Cell Membrane Structure and Transport

Cell Membrane Structure

• Phospholipid bilayer with hydrophilic heads and hydrophobic tails
• Embedded proteins: integral (crossing the entire membrane) and peripheral (attached to the membrane surface)
• Selectively permeable, allowing some substances to pass through more easily than others

Mechanisms of Membrane Transport

• Passive transport: diffusion (down concentration gradient) and facilitated diffusion (using channel or carrier proteins)
• Active transport: moving molecules against their concentration gradient, requiring energy (ATP)

Types of Membrane Transport

• Diffusion: movement from high to low concentration
• Facilitated diffusion: passive transport using protein channels or carrier proteins
• Active transport: movement against concentration gradient, requiring energy input
• Osmosis: movement of water across a semi-permeable membrane from high to low concentration
• Ion channels: selective transport of ions down their electrochemical gradient
• Transporters (carriers): binding to specific molecules, transporting them across the membrane

Factors Affecting Diffusion

• Amount of substance: more substance, faster diffusion
• Concentration gradient: steeper gradient, faster diffusion
• Temperature: higher temperature, faster diffusion
• Surface area: larger surface area, faster diffusion
• Diffusion distance: shorter distance, faster diffusion

Tonicity

• Isotonic solution: same solute concentration as the cell, no change in size
• Hypotonic solution: lower solute concentration, water flows into the cell, causing it to swell
• Hypertonic solution: higher solute concentration, water flows out of the cell, causing it to shrink

Primary Active Transport Mechanisms (Pumps)

• E.g. Sodium-Potassium pump (Na-K-ATPase)
• Requires energy for solutes moving against their concentration gradient
• Requires energy through ATP hydrolysis
• Exhibits saturation kinetics
• Crucial for maintaining cell volume and ionic gradients

Vesicular Transport

• Vesicle: a small spherical sac formed by budding off from a membrane
• Endocytosis: materials move into a cell in a vesicle
• Exocytosis: vesicles fuse with the plasma membrane, releasing their contents into the extracellular fluid
• Transcytosis: a combination of endocytosis and exocytosis

Phagocytosis

• Form of endocytosis where a cell engulfs large particles
• Two main phagocytes: macrophages and neutrophils

Bulk-phase Endocytosis (Pinocytosis)

• No receptor proteins involved
• Transport of extracellular fluid
• Plasma membrane folds inward

Receptor-mediated Endocytosis

• Receptor protein recognizes and binds a specific particle
• Clathrin molecules form a basket-like structure on the cytosolic side of the membrane
• Fuses with endosome, receptors recycled, and vesicles bud off endosome

Exocytosis

• Releases materials from cells
• Important for neurotransmitter signaling

Secondary Active Transport Mechanisms

• Use energy stored in Na+ or H+ concentration gradients to drive transport of other solutes against their concentration gradients

Cilia and Flagella

• Cilia: short, hair-like projections from the cell surface, moving fluids along a cell surface
• Flagella: longer than cilia, moving an entire cell (e.g. sperm cell's tail)

Endoplasmic Reticulum

• Network of membranes in the shape of flattened sacs or tubules
• Rough ER: connected to the nuclear envelope, produces secretory, membrane, and organellar proteins
• Smooth ER: synthesizes fatty acids and steroids, detoxifies certain drugs

Ribosomes

• Sites of protein synthesis
• Contain large amounts of ribosomal RNA (rRNA)
• Attached to the outer surface of the nuclear membrane and Endoplasmic Reticulum

Golgi Complex

• Consists of 3-20 flattened, membranous sacs called cisternae
• Modifies, sorts, and packages proteins for transport to different destinations

Lysosomes, Peroxisomes, and Proteasomes

• Lysosomes: vesicles that form from the Golgi complex, containing powerful digestive enzymes
• Peroxisomes: smaller than lysosomes, detoxify toxic substances using oxidase enzymes
• Proteasomes: continuously destroy unneeded, damaged, or faulty proteins

Mitochondria

• Generate ATP by aerobic respiration
• Prevalent in active cells: muscles, liver, and kidneys
• Self-replicate during times of increased cellular demand or before cell division
• Contain own DNA, inherited only from the mother
• Plays an important role in apoptosis

Nucleus

• Spherical or oval shaped
• Nuclear envelope: a double membrane separating the nucleus from the cytoplasm
• Nuclear pores: numerous openings in the nuclear envelope, controlling movement of substances
• Nucleolus: spherical body producing ribosomes