Concept 6.1: Biologists Use Microscopes and Biochemistry to Study Cells

Microscopy

Microscopes are essential tools for visualizing cells, which are typically too small to be seen by the naked eye. The study of cells relies on both microscopy and biochemical techniques.

• Light Microscopes (LM): Use visible light passed through a specimen and glass lenses to magnify images. Magnification, resolution, and contrast are key parameters.

• Magnification: Ratio of image size to real size.

• Resolution: Minimum distance between two distinguishable points.

• Contrast: Differences in brightness between parts of the sample.

• Electron Microscopes (EM): Use electron beams for higher resolution.

  • Scanning Electron Microscope (SEM): Provides 3-D images of specimen surfaces.

  • Transmission Electron Microscope (TEM): Reveals internal cell structures.

• Recent Advances: Fluorescent labeling, confocal microscopy, and cryo-electron microscopy enhance detail and preserve cellular environments.

Cell Fractionation

Cell fractionation separates cellular components using centrifugation, allowing scientists to study organelle functions and correlate structure with function.

Concept 6.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Comparing Prokaryotic and Eukaryotic Cells

All cells share certain features, but prokaryotic and eukaryotic cells differ in complexity and organization.

• Basic Features: Plasma membrane, cytosol, chromosomes, ribosomes.

• Prokaryotic Cells:

  • No nucleus; DNA in nucleoid region.

  • No membrane-bound organelles.

  • Cytoplasm bound by plasma membrane.

• Eukaryotic Cells:

  • DNA in nucleus (double membrane).

  • Membrane-bound organelles.

  • Cytoplasm between plasma membrane and nucleus.

  • Generally larger than prokaryotic cells.

• Surface Area to Volume Ratio: Limits cell size; as cells grow, volume increases faster than surface area, affecting nutrient and waste exchange.

Concept 6.3: The Eukaryotic Cell’s Genetic Instructions Are Housed in the Nucleus and Carried Out by the Ribosomes

The Nucleus

The nucleus is the cell's information center, containing most of its genes and surrounded by a double membrane (nuclear envelope).

• Nuclear Envelope: Double membrane with nuclear pores for molecular transport.

• Nuclear Lamina: Protein network supporting nuclear shape.

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

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

Ribosomes

Ribosomes are complexes of rRNA and protein, responsible for protein synthesis.

• Free Ribosomes: Located in cytosol; synthesize proteins for use within the cell.

• Bound Ribosomes: Attached to endoplasmic reticulum or nuclear envelope; synthesize proteins for secretion or membrane insertion.

Concept 6.4: The Endomembrane System Regulates Protein Traffic and Performs Metabolic Functions

Components of the Endomembrane System

The endomembrane system includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane. These components are interconnected or transfer materials via vesicles.

Endoplasmic Reticulum (ER)

• Smooth ER: Synthesizes lipids, detoxifies drugs, stores calcium ions.

• Rough ER: Studded with ribosomes; synthesizes glycoproteins, distributes transport vesicles, and produces membranes.

Golgi Apparatus

• Consists of cisternae (flattened sacs).

• Modifies ER products, manufactures macromolecules, sorts and packages materials into vesicles.

Lysosomes

• Membranous sacs of hydrolytic enzymes for digesting macromolecules.

• Enzymes function best in acidic environments.

• Participate in phagocytosis and autophagy (recycling cell components).

Vacuoles

• Large vesicles with diverse functions: food storage, water regulation (contractile vacuoles), and ion storage (central vacuole in plants).

Concept 6.5: Mitochondria and Chloroplasts Change Energy from One Form to Another

Mitochondria

Mitochondria are the sites of cellular respiration, converting oxygen and nutrients into ATP.

• Structure: Smooth outer membrane, folded inner membrane (cristae), intermembrane space, and matrix.

• Function: ATP synthesis; metabolic reactions occur in the matrix.

Chloroplasts

Chloroplasts are found in plants and algae, responsible for photosynthesis.

• Structure: Double membrane, thylakoids (stacked as granum), stroma (internal fluid), contains DNA and ribosomes.

• Function: Capture light energy to synthesize organic molecules.

Peroxisomes

• Specialized metabolic compartments with enzymes that transfer hydrogen to oxygen, forming hydrogen peroxide.

• Functions include fatty acid breakdown, detoxification, and conversion of fatty acids to sugars in plants.

Concept 6.6: The Cytoskeleton: Support, Motility, and Regulation

Roles of the Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and enables movement of organelles and vesicles via motor proteins.

Components of the Cytoskeleton

• Microtubules: Hollow tubes of tubulin; shape cell, guide organelle movement, separate chromosomes.

• Microfilaments (Actin Filaments): Twisted double chains of actin; support cell shape, form cortex, core of microvilli, involved in motility.

• Intermediate Filaments: Fibrous proteins; provide structural stability, anchor organelles.

Centrosomes and Centrioles

• Microtubules grow from centrosomes in animal cells, which contain centrioles arranged in nine triplets.

Cilia and Flagella

• Microtubule-containing extensions for cell movement; cilia are numerous, flagella are few.

• Structure: Nine doublets in a ring, two singles in center, anchored by basal body, movement driven by dynein motor proteins.

Microfilaments (Actin Filaments)

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

• Involved in motility (with myosin), pseudopodia, and cytoplasmic streaming.

Intermediate Filaments

• Permanent fixtures; support cell shape, anchor organelles, form nuclear lamina.

Concept 6.7: Extracellular Components and Connections Between Cells Help Coordinate Cellular Activities

Cell Walls of Plants

Plant cell walls are extracellular structures made of cellulose, providing protection, shape, and preventing excessive water uptake. Multiple layers include primary wall, middle lamella (pectins), and secondary wall.

Extracellular Matrix (ECM) of Animal Cells

• Animal cells lack cell walls but have ECM made of glycoproteins (collagen, proteoglycans, fibronectin).

• ECM proteins bind to integrins in plasma membrane, influencing cell behavior and gene activity.

Cell Junctions

• Plasmodesmata (Plants): Channels connecting plant cells, allowing passage of water, solutes, proteins, and RNA.

• Tight Junctions (Animals): Seal cells together, prevent fluid leakage.

• Desmosomes: Anchor cells into sheets.

• Gap Junctions: Provide cytoplasmic channels for communication.

Concept 6.8: A Cell Is Greater Than the Sum of Its Parts

Cellular functions require coordination among all components. For example, macrophages use cytoskeleton, lysosomes, and plasma membrane to engulf and destroy bacteria.