Glossary

2.2 Cell membrane and transport mechanisms

6 min read•august 14, 2024

Cell membranes are crucial gatekeepers, controlling what enters and exits cells. They're made of a with embedded proteins, acting as a selective barrier. This structure allows cells to maintain their internal environment and communicate with the outside world.

Transport across cell membranes happens through various mechanisms. , like diffusion, doesn't need energy. uses ATP to move substances against concentration gradients. Understanding these processes is key to grasping how cells function and interact with their surroundings.

Cell Membrane Structure and Composition

Phospholipid Bilayer

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  • The cell membrane is a phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward, creating a semipermeable barrier
  • This arrangement of phospholipids allows the membrane to be selectively permeable, controlling the movement of substances in and out of the cell
  • The hydrophobic interior of the membrane prevents the passage of polar or charged molecules, while allowing nonpolar molecules to pass through more easily
  • The fluid nature of the phospholipid bilayer enables the membrane to be flexible and adaptable to changes in cell shape

Membrane Proteins and Cholesterol

  • The membrane contains embedded within the phospholipid bilayer and peripheral proteins attached to the surface
  • Integral proteins span the entire thickness of the membrane and may function as channels, carriers, receptors, or enzymes
  • Peripheral proteins are attached to the membrane surface through interactions with integral proteins or lipids and can serve as structural components or signaling molecules
  • Cholesterol molecules are interspersed among the phospholipids, providing stability and fluidity to the membrane
  • Cholesterol helps to regulate membrane fluidity by interacting with the fatty acid tails of phospholipids, preventing them from packing too tightly or becoming too fluid

Glycoproteins and Glycolipids

  • and on the extracellular surface of the membrane play a role in cell recognition and adhesion
  • Glycoproteins are proteins with attached carbohydrate chains that extend from the cell surface and participate in cell-cell interactions (cell adhesion molecules)
  • Glycolipids are lipids with attached carbohydrate chains that also contribute to cell recognition and serve as receptors for extracellular signaling molecules
  • The unique combinations of glycoproteins and glycolipids on the cell surface give each cell type a distinct "fingerprint" that allows it to be recognized by other cells

Fluid Mosaic Model

  • The describes the dynamic nature of the cell membrane, with components able to move laterally within the plane of the membrane
  • Phospholipids and membrane proteins are not fixed in place but can diffuse laterally within the membrane, allowing for flexibility and adaptability
  • This lateral movement enables the formation of microdomains, such as lipid rafts, which are enriched in specific lipids and proteins and serve as platforms for signaling and membrane trafficking
  • The fluid mosaic model emphasizes the heterogeneous and dynamic nature of the cell membrane, with its composition and organization constantly changing in response to cellular needs

Principles of Cell Transport

Concentration and Electrochemical Gradients

  • The is the difference in the concentration of a substance between two regions, such as the intracellular and extracellular spaces
  • Substances tend to move down their concentration gradient, from regions of high concentration to regions of low concentration, in order to reach equilibrium
  • The is the combined effect of the concentration gradient and the electrical potential difference across the membrane
  • Ions, such as Na+ and K+, are influenced by both the concentration gradient and the electrical charge difference across the membrane, which arises from the unequal distribution of ions

Passive and Active Transport

  • Passive transport occurs down the concentration gradient without the input of cellular energy (ATP), while active transport requires energy to move substances against the concentration gradient
  • Passive transport mechanisms include , , and , which rely on the kinetic energy of molecules and the permeability of the membrane
  • Active transport mechanisms, such as primary and , use cellular energy to move substances against their concentration gradient, often coupled to the movement of ions down their electrochemical gradient
  • The is an example of that maintains the electrochemical gradient across the membrane by pumping Na+ out of the cell and K+ into the cell

Membrane Permeability

  • Membrane permeability determines the ease with which a substance can cross the membrane, depending on factors such as size, charge, and polarity
  • Small, nonpolar molecules (O2, CO2) can easily pass through the hydrophobic core of the phospholipid bilayer, while larger or polar molecules require specific transport proteins
  • The selectivity of membrane permeability allows the cell to control the entry and exit of substances, maintaining the proper intracellular environment for cellular functions
  • Changes in membrane permeability, such as those caused by ion channels opening or closing, can rapidly alter the concentration of specific ions or molecules within the cell, leading to changes in cell function

Diffusion vs Active Transport

Simple and Facilitated Diffusion

  • Simple diffusion is the passive movement of small, nonpolar molecules (O2, CO2) directly through the phospholipid bilayer, driven by the concentration gradient
  • This process does not require any specific membrane proteins or energy input, as the molecules can easily pass through the hydrophobic core of the membrane
  • Facilitated diffusion is the passive movement of larger or polar molecules (glucose) through specific membrane protein channels or carriers, still driven by the concentration gradient
  • Channels are protein pores that allow specific molecules or ions to pass through the membrane down their concentration gradient, while carriers bind to the molecule and undergo a conformational change to transport it across the membrane
  • Examples of facilitated diffusion include the movement of glucose through GLUT transporters and the movement of ions through ion channels (K+ leak channels)

Primary and Secondary Active Transport

  • Active transport is the movement of molecules against the concentration gradient, requiring energy input from ATP hydrolysis
  • Primary active transport directly uses ATP to power the movement of ions or molecules across the membrane (Na+/K+ ATPase pump)
  • The Na+/K+ ATPase pump maintains the electrochemical gradient by pumping 3 Na+ ions out of the cell and 2 K+ ions into the cell for each ATP molecule hydrolyzed
  • Secondary active transport uses the electrochemical gradient created by primary active transport to move other substances against their concentration gradients (Na+/glucose cotransporter)
  • The Na+/glucose cotransporter (SGLT) uses the Na+ gradient created by the Na+/K+ ATPase pump to transport glucose into the cell against its concentration gradient
  • Other examples of secondary active transport include the Na+/Ca2+ exchanger and the H+/amino acid symporter

Membrane Proteins in Transport and Signaling

Channel and Carrier Proteins

  • Channel proteins form hydrophilic pores that allow specific ions or water molecules to pass through the membrane down their concentration gradients
  • Ion channels can be gated by various stimuli, such as voltage changes (voltage-gated channels), binding (ligand-gated channels), or mechanical stress (mechanosensitive channels)
  • are channel proteins that selectively allow water molecules to pass through the membrane, important for maintaining cell volume and water homeostasis
  • undergo conformational changes to bind and transport specific molecules across the membrane, either through facilitated diffusion or active transport
  • Examples of carrier proteins include the glucose transporter GLUT1, which facilitates the diffusion of glucose into cells, and the Na+/K+ ATPase pump, which actively transports Na+ and K+ ions across the membrane

Receptor Proteins and Cell Signaling

  • bind to specific ligands (hormones, neurotransmitters) on the extracellular surface, triggering intracellular signaling cascades that alter cell function
  • Ligand binding induces a conformational change in the receptor, which can lead to the activation of intracellular signaling pathways or the opening of ion channels
  • Enzyme-linked receptors have intracellular domains with enzymatic activity (tyrosine kinase) that is activated upon ligand binding, initiating signaling pathways
  • For example, the insulin receptor is a tyrosine kinase that, when activated by insulin binding, phosphorylates intracellular proteins to regulate glucose uptake and metabolism
  • G protein-coupled receptors interact with G proteins on the intracellular surface, which then activate various effector molecules (adenylyl cyclase, phospholipase C) to amplify the signal
  • The activation of G protein-coupled receptors can lead to the production of second messengers, such as cAMP or IP3, which further propagate the signal and induce cellular responses (neurotransmitter receptors, olfactory receptors)

Key Terms to Review (28)

Active transport: Active transport is a cellular process that moves molecules across a membrane against their concentration gradient, utilizing energy, usually in the form of ATP. This mechanism is crucial for maintaining cellular homeostasis and allows for the selective uptake of essential substances, such as ions and nutrients, while removing waste products. Active transport plays an important role in various physiological functions, including those in the kidneys and cellular membranes.
Aquaporins: Aquaporins are specialized membrane proteins that facilitate the transport of water across cell membranes. They form channels that selectively allow water molecules to pass while blocking ions and other solutes, ensuring efficient water movement in response to osmotic gradients. This ability is crucial for maintaining cellular homeostasis and regulating fluid balance in various tissues throughout the body.
Carrier proteins: Carrier proteins are specialized membrane proteins that facilitate the transport of specific substances across the cell membrane. They play a crucial role in moving molecules that cannot easily pass through the lipid bilayer, such as ions, glucose, and amino acids, either by passive or active transport mechanisms. This transport is vital for maintaining cellular homeostasis and enabling various physiological processes.
Concentration gradient: A concentration gradient refers to the difference in the concentration of a substance between two areas. This difference drives the movement of substances, allowing them to flow from regions of higher concentration to regions of lower concentration, a process that is crucial for various physiological functions. Understanding concentration gradients helps in grasping how materials are exchanged across cell membranes and how substances are filtered and reabsorbed in renal structures.
Electrochemical gradient: An electrochemical gradient is a difference in the concentration of ions and their charge across a cell membrane. This gradient is crucial for various cellular processes, as it influences the movement of ions and other substances in and out of cells, impacting functions like nerve impulse transmission and muscle contraction.
Endocytosis: Endocytosis is the process by which cells internalize substances from their external environment by engulfing them in a section of the cell membrane. This mechanism is crucial for transporting large molecules, nutrients, and even pathogens into the cell, allowing it to maintain homeostasis and respond to changing conditions. Endocytosis is closely related to the cell membrane's structure and function, as it relies on membrane dynamics and various transport proteins to facilitate this uptake.
Facilitated diffusion: Facilitated diffusion is a process by which molecules move across a cell membrane through specific transport proteins, down their concentration gradient, without the expenditure of energy. This mechanism is crucial for transporting substances that cannot freely pass through the lipid bilayer of the cell membrane, such as polar or large molecules. The process ensures that essential nutrients and ions can enter cells efficiently while maintaining homeostasis.
Fluid mosaic model: The fluid mosaic model describes the structure of cell membranes as a dynamic and flexible arrangement of various molecules, including phospholipids, proteins, cholesterol, and carbohydrates. This model emphasizes that the membrane is not a rigid structure but rather a fluid layer where components can move laterally, allowing for interactions essential for cell function, signaling, and transport mechanisms.
Glycolipids: Glycolipids are lipids with a carbohydrate attached, found in the cell membrane and playing a crucial role in cell recognition and communication. These molecules contribute to the structural integrity of the membrane while also serving as markers that help cells identify and interact with each other, which is essential for various physiological processes.
Glycoproteins: Glycoproteins are molecules that consist of a protein component and carbohydrate chains attached to them. These structures play a critical role in cell-cell recognition, signaling, and adhesion, serving as important markers on cell membranes. The presence of carbohydrates on glycoproteins enhances their functionality, influencing various biological processes such as immune responses and the transport of molecules across cell membranes.
Integral proteins: Integral proteins are a type of membrane protein that is permanently attached to the cell membrane and spans across its lipid bilayer. These proteins play crucial roles in various functions, including transport, acting as channels or carriers for molecules, and serving as receptors for signaling molecules. Their positioning within the membrane allows them to interact with both the extracellular environment and the cytoplasm, which is essential for cell communication and nutrient transport.
Ligand: A ligand is a molecule that binds to a specific site on a target protein, often causing a change in the protein's activity. This interaction is crucial for many biological processes, including signal transduction and cell communication. Ligands can be small molecules, peptides, or larger biomolecules that play key roles in modulating physiological functions by activating or inhibiting receptor proteins on the cell membrane.
Na+/K+ ATPase Pump: The Na+/K+ ATPase pump is a vital membrane protein that actively transports sodium (Na+) out of and potassium (K+) into cells, utilizing energy derived from ATP hydrolysis. This pump plays a crucial role in maintaining the electrochemical gradient across the cell membrane, which is essential for various cellular functions including nerve impulse transmission and muscle contraction.
Osmosis: Osmosis is the movement of water molecules across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This process is vital for maintaining the proper balance of fluids and electrolytes in cells and organs, which plays a key role in processes such as urine formation, fluid regulation, and cell membrane function.
Passive transport: Passive transport is the movement of molecules across a cell membrane without the need for energy input from the cell. This process relies on the natural kinetic energy of molecules and occurs along the concentration gradient, meaning substances move from areas of higher concentration to areas of lower concentration. It is a fundamental mechanism that allows cells to maintain homeostasis by regulating the internal environment through selective permeability of the cell membrane.
Permeability coefficient: The permeability coefficient is a numerical value that quantifies the ability of a substance to pass through a membrane, specifically relating to the rate at which solutes or solvents diffuse across cell membranes. This coefficient is influenced by various factors, including the nature of the membrane, the size and charge of the molecules, and environmental conditions such as temperature. Understanding the permeability coefficient is essential for grasping how substances move in and out of cells, which is critical for maintaining cellular homeostasis.
Phospholipid bilayer: The phospholipid bilayer is a fundamental structure of cell membranes, consisting of two layers of phospholipids arranged tail-to-tail. This unique arrangement creates a hydrophobic core that serves as a barrier to most water-soluble substances, while allowing lipid-soluble molecules to pass through. The bilayer's composition and fluidity are crucial for various cellular processes, including transport mechanisms, signaling, and maintaining the integrity of the cell.
Primary active transport: Primary active transport is a cellular process that uses energy, typically derived from ATP, to move ions or molecules against their concentration gradient across a cell membrane. This mechanism is crucial for maintaining cellular homeostasis and allows cells to accumulate essential substances or expel waste products, ensuring proper physiological functions.
Receptor proteins: Receptor proteins are specialized proteins located on cell membranes that play a critical role in cellular communication by binding to specific signaling molecules, such as hormones or neurotransmitters. When these molecules bind to receptor proteins, they trigger a series of intracellular responses, enabling cells to respond to their environment and maintain homeostasis. This interaction is crucial for processes like cell signaling, immune response, and neurotransmission.
Receptor-ligand interactions: Receptor-ligand interactions refer to the specific binding of signaling molecules (ligands) to their corresponding receptors on the surface of cells, triggering a cascade of biological responses. These interactions are crucial for communication between cells, influencing processes like cellular signaling, immune response, and metabolic regulation. The affinity and specificity of these interactions determine how effectively cells can respond to various stimuli.
Resting Membrane Potential: Resting membrane potential refers to the electrical charge difference across a cell membrane when the cell is not actively sending signals. This potential is crucial for maintaining the stability of the cell's environment, allowing for the proper functioning of neurons and muscle cells, and plays a significant role in the processes of action potentials and signal transmission.
Secondary active transport: Secondary active transport is a cellular process that moves molecules across the cell membrane against their concentration gradient, utilizing the energy stored in the form of an electrochemical gradient created by primary active transport. This method relies on the movement of one molecule down its gradient to drive the transport of another molecule against its gradient, allowing for the uptake of essential nutrients and ions. It plays a crucial role in maintaining cellular homeostasis and the overall function of various physiological processes.
Selective Permeability: Selective permeability refers to the property of cell membranes that allows certain molecules or ions to pass through while restricting others. This characteristic is essential for maintaining homeostasis within the cell, enabling it to control its internal environment by regulating what enters and exits. The structure of the cell membrane, particularly the phospholipid bilayer and embedded proteins, plays a crucial role in this selective process.
Signal Transduction: Signal transduction is the process by which cells respond to external signals, converting these signals into a functional response. This intricate communication system involves various molecular pathways, allowing cells to interpret stimuli from their environment, which is crucial for maintaining homeostasis and coordinating physiological functions.
Simple diffusion: Simple diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration without the need for energy input or assistance from transport proteins. This natural tendency of molecules to spread out evenly in a given space is crucial for maintaining cellular homeostasis and facilitating the movement of essential substances across cell membranes.
Solute: A solute is a substance that is dissolved in a solvent, resulting in a solution. In biological systems, solutes can include ions, small molecules, and larger biomolecules that play critical roles in cellular function. Understanding solutes and their interactions with solvents is essential for grasping how substances move across cell membranes and how transport mechanisms operate.
Vesicles: Vesicles are small, membrane-bound sacs that transport and store substances within cells. They play a crucial role in cellular processes such as secretion, metabolism, and the transport of proteins and lipids. By encapsulating various materials, vesicles facilitate their movement to different parts of the cell or even outside the cell.
Water channels: Water channels are specialized proteins embedded in cell membranes that facilitate the rapid transport of water molecules across the membrane. These channels, also known as aquaporins, are essential for maintaining cellular water balance and play a critical role in various physiological processes, including kidney function and plant water regulation.
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