Cell Membranes and Membrane Lipids

• Cell biology focuses on the structure, function, and behavior of cells.
• It encompasses various sub-disciplines including the study of cell movement and transport across membranes.

Cell Membranes

• Cell membranes are selectively permeable barriers.
• They separate the internal cell environment from the external environment.
• Membranes are composed primarily of a lipid bilayer, made of phospholipids, with embedded proteins.
• This structure is described by the fluid mosaic model.
• The lipid bilayer provides a flexible structure that is impermeable to most water-soluble molecules.
• Membrane proteins carry out specific functions, including transport, signaling, and cell adhesion.

Membrane Lipids

• Phospholipids are amphipathic molecules.
• They have a hydrophilic (polar) head and hydrophobic (nonpolar) tails.
• In an aqueous environment, phospholipids spontaneously arrange into a bilayer.
• Hydrophobic tails face inward, away from water.
• Hydrophilic heads face outward, interacting with water.
• Cholesterol is another lipid component of animal cell membranes.
• It affects membrane fluidity, making it less fluid at high temperatures and more fluid at low temperatures.
• Glycolipids are lipids with attached carbohydrate groups, found on the outer membrane surface.
• They play a role in cell recognition and signaling.

Membrane Proteins

• Membrane proteins can be classified as integral or peripheral.
• Integral membrane proteins are embedded within the lipid bilayer.
• They have hydrophobic regions that interact with the lipid tails.
• Transmembrane proteins span the entire membrane.
• Peripheral membrane proteins are associated with the membrane surface.
• They bind to integral proteins or lipid heads.
• Membrane proteins perform diverse functions.
• They act as transporters, channels, receptors, enzymes, and anchors.

Membrane Fluidity

• Membrane fluidity refers to the viscosity of the lipid bilayer.
• It affects the movement of lipids and proteins within the membrane.
• Temperature, lipid composition, and cholesterol content influence membrane fluidity.
• Unsaturated fatty acids increase fluidity due to kinks in the hydrocarbon tails.
• Saturated fatty acids decrease fluidity due to straight hydrocarbon tails that allow tight packing.
• Shorter fatty acid tails increase fluidity.
• Longer fatty acid tails decrease fluidity.

Transport Across Membranes

• Cell membranes are selectively permeable.
• They allow some substances to cross more easily than others.
• Transport mechanisms include passive transport and active transport.

Passive Transport

• Passive transport does not require energy input from the cell.
• Substances move down their concentration gradient.
• Types of passive transport include simple diffusion, facilitated diffusion, and osmosis.

Simple Diffusion

• Simple diffusion is the movement of a substance across a membrane.
• It goes down its concentration gradient.
• Small, nonpolar molecules (e.g., O2, CO2) can diffuse directly across the lipid bilayer.

Facilitated Diffusion

• Facilitated diffusion requires the assistance of membrane proteins.
• Transport proteins speed the movement of molecules.
• Channel proteins form hydrophilic pores across the membrane.
• Carrier proteins bind to molecules.
• This induces a conformational change and releases the molecules on the other side.
• Examples include glucose transporters and ion channels.

Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane.
• Water moves from an area of high water concentration (low solute concentration) to an area of low water concentration (high solute concentration).
• Osmosis is crucial for maintaining cell turgor and volume.

Tonicity

• Tonicity refers to the ability of a surrounding solution to cause a cell to gain or lose water.
• Isotonic solutions have the same solute concentration as the cell.
• There is no net water movement across the membrane.
• Hypotonic solutions have a lower solute concentration than the cell.
• Water moves into the cell, causing it to swell (or lyse in animal cells).
• Hypertonic solutions have a higher solute concentration than the cell.
• Water moves out of the cell, causing it to shrink (crenation in animal cells).

Active Transport

• Active transport requires energy input, usually in the form of ATP.
• Substances move against their concentration gradient.
• Active transport is carried out by carrier proteins.
• Types of active transport include primary active transport and secondary active transport.

Primary Active Transport

• Primary active transport directly uses ATP to move molecules.
• ATP hydrolysis provides the energy for the conformational change of the transport protein.
• The sodium-potassium pump (Na+/K+ pump) is a prime example.
• The pump transports Na+ out of the cell and K+ into the cell, both against their concentration gradients.

Secondary Active Transport

• Secondary active transport uses the electrochemical gradient created by primary active transport.
• It drives the movement of other molecules.
• Symport is the movement of two or more different molecules or ions in the same direction across a membrane.
• Antiport is the movement of two or more different molecules or ions in opposite directions across a membrane.
• Glucose transport in the small intestine is an example of secondary active transport.
• The Na+ gradient drives the uptake of glucose into the cell.

Vesicular Transport

• Vesicular transport involves the movement of large particles and macromolecules across the plasma membrane.
• It occurs via vesicles, small membrane-bound sacs.
• Types of vesicular transport include endocytosis and exocytosis.

Endocytosis

• Endocytosis is the uptake of substances into the cell.
• The plasma membrane invaginates, forming a vesicle that encloses the material.
• Types of endocytosis include phagocytosis, pinocytosis, and receptor-mediated endocytosis.
• Phagocytosis is the engulfment of large particles or cells.
• Pinocytosis is the uptake of extracellular fluid and small solutes.
• Receptor-mediated endocytosis relies on specific receptors on the cell surface.
• These receptors bind to target molecules, triggering vesicle formation.

Exocytosis

• Exocytosis is the release of substances from the cell.
• Vesicles fuse with the plasma membrane, releasing their contents into the extracellular space.
• It is used for secretion of hormones, neurotransmitters, and waste products.

Cell Movement

• Cell movement is essential for various biological processes.
• These include embryonic development, wound healing, immune responses, and cancer metastasis.
• Cells can move individually or collectively.
• Mechanisms of cell movement involve the cytoskeleton, adhesion molecules, and signaling pathways.

Cytoskeleton

• The cytoskeleton is a network of protein filaments within the cell.
• It provides structural support and facilitates cell movement.
• Major components of the cytoskeleton include actin filaments, microtubules, and intermediate filaments.
• Actin filaments are involved in cell shape changes, lamellipodia formation, and muscle contraction.
• Microtubules are involved in cell polarity and cell division.
• Intermediate filaments provide mechanical strength and stability.

Cell Adhesion Molecules

• Cell adhesion molecules (CAMs) mediate cell-cell and cell-matrix interactions.
• They are crucial for cell migration and tissue organization.
• Major types of CAMs include cadherins, integrins, selectins, and immunoglobulin superfamily proteins.
• Cadherins mediate calcium-dependent cell-cell adhesion.
• Integrins mediate cell adhesion to the extracellular matrix.
• Selectins mediate cell adhesion to leukocytes and endothelial cells during inflammation.

Signaling Pathways

• Signaling pathways regulate cell movement in response to external cues.
• Growth factors, chemokines, and other signaling molecules can stimulate cell migration.
• These signals activate intracellular signaling cascades.
• They regulate the activity of cytoskeletal proteins and CAMs.
• Rho GTPases (Rho, Rac, Cdc42) are key regulators of actin cytoskeleton dynamics and cell motility.

Mechanisms of Cell Movement

• Cell movement typically involves a cycle of protrusion, adhesion, contraction, and detachment.
• Protrusion involves the extension of the cell membrane at the leading edge.
• This forms lamellipodia or filopodia.
• Adhesion involves the attachment of the cell to the substrate via CAMs.
• Contraction involves the generation of force to pull the cell body forward.
• Detachment involves the release of adhesions at the rear of the cell.
• Chemotaxis is the movement of cells towards or away from a chemical gradient.
• It is guided by chemoattractants or chemorepellents.