General Biology: Cell Structure, Membrane Dynamics, and Bioenergetics Study Guide

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Cell Structure and Function

Limitations on Cell Size

Cell size is constrained by several physical and biological factors, which explain why most cells are small.

Surface Area-to-Volume Ratio: As cells grow larger, their volume increases faster than their surface area, limiting efficient exchange of materials.
Diffusion Limitations: Larger cells have difficulty transporting nutrients and waste efficiently.
Support for Cell Functions: Larger cells require more structural support and energy.
Examples: Most prokaryotic cells are smaller than eukaryotic cells due to these limitations.

Microscopy: Light vs. Electron

Different cellular structures can be visualized using light and electron microscopy, each with distinct capabilities.

Light Microscopy: Used for viewing living cells and general cell structure.
Transmission Electron Microscopy (TEM): Provides detailed images of internal cell structures.
Scanning Electron Microscopy (SEM): Visualizes cell surfaces in high detail.
Example: TEM is used to observe organelles like mitochondria and the nucleus.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structure and complexity.

Prokaryotic Cells: Lack membrane-bound organelles; generally smaller and simpler.
Eukaryotic Cells: Possess membrane-bound organelles and compartmentalized functions; generally larger and more complex.
Example: Animal and plant cells are eukaryotic; bacteria are prokaryotic.

Ribosomes: Structure and Function

Ribosomes are the site of protein synthesis in all cells.

Structure: Composed of ribosomal RNA and proteins; found in cytosol or attached to the endoplasmic reticulum.
Function: Translate mRNA into polypeptides.
Example: Free ribosomes synthesize cytosolic proteins; bound ribosomes synthesize secretory and membrane proteins.

Endoplasmic Reticulum (ER)

The ER is a network of membranes involved in protein and lipid synthesis.

Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.
Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.
Example: Liver cells have abundant smooth ER for detoxification.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

Function: Glycosylation of proteins, sorting, and vesicle formation.
Pathway: Proteins move from ER → Golgi → plasma membrane or other destinations.
Example: Secretory proteins are processed in the Golgi before exocytosis.

Lysosomes, Peroxisomes, and Mitochondria

These organelles perform specialized functions essential for cell survival.

Lysosomes: Contain digestive enzymes for breaking down macromolecules.
Peroxisomes: Break down fatty acids and detoxify harmful substances.
Mitochondria: Site of cellular respiration and ATP production.
Chloroplasts: (in plants) Site of photosynthesis.

Cytoskeleton and Motor Proteins

Cytoskeletal Components

The cytoskeleton provides structural support and facilitates movement within cells.

Microtubules: Hollow tubes; involved in cell shape, transport, and division.
Microfilaments (Actin Filaments): Thin fibers; support cell shape and enable movement.
Intermediate Filaments: Provide mechanical strength.

Component	Structure	Subunit	Primary Roles
Microtubules	Hollow tubes	Tubulin	Cell shape, transport, division
Microfilaments	Thin fibers	Actin	Cell movement, shape
Intermediate Filaments	Rope-like fibers	Various proteins	Mechanical strength

Motor Proteins

Motor proteins interact with cytoskeletal components to produce movement.

Kinesin and Dynein: Move along microtubules; transport vesicles and organelles.
Myosin: Moves along actin filaments; involved in muscle contraction and cell movement.

Cell Membrane Structure and Function

Fluid-Mosaic Model

The cell membrane is a dynamic structure composed of lipids, proteins, and carbohydrates.

Phospholipid Bilayer: Provides a semi-permeable barrier.
Integral and Peripheral Proteins: Facilitate transport and communication.
Cholesterol: Modulates membrane fluidity.

Membrane Fluidity

Membrane fluidity is influenced by lipid composition and temperature.

Unsaturated Fatty Acids: Increase fluidity.
Saturated Fatty Acids: Decrease fluidity.
Cholesterol: Buffers fluidity changes.
Example: Cells adjust membrane composition in response to temperature changes.

Membrane Proteins

Proteins are associated with the membrane in different ways.

Integral Proteins: Span the membrane; involved in transport and signaling.
Peripheral Proteins: Attached to the membrane surface.
Lipid-Anchored Proteins: Covalently attached to lipids within the membrane.

Transport Across Membranes

Selective Permeability

The cell membrane allows selective passage of substances.

Small, nonpolar molecules: Pass easily.
Large or charged molecules: Require transport proteins.
Example: Oxygen and carbon dioxide diffuse freely; ions require channels.

Passive and Active Transport

Transport mechanisms are classified as passive or active based on energy requirements.

Passive Transport: Movement down concentration gradient; no energy required.
Active Transport: Movement against gradient; requires ATP.
Facilitated Diffusion: Passive transport via proteins.
Example: Sodium-potassium pump uses ATP for active transport.

Osmosis and Diffusion

Osmosis is the movement of water across membranes; diffusion is the movement of solutes.

Osmosis: Water moves from low solute to high solute concentration.
Diffusion: Solutes move from high to low concentration.
Example: Red blood cells in hypotonic solution swell; in hypertonic solution shrink.

Bioenergetics and Thermodynamics

Energy in Biological Systems

Cells require energy to perform work, which is governed by the laws of thermodynamics.

First Law: Energy cannot be created or destroyed.
Second Law: Entropy increases in energy transformations.
Example: Cellular respiration converts glucose to ATP, releasing heat.

Free Energy and Spontaneity

Free energy changes () determine whether reactions are spontaneous.

Spontaneous Reactions: Occur without energy input; .
Nonspontaneous Reactions: Require energy input; .
Example: Hydrolysis of ATP is spontaneous.

Enzymes and Catalysis

Enzymes are biological catalysts that speed up reactions by lowering activation energy.

Active Site: Region where substrate binds.
Specificity: Enzymes are specific to substrates.
Inhibition: Inhibitors reduce enzyme activity; measured by changes in and .
Example: Competitive inhibitors increase but do not affect .

Energy Diagrams

Energy diagrams illustrate the energy changes during chemical reactions.

Activation Energy: Energy required to start a reaction.
Transition State: High-energy intermediate.
Enzyme Catalysis: Lowers activation energy, increasing reaction rate.

Summary Table: Types of Transport

Type	Energy Required	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via GLUT transporter
Active Transport	Yes (ATP)	Against gradient	Na+/K+ pump
Osmosis	No	Water down gradient	Water across aquaporins

Key Equations

Free Energy Change:
Rate of Diffusion:

Additional info:

Some context and explanations have been expanded for clarity and completeness.
Tables have been recreated and summarized for study purposes.