Cell Structure and Function: Study Notes for College Cell Biology

Cell Structure and Function

Introduction

Cell biology explores the structure and function of cells, the fundamental units of life. Understanding the organization and roles of subcellular components and organelles is essential for grasping how cells maintain homeostasis, interact with their environment, and carry out complex biological processes.

2.1 Cell Structure and Function

This topic covers the major subcellular components and organelles, focusing on their structure and function within the cell.

• Ribosomes: Non-membrane-bound structures composed of ribosomal RNA (rRNA) and protein. They synthesize proteins by translating messenger RNA (mRNA) sequences. Ribosomes are found in all forms of life, reflecting common ancestry.

• Endomembrane System: A group of membrane-bound organelles that work together to modify, package, and transport lipids and proteins. Includes the endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles.

• Endoplasmic Reticulum (ER): Provides mechanical support and plays a role in intracellular transport. Rough ER is studded with ribosomes and is involved in protein synthesis and compartmentalization. Smooth ER is involved in lipid synthesis and detoxification.

• Golgi Complex: A membrane-bound structure consisting of flattened membrane sacs. Functions include modifying newly synthesized cellular products and packaging proteins for trafficking.

• Mitochondria: Double-membrane organelles that provide compartments for metabolic reactions, including cellular respiration. The inner membrane (cristae) increases surface area for ATP production.

• Lysosomes: Membrane-enclosed sacs containing hydrolytic enzymes for digestion and programmed cell death (apoptosis).

• Vacuoles: Membrane-bound sacs for storage and maintaining turgor pressure, especially in plant cells.

• Chloroplasts: Double-membrane organelles found in plants and algae, serving as the site of photosynthesis.

2.2 Cell Size

Cell size affects the exchange of materials between cells and their environment. The surface area-to-volume ratio is a critical factor in determining the efficiency of this exchange.

• Surface Area-to-Volume Ratio: As cells increase in size, their volume grows faster than their surface area, reducing the efficiency of material exchange.

• Formula for Volume of a Cube:

$V = s^3$ where $s$ is the length of a side of the cube.

• Formula for Surface Area of a Cube:

$A = 6s^2$

• Surface Area-to-Volume Ratio:

$\text{SA:V Ratio} = \frac{6s^2}{s^3} = \frac{6}{s}$

• Smaller cells have a higher SA:V ratio, facilitating efficient exchange of materials.

2.3 Plasma Membrane

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell.

• Fluid Mosaic Model: Describes the plasma membrane as a dynamic structure composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming the bilayer.

• Membrane Proteins: Integral and peripheral proteins serve functions such as transport, signaling, and structural support.

2.4 Membrane Permeability

Membrane permeability refers to the ability of the plasma membrane to allow certain substances to pass while restricting others.

• Selective Permeability: The membrane allows small, nonpolar molecules (e.g., O2, CO2) to pass freely, while ions and large polar molecules require transport proteins.

• Role in Cell Function: Maintains homeostasis by controlling the internal environment.

2.5 Membrane Transport

Cells use various mechanisms to transport substances across the plasma membrane.

• Passive Transport: Movement of molecules down their concentration gradient without energy input (e.g., diffusion, osmosis).

• Facilitated Diffusion: Passive transport of molecules via membrane proteins.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (ATP).

• Bulk Transport: Endocytosis and exocytosis for large molecules.

2.6 Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules across the membrane via specific transport proteins.

• Channel Proteins: Provide corridors for specific molecules or ions.

• Carrier Proteins: Bind and transport substances across the membrane.

2.7 Tonicity and Osmoregulation

Tonicity describes the ability of a solution to cause a cell to gain or lose water. Osmoregulation is the process by which organisms maintain water and solute balance.

• Hypertonic Solution: Higher solute concentration outside the cell; cell loses water.

• Hypotonic Solution: Lower solute concentration outside the cell; cell gains water.

• Isotonic Solution: Equal solute concentration; no net water movement.

2.8 Mechanisms of Transport

Cells use various mechanisms to move substances across membranes.

• Simple Diffusion: Movement of small, nonpolar molecules.

• Facilitated Diffusion: Movement via transport proteins.

• Active Transport: Requires energy to move substances against their gradient.

• Endocytosis/Exocytosis: Bulk transport of large molecules.

2.9 Cell Compartmentalization

Compartmentalization refers to the division of the cell into distinct organelles, each with specialized functions.

• Membrane-bound Organelles: Allow for separation of metabolic processes and increased efficiency.

• Examples: Nucleus, mitochondria, chloroplasts, ER, Golgi apparatus.

2.10 Origins of Cell Compartmentalization

The origin of membrane-bound organelles in eukaryotic cells is explained by the endosymbiotic theory.

• Endosymbiotic Theory: Proposes that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA, and similarities to prokaryotic cells.

Table: Comparison of Organelle Structure and Function

Organelle	Structure	Function
Ribosome	Non-membrane-bound, rRNA and protein	Protein synthesis
Endoplasmic Reticulum (Rough)	Membrane-bound, studded with ribosomes	Protein synthesis, compartmentalization
Endoplasmic Reticulum (Smooth)	Membrane-bound, no ribosomes	Lipid synthesis, detoxification
Golgi Complex	Flattened membrane sacs	Modification and packaging of proteins
Mitochondria	Double membrane, inner folds (cristae)	ATP production, cellular respiration
Lysosome	Membrane-bound sac with enzymes	Digestion, apoptosis
Vacuole	Membrane-bound sac	Storage, turgor pressure
Chloroplast	Double membrane, internal thylakoids	Photosynthesis

Additional info:

• These notes expand on the brief syllabus points to provide definitions, examples, and context suitable for college-level cell biology students.

• Formulas for surface area and volume are included for understanding cell size constraints.

• The table summarizes key organelles for quick comparison.