Cell Structure, Membranes, and Cellular Processes: Study Notes for General Biology

Cellular Metabolism

Overview of Chemical Reactions in Cells

Cells carry out a variety of chemical reactions to build up or break down molecules, which are essential for life processes.

• Metabolism: The sum of all chemical reactions in a cell or organism.

• Anabolism: Phase of metabolism where complex molecules are formed from simpler ones, requiring energy.

• Catabolism: Phase of metabolism where complex molecules are broken down into simpler ones, releasing energy.

Cell Structure and Organelles

Nucleus and Ribosomes

The nucleus and ribosomes are key organelles in eukaryotic cells, responsible for storing genetic material and synthesizing proteins.

• Nucleus: Houses most of the cell's DNA.

• Ribosomes: Build proteins using information from DNA.

Endomembrane System

The endomembrane system is a complex network of membranes within eukaryotic cells that compartmentalizes cellular functions.

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

• Interaction: Components are either continuous or connected via transfer by vesicles.

Endoplasmic Reticulum (ER)

The ER is a biosynthetic factory that accounts for more than half of the total membrane in many eukaryotic cells.

• Smooth ER: Synthesizes lipids, detoxifies drugs and poisons, stores calcium ions.

• Rough ER: Secretes glycoproteins, distributes transport vesicles, serves as a membrane factory.

Golgi Apparatus

The Golgi apparatus consists of flattened membranous sacs called cisternae and is involved in modifying, sorting, and packaging macromolecules.

• Functions: Modifies products of the ER, manufactures certain macromolecules, sorts and packages materials into transport vesicles.

Lysosomes

Lysosomes are membranous sacs of hydrolytic enzymes that digest macromolecules.

• Function: Digest engulfed cells and recycle the cell's own organelles and macromolecules (autophagy).

• Phagocytosis: Forms a food vacuole that fuses with a lysosome to digest its contents.

Vacuoles

Vacuoles are large vesicles derived from the ER and Golgi apparatus, performing various functions in different cells.

• Function in Plants: Play a major role in plant cell growth.

Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-converting organelles found in eukaryotic cells.

• Mitochondria: Sites of cellular respiration, generating ATP using oxygen.

• Chloroplasts: Sites of photosynthesis in plants and algae.

Endosymbiont Theory

This theory suggests that an early ancestor of eukaryotes engulfed a prokaryotic cell, which evolved into mitochondria and chloroplasts.

• Similarities with Bacteria: Both are enveloped by a double membrane, contain free ribosomes and circular DNA, and can grow and reproduce independently.

Peroxisomes

Peroxisomes are specialized metabolic compartments that contain enzymes for oxidation reactions.

• Function: Detoxify alcohol and other harmful compounds, especially in the liver.

Cytoskeleton

Components and Functions

The cytoskeleton is a network of fibers that organizes structures and activities in the cell, providing structural support and facilitating movement.

• Microtubules: Thickest components, constructed of α-tubulin and β-tubulin dimers, diameter of 25 nm. Shape the cell, guide movement of organelles, and separate chromosomes during cell division.

• Intermediate Filaments: Fibrous proteins coiled into cables, diameter 8-12 nm. Support cell shape and fix organelles in place.

• Microfilaments (Actin Filaments): Thinnest components, diameter 7 nm. Maintain cell shape, allow changes in cell shape, facilitate muscle contraction, and enable cell motility.

Cilia and Flagella

Cilia and flagella are extensions of the plasma membrane containing microtubules arranged in a pattern of nine doublets surrounding two single microtubules.

• Function: Enable movement and are anchored by a basal body.

Extracellular Matrix (ECM) and Cell Junctions

The ECM regulates cell behavior, communicates with cells through integrins, and influences gene activity in the nucleus.

• Types of Cell Junctions in Epithelial Tissues:

  • Tight Junctions: Prevent leakage of extracellular fluid by pressing membranes of neighboring cells together.

  • Desmosomes: Fasten cells together into strong sheets, providing structural integrity.

  • Gap Junctions: Provide cytoplasmic channels between adjacent cells, allowing communication.

• Plasmodesmata: Channels that connect plant cells, allowing passage of water, solutes, and some proteins and RNA.

Plant Cell Walls

Plant cells have a primary cell wall, a middle lamella (containing pectins), and sometimes a secondary cell wall for additional support.

Cell Membranes and Transport

Plasma Membrane Structure and Function

The plasma membrane separates the living cell from its surroundings and controls the exchange of materials.

• Selective Permeability: Allows some substances to cross more easily than others.

• Main Components: Lipids (phospholipids), proteins, and carbohydrates.

• Phospholipids: Amphipathic molecules with hydrophobic (water-fearing) tails and hydrophilic (water-loving) heads.

• Fluid Mosaic Model: Describes the membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Membrane Asymmetry: Distinct inside and outside faces with different compositions.

Membrane Proteins

Proteins in the membrane perform various functions, including transport, enzymatic activity, signal transduction, cell-cell recognition, and attachment to the cytoskeleton and ECM.

• Peripheral Proteins: Bound to the surface of the membrane.

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

Carbohydrates in Membranes

• Glycolipids: Carbohydrates bonded to lipids.

• Glycoproteins: Carbohydrates bonded to proteins.

Membrane Fluidity

Membrane fluidity is affected by temperature, cholesterol content, and the percentage of unsaturated phospholipids.

• Cholesterol restrains movement at warm temperatures and maintains fluidity at cool temperatures.

• Organisms adjust membrane lipid composition in response to environmental temperature changes.

Transport Across Membranes

Cells transport substances across membranes using passive and active mechanisms.

• Passive Transport: Movement of small molecules without energy input (e.g., diffusion, facilitated diffusion).

• Bulk Transport: Movement of large molecules in and out of the cell via exocytosis and endocytosis.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy and transport proteins.

Diffusion and Osmosis

• Diffusion: Movement of particles from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Passive movement aided by transport proteins (e.g., channel proteins, aquaporins).

Electrochemical Gradients and Pumps

• Electrochemical Gradient: Combined forces of chemical concentration and electrical charge across a membrane.

• Sodium-Potassium Pump: Main electrogenic pump in animal cells, generating membrane potential.

$\text{Na}^+/\text{K}^+ \text{ pump:} \, 3\, \text{Na}^+ \text{ out}, \, 2\, \text{K}^+ \text{ in}$

Cell Signaling

Types and Stages of Cell Signaling

Cells communicate via chemical signals, which can be local or long-distance.

• Reception: Cell detects a signaling molecule via a receptor protein.

• Transduction: Signal is converted to a form that can bring about a cellular response, often involving a cascade of molecular interactions.

• Response: The transduced signal triggers a specific cellular activity.

Receptors and Signal Transduction

• G Protein-Coupled Receptors (GPCRs): Largest family of cell surface receptors, work with the help of G proteins.

• Receptor Tyrosine Kinases (RTKs): Membrane receptors that catalyze the transfer of phosphate groups from ATP to proteins.

• Ligand-Gated Ion Channels: Open or close in response to binding of a signaling molecule, allowing ions to pass through.

Second Messengers

• cAMP (Cyclic AMP): Produced from ATP, activates protein kinase A.

• Ca2+ ions: Trigger various cellular responses.

$\text{ATP} \rightarrow \text{cAMP} + \text{PP}_i$

Signal Amplification and Termination

• Amplification: Signal transduction cascades amplify the cellular response.

• Termination: Mechanisms ensure signals are not perpetually active.

Apoptosis

Apoptosis is programmed cell death, involving a cascade of specific cellular events.

• Caspases: Enzymes that carry out the death program by chopping proteins.

• Interference with apoptosis may contribute to cancer development.

Examples and Applications

• Quorum Sensing in Bacteria: Bacteria sense local population density and coordinate behavior.

• Synaptic Signaling: Neurons release neurotransmitters to communicate across synapses.

• Hormonal Signaling: Specialized cells release hormones that travel through the circulatory system to target cells.

Summary Table: Cell Junctions in Animal Cells

Junction Type	Structure	Function
Tight Junction	Membranes pressed together	Prevents leakage of extracellular fluid
Desmosome	Strong sheets, anchored by intermediate filaments	Provides structural integrity
Gap Junction	Channels between cells	Allows communication

Summary Table: Cytoskeletal Elements

Element	Diameter	Main Protein	Function
Microtubules	25 nm	α- and β-tubulin	Cell shape, organelle movement, chromosome separation
Intermediate Filaments	8-12 nm	Various fibrous proteins	Cell shape, organelle anchoring
Microfilaments	7 nm	Actin	Cell shape, muscle contraction, cell motility

Key Equations

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

• Electrochemical Gradient: $\Delta G = RT \ln \frac{[C]_\text{in}}{[C]_\text{out}} + zF\Delta \Psi$

Additional info: Some explanations and terminology have been expanded for clarity and completeness, including definitions, examples, and equations relevant to General Biology.