Cell Membrane Permeability and Transport Mechanisms

Permeability of Cell Membranes

• Freely Permeable: Allows everything to pass through.
• Semi-Permeable (Selectively Permeable):
  • Allows only certain substances to pass through.
  • Some substances pass through more easily than others.
  • Some substances cannot pass through at all.
  • Selectively permeable based on the needs of the cell (e.g., nutrients, size).
• Impermeable: Allows nothing to pass through.
• Selective Permeability:
  • Nutrients can enter the cell.
  • Protected by the cell.
  • Size: Small molecules (e.g., ions, water) pass through more easily; large molecules (e.g., proteins) have difficulty.
    • Water passes through most easily.
    • Ions (e.g., salts) are smaller than proteins
    • Proteins - very large structure
  • Examples:
    • Cerebral Spinal Fluid: Protein should not be present; its presence indicates a problem.
    • Blood/Urine Samples: Protein should not be present (exercise can force protein into urine).
  • Sugars:
    • Starch (large) should not pass.
    • Glucose (small) can pass, sometimes needing help.

Fluid Mosaic Model of Cell Membrane

• Early models suggested a rigid arrangement of proteins and lipids.
• The Fluid Mosaic Model (1970s):
  • Describes the membrane as a double lipid layer with interspersed proteins.
    • Fluid: Lipid layer has a fluid consistency.
    • Mosaic: Proteins are scattered throughout.
  • Composed of a phospholipid bilayer.
• Phospholipid Composition:
  • Typical fat contains three fatty acids
  • Double lipid layer with proteins interspersed.
  • Proteins can span the entire membrane or be located on the inner or outer surface.
• Types of Membrane Proteins:
  • Integral Proteins: Span the entire membrane from one side to the other.
  • Peripheral Proteins: Located on the inside or outside surface of the membrane; lend stability.
• Carbohydrates:
  • Small amounts may attach to proteins (glycoproteins) or lipids (glycolipids).
  • Act as identifying features to distinguish one cell from another.
    *Illustration:
  • This glycolipid is attached to a lipid portion.
  • This glycoprotein is attached to a protein portion.

Functions of Membrane Proteins

• Channel Proteins:
  • Form a channel through which molecules can pass.
  • Allow molecules that cannot pass through the lipid portion to be carried through the protein portion.
• Transport Proteins:
  • Carrier proteins carry materials across the membrane.
  • Bind to a substance to be carried, move across the membrane, and deposit the substance on the other side.
• Enzymes:
  • Biological catalysts that speed up reactions without being changed by the reaction.
    Enzyme + Substrate \rightarrow Products
  • Peripheral proteins can act as enzymes, providing a site for chemical reactions on the membrane surface.
  • The endoplasmic reticulum, which has the same structure as the cell membrane, facilitates many chemical reactions across its surface.
• Receptor Sites:
  • Attract specific chemicals, such as hormones, to the membrane.
• Identifying features:
• Anchoring:
  • Anchor internal cellular structures (e.g., microtubules) by attaching to protein portions of the membrane.

Membrane Transport

• The most important function of the cell membrane is its transport function, allowing it to bring things into and out of the cell.
• Semipermeability: Allows the cell membrane to selectively carry certain substances in or out.
• Two types of transport:
  • Passive Transport.
  • Active Transport.

Passive Transport

• Definition: Movement of molecules from an area of higher concentration to an area of lower concentration (i.e., down the concentration gradient).
• Energy Requirement: Does not require energy (ATP) expenditure by the cell.

Active Transport

• Definition: Movement of molecules from an area of lower concentration to an area of higher concentration (i.e., against the concentration gradient).
• Energy Requirement: Requires energy (ATP) expenditure by the cell.
• Protein Requirement: Also requires protein involvement.

Types of Passive Transport

• Diffusion.
• Facilitated Diffusion.
• Filtration.
• Dialysis.
• Note: All the types of transport move substances from higher to lower concentration without using ATP.

Diffusion

• Definition: Movement of molecules from an area of higher concentration to an area of lower concentration; a fundamental type of passive transport.
• Examples: Smelling Coffee Brewing.
• Occurs in all three states of matter: solid, liquid, and gas.
• Diffusion rates vary: gas > liquid > solid.

Factors Affecting the Rate of Diffusion

• Molecular Weight:
  • Inverse relationship: \uparrow Molecular \, Weight \implies \downarrow Rate
  • Higher molecular weight results in a lower rate of diffusion.
• Energy (e.g., Heat):
  • \uparrow Energy \implies \uparrow Rate
  • Increased energy (e.g., heat) increases the rate of diffusion (e.g., sugar dissolving faster in hot tea).
• Concentration Gradient:
  • \uparrow Concentration \, Gradient \implies \uparrow Rate
  • The greater the concentration difference across a membrane, the greater the rate of diffusion.

Examples of Diffusion

• Opening a bottle of ammonia or perfume: The scent spreads from a higher to a lower concentration.
• Glass Tube Experiment:
  • Hydrochloric acid (HCl) and ammonium hydroxide (NH4OH) are placed at opposite ends of a glass tube.
  • They react to form ammonium chloride (NH4Cl) and water.
    HCl + NH4OH \rightarrow NH4Cl + H_2O
  • The reaction occurs closer to the end with the heavier molecular weight (HCl).
  • Molecular weight of chloride ion (Cl^-) is 35.
  • Molecular weight of ammonium (NH_4^+) is approximately 18.
  • Ammonium diffuses faster due to its lower molecular weight.
• Petri Dish Experiment:
  • Agar is placed in a petri dish; two dyes are added.
  • Potassium permanganate (molecular weight = 158) and methylene blue (molecular weight = 374) are used.
  • Potassium permanganate diffuses faster due to its lower molecular weight.
• Examples in the body:
  • Gas exchange in the lungs is a vital example in the body
  • Oxygen moves into the bloodstream and Carbon Dioxide moves from the blood stream to exit the body

Diffusion with/without a Membrane

• Diffusion can occur with or without a cell membrane.
• Examples of diffusion without a membrane:
  • Sugar dissolves in coffee or tea.
  • Perfume scent spreading in a room.
• Respiration Example:
  • Oxygen moves from a higher concentration (inhaled air) to a lower concentration (blood).
  • Carbon dioxide moves from a higher concentration (blood) to a lower concentration (exhaled air).

Osmosis

• Definition: The movement of water across a semipermeable membrane from an area of more water to an area of less water (i.e., from a less concentrated solution to a more concentrated solution).
• Requires a membrane, specifically for water movement.

Solutions

• Solutions are composed of a solute and a solvent.
  • Solute: The substance being dissolved.
  • Solvent: The substance doing the dissolving.
• Solution Concentration: Expressed as a percentage of solute (e.g., 5% salt solution).
• More Water: Less concentrated solution.
• Less Water: More concentrated solution.

Osmosis in Animal Cells (Red Blood Cells)

• Red blood cells (erythrocytes) have a specific biconcave disc shape, with indentations on both sides.
• Shape importance:
  • Provides a large surface area for oxygen transport.
  • Allows passage through small capillaries.
  • Ex: Sickle cell anemia where blood cells are deformed (sickle-shaped), which cuts down on surface area and oxygen transport.

Isotonic Solutions

• Red blood cells contain approximately 0.9% salt.
• To maintain cell shape, the cell must be in a solution of equal concentration (0.9% salt).
• Isotonic Definition: The word translates to maintaining its equilibrium, the same concentration.
• Water molecules flow evenly through the membrane thus the cell keeps the same shape.
• Examples:
  • Nasal sprays and contact lens solutions are often isotonic to prevent cell rupture.
• This maintains the shape of the cell.

Hypertonic Solutions

• Example: Placing a red blood cell in a 5% salt solution.
• Definition: It means that it is a solution with a higher concentration than a red blood cell.
  *To return this concentration within the cell to normal equilibrium water will rush out of the area within the cell so the higher concentration of solute out side the cell will decrease
• Crenation: The water moves out, and the cell shrinks.
• Not desirable, as it changes cell shape and reduces surface area for gas exchange and ability to pass through capillaries.
• Historical Example:
  • Salting foods to preserve them.
  • Salt draws water out, killing bacteria.

Hypotonic Solutions

• Example: Placing a red blood cell in plain water.
• Definition: A solution with a lower concentration than a red blood cell. It is the opposite of a Hypertonic Solution
• To return to normal water will go into the cell
• Hemolysis: The cell gains water, swells, and eventually bursts; lysis refers to breaking apart.
• Not Desirable: Should be avoided when using intravenous solutions.

Importance of Isotonicity

• Carefully regulated, particularly with intravenous fluids.
• Physiology Labs: Frog muscles must be bathed in an isotonic solution to maintain viability during experiments.
• For Lawn Care:
  • Too much Fertilizer and a lack of rain can draw all the water out of the soil.
  • Sprinkling salt on roads in the winter can kill grass; the water draws out of the soil because of the hypertonic environment that is created.
  • This can also be shown with potatoes in salt water. Placing a potato in high salt concentration will draw water, whereas in plain water, it will keep the water, which demonstrates water absorption.

Water Movement and Concentration

• If something has more water, it is less concentrated (hypotonic).
• If something has less water, it is more concentrated (hypertonic).
• The movement of water does not mean that the solute will also follow.
  * This is because most of the time water carries easier than the solute.

Osmotic and Hydrostatic Pressure

• Hydrostatic Pressure: Pressure of water.
• Osmotic Pressure: Pressure of the solvents involved.
• Water moves from a lower to a higher osmotic pressure.