Cell Membrane Structure and Function: Study Notes

Cell Membrane Structure and Function

Introduction to the Cell Membrane

The cell membrane, also known as the plasma membrane, is typically the outermost barrier of the cell. It plays a crucial role in maintaining the integrity of the cell and regulating the movement of substances in and out of the cell.

• Cell Wall Exception: Some cells, such as plant, fungal, and prokaryotic cells, possess a cell wall outside the plasma membrane. The cell wall provides structural support and protection but is not a selective barrier.

• Plasma Membrane: Present in all cells, it acts as a selective barrier that regulates the cell's chemical environment.

The Plasma Membrane: Structure and Functions

The plasma membrane is a dynamic structure that allows the cell to interact with its environment while maintaining internal stability.

• Functions of the Plasma Membrane:

  1. Isolates the cell environment

  2. Regulates exchange between inside and outside

  3. Communicates with other cells

  4. Identifies the type of cell

• Major Components:

  • Lipids: Form the basic structure of the membrane.

  • Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails, forming a phospholipid bilayer.

  • Proteins: Embedded within or attached to the bilayer, serving various functions.

  • Carbohydrates: Often attached to proteins or lipids, involved in cell recognition.

Fluid Mosaic Model

The Fluid Mosaic Model describes the structure of the plasma membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids (Singer and Nicolson, 1972).

• Fluidity: The membrane is flexible due to the lateral movement of lipids and proteins. Factors influencing fluidity include:

  • Lipid composition: More unsaturated fatty acids increase fluidity; more saturated fatty acids decrease fluidity.

  • Temperature: Higher temperatures increase fluidity.

• Mosaic: Refers to the presence of various proteins interspersed within the lipid bilayer.

Membrane Proteins: Types and Functions

• Transport Proteins: Facilitate the movement of substances across the membrane.

  • Channel Proteins: Form pores for specific molecules.

  • Carrier Proteins: Bind and transport substances across the membrane.

• Receptors: Receive and transmit signals from the environment.

• Cell-to-Cell Recognition: Glycoproteins serve as identification tags for cell recognition.

Transport Across Membranes

Overview of Membrane Transport

Transport refers to the movement of molecules across the cell membrane. This process is essential for maintaining homeostasis and allowing communication with the environment.

• Movement of Molecules Requires:

  1. A fluid medium (liquid or gas)

  2. A concentration gradient (difference in concentration across a space)

• Concentration Gradient: A region where the concentration of a substance changes over a distance.

Passive Transport

Passive transport is the movement of substances across the membrane without the expenditure of cellular energy (ATP). It relies on the concentration gradient.

• Diffusion: The net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached.

  • Faster with greater concentration differences.

  • Occurs over short distances.

• Osmosis: The diffusion of water across a selectively permeable membrane.

Selectively Permeable Membranes

The cell membrane is selectively permeable, allowing some substances to cross more easily than others.

• Factors Influencing Permeability:

  • Solubility: Lipid-soluble substances cross more easily.

  • Transport Proteins: Facilitate the movement of specific molecules.

Facilitated Diffusion

Facilitated diffusion is a type of passive transport where specific molecules move across the membrane with the help of transport proteins.

• Types of Facilitated Diffusion:

  1. Bind-and-Release (Carrier Proteins): The protein binds the molecule, changes shape, and releases it on the other side.

  2. Selective Channel: Acts like a door, allowing only specific substances to pass.

  3. Gated Channel: Opens or closes in response to a signal, controlling the passage of substances.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	Oxygen, CO2
Facilitated Diffusion	No	High to Low	Glucose via carrier protein
Osmosis	No	High to Low (water)	Water through aquaporins
Active Transport	Yes (ATP)	Low to High	Sodium-potassium pump

Key Equations

• Fick's Law of Diffusion:

$J = -D \frac{dC}{dx}$

• Where J is the rate of diffusion, D is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Additional info: These notes expand on the provided content by including definitions, examples, and a summary table for clarity and completeness.