Cell Structure and Membrane Functions

Overview of Cell Structures and Functions

Nucleus

• Structure of the Nucleus

  • Surrounded by a double membrane (nuclear envelope)

  • Composed of two lipid bilayers

• Functionality

  • Contains nuclear pores for selective transport in and out of the nucleus

  • Protects DNA from degradation

Endoplasmic Reticulum (ER)

• Structure

  • Composed of a complex network of membranes

• Functions

  • Synthesizes lipids and proteins

  • Types of ER:

  • Rough ER (RER): Contains ribosomes, involved in protein synthesis.

  • Smooth ER (SER): Lacks ribosomes, involved in lipid synthesis and detoxification.

• Product transportation

  • Sends synthesized products to the Golgi apparatus.

Golgi Apparatus

• Description: The UPS of the cell

• Function

  • Receives, sorts, and dispatches products from the ER

  • Processes proteins and lipids for transport to various destinations.

Lysosome

• Functionality

  • Contains digestive enzymes for breaking down macromolecules

  • Acts as the recycling center for the cell, dismantling and repurposing cellular debris.

Mitochondria

• Primary Role

  • Site of ATP (adenosine triphosphate) production, which serves as cellular energy currency.

Cytoskeleton

• Structure

  • Composed of a network of protein filaments and tubules.

• Functions

  • Provides structural support and shape to the cell.

  • Facilitates movement (e.g., cell division).

Extracellular Matrix (ECM)

• Definition

  • Network of proteins and carbohydrates located outside the cell.

• Functions

  • Provides structural support for tissues and helps anchor cells in place.

Plasma Membrane (Chapter 7)

Definition and Composition

• Also known as cell membrane or phospholipid bilayer.

• Structure

  • Composed of phospholipids arranged in a bilayer with embedded proteins and carbohydrates.

Phospholipid Bilayer

• Properties

  • Hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

  • Spontaneously forms a bilayer in aqueous environments.

• Dynamics

  • Fluid mosaic model: Membrane is adaptable; proteins and lipids move laterally within the layer.

Fluid Mosaic Model

• Explanation

  • Describes the structure of the plasma membrane as a mosaic of different components (lipids, proteins, carbohydrates) that can move freely.

• Evidence

  • Freeze-fracture technique allowed scientists to observe proteins' distribution within the membrane.

Phospholipid Movement

• Types of Movements

  • Lateral movement (quickly, 10 million times per second).

  • Flip-flop movement (slow, cannot easily cross the hydrophobic core).

• Role of cholesterol

  • Stabilizes the membrane structure, impacting fluidity.

Proteins in the Plasma Membrane

• Types of Membrane Proteins

  • Integral (transmembrane) proteins: Span the entire membrane.

  • Peripheral proteins: Loosely attached to the membrane surface.

• Functions of Membrane Proteins

  • Transport, enzymatic activity, signal transduction, cell-cell recognition, and attachment to the cytoskeleton.

• Carbohydrates on Membrane

  • Glycoproteins and glycolipids participate in cell-surface recognition.

Transport Mechanisms

Selective Permeability

• The membrane allows certain substances to pass while restricting others.

Passive Transport

• Definition

  • Movement of molecules from high concentration to low concentration without energy use.

• Types

  • Simple diffusion: Direct movement through the membrane (e.g., oxygen).

  • Facilitated diffusion: Requires specific proteins (e.g., glucose, ions).

Osmosis

• Definition

  • Movement of water across a selectively permeable membrane.

• Terms

  • Hypertonic: Solution with a higher concentration of solute.

  • Hypotonic: Solution with a lower concentration of solute.

  • Isotonic: Solutions with equal solute concentrations.

Active Transport

• Definition

  • Movement of molecules from low concentration to high concentration using energy (typically ATP).

• Example: Sodium-potassium pump

  • Transports sodium out of the cell and potassium into the cell against their concentration gradients using ATP.

Summary of Key Concepts

• Passive transport: High to low concentration, energy not required.

• Active transport: Low to high concentration, energy required.

• Osmosis is crucial for maintaining water balance within animal and plant cells:

  • Animal cells need an isotonic environment to prevent lysis or shrinking.

  • Plant cells thrive in a hypotonic environment, maintaining turgor pressure due to their cell wall structure.