Transport of Substances Across Membranes

Membrane Structure and Properties

Biological membranes are primarily composed of lipid bilayers, which provide a selectively permeable barrier for cells. The arrangement of hydrophilic heads and hydrophobic tails of phospholipids allows for dynamic movement and selective transport of substances.

• Lipid bilayer: Two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails inward.

• Selective permeability: Small, uncharged molecules cross easily; ions and large molecules require assistance from membrane proteins.

• Fluidity: Increased temperature raises membrane fluidity; phospholipids move laterally but rarely flip-flop.

Diffusion and Rate of Diffusion

Diffusion is the passive movement of molecules from regions of high concentration to low concentration, driven by random molecular motion. It does not require energy input.

• Concentration gradient: Drives net movement of solutes until equilibrium is reached.

• Equation for rate of diffusion:

• F: Rate of diffusion

• k: Diffusion constant (depends on solute, membrane, temperature)

• A: Surface area for diffusion

• \Delta c: Change in concentration

• d: Distance for diffusion (e.g., membrane thickness)

Electrochemical Gradients

For charged particles, diffusion is influenced by both concentration and electrical gradients, forming an electrochemical gradient. Equilibrium is reached when both gradients are balanced.

Consequences of Size and Shape

As organisms increase in size, their surface area to volume ratio decreases, reducing the efficiency of diffusion. Adaptations include subdivision into smaller units, increased surface complexity, and elongated or flattened shapes.

Osmosis

Osmosis is the diffusion of water across a semi-permeable membrane. Water moves from regions of high water potential (low solute concentration) to low water potential (high solute concentration).

• Water is polar: Forms hydrogen bonds with other molecules.

• Semi-permeable membrane: Allows water but not all solutes to cross.

Passive and Active Transport

• Passive transport: Movement along the electrochemical gradient without energy input (includes simple diffusion, facilitated diffusion via channel or carrier proteins).

• Active transport: Movement against the electrochemical gradient, requiring energy (often from ATP). Includes primary (direct ATP use) and secondary (uses gradients established by pumps) active transport.

Diversification of Eukaryotes and Protists

Tree of Life and Domains

Life is classified into three domains: Bacteria, Archaea, and Eukarya. Eukaryotes possess a nucleus and complex organelles, while prokaryotes (Bacteria and Archaea) do not.

Protists

Protists are a paraphyletic group of eukaryotes, excluding land plants, fungi, and animals. They exhibit diverse forms, modes of nutrition (autotrophic and heterotrophic), and life cycles (asexual and sexual reproduction).

• Amoeboid motion: Movement via pseudopodia.

• Cilia: Short projections for movement or fluid circulation.

Role in Ecosystems

Protists are primary producers in aquatic ecosystems, forming the base of the food chain and playing a key role in the global carbon cycle.

Plant Diversity and Evolution

Clades and Adaptations

Plants are organized into nested clades, with adaptations for terrestrial life such as cuticles, pores, and stomata for gas exchange. Land plants (embryophytes) protect their embryos and exhibit alternation of generations.

Alternation of Generations

Plants alternate between haploid (gametophyte) and diploid (sporophyte) generations. Meiosis produces spores; mitosis produces gametes.

• Sporophyte: Diploid, produces spores.

• Gametophyte: Haploid, produces gametes.

• Syngamy: Fusion of gametes to form a zygote.

Vascular and Seed Plants

Vascular plants have specialized tissues for transport (xylem and phloem). Seed plants (gymnosperms and angiosperms) have pollen and seeds, with reduced gametophytes.

Plant Form and Function

Plant Body Organization

The plant body is divided into the shoot system (above ground) and root system (below ground), each with specialized functions.

• Root system: Water and nutrient uptake, anchorage, storage.

• Shoot system: Light and CO2 harvesting, gas exchange, transport, reproduction.

Root System

• Primary root (taproot): Main root, with lateral roots branching off.

• Root hairs: Increase surface area for absorption.

• Root cap: Protects growing tip, senses gravity.

• Phenotypic plasticity: Root form changes with environment.

Shoot System and Leaves

• Stems, leaves, flowers: Main components of the shoot system.

• Leaf structure: Blade (expanded portion), petiole (stalk), various adaptations (e.g., bulbs, tendrils).

• Surface area to volume ratio: Maximized in leaves for efficient absorption of sunlight and gas exchange.

Plant Cells and Tissues

• Cell wall: Provides support and defines shape; primary wall (cellulose, pectin), secondary wall (lignin, rigid).

• Plasmodesmata: Cytoplasmic channels connecting adjacent cells.

• Plastids: Chloroplasts (photosynthesis), amyloplasts (starch storage), chromoplasts (pigments).

• Vacuole: Stores water, ions, pigments; maintains turgor pressure.

Types of Plant Tissues

• Dermal tissue: Outer protective layer (epidermis, periderm), includes cuticle, guard cells, trichomes.

• Ground tissue: Photosynthesis, storage, support (parenchyma, collenchyma, sclerenchyma).

• Vascular tissue: Transport of water, nutrients, and sugars (xylem, phloem).

Primary Growth and Meristems

Primary growth increases plant length via apical meristems at root and shoot tips. Meristematic cells are totipotent and give rise to all cell types.

Secondary Growth

Secondary growth increases plant girth via lateral meristems (vascular cambium and cork cambium). Produces secondary xylem (wood), secondary phloem, cork, and phelloderm.

Water and Sugar Transport in Plants

Water Potential

Water potential (ψ) is the potential energy of water in a system, determining the direction of water movement. It is measured in megapascals (MPa).

• Water moves from high to low water potential.

• Solute potential (osmotic potential): Lowered by higher solute concentration.

• Pressure potential: Physical pressure on water; turgor pressure in plant cells maintains rigidity.

• Ψ: Water potential

• Ψs: Solute potential

• Ψp: Pressure potential

Water Uptake and Transport

• Water enters roots by osmosis, moves through symplast (cytoplasm) and apoplast (cell wall matrix).

• Casparian strip in endodermis blocks apoplastic flow, forcing water through cell membranes.

• Water is transported upward via xylem, driven by transpiration and cohesion-tension mechanism.

Transpiration and Stomata

• Transpiration: Evaporation of water from leaf surfaces, creating negative pressure that pulls water upward.

• Stomata: Pores regulated by guard cells, control gas exchange and water loss.

Mechanisms to Prevent Water Loss

• Sunken stomata, trichomes, thick cuticle, reduced surface area, and timing of stomatal opening (e.g., CAM plants) reduce water loss.

Translocation of Sugars

Translocation is the bulk flow of sugars from sources (production/storage sites) to sinks (utilization/storage sites) via phloem, driven by pressure gradients (pressure-flow model).

• Phloem loading (active) increases turgor pressure at source; unloading (passive or active) decreases pressure at sink.

Plant Nutrition

Essential Nutrients

Plants require essential elements for growth and reproduction. These include macronutrients (needed in large amounts) and micronutrients (needed in trace amounts).

Nutrient Uptake and Transport

• Root hairs: Increase surface area for absorption; contain transport proteins.

• Proton pumps: Establish electrochemical gradients for nutrient uptake.

• Cotransporters: Move anions into cells by coupling with proton movement.

• Casparian strip: Forces selective uptake through endodermal cells.

Nitrogen Fixation and Crop Rotation

• Some bacteria and archaea convert atmospheric N2 to usable forms (ammonium, nitrate) via nitrogenase.

• Legumes form symbiotic relationships with rhizobia in root nodules, aided by leghemoglobin to protect nitrogenase from O2.

• Crop rotation with legumes restores soil nitrogen and improves sustainability.

Specialized Plant Nutrition Strategies

• Parasitic plants: Obtain water/nutrients from host plants via haustoria.

• Epiphytes: Grow on other plants, absorb water/nutrients from the environment.

• Carnivorous plants: Trap and digest insects to supplement nitrogen intake.

• Phytoremediation: Use of plants to clean up environmental contaminants.