Biological Transport

Cell Membrane Structure and Function

The cell membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell. It is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Selective Permeability: Allows certain molecules to pass while restricting others, maintaining homeostasis.

• Fluid Mosaic Model: Describes the dynamic and flexible nature of the membrane, with proteins floating in or on the fluid lipid bilayer.

• Functions: Protection, communication, transport of materials, and cell recognition.

Diffusion and Factors Affecting Diffusion

Diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration.

• Factors Affecting Rate of Diffusion:

  • Size of Molecule: Smaller molecules diffuse faster than larger ones.

  • Charge: Uncharged (nonpolar) molecules diffuse more easily through the lipid bilayer than charged (polar) molecules.

  • Temperature: Higher temperatures increase the kinetic energy of molecules, thus increasing the rate of diffusion.

Osmosis is the diffusion of water across a selectively permeable membrane.

Osmotic Behavior in Plant and Animal Cells

• Isotonic Solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypotonic Solution: Lower solute concentration outside; water enters the cell.

  • Animal Cells: May burst (lysis).

  • Plant Cells: Become turgid (normal state due to cell wall).

• Hypertonic Solution: Higher solute concentration outside; water leaves the cell.

  • Animal Cells: Shrivel (crenation).

  • Plant Cells: Undergo plasmolysis (cell membrane pulls away from cell wall).

Enzymes

Function and Regulation of Enzymes

Enzymes are biological catalysts that speed up chemical reactions without being consumed. They lower the activation energy required for reactions.

• Active Site: The region on the enzyme where the substrate binds.

• Specificity: Each enzyme is specific to its substrate.

• Regulation: Enzyme activity can be regulated by inhibitors (competitive and noncompetitive) and by allosteric regulation.

Effects of Temperature and pH on Enzyme Activity

• Temperature: Each enzyme has an optimal temperature. High temperatures can denature enzymes, reducing activity.

• pH: Each enzyme has an optimal pH. Extreme pH values can denature enzymes.

Example: Human amylase works best at pH 7, while pepsin (stomach enzyme) works best at pH 2.

Photosynthesis

Principles and Requirements

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy (glucose).

• Requirements: Water (H2O), carbon dioxide (CO2), and light energy.

• Products: Glucose (C6H12O6) and oxygen (O2).

Overall Equation:

$6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{light energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

Photosynthetic Pigments

• Chlorophyll a: Main pigment involved in light absorption.

• Chlorophyll b: Accessory pigment that broadens the spectrum of light absorbed.

• Carotenoids: Accessory pigments that protect against excess light.

Leaf Structure: Stomata and Associated Cells

• Stomata (Stomates): Pores on the leaf surface that allow gas exchange.

• Guard Cells: Specialized cells that regulate the opening and closing of stomata.

• Epidermal Cells: Outer layer of cells that protect the leaf.

Example: During the day, guard cells swell with water, opening the stomata for gas exchange.

Cellular Respiration

Reactants and Products

Cellular respiration is the process by which cells extract energy from glucose.

• Aerobic Respiration: Requires oxygen.

  • Reactants: Glucose and oxygen.

  • Products: Carbon dioxide, water, and ATP (energy).

• Anaerobic Respiration (Fermentation): Occurs without oxygen.

  • In Plants and Yeast: Produces ethanol, CO2, and ATP.

  • In Animals: Produces lactic acid and ATP.

Overall Equation for Aerobic Respiration:

$\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2 \rightarrow 6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{ATP}$

Mitosis

Stages of the Cell Cycle

The cell cycle consists of interphase (cell growth and DNA replication) and mitosis (nuclear division).

• Interphase: Cell grows, performs normal functions, and duplicates DNA.

• Mitosis: Division of the nucleus into two genetically identical daughter nuclei.

  • Prophase: Chromosomes condense, spindle forms.

  • Metaphase: Chromosomes align at the cell's equator.

  • Anaphase: Sister chromatids separate to opposite poles.

  • Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Example: In animal cells, cytokinesis occurs by cleavage; in plant cells, by formation of a cell plate.

Additional info: Students should be able to apply these concepts to laboratory scenarios, interpret experimental results, and understand the significance of these processes in living organisms.