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Cell Membrane and Transport

Introduction​

The cell membrane, also known as the plasma membrane, is a crucial component of eukaryotic cells. It serves as the boundary between the interior of the cell and its external environment. The cell membrane plays a vital role in maintaining cellular homeostasis, regulating what enters and leaves the cell, and facilitating communication between cells.

This chapter explores the structure of the cell membrane, its functions, and various transport mechanisms used by cells to exchange materials with their surroundings.

Structure of the Cell Membrane​

The cell membrane consists of three main components:

  1. Phospholipid bilayer
  2. Embedded proteins
  3. Cholesterol molecules

Phospholipid Bilayer​

The phospholipid bilayer is the primary structural component of the cell membrane. It consists of two layers of lipid molecules arranged head-to-tail:

  • The hydrophilic (water-loving) heads face outwards to water
  • The hydrophobic (water-fearing) tails face inwards away from water

This arrangement creates a barrier that separates the cell's internal environment from its external surroundings.

Embedded Proteins​

Proteins embedded within the phospholipid bilayer serve various functions:

  • Transport proteins facilitate the movement of molecules across the membrane
  • Receptor proteins receive signals from outside the cell
  • Enzymes catalyze chemical reactions at the membrane surface

Some common types of membrane proteins include:

  • Channel proteins: Form pores that allow specific ions or molecules to pass through
  • Carrier proteins: Bind to molecules one side of the membrane and release them on the other after undergoing a conformational change
  • Adhesion proteins: Help cells stick together and interact with their environment

Cholesterol Molecules​

Cholesterol molecules are interspersed between the phospholipids in the membrane. They play several important roles:

  • Stabilize the fluidity of the membrane
  • Regulate the activity of membrane proteins
  • Influence the permeability of the membrane to certain substances

Functions of the Cell Membrane​

The cell membrane performs several critical functions:

  1. Selective permeability: Controls what enters and leaves the cell
  2. Cellular communication: Receives signals from outside the cell
  3. Maintenance of cellular homeostasis: Regulates ion balance and pH levels
  4. Support structure: Provides mechanical support and shape to the cell
  5. Storage: Can store small molecules like calcium ions

Types of Transport Mechanisms​

Cells employ various transport mechanisms to exchange materials with their surroundings. These can be broadly categorized into passive and active transport.

Passive Transport​

Passive transport does not require energy input from the cell. There are two main types:

Diffusion​

Diffusion is the random movement of particles from an area of higher concentration to an area of lower concentration until equilibrium is reached. This process occurs without the need for energy input.

Examples:

  • Oxygen diffuses into cells
  • Carbon dioxide diffuses out of cells

Osmosis​

Osmosis is the movement of water molecules from an area of higher concentration to an area of lower concentration through a selectively permeable membrane. This process helps maintain proper hydration of cells.

Example:

  • Plant roots absorb water through osmosis

Active Transport​

Active transport requires energy input from the cell to move molecules against their concentration gradient. There are three main types:

Primary Active Transport​

Primary active transport uses ATP directly to pump molecules across the membrane.

Example:

  • Sodium-potassium pump moves sodium out of and potassium into cells

Secondary Active Transport​

Secondary active transport couples the movement of one molecule down its concentration gradient to drive the movement of another molecule up its concentration gradient.

Example:

  • Sodium-glucose cotransport moves glucose into intestinal cells using the sodium gradient established by the sodium-potassium pump

Facilitated Diffusion​

Facilitated diffusion involves carrier proteins or channel proteins to assist in the movement of molecules across the membrane.

Example:

  • GLUT4 protein facilitates glucose uptake in muscle cells

Examples of Cell Membrane Function​

  1. Ion Channels:

    • Sodium channels in neurons allow rapid depolarization during action potentials
    • Potassium channels help repolarize the neuron after an action potential

  2. Receptor-Mediat Endocytosis:

    • LDL receptors remove low-density lipoproteins from the bloodstream
    • Insulin receptors regulate blood sugar levels

  3. Gap Junctions:

    • Allow direct communication between adjacent cells
    • Important in cardiac tissue for coordinated contraction

  4. Cell Signaling:

    • G-protein coupled receptors respond to hormones and neurotransmitters
    • Toll-like receptors recognize pathogens and trigger immune responses

  5. Antigen Presentation:

    • Major Histocompatibility Complex (MHC) molecules display peptides from inside the cell to T-cells
    • B-cell receptors bind antigens for antibody production

Illustrations​

Here are some simple illustrations to help visualize key concepts:

  1. Cell membrane structure: