BI222 Midterm Study Guide: Membranes, Cells, Energy, Signaling, Plant Form & Function, Nutrition, Transport, and Plant Responses

Membranes and Transport

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a dynamic combination of lipids, proteins, and carbohydrates. The membrane is 'fluid' because the lipid molecules move laterally within the layer, and 'mosaic' due to the patchwork of proteins embedded or attached to the bilayer.

• Phospholipid bilayer: Provides the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins or lipids, important for cell recognition.

Selective Permeability

Selective permeability means the membrane allows some substances to cross more easily than others.

• Lipid bilayer: Permeable to small, nonpolar molecules (e.g., O2, CO2), but not to ions or large polar molecules.

• Transport proteins: Facilitate movement of specific ions and molecules across the membrane.

Types of Membrane Transport

• Diffusion (Passive Transport): Movement of molecules from high to low concentration without energy input.

• Facilitated Diffusion: Passive movement via transport proteins (channels or carriers).

• Active Transport: Movement against the concentration gradient, requiring energy (usually ATP).

Key Differences: Passive transport does not require energy; active transport does.

Electrochemical Gradients and Membrane Potential

• Electrochemical gradient: Combination of concentration gradient and electrical charge difference across the membrane.

• Membrane potential: Voltage across a membrane, important for processes like nerve impulse transmission.

• Co-transport: Uses the energy of one molecule moving down its gradient to transport another molecule against its gradient.

Example: The sodium-potassium pump establishes an electrochemical gradient used for glucose co-transport in animal cells.

Cells and Energy

Cell Theory and Multicellularity

• All living things are composed of cells.

• Cells are the basic unit of structure and function in living organisms.

• All cells arise from pre-existing cells.

• Multicellular organisms are constructed from many cells that may specialize for different functions.

Cell Types and Organelles

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles (e.g., bacteria).

• Eukaryotic cells: Have a nucleus and organelles (e.g., plants, animals, fungi, protists).

• Organelle functions:

  • Nucleus: Stores genetic material; most abundant in cells with high transcriptional activity.

  • Mitochondria: Site of ATP production; abundant in muscle cells.

  • Chloroplasts: Photosynthesis; found in plant cells.

  • Endoplasmic reticulum (ER): Protein and lipid synthesis.

  • Golgi apparatus: Modifies, sorts, and packages proteins and lipids.

  • Lysosomes: Digestion of macromolecules.

  • Vacuoles: Storage and structure in plant cells.

ATP: The Energy Currency

• ATP (adenosine triphosphate): Main energy source for cellular work.

• ATP hydrolysis releases energy used for cellular processes, including phosphorylation of proteins to change their activity.

Equation:

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

Cell Signaling

Signal Transduction Pathways

A signal transduction pathway is a series of steps by which a signal on a cell's surface is converted into a specific cellular response.

• Reception: Signal molecule binds to receptor.

• Transduction: Relay molecules transmit the signal.

• Response: Cellular activity is altered.

Classes of Membrane-Associated Receptors

• G protein-coupled receptors (GPCRs): Activate G proteins to relay signals.

• Receptor tyrosine kinases (RTKs): Dimerize and autophosphorylate to activate signaling cascades.

• Ion channel receptors: Open or close in response to ligand binding, allowing ion flow.

Phosphorylation Cascades vs. Second Messenger Pathways

• Phosphorylation cascade: Series of protein kinases activate each other by adding phosphate groups.

• Second messenger-mediated transduction: Small molecules (e.g., cAMP, Ca2+) relay signals inside the cell.

Signal Transduction and Gene Expression

• Signal transduction can lead to changes in gene expression by activating transcription factors.

Development

Plant vs. Animal Development and Pattern Formation

• Both plants and animals undergo development involving cell division, differentiation, and pattern formation.

• Pattern formation: The process by which cells acquire different identities according to their spatial position.

• Plants can form new organs throughout life; animals have more fixed body plans.

Differentiation vs. Determination

• Determination: Commitment of a cell to a specific fate.

• Differentiation: Process by which a determined cell develops its specialized structure and function.

Development and Signal Transduction

• Signals (hormones, morphogens) guide development by influencing gene expression and cell behavior.

Plant Form and Function

Plant Organs, Tissues, and Cells

• Organs: Roots, stems, leaves, flowers.

• Tissues: Dermal (protection), vascular (transport), ground (photosynthesis, storage, support).

• Cells: Parenchyma (photosynthesis, storage), collenchyma (support), sclerenchyma (strength).

Growth Patterns

• Determinant growth: Growth stops after reaching a certain size (e.g., leaves, flowers).

• Indeterminant growth: Growth continues throughout life (e.g., roots, stems).

Primary and Secondary Growth

• Primary growth: Lengthening of roots and shoots from apical meristems.

• Secondary growth: Increase in girth from lateral meristems (vascular cambium, cork cambium).

Soil and Nutrition

Soil Properties and Plant Nutrition

• Texture: Proportion of sand, silt, clay affects water and nutrient retention.

• Structure: Arrangement of soil particles influences root penetration and aeration.

• Chemistry: pH and mineral content affect nutrient availability.

Symbiotic Nitrogen Fixation

• Reciprocal signaling between plants and bacteria leads to nodule formation and nitrogen fixation.

• Signal transduction pathways mediate communication and development of symbiosis.

Mycorrhizae

• Fungal associations with plant roots increase surface area for water and nutrient absorption.

Long Distance Transport in Plants

Water Potential and Movement

• Water potential (Ψ): Predicts the direction water will move; water moves from high to low Ψ.

Equation:

$\Psi = \Psi_s + \Psi_p$

where $\Psi_s$ is solute potential and $\Psi_p$ is pressure potential.

• Stomata open in response to light, low CO2, and internal signals; close due to drought, darkness, or abscisic acid.

• Short-distance movement: via cell-to-cell (apoplast, symplast, transmembrane routes).

• Long-distance movement: via xylem (water, minerals) and phloem (sugars).

• Pressure flow hypothesis explains phloem transport of sugars.