplasmic Reticulum (ER)

• Rough ER (RER):

  • Embedded ribosomes; aids in protein folding and glycoprotein formation.

• Smooth ER (SER):

  • Lacks ribosomes; involved in lipid synthesis and detoxification processes.

The Golgi Apparatus

• Description:

  • "The post office" of the cell, consists of flattened sacs called cisternae.

• Functions:

  • Receives vesicles from the ER, packages and sorts contents for their destination.

Lysosomes

• Functionality:

  • Recycle materials within cells; vesicles containing digestive enzymes.

  • Break down damaged substances, providing necessary building blocks back to the cell.

Lysosomal Disorders

• Examples:

  • Tay-Sachs disease: genetic disorder; lysosomes in brain cells fail to degrade a lipid (ganglioside), leading to neurological decline and early death.

  • Many lysosomal storage disorders exist with varying incidence and effects.

Cells Need Energy

• Importance of Energy:

  • Required for growth, reproduction, movement and response to stimuli.

• Energy Sources:

  • Glucose breakdown in mitochondria (all eukaryotes).

  • Photosynthesis in chloroplasts (in photosynthetic organisms).

  • Plasma membrane in prokaryotes performs energy-harvesting functions.

Mitochondria

• Functionality:

  • Primary site for glucose breakdown and ATP generation; size ranges from 2-8 µm.

  • Found abundantly in cells with high energy requirements; can number from 1 to 100,000.

• Structural Features:

  • Two membranes: outer (smooth) and inner (highly folded) control substance entry/exit.

Mitochondria Cont’d

• Independence:

  • Mitochondria can divide independently; believed to be ancient symbiotic organisms.

Chloroplasts

• Characteristics:

  • Present only in plants and some protists; site of photosynthesis.

  • Contain chlorophyll, essential for capturing sunlight energy.

Chloroplast Structure

• Membrane Arrangement:

  • Two outer membranes enclose the stroma; inner thylakoid membranes contain photosynthetic pigments organized in stacks (grana).

• Autonomy:

  • Like mitochondria, chloroplasts contain DNA and can divide independently.

Vacuoles

• Functions:

  • Found in plants, fungi, and protists; function in storage and support.

  • Can occupy up to 90% of a plant cell, providing structural integrity through water uptake.

  • Role in reproduction by attracting pollinators via pigments.

The Cytoskeleton

• Definition and Functions:

  • A meshwork of filaments providing shape, support, and holding organelles.

  • Interactions with extracellular structures aid in anchoring cells.

• Components of Eukaryotic Cytoskeleton:

  • Microfilaments, intermediate filaments, and microtubules, each with specific roles.

Microfilaments

• Composition:

  • Formed from actin protein monomers into double helical chains.

• Functions:

  • Enable cell movement, muscle contraction, and changes in cell shape.

  • Assist in cellular division processes.

Intermediate Filaments

• Characteristics:

  • Medium sized; diverse types specific to cell types.

• Functionality:

  • Provide structural stability, anchor organelles, and maintain tissue integrity.

Microtubules

• Structure:

  • Largest filament type; hollow and unbranched, made up of tubulin protein.

• Functions:

  • Serve as tracks for motor proteins, facilitate chromosome separation during cell division, and are affected by certain anti-cancer drugs.