Cell Structure and Function: Study Guide

Introduction to Cell Biology

This study guide covers foundational concepts in cell biology, including scientific theories, microscopy, cell structure, and the organization of prokaryotic and eukaryotic cells. Understanding these topics is essential for exploring the complexity and diversity of life at the cellular level.

Scientific Theories

What is a Scientific Theory?

• Scientific Theory: A well-substantiated explanation of some aspect of the natural world, based on a body of evidence that has been repeatedly confirmed through observation and experimentation.

• Difference from Hypothesis: A hypothesis is a testable prediction or explanation, while a theory is broader, supported by extensive evidence, and has withstood repeated testing.

• Difference from Guess/Opinion: Guesses and opinions lack systematic evidence and testing.

• Key Features: Evidence-based, subject to peer review, refined over time as new data emerges.

• Example: The Cell Theory is a scientific theory that describes the properties of cells.

Cell Theory

Major Components of Cell Theory

• All living organisms are composed of one or more cells.

• The cell is the basic unit of structure and function in living things.

• All cells arise from pre-existing cells.

• Additional info: Modern cell theory also includes that cells contain hereditary information (DNA) and that energy flow occurs within cells.

Microscopes & History

Types of Microscopes

• Light Microscope (LM): Uses visible light to illuminate specimens; suitable for viewing live cells and tissues.

• Scanning Electron Microscope (SEM): Uses electrons to scan the surface of a specimen, producing detailed 3D images of surfaces.

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

• Image Identification: LM images are colored and less detailed; SEM images show surface texture in 3D; TEM images are black-and-white and show internal cell structures.

Key Terms

• Magnification: The process of enlarging the appearance of an object.

• Resolution: The ability to distinguish two points as separate; higher resolution reveals more detail.

Historical Contributions

• Robert Hooke: First to observe and name "cells" in cork tissue using a light microscope (1665).

• Anton van Leeuwenhoek: Improved microscope lenses and was first to observe living microorganisms (1670s).

Surface Area to Volume Ratio

Definitions and Importance

• Surface Area: The total area of the cell's outer membrane.

• Volume: The amount of space inside the cell.

• Surface-Area-to-Volume Ratio: The ratio of a cell's surface area to its volume; calculated as $\text{SA:V} = \frac{\text{Surface Area}}{\text{Volume}}$

• Why Cells Must Remain Small: As cells grow, volume increases faster than surface area, limiting the rate of diffusion of materials in and out of the cell.

• Diffusion Limitation: Efficient exchange of nutrients and waste is only possible in small cells due to the high surface-area-to-volume ratio.

• Microvilli: Finger-like projections that increase surface area without significantly increasing volume, enhancing absorption (e.g., in intestinal cells).

Prokaryotic Cells

Definition and Domains

• Prokaryotic Cells: Cells lacking a nucleus and membrane-bound organelles; structurally simpler than eukaryotic cells.

• Domains: Bacteria and Archaea are the two prokaryotic domains.

• Main Difference: Prokaryotes lack a true nucleus and complex organelles.

Prokaryotic Cell Structures

• Capsule: Outer protective layer; prevents desiccation and helps evade immune response.

• Cell Wall: Provides structural support and shape; made of peptidoglycan in bacteria.

• Plasma Membrane: Selectively permeable barrier controlling entry and exit of substances.

• Cytoplasm: Gel-like substance inside the cell where metabolic reactions occur.

• Nucleoid: Region containing the cell's circular DNA; not membrane-bound.

• Plasmids: Small, circular DNA molecules carrying accessory genes.

• Ribosomes: Sites of protein synthesis; smaller than eukaryotic ribosomes.

• Fimbriae: Hair-like projections for attachment to surfaces.

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

• Shared Structures with Eukaryotes: Plasma membrane, cytoplasm, ribosomes.

• Shape and Size Constraints: Prokaryotic cells are small due to surface-area-to-volume ratio limitations.

Eukaryotic Cells

Organisms and Features

• Organisms: Protists, fungi, plants, and animals are composed of eukaryotic cells.

• Defining Features: Presence of a nucleus, membrane-bound organelles, and greater structural complexity.

The Nucleus and Ribosomes

Nucleus

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

• Nuclear Pore: Openings that regulate movement of molecules between nucleus and cytoplasm.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

• Nucleolus: Dense region where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Stored in Nucleus: Genetic material (DNA).

• Connection to E.R.: The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum (E.R.).

Ribosomes

• Function: Synthesize proteins by translating mRNA.

• Location: Found free in cytoplasm or bound to rough E.R.; present in both prokaryotes and eukaryotes.

• Made in: Nucleolus (in eukaryotes).

The Endomembrane System

Purpose and Components

• Purpose: Synthesis, modification, packaging, and distribution of molecules within the cell.

Rough Endoplasmic Reticulum (Rough E.R.)

• Structure: Network of membranes studded with ribosomes, giving a "rough" appearance.

• Role: Protein folding and processing; initial site for secretory and membrane protein synthesis.

• Vesicle Formation: Proteins are packaged into vesicles for transport to the Golgi apparatus.

Smooth Endoplasmic Reticulum (Smooth E.R.)

• Lipid Synthesis: Produces lipids, including phospholipids and steroids.

• Detoxification: Breaks down toxins and drugs (especially in liver cells).

• Calcium Storage: Stores calcium ions, important for muscle contraction and signaling.

Vesicles

• Formation: Bud off from E.R. or Golgi; transport materials within the cell.

• Fusion: Can fuse with membranes to deliver contents or secrete substances (exocytosis).

Golgi Apparatus

• Structure: Stacked, flattened sacs (cisternae).

• Role: Modifies, sorts, and ships proteins and lipids; acts as the "UPS" of the cell.

• Receives: Vesicles from E.R.; produces lysosomes.

Lysosomes

• Function: Digestive and recycling centers; contain hydrolytic enzymes.

• Role in Endocytosis: Fuse with vesicles containing ingested material to break them down.

Peroxisomes

• Difference from Lysosomes: Peroxisomes are involved in detoxification and fatty acid breakdown, not general digestion.

• Function: Break down fatty acids and detoxify harmful substances; produce hydrogen peroxide as a byproduct, which is then broken down by catalase.

Energy-Converting Organelles

Chloroplasts

• Function: Site of photosynthesis; convert solar energy to chemical energy (glucose).

• Organisms: Found in plants and some protists (e.g., algae).

• Structure: Surrounded by three membranes; contain thylakoids, stroma, and grana.

• Genetic Material: Contain their own DNA and ribosomes (evidence for endosymbiotic origin).

Mitochondria

• Function: Site of aerobic respiration; produce ATP from glucose.

• Organisms: Found in nearly all eukaryotic cells.

• Structure: Double-membrane organelle; inner membrane folded into cristae.

• Genetic Material: Contain their own DNA and ribosomes.

The Cytoskeleton

Functions

• Maintains cell shape, enables movement, and organizes cell components.

Actin Filaments (Microfilaments)

• Structure: Thin filaments made of actin protein.

• Role in Microvilli: Support the structure of microvilli.

• Amoeboid Movement: Enable cell movement by rapid assembly/disassembly.

• Interaction with Myosin: Actin and myosin interact for muscle contraction and cell movement.

Intermediate Filaments

• Structural Strength: Provide mechanical support; more stable than actin filaments.

• Anchoring Organelles: Help position organelles within the cell.

• Cell Junction Support: Reinforce cell-to-cell junctions (e.g., desmosomes).

Microtubules

• Structure: Hollow tubes made of α- and β-tubulin dimers.

• Centrosomes & Centrioles: Organize microtubules; centrioles are found in animal cells.

• Cilia and Flagella: Microtubule-based structures for movement.

• Intracellular Transport: Serve as tracks for vesicle and organelle movement.

• Spindle Apparatus: Form the mitotic spindle during cell division.

Cell Connections

Membrane Proteins

• Anchor cells to each other and to the extracellular matrix.

• Facilitate cell communication and attachment.

Cell-to-Cell Junctions

• Adhesion Junctions (Desmosomes): Strong connections that anchor cells together; found in tissues subject to stress (e.g., skin).

• Tight Junctions: Create waterproof seals between cells; prevent leakage (e.g., in intestinal lining).

• Gap Junctions: Open channels allowing ions and small molecules to pass; enable cell communication. In plants, similar structures are called plasmodesmata.

Key Terms and Definitions

Term	Definition
Adhesion junction (Desmosome)	Cell junction providing strong adhesion between cells
Archaea	Domain of prokaryotic microorganisms distinct from bacteria
Bacteria	Domain of prokaryotic microorganisms with peptidoglycan cell walls
Capsule	Protective outer layer of some prokaryotes
Cell theory	Scientific theory stating all living things are made of cells
Centrosome	Microtubule-organizing center in animal cells
Centrioles	Cylindrical structures in centrosomes; help organize spindle fibers
Chloroplast	Organelle for photosynthesis in plants and algae
Cilia	Short, hair-like projections for movement
Cytoskeleton	Network of protein filaments for cell shape and movement
Desmosome	See adhesion junction
Diffusion	Movement of molecules from high to low concentration
Endocytosis	Process of taking substances into the cell by engulfing them
Endomembrane system	Group of organelles for synthesis and transport of cell products
Exocytosis	Release of substances from the cell via vesicle fusion
Fimbriae	Attachment structures on prokaryotes
Flagella	Long, whip-like structures for cell movement
Gap junction	Channels between animal cells for communication
Golgi apparatus	Organelle for modifying, sorting, and shipping proteins/lipids
Intermediate filaments	Protein fibers providing structural support
Lysosome	Organelle with digestive enzymes for breakdown of materials
Magnification	Increase in apparent size of an object
Microtubules	Hollow tubes of tubulin for structure and transport
Microvilli	Projections that increase cell surface area
Mitochondria	Organelle for ATP production via aerobic respiration
Nuclear envelope	Double membrane surrounding the nucleus
Nuclear pore	Opening in nuclear envelope for molecular transport
Nucleoid	Region in prokaryotes containing DNA
Nucleolus	Site of rRNA synthesis and ribosome assembly in nucleus
Peroxisome	Organelle for detoxification and fatty acid breakdown
Plasma membrane	Phospholipid bilayer enclosing the cell
Plasmid	Small, circular DNA in prokaryotes
Resolution	Ability to distinguish two points as separate
Ribosome	Structure for protein synthesis
Rough ER	Endoplasmic reticulum with ribosomes; protein synthesis
Smooth ER	Endoplasmic reticulum without ribosomes; lipid synthesis
Spindle apparatus	Microtubule structure for chromosome movement in cell division
Surface area-to-volume ratio	Ratio affecting cell size and efficiency
Tight junction	Seal between cells preventing leakage
Vesicle	Small membrane-bound sac for transport

Additional info: This guide expands on brief points with academic context to ensure clarity and completeness for college-level biology students.