Biology of the Cell - Chapter 4 Notes PDF

# Chapter 4 Biology of the Cell ## Cytology - Study of Cells - Microscopes necessary ## Microscopy Using a microscope to view small-scale structures. Staining techniques provide contrast. ### Types of Microscopy 1. **Light microscope (LM)** - Produces a two-dimensional image - Passes li...

# Chapter 4 Biology of the Cell ## Cytology - Study of Cells - Microscopes necessary ## Microscopy Using a microscope to view small-scale structures. Staining techniques provide contrast. ### Types of Microscopy 1. **Light microscope (LM)** - Produces a two-dimensional image - Passes light through a specimen 2. **Electron microscope (EM)** - Beam of electrons illuminates specimen - Greater magnification and resolution than light microscope 3. **Transmission electron microscope (TEM)** - Directs an electron beam through thin-cut sections; get 2-D images 4. **Scanning electron microscope (SEM)** - Directs an electron beam across surface of specimen; get 3-D images ## Cell Size and Shape Cells vary greatly in size and shape. - Most are microscopic. - Shapes vary: spherical, cube-like, column-like, cylindrical, disc-shaped, or irregular. ## Common Features and General Functions 1. **Plasma membrane** - Forms outer, limiting barrier separating internal contents from external environment. - Modified extensions of plasma membrane include cilia, flagella, microvilli. 2. **Nucleus** - Largest structure in cell; enclosed by a nuclear envelope. - Contains genetic material (DNA); also contains a nucleolus. - Nucleoplasm - inner fluid. 3. **Cytoplasm** - Cellular contents between plasma membrane and nucleus. - Includes: cytosol, organelles, and inclusions. ### Cytoplasmic components 1. **Cytosol (intracellular fluid)** - Viscous fluid of cytoplasm; high water content. - Contains dissolved macromolecules and ions. 2. **Organelles ("little organs")** - Organized structures within cells, varied functions. - Two categories: - Membrane-bound organelles - Non-membrane-bound organelles #### a. Membrane-bound organelles - Enclosed by a membrane. - Separates contents from cytosol. - Includes endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria #### b. Non-membrane-bound organelles - Not enclosed within a membrane. - Composed of protein. - Includes ribosomes, cytoskeleton, centrosome, proteasomes #### c. Inclusions: - Cytosol stores temporarily. - Not considered organelles. - Molecules added to and removed from continuously. - For example, pigments, glycogen, triglycerides. ## Common Features and General Functions of the Cell Cells perform general functions: 1. Maintain integrity and shape of a cell. 2. Obtain nutrients and form chemical building blocks. 3. Dispose of wastes. 4. Cell division (some cells). 5. Provide cells for new growth and to replace dead cells ## Lipid Components of the Cell 1. **Plasma membrane** - Fluid mixture composed of equal parts lipid and protein by weight. - Regulates movement of most substances in and out of cell. - Contains several different types of lipids: - Phospholipids - Cholesterol - Glycolipids #### a. Phospholipids - "Balloon with two tails". - Polar and hydrophilic "head"; two nonpolar and hydrophobic “tails”. - Form two parallel sheets of molecules lying tail to tail. - Hydrophobic tails form internal environment of membrane. - Hydrophilic polar heads directed outward. - Phospholipid bilayer is the basic structure of the framework. - Ensures cytosol remains inside the cell. - Ensures interstitial fluid remains outside. #### b. Cholesterol - Four-ring lipid molecule scattered within phospholipid bilayer. - Strengthens membrane. - Stabilizes membrane against temperature extremes. #### c. Glycolipids - Lipids with attached carbohydrate groups. - Located on outer phospholipid region only. - Helps form glycocalyx. ## Membrane Proteins - Half of plasma membrane by weight. - Float and move in fluid bilayer. - Performs most of membrane's functions. - Two structural types: - Integral - Peripheral #### a. Integral proteins - Embedded within, and extend across, phospholipid bilayer. - Hydrophobic regions interact with hydrophobic interior. - Hydrophilic regions are exposed to aqueous environments on either side of membrane. - Many are glycoproteins with attached carbohydrate groups. #### b. Peripheral proteins - Not embedded in lipid bilayer. - Loosely attached to external or interior surfaces of membrane. ## Classification of Proteins by Function Proteins are also categorized functionally: 1. **Transport proteins** - Regulate movement of substances across membrane. - For example, channels, carrier proteins, pumps, symporters, and antiporters. 2. **Cell surface receptors** - Bind molecules called ligands. - For example, neurotransmitters released from a nerve cell that binds to a muscle cell to initiate contraction. 3. **Identity markers** - Communicate to other cells that they belong to the body. - These markers are used to distinguish healthy cells from cells to be destroyed. 4. **Enzymes** - May be attached to either internal or external surface of a cell. - Catalyze chemical reactions i.e. they speed up chemical reactions by lowering activation energy. - Enzymes typically end with the suffix "-ase”. 5. **Anchoring sites** - Secure cytoskeleton to plasma membrane. 6. **Cell-adhesion proteins** - Perform cell-to-cell attachments. ## Membrane Transport **Plasma membrane:** - Serves as physical barrier between cell and fluid that surround it (interstitial fluid). - Regulates movement into and out of a cell. - Is semi-permeable. - Establishes and maintains electrochemical gradient. - Functions in cell communication. Membrane transport is the process of obtaining and eliminating substance across the plasma membrane. - Two categories - Passive processes - Active processes #### a. Passive processes of membrane transport - Do not require energy. - Depend on substances moving down concentration gradient. - Move from an area of high concentration to an area of low concentration. - Two types: - Diffusion - Osmosis #### b. Active processes of membrane transport - Require energy. - Substance must be moved against its concentration gradient (active transport). - Membrane-bound vesicle must be released (vesicular transport). ## Passive Processes: Diffusion and Osmosis 1. **Diffusion** - Net movement of ions or molecules from area of greater concentration to area of lesser concentration i.e. down the concentration gradient. - Due to kinetic energy (energy of motion) of ions/molecules. - Increased temperature = Increased rate of diffusion. - Increased concentration = Increased rate of diffusion. - Increased particle size = Decreased rate of diffusion. - Diffusion continues until substance reaches equilibrium. ### Types of Diffusion: 1. **Simple diffusion** - Molecules move unassisted between phospholipid molecules. - Small and nonpolar solutes. - Include: respiratory gases (O2 and CO2), some fatty acids, ethanol, urea. 2. **Facilitated diffusion** - Transport process for small charged or polar solutes requires assistance from plasma membrane proteins. - Two types: - Channel-mediated diffusion - Carrier-mediated diffusion #### a. Channel-mediated diffusion - Movement of small ions through water-filled protein channels. - Channels specific for one ion type. - Leak channels - Continuously open - Gated channels - Usually closed - Opens in response to stimulus for fraction of second. - Important in normal function of muscle and nerve cells. #### b. Carrier-mediated diffusion - Small polar molecules assisted across membrane by carrier protein. - Binding of substance causing change in carrier protein shape. - Releases substances on other side of membrane. - Moves substances down their gradient. - Uniporter-carrier transporting only one substance. - The number of channels and carriers determines the max rate of substance transport. 2. **Osmosis** - Movement of water, not solutes. - Passive movement of water through semipermeable membrane. - Membrane allows passage of water, prevents passage of most solutes. - Membrane is also selectively permeable – regulates movement of specific solutes. - Osmosis is promoted by differences in water concentration on either side of a membrane. ### Two ways water crosses membrane: 1. Slips between molecules of phospholipid bilayer. 2. Moves through integral protein water channels- aquaporins. ### Two types of solutes: #### a. Permeable solutes - Pass through bilayer. - For example, small and nonpolar solutes such as oxygen, carbon dioxide, urea. #### b. Nonpermeable solutes - Prevented from passing through bilayer. - For example, charged, polar, or large solutes such as ions, glucose, proteins. ## Concentration gradients across the plasma membrane - Between cytosol and interstitial fluid. - Solutes are prevented from moving across the bilayer. - Cause water concentrations to exist. - Greater concentration of solutes contains lower concentration of water. ## Movement of water by osmosis - Dependent upon concentration gradient between cytosol and solution surrounding cell. - Water moves down gradient until equilibrium is reached. - For example, moves from solution of 1% solutes (99% water) into solution containing 3% solutes (97% water). ### Osmotic pressure - "Pulling pressure" - Pressure exerted by movement of water across semipermeable membrane. - Due to difference in solution concentration. - Steeper gradient, more water moved by osmosis and greater osmotic pressure. ### Hydrostatic pressure— “Pushing pressure" - Pressure exerted by a fluid on the inside wall of its container. ## Osmosis and tonicity - Cell gains or loses water with osmosis along with a change in cell volume and osmotic pressure. - Tonicity-ability of a solution to change the volume or pressure of a cell by osmosis. - Terms that describe relative concentration of solutions: - Isotonic - Hypotonic - Hypertonic #### A. Isotonic solution - Both cytosol and solution have same relative concentration of solutes. - For example, normal saline with a concentration of 0.9% NaCl. - Commonly used in IV solutions. - No net movement of water. #### B. Hypotonic solution - Solution has a lower concentration of solutes, higher concentration of water than in cytosol. - For example, erythrocytes in pure water. - Water moves down concentration gradient from outside cell to inside. - Increases volume and pressure of cell. - Lysis-rupturing of red blood cells occurs if difference is large enough. - Hemolysis—rupturing erythrocytes. #### C. Hypertonic solution - Solution with a higher concentration of solutes than cytosol. - For example, erythrocytes in 3% NaCl water solution. - Water moves down concentration gradient. - Moves from inside cell to outside. - Decreases volume and pressure of cell. - Crenation-cell shrinks. ## Active Processes Active processes are organized into active transport and vesicular transport. 1. **Active transport** - Movement of a solute against its concentration gradient (that is, from lower to higher concentration). - Maintains gradient between cell and interstitial fluid. - Types: - Primary Active - Secondary Active - The source of energy determines whether movement is primary or secondary. #### 1. Primary active transport - Uses energy directly from breakdown of ATP. - Phosphorylation of carrier occurs. - Breakdown of ATP results in phosphate group added to transport protein. - Changes protein's shape and results in movement of substance across the membrane. ##### Ion pumps - Cellular protein pumps that move ions across membrane. - Maintain internal concentrations of ions. - For example, Ca2+ pumps in plasma membrane of erythrocytes. - Prevent cell rigidity from accumulated calcium. - Erythrocytes remain flexible enough to move. - **Sodium-potassium (Na+/K+) pump** - Type of exchange pump. - Moves one type of ion into cell against gradient, while moving another type of ion out of cell against gradient. - Continuously exports Na+ out of the cell and moves K+ into the cell. #### 2. Secondary active transport - Moves substance against concentration gradient. - Uses energy from movement of second substance down its gradient. - Kinetic energy providing “power” to pump other substance. - Na+ moves down concentration gradient. - Dependent on Na+/K+ pumps for energy. - Two types: - Symport - Antiport ##### a. Symport - Two substances moved in same direction—symporters. - Process is called symport secondary active transport. ##### b. Antiport - Two substances move in opposite directions—antiporters. - Process is called antiport secondary active transport. 2. **Vesicular transport** - Also called bulk transport. - Involves energy input to transport large substances across the plasma membrane by a vesicle. - Vesicle = membrane-bounded sac filled with materials. - Organized into processes of: - Exocytosis - Endocytosis #### a. Exocytosis - Large substances secreted from cell. - Macromolecules too large to be moved across membrane. - Material packed within intracellular transport vehicles. - Vesicle and plasma membrane fusion. - Requires ATP. - Contents released to outside of cell following fusion. - For example, release of neurotransmitters from nerve cells. #### 2. Endocytosis - Cellular uptake of large substances from external environment. - Steps of endocytosis are similar to exocytosis, but in reverse. - Pocket (invagination) forms, pinches off to form vesicle. - Used for: - Uptake of materials for digestion. - Retrieval of membrane regions from exocytosis. - Regulation of membrane protein composition to alter cellular processes. ### There are three types of endocytosis: 1. **Phagocytosis** 2. **Pinocytosis** 3. **Receptor-mediated endocytosis** #### a. Phagocytosis - Cellular eating. - Occurs when a cell engulfs a large particle external to cell. - Forms large extensions called pseudopodia. - They surround particle, enclosing it in a membrane sac. - Sac is internalized, contents digested after fusing with lysosome. - Only a few cell types perform this, e.g. A white blood cell engulfs and digests a microbe. #### b. Pinocytosis - Cellular drinking. - Internalization of droplets of interstitial fluid containing dissolved solutes. - Multiple, small vesicles formed. - Performed by most cells. #### c. Receptor-mediated endocytosis - Uses receptors on plasma membrane to bind molecules within interstitial fluid and bring the molecules into cell. - Enables the cell to obtain bulk quantities of substances. - For example, transport of cholesterol from blood to a cell. - Cholesterol is bound to low-density lipoproteins (LDLs). - These move from the blood into the interstitial fluid, then bind to LDL receptors in the plasma membrane. - LDLs are then internalized. ## Clinical View: Familial Hypercholesteremia - Inherited genetic disorder. - Defects in LDL receptor or proteins of LDLs. - Interfere with normal receptor-mediated endocytosis of cholesterol. - Results in greatly elevated cholesterol. - Can lead to atherosclerosis, greatly increased risk of heart attack. ## Resting Membrane Potential Plasma membrane establishes and maintains electrochemical gradient-resting membrane potential (RMP). - Essential for muscle and nerve cell function. ### Electrical charge difference at plasma membrane - Membrane potential—potential energy of charge difference. - Resting membrane potential (RMP)—potential when a cell is at rest Two conditions for RMP: 1. **Unequal distribution of ions/molecules across plasma membrane** - More K+ in cytoplasm than in interstitial fluid. - More Na+ in interstitial fluid than in cytoplasm. - Due to Na+/K+ pumps. 2. **Unequal relative amounts of positive and negative charges** - More positive on outside than inside of cell ### Most important ions = Na+ and K+ Movement depends on electrochemical gradient. ### The role of K+ - Most important determinant in specific value of RMP. - K+ moves down steep concentration gradient through leak channels from cytosol to interstitial fluid. - Negatively charged proteins remain inside cell. - Electrochemical gradient: - Positive charge outside repels movement of K+ out. - Negative charge on inside attracts K+ inward. - K+ moves until equilibrium is reached. ### The role of Na+ - Na+ diffuses into cells from interstitial fluid to cytosol simultaneous to the loss of K+. - Enters through Na+ leak channels. - Down concentration gradient. - Pulled by electrical gradient. - Leak channels prevent as much Na+ into the neuron a K+ out. - Inside becomes more positive. ### Maintaining an RMP - Na+/K+ pumps significant. - Maintains K+ and Na+ gradients following their diffusion. - Na+ pumped out. - K+ pumped in. - Opposite directions. - Against concentration gradient. ## Cell Communication Plasma membrane serves an important role in cell communication. Structures such as glycolipids and glycoproteins facilitate both direct interaction between cells as well as recognition and response to external molecular signals. ### Direct Contact Between Cells Direct contact between cells is important for some cells to function Examples: - Cells of immune system. - Distinguishes normal cells from unhealthy cells. - Sperm and oocyte. - Egg with unique glycocalyx. - Allows for recognition by sperm during fertilization. - Cellular regrowth following injury. - Damaged tissue replaced by cell division in epidermis. - Cellular contact prevents overgrowth. ### Ligand-Receptor Signaling Most cell communication occurs through ligands: - Molecules that bind with macromolecules. - Neurotransmitters from nerve cells and hormones from endocrine cells. - Important for controlling growth, reproduction, and other cellular processes. - 3 types of receptors that bind ligands: - Channel-linked receptors - Enzymatic receptors - G protein-coupled receptors #### Channel-Linked Receptors - Permit ion passage into or out of cells. - Occurs in response to neurotransmitter binding. - Help initiate electrical changes to RMP in muscle and nerve cells. #### Enzymatic Receptors - Protein kinase enzymes. - Activated to phosphorylate other enzymes within the cell. - Provides mechanism for altering enzymatic activity. #### G Protein-Coupled Receptors - Indirectly activate protein kinase enzymes. ## Cellular Structures - Membrane-bound organelles - Non-membrane-bound organelles - Vesicles for transport - Structures extending from cell surface - Surrounded by membrane - Allows activities in isolated environment ### Endoplasmic Reticulum (ER) - Extensive interconnected membrane network. - Varies in shape, but one continuous lumen. - Extends from nuclear envelope to plasma membrane. - Composes about half of membrane within cell. - Point of attachment for ribosomes. - With ribosomes—rough ER - Without ribosomes-smooth ER #### a. Rough ER - Protein production by ribosomes, inserted into ER. - Original structure of protein changed. - Transported out in enclosed membrane sacs. - Transport vesicles shuttle proteins from rough ER lumen to Golgi apparatus. #### b. Smooth ER - Diverse metabolic processes vary by cell. - Functions: - Synthesis, transport, and storage of lipids. - Carbohydrate metabolism. - Detoxification of drugs and poisons. ### Golgi apparatus - Composed of cisternae, elongated saclike membranous structures. - Exhibits polarity. - Cis-face — Proximal to ER. - Trans-face — Distal from ER. - Functions: modification, packaging, and sorting of proteins. - Formation of secretory vesicles. - Some vesicles become part of plasma membrane. - Others release contents outside cell. ### Lysosomes - Small, membranous sacs. - Contain digestive enzymes formed by Golgi. - Participate in digestion of unneeded substances. - Digest contents of endocytosed vesicles. ### Peroxisomes - Membrane-enclosed sacs, smaller than lysosomes. - Pinched off vesicles from rough ER. - Proteins are incorporated to serve as their enzymes. - Metabolic functions include: - Role in chemical digestion. - Beta oxidation. - Lipid synthesis. ### Clinical View: Lysosomal Storage Diseases - Group of heritable disorders. - Accumulation of incompletely digested molecules within lysosomes. - Mutation in genes for lysosomal enzymes. - For example, Tay-Sachs disease: - Lipids accumulate within nerve cells. - Paralysis, blindness, deafness, followed by death by age 4. ### Endomembrane system - Includes ER, Golgi apparatus, vesicles, lysosomes, peroxisomes, plasma membrane and nuclear envelope. - Connected directly or through vesicles moving between them. - Provides means of transporting substances within cells. ### Mitochondria - Oblong shaped organelles with double membrane. - "Powerhouses" of cell. - Synthesize ATP through aerobic cellular respiration - Oxygen is required. ### Ribosomes - Contain protein and ribonucleic acid. - Arranged into large subunit with A, P and E sites and a small subunit. - Made in nucleolus; assembled in cytoplasm. - Bound ribosomes - attached to RER membrane. - Synthesize proteins for export, become part of plasma membrane, or serve as enzymes in lysosomes. - Free ribosomes suspended within cytosol. - All other proteins within cell synthesized here. ### Centrosome - Pair of perpendicularly oriented cylindrical centrioles. - Primary function: organizes microtubules within cytoskeleton. - Functions in cellular division (help to move chromosomes around cell). ### Proteasomes - Located in cytosol and cell nucleus. - Degrade cell proteins through ATP-dependent pathway. - For example, damaged proteins, incorrectly folded proteins, proteins no longer needed. - Proteins marked with ubiquitin tag for disposal. - With age, may be unable to normally remove proteins. ### Cytoskeleton - Plays roles in: - Intracellular support - Organization of organelles - Cell division - Movement of materials - Extends throughout cell interior; anchor proteins in membrane Includes: - Microfilaments - Intermediate filaments - Microtubules #### a. Microfilaments - Smallest components of cytoskeleton. - Actin protein monomers in two twisted filaments. - Functions - maintain cell shape, internal support, cell division. #### b. Intermediate filaments - Intermediate-sized; more rigid than microfilaments. - Functions – structural support, cell junctions. #### c. Microtubules - Largest components of cytoskeleton; composed of tubulin. - May be elongated or shortened as needed. - Functions - maintain shape, cell transport, cell division. ## Structures of the Cell's External Surface ### Cilia - Hair-like projections that move substances along cell surface. - Longer and wider than cilia; propels entire cell. ### Flagella - Extensions of plasma membrane that increase surface area of a cell. ## Membrane Junctions ### Tight junctions - Strands or rows of proteins linking cells. - Prevent substances from passing between cells. - Requires materials to move through, rather than between cells. - Maintain polarity of epithelia. ### Desmosomes - Composed of proteins that bind neighboring cells. - Hemidesmosomes anchor basal layer of cells of epidermis to underlying components. ### Gap junctions - Form tiny, fluid-filled tunnels. - Provide direct passageway for substances to travel between cells e.g. ions between cells in cardiac muscle).