Chapter 3: Membranes

General characteristics of cell membranes

Maintains integrity of a cell, controls entry and exit (selectively permeable), responsible for compartmentalization

Maintains integrity of a cell

The membrane forms a physical boundary that holds in cellular contents and separates the intracellular fluid (ICF), or cytosol, from the extracellular fluid (ECF)

Cytoskeleton helps the cell maintain its structural integrity

Selective permeability

It is selectively permeable, meaning it controls which substances enter and leave the cell, allowing some to pass through while excluding others

Signal transduction

The membrane functions in signal transduction, a process that allows the cell to receive and respond to incoming messages from the outside environment

Chemical composition

The membrane is mainly composed of lipids and proteins, with a small amount of carbohydrates. The basic framework is a phospholipid bilayer, which is self-assembled with water-soluble "heads" on the surfaces and water-insoluble "tails" making up the interior

Self-sealing capability

Due to its fluid and flexible structure, the membrane can quickly seal tiny breaks. However, if the membrane is extensively damaged, the cell will die

Increasing surface area

Features such as outpouchings and infoldings (like microvilli) can significantly increase the membrane's surface area, which is often related to specialized functions like absorption

Compartmentalization

It is responsible for compartmentalization, effectively partitioning the cell and its organelles from the surrounding environment

Cell membrane structure

Phospholipid bilateral, hydrophobic “tails”, hydrophilic “heads”

Phospholipid bilateral

Double layer of phospholipid molecules that self-assemble into a thin, stable, and flexible film. It is composed of two distinct regions, hydrophilic and hydrophobic

Hydrophobic “tails”

These are water-insoluble fatty acid chains that make up the middle portion of the membrane. They face each other, creating an oily interior that is shielded from the water on either side

Hydrophilic “heads”

These are the water-soluble portions of the molecules that contain phosphate groups. They form the inner and outer surfaces of the membrane, facing the watery intracellular fluid (ICF) and extracellular fluid (ECF)

Cell membrane structure: Proteins

Proteins are embedded within the bilayer and are classified by their position and function

Proteins by position: Integral protein

These extend into the lipid bilayer. If they span the entire thickness and protrude from both sides, they are called transmembrane proteins

Proteins by position: Peripheral proteins

These are associated with only one side of the bilayer and are often attached to the internal cytoskeleton to help maintain the cell's shape

Proteins by function: Receptors

A specialized type of membrane protein designed to respond to extracellular signals. They are often transmembrane proteins that take the form of tightly coiled rods, with portions exposed on the cell's outer surface

Proteins by function: Enzymes

Are a functional class of membrane proteins that catalyze chemical reactions. They are characterized by having active sites exposed to substances in either the intracellular or extracellular fluid

Proteins by function: Cell surface proteins (self identification)

Are essential for establishing "self" and allowing the body to distinguish between its own cells and foreign entities

Proteins by function: Cellular adhesion molecules

Are specialized membrane proteins that enable cells to stick to one another by "hooking" together to form intercellular junctions

Cell membrane structure: Cholesterol and membrane fluidity

Are a vital lipid component of the cell membrane, specifically embedded within the hydrophobic interior (the middle) of the phospholipid bilayer

Cholesterol molecules

Have a relatively rigid structure that helps to stabilize the cell membrane, making the otherwise thin and oily film more robust, essential for maintaining proper membrane fluidity, ensuring the membrane remains flexible and functional under varying conditions, helps make the membrane impermeable to water-soluble substances, further contributing to the membrane's role as a selective barrier

Molecular Transport

Various mechanisms by which substances move across the selectively permeable cell membrane to maintain homeostasis. These processes are generally categorized into passive mechanisms, which do not require cellular energy, and active mechanisms, which require energy in the form of adenosine triphosphate (ATP)

Molecular Transport: Random molecular motion

Molecules, atoms, and ions are in constant, random motion due to their kinetic energy, which drives many transport processes

Molecular Transport: Differences in concentration establish a gradient

A gradient exists when a substance is at a higher concentration in one area than another. Substances typically move "down" or "with" this gradient from high to low concentration

Molecular Transport: Membrane permeability —> Non-polar substances (gases, lipids)

Molecules like oxygen, carbon dioxide, and steroid hormones are lipid-soluble and can easily pass through the phospholipid bilayer

Molecular Transport: Membrane permeability —> Polar substances (carbohydrates, proteins, charged ions)

Water-soluble molecules such as sugars, proteins, and charged ions (like Na+ or K+) are generally impermeable to the bilayer and require specialized proteins to cross

(Simple) diffusion

Movement is with (or down) concentration gradient (high to low), requires no cellular energy (ATP) since molecules are moving down their concentration gradient (lowkey just needs a door to open for them), physiological “steady state” reached where concentration remain unequal but stable to due to continuous metabolic processes

Factors that influence rate of diffusion (“flux”)

Distance (membrane thickness), size of contraction gradient, temperature

Facilitated diffusion

Specialized transport protein required, movement with concentration gradient

Facilitated diffusion in transporting proteins

Channel proteins, carrier proteins

Channel proteins (in facilitated diffusion)

Mechanism: Form water-filled "pores" or tunnels through the phospholipid bilayer

Gating: Many channels are "gated," meaning they can open or close in response to specific triggers such as electrical changes (voltage), the binding of a molecule (ligand), or mechanical force

Gated, respond to ligand, voltage, or mechanical force

Carrier proteins

Must physically bind to a specific solute at the membrane surface. This union causes the protein to undergo a conformational shape change, which moves the substance to the other side of the membrane where it is released

Move larger water-soluble molecules that are too big for channels, such as glucose and amino acids

Can become saturated

Used both in facilitated diffusion (passive) and active transport

Osmosis

Passive movement of water molecules through a porous membrane with the concentration gradient

High (water) —> low (water)

Low (solute) —> high (solute)

Osmotic pressure

Ability to left volume of water, greater (solutes) = greater osmotic pressure

Relative solutions

Hypertonic, hypotonic, isotonic

Hypertonic

Relatively higher osmotic pressure (relatively more solutes)

Has a higher concentration of solutes (higher osmotic pressure) than the fluid inside the cell

Hypotonic

Relatively lower osmotic pressure (relatively fewer solutes)

Has a lower concentration of solutes (lower osmotic pressure) than the fluid inside the cell

Isotonic

Similar osmotic pressure (similar concentration of solutes)

Has the same concentration of non-penetrating solutes (and therefore the same osmotic pressure) as the fluid inside the cell

Filtration

Molecules (the filtrate) forced through porous membranes, concentration difference is not important; size of molecule is, hydrostatic forces apply, movement is bulk flow (not random)