2. Membrane Structure
• Life, as we know it, depends on a thin membrane that separates each
cell from the surrounding world
• These membranes, composed of two layers of lipids, are generally
impermeable to ions and macromolecules
• Proteins embedded in the lipid membrane facilitate the movement of
ions, allowing cells to create an internal environment different from
that outside
• Membranes are a planar sandwich of two layers of lipids that behave
like two-dimensional fluids. Each lipid has a polar group coupled to
hydrocarbon tails that are insoluble in water(non polar)
3.
4. Membrane Dynamics
• The hydrophobic interior of the bilayer is poorly permeable to ions
and macromolecules
• This impermeability makes it possible for cellular membranes to form
barriers between the external environment, cytoplasm, and
organelles
• The selectively permeable membrane around each organelle allows
the creation of a unique interior space
• Peripheral membrane proteins found on the surfaces of the bilayer
often participate in enzyme and signaling reactions
6. Membrane Pumps
• Lipid bilayers provide a barrier to diffusion of ions and polar molecules
larger than about 150 Da
• Transmembrane proteins are required for selective passage for ions, and
other larger molecules across membranes
• Pumps are enzymes that use energy from ATP, light, or (rarely) other
sources to move ions
• Pumps are also called primary active transporters, because they transduce
electromagnetic or chemical energy
8. Carrier proteins
• Carrier proteins facilitate the movement of ions and nutrients across
membranes, allowing them to move down concentration gradients
faster
• Some carriers couple movement of an ion such as Na+ down its
concentration gradient to the movement of a solute such as glucose
up a concentration gradient into the cell.
• Carriers change their shape reversibly, opening and closing “gates” to
transport their cargo across the membrane one molecule at a time
10. Channels
• Channels are transmembrane proteins with selective pores that allow
ions, water, glycerol, or ammonia to move very rapidly down
concentration gradients across membranes
• Taking advantage of ion gradients created by pumps and carriers, cells
selectively open ion channels to create electrical potentials across the
plasma membrane and some organelle membranes
• The electrical potential across the membrane regulates voltage-gated
cation channels
• Binding of a chemical ligand opens other channels.
(neurotransmitters)
16. Cellular Organelles
• Eukaryotic cells evolved membrane-bounded compartments
• These subcellular compartments, called organelles, have distinctive
chemical compositions
• Organelles vary in abundance and size in different cell types, because each
tissue and organ has specialized functions
• Each organelle membrane establish an internal chemical environment (pH,
divalent cation concentration, reduction–oxidation [redox] potential) that
is appropriate for particular biochemical functions
18. Mitochondria
• Inner membrane forms folds called cristae that are specialized for
converting energy provided by breakdown of nutrients in the matrix
into ATP
• Mitochondria use energy extracted from the chemical bonds of
nutrients to generate a proton gradient across the inner membrane
• Mitochondria receive energy-yielding chemical intermediates from
two metabolic pathways, glycolysis and fatty acid oxidation
22. Endoplasmic Reticulum
• ER is the largest membrane-delineated intracellular compartment
within eukaryotic cells
• Performs many essential cellular functions, including protein
synthesis and processing, lipid synthesis, ect
• It also has roles in the biogenesis of the Golgi apparatus,
peroxisomes, and helps mitochondria to divide.
23.
24.
25. Endocytosis
• Macromolecules can enter cells only by being captured and enclosed
within membrane-bound carriers that invaginate and pinch off the plasma
membrane in a process known as endocytosis.
• Cells use endocytosis to feed themselves, to defend themselves, and to
maintain homeostasis
• Some toxins, viruses, pathogenic bacteria, and protozoa “hijack” this
process to enter cells
• Endocytosis was discovered more than a century ago in white blood cells
(macrophages and neutrophils), the body’s “professional phagocytes”
• Cells use many different mechanisms for endocytosis
Membranes are a planar sandwich of two layers of lipids that behave like two-dimensional fluids. Each lipid has a polar group coupled to hydrocarbon tails that are insoluble in water
Membranes also subdivide the cytoplasm of eukaryotic cells into compartments called organelles
Others form a membrane skeleton on the cytoplasmic surface that reinforces the fragile lipid bilayer and attaches it to cytoskeletal filaments
Others serve as adhesion proteins that allow cells to interact with each other or extracellular substrates
The energy from binding is used to transmit a signal across the membrane and regulate biochemical reactions in the cytoplasm
for specialized biochemical reactions that contribute to the life of the cell
introduces three families of pumps that use adenosine triphosphate (ATP) hydrolysis as the source of energy to transport ions or solutes up concentration gradients across membranes. For example, pumps in the plasma membranes of animal cells use ATP hydrolysis to expel Na+ and concentrate K+ in the cytoplasm. Another type of pump creates the acidic environment inside lysosomes. A related pump in mitochondria runs in the opposite direction, taking advantage of a proton gradient across the membrane to synthesize ATP. A third family, called ABC transporters, use ATP hydrolysis to move a wide variety of solutes across plasma membranes
directly into transmembrane concentration gradients between membrane-bound compartments.
Carriers provide passive pathways for solutes to move across membranes down their concentration gradients from a region of higher concentration to one of lower concentration. Each conformational change in a carrier protein translocates a limited number of small molecules across the membrane. Carriers use ion gradients created by pumps as a source of energy to perform a remarkable variety of work. Translocation of an ion down its concentration gradient can drive another ion or solute up a concentration gradient, so these are called secondary transporters
All living organisms depend on combinations of pumps, carriers, and channels for many physiological functions
Cells use ion concentration gradients produced by pumps as a source of potential energy to drive the uptake of nutrients through plasma membrane carriers.
For instance, nerve cells secrete small organic ions (called neurotransmitters) to stimulate other nerve cells and muscles by binding to an extracellular domain of cation channels
Channels are ion-specific pores that typically open and close transiently in a regulated manner. Open channels are highly specific but passive transporters, allowing a flood of an ion or other small solute to pass quickly across the membrane, driven by electrical and concentration gradients.
specialized to provide energy; to synthesize lipids, carbohydrates, proteins, and nucleic acids; and to degrade cellular constituents.
An organelle often holds a monopoly on performing a given task; for example, endoplasmic reticulum (ER) synthesizes membrane proteins and certain membrane lipids, lysosomes contain enzymes to degrade many macromolecules, and mitochondria convert energy derived from the covalent bonds of nutrients into adenosine triphosphate (ATP) to provide energy for diverse cellular functions
Mitochondria consist of two membrane-bounded compartments, one inside the other
outer membrane surrounds the intermembrane space. The inner membrane surrounds the matrix. Each membrane and compartment has a distinct protein composition and functions.
Porins in the outer membrane provide nonspecific channels for passage of molecules of less than 5000 Da, including most metabolites required for ATP synthesis
The ER is organized as an extensive array of tubules and flat saccules called cisternae (cisterna means “reservoir”) that form an interconnected and contiguous threedimensional network (a reticulum) stretching from the nuclear envelope to the cell surface
The ER that flattens around the cell nucleus to form a double membrane bilayer barrier is called the nuclear envelope
The peripheral ER that extends from the nuclear envelope is comprised of both a network of tubules and flat, stacked membrane cisternae close to the nucleus.
The stacked cisternae are covered with ribosomes for the synthesis, import, and folding of membrane, luminal, and secreted proteins.
compartmentalization of the nucleus, calcium (Ca2+ ) storage and release, detoxification of compounds, and lipid transfer and signaling to other organellesThe ER retains some of these imported proteins for its own functions, some are degraded, and others are exported into the secretory pathway (see Chapter 21) for targeting to other compartments within the cell.
The peripheral ER that extends from the nuclear envelope is comprised of both a polygonal network of tubules and flat, stacked membrane cisternae close to the nucleus.
The stacked cisternae are covered with ribosomes for the synthesis, import, and folding of membrane, luminal, and secreted proteins.
The Golgi apparatus (Fig. 21.18) performs three primary functions within the secretory membrane system. First, it is a factory to synthesize the carbohydrate chains of glycoproteins, proteoglycans, and polysaccharides secreted by plants (eg, inulin) in preparation for their biological functions at the cell surface. Second, the Golgi apparatus is a protein-sorting station for the delivery to many cellular destinations. This includes transport to the plasma membrane, secretion to the cell exterior, sorting to the endosome/lysosomal system, or retrieval back to the ER. Third, the Golgi apparatus synthesizes sphingomyelin and glycosphingolipids. These lipids associate with cholesterol and influence protein sorting in the Golgi apparatus and plasma membrane
These differ in mode of uptake and in the type and intracellular fate of internalized cargo. The mechanisms include phagocytosis, macropinocytosis, clathrin-mediated endocytosis, caveolae-dependent uptake, and nonclathrin/noncaveolae endocytosis
Phagocytosis is the ingestion of large particles such as bacteria, foreign bodies, and remnants of dead cells
Cells use the actin cytoskeleton to push a protrusion of the plasma membrane that surrounds these particles
Phagocytosis proceeds through four steps: attachment, engulfment, fusion with lysosomes, and degradation (Fig. 22.3). These steps are highly regulated by cell surface receptors, polyphosphatidylinositides, and signaling cascades mediated by Rho-family guanosine triphosphatases (GTPases)
