Cell Biology II.

Cell Biology - Membranes of Living Organisms

Overview

  • Membranes separate the extracellular environment from the intracellular environment.

  • Cell membranes are primarily composed of a phospholipid bilayer, which is ubiquitous across almost all cells, except for Archaea which have monolayers.

Membrane Structure

Phospholipids

  • Lipid Molecules: Comprise fats and oils; essential components of cell membranes.

    • Hydrophilic Head:

    • Loves water due to its polar nature.

    • Contains a phosphate group and glycerol.

    • Hydrophobic Tail:

    • Repels water because it is apolar.

    • Comprises fatty acid chains.

Membrane Formation

  • When phospholipids are placed in water:

    • The hydrophilic heads align with water, while hydrophobic tails orient away from it.

    • This arrangement leads to the formation of a membranous structure when in abundance.

  • Contrarily, in oil, the orientation reverses with heads pointing outward, emphasizing the bilayer's importance in aqueous environments.

Transport Mechanisms

Types of Transport

  • Passive Transport:

    • Does not require energy; driven by environmental gradients (e.g., concentration, charge).

  • Active Transport:

    • Requires energy from the organism to move substances against their gradient.

Gradient Concept

  • Gradient represents the difference in concentration across a membrane; substances will move from a region of higher concentration to lower concentration to equalize distribution (e.g., Mg2+ ions).

Semipermeable Nature of Membranes

  • Membranes prevent unrestricted movement due to their phospholipid bilayer, especially for water-soluble substances that cannot traverse the hydrophobic core.

  • Membrane proteins facilitate the transport process.

Membrane Proteins

Categories and Functions

  • Peripheral Proteins:

    • Located on the membrane's surface.

  • Integral Proteins:

    • Span the entire membrane, providing pathways for transport.

  • Functions:

    • Transport:

      • Can be passive (channels) or active (transporters).

    • Signaling:

      • Receive and transmit signals within cellular pathways.

Passive Transport Mechanisms

Types

  • Diffusion:

    • Movement of dissolved particles along a concentration gradient.

  • Osmosis:

    • Movement of water according to its own concentration gradient.

Tonicity Definitions

  • Hypertonic Solution:

    • Higher concentration of solutes compared to inside the cell, leading to water exiting the cell.

  • Hypotonic Solution:

    • Lower concentration of solutes, causing water to enter the cell.

  • Isotonic Solution:

    • Equal solute concentrations, resulting in no net water movement.

Cell Types and Membranes

Differences Between Animal and Plant Cells

  • Animal Cell: Has a cell surface membrane without a rigid structure (cell wall).

  • Plant Cell: Has both a cell surface membrane and a rigid cell wall, which provides additional structural support.

Cholesterol in Cell Membranes

Role and Health Implications

  • Present only in eukaryotic cells (mainly animal cells).

  • Increases membrane integrity, compensating for the absence of cell walls.

  • Excess dietary cholesterol can lead to health issues like high blood pressure.

Cellular Organelles and Structures

Eukaryotic Cells Overview

  • Comprise four kingdoms: unicellular eukaryotes (protists), plants, animals, and fungi.

  • Characterized by a defined nucleus and internal membrane-bound organelles.

Nucleus

  • Enclosed by a nuclear envelope, housing DNA that controls cell metabolism, structure, and reproduction processes.

  • Nuclear pores facilitate the movement of RNA (not DNA) from the nucleus.

Endoplasmic Reticulum (ER)

Types

  • Rough ER (RER): Studded with ribosomes, synthesizing proteins and lipids.

    • Continuous with the nuclear envelope.

  • Smooth ER (SER): Lacks ribosomes; involved in metabolism and detoxification processes.

Ribosomes

  • Sites of protein synthesis; can be free-floating in the cytoplasm or attached to the RER.

Golgi Apparatus

  • Functions in packaging synthesized proteins into vesicles before transport.

  • Essential for transporting proteins destined for secretion or use within the cell.

Lysosomes

  • Organelles responsible for breaking down macromolecules and recycling cellular components.

  • Merges with vesicles containing food for digestion, utilizing digestive enzymes.

Endocytosis and Exocytosis

Processes

  • Endocytosis: Enveloping of large molecules by the cell membrane, forming vesicles for transport (e.g., immune cells neutralizing pathogens).

  • Exocytosis: Removal of waste or unnecessary materials from the cell via vesicles merging with the membrane and releasing contents outside.

Mitochondria

  • Enveloped by two membranes, contains its DNA, ribosomes, and enzymes.

  • Functions as the powerhouse of the cell, producing ATP through oxidation of glucose and oxygen.

  • Bacterial origin; essential for cellular energy production.

Chloroplasts

  • Site of photosynthesis in plants; contains chlorophyll.

  • Converts CO2 and water into sugars using sunlight.

  • Enveloped by two membranes, similar to mitochondria.

Vacuoles

  • Central vacuole in plant cells stores water, nutrients, and waste and helps maintain cell shape.

Cell Wall

  • Provides structural support to plant and fungal cells.

  • Composed of cellulose (plants) and chitin (fungi).

Cell Movement Structures

Components

  • Flagella: Long, whip-like structures promoting movement.

  • Cilia: Short, hair-like structures providing a coordinated movement.

  • Pseudopodia: Extensions of cytoplasm aiding in movement and engulfing food.

Cytoskeleton

  • Network of protein filaments maintaining cell shape and enabling movement and material transfer.