Chapter 4 Bio

Objectives

• Demonstrate knowledge of cells as the basic units of life.

• Differentiate between prokaryotic and eukaryotic cell structure.

• Differentiate between plant and animal cell structure.

• Describe typical organelles found in cells and explain the basic functions of each.

Study tip: Making a flash card for vocabulary terms is recommended.

Cells Overview

Prokaryotes

Prokaryotic cells are among the simplest forms of life and lack a nucleus and most organelles found in eukaryotic cells.

• Features shared with eukaryotic cells: Both have a plasma membrane, cytoplasm, ribosomes, and genetic material (DNA).

• What prokaryotic cells lack: They do not have a membrane-bound nucleus or membrane-bound organelles such as mitochondria or endoplasmic reticulum.

• Example: Escherichia coli is a common prokaryote.

Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds all cells, controlling the movement of substances in and out.

• Purpose: Maintains the internal environment of the cell and facilitates communication and transport.

• Structure: Composed of a phospholipid bilayer with embedded proteins. The bilayer has hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Surface area vs. volume: As a cell grows, its volume increases faster than its surface area, limiting the efficiency of transport across the membrane.

• Cell division: Cells divide to maintain a favorable surface area-to-volume ratio, ensuring efficient exchange of materials.

Organelles and Cell Structure

Organelles

Organelles are specialized structures within eukaryotic cells that perform distinct functions necessary for cell survival.

• Definition: Membrane-bound compartments within eukaryotic cells.

• Purpose: Compartmentalize cellular processes, increasing efficiency and organization.

• Examples: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus.

Building Proteins and the Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

Flow of Information in Cells

• Central Dogma: The flow of genetic information is: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Nucleus

• Organization: DNA is organized around proteins called histones, forming chromatin.

• Function: The nucleus stores genetic information and is the site of transcription (DNA to RNA).

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

• Nuclear envelope: Double membrane that connects to the endoplasmic reticulum.

Ribosomes

• Role: Synthesize proteins by translating messenger RNA (mRNA).

• Structure: Composed of two subunits (large and small) made of rRNA and proteins.

• Location: Found free in the cytoplasm or bound to the rough endoplasmic reticulum (RER).

Rough Endoplasmic Reticulum (RER)

• Purpose: Synthesizes and processes proteins destined for membranes, lysosomes, or secretion.

• Location: Continuous with the nuclear envelope, studded with ribosomes.

• Appearance: Flattened sacs with ribosomes on the surface.

Smooth Endoplasmic Reticulum (SER)

• Purpose: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons.

• Location: Lacks ribosomes, often found near the RER.

• Appearance: Tubular network without ribosomes.

Transport and Storage Organelles

Vesicles

• Structure: Small, membrane-bound sacs.

• Purpose: Transport materials within the cell.

Vacuoles

• Structure: Large, membrane-bound sacs, especially prominent in plant cells.

• Purpose: Storage of water, nutrients, and waste products.

• Central Vacuole: Important for maintaining turgor pressure in plant cells.

Energy Processing Organelles

Lysosome

• Purpose: Digests macromolecules, old organelles, and foreign substances.

• Appearance: Small, spherical vesicles containing hydrolytic enzymes.

• Contents: Acidic environment with digestive enzymes.

Peroxisome

• Purpose: Breaks down fatty acids and detoxifies harmful substances.

• Appearance: Small, membrane-bound organelles.

• Contents: Enzymes that produce and degrade hydrogen peroxide ($H_2O_2$).

Chloroplasts and Mitochondria Comparison Table

Chloroplasts and mitochondria are organelles involved in energy transformation in eukaryotic cells.

Organelle	Found in what cell types?	Name of process that occurs	Energy transformation
Chloroplast	Plant cells, some protists	Photosynthesis	Light energy to chemical energy (glucose)
Mitochondria	All eukaryotic cells	Cellular respiration	Chemical energy (glucose) to ATP

Organization and Building: The Cytoskeleton

Inside the Cell: Cytoskeleton

• Intermediate Filaments: Provide mechanical support for the cell.

• Microfilaments: Composed of actin; involved in cell movement and shape.

• Microtubules:

  • Purpose: Maintain cell shape, facilitate intracellular transport, and separate chromosomes during cell division.

  • Distribution: Radiate from the centrosome in animal cells.

  • Organization: Made of tubulin proteins; can be labeled and visualized in diagrams.

  • Centrosomes: Microtubule-organizing centers found in animal cells.

Outside the Cell

Cell part	Type of cell	Purpose	Made of
Cell Wall	Plant, fungi, bacteria	Provides structural support and protection	Cellulose (plants), chitin (fungi), peptidoglycan (bacteria)
Extracellular Matrix	Animal cells	Supports cell structure, cell signaling	Proteins (collagen, elastin), glycoproteins

Additional info: Expanded explanations and examples were added for clarity and completeness.