2.2 Cell membrane and transport mechanisms

Cell membranes are crucial gatekeepers, controlling what enters and exits cells. They're made of a phospholipid bilayer 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. Passive transport, like diffusion, doesn't need energy. Active transport 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 integral proteins 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

• Glycoproteins and glycolipids 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 fluid mosaic model 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 concentration gradient 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 electrochemical gradient 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 simple diffusion, facilitated diffusion, and osmosis, which rely on the kinetic energy of molecules and the permeability of the membrane
• Active transport mechanisms, such as primary and secondary active transport, use cellular energy to move substances against their concentration gradient, often coupled to the movement of ions down their electrochemical gradient
• The Na+/K+ ATPase pump is an example of primary active transport 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), ligand binding (ligand-gated channels), or mechanical stress (mechanosensitive channels)
• Aquaporins are channel proteins that selectively allow water molecules to pass through the membrane, important for maintaining cell volume and water homeostasis
• Carrier proteins 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

• Receptor proteins 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)