Ch.6 study guide

Cell Theory and the Common Origin of Cells

Major Lines of Evidence for Common Origin

All presently living cells share several fundamental characteristics, supporting the idea of a common evolutionary origin.

• Universal Biochemical Pathways: All cells use similar metabolic pathways and energy conversion reactions.

• Genetic Material: All cells store and transfer genetic information in DNA.

• Cell Structure: All cells are surrounded by a plasma membrane and contain ribosomes for protein synthesis.

• Molecular Similarities: Many molecular components, such as ATP, are universal among cells.

Cell Theory

• All living organisms are composed of cells.

• Cells are the basic unit of structure and function in organisms.

• All cells arise from pre-existing cells.

Cell Size and Surface Area to Volume Ratio

Definition and Importance

The surface area to volume ratio (SA:V) is the relationship between the surface area of a cell and its volume. It is a critical factor that limits cell size.

• Formula: For a sphere, $\text{SA:V} = \frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r}$

• Importance: A high SA:V ratio allows efficient exchange of materials (nutrients, gases, wastes) with the environment.

• As cells grow larger, their volume increases faster than their surface area, decreasing the SA:V ratio.

• Low SA:V ratio limits the rate of exchange, restricting cell size.

Effect of Cell Growth on SA:V Ratio

• As a cell increases in size, its volume grows faster than its surface area, causing the SA:V ratio to decrease.

• This limits the maximum size a cell can attain while maintaining efficient transport.

Microscopy: LM, EM, SEM, and TEM

Types of Microscopes

• Light Microscope (LM): Uses visible light to illuminate specimens; can magnify up to 1000x; resolution limited by the wavelength of light (~200 nm).

• Electron Microscope (EM): Uses electron beams for much higher resolution (up to 2 nm); includes Transmission (TEM) and Scanning (SEM) types.

SEM vs. TEM

• SEM (Scanning Electron Microscope): Scans the surface of a specimen, producing 3D images of surface structures.

• TEM (Transmission Electron Microscope): Transmits electrons through thin sections of specimens, revealing internal structures in high detail.

• Resolution: Ability to distinguish two points as separate; higher in EM than LM.

• Magnification: Degree to which the image size is increased; higher in EM than LM.

• contrast visible differences in parts of the cell

Comparison Table

Microscope	Resolution	Magnification	Image Type
LM	~200 nm	Up to 1000x	Color, 2D
SEM	~2 nm	Up to 100,000x	3D, black & white
TEM	~2 nm	Up to 500,000x	2D, black & white

Cell Fractionation

Purpose and Process

Cell fractionation is a laboratory technique used to separate cellular components for study.

• Lyse: Cells are broken open using mechanical disruption or detergents.

• Centrifugation: The cell lysate is spun at high speeds to separate components by size and density.

• Pellet: Heavier components form a pellet at the bottom of the tube.

• Supernatant: Lighter components remain in the liquid above the pellet.

This process allows isolation of organelles such as nuclei, mitochondria, and ribosomes for further analysis.

Prokaryotic vs. Eukaryotic Cells

Size and Organization

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles; typically 1-10 μm in diameter; DNA is in a nucleoid region.

• Eukaryotic Cells: Have a nucleus and membrane-bound organelles; typically 10-100 μm in diameter; compartmentalized internal structure.

Comparison Table

Feature	Prokaryotic	Eukaryotic
Nucleus	No	Yes
Organelles	No (few)	Yes
Size	1-10 μm	10-100 μm
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Cellular Components and Organelles

Cytoplasm, Cytosol, Nucleoplasm, and Membranes

• Cytoplasm: Everything inside the plasma membrane except the nucleus.

• Cytosol: The fluid portion of the cytoplasm.

• Nucleoplasm: The substance within the nucleus.

• Membranes: Separate cellular compartments and regulate transport.

Major Organelles: Structure and Function

• Nucleus: Contains DNA; surrounded by a double membrane (nuclear envelope) with nuclear pores for transport.

• Ribosomes: Sites of protein synthesis; composed of rRNA and proteins.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER has ribosomes (protein synthesis), smooth ER (lipid synthesis, detoxification).

• Golgi Apparatus: Stack of flattened membranes; modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes; break down waste and cellular debris.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Vacuoles: Storage organelles; large central vacuole in plants for water and solute storage.

• Mitochondria: Site of aerobic respiration; produce ATP.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Microbodies: General term for small, enzyme-containing organelles (e.g., peroxisomes, glyoxysomes).

Animal vs. Plant Cell Organelles

Organelle	Animal Cell	Plant Cell
Cell Wall	No	Yes
Chloroplasts	No	Yes
Central Vacuole	No (small vacuoles)	Yes (large)
Lysosomes	Yes	Rare

Nucleus and Genetic Material

Structure and Function

• Nuclear Envelope: Double membrane surrounding the nucleus; contains nuclear pores for transport.

• Nuclear Pores: Protein complexes that regulate passage of molecules.

• Chromatin: DNA-protein complex; less condensed form of genetic material.

• Chromosomes: Condensed, organized structures of DNA during cell division.

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

Endomembrane System

Components and Functions

• ER (Endoplasmic Reticulum): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.

• Vesicles: Small membrane-bound sacs for transport within the cell.

• Vacuoles: Storage and maintenance of cell turgor (especially in plants).

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Microbodies: Includes peroxisomes and glyoxysomes; involved in metabolism and detoxification.

• Lysosomes: Digestive organelles containing hydrolytic enzymes.

• Peroxisomes: Break down fatty acids and detoxify hydrogen peroxide.

• Glyoxysomes: Specialized peroxisomes in plants for converting fats to sugars.

Energy-Converting Organelles

Mitochondria

• Double-membraned organelle; site of aerobic respiration.

• Inner membrane is highly folded into cristae, increasing surface area for ATP production.

• Matrix contains enzymes for the citric acid cycle.

• Contain their own DNA and ribosomes; replicate independently.

• Equation for aerobic respiration:

$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} + \text{energy (ATP)}$

Chloroplasts

• Double-membraned organelle; site of photosynthesis in plants and algae.

• Contains thylakoids (stacked into grana) where light-dependent reactions occur.

• Stroma is the fluid-filled space containing enzymes for the Calvin cycle.

• Contain their own DNA and ribosomes.

• Equation for photosynthesis:

$\text{carbon dioxide} + \text{water} + \text{light energy} \rightarrow \text{glucose} + \text{oxygen}$

Endosymbiont Theory

Origin of Mitochondria and Chloroplasts

• Proposes that mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA (circular), ribosomes similar to bacteria, replicate independently.

• Genetic and biochemical similarities to certain bacteria (e.g., mitochondria to proteobacteria, chloroplasts to cyanobacteria).

Cytoskeleton

Functions

• Maintains cell shape, enables movement, organizes organelles, and facilitates intracellular transport.

Main Types

• Microfilaments (Actin Filaments): Thin, flexible fibers; involved in cell movement and muscle contraction.

• Intermediate Filaments: Provide mechanical support and maintain cell shape.

• Microtubules: Hollow tubes; involved in cell division, organelle movement, and cilia/flagella structure.

Motor Proteins

• Proteins (e.g., kinesin, dynein, myosin) that move along cytoskeletal filaments, transporting vesicles and organelles.

Specialized Cell Types and Structures

Types of Cells

• Prokaryotic Cell: Simple structure, no nucleus, few organelles.

• Plant Cell: Has cell wall, chloroplasts, large central vacuole.

• Fungal Cell: Has cell wall (chitin), no chloroplasts, vacuoles present.

• Animal Cell: No cell wall, small vacuoles, lysosomes present.

Glycocalyx and Extracellular Matrix (ECM) in Animal Cells

Structure and Function

• Glycocalyx: Carbohydrate-rich layer on the cell surface; involved in cell recognition and protection.

• ECM (Extracellular Matrix): Network of proteins and polysaccharides outside the cell; provides structural support and mediates cell signaling.

• Key Components: Collagen (strength), fibronectin (adhesion), integrin (cell-ECM connection).

Interaction: Integrins in the plasma membrane connect the cytoskeleton to the ECM via fibronectin and collagen, facilitating communication and structural integrity.

Additional info: Some diagrams and drawings were referenced in the original material but are not included here. Students are encouraged to consult their textbook or lecture slides for visual representations of cell types and organelles.