Membrane Transport

Structure and Function of Biological Membranes

Biological membranes are primarily composed of a phospholipid bilayer with embedded proteins, forming a selectively permeable barrier that regulates the movement of substances into and out of cells.

• Phospholipid Bilayer: Consists of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, creating a planar, double-layered structure.

• Membrane Proteins: Include channel proteins, carrier proteins, and pumps, each facilitating different types of transport.

Diffusion and Gradients

Diffusion is the passive movement of molecules from regions of higher concentration to lower concentration, driven by concentration and electrochemical gradients.

• Net Movement: Refers to the overall movement of molecules in one direction, even though individual molecules move randomly.

• Equilibrium: Achieved when the concentration of molecules is equal throughout a space, resulting in no net movement.

• Factors Affecting Diffusion Rate: Temperature, molecule size, membrane permeability, and gradient steepness.

Surface Area to Volume Ratio

As cells increase in size, their volume grows faster than their surface area, reducing the surface area-to-volume ratio. This limits the efficiency of diffusion and transport.

• Problem: Large cells may not exchange materials efficiently.

• Solutions: Cells may divide, develop folds, or specialize structures to increase surface area.

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane. The tonicity of a solution affects cell volume and function.

• Hypertonic Solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

• Hypotonic Solution: Lower solute concentration outside; water enters the cell, causing it to swell.

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

Passive vs. Active Transport

• Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., simple diffusion, facilitated diffusion via channel/carrier proteins).

• Active Transport: Movement against a gradient, requiring energy (usually ATP). Includes primary (direct ATP use) and secondary (uses gradient established by primary transport) active transport.

Membrane Proteins and Pumps

• Channel Proteins: Form pores for specific molecules/ions to pass through (e.g., aquaporins for water).

• Carrier Proteins: Bind and transport substances across the membrane by changing shape.

• Pumps: Use energy to move substances against gradients (e.g., sodium-potassium pump).

• Sodium-Potassium Pump: Moves 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.

• Secondary Active Transport: Uses the energy from one gradient to drive another (e.g., symporters and antiporters).

Example: The sodium-potassium pump maintains electrochemical gradients essential for nerve impulse transmission.

Diversification of Eukaryotes (Protists)

Characteristics of Protists

Protists are a diverse group of mostly unicellular eukaryotes that are not plants, animals, or fungi.

• Eukaryotic: Have membrane-bound organelles, including a nucleus.

• Paraphyletic: Do not form a single clade; grouped by exclusion.

• Movement: Use flagella, cilia, or pseudopodia.

• Nutrient Acquisition: Can be autotrophic, heterotrophic, or mixotrophic.

• Protection: Some have cell walls, shells, or pellicles.

Endosymbiosis Theory

The theory of endosymbiosis explains the origin of mitochondria and plastids (e.g., chloroplasts) as formerly free-living bacteria engulfed by ancestral eukaryotes.

• Evidence: Double membranes, own DNA, ribosomes similar to bacteria, replicate independently.

Protist Roles and Life Cycles

• Ecological Roles: Primary producers, decomposers, pathogens, and symbionts.

• Life Cycles: Can reproduce asexually (mitosis) or sexually (meiosis and syngamy).

• Chromosome Number: Mitosis maintains, meiosis halves, syngamy restores diploid number.

Photosynthesis and the Carbon Cycle

• Inputs: CO2, H2O, light energy.

• Outputs: Glucose, O2.

• Consequences: Carbon fixation, oxygen evolution, and influence on global carbon cycle.

Plant Diversity

Major Plant Groups and Adaptations

• Groups: Bryophytes (mosses), ferns, gymnosperms, angiosperms.

• Key Features: Chloroplasts (all), vascular tissue (ferns, gymnosperms, angiosperms), seeds/pollen (gymnosperms, angiosperms), flowers (angiosperms).

• Adaptations for Land: Cuticle, stomata, vascular tissue, seeds, pollen, flowers.

Alternation of Generations

Plants alternate between multicellular haploid (gametophyte) and diploid (sporophyte) generations.

• Bryophytes: Gametophyte dominant.

• Ferns: Sporophyte dominant, independent gametophyte.

• Seed Plants: Sporophyte dominant, reduced gametophyte.

Plant Form and Function

Root and Shoot Systems

• Root System: Anchors plant, absorbs water and minerals.

• Shoot System: Stems and leaves; supports, photosynthesizes, and reproduces.

• Leaf Parts: Blade, petiole, veins.

• Modifications: Storage roots, tendrils, spines, tubers.

Plant Cell Walls and Connections

• Primary Cell Wall: Flexible, made of cellulose.

• Secondary Cell Wall: Rigid, contains lignin.

• Plasmodesmata: Channels connecting plant cells for communication.

Cellular Components and Functions

• Cell Wall: Structure and protection.

• Plasmodesmata: Intercellular communication.

• Chloroplast: Photosynthesis.

• Amyloplast: Starch storage.

• Chromoplast: Pigment synthesis and storage.

• Vacuole: Storage, turgor pressure maintenance.

Plant Tissues

• Simple Tissues: One cell type (e.g., parenchyma).

• Complex Tissues: Multiple cell types (e.g., xylem, phloem).

• Three-Tissue System: Dermal (protection), ground (photosynthesis, storage), vascular (transport).

Plant Growth and Meristems

• Meristems: Regions of undifferentiated cells; apical (primary growth), lateral (secondary growth).

• Primary Growth: Lengthening of roots and shoots.

• Secondary Growth: Increase in girth; production of wood and bark.

• Root Hair: Increases surface area for absorption.

• Root Cap: Protects root tip during growth.

Water and Sugar Transport in Plants

Water Potential and Movement

Water potential () determines the direction of water movement and is influenced by solute potential () and pressure potential ():

• Cell Wall: Contributes to pressure potential (turgor pressure).

• Turgor Pressure: Pressure of cell contents against the wall; supports plant structure.

Stomata and Water Transport

• Stomata: Regulate gas exchange and water loss.

• Water Entry: Water enters roots via osmosis, moves through apoplast, symplast, or transmembrane pathways to the xylem.

• Casparian Strip: Forces water through cell membranes, regulating uptake.

• Root Pressure: Pushes water upward in small plants.

• Transpiration-Cohesion-Tension Mechanism: Water pulled up by evaporation at leaves, cohesion and adhesion maintain column.

• Prevention of Water Loss: Cuticle, closing stomata, leaf modifications.

Sugar Transport (Phloem)

• Sources and Sinks: Sources produce sugars (leaves), sinks consume/store (roots, fruits); roles can change seasonally.

• Phloem Loading/Unloading: Active or passive transport of sugars into/out of phloem.

Plant Nutrition

Essential Nutrients and Uptake

• Essential Nutrient: Required for plant growth and reproduction.

• Macronutrients: Needed in large amounts (e.g., N, P, K).

• Micronutrients: Needed in trace amounts (e.g., Fe, Zn).

• Limiting Nutrient: Most likely to be in short supply (e.g., N, P).

• Mobile vs. Immobile Nutrients: Mobile can be relocated within plant; deficiency symptoms appear in older leaves first.

• Uptake Location: Occurs at root hairs.

• Passive Exclusion: Roots exclude some ions by lack of transporters.

Nitrogen Fixation and Symbiosis

• Nitrogen Fixation: Conversion of N2 to ammonia by bacteria (enzyme: nitrogenase).

• Root Nodules: House nitrogen-fixing bacteria (e.g., Rhizobium).

• Leghemoglobin: Binds oxygen, protects nitrogenase.

• Crop Rotation: Alternating crops to maintain soil fertility.

• Parasitic Plants: Obtain nutrients from host plants.

• Epiphytes: Grow on other plants, not parasitic.

• Carnivorous Plants: Trap and digest insects for nutrients.

Example: The "three sisters" (corn, beans, squash) support each other's growth and nutrient needs.

Sensory Systems, Signals, and Responses in Plants

Signal Perception and Transduction

• Hormones: Chemical messengers regulating growth and responses.

• Phototropism: Growth toward light; stimulus perceived at tip, signal transmitted (Cholodny-Went hypothesis: auxin redistribution).

• Acid Growth Hypothesis: Auxin stimulates proton pumps, loosening cell wall for expansion.

• Stomatal Opening: Triggered by blue light; closing by drought (ABA hormone).

• Phytochrome: Light receptor acting as molecular switch; regulates germination, flowering.

• Etiolation: Growth in darkness; adaptive for reaching light.

• Gravitropism: Growth in response to gravity; statolith hypothesis (amyloplasts settle in response to gravity).

• Nastic Movements: Non-directional responses to stimuli (e.g., touch).

• Hormones: Auxin (growth), ethylene (senescence, fruit ripening), ABA (stress responses).

• Defense Strategies: Mechanical (thorns), chemical (toxins), inducible defenses (produced in response to attack).

Plant Reproduction

Flower Structure and Life Cycle

• Flower Parts: Sepals, petals, stamens (male), carpels (female).

• Symmetry: Radial (actinomorphic) vs. bilateral (zygomorphic).

• Monoecious: Both sexes on one plant; Dioecious: Separate male and female plants.

• Alternation of Generations: Flowering plant life cycle includes both sporophyte and gametophyte stages.

• Pollen: Male gametophyte; Embryo Sac: Female gametophyte.

• Pollen Tube: Delivers sperm to egg via micropyle.

• Sporopollenin: Tough outer layer of pollen grains.

• Pollination Syndromes: Adaptations for specific pollinators; result of coevolution.

• Double Fertilization: One sperm fertilizes egg (zygote), other fuses with polar nuclei (endosperm).

Bioskills 16: Primary Literature

• Primary Literature: Original research articles presenting new data.

• Sections: Abstract, Introduction, Methods, Results, Discussion, References.

• Citation: Essential for academic integrity and acknowledging sources.

Bioskills 9: Microscopy

• Light Microscopy: Used for live cells, general structure; fluorescent microscopy for specific molecules.

• Electron Microscopy: Higher resolution; Transmission EM (TEM): Internal structures; Scanning EM (SEM): Surface details, 3-D images.

• Live Cell Imaging: Achieved with light/fluorescent microscopy.

• 3-D Structures: Visualized with SEM or advanced light microscopy techniques.