Chapter 22: Passive and Active Transport - Detailed Notes

Learning Objectives

Explain how the size, polarity, and charge of a molecule affects its ability to cross a phospholipid membrane.
Compare the processes of diffusion, osmosis, and facilitated diffusion, and provide biological examples that illustrate each process.
Define passive, active, and secondary active transport and explain the role of channels, carriers, pumps, and co-transporters in transport.

Introduction

The plasma membrane (cell membrane) defines cell borders and maintains cell functionality.
It is selectively permeable, allowing some materials to pass freely while others require specialized structures or energy.

Cell Membrane Components and Structure

The plasma membrane defines the cell, outlines its borders, and controls interaction with the environment.
It allows cells to exclude some substances, take in others, and excrete still others, all in controlled quantities.
The plasma membrane is flexible, allowing cells like red and white blood cells to change shape when passing through narrow capillaries.
The plasma membrane's surface carries markers for cell recognition, crucial for tissue and organ formation during early development and immune response.

Fluid Mosaic Model

The fluid mosaic model (Fig 22.1) describes the plasma membrane as a mosaic of components, including phospholipids, cholesterol, proteins, and carbohydrates, giving the membrane a fluid character.
Plasma membranes are typically 5 to 10 nm thick.
Human red blood cells are approximately 8 μm wide, about 1,000 times wider than a plasma membrane.

Components

The main components are lipids (phospholipids and cholesterol), proteins, and carbohydrates (attached to lipids or proteins).
A phospholipid consists of glycerol, two fatty acids, and a phosphate-linked head group.
Cholesterol, a lipid with four fused carbon rings, is located alongside phospholipids in the membrane's core.
Carbohydrates are on the plasma membrane’s exterior surface, forming glycoproteins (attached to proteins) or glycolipids (attached to lipids).
Typical human cell composition: proteins (50% by mass), lipids (40%), carbohydrates (10%). Protein and lipid concentration varies in different cell membranes.

Phospholipids

The membrane's primary structure is made of amphiphilic phospholipid molecules.
Hydrophilic (“water-loving”) areas are in contact with aqueous fluid inside and outside the cell.
Hydrophobic (“water-fearing”) molecules are non-polar and do not interact with polar molecules, tending to cluster in water due to the hydrophobic effect.
Phospholipids’ hydrophilic regions form hydrogen bonds with water and other polar molecules on the cell's exterior and interior, making these surfaces hydrophilic.
The cell membrane's interior is hydrophobic and does not interact with water.
Phospholipids form a two-layer cell membrane separating fluid within the cell from the fluid outside.
In water, phospholipids arrange with hydrophobic tails facing each other and hydrophilic heads facing out, forming a lipid bilayer.

Proteins

Proteins are the second major component.
Integral proteins integrate completely into the membrane structure, with hydrophobic regions interacting with the phospholipid bilayer’s hydrophobic region.
Single-pass integral membrane proteins have a hydrophobic transmembrane segment.
Some proteins span only part of the membrane, while others stretch from one side to the other.
Complex proteins may consist of up to 12 protein segments, extensively folded and embedded in the membrane.
These proteins have hydrophilic regions and one or more mildly hydrophobic regions, orienting them alongside phospholipids with hydrophobic regions adjacent to the phospholipids’ tails and hydrophilic regions protruding from the membrane.
Peripheral proteins are on the membrane's exterior and interior surfaces, attached to integral proteins or phospholipids.
They may serve as enzymes, structural attachments for the cytoskeleton’s fibers, or parts of the cell’s recognition sites (“cell-specific” proteins).
The body recognizes its own proteins and attacks foreign proteins associated with invasive pathogens.

Carbohydrates

Carbohydrates are the third major component, always on the cell's exterior surface.
They bind to proteins (glycoproteins) or lipids (glycolipids).
Along with peripheral proteins, carbohydrates form specialized sites on the cell surface for cell recognition.
These sites have unique patterns enabling cell recognition, similar to facial features for individual recognition.
This recognition is important for the immune system to differentiate between body cells (“self”) and foreign cells or tissues (“non-self”).
Similar glycoprotein and glycolipid types are on the surfaces of viruses and may change frequently, preventing immune cells from recognizing and attacking them.

Passive Transport

In passive transport, substances move from an area of higher concentration to an area of lower concentration.
It is a naturally occurring phenomenon that does not require the cell to expend energy.
A concentration gradient exists in a physical space with a single substance concentration range.
Molecules move from areas of higher concentration to lower concentration, “down” the concentration gradient, without energy input.

Diffusion

Diffusion occurs when molecules move from a high concentration to a low concentration area until the concentration is equal.
Example: Ammonia gas diffusing from a bottle opened in a room.
Concentration gradients are a form of potential energy that dissipates as the gradient is eliminated.
Plasma membranes are selectively permeable, allowing some substances to pass through but not others.
Loss of selectivity would destroy the cell.
Plasma membranes are amphiphilic, aiding the movement of some materials but hindering others.
Nonpolar (lipid-soluble) molecules with a low molecular weight can easily pass through the membrane’s hydrophobic lipid core.
Fat-soluble vitamins A, D, E, and K readily pass through plasma membranes.
Fat-soluble drugs and hormones also gain easy entry into cells.
Oxygen and carbon dioxide molecules pass through membranes by simple diffusion.
Small and lipid-soluble (hydrophobic) molecules can pass directly through the membrane down their concentration gradient without protein facilitation or energy input, described as simple diffusion.

Facilitated Transport

Polar and charged substances face challenges crossing the membrane.
Some polar molecules connect easily with the cell’s outside but cannot pass through the lipid core.
Small ions could slip through spaces in the membrane but their charge prevents them from doing so.
Ions like sodium, potassium, calcium, and chloride need special ways to penetrate plasma membranes.
Molecules that cannot pass directly through the membrane require transmembrane proteins.
In facilitated transport, materials passively transport across the plasma membrane with the help of membrane proteins.
A concentration gradient allows materials to pass into the cell without expending cellular energy, but these materials are polar or charged molecules that the cell membrane’s hydrophobic parts repel, or they are too large.
Facilitated transport proteins shield these materials from the membrane’s repulsive force or create a large enough passageway.

Transport Proteins

Integral proteins involved in facilitated transport are transport proteins, of two types: channels or carriers.
Both types are transmembrane proteins and are specific to transporting a particular molecule.

Channel proteins

Channel proteins have hydrophilic domains exposed to intracellular and extracellular fluids.
They have a hydrophilic channel through their core, providing a hydrophilic opening through the hydrophobic lipid bilayer interior.
Passage through the channel allows polar or charged compounds to avoid the nonpolar central layer.
Aquaporins allow water to pass through the membrane at a very high rate.
Channel proteins can be open at all times or “gated,” controlling the channel’s opening.
Opening may be controlled by a particular ion attaching or other mechanisms/substances.
In some tissues, sodium and chloride ions pass freely through open channels, while in others, a gate must open.

Carrier proteins

Carrier proteins facilitate transport by binding to a molecule, triggering a shape change that moves the molecule from one side of the membrane to the other.
A channel protein’s shape is static, whereas a carrier protein changes shape.
Each carrier protein is specific to one substance, and there are a finite number in any membrane.
When all proteins are bound to their ligands, they are saturated, and the transport rate is at its maximum. Increasing the concentration gradient will not increase the transport rate.
Example: Glucose reabsorption in the kidney. Excess glucose is excreted through urine if there are not enough carrier proteins, common in diabetes.

Active Transport

Some cells need larger amounts of specific substances and obtain them from extracellular fluids.
Certain materials move back and forth passively, or the cell may have special mechanisms that facilitate transport.
Some materials are so important that the cell spends energy, hydrolyzing adenosine triphosphate (ATP), to obtain them.
Active transport mechanisms require the cell’s energy, usually in the form of ATP.
If a substance must move into the cell against its concentration gradient, the cell must use energy.
Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane; others transport much larger molecules.

Electrochemical Gradients

Concentration gradients, i.e. differential concentrations across a space or a membrane.
Living systems also have electrical gradients: a difference of charge across the plasma membrane, because ions move and cells contain proteins that do not move across the membrane and are mostly negatively charged.
The interior of living cells is electrically negative with respect to the extracellular fluid.
Cells have higher concentrations of potassium $(K^+)$ and lower concentrations of sodium $(Na^+)$ than the extracellular fluid.
The concentration gradient of $(Na^+)$ tends to drive it into the cell, and its electrical gradient also drives it inward to the negatively charged interior.
The electrical gradient of $K^+$ , a positive ion, also drives it into the cell, but the concentration gradient of $K^+$ drives $K^+$ out of the cell.
The combined concentration gradient and electrical charge that affects an ion is its electrochemical gradient.

Moving Against a Gradient Requires Specialized Proteins

To move substances against a concentration or electrochemical gradient, the cell must use energy from ATP generated through metabolism.
Active transport proteins, called pumps, move ions or molecules against their electrochemical gradients.
Active transport is required to maintain concentrations of ions and other substances that living cells require, different from the external environment.
A cell may spend much of its metabolic energy maintaining these processes.
Example: A red blood cell uses most of its metabolic energy to maintain the imbalance between exterior and interior sodium and potassium levels.

Transporters

Three types of proteins transport materials across the cell membrane that cannot diffuse across it, collectively called transporters:
- A uniporter carries one specific ion or molecule.
- A symporter carries two different ions or molecules, both in the same direction.
- An antiporter carries two different ions or molecules, but in different directions.
When these transporters are moving material against the electrochemical gradient, they require energy and are carrying out active transport.
When they are moving material with the electrochemical gradient, they do not require energy and are carrying out facilitated diffusion.

Transport Mechanisms

Two mechanisms exist for transporting small-molecular weight material and small molecules:
- Primary active transport moves ions across a membrane and creates a difference in charge across that membrane, which is directly dependent on ATP.
- Secondary active transport does not directly require ATP: instead, it is the movement of material due to the electrochemical gradient established by primary active transport.

Primary Active Transport

One of the most important pumps in animals cells is an antiporter that caries out active transport: the sodium potassium pump ( $Na^+-K^+$ ATPase), which pumps sodium ions out of the cell and potassium ions into the cell
The sodium-potassium pump is an example of primary active transport that moves ions, sodium and potassium ions in this instance, across a membrane against their concentration gradients. The energy is provided by the hydrolysis of ATP.
Three sodium ions are moved out of the cell for every 2 potassium ions that are brought into the cell. This creates an electrochemical gradient that is crucial for living cells.