Cell Structure and Function: Study Notes

Chapter 6: Cell Structure and Function

Cell as the Basic Unit of Life

The cell is the fundamental structural and functional unit of all living organisms. The discovery of cells was made possible by advancements in microscopy.

• Robert Hooke (1665): First observed cells in cork using a microscope.

• Cell Theory (Schleiden & Schwann, 1838):

  • All living things are composed of cells.

  • Cells are the smallest units of life.

  • New cells arise from division of existing cells.

Features of Cells

Cells vary in size, shape, and internal organization, reflecting their diverse functions.

• Size Range: Most cells are between 1–100 μm and require a light microscope to be seen. Organelles within cells often require electron microscopy.

• Structure = Function: The unique shapes of cells are closely related to their specific functions (e.g., neurons, muscle cells).

• Surface Area to Volume Ratio: Smaller cells have a higher surface area-to-volume ratio, facilitating efficient exchange of materials.

Types of Cells

Cells are classified into two major types based on the presence or absence of a nucleus.

Type	Features	Examples	Size
Prokaryotic	No nucleus, nucleoid region, no membrane-bound organelles	Bacteria, Archaea	1–10 μm
Eukaryotic	True nucleus with nuclear membrane, membrane-bound organelles	Protists, animals, plants, fungi	10–100 μm

Cell Structure Overview

Plasma Membrane

All cells possess a plasma membrane, which acts as a selective barrier regulating the movement of substances in and out of the cell.

• Structure: Composed of a phospholipid bilayer with embedded proteins and carbohydrates.

• Function: Maintains homeostasis, facilitates communication, and provides structural support.

Eukaryotic Cell Organelles

Eukaryotic cells contain membrane-bound organelles that compartmentalize cellular functions.

• Nucleus: Contains genetic material (DNA), site of transcription.

• Ribosomes: Sites of protein synthesis; found free in cytoplasm or attached to endoplasmic reticulum.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles. Functions in synthesis, modification, and transport of cellular materials.

• Mitochondria: Powerhouse of the cell; site of cellular respiration. Contains its own DNA and ribosomes. Has double membrane (outer and inner), cristae, and matrix.

• Chloroplasts: Found in plant cells; site of photosynthesis. Contains its own DNA and ribosomes. Has double membrane, thylakoids, grana, and stroma.

• Peroxisomes: Single-membrane organelles containing enzymes that generate and degrade hydrogen peroxide ($H_2O_2$), detoxify compounds, and break down fatty acids.

The Cytoskeleton

The cytoskeleton provides mechanical support, maintains cell shape, and enables cell movement and division.

• Microtubules: Hollow rods (25 nm diameter) made of tubulin. Functions include:

  • Serving as tracks for molecular motor proteins (e.g., kinesin, dynein).

  • Formation of centrioles for chromosome movement during cell division.

  • Providing structural support against compression forces.

  • Composing cilia and flagella (9+2 arrangement).

• Microfilaments: Solid rods (7 nm diameter) made of actin. Functions include:

  • Maintaining cell shape by resisting tension (pulling forces).

  • Cell motility (muscle contraction with myosin, cleavage furrow during cell division, pseudopodia, cytoplasmic streaming).

• Intermediate Filaments: Cables (8–12 nm diameter) made of keratin. Functions include:

  • Maintaining cell shape.

  • Anchoring organelles.

  • Not typically re-arranged like microtubules or microfilaments.

Cell Surface Structures

Cell Wall

Found in plants and algae, the cell wall provides protection and structural support.

• Composition: Cellulose, polysaccharides, protein.

• Layers: Primary cell wall, middle lamella (pectin), secondary cell wall.

Extracellular Matrix (ECM)

In animal cells, the ECM provides structural support and mediates cell signaling.

• Components: Glycoproteins (collagen, proteoglycans), fibronectin, integrins.

• Functions: Attaches ECM to cell, connects ECM to cytoskeleton.

Intracellular Junctions

Plasmodesmata

Found in plant cells, plasmodesmata are plasma membrane channels that allow passage of small molecules between cells.

Junctions in Animal Cells

Junction Type	Structure	Function
Tight Junctions	Adjacent cells held tightly together, forming a belt	Creates a water-tight barrier; prevents leakage (e.g., skin, intestinal lining)
Desmosomes	Anchoring junctions, reinforced by intermediate filaments	Fasten cells together into sheets; provide mechanical strength
Gap Junctions	Channels connecting adjacent cells	Allow passage of ions and small molecules; enable cell communication

Review Questions and Applications

• Which cytoskeletal element is responsible for cytoplasmic streaming? Answer: Microfilaments (actin).

• Which cytoskeletal element is made up of tubulin? Answer: Microtubules.

• What sugar is found in RNA? Answer: Ribose.

• What type of bond connects the monomers of nucleic acids into polymers? Answer: Phosphodiester bond.

• Your skin is waterproof. What type of cell-cell junction is most likely involved in creating a water-tight barrier between the outside and inside of your body? Answer: Tight junctions.

Summary Table: Cell Junctions

Junction	Formed by	Function	Similar to Plasmodesmata?
Tight Junctions	Membrane proteins	Water-tight barrier	No
Desmosomes	Intermediate filaments	Anchoring, sheet formation	No
Gap Junctions	Channel proteins	Communication, passage of materials	Yes

Additional info: These notes expand on the provided slides and images, adding definitions, examples, and academic context for clarity and completeness.