Chapter 3 Cellular Form and Function

• Physiological processes occur at the surface of a cell.
• The boundaries of the cell are defined by the plasma membrane.
• The side facing the cytoplasm is the intracellular face.
• The side facing outward is the extracellular face.
• The plasma membrane contains lipids and proteins.
• Phospholipids make up about 75% of membrane lipids.
• Phospholipids in the bilayer are arranged with their hydrophilic phosphate heads facing water on either side of the membrane and their hydrophobic fatty acid tails directed toward the center, avoiding water.
• Cholesterol constitutes about 20% of the membrane lipids.
• Glycolipids make up the remaining 5% of the lipids found in the bilayer, which help form the glycocalyx.
• Proteins make up about 50% of the membrane's weight.
• Transmembrane proteins pass through the membrane.
• Most transmembrane proteins are glycoproteins.
• Peripheral proteins are found on the membrane surface, tethered to the cytoskeleton.
• All animal cells have a glycocalyx external to the plasma membrane, consisting of carbohydrates.
• Surface extensions that increase surface area include microvilli and cilia.
• Microvilli are found on absorptive cells, like those of the intestines and kidneys.
• Motile cilia are found in the respiratory tract, uterine tubes, ventricles of the brain, and ducts of the testes.
• In saline, cilia beat within a saline layer on the cell’s surface
• Water moves to hydrate the cell, sodium and chloride are affected by the saline layer
• Flagella occur in humans.
• Pseudopods are temporary cytoplasmic extensions used for motility.

Membrane Transport

• Selective permeability means that a plasma membrane allows some things through, but prevents others from passing through.
• Filtration is driven by hydrostatic pressure; an example in the human body is when water and small solutes filter from the capillaries into the tissue fluid in the kidneys.
• Simple diffusion refers to the net movement of particles from a place of high concentration to a place of lower concentration.
• Substances move down a concentration gradient.
• Diffusion occurs where there is a concentration gradient.
• The rate of diffusion depends on temperature, molecular weight, steepness of the concentration gradient, membrane surface area, and membrane permeability.
• Osmosis is the net flow of water through a selectively permeable membrane from a region of high water concentration to one of lower water concentration.
• Osmosis moves towards the the higher concentration of solutes.
• An aquaporin is a protein channel in the membrane that allows water molecules to pass through.
• If a solution contains nonpermeating molecules on one side of a membrane, water will move toward that side.
• Osmolality is the molar concentration of solute within a solution.
• Tonicity is the ability of a solution to affect fluid volume and pressure in a cell.
• A hypotonic solution has a lower concentration of nonpermeating solutes than the intracellular fluid (ICF), causing cells to absorb water, swell, and potentially burst (lyse).
• A hypertonic solution has a higher concentration of nonpermeating solutes than the ICF, causing cells to lose water and shrivel (crenate).
• An isotonic solution has the same concentration of nonpermeating solutes as the ICF, causing no change in cell volume or shape.
• Transport proteins are responsible for carrying solutes across the membrane that cannot cross on their own.
• A carrier is similar to an enzyme because it binds a solute and releases it on the other side of the membrane.
• A carrier must exhibit specificity for a particular solute.
• Carriers can exhibit saturation, meaning that as the solute concentration increases, the rate of transport increases up to a maximum point.

Transport Mechanisms

• Facilitated diffusion is carrier-mediated transport of a solute through a membrane down its concentration gradient and does not consume ATP.
• Primary active transport is carrier-mediated transport of a solute through a membrane up its concentration gradient using energy from ATP.
• The sodium-potassium (Na+ K+ pump) is an example of active transport
• Each cycle of the pump hydrolyzes one ATP and exchanges three Na+ for two K+.
• The sodium-potassium pump compensates for the leakage of Na+ into the cell and K+ out of the cell.
• The four main functions of the Na+ K+ pump are secondary transport, regulation of cell volume, maintenance of a negative charge inside the cell, and heat production.
• Vesicular transport moves large particles, droplets of fluid, or numerous molecules at once through the membrane in vesicles.
• Endocytosis is vesicular transport into the cell using ATP.
• Exocytosis is vesicular transport out of the cell using ATP.
• Phagocytosis is cell eating such as engulfing bacteria or cellular debris.

The Cell Interrior

• Neutrophils engage in phagocytosis of bacteria and cellular debris.
• A phagosome merges with a lysosome, which contains enzymes to destroy the invader.
• Pinocytosis or "cell drinking" takes in droplets of ECF containing molecules useful in the cell.
• Receptor-mediated endocytosis is where specific molecules bind to receptors on the cell surface, are taken into the cell, and are selective.
• Transcytosis transports molecules across the cell.
• Exocytosis discharges material from the cell.
• The three structures in the cytoplasm embedded in the cytosol are the cytoskeleton, organelles, and inclusions.
• The cytoskeleton is made up of intermediate filaments, microfilaments, and microtubules and it determines cell shape.
• Microfilaments are made of actin and support microvilli.
• Intermediate filaments are thicker than actin filaments and give cells shape and physical support, they are found in epidermal cells and are made of keratin.
• Microtubules radiate from the centrosome and maintain cell shape and rigidity.
• Organelles are internal structures in a cell that carry out specialized metabolic tasks.
• The nucleus is the largest organelle.
• Mature red blood cells lack the nuclei.
• The nucleus is surrounded by a nuclear envelope and it is perforated with nuclear pores.
• The material inside the organelle includes chromatin and nucleoli.
• The ER synthesizes steroids and other lipids, detoxifies alcohol and other drugs, and manufactures all membranes of the cell.
• The endoplasmic reticulum (ER) is a network of interconnected channels and sacs, and it is continuous with the nuclear envelope.
• Synthesis of proteins occurs on the rough ER.
• The smooth endoplasmic reticulum consists of branched cisternae and lacks ribosomes.
• The Golgi complex synthesizes carbohydrates and puts the finishing touches on protein and glycoprotein synthesis.
• The Golgi complex receives completed proteins from the rough ER.
• Lysosomes contain enzymes to degrade proteins, nucleic acids, carbohydrates, phospholipids, and other matter.
• Peroxisomes, produced in the ER, oxidize organic molecules.
• Proteasomes degrade proteins that are no longer needed or that are misfolded or damaged.
• Mitochondria produces ATP.
• The matrix contains ribosomes, enzymes used in ATP synthesis, and mitochondrial DNA.
• A centriole is a short cylindrical assembly of microtubules.
• The centrosome, where two centrioles lie perpendicular to each other, plays an important role in cell division.

Explore the cell membrane, its structure, and function. Learn about the lipid bilayer composed of phospholipids, cholesterol, and glycolipids. Understand the roles of transmembrane and peripheral proteins, as well as the glycocalyx.