Cell Structure and Function

Microscopes

Microscopes are essential tools for studying cells and their components. They vary in their mechanisms and capabilities.

• Scanning Electron Microscope (SEM): Produces detailed images of cell surfaces by scanning with a focused beam of electrons.

• Transmission Electron Microscope (TEM): Passes electrons through thin sections of specimens to reveal internal structures.

• Resolution: The ability to distinguish two close objects as separate; higher in electron microscopes than light microscopes.

• Magnification: The process of enlarging the appearance of an object.

• Example: SEM is used to view the surface of pollen grains at high magnification.

Cell Fractionation

Cell fractionation is a laboratory technique used to separate cellular components for study.

• Purpose: To isolate organelles and macromolecules for biochemical analysis.

• How it works: Cells are broken apart, and components are separated by centrifugation based on size and density.

• Example: Isolation of mitochondria to study ATP production.

Cell Size and Surface Area

The size and shape of cells are limited by the relationship between surface area and volume.

• Surface Area: Determines the rate of exchange of materials with the environment.

• Volume: Determines the amount of metabolic activity a cell can support.

• Limitation: As cells grow, volume increases faster than surface area, limiting cell size.

• Formula: ;

Prokaryotes vs. Eukaryotes

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

• Prokaryotes: Lack a nucleus; DNA is in a region called the nucleoid. Examples: Bacteria, Archaea.

• Eukaryotes: Have a true nucleus and membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

• Comparison Table:

Chromatin

Chromatin is the complex of DNA and proteins found in the nucleus of eukaryotic cells.

• Function: Packages DNA into a compact form and regulates gene expression.

• Example: Chromatin condenses to form chromosomes during cell division.

Common Components of All Cells

Despite differences, all cells share certain features.

• Plasma membrane

• Cytoplasm

• Ribosomes

• DNA

Organelles: Structure and Function

Eukaryotic cells contain specialized organelles with distinct functions.

• Nucleus and Nucleolus: Stores genetic material; nucleolus synthesizes ribosomal RNA.

• Mitochondria: Site of cellular respiration and ATP production.

• Chloroplasts: Site of photosynthesis in plant cells.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids and detoxifies.

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

• Lysosome: Contains hydrolytic enzymes for intracellular digestion.

Information Flow in Cells

Cells synthesize proteins using ribosomes, which may be free in the cytoplasm or bound to the ER.

• Free Ribosomes: Produce proteins for use within the cell.

• Bound Ribosomes: Produce proteins for export or for use in membranes.

Autophagy

Autophagy is the process by which cells degrade and recycle their own components.

• Function: Maintains cellular homeostasis and removes damaged organelles.

Endosymbiosis Theory

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

• Key Point: These organelles originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

Cytoskeleton

The cytoskeleton provides structural support and facilitates movement within cells.

• Components: Microfilaments, microtubules, intermediate filaments.

• Functions: Maintains cell shape, enables movement, assists in cell division.

Cell Junctions

Cell junctions connect cells and facilitate communication.

• Plasmodesmata: Channels between plant cells for transport and communication.

• Desmosomes: Anchor cells together in animal tissues.

• Tight Junctions: Prevent leakage of extracellular fluid.

• Gap Junctions: Allow ions and small molecules to pass between animal cells.

Cell Membranes and Transport

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which depicts a dynamic structure of lipids and proteins.

• Phospholipid Bilayer: Forms the basic structure; hydrophilic heads face outward, hydrophobic tails face inward.

• Proteins: Embedded or attached; serve as channels, receptors, or enzymes.

Cholesterol

Cholesterol is interspersed within the membrane, affecting fluidity and stability.

• Function: Maintains membrane fluidity at varying temperatures.

• Example: Antarctic fish have membranes with more unsaturated fatty acids for fluidity in cold environments.

Saturated vs. Unsaturated Phospholipids

The type of fatty acids in phospholipids influences membrane properties.

• Saturated: No double bonds; pack tightly, less fluid.

• Unsaturated: Double bonds; kinked tails, more fluid.

Membrane Proteins

Proteins in the membrane are classified by their association with the lipid bilayer.

• Integral/Transmembrane Proteins: Span the membrane; involved in transport and signaling.

• Peripheral Proteins: Attached to the surface; involved in cell signaling and structure.

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions.

Membrane Transport Mechanisms

Cells exchange materials with their environment through various transport mechanisms.

• Diffusion/Passive Transport: Movement of molecules from high to low concentration without energy input.

• Facilitated Diffusion: Passive transport via membrane proteins.

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

• Active Transport: Movement against a concentration gradient using energy (ATP).

• Example Equation:

Types of Molecules Crossing the Membrane

Membrane permeability depends on molecule size, polarity, and charge.

• Can Pass: Small, nonpolar molecules (e.g., O2, CO2).

• Cannot Pass: Large, polar molecules and ions (e.g., glucose, Na+).

Membrane Potential

Membrane potential is the voltage difference across the plasma membrane due to ion distribution.

• Maintained by: Ion pumps (e.g., sodium-potassium pump).

• Equation:

Bulk Transport

Cells transport large molecules via vesicles in processes called endocytosis and exocytosis.

• Pinocytosis: Uptake of fluids and dissolved substances.

• Phagocytosis: Engulfment of large particles or cells.

• Receptor-Mediated Endocytosis: Specific uptake of molecules via receptor proteins.

Organelle Functions and Cellular Processes

Chloroplasts

Chloroplasts are organelles in plant cells responsible for photosynthesis.

• Function: Convert light energy into chemical energy (glucose).

• Components: Thylakoids, stroma, DNA, ribosomes.

Mitochondria

Mitochondria are the powerhouses of the cell, generating ATP through cellular respiration.

• Structure: Double membrane, cristae (folds), matrix.

• Function: ATP synthesis.

• Labeling Example:

Endoplasmic Reticulum (ER)

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

• Rough ER: Studded with ribosomes; synthesizes proteins.

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies chemicals.

Golgi Apparatus

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

• Function: Forms lysosomes, buds off vesicles.

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes.

• Function: Breakdown of macromolecules, autophagy, and defense against pathogens.

• Example: Tay-Sachs disease results from lysosomal enzyme deficiency.

Vacuoles

Vacuoles are large vesicles in plant and some animal cells for storage and waste disposal.

• Central Vacuole: Maintains cell turgor and stores nutrients in plant cells.

Cellular Transport and Membrane Dynamics

Osmosis and Tonicity

Osmosis affects cell volume and shape depending on the surrounding solution's tonicity.

• Hypotonic: Lower solute concentration outside; cell swells.

• Hypertonic: Higher solute concentration outside; cell shrinks.

• Isotonic: Equal solute concentration; no net water movement.

Sodium-Potassium Pump

The sodium-potassium pump is an electrogenic pump that maintains ion gradients across the membrane.

• Function: Pumps 3 Na+ out and 2 K+ in per ATP hydrolyzed.

• Equation:

Phospholipids and Membrane Components

Phospholipids are amphipathic molecules forming the bilayer; other components include cholesterol and glycolipids.

• Phospholipids: Hydrophilic head, hydrophobic tail.

• Cholesterol: Modulates fluidity.

• Glycolipids: Lipids with carbohydrate chains; involved in cell recognition.

Carrier Proteins

Carrier proteins facilitate the transport of specific molecules across the membrane.

• Specificity: Each carrier protein transports a particular molecule.

• Energy: Some require energy (active transport), others do not (facilitated diffusion).

Bulk Flow and Vesicular Transport

Cells use vesicles for bulk transport of materials.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Endocytosis: Cell engulfs material by forming vesicles from the membrane.

• Phagocytosis: Engulfment of large particles; used by white blood cells.

• Pinocytosis: Uptake of fluids and small molecules.

• Receptor-Mediated Endocytosis: Specific uptake via receptor proteins.

Additional info:

• Some content inferred from standard General Biology curriculum and context of questions and outline.

• Tables and diagrams described in text for clarity and completeness.