Cell Structure, Function, and Membrane Transport: Study Notes for General Biology

Chapter 6: Cell Structure and Function

Cell Fractionation

Cell fractionation is a laboratory technique used to separate cellular components while preserving their individual functions. This process allows scientists to study the functions of specific organelles in isolation.

• Definition: The process of breaking up cells and separating their components by centrifugation.

• Purpose: To analyze the structure and function of organelles such as nuclei, mitochondria, and ribosomes.

• Steps:

  1. Homogenization: Breaking open the cells.

  2. Centrifugation: Spinning the homogenate at various speeds to separate organelles by size and density.

• Example: Isolating mitochondria to study cellular respiration.

Microscopy

Microscopes are essential tools for visualizing cells and their structures. Different types of microscopes offer various advantages and limitations.

• Light Microscope (LM):

  • Uses visible light to illuminate specimens.

  • Advantages: Can observe living cells, relatively inexpensive.

  • Disadvantages: Limited resolution (~200 nm), cannot view most organelles in detail.

• Compound Light Microscope: A type of light microscope with multiple lenses for higher magnification.

• Electron Microscopes (EM):

  • Use beams of electrons for much higher resolution.

  • Transmission Electron Microscope (TEM): Provides detailed images of internal cell structures.

  • Scanning Electron Microscope (SEM): Produces 3D images of cell surfaces.

  • Advantages: High resolution (up to 0.1 nm).

  • Disadvantages: Specimens must be dead, expensive equipment.

Surface Area to Volume Ratio

The surface area to volume ratio is a key factor that limits cell size. As a cell grows, its volume increases faster than its surface area, making it harder to exchange materials efficiently.

• Formula: $\text{Surface Area} = 6a^2$, $\text{Volume} = a^3$ (for a cube of side a)

• Implication: Smaller cells have a higher surface area to volume ratio, facilitating efficient transport of materials.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on the presence of a nucleus and membrane-bound organelles.

• Prokaryotic Cells: Lack a nucleus; DNA is in the nucleoid region. No membrane-bound organelles. Examples: Bacteria, Archaea.

• Eukaryotic Cells: Have a nucleus containing DNA. Possess membrane-bound organelles. Examples: Plants, Animals, Fungi, Protists.

DNA Location

• In eukaryotic cells, most DNA is located in the nucleus.

• In prokaryotic cells, DNA is found in the nucleoid region.

Ribosomes and Protein Synthesis

Ribosomes are the sites of protein synthesis in all cells.

• Free Ribosomes: Float in the cytosol; synthesize proteins that function within the cytosol.

• Bound Ribosomes: Attached to the endoplasmic reticulum (ER); synthesize proteins for membranes, organelles, or export.

Chloroplast Structure

Chloroplasts are the sites of photosynthesis in plant cells.

• Key Structures:

  • Thylakoid: Flattened sacs where light-dependent reactions occur.

  • Thylakoid Space: Internal compartment of the thylakoid.

  • Granum: Stack of thylakoids.

  • Stroma: Fluid surrounding the thylakoids; site of the Calvin cycle.

Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Includes: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and plasma membrane.

Cytoskeleton

The cytoskeleton provides structural support, maintains cell shape, and facilitates movement.

• Components:

  • Microtubules: Hollow tubes; maintain cell shape, aid in chromosome movement, and serve as tracks for organelle movement.

  • Microfilaments (Actin Filaments): Thin rods; involved in cell movement and muscle contraction.

  • Intermediate Filaments: Provide mechanical support for the cell.

Comparison of Plant and Animal Cells

Plant and animal cells share many organelles but also have unique structures.

Feature	Plant Cells	Animal Cells
Cell Wall	Present	Absent
Chloroplasts	Present	Absent
Central Vacuole	Large, present	Small or absent
Lysosomes	Rare	Common

Cell Junctions

Cell junctions connect cells and facilitate communication.

• Tight Junctions: Seal cells together to prevent leakage (animal cells).

• Desmosomes: Anchor cells together (animal cells).

• Gap Junctions: Allow passage of ions and small molecules between cells (animal cells).

• Plasmodesmata: Channels between plant cells for transport and communication.

Chapter 7: Membrane Structure and Function

Plasma (Cell) Membrane Structure

The plasma membrane is a selectively permeable barrier that surrounds the cell, composed mainly of a phospholipid bilayer with embedded proteins.

• Components:

  • Phospholipids (form the bilayer)

  • Proteins (integral and peripheral)

  • Cholesterol (modulates fluidity)

  • Carbohydrates (attached to lipids and proteins for cell recognition)

• Function: Controls the movement of substances in and out of the cell.

Role of Unsaturated Fats in the Cell Membrane

• Unsaturated fatty acids in phospholipids prevent tight packing, increasing membrane fluidity.

• This fluidity is essential for membrane function and flexibility.

Selective Permeability of the Membrane

The cell membrane allows some substances to pass more easily than others.

• Most Easily Passed: Small, nonpolar (hydrophobic) molecules (e.g., O2, CO2).

• Less Easily Passed: Large polar molecules, ions, and glucose require transport proteins.

Amphipathic Molecules and Integral Proteins

• Amphipathic: Molecules with both hydrophobic and hydrophilic regions (e.g., phospholipids, integral membrane proteins).

• Integral Proteins: Span the membrane and interact with both the hydrophobic core and hydrophilic surfaces.

Membrane Sidedness

• The plasma membrane has distinct inside (cytoplasmic) and outside (extracellular) faces, with specific protein and lipid compositions.

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane. Tonicity describes the ability of a solution to cause a cell to gain or lose water.

• Hypertonic Solution: Higher solute concentration outside; cell loses water and shrinks.

• Hypotonic Solution: Lower solute concentration outside; cell gains water and swells.

• Isotonic Solution: Equal solute concentration; no net water movement.

• Plant Cells: Prefer hypotonic environments (turgid).

• Animal Cells: Prefer isotonic environments.

Passive Transport

Passive transport is the movement of substances across the membrane without energy input.

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a membrane.

• Facilitated Diffusion: Transport of substances via channel or carrier proteins (e.g., glucose transporters).

Water Potential

Water potential predicts the direction water will move. Water moves from areas of higher to lower water potential.

• Formula: $\Psi = \Psi_s + \Psi_p$ (Water potential = solute potential + pressure potential)

• Higher solute concentration lowers water potential.

• Hypotonic solutions have higher water potential; hypertonic solutions have lower water potential.

Active Transport

Active transport moves substances against their concentration gradient using energy (usually ATP).

• Example: Sodium-potassium pump ($\text{Na}^+/\text{K}^+$ pump) exchanges 3 Na+ out and 2 K+ in per ATP hydrolyzed.

• Equation: $3\ \text{Na}^+_{\text{in}} + 2\ \text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\ \text{Na}^+_{\text{out}} + 2\ \text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

Electrogenic Pumps and Membrane Potential

• Electrogenic Pump: A transport protein that generates voltage across a membrane (e.g., Na+/K+ pump).

• Membrane Potential: The voltage difference across a membrane, important for nerve impulse transmission.

Cotransport

Cotransport involves the coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.

• Components: Symporters and antiporters (types of transport proteins).

• Example: Sucrose-H+ cotransporter in plant cells.

Bulk Transport

Bulk transport moves large molecules or particles across the membrane via vesicles.

• Phagocytosis: "Cell eating"; cell engulfs large particles.

• Pinocytosis: "Cell drinking"; cell engulfs extracellular fluid.

• Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to receptors.