Cells: Structure and Function

Introduction to Cells

Cells are the fundamental units of life, possessing all the properties necessary for living organisms. Their structure and function are closely linked to their ability to maintain homeostasis and interact with their environment.

• Definition: The cell is the smallest functional unit that contains all the properties of life.

Plasma Membrane

Structure of the Plasma Membrane

The plasma membrane is a selectively permeable barrier that surrounds the cell, composed of several key components:

• Phospholipid Bilayer:

  • Hydrophilic phosphate heads face both the interior and exterior aqueous environments.

  • Hydrophobic fatty acid tails orient toward each other, forming the membrane's core.

• Proteins:

  • Provide structural stability.

  • Facilitate the transport of molecules across the membrane.

• Carbohydrates:

  • Located on the external surface; involved in cell recognition and signaling.

Functions of the Plasma Membrane

• Boundary Formation: Separates the cell from its external environment.

• Selective Permeability: Regulates the entry and exit of substances via different transport mechanisms:

  • Diffusion

  • Facilitated diffusion

  • Active transport

Transport Across the Plasma Membrane

Simple Diffusion

Simple diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient.

• Examples: Oxygen (O2) and carbon dioxide (CO2) diffuse directly across the membrane.

• Factors Affecting Diffusion Rate:

  • Temperature (higher temperature increases rate)

  • Magnitude of concentration gradient

  • Size of molecules (smaller molecules diffuse faster)

  • Polarity (nonpolar molecules diffuse more easily)

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane from an area of low solute concentration (high water potential) to an area of high solute concentration (low water potential).

• Direction: Water moves from high [water] to low [water].

• Tonicity: Describes the relative solute concentration of solutions separated by a membrane.

  • Hypertonic: Solution with higher solute concentration compared to the cell; water moves out of the cell.

  • Hypotonic: Solution with lower solute concentration compared to the cell; water moves into the cell.

  • Isotonic: Solutions have equal solute concentrations; no net water movement.

Facilitated Diffusion

Facilitated diffusion is a passive process that moves molecules down their concentration gradient with the help of specific membrane proteins.

• Used for: Large or polar molecules that cannot diffuse directly through the lipid bilayer.

• Proteins involved: Channel proteins and carrier proteins.

• Energy requirement: None (passive process).

Active Transport

Active transport moves molecules against their concentration gradient, from low to high concentration, requiring energy input (usually ATP).

• Proteins involved: Specific membrane proteins (e.g., sodium-potassium pump).

• Energy requirement: ATP is hydrolyzed to provide energy.

Example: The sodium-potassium pump maintains electrochemical gradients in animal cells by transporting Na+ out and K+ in.

Equation for ATP hydrolysis:

Laboratory Applications

Testing Movement Across a Semipermeable Membrane

Dialysis tubing can simulate a cell membrane to study the movement of various solutes by diffusion and osmosis.

• Dialysis tubing is filled with a solution containing different solutes (e.g., starch, chloride ions, sulfate ions, protein, reducing sugars).

• The tubing is placed in another solution, and after a set period, tests are performed to determine which molecules have crossed the membrane.

Surface Area and Volume in Cells

The efficiency of diffusion in cells is influenced by the ratio of surface area to volume.

• As a cell grows, its volume increases faster than its surface area.

• Cells remain small to maximize surface area relative to volume, ensuring efficient exchange of materials and preventing bursting.

Mathematical relationship:

For a cube-shaped cell of side length l:

• Surface area =

• Volume =

• Surface area to volume ratio =

This ratio decreases as cell size increases, limiting cell size.

Summary Table: Types of Membrane Transport

Key Terms

• Plasma membrane: The cell's outer boundary, controlling movement of substances.

• Diffusion: Passive movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a membrane.

• Tonicity: Relative solute concentration of solutions.

• Facilitated diffusion: Passive transport via membrane proteins.

• Active transport: Energy-requiring movement against a gradient.

• Surface area to volume ratio: Determines efficiency of material exchange in cells.

Additional info: Academic context and equations have been added to clarify and expand upon the original notes for comprehensive exam preparation.