U2 Reading Guide PDF - Cell Structure and Function

UNIT READING GUIDE READ AND ANNOTATE THE FOLLOWING INFORMATION LT2.1 CELL STRUCTURE AND FUNCTION Cells are the fundamental units of life, forming the basic structure and function of all living organisms. They arise from pre-existing...

UNIT READING GUIDE READ AND ANNOTATE THE FOLLOWING INFORMATION LT2.1 CELL STRUCTURE AND FUNCTION Cells are the fundamental units of life, forming the basic structure and function of all living organisms. They arise from pre-existing cells and carrying out essential processes that sustain life. This principle is central to the Cell Theory, which states: 1. All living organisms are composed of one or more cells. 2. The cell is the basic unit of life. 3. All cells arise from pre-existing cells. Types of Cells Prokaryotic cells are simpler and smaller, typically found in bacteria and archaea. They lack a nucleus and membrane-bound organelles (structures that perform specialized functions). Their DNA is located in a nucleoid region, and they often have additional structures like a cell wall, plasma membrane, ribosomes, and sometimes flagella or pili for movement and attachment. Eukaryotic cells are more complex and larger, found in plants, animals, fungi, and protists. They have a true nucleus enclosed by a nuclear membrane, and various membrane-bound organelles that perform specific functions. These cells exhibit a higher level of compartmentalization, which allows for specialized functions and efficiency. Unit 2: The Basic Unit of Life 1  Key Organelles Description The control center of the cell, housing the cell's DNA and coordinating activities like Nucleus growth, metabolism, and reproduction. Sites of protein synthesis, found either floating freely in the cytoplasm or attached to Ribosomes the rough endoplasmic reticulum. Endoplasmic Rough ER: Studded with ribosomes, involved in protein synthesis and modification. Reticulum Smooth ER: Lacks ribosomes, involved in lipid synthesis and detoxification. Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell. Golgi Contains flattened sacs called cisternae. Has a cis side, which receives vesicles from Apparatus the ER, and a trans side, which ships vesicles to their final destination. Known as the powerhouse of the cell, generating ATP through cellular respiration. Mitochondria have a double membrane structure: an outer membrane and a highly Mitochondria folded inner membrane called cristae, which increase the surface area for ATP production. Sites of photosynthesis, converting light energy into chemical energy stored in glucose. Chloroplasts They have a double membrane, and inside, they contain thylakoids (stacked into (in plant cells) grana) where the light-dependent reactions occur, and stroma, where the Calvin cycle takes place. Lysosomes Contain digestive enzymes that break down waste materials and cellular debris. Contain enzymes that detoxify harmful substances and break down fatty acids, Peroxisomes producing hydrogen peroxide as a byproduct, which is then converted to water. Storage organelles. In plant cells, a large central vacuole maintains turgor pressure, Vacuoles while in animal cells, smaller vacuoles store various substances. Provides structural support, aids in cell division, and enables cell movement. Microfilaments: Thin, actin filaments that support cell shape and enable movement. Microtubules: Hollow tubes made of tubulin that help with cell shape, transport within Cytoskeleton the cell, and cell division. Intermediate Filaments: Fibrous proteins that provide mechanical support and help maintain cell shape. Endomembrane System vs. Energy Organelles The endomembrane system includes organelles involved in the synthesis, modification, and transport of proteins and lipids. It consists of the nuclear envelope, rough and smooth ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane. This system ensures that cellular products are properly processed and directed to their destinations. Energy organelles, such as mitochondria and chloroplasts, are primarily involved in energy conversion. Mitochondria perform cellular respiration, producing ATP, while chloroplasts carry out photosynthesis in plant cells. Unit 2: The Basic Unit of Life 2  Structure of the Mitochondria: Structure of the Chloroplast: Compartmentalization in Cells Compartmentalization refers to the presence of membrane-bound organelles in eukaryotic cells, which creates distinct environments within the cell. This separation allows for specific biochemical reactions to occur in isolated areas, enhancing efficiency and regulation. For example, the lysosomes' acidic environment is optimal for enzyme activity, preventing damage to other cellular components. A second example can be seen with the mitochondria. Mitochondria exemplify compartmentalization in cells by housing distinct metabolic processes within their double membrane structure: the inner membrane's folds (cristae) provide a large surface area for the electron transport chain, while the mitochondrial matrix contains enzymes for the Krebs cycle, enabling efficient ATP production (see “Structure of the Mitochondria” image above). Differences Between Animal and Plant Cells While plant cells and animal cells share many of the same organelles, such as mitochondria, nucleus, ER, golgi, and more, they possess key different structures. Plant Cells Cell Wall: Provides additional support and protection. Chloroplasts: Structures that perform photosynthesis. Large Central Vacuole: Maintains turgor pressure (rigidity) for structural support. Plasmodesmata: Channels that allow communication and transport between plant cells. Animal Cells Lysosomes: More prominent in animal cells, involved in waste digestion. Centrioles: Play a role in cell division. Cilia and Flagella: Hair-like structures that protrude from the cell that aid in movement. Unit 2: The Basic Unit of Life 3  LT2.2 CELL SIZE Cells, the fundamental units of life, vary greatly in size. Despite this variation, all cells must be small enough to efficiently carry out necessary functions. The size of a cell impacts its metabolism, surface area to volume ratio, and overall efficiency. Understanding these concepts is crucial for grasping how cells operate and why they are structured the way they are. Cell Metabolism and Size Cell metabolism refers to the chemical reactions that occur within a cell to maintain life. These reactions include processes like breaking down nutrients for energy, synthesizing proteins, and removing waste products. The efficiency of these metabolic processes is influenced by the cell’s size. Smaller cells have a higher surface area relative to their volume, which allows for more efficient exchange of materials (such as oxygen, nutrients, and waste products) between the cell and its environment. Surface Area to Volume Ratio The surface area to volume ratio (SA:V) is a crucial concept in understanding cell size. It is a measure of how much surface area a cell has relative to its volume. A high SA:V ratio means that a cell has a large surface area compared to its volume, which is beneficial for exchanging materials quickly and efficiently. Importance of SA:V Ratio Exchange of Materials: Cells rely on diffusion to exchange materials with their surroundings. A high SA:V ratio ensures that substances can diffuse in and out of the cell quickly. Metabolic Efficiency: Smaller cells with a high SA:V ratio can efficiently distribute nutrients and remove waste, supporting higher metabolic rates. Temperature Regulation: Cells can lose heat more rapidly with a high SA:V ratio, helping to maintain optimal temperatures for enzymatic activities. Formulas for Surface Area and Volume Cells can be cuboidal or spherical. See the formulas for calculating surface area (SA) and volume (V) for each of these cells to the right. In these equations: s = side length (for the cuboidal cell) r = radius (for the spherical cell) By calculating these values, we can see that as the size (side length for cuboidal or radius for spherical) increases, the volume grows faster than the surface area. This decrease in the SA:V ratio with increasing size explains why larger cells face challenges in material exchange and metabolism. Unit 2: The Basic Unit of Life 4 Why Cell Size Matters Cells must maintain an optimal size to function efficiently. If a cell grows too large, it's SA ratio decreases, leading to insufficient surface area for exchanging materials relative to its volume. This can result in: Slower Diffusion Rates: Larger cells struggle to obtain enough nutrients and oxygen or to expel waste quickly. Metabolic Inefficiencies: Reduced SA ratio can hinder the cell's ability to support high metabolic activity, affecting growth and reproduction. Strategies to Increase SA:V Ratio Cells have evolved several strategies to maintain a high SA:V ratio: Microvilli: Many cells have extensions like microvilli to increase their surface area without significantly increasing volume. Flattened Shapes: Some cells adopt flattened shapes to maximize surface area relative to volume. Compartmentalization: Eukaryotic cells have internal membranes and organelles that increase the internal surface area available for metabolic activities. LT2.3 CELL MEMBRANE The plasma membrane, also known as the cell membrane, is a vital structure that surrounds the cell, providing protection and support. It plays a crucial role in controlling what enters and leaves the cell, maintaining homeostasis. Understanding its structure and function is essential for grasping how cells interact with their environment. Unit 2: The Basic Unit of Life 5 Structure and Function of the Plasma Membrane The plasma membrane is primarily composed of a phospholipid bilayer, proteins, and carbohydrates. Each component has specific roles that contribute to the membrane's overall function. Phospholipid Structure and Function Phospholipids are the main building blocks of the plasma membrane. Each phospholipid molecule has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. This amphipathic nature allows phospholipids to form a bilayer, with the hydrophobic tails facing inward, shielded from water, and the hydrophilic heads facing outward, interacting with the aqueous environment inside and outside the cell. Selective Permeability The plasma membrane is selectively permeable, meaning it allows some substances to pass through while blocking others. This selective permeability is crucial for maintaining the internal conditions necessary for the cell's survival and function. The Fluid Mosaic Model The fluid mosaic model describes the structure of the plasma membrane as a mosaic of various proteins that float in or on the fluid lipid bilayer. This model highlights two key aspects: Fluidity: The lipid bilayer is flexible, allowing lipids and proteins to move laterally within the layer. Mosaic Nature: The membrane is composed of different molecules, including lipids, proteins, and carbohydrates, arranged in a mosaic-like pattern. Factors Affecting Membrane Fluidity Membrane fluidity is essential for proper membrane function and is influenced by several factors: Temperature: Higher temperatures increase fluidity, while lower temperatures decrease it. Fatty Acid Composition: Unsaturated fatty acids increase fluidity due to the kinks in their tails, while saturated fatty acids decrease fluidity. Cholesterol: Cholesterol plays a dual role in membrane fluidity. At high temperatures, it stabilizes the membrane, reducing fluidity. At low temperatures, it prevents tight packing of phospholipids, increasing fluidity. Unit 2: The Basic Unit of Life 6 Membrane Proteins Membrane proteins are integral to the membrane's function. They can be classified into two types: Integral Proteins: These proteins penetrate the hydrophobic core of the lipid bilayer. Some span the entire membrane, acting as channels or transporters for molecules. Peripheral Proteins: These are attached to the exterior or interior surfaces of the membrane and often play a role in signaling or maintaining the cell's shape. Membrane Carbohydrates Carbohydrates attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular side of the membrane play crucial roles in cell recognition and communication. They help cells identify each other and interact appropriately within a tissue or organism. Plant Cell Walls In addition to the plasma membrane, plant cells have a cell wall. The cell wall provides additional support and protection. It is primarily composed of cellulose, a complex carbohydrate. The cell wall maintains the shape of plant cells, prevents excessive water uptake, and provides rigidity. Structure and Function of the Cell Wall Structure: The cell wall is a rigid layer made up of cellulose fibers embedded in a matrix of other polysaccharides and proteins. Function: It offers structural support, maintains cell shape, and prevents bursting in hypotonic environments by providing a counter-pressure. Unit 2: The Basic Unit of Life 7  LT2.4 CELL TRANSPORT The plasma membrane is selectively permeable, meaning it controls what can enter and exit the cell. Can diffuse easily: Small, nonpolar molecules like oxygen and carbon dioxide. More difficult to diffuse: Larger, polar molecules and ions. Passive Transport Passive transport is the movement of substances across the membrane without the use of energy. There are three types of passive transport: simple diffusion, osmosis, and facilitated diffusion. Simple diffusion is the process where molecules move from an area of higher concentration to an area of lower concentration until they are evenly distributed. This movement happens naturally and does not require energy. Osmosis is a type of diffusion specifically for water. Water molecules move across a semipermeable membrane from an area of lower solute concentration (more water) to an area of higher solute concentration (less water). Facilitated diffusion is a type of passive transport that uses proteins to help larger or polar molecules cross the membrane. These proteins are hydrophobic so they are able to embed in the plasma membrane. Channel Proteins: These proteins form pores in the membrane, allowing specific molecules or ions to pass through. ○ For example, aquaporins are channel proteins that facilitate the rapid passage of water molecules. Carrier Proteins: These proteins bind to the molecule they transport, change shape, and release the molecule on the other side of the membrane. Active Transport Active transport requires energy (ATP) to move substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. Pumps Sodium-Potassium Pump: This pump moves three sodium ions out of the cell and two potassium ions into the cell, using one molecule of ATP for each cycle. This process helps maintain the cell's membrane potential, an electrical gradient across the membrane. Proton Pump: This pump moves protons (H⁺ ions) out of the cell, creating a gradient that can be used for other cellular processes. Proton pumps are crucial for processes like ATP synthesis in cellular respiration. Unit 2: The Basic Unit of Life 8 Cotransport Cotransport involves the simultaneous movement of two substances across the membrane: Symport: Both substances move in the same direction. Antiport: Substances move in opposite directions. In plants, the sucrose-proton cotransport system uses the proton gradient created by the proton pump to bring sucrose into the cell against its concentration gradient. Exocytosis and Endocytosis These processes move large molecules or particles into and out of the cell using vesicles. Exocytosis: Cells expel materials by merging vesicles with the plasma membrane, releasing their contents outside the cell. This process is used to secrete hormones, neurotransmitters, and digestive enzymes. Endocytosis: Cells take in materials by engulfing them in a vesicle formed from the plasma membrane. There are three types of endocytosis: ○ Phagocytosis: "Cell eating," where the cell engulfs large particles like food or pathogens. ○ Pinocytosis: "Cell drinking," where the cell takes in extracellular fluid and dissolved substances. ○ Receptor-Mediated Endocytosis: Specific molecules are taken into the cell after binding to receptors on the cell surface, forming a vesicle around the bound molecules. How ATP Functions in Active Transport ATP (adenosine triphosphate) provides the energy needed for active transport. When ATP is broken down into ADP (adenosine diphosphate) and a phosphate group, energy is released, which is used to power various transport proteins and pumps. Unit 2: The Basic Unit of Life 9  LT2.5 TONICITY AND OSMOREGULATION Osmosis is the movement of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. It is a crucial process for maintaining cell homeostasis, ensuring that cells neither swell excessively nor shrink too much, which can disrupt cellular functions and lead to cell death. Tonicity and Water Balance Tonicity refers to the ability of a surrounding solution to cause a cell to gain or lose water. It is determined by the relative concentrations of solutes inside the cell compared to outside. There are three types of solutions we will focus on: isotonic, hypotonic, and hypertonic solutions. Isotonic Solution (iso- = same) What is it? In an isotonic solution, the solute concentration is equal inside and outside the cell. Example: When a cell is placed in a saline solution with the same concentration as its cytoplasm, water moves in and out of the cell at equal rates, and the cell remains unchanged. Hypotonic Solution (hypo- = low) What is it? In a hypotonic solution, the solute concentration outside the cell is lower than inside the cell. Example: When a cell is placed in pure water, water enters the cell due to osmosis, causing the cell to swell and possibly burst (lysis). Effect on Plant Cells: Plant cells become turgid (firm) due to the inflow of water, which is ideal for maintaining structural integrity. Hypertonic Solution (hyper- = high) What is it? In a hypertonic solution, the solute concentration outside the cell is higher than inside the cell. Example: When a cell is placed in a saltwater solution, water leaves the cell, causing it to shrink (crenate in animal cells or undergo plasmolysis in plant cells). Effect on Plant Cells: Plant cells lose water, and the plasma membrane pulls away from the cell wall, leading to plasmolysis, which can be damaging. Water Potential Water potential (𝚿) is a measure of the potential energy in water and is used to predict the direction of water movement. Water moves from regions of higher water potential to regions of lower water potential. The water potential will be equal to the solute potential of a solution in an open container because the pressure potential of the solution in an open container is zero. Unit 2: The Basic Unit of Life 10 The formula for water potential (𝚿) is given below: 𝚿 = 𝚿P + 𝚿s 𝚿 = water potential 𝚿P = pressure potential 𝚿s = solute potential Solute Potential Solute potential (𝚿s), also known as osmotic potential, is the effect of solute concentration on the water potential. It is always negative or zero, as the addition of solutes lowers the water potential. The formula for solute potential (𝚿s) is given below: 𝚿s = -iCRT i = ionization constant (the number of particles the solution dissociates to) C = molar concentration of the solute R = pressure constant (0.0831 liter bar per mole K) T = temperature in Kelvin (K = °C + 273) Pressure Potential Pressure potential (𝚿P) is the physical pressure exerted on or by the cell. In plant cells, this is often positive due to turgor pressure from the cell wall, which helps maintain rigidity. Unit 2: The Basic Unit of Life 11