Eukaryotic Cell Structure, Membranes, and Transport: Study Guide

A Tour of the Cell

Cell Theory and Types of Cells

The cell theory is a foundational concept in biology, stating that all living organisms are composed of cells, which are the smallest units of life. Cells arise from pre-existing cells and carry out all necessary functions for life.

• Cell Theory: All living things are made of cells; cells are the basic units of structure and function in living organisms.

• Hierarchy: Atoms → Molecules → Cells → Tissues → Organs → Organisms.

• Two Major Categories:

  • Prokaryotes: Bacteria and Archaea; lack a nucleus and membrane-bound organelles; DNA is in a nucleoid region.

  • Eukaryotes: Eukarya; have a true nucleus and membrane-bound organelles; DNA is linear and enclosed within the nucleus.

• Shared Features of All Cells:

  • Cytoplasm (cytosol): Semi-fluid substance where subcellular components are suspended.

  • Plasma Membrane: Selective barrier regulating passage of substances.

  • Chromosomes: Carry genetic information (DNA).

  • Ribosomes: Sites of protein synthesis.

Comparison of Prokaryotic and Eukaryotic Cells

• Prokaryotic Cells:

  • Smaller (1-10 μm), no nucleus, no membrane-bound organelles, circular DNA in nucleoid.

  • First to evolve; structurally simpler.

• Eukaryotic Cells:

  • Larger (10-100 μm), true nucleus, membrane-bound organelles (e.g., mitochondria, chloroplasts), linear DNA.

  • Structurally more complex due to compartmentalization.

Cell Size and Surface Area-to-Volume Ratio

• Cells remain small to maintain a high surface area-to-volume ratio, which facilitates efficient exchange of materials.

• As cell size increases, volume grows faster than surface area, limiting efficiency.

• Microvilli increase surface area without significantly increasing volume (e.g., in intestinal cells).

Why Organelles Allow Larger Cell Size

• Membrane-bound organelles compartmentalize functions, allowing specialization and increased efficiency.

• Organelles perform specific roles: control, manufacture, energy processing, structural support, movement, and communication.

The Nucleus and Ribosomes

Nucleus: Genetic Control Center

• Nucleus: Contains most of the cell's genes; controls cellular activities by directing protein synthesis.

• Nuclear Envelope: Double membrane (two phospholipid bilayers) with nuclear pores for molecular transport; continuous with the endoplasmic reticulum (ER).

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

• Chromosomes: Structures carrying genetic information; each consists of one long DNA molecule and associated proteins.

• Nuclear Lamina: Protein network supporting nuclear shape.

Role of DNA in Protein Synthesis

• DNA is transcribed into messenger RNA (mRNA) in the nucleus.

• mRNA exits the nucleus via nuclear pores and is translated by ribosomes in the cytoplasm into proteins.

Ribosomes: Protein Factories

• Composed of rRNA and proteins; not membrane-bound (not organelles).

• Two types:

  • Free Ribosomes: Suspended in cytosol; synthesize proteins for use within the cell.

  • Bound Ribosomes: Attached to rough ER or nuclear envelope; synthesize proteins for membranes, organelles, or secretion.

• Structurally identical; can switch roles.

The Endomembrane System

Components and Functions

The endomembrane system is a network of membranes within eukaryotic cells that regulates protein traffic and performs metabolic functions.

• Includes: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles/vacuoles, and plasma membrane.

• Functions:

  • Protein synthesis and transport

  • Lipid metabolism and movement

  • Detoxification of poisons

Pathway of Molecules Through the Endomembrane System

1. Rough ER → 2. Transport Vesicles → 3. Golgi Apparatus → 4. Vesicles → 5. Final Destination (plasma membrane, secretion, or lysosomes)

Three Destinations for Endomembrane Products

• Fusion with plasma membrane

• Secretion outside the cell

• Delivery to organelles (e.g., lysosomes)

Organelle Functions

• Rough ER (RER): Studded with ribosomes; synthesizes proteins (especially secretory and membrane proteins); produces membranes.

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids (oils, steroids, phospholipids), metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions.

• Vesicles: Membrane-bound sacs for storage and transport; can fuse with other organelles or the plasma membrane.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids; consists of cis (receiving) and trans (shipping) faces; manufactures some macromolecules (e.g., polysaccharides).

• Lysosomes: Membranous sacs of hydrolytic enzymes; digest macromolecules, recycle cell components (autophagy), and participate in defense (e.g., macrophages).

Table: Main Organelles of the Endomembrane System

Organelle	Main Function
Rough ER	Protein synthesis, membrane production
Smooth ER	Lipid synthesis, detoxification, Ca2+ storage
Golgi Apparatus	Modification, sorting, packaging of proteins/lipids
Lysosome	Digestion of macromolecules, recycling
Vesicles	Transport and storage

Energy-Transforming Organelles

Mitochondria

• Site of cellular respiration; generates ATP by extracting energy from sugars, fats, and other fuels using oxygen.

• Double membrane: outer (smooth) and inner (folded into cristae).

• Compartments: intermembrane space and mitochondrial matrix (contains enzymes, DNA, ribosomes).

• Contain their own circular DNA and ribosomes; evidence for endosymbiotic origin.

Peroxisomes

• Contain enzymes that transfer hydrogen from substrates to oxygen, producing hydrogen peroxide (H2O2), which is then converted to water.

• Detoxify harmful substances (e.g., alcohol in liver cells).

The Cytoskeleton

Structure and Function

• Network of protein fibers providing structural support, shape, and facilitating movement.

• Dynamic; can assemble/disassemble to reshape the cell.

• Interacts with motor proteins for cell motility and intracellular transport.

Three Types of Cytoskeletal Fibers

Fiber Type	Diameter	Protein Subunit	Main Functions
Microtubules	Thickest	Tubulin	Shape, support, tracks for movement, chromosome separation
Microfilaments	Thinnest	Actin	Shape, muscle contraction, cell division, microvilli formation
Intermediate Filaments	Intermediate	Various (e.g., keratin)	Structural support, anchoring organelles

• Microvilli: Formed by bundles of microfilaments; increase surface area for absorption (e.g., in the digestive tract).

• Cilia and Flagella: Composed of microtubules; cilia move substances over cell surfaces (e.g., respiratory tract), flagella propel cells (e.g., sperm).

Extracellular Components and Cell Junctions

Extracellular Matrix (ECM)

• Composed of glycoproteins (e.g., collagen, elastin, proteoglycans, fibronectin) secreted by cells.

• Provides structural support, regulates cell behavior, and facilitates communication.

Cell Junctions

Junction Type	Structure	Main Function
Tight Junctions	Seals cells together	Prevents leakage of extracellular fluid
Desmosomes	Button-like rivets	Anchors cells together (e.g., skin)
Gap Junctions	Channels between cells	Allows passage of ions and small molecules

Membrane Structure and Function

Fluid Mosaic Model

• Membranes are composed of a phospholipid bilayer with embedded proteins and carbohydrates.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails; form bilayers in aqueous environments.

• Fluid Mosaic: Lipids and proteins move laterally within the layer; membrane is dynamic, not static.

• Cholesterol: Modulates membrane fluidity; restrains movement at high temperatures, prevents solidification at low temperatures.

Membrane Proteins

• Integral Proteins: Penetrate the hydrophobic core; often span the membrane (transmembrane proteins).

• Peripheral Proteins: Loosely bound to the membrane surface; often attached to integral proteins.

• Six Major Functions:

  1. Transport (channels and carriers)

  2. Enzymatic activity

  3. Signal transduction

  4. Cell-cell recognition

  5. Intercellular joining

  6. Attachment to cytoskeleton and ECM

Membrane Carbohydrates

• Short, branched chains attached to lipids (glycolipids) or proteins (glycoproteins).

• Function as cell markers for recognition (e.g., blood types).

• Located on the extracellular surface of the plasma membrane.

Membrane Permeability and Transport

Selective Permeability

• Plasma membrane allows some substances to cross more easily than others.

• Essential for maintaining homeostasis.

Types of Molecules and Their Permeability

Molecule Type	Permeability	Examples
Small, nonpolar (hydrophobic)	Cross easily	O2, CO2, lipids
Small, uncharged polar	Cross slowly	H2O
Large, uncharged polar	Do not cross easily	Glucose
Ions	Do not cross	Na+, K+, Cl-

Transport Proteins

• Channel Proteins: Provide hydrophilic channels for specific molecules or ions (e.g., aquaporins for water).

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

• Both types facilitate the movement of substances that cannot diffuse through the lipid bilayer.

Summary Table: Types of Membrane Transport

Transport Type	Energy Required?	Protein Involved?	Example
Simple Diffusion	No	No	O2, CO2
Facilitated Diffusion	No	Yes (channel/carrier)	Glucose, ions
Active Transport	Yes (ATP)	Yes (pump)	Na+/K+ pump

Key Equations

• Surface Area of a Sphere: $SA = 4\pi r^2$

• Volume of a Sphere: $V = \frac{4}{3}\pi r^3$

• Surface Area to Volume Ratio: $\text{Ratio} = \frac{SA}{V}$

Example: As a cell's radius increases, its surface area to volume ratio decreases, limiting efficient exchange of materials.

Additional info: The notes above expand on the original content by providing definitions, examples, and tables for clarity and completeness, as well as equations relevant to cell size and transport.