Cell Structure and Function: Study Guide for General Biology

Microscopy and Cell Size

Principles and Types of Microscopes

Microscopes are essential tools for studying cells and their structures. Different types of microscopes offer varying levels of magnification and resolution.

• Light Microscope: Uses visible light to illuminate specimens. Allows observation of living cells and tissues up to about 1000x magnification. Advantages: Simple, can view live cells. Limitations: Limited resolution (~0.2 μm).

• Transmission Electron Microscope (TEM): Passes electrons through thin sections of specimens. Advantages: High resolution (up to 0.002 μm), detailed internal structures. Limitations: Requires extensive sample preparation, only dead specimens can be observed.

• Scanning Electron Microscope (SEM): Scans the surface of specimens with electrons, producing 3D images. Advantages: Detailed surface structures. Limitations: Only surface can be viewed, specimens must be coated and are not alive.

Limits to Cell Size

Cell size is constrained by physical and biological factors.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting efficient exchange of materials.

• Diffusion Rates: Larger cells have slower diffusion rates, affecting nutrient and waste transport.

• Genetic Control: The amount of DNA limits the rate of protein synthesis and cell function.

Cell Types and Structures

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their internal organization.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in the nucleoid region. Examples: Bacteria, Archaea.

• Eukaryotic Cells: Have a true nucleus and membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

Nucleoid vs. Nucleus

• Nucleoid: Region in prokaryotic cells where DNA is concentrated, not surrounded by a membrane.

• Nucleus: Membrane-bound organelle in eukaryotic cells containing genetic material.

Common Features of All Cells

• Plasma membrane

• Cytoplasm

• Ribosomes

• Genetic material (DNA)

Prokaryotic Structures and Functions

• Flagella: Long, whip-like structures for movement.

• Fimbriae: Short, hair-like structures for attachment to surfaces.

• Capsule: Protective outer layer aiding in defense and adherence.

Structures Unique to Plant Cells

• Cell wall

• Chloroplasts

• Central vacuole

• Plasmodesmata

Nucleus and Ribosomes

Nuclear Envelope Structure and Function

The nuclear envelope is a double membrane that surrounds the nucleus, separating it from the cytoplasm.

• Contains nuclear pores for transport of molecules.

• Maintains the environment for DNA replication and transcription.

Functions of the Nucleus

• Stores genetic material (DNA).

• Controls cellular activities by regulating gene expression.

• Coordinates cell division.

Role of the Nucleolus

• Site of ribosomal RNA (rRNA) synthesis.

• Assembly of ribosome subunits.

Free vs. Bound Ribosomes

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

• Bound Ribosomes: Attached to the endoplasmic reticulum; synthesize proteins for export or for membranes.

Endomembrane System

Components and Functions

The endomembrane system is a network of membranes within eukaryotic cells that work together in the synthesis, transport, and processing of proteins and lipids.

• Nuclear Envelope: Encloses the nucleus.

• Endoplasmic Reticulum (ER): Smooth ER synthesizes lipids; rough ER has ribosomes and synthesizes proteins.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes for breakdown of macromolecules.

• Vacuoles: Storage and transport; central vacuole in plants maintains turgor pressure.

• Plasma Membrane: Controls entry and exit of substances.

Smooth vs. Rough ER

• Smooth ER: Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

Function of Vacuoles

• Storage of nutrients, waste products, and water.

• Maintenance of cell shape and turgor pressure in plants.

Energy Converting Organelles

Mitochondria and Chloroplasts

These organelles are responsible for energy conversion in eukaryotic cells.

• Mitochondria: Site of cellular respiration; converts glucose and oxygen into ATP.

• Chloroplasts: Site of photosynthesis; converts light energy, water, and CO2 into glucose and oxygen.

Cell Types Containing Mitochondria and Chloroplasts

• Mitochondria: Found in nearly all eukaryotic cells (plants, animals, fungi, protists).

• Chloroplasts: Found in plant cells and some protists.

Parts of Mitochondria

• Outer membrane

• Inner membrane (folded into cristae)

• Matrix

• Intermembrane space

Parts of Chloroplast

• Outer membrane

• Inner membrane

• Stroma

• Thylakoid membranes (stacked into grana)

Semiautonomous Organelles

• Mitochondria and chloroplasts contain their own DNA and ribosomes.

• They can replicate independently within the cell.

• Endosymbiotic Theory: Suggests these organelles originated from free-living prokaryotes engulfed by ancestral eukaryotes.

Role of Peroxisomes

• Break down fatty acids and detoxify harmful substances.

• Produce hydrogen peroxide, which is then converted to water.

Cytoskeleton and Cell Surfaces

Functions of the Cytoskeleton

The cytoskeleton provides structural support, facilitates cell movement, and organizes cellular components.

• Maintains cell shape

• Anchors organelles

• Enables intracellular transport

• Facilitates cell division

Types of Cytoskeletal Elements

Type	Structure	Function
Microtubules	Hollow tubes of tubulin	Cell shape, movement, chromosome separation
Microfilaments	Thin filaments of actin	Cell shape, muscle contraction, cytoplasmic streaming
Intermediate Filaments	Fibrous proteins	Cell strength, organelle anchoring

Cilia, Centrioles, and Basal Bodies

• Cilia: Short, numerous projections for movement or fluid transport.

• Flagella: Longer, fewer projections for cell movement.

• Centrioles: Cylindrical structures involved in cell division.

• Basal Bodies: Anchor cilia and flagella to the cell.

Movement of Cilia and Flagella

• Powered by the sliding of microtubules using motor proteins (dynein).

• Requires ATP for movement.

Motor Proteins and Molecular Motion

• Motor proteins (e.g., kinesin, dynein, myosin) move along cytoskeletal filaments.

• Transport vesicles, organelles, and chromosomes within the cell.

Extracellular Matrix and Cell Junctions

Structure and Roles of the Extracellular Matrix (ECM)

The ECM is a network of proteins and carbohydrates outside animal cells that provides structural support and regulates cell behavior.

• Composed of collagen, proteoglycans, and fibronectin.

• Influences cell adhesion, migration, and differentiation.

Types of Cell Junctions and Their Functions

Junction Type	Function
Plasmodesmata	Channels between plant cells for transport and communication
Gap Junction	Channels between animal cells for communication
Desmosome (Anchoring Junction)	Fasten cells together, provide mechanical strength
Tight Junction	Seal cells together, prevent leakage of extracellular fluid

Additional info: Where questions requested drawings, descriptive text was provided instead. All content is expanded for clarity and completeness.