General Biology Study Guide: Membranes, Protists, Plants, and Plant Structure

Membrane Structure and Function

Planar Bilayer Structure of Membranes

The plasma membrane is a fundamental structure in all cells, composed of a phospholipid bilayer that separates the internal environment from the external environment.

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

• Selectively permeable: Small, nonpolar molecules cross easily; large or charged molecules and proteins require transport mechanisms.

Diffusion and Electrochemical Gradients

Diffusion and the movement of molecules across membranes are governed by concentration and electrochemical gradients.

• Diffusion: Molecules move passively from high to low concentration.

• Electrochemical gradient: Combination of concentration gradient and electrical gradient.

• Net movement: Refers to the overall movement of molecules from high to low concentration until equilibrium is reached.

Equilibrium

At equilibrium, molecules are evenly distributed, and there is no net movement, though random movement continues in both directions.

Factors Affecting Diffusion Rate

• Diffusion constant (D): Depends on solute, membrane, temperature.

• Surface area (A): More surface = faster diffusion.

• Concentration difference (ΔC): Greater difference = faster rate.

• Distance (d): Thicker barriers = slower diffusion.

Surface Area to Volume Ratio (SA:V)

As cell size increases, the SA:V ratio decreases, making diffusion less efficient. Solutions include:

• Surface folds (e.g., microvilli) to increase surface area.

• Large vacuoles in plants to reduce active volume.

Osmosis and Tonicity

Osmosis is the diffusion of free water across a semipermeable membrane, with important consequences for cells in different solutions.

• Osmosis: Water moves from high free water (low solute) to low free water (high solute).

• Hypertonic: More solute outside; cell loses water and shrinks (crenation/plasmolysis).

• Hypotonic: Less solute outside; cell gains water and swells (lysis in animals, turgid in plants).

• Isotonic: Equal solute inside and out; no net water movement.

Water Molecules and Membrane Transport

• Water is polar and forms hydrogen bonds with other molecules.

• Diffuses slowly across membranes or rapidly via aquaporins (water-specific channels).

Passive vs. Active Transport

• Passive transport: No energy required; moves down gradient (e.g., simple diffusion, channel proteins, carrier proteins).

• Active transport: Requires energy (usually ATP); moves against gradient.

  • Primary active transport: ATP pumps.

  • Secondary active transport: Uses electrochemical gradients.

Membrane Transport Proteins

• Channel proteins: Form selective pores for ions/molecules.

• Carrier proteins: Change shape to move substances.

• Aquaporins: Water-specific channels.

• Gated channels: Open/close in response to signals (e.g., voltage).

Pumps and the Sodium-Potassium Pump

• Pump: Membrane protein using energy (often ATP) to move molecules against their gradient.

• Sodium-potassium pump: Uses ATP to pump 3 Na+ out and 2 K+ in, maintaining cell electrochemical gradient and resting potential.

Symporters and Antiporters

• Symporters: Move two molecules in the same direction (one down gradient, one against).

• Antiporters: Move molecules in opposite directions.

Eukaryotes and Protists

Protists: Eukaryotic Cell Structure and Function

• Eukaryotic: Have nucleus, organelles, cytoskeleton.

• Paraphyletic: Common ancestor + some descendants (not all).

• Movement: Flagella, cilia, amoeboid motion (pseudopodia).

• Nutrition: Autotrophs (photosynthesis), heterotrophs (ingestive, absorptive).

• Protection: Cell walls, shells, toxins.

Mitochondria and Endosymbiosis

• Mitochondria: Organelle that makes ATP using pyruvate and oxygen.

• Divides by fission, has own ribosomes, circular DNA, double membrane.

• Similar in size to α-proteobacteria.

• Endosymbiosis theory: Mitochondria originated when a bacterial cell was engulfed by a host cell and became permanent.

• Evidence: Replicate via fission, have circular DNA, double membrane, similarities to bacteria.

Major Protist Groups and Roles

• Diatoms: Silica cell walls, major plankton.

• Foraminiferans: Calcium carbonate shells.

• Dinoflagellates: Flagella, some toxic, cause red tides.

• Plankton: Base of aquatic food chains.

Protists in Ecosystems

• Primary producers: Photosynthetic protists fix carbon and produce oxygen.

• Carbon cycle regulators: Half of Earth's CO2 fixation by marine protists.

• Carbon sinks: Shells, petroleum deposits.

• Food web: Base of aquatic food chains.

Asexual and Sexual Life Cycles

• Asexual: Identical offspring, fast reproduction.

• Sexual: Increases genetic diversity.

• Life cycles: Vary in timing of mitosis vs. meiosis.

Meiosis, Mitosis, and Syngamy

• Mitosis: Chromosome number maintained (diploid → diploid).

• Meiosis: Chromosome reduction (diploid → haploid).

• Syngamy: Fusion of haploid gametes restores diploid state.

Photosynthesis: Inputs, Outputs, and Consequences

• Equation:

$6CO_2 + 6H_2O + light \rightarrow C_6H_{12}O_6 + 6O_2$

• Inputs: CO2, H2O, light

• Outputs: Glucose (C6H12O6), O2

• Consequences: Produces atmospheric oxygen, stores energy in organic molecules, forms basis of food chains.

The Carbon Cycle

• Photosynthesis: Fixes CO2 into organic molecules.

• Respiration: Releases CO2 back into atmosphere.

• Protists: Major contributors to carbon fixation and long-term storage in sediments.

Plant Diversity

Major Plant Groups

• Green algae: Aquatic, close relatives of land plants.

• Non-vascular plants (Bryophytes): Mosses, liverworts, hornworts.

• Seedless vascular plants: Ferns, lycophytes, whisk ferns, horsetails.

• Gymnosperms: Cycads, ginkgo, conifers, gnetophytes.

• Angiosperms: Flowering plants (monocots, eudicots).

Plant Structures: Chloroplasts, Vascular Tissue, Seeds, Pollen, Flowers

• Chloroplasts: All green plants.

• Vascular tissue: Seedless vascular plants, gymnosperms, angiosperms.

• Seeds & pollen: Gymnosperms, angiosperms.

• Flowers & fruits: Angiosperms only.

Adaptations for Life on Land

• Cuticle: Waxy coating prevents drying out.

• Pores/Stomata: Allow gas exchange.

• Multicellular embryo: Retained and protected by parent plant.

• Vascular tissue: Transport and support.

• Seeds & pollen: Dispersal and reproduction without water.

• Flowers & fruits: Reproduction and dispersal (angiosperms).

Alternation of Generations

• Plants alternate between two multicellular stages:

  • Sporophyte (diploid): Produces haploid spores by meiosis.

  • Gametophyte (haploid): Produces gametes by mitosis.

  • Gametes fuse (syngamy) → zygote (2n) → develops into sporophyte.

Sporophyte and Gametophyte Relationships

• Bryophytes: Gametophyte is dominant, sporophyte is small, attached, and dependent.

• Seedless vascular plants: Sporophyte is dominant and independent, gametophyte is small but free-living.

• Seed plants: Sporophyte is dominant, gametophyte is reduced and dependent (pollen, ovule).

Plant Form and Function

Surface Area to Volume Ratio in Plants

• Shape and size influence material exchange with the environment.

• Thin/flat structures (leaves) have high SA:V for absorption of light, CO2, and water.

• Thicker/longer structures (stems, roots) have low SA:V for support and storage.

• As size increases, SA:V decreases, making exchange less efficient.

Root and Shoot Systems

• Root system: Absorbs water/nutrients, anchors plant, stores carbohydrates, supports growth.

• Shoot system: Includes stems, leaves, flowers; harvests light/CO2, supports reproduction, facilitates gas exchange.

Leaf Structure and Adaptations

• Node: Where leaf attaches to stem.

• Simple leaf: Undivided blade.

• Compound leaf: Divided into leaflets.

• Needle-like leaf: Adapted to conserve water in dry/cold environments.

Root, Shoot, and Leaf Modifications

• Roots: Storage (carrots), support (prop roots), pneumatophores (gas exchange), aerial roots.

• Shoots: Tubers (potatoes), rhizomes, stolons, thorns.

• Leaves: Spines (protection), tendrils (climbing), succulent leaves (water storage), reproductive leaves.

Plant Cell Walls and Connections

• Primary cell wall: All plant cells; thin, flexible, high in cellulose and pectin.

• Secondary cell wall: Some cells; thick, rigid, contains lignin for extra strength.

• Plasmodesmata: Cytoplasmic channels connecting adjacent cells for transport and signaling.

Functions of Plant Organelles

• Cell wall: Support, structure, protection.

• Plasmodesmata: Connect plant cells, allow material passage.

• Chloroplast: Site of photosynthesis.

• Amyloplast: Stores starch.

• Chromoplast: Stores pigments, gives color to fruits/flowers.

• Vacuole: Stores water, supports structure, protects, and defines cell shape.