Cell Membrane Structure and Function: Study Notes

Cell Membrane Structure and Function

Cell Wall vs. Cell Membrane

The cell membrane is typically the outermost barrier of the cell, but some cells also possess a cell wall. The cell wall provides structural support and protection, but usually does not act as a barrier to molecules.

• Cell wall is found in:

  • Plants – made of cellulose

  • Fungi – made of chitin

  • Prokaryotes – composed of a variety of macromolecules

• The plasma membrane surrounds all cells and regulates interaction with the environment.

The Plasma Membrane

The plasma membrane is a selectively permeable boundary that allows the cell to interact with its environment while maintaining internal conditions.

• Functions of the plasma membrane:

  1. Isolate the cell environment

  2. Regulate exchange between inside and outside

  3. Communicate with other cells

  4. Identify the cell type

Membrane Composition

Cell membranes are primarily composed of lipids and proteins. The main lipid component is the phospholipid bilayer, which forms the basic structure of the membrane.

• Phospholipids can form membranes naturally due to their amphipathic nature (having both hydrophilic and hydrophobic regions).

• Biological membranes also contain proteins embedded within or attached to the bilayer.

Fluid Mosaic Model

The Fluid Mosaic Model describes the structure of the plasma membrane as a dynamic arrangement of phospholipids and proteins.

• Fluid – Lateral movement of lipids and proteins within the bilayer; transverse movement (flip-flop) is rare.

• Mosaic – Proteins are interspersed among the lipids, creating a mosaic pattern.

• Proteins in the membrane have hydrophilic (polar head groups) and hydrophobic (nonpolar tails) regions.

Membrane Proteins and Glycoproteins

Membrane proteins perform various functions essential for cell survival and communication.

• Transport proteins – Facilitate movement of substances across the membrane.

• Channel proteins – Form pores for specific molecules to pass through.

• Carrier proteins – Selectively transport molecules by changing shape.

• Receptors – Receive and transmit signals from the environment.

• Cell-to-cell recognition – Glycoproteins (proteins with attached carbohydrates) serve as identification markers.

Transport Across Membranes

General Principles

Transport across membranes is essential for maintaining cellular homeostasis. Movement of molecules requires:

• Passage through a fluid (liquid or gas)

• A concentration gradient (difference in the amount of molecules from one place to another)

Passive Transport

Passive transport is the movement of substances down a concentration gradient, without the need for cellular energy.

• Diffusion – Net movement of molecules from higher to lower concentration.

  • The greater the concentration difference, the faster the diffusion.

  • Net movement continues until equilibrium is reached.

  • Usually occurs over short distances.

• Selective permeability – Biological membranes allow some substances to cross more easily than others.

Facilitated Diffusion

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

• Occurs via carrier proteins or channel proteins.

• Three types:

  1. Bind-and-release (carrier proteins)

  2. Selective channel (channel proteins)

  3. Gated channel (channels that open/close like a door)

Osmosis and Osmotic Pressure

Osmosis is the passive diffusion of water across a differentially permeable membrane.

• Water moves from high concentration to low concentration.

• Influenced by solute concentration and membrane permeability.

• Osmotic pressure is the tendency of a solution to take up water when separated from pure water by a membrane.

Principles of Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane.

• Water moves from high concentration to low concentration.

Cellular Water Balance

Cells must balance water movement to survive. The environment can be:

Environment	Effect on Cell
Isotonic	No net movement of water; cell volume remains stable.
Hypertonic	Water moves out; cell shrivels.
Hypotonic	Water moves in; cell swells and may burst.

Isotonic – Equal concentration of solute inside and outside the cell. Hypertonic – Greater concentration of solute outside the cell. Hypotonic – Lower concentration of solute outside the cell.

Cells with walls (e.g., plant cells) are more tolerant to excessive water movements and become turgid in hypotonic environments, providing mechanical support.

Energy-Requiring Transport Across Membranes

Active Transport

Active transport is the movement of solutes against a concentration gradient, requiring energy input from the cell.

• Uses energy from ATP.

• Maintains ion gradients (e.g., Ca2+ ions).

Example equation:

$\text{Active Transport:} \, \text{Solute}_{\text{low}} \xrightarrow{\text{ATP}} \text{Solute}_{\text{high}}$

Endocytosis and Exocytosis

Cells transport large molecules via endocytosis and exocytosis.

• Endocytosis – Cellular uptake of large molecules or particles by engulfing them in vesicles.

  • Phagocytosis – "Cell eating"; uptake of solid particles, often involving pseudopodia.

  • Pinocytosis – "Cell drinking"; uptake of fluids and dissolved substances.

  • Receptor-mediated endocytosis – Import of specific macromolecules via binding to cell surface receptors and formation of coated vesicles.

• Exocytosis – Secretion of large molecules by fusion of vesicles with the plasma membrane.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Passive Transport	No	Down concentration gradient	Diffusion, Osmosis
Facilitated Diffusion	No	Down concentration gradient	Glucose transport via carrier protein
Active Transport	Yes (ATP)	Against concentration gradient	Sodium-potassium pump
Endocytosis	Yes	Into cell	Phagocytosis of bacteria
Exocytosis	Yes	Out of cell	Secretion of hormones

Additional info: These notes expand on the original content by providing definitions, examples, and summary tables for clarity and completeness.