Rucete ✏ AP Biology In a Nutshell
3. Cell Organelles, Membranes, and Transport
This chapter reviews the structure and function of cellular organelles, the structure of membranes, mechanisms of material transport, and key principles such as surface area-to-volume ratios and compartmentalization in cells.
Cell Organelles and Their Functions
• All cells have DNA, ribosomes, cytosol, and plasma membranes.
• Prokaryotic cells: lack membrane-bound organelles; DNA is circular in the nucleoid region; may contain plasmids.
• Eukaryotic cells: have membrane-bound organelles; DNA is linear and housed in the nucleus.
• Ribosomes: present in all cells, synthesize proteins using mRNA. Free ribosomes float in cytosol; bound ribosomes are attached to rough ER.
• Endoplasmic Reticulum (ER): – Rough ER has ribosomes and synthesizes proteins. – Smooth ER synthesizes lipids and detoxifies substances.
• Golgi Complex: modifies and packages proteins into vesicles for transport.
• Lysosomes: contain hydrolytic enzymes for digestion, waste removal, and apoptosis.
• Vacuoles: store water, nutrients, or waste; help maintain turgor pressure in plant cells.
• Mitochondria: produce ATP via cellular respiration; have double membranes, their own DNA (mtDNA), and ribosomes.
• Chloroplasts (in plants/algae): conduct photosynthesis; contain thylakoids, grana, stroma, cpDNA, and ribosomes.
• Centrosome: forms spindle fibers for cell division in animals.
• Amyloplasts (in plants): store starch from excess glucose.
• Other structures: – Peroxisomes: break down toxins via oxidation. – Nucleolus: assembles ribosomes inside the nucleus. – Cytoskeleton: maintains shape and supports movement.
Endosymbiosis Hypothesis
• Suggests mitochondria and chloroplasts originated as engulfed prokaryotes.
• Supporting evidence: – Both have circular DNA like bacteria. – Both have prokaryote-like ribosomes. – Both reproduce via binary fission.
• Modern example: Paramecium bursaria retains active chloroplasts from engulfed algae (kleptoplasty).
The Advantages of Compartmentalization
• Membrane-bound organelles allow specialization and efficiency.
• Separates metabolic processes to prevent interference and improve regulation.
• Folding internal membranes increases surface area (e.g., cristae in mitochondria).
• Prokaryotes increase surface area by folding their plasma membranes.
The Importance of Surface Area to Volume Ratios
• Larger surface area to volume ratio = more efficient exchange of materials.
• As cell size increases, efficiency decreases due to reduced relative surface area.
• Folding membranes (e.g., villi, cristae) increases surface area and absorption efficiency.
Plasma Membrane Structure
• Plasma membranes are selectively permeable phospholipid bilayers.
• Phospholipids: hydrophilic heads (outward), hydrophobic tails (inward).
• Fluid mosaic model: includes embedded proteins, cholesterol, and carbohydrates.
• Membrane proteins: – Transport proteins (channels/pumps) – Signal transduction receptors – Anchors for cytoskeleton – Cell-cell recognition (glycoproteins)
• Cholesterol maintains membrane fluidity, especially at temperature extremes.
Selective Permeability
• Small, nonpolar molecules (O₂, CO₂, N₂) freely diffuse across membrane.
• Small polar molecules (H₂O) can pass in small amounts.
• Large, charged, or polar molecules (glucose, ions) require membrane proteins for transport.
• The membrane’s selectivity maintains internal cellular homeostasis.
Types of Transport Across Membranes
• Passive Transport: no energy required.
– Simple diffusion: movement down the concentration gradient (e.g., O₂, CO₂).
– Facilitated diffusion: uses channel or carrier proteins for charged/large molecules (e.g., glucose, ions).
– Osmosis: diffusion of water across membranes toward higher solute concentration.
• Active Transport: requires ATP to move substances against their concentration gradient.
– Uses pumps (e.g., sodium-potassium pump) or vesicle-mediated transport.
Bulk Transport (Vesicular Transport)
• Endocytosis: cell engulfs material into vesicles.
– Phagocytosis: "cell eating" (large solids).
– Pinocytosis: "cell drinking" (liquids).
– Receptor-mediated endocytosis: specific uptake using receptor proteins.
• Exocytosis: vesicles fuse with membrane to release contents outside cell (e.g., hormones, neurotransmitters).
Water Potential and Osmoregulation
• Water moves from high water potential to low water potential.
• Water potential (Ψ) = pressure potential + solute potential; measured in bars.
• Solute potential is negative; more solute = lower Ψ.
• Cells in different solutions: – Hypertonic: cell loses water, shrinks. – Hypotonic: cell gains water, swells (plants become turgid). – Isotonic: no net water movement.
• Osmoregulation: maintaining internal water balance.
– Contractile vacuoles in protists; cell walls in plants resist excessive water influx.
In a Nutshell
Cells are organized into membrane-bound compartments that specialize in various functions. The plasma membrane’s selective permeability allows for regulated transport of materials, using both passive and active mechanisms. Surface area, compartmentalization, and transport systems like osmosis and active transport are essential for maintaining internal balance and efficient function.