Chapter 6

Cellular Level of Organization

Cell Theory

The cell theory is a fundamental concept in biology, stating that all living organisms are composed of cells, cells are the basic units of structure and function in organisms, and all cells arise from pre-existing cells. This theory was first articulated by Schleiden and Schwann in the 19th century.

• All organisms are composed of one or more cells.

• Cells are the basic unit of structure and function in living things.

• Cells arise only from pre-existing cells by division.

Microscopy and Cell Study

Types of Microscopes

Microscopes are essential tools for studying cells and their components. There are three main types:

• Light Microscope (LM): Uses visible light to magnify specimens up to 1000x. Can be used to view living cells.

• Scanning Electron Microscope (SEM): Uses electron beams to scan the surface of a specimen, providing detailed images of cell surfaces. High resolution, but cannot view living cells.

• Transmission Electron Microscope (TEM): Passes electrons through thin sections of specimens to reveal internal structures at very high resolution. Not suitable for living cells.

Cell Fractionation: A laboratory technique that separates cellular components by centrifugation, allowing scientists to study the function of individual organelles.

Cell Structure and Function

Basic Features of All Cells

• Plasma Membrane: A selective barrier composed of a phospholipid bilayer with embedded proteins and cholesterol. Regulates the passage of substances in and out of the cell.

• Cytosol: The semi-fluid, jelly-like substance inside the cell where organelles are suspended.

• Chromosomes: Structures carrying genetic information (DNA).

• Ribosomes: Complexes of rRNA and protein that synthesize proteins.

Prokaryotic vs. Eukaryotic Cells

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

• Eukaryotic Cells: Have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Examples: Protists, fungi, plants, and animals.

Plant vs. Animal Cells:

• Plant Cells: Have a cell wall (cellulose), chloroplasts, and a central vacuole.

• Animal Cells: Have centrioles and lysosomes, which are generally absent in plant cells.

Cell Size and Surface Area-to-Volume Ratio

Cells are small to maximize the surface area-to-volume ratio, which facilitates efficient exchange of materials with the environment. As a cell grows, its volume increases faster than its surface area, limiting the size a cell can attain.

The Nucleus and Ribosomes

The Nucleus

• Nucleus: Contains most of the cell's genetic material (DNA) organized as chromosomes. Surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for material exchange.

• Nucleolus: A dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and ribosome assembly begins.

• Chromatin: The complex of DNA and proteins that forms chromosomes. Condenses during cell division.

Ribosomes

• Structure: Composed of rRNA and proteins.

• Function: Protein synthesis.

• Locations: Free in the cytosol (make proteins for use in the cytosol) or bound to the rough endoplasmic reticulum (make proteins for membranes or export).

The Endomembrane System

Components and Functions

• Nuclear Envelope

• Endoplasmic Reticulum (ER): Network of membranes; two types:

  • Rough ER: Studded with ribosomes; synthesizes secretory proteins and membranes.

  • Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs, stores calcium ions.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

• Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules; unique to animal cells.

• Vacuoles: Large vesicles with varied functions (storage, waste disposal, water balance). Central vacuole in plants stores water and maintains turgor pressure.

• Plasma Membrane: Regulates entry and exit of substances.

• Vesicles: Small membrane-bound sacs that transport materials between organelles.

Protein Pathway for Secretion

1. Protein synthesis begins on ribosomes bound to the rough ER.

2. Proteins are packaged into transport vesicles and sent to the Golgi apparatus.

3. Golgi modifies and sorts proteins, then packages them into vesicles for delivery to the plasma membrane or other destinations.

4. Vesicles fuse with the plasma membrane, releasing proteins outside the cell (exocytosis).

Energy Organelles

Mitochondria and Chloroplasts

• Mitochondria: Sites of cellular respiration; convert chemical energy in food to ATP. Present in both plant and animal cells.

• Chloroplasts: Sites of photosynthesis; convert solar energy to chemical energy in plants and algae.

• Structure:

  • Mitochondria: Double membrane; inner membrane folded into cristae; contains matrix.

  • Chloroplasts: Double membrane; internal thylakoid membranes stacked into grana; stroma is the fluid outside thylakoids.

• Endosymbiont Theory: Proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells. Evidence includes their double membranes, own DNA, and ribosomes.

Peroxisomes

• Function: Break down fatty acids and detoxify harmful substances. Produce hydrogen peroxide, which is then converted to water.

• Structure: Single membrane-bound organelles found in both plant and animal cells.

The Cytoskeleton

Structure and Function

The cytoskeleton is a dynamic network of protein fibers that provides structural support, maintains cell shape, and facilitates movement of organelles and the cell itself.

• Microtubules: Hollow tubes made of tubulin; maintain cell shape, guide organelle movement, separate chromosomes during cell division, form cilia and flagella.

• Microfilaments (Actin Filaments): Thin, solid rods; involved in cell shape, muscle contraction, and cell movement.

• Intermediate Filaments: Fibrous proteins; provide mechanical support and maintain cell shape.

Centrosome and Centrioles

• Centrosome: Microtubule-organizing center in animal cells, located near the nucleus.

• Centrioles: Pair of structures within the centrosome, each composed of nine triplets of microtubules arranged in a ring.

Cilia and Flagella

• Cilia: Short, numerous projections that move with a back-and-forth motion.

• Flagella: Longer, usually one or a few per cell, move with an undulating motion.

• Basal Body: Anchors cilia and flagella to the cell; structurally similar to centrioles.

Cellular Surfaces and Junctions

Cell Walls

• Plant Cell Wall: Rigid structure made of cellulose; provides protection, maintains shape, and prevents excessive water uptake.

• Plasmodesmata: Channels between plant cells that allow for transport and communication.

Extracellular Matrix (ECM)

• ECM: Network of glycoproteins (e.g., collagen) and other molecules outside animal cells; provides structural support, regulates cell behavior, and facilitates communication.

• Integrins: Receptor proteins in the plasma membrane that connect the ECM to the cytoskeleton.

Cell Junctions

Junction Type	Structure	Function	Location
Tight Junctions	Membranes pressed together	Prevent leakage of extracellular fluid	Animal cells (epithelial tissue)
Desmosomes	Anchoring junctions	Fasten cells together into strong sheets	Animal cells
Gap Junctions	Communicating junctions	Provide cytoplasmic channels between cells	Animal cells
Plasmodesmata	Channels through cell walls	Allow passage of materials between plant cells	Plant cells

Key Terms and Definitions

• Hydrolysis: Chemical breakdown of a compound due to reaction with water.

• Phagocytosis: Process by which a cell engulfs particles to form an internal compartment (food vacuole).

• Autophagy: Lysosomal process of recycling the cell's own organelles and macromolecules.

• Contractile Vacuole: Organelle in freshwater protists that pumps excess water out of the cell.

• Glycoprotein: Protein with carbohydrate chains attached; important in cell recognition and signaling.

• Cisternae: Flattened membranous sacs in the ER and Golgi apparatus.

• Matrix: The innermost compartment of the mitochondrion.

• Stroma: Fluid-filled space in chloroplasts surrounding the thylakoids.

• Thylakoids: Flattened sacs inside chloroplasts, site of the light-dependent reactions of photosynthesis.

• Granum (Grana): Stack(s) of thylakoids in chloroplasts.

• Plastids: Family of plant organelles including chloroplasts, amyloplasts (store starch), and chromoplasts (store pigments).

Summary Table: Plant vs. Animal Cell Structures

Structure	Plant Cell	Animal Cell
Cell Wall	Present	Absent
Chloroplasts	Present	Absent
Central Vacuole	Large, central	Small or absent
Lysosomes	Rare	Common
Centrioles	Absent	Present

Additional info:

• Surface area to volume ratio formula: $\text{Surface Area} = 6a^2$, $\text{Volume} = a^3$ for a cube of side a. As a cell increases in size, the volume increases faster than the surface area, limiting cell size.

• Endosymbiont theory is supported by the presence of circular DNA and prokaryote-like ribosomes in mitochondria and chloroplasts.

• Animal cells rely on the ECM for structural support and communication, while plant cells rely on the cell wall and plasmodesmata.