Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotic cells lack a nucleus and membrane-bound organelles. Their DNA is located in the nucleoid region. Example: Escherichia coli.

• Eukaryotic cells have a true nucleus and various membrane-bound organelles. Example: Homo sapiens (human cells).

• Key differences: Eukaryotes are generally larger, more complex, and can be multicellular.

Plant vs. Animal Cells

Plant and animal cells share many features but also have distinct differences.

• Plant cells have a cell wall, chloroplasts, and large central vacuole.

• Animal cells lack cell walls and chloroplasts but have centrioles and lysosomes.

• Example: Photosynthesis occurs in plant cells due to chloroplasts.

Cell Surface Area and Volume

The ratio of surface area to volume affects cell efficiency in exchanging materials.

• As cells grow, volume increases faster than surface area.

• Smaller cells have a higher surface area-to-volume ratio, facilitating efficient transport.

• Formula: , for a cube of side a.

Cellular Organelles and Their Functions

Organelles perform specialized functions within eukaryotic cells.

• Nucleus: Stores genetic material and coordinates cell activities.

• Mitochondria: Site of cellular respiration and ATP production.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Digest cellular waste.

• Ribosomes: Synthesize proteins.

Endomembrane System

The endomembrane system coordinates the synthesis, modification, and transport of cellular products.

• Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles.

• Functions in protein and lipid transport.

Endosymbiotic Theory

This theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Proposes that these organelles originated from free-living prokaryotes engulfed by ancestral eukaryotes.

• Supported by double membranes and their own DNA.

Mitochondria and Chloroplasts

Both organelles are essential for energy conversion in cells.

• Mitochondria: Convert glucose to ATP via cellular respiration.

• Chloroplasts: Convert solar energy to chemical energy via photosynthesis (in plants).

Ribosomes

Ribosomes are the site of protein synthesis.

• Composed of rRNA and proteins.

• Found free in cytoplasm or attached to rough ER.

Cytoskeleton

The cytoskeleton provides structural support and facilitates movement.

• Composed of microfilaments, intermediate filaments, and microtubules.

• Functions in cell shape, transport, and division.

Cytoskeleton Components

Cilia and Flagella

These structures enable cell movement.

• Cilia: Short, numerous, move fluid over cell surface.

• Flagella: Long, few, propel cells (e.g., sperm).

Centrioles and Centrosomes

Centrioles are involved in cell division and organization of microtubules.

• Centrosome is the microtubule organizing center.

• Centrioles help form spindle fibers during mitosis.

Cell Membrane Structure and Function

Cell Membrane Structure

The cell membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins.

• Phospholipids: Form the bilayer; hydrophilic heads and hydrophobic tails.

• Proteins: Integral and peripheral; function in transport, signaling, and structure.

• Cholesterol: Modulates fluidity.

Fluid Mosaic Model

This model describes the dynamic nature of the cell membrane.

• Membrane components move laterally within the bilayer.

• Proteins and lipids are not fixed in place.

Membrane Permeability

Selective permeability allows certain substances to cross more easily than others.

• Small, nonpolar molecules pass easily; ions and large polar molecules require transport proteins.

Membrane Protein Functions

Diffusion and Osmosis

Movement of molecules across membranes occurs via diffusion and osmosis.

• Diffusion: Movement from high to low concentration.

• Osmosis: Diffusion of water across a membrane.

Active and Passive Transport

Cells use different mechanisms to move substances across membranes.

• Passive transport: No energy required (diffusion, facilitated diffusion, osmosis).

• Active transport: Requires energy (ATP) to move substances against concentration gradient.

• Example: Sodium-potassium pump ( ATPase).

Electrochemical Gradient and Membrane Potential

Membrane potential is created by differences in ion concentration and charge across the membrane.

• Electrochemical gradient: Combination of concentration and electrical gradients.

• Membrane potential: Voltage across the membrane, important for nerve impulses.

Cell Signaling and Communication

Receptors and Ligands

Cells communicate via signaling molecules (ligands) and receptors.

• Membrane receptors: Bind ligands outside the cell.

• Intracellular receptors: Bind ligands inside the cell.

• Ligand: A molecule that binds to a receptor to initiate a response.

G-Proteins and Ligand-Gated Channels

G-proteins and ligand-gated channels are important in signal transduction.

• G-proteins: Relay signals from receptors to target molecules inside the cell.

• Ligand-gated channels: Open or close in response to ligand binding.

Signal Transduction Pathways

Signal transduction involves a cascade of molecular events.

• Often includes protein kinases and phosphatases.

• Second messengers (e.g., cAMP) amplify the signal.

Bioenergetics and Enzyme Function

Thermodynamics in Biology

Biological processes obey the laws of thermodynamics.

• First law: Energy cannot be created or destroyed.

• Second law: Entropy (disorder) increases in spontaneous processes.

• Entropy (): Measure of disorder.

Anabolic vs. Catabolic Pathways

Metabolic pathways are classified as anabolic or catabolic.

• Anabolic: Build complex molecules from simpler ones (requires energy).

• Catabolic: Break down molecules to release energy.

Kinetic vs. Potential Energy

Energy exists in different forms.

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy due to position or structure.

Endergonic vs. Exergonic Reactions

Reactions are classified by energy change.

• Endergonic: Absorb energy ().

• Exergonic: Release energy ().

ATP Structure and Function

ATP (adenosine triphosphate) is the cell's main energy currency.

• Composed of adenine, ribose, and three phosphate groups.

• Hydrolysis of ATP releases energy for cellular work.

• Equation:

Energy Coupling

Cells couple exergonic and endergonic reactions to drive necessary processes.

• ATP hydrolysis powers endergonic reactions.

Phosphorylation

Phosphorylation is the addition of a phosphate group to a molecule, often regulating activity.

• Protein kinases catalyze phosphorylation.

• Phosphorylation can activate or deactivate enzymes.

Enzyme Structure and Function

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

• Active site: Region where substrate binds.

• Substrate: Reactant acted upon by enzyme.

• Activation energy (): Energy required to start a reaction.

Enzyme Regulation

Enzyme activity is regulated by various mechanisms.

• Competitive inhibition: Inhibitor binds active site.

• Noncompetitive inhibition: Inhibitor binds elsewhere, changing enzyme shape.

• Allosteric regulation: Regulation at a site other than the active site.

• Feedback inhibition: End product inhibits an earlier step in the pathway.

Factors Affecting Enzyme Activity

• Temperature, pH, and local conditions affect reaction rates.

• Enzymes have optimal conditions for activity.

Cellular Work

Cells use energy for mechanical, transport, and chemical work.

• ATP powers muscle contraction, active transport, and biosynthesis.

Summary Table: Key Cell Structures and Functions

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard General Biology curriculum.