Chapter 4 – A Tour of the Cell: General Biology Study Notes

Introduction to the Cell

Cell Theory

The cell theory is a fundamental concept in biology, stating that all living things are composed of cells, and all cells arise from preexisting cells.

• Definition: The cell is the basic unit of life.

• Key Points:

  • All organisms are made of one or more cells.

  • Cells arise only from preexisting cells by division.

• Examples: Humans are multicellular organisms made up of trillions of cells. Bacteria reproduce by splitting into two cells (binary fission).

Microscopy

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

• Light Microscope (LM): Uses visible light and glass lenses to magnify specimens up to ~1,000×. Examples: Viewing onion skin cells; observing pond water protists.

• Electron Microscope (EM): Uses a beam of electrons for much higher resolution images. Examples: Seeing viruses; observing internal structures of mitochondria.

• Scanning Electron Microscope (SEM): Produces 3D images of cell surfaces. Examples: Studying the surface of an insect; looking at microvilli on intestinal cells.

• Transmission Electron Microscope (TEM): Produces 2D images of thin cell slices, showing internal structure. Examples: Viewing ribosomes inside the ER; observing chloroplast membranes.

Cell Structure

Cells have various structures that perform specialized functions, contributing to the cell's survival and efficiency.

• Cell Wall: Rigid protective layer outside plasma membrane in plants, fungi, and bacteria. Examples: Plant cell walls made of cellulose; bacterial cell walls provide protection.

• Plasma Membrane: Flexible boundary made of phospholipids and proteins; regulates what enters/exits. Examples: Nerve cells transmitting signals; plant cells maintaining homeostasis.

• Cytoplasm: Jelly-like fluid plus organelles inside the plasma membrane. Examples: Cytoplasmic streaming moves nutrients; metabolic reactions occur here.

Cell Types

• Prokaryotic Cell: Simple cell without nucleus or membrane-bound organelles. Examples: E. coli; Streptococcus bacteria.

• Eukaryotic Cell: Complex cell with nucleus and membrane-bound organelles. Examples: Animal cells; plant cells.

Cytosol

The fluid portion of cytoplasm, excluding organelles.

• Examples: Glycolysis occurs in cytosol; proteins fold here before entering organelles.

Genetic Material

• Chromosome: DNA molecules packed with proteins carrying genetic instructions. Examples: Humans have 46 chromosomes; fruit flies have 8.

• Chromatin: DNA and proteins in an uncondensed form (found in the nucleus). Examples: Chromatin during interphase; euchromatin regions actively transcribed.

Nucleus and Nuclear Structures

• Nucleus: Control center containing DNA. Examples: White blood cells with large nuclei; plant cell nuclei controlling photosynthesis genes.

• Nuclear Envelope: Double membrane around the nucleus with pores for transport. Examples: RNA leaving the nucleus; proteins entering to regulate transcription.

• Nucleolus: Site of ribosome production inside the nucleus.

The Nucleus and Ribosomes

Ribosome

Ribosomes are tiny structures that build proteins from amino acids.

• Examples: Free ribosomes making enzymes in cytosol; ER-bound ribosomes making membrane proteins.

The Endomembrane System

Endomembrane System

A network of membranes involved in making, packaging, and transporting proteins and lipids.

• Examples: Rough ER makes proteins → Golgi ships them; lysosomes digest food.

Endoplasmic Reticulum (ER)

• Smooth ER: Lacks ribosomes; makes lipids, detoxifies drugs. Examples: Liver cells detoxifying alcohol; ovarian cells producing estrogen.

• Rough ER: Studded with ribosomes; makes proteins. Examples: Pancreatic cells making insulin; antibody-producing immune cells.

Golgi Apparatus

The Golgi apparatus is the "shipping center" that modifies, sorts, and ships proteins.

• Examples: Salivary glands releasing enzymes; plant cells sending pectin to the cell wall.

Lysosome

Lysosomes are organelles containing digestive enzymes to break down macromolecules.

• Examples: White blood cells digesting bacteria; recycling old cell parts.

Vacuole

Vacuoles are storage organelles found in plant and protist cells.

• Examples: Plant central vacuole stores water; protist contractile vacuole pumps water out.

Central Vacuole

Large vacuole in plant cells for water and nutrient storage.

• Examples: Turgor pressure keeps plants rigid; storing pigments in flowers.

Transport Vesicle

Small membrane sacs that move molecules around the cell.

• Examples: Proteins moving ER → Golgi; Golgi → plasma membrane transport.

Peroxisome

Peroxisomes break down toxins and fatty acids.

• Examples: Liver detoxifying alcohol; breaking down hydrogen peroxide.

Energy-Converting Organelles

Mitochondrion (plural: mitochondria)

Mitochondria are the "powerhouse" of the cell, the site of ATP production.

• Examples: Muscle cells with many mitochondria; brain neurons needing energy.

Chloroplast

Site of photosynthesis in plants and algae.

• Examples: Leaf mesophyll cells; algae making sugars in oceans.

Thylakoid

Flattened sacs in chloroplasts where light reactions occur.

• Examples: Photosystem II in thylakoids; ATP synthase embedded in thylakoid membrane.

Granum (plural: grana)

Stacked thylakoids where the Calvin cycle occurs.

• Examples: Carbon fixation producing glucose; enzyme rubisco found here.

Endosymbiont Theory

The endosymbiont theory proposes that mitochondria and chloroplasts originated from free-living bacteria.

• Examples: Both have their own DNA; both reproduce independently inside cells.

Cytoskeleton, Cell Surfaces, and Junctions

Cytoskeleton

The cytoskeleton is a network of protein fibers providing shape and movement.

• Examples: Muscle contraction; cell division spindle fibers.

Microtubule

Hollow protein tubes giving cell shape and moving organelles.

• Examples: Spindle fibers separating chromosomes; cilia movement.

Microfilament

Thin actin filaments for support and motility.

• Examples: Amoeba crawling using actin; microvilli structure.

Intermediate Filament

Proteins anchoring organelles and maintaining shape.

• Examples: Keratin in skin cells; nuclear lamina support.

Motor Protein

Proteins that move cargo along the cytoskeleton.

• Examples: Kinesin carrying vesicles; dynein moving cilia.

Cilium (plural: cilia)

Short hairlike structures for movement.

• Examples: Cilia in lungs moving mucus; protist Paramecium swimming.

Extracellular Matrix (ECM)

Network outside animal cells for support.

• Examples: Collagen in connective tissue; bone cells embedded in ECM.

Integrin

Proteins in the membrane connecting ECM to cytoskeleton.

• Examples: White blood cells moving through tissues; cell adhesion in development.

Plasmodesma (plural: plasmodesmata)

Channels between plant cells for transport.

• Examples: Water moving cell-to-cell in leaves; nutrient sharing in roots.

Glycoprotein

Protein with sugar chains, used for cell recognition.

• Examples: Antigens on red blood cells; immune cell communication.

Collagen

Structural protein in ECM.

• Examples: Tendons; cartilage.

Types of Cell Junctions

Junction Type	Function	Location/Structure
Tight Junction	Creates a seal between adjacent cells, preventing passage of molecules and ions.	Commonly found in epithelial tissues, such as the intestinal lining and blood-brain barrier.
Adherens Junction	Provides strong mechanical attachments between adjacent cells by connecting to the actin cytoskeleton.	Formed by cadherin proteins that link to the cytoskeleton.
Gap Junction	Allows direct communication between adjacent cells by forming channels that permit the passage of ions and small molecules.	Facilitates coordinated responses in tissues.

Chapter Objectives

• Compare designs and images produced by LM, SEM, TEM.

• Explain limits of cell size: Small size maximizes surface area-to-volume ratio. Formula: $\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

• Structure and function of plasma membrane: Phospholipid bilayer with proteins; regulates traffic.

• Distinguish prokaryotic vs. eukaryotic cells: Prokaryotes lack nucleus/organelles; eukaryotes have them.

• Compartmentalization: Allows specialization and efficiency; example: chloroplasts for photosynthesis.

• Functions of nucleus/nucleolus/ribosomes: Nucleus stores DNA; nucleolus makes ribosomes; ribosomes make proteins.

• Endomembrane components: Smooth ER (lipids), rough ER (proteins), Golgi (ships), lysosome (digests), vacuole (stores), transport vesicle (moves).

• Energy conversion: Chloroplasts (photosynthesis), mitochondria (ATP).

• Evidence for endosymbiosis: DNA, ribosomes, double membranes, division like bacteria.

• Cell junctions: Specialized structures that connect cells, maintain tissue integrity, and allow communication.