Cell Biology: Organelles and Membranes

Plant Cell Organelles

• Centrioles are absent in plant cells.
• Plant cells do contain mitochondria and microtubules.

Membrane Lipids

• Enzymes for the biosynthesis of membrane lipids are located in the endoplasmic reticulum.

Membrane Proteins

• A membrane protein synthesized in the rough ER can be directed to peroxisomes, lysosomes, and mitochondria.

Brefeldin A

• Brefeldin A is a drug disrupts transport from the ER to the Golgi apparatus.
• Other organelles and membranes affected by Brefeldin A includes lysosomes, vacuoles, and the plasma membrane.

Taxol

• Taxol, a drug for breast cancer treatment, prevents depolymerization of microtubules.
• Taxol might interfere with chromosome movements in cell division and cell division (cleavage furrow formation), which strongly affect cancer cells.

Animal versus Plant Cells

• The difference in the structures of animal cell extracellular matrix and plant cell walls reflect fundamental adaptive differences.
• Animal cells must move more than plant cells.

Cytoskeleton Fibers

• Both microfilaments and microtubules have different motor proteins that interact with each type of fiber.
• Actin and tubulin subunits found in these fibers are coded for by genes in the nucleus.
• Both types of fibers can be in a stable state or a dynamic state of turnover.
• Subunits assemble into each fiber interacting with noncovalent interactions.
• Both do not make up part of the elements present in basal bodies.

Kartagener's Syndrome

• Males afflicted with Kartagener's syndrome are sterile because of immotile sperm and suffer from lung infections.
• The genetic basis of the disorder may relate to defects in the structure of cilia or flagella.

Membrane Structure and Function

• Cellular membranes are fluid mosaics of lipids and proteins.
• Membrane structure results in selective permeability.
• Passive transport is difussion without energy investment.
• Active transport uses energy to move solutes against their gradients.
• Bulk transport across the plasma membrane occurs by exocytosis and endocytosis.

Cellular Membranes

• Cellular membranes are fluid mosaics of lipids and proteins.

Lipid Part

• Phospholipids are amphipathic molecules.

Protein Part

• Proteins are also amphipathic molecules.

Fluidity

• Membranes are very dynamic and fluid.
• They are held together by weak, non-covalent interactions, mainly hydrophobic interactions.

Movement of Lipids

• Lateral movement of lipids occurs 107 times per second.
• Flip-flopping of lipids across the membrane is rare, about once per month.

Factors Affecting Fluidity

• Different organisms have different lipid compositions depending on their environment.
• Cholesterol reduces membrane fluidity at high temperatures by preventing movement.
• Cholesterol also hinders solidification by preventing the tight packaging of phospholipids (increases fluidity) at lower temperatures.

Membrane Proteins

• Cells have many different membrane proteins in two classes: peripheral and integral.
• Transmembrane proteins are composed of α-helices.

Functions of Membrane Proteins

• Membrane proteins function as transporters, enzymes, receptors, for cell signaling, for intercellular joining and for attachment.

Genetics of HIV Resistance

• Researches could design a drug for HIV by targeting the CD4 molecule, CCR5 co-receptor, or developing entry inhibitors.
• 2007 - maraviroc (Selzentry) a class of drugs targetting the co-receptor was developed

Membrane Structure

• Cellular membranes are fluid mosaics of lipids and proteins.
• Membrane structure results in selective permeability.
• Passive transport is diffusion of a substance across a membrane with no energy investment.
• Active transport uses energy to move solutes against their gradients.
• Bulk transport across the plasma membrane occurs by exocytosis and endocytosis.

Membrane Permeability

• Cells exchange materials with their surroundings, which is controlled by plasma membrane.
• Plasma membranes are selectively permeable, regulating molecular traffic.
• Hydrophobic (nonpolar) molecules, such as hydrocarbons, CO2, and O2, dissolve in the lipid bilayer and rapidly pass through.
• Polar molecules, like sugars, do not cross the membrane easily, they require transport aide.

Factors controlling Rate

• Molecular size and solubility control the rate at which molecules pass through a membrane.

Transport Proteins

• Transport proteins allow the passage of hydrophilic substances across the membrane.
• Channel proteins have a hydrophilic channel that molecules or ions can use as a tunnel.
• Aquaporins facilitate the passage of water.
• Carrier proteins bind to molecules and change shape to shuttle them across.
• Transport proteins are specific for the substance they move.
• Poring and Aquaporins are always open, Channels proteins are open for short time and Carrier Protein requires conformational changes.

Membrane Transport Questions

• Lipid's affinity for the lipid bilayer allows O₂ and CO₂ to cross without membrane proteins.
• Transport of water through a membrane require transport protein.
• Aquaporins, are specialized to prevent the passage of hydronium ions (H₃O⁺) while allowing glycerol, a three-carbon alcohol, and H₂O.
• The underlying mechanism is due to the smaller size of water and the repulsion of positive charges from the channel.
• Polar and charged molecules require mechanisms to get in and out.

Factors Controlling Movement

• What enters or leaves the cell is determined by transmembrane domains, channel domains, and the electrochemical gradient.
• Mechanisms used are passive and active diffusion
• What gets into the cell is determined by the cell membrane and permeability.

Membrane Related Questions

• If a cell membrane were composed of only a phospholipid bilayer, it would have limited permeability and lack specific transport mechanisms.
• Membrane proteins serve roles in transport, enzymatic activity, cell signaling, intercellular joining, and attachment.
• Some cell types are more permeable to specific substances due to different compositions and kinds of transport proteins.

Types of Diffusion

• Substances move across the membrane via simple diffusion, facilitated diffusion, or active transport.

What to Observe?

• Simple diffusion needs a concentration gradient.
• Facilitated diffusion needs transport molecules.
• Active transport needs to be coupled with ATP hydrolysis.

Passive Transport

• Passive transport involves no energy requirement and moves down the concentration gradient.
• An example of passive transport is the movement of O₂ into cells for cellular respiration.

Osmosis

• Osmosis is the passive diffusion of water across a semi-permeable membrane.
• The movement of water is defined by the solute concentration of the cell's environment, called tonicity.

Water Transfer

• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water.
• In a hypotonic solution, water enters the cell, causing it to lyse.
• In an isotonic solution, there is no net water movement.
• In a hypertonic solution, water leaves the cell, causing it to shrivel.
• In plant cells, turgid is normal in hypotonic solution, flaccid in isotonic and plasmolyzed in hypertonic solution.

Facilitated Diffusion

• Facilitated diffusion is passive transport aided by proteins.
• Examples include aquaporins and ion channels.
• Facilitated diffusion requires no energy and follows the concentration gradient.
• Both are very specific for what they transport.

Active Transport

• Active transport moves substances against their concentration gradients.
• This process requires energy, usually in the form of ATP.
• It involves specific carrier proteins embedded in the membranes to maintain concentration gradients.
• The sodium-potassium pump is a key example.
• There is also secondary active transport which uses the gradient of one molecule to move the next.

Passive versus Active Transfer

• Passive transport is via diffusion, active transport requires energy.

Bulk Transport

• Bulk transport across the plasma membrane occurs by exocytosis and endocytosis.

Exocytosis

• Has two types: constitutive and regulated.

Endocytosis

• Has three types: phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Cell Surface

• The cell surface is covered with carbohydrates (~15 sugar units), in the form of glycolipids and glycoproteins.
• Composition differs and is unique to species and even individuals.
• It's important for cell-to-cell recognition.
• It is critical for immune defense (self vs. foreign), blood types, and organization (sorting) of cells during embryogenesis and Basis for tissue rejection.

Membrane Synthesis

• Transmembrane glycoproteins and glycolipids go to the plasma membrane face.
• Glycolipids and glycoproteins are synthesized in the ER and Golgi apparatus, then transported via vesicles to the plasma membrane.