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.

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.

Surface Area and Volume of Cells

The ratio of surface area to volume is a key factor limiting cell size and efficiency.

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

• As cells grow, volume increases faster than surface area, limiting transport and metabolic rates.

Prokaryotes vs. Eukaryotes

Cells are classified as prokaryotic or eukaryotic based on structural differences.

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

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

Chromatin

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

• Condenses to form chromosomes during cell division.

Common Components of All Cells

• Plasma membrane

• Cytoplasm

• Ribosomes

• Genetic material (DNA)

Structural Features and Functions of Organelles

Organelles perform specialized functions within eukaryotic cells.

• Nucleus and Nucleolus: Contains DNA; 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

• DNA → RNA → Protein (Central Dogma)

• Ribosomes translate mRNA into polypeptides.

Bound vs. Free Ribosomes

• Free ribosomes: Synthesize proteins for use within the cytoplasm.

• Bound ribosomes: Attached to the ER; synthesize proteins for secretion or for organelles.

Autophagy

• Process by which cells digest and recycle their own components using lysosomes.

Endosymbiosis Theory

• Explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

Cytoskeleton

The cytoskeleton provides structural support and facilitates movement.

• Microtubules: Hollow tubes; involved in cell division and transport.

• Microfilaments: Thin fibers; involved in cell shape and movement.

• Intermediate filaments: Provide mechanical strength.

Muscle Contraction

• Involves actin and myosin filaments sliding past each other.

Cell Junctions

Cell junctions connect cells and facilitate communication.

• Plasmodesmata: Channels between plant cells.

• Desmosomes: Anchor cells together.

• Tight junctions: Prevent leakage between cells.

• Gap junctions: Allow passage of ions and small molecules between animal cells.

Cell Membranes and Transport

Fluid Mosaic Model of Membranes

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

• Phospholipid bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

• Proteins: Integral (transmembrane) and peripheral; perform transport, signaling, and structural roles.

• Cholesterol: Modulates membrane fluidity and stability.

Saturated vs. Unsaturated Phospholipids

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

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

Integral vs. Peripheral Membrane Proteins

• Integral (Transmembrane): Span the membrane; involved in transport and signaling.

• Peripheral: Attached to membrane surface; involved in signaling and structural support.

Amphipathic Molecules

• Contain both hydrophilic and hydrophobic regions (e.g., phospholipids).

Diffusion and Passive Transport

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

• Facilitated diffusion: Passive transport via membrane proteins.

Osmosis

• Diffusion of water across a selectively permeable membrane.

Active Transport

• Movement of substances against their concentration gradient using energy (ATP).

• Example: Sodium-potassium pump ( out, in per ATP$).

Types of Molecules Crossing Membranes

• Small, nonpolar molecules (e.g., , ) cross easily.

• Large or charged molecules require transport proteins.

Membrane Potential

• Voltage difference across the membrane due to ion distribution.

Endocytosis and Exocytosis

• Pinocytosis: Uptake of fluids and small molecules.

• Phagocytosis: Engulfment of large particles or cells.

• Receptor-mediated endocytosis: Specific uptake via receptor binding.

Organelle Functions and Cell Processes

Organelle Functions Table

Cellular Transport Table

Examples and Applications

• Example: Tay-Sachs disease is caused by lysosomal enzyme deficiency, leading to accumulation of undigested lipids.

• Example: Mitochondria have cristae to increase surface area for ATP production.

• Example: White blood cells use phagocytosis to engulf pathogens.

Key Equations

• Surface Area of a Sphere:

• Volume of a Sphere:

• Membrane Potential (Nernst Equation):

Additional info:

• Cell junctions are critical for tissue integrity and communication in multicellular organisms.

• Fluidity of membranes is affected by temperature, cholesterol, and fatty acid composition.

• Endosymbiosis theory is supported by the presence of double membranes and independent DNA in mitochondria and chloroplasts.