Unit 2 – Cell Structure and Function

Comprehensive Notes: Unit 2 – Cell Structure and Function

1. Structure and Function of Subcellular Components and Organelles

Cells are composed of various subcellular components and organelles, each with distinct structures that enable specific functions essential for cellular life:

  • Nucleus:

    • Contains genetic material (DNA)

    • Controls cellular activities.

  • Ribosomes:

    • Sites of protein synthesis.

    • Can be free in the cytoplasm or bound to the endoplasmic reticulum (ER).

  • Endoplasmic Reticulum (ER):

    • Rough ER:

    • Studded with ribosomes.

    • Modifies proteins.

    • Smooth ER:

    • Lacks ribosomes.

    • Synthesizes lipids and detoxifies substances.

  • Golgi Apparatus:

    • Modifies, sorts, and packages proteins and lipids for transport.

  • Mitochondria:

    • Produce ATP (adenosine triphosphate) through cellular respiration.

    • Feature a double membrane and possess their own DNA, which reflects their evolutionary history (likely derived from prokaryotes).

  • Chloroplasts (in plants):

    • Capture light energy for photosynthesis.

    • Possess thylakoid membranes and their own DNA.

  • Lysosomes and Peroxisomes:

    • Break down macromolecules (like lipids, carbohydrates, and proteins).

    • Detoxify harmful substances.

  • Cytoskeleton:

    • Network of protein fibers that maintains cell shape, supports movement, and aids intracellular transport.

  • Plasma Membrane:

    • A selectively permeable barrier controlling the entry and exit of substances.

2. Cell Structures and Energy Capture, Storage, and Use

Organelles such as mitochondria and chloroplasts have specialized structures that optimize energy capture and conversion:

  • Mitochondria:

    • Cristae:

    • Folded inner membrane increases surface area for ATP-generating enzymes, enhancing energy production.

  • Chloroplasts:

    • Thylakoid membranes:

    • House photosynthetic pigments (such as chlorophyll).

    • Stroma:

    • Contains enzymes essential for sugar synthesis.

    • These specialized features facilitate efficient energy transduction vital for cellular metabolism.

3. Surface Area-to-Volume Ratio and Material Exchange

The ratio of surface area to volume is critical for cellular efficiency:

  • As cell size increases, volume grows faster than surface area, which limits the cell’s ability to exchange materials efficiently.

  • Cells optimize exchange through structures that increase surface area, such as:

    • Microvilli in animal cells.

    • Flattened and elongated shapes.

  • Effective surface area supports diffusion of nutrients, gases, and waste, thus maintaining cellular homeostasis.

4. Specialized Structures and Strategies for Molecular Exchange

Organisms employ various adaptations to enhance exchange of molecules with their environment:

  • Cell membrane folding (e.g., microvilli):

    • Increases surface area for absorption.

  • Membrane channels and transport proteins:

    • Facilitate selective nutrient uptake and waste removal.

  • In multicellular organisms, specialized tissues and organ systems enhance material transport and exchange.

5. Cell Membrane Components and the Fluid Mosaic Model

The cell membrane is a dynamic structure described by the Fluid Mosaic Model, which highlights the following features:

  • Lipid Bilayer:

    • Composed of phospholipids with hydrophilic heads and hydrophobic tails, forming a semi-permeable barrier.

  • Proteins:

    • Integral and peripheral proteins assist in transport, signal transduction, and provide structural support.

  • Cholesterol:

    • Regulates membrane fluidity and stability.

  • Carbohydrates:

    • Attached to lipids or proteins; function in cell recognition and adhesion.

6. Selective Permeability of Biological Membranes

The structure of membranes directly influences selective permeability, allowing cells to control their internal environments:

  • Small, nonpolar molecules (e.g., O2, CO2):

    • Pass freely through the lipid bilayer.

  • Polar and charged molecules:

    • Require transport proteins to cross membranes.

  • Membrane proteins:

    • Provide specificity, enabling regulated passage of ions and molecules.

7. Role of the Cell Wall

Found in bacteria, plants, fungi, and some protists, the cell wall:

  • Provides structural support and protection.

  • Maintains cell shape.

  • Prevents excessive water uptake due to osmotic pressure.

  • Composed of different molecules depending on the cell type; for example:

    • Cellulose in plants.

    • Peptidoglycan in bacteria.

8. Osmoregulatory Mechanisms in Organisms

Organisms regulate water and solute concentrations to maintain cellular homeostasis and health:

  • Osmosis:

    • Movement of water across selectively permeable membranes from low to high solute concentration.

  • Mechanisms include:

    • Contractile vacuoles in protists that expel excess water.

    • Kidneys in animals that regulate water and salt balance.

    • Plant cells rely on turgor pressure for structural integrity.

9. Membrane Transport Mechanisms

Various methods enable substances to cross plasma membranes:

  • Passive Transport:

    • Does not require energy; includes:

    • Diffusion.

    • Facilitated diffusion:

      • Via channel or carrier proteins.

    • Osmosis.

  • Active Transport:

    • Energy-dependent movement of molecules against concentration gradients using transport proteins (e.g., ion pumps).

  • Bulk Transport:

    • Endocytosis:

    • Phagocytosis: ingestion of large particles ("cell eating").

    • Pinocytosis: ingestion of liquids or small particles ("cell drinking").

    • Exocytosis:

    • Moves large molecules or particles out of the cell.

10. Effect of Molecular Structure on Membrane Passage

Structural characteristics of molecules influence their ability to cross membranes:

  • Small, nonpolar molecules:

    • Pass easily through the lipid bilayer.

  • Large, polar, or charged molecules:

    • Generally impermeable without facilitation through transport mechanisms.

  • Molecular shape and polarity:

    • Affect interaction with membrane proteins and the lipid bilayer, influencing transport efficiency.

11. Concentration Gradients and Molecular Movement

Molecules move according to gradients:

  • Move from regions of higher to lower concentration (down the gradient) during passive transport.

  • Move against the gradient during active transport, requiring energy (ATP).

  • Gradients also drive secondary active transport or coupled transport mechanisms.

12. Membrane-Bound Structures and Compartmentalization in Eukaryotic Cells

Eukaryotic cells contain extensive internal membranes creating distinct compartments, allowing specialization of functions:

  • Organelles such as the nucleus, ER, Golgi apparatus, lysosomes, and mitochondria each conduct specific biochemical processes.

  • Compartmentalization:

    • Increases efficiency by segregating incompatible reactions and localizing enzymes.

  • Membrane-bound compartments regulate molecular traffic and signaling within the cell.

13. Similarities and Differences in Compartmentalization Between Prokaryotic and Eukaryotic Cells

  • Prokaryotic Cells:

    • Lack membrane-bound organelles.

    • Metabolic processes occur in the cytoplasm or at the plasma membrane.

  • Eukaryotic Cells:

    • Possess numerous membrane-bound organelles and complex internal structures.

  • Both have:

    • Plasma membranes.

    • Ribosomes.

    • Cytoskeletal elements, but differ significantly in internal organization.

14. Endosymbiotic Theory and Organellar Ancestry

The endosymbiotic theory explains the origin of mitochondria and chloroplasts:

  • Proposes that these organelles were formerly free-living prokaryotes engulfed by ancient eukaryotic cells.

  • Evidence supporting this theory includes:

    • Both organelles have double membranes.

    • Possess own circular DNA and replicate independently.

    • Retain similarities to their free-living bacterial ancestors in structure and genetics.

  • This relationship helps explain the complexity of eukaryotic cells and their energy metabolism.

15. Interpretation of Data Sets and Graphs

Understanding and analyzing data related to cell structure and function are essential skills:

  • Analyzing graphs showing rates of diffusion, osmosis, or transport across membranes under varying conditions.

  • Interpreting microscopy images and relating structural variations to functional differences.

  • Evaluating experimental data on membrane permeability, organelle function, or energy capture.

  • Drawing conclusions connecting experimental evidence to theoretical models such as the Fluid Mosaic Model or endosymbiotic theory.