Cell Structure, Organelles, and Transport: General Biology Study Guide

Cell Structure and Function

Single Cells & Examples

Cells are the basic functional units of life, and their size and shape can vary widely depending on their type and function.

• Eggs (e.g., chicken, ostrich): Before fertilization, eggs are single cells, among the largest known cells.

• Neurons: Can be extremely long and narrow. Example: Giraffe neck neurons can be up to 2 meters long.

• Fertilized eggs: Once cell division begins, the zygote is no longer a single cell.

Prokaryotic vs. Eukaryotic Cells

Cells are classified based on the presence or absence of a nucleus and other membrane-bound organelles.

• Eukaryotic Cells:

  • Have a nucleus with linear DNA.

  • Possess membrane-bound organelles (e.g., mitochondria, ER).

  • Multiple chromosomes.

• Prokaryotic Cells:

  • No nucleus; DNA is circular and located in the nucleoid.

  • Fewer organelles.

• Key distinction: Presence/absence of a nucleus.

Cell Boundaries and Support Structures

Cell Wall (Plants, Fungi, Algae, Some Prokaryotes)

The cell wall provides structural support and protection to certain cells.

• Plant Cell Wall:

  • Built from cellulose microfibrils and macrofibrils.

  • Layers:

    1. Middle lamella – pectin-rich "glue" between cells.

    2. Primary wall – thin, flexible.

    3. Secondary wall – thick, rigid, structural support.

• Fungal Cell Wall: Made of chitin (nitrogen-containing glucose derivative).

• Functions:

  • Mechanical support, maintain shape.

  • Prevent cell bursting (osmotic regulation).

  • Provide pathogen defense.

  • Enable recognition (e.g., pollen) and symbiosis (e.g., nitrogen-fixing bacteria).

Extracellular Matrix (ECM) – Animal Cells

Animal cells lack cell walls but have an ECM for structure and communication.

• Components:

  • Proteoglycans: Up to 95% carbohydrate; form gel-like matrix.

  • Collagen & Fibrin: Strong fibers for strength & flexibility.

  • Fibronectin: Links ECM to integrins in the plasma membrane.

• Functions:

  • Coordinates cells (e.g., synchronized heartbeats).

  • Maintains structural integrity and adhesion.

Cell Membranes and Junctions

Plasma Membrane

The plasma membrane is a selective barrier found in all cells, crucial for maintaining homeostasis.

• Composition: Phospholipids, cholesterol, proteins, carbohydrates.

• Functions:

  • Selective barrier for transport.

  • Sites for chemical reactions.

  • Communication/signaling & recognition.

  • Maintains shape and stability.

• Proteins:

  • Integral proteins (integrins): Span the membrane, connect ECM → cytoskeleton.

  • Peripheral proteins: Loosely attached; signaling/structural roles.

• Carbohydrates: Glycoproteins & glycolipids act as markers for cell recognition.

Cell Junctions

Cell junctions connect cells and regulate the passage of materials.

• Tight junctions: Seal cells into sheets; prevent leakage.

• Desmosomes (anchoring junctions): Act like rivets; fasten cells into strong sheets.

• Gap junctions: Protein-lined pores allowing ions & molecules to pass; enable electrical/chemical communication.

Transport Mechanisms

Gradients & Transport

Transport across membranes is driven by concentration gradients and can be passive or active.

• Gradient analogy: Like water behind a dam (pressure difference drives flow).

• Passive Transport:

  • Simple diffusion: Movement down concentration gradient (no energy).

  • Facilitated diffusion: Uses channel/carrier proteins; still passive.

• Active Transport:

  • Moves substances against gradient using ATP.

  • Example: Sodium-Potassium Pump: $3 \text{Na}^+ \text{ out}, 2 \text{K}^+ \text{ in per cycle}$ Essential for nerve signaling (rapid pump cycles send signals in reflexes).

• Osmosis: Diffusion of water across a membrane.

  • Hypertonic: Higher solute outside → water leaves cell → shrinks.

  • Hypotonic: Lower solute outside → water enters → swells/bursts.

  • Isotonic: Equal solute inside/outside → no net movement.

Clinical note: IV bags contain 0.9% saline (isotonic to blood) to prevent RBC damage.

Endocytosis & Exocytosis

Cells import and export large materials via vesicle-mediated processes.

• Exocytosis: Export of large/bulk materials.

• Endocytosis: Import of materials.

  • Pinocytosis ("cell drinking"): Fluids.

  • Receptor-mediated endocytosis: Highly specific uptake of molecules via receptor proteins.

Cellular Organelles

Major Classes of Organic Molecules

Cells are composed of four major classes of organic molecules:

• Carbohydrates: Energy & structure (e.g., glucose, cellulose).

• Lipids: Long-term energy storage, membranes (e.g., fats, phospholipids).

• Proteins: Structure, enzymes, transport, communication.

• Nucleic acids: DNA & RNA store genetic information. Additional info: Nucleic acids are often included as a fourth major biomolecule.

Golgi Apparatus (Dictyosomes)

The Golgi apparatus acts as the cell's shipping and packaging department.

• Structure: Series of flattened membrane sacs called cisternae.

• Two distinct sides:

  • Cis face: Receiving side (accepts vesicles from rough ER).

  • Trans face: Shipping side (sends out vesicles).

• Function:

  • Processes, sorts, and modifies materials from the ER.

  • Packages proteins and lipids into vesicles.

  • Ships materials to other organelles (e.g., lysosomes) or the plasma membrane for secretion (exocytosis).

• Key Notes: Each vesicle that buds off the Golgi contains substances destined for a specific location.

Lysosomes

Lysosomes are the cell's recycling center and waste disposal unit, found only in animal cells.

• Structure: Membrane-bound sacs containing hydrolytic enzymes.

• Function:

  • Digest macromolecules (proteins, polysaccharides, lipids, nucleic acids).

  • Break down worn-out organelles (cellular recycling).

  • Fuse with food vacuoles to digest ingested material.

  • Destroy pathogens (defense mechanism).

• Why Acidic?:

  • Enzymes function best at low pH.

  • Acidic interior keeps destructive enzymes from harming the rest of the cell.

Peroxisomes

Peroxisomes are the cell's detox centers, breaking down fatty acids and neutralizing hydrogen peroxide.

• Structure: Membrane-bound sacs containing oxidative enzymes (e.g., catalase).

• Function:

  • Break down fatty acids for energy.

  • Detoxify harmful substances (e.g., alcohol, hydrogen peroxide).

  • Produce hydrogen peroxide (H2O2) as a byproduct, then convert it into water & oxygen.

  • In plants: Specialized peroxisomes (glyoxysomes) aid seed germination.

Hydrogen Peroxide (H2O2) in Biology & Applications

Hydrogen peroxide is produced in cells during metabolism and must be quickly broken down to prevent oxidative damage.

• In Cells: Produced in peroxisomes during metabolism; quickly broken down.

• In Medicine:

  • Household use: 3% solution.

  • Reacts with tissue – bubbles form (O2 release).

  • Kills bacteria and healthy cells – slows wound healing.

  • Not recommended for long-term wound care.

• Storage & Chemistry: Kept in brown bottles (light breaks it down into water + oxygen).

• Industrial/technical uses: Used at much higher concentrations (e.g., 70%) for etching metals (titanium, tungsten, microcircuits).

Material Science Side Notes

Connections between cell biology and material science are seen in the use of hydrogen peroxide for etching metals.

• Tungsten: Brittle but can hold a sharp edge (e.g., razor blades, drill bits).

• Titanium: Very tough, corrosion-resistant; used in medical implants and consumer products.

• Connection to peroxisomes/hydrogen peroxide: Industrial H2O2 is used in etching these metals.

Cytoskeleton

Cytoskeleton

The cytoskeleton is a dynamic framework that gives structure, movement, and intracellular transport to cells.

• Components:

  • Microtubules (25 nm): Hollow tubes made of tubulin; resist compression; involved in motility, intracellular transport, and chromosome movement.

    • Arrangements: 9+2 (cilia/flagella), 9 triplets (centrioles/basal bodies).

  • Microfilaments (7 nm): Made of actin; resist tension/stretching; roles in cell shape, motility, cytokinesis, microvilli reinforcement, and cytoplasmic streaming.

  • Intermediate filaments (12 nm): Fibrous proteins (e.g., keratin); provide tension resistance, anchor organelles, and form the nuclear lamina.

Summary Table: Cell Organelles and Functions

Organelle	Main Function	Key Features
Golgi Apparatus	Processes, sorts, packages, and ships proteins & lipids	Flattened cisternae; cis/trans faces
Lysosome	Digests macromolecules, recycles components	Hydrolytic enzymes; acidic pH
Peroxisome	Detoxifies harmful substances, breaks down fatty acids	Oxidative enzymes; produces H2O2
Plasma Membrane	Selective barrier, communication	Phospholipid bilayer, proteins, carbohydrates
Cell Wall	Structural support, protection	Cellulose (plants), chitin (fungi)
ECM	Support, adhesion, signaling	Proteoglycans, collagen, fibronectin

Key Concepts & Takeaways

• Cell size/shape varies widely: eggs (large), neurons (long), bacteria (tiny).

• Cell walls vs. ECM: Plants/fungi use rigid walls; animals rely on ECM.

• Connectivity: Integrins link the ECM → cytoskeleton for coordination.

• Transport: Passive (diffusion) vs. active (ATP-driven).

• Tonicity: Relative solute concentrations determine water movement.

• Cytoskeleton: Dynamic framework giving structure, movement, and intracellular transport.

Additional info: Nucleic acids (DNA & RNA) are often included as a fourth major biomolecule in cell biology.