Plant Cells PDF

Science PLANT CELLS Mermaid’s wineglass (Acetabularia) three parts: a delicate base that anchors it to a rock or piece of coral, a long slender stalk, and a cap (A. mediterranea) is smooth and cup shaped, and another (A. crenulata) is rounded with a series of fingerlike projections INTRODUCTION...

Science PLANT CELLS Mermaid’s wineglass (Acetabularia) three parts: a delicate base that anchors it to a rock or piece of coral, a long slender stalk, and a cap (A. mediterranea) is smooth and cup shaped, and another (A. crenulata) is rounded with a series of fingerlike projections INTRODUCTION In the 1600s, Anton van Leeuwenhoek (1632– 1723) perfected the art of grinding lenses (pieces of glass with curved surfaces) and used them in microscopes of his own design to produce clear, magnified images. In 1665, Robert Hooke used a microscope to examine a cork sliver, observed tiny compartments he named cells, and recognized that while he saw only the cell walls of dead cells, the contents of living cells were also important. 19th century so that biologists were able to observe tiny structures within cells, which they called organelles (little organs). 1830 Robert Brown a Scottish botanist, first identified and named the cell’s nucleus (an organelle that serves as its control center). INTRODUCTION By the late 1830s, after examining various tissues, biologists concluded that all organisms are composed of cells, a concept formally stated by German biologists Matthias Schleiden and Theodor Schwann in 1838 and 1839, respectively, which became known as the cell theory. Another German scientist, Rudolf Virchow, extended the cell theory in 1855 by stating that all cells come from preexisting cells. That is, cells divide to give rise to new cells. In 1880 August Weismann pointed out that since cells come from preexisting cells, all cells in existence today trace their origins back to ancient cells. PLANT BIOLOGIST TODAY USE A VARIETY OF METHODS TO STUDY CELLS EUKARYOTIC CELLS AND PROKARYOTIC CELLS Cells are composed of cytoplasm surrounded by a plasma membrane, which serves as the outer boundary of the cell, though plant cells also have a cell wall. All cells contain DNA, the genetic material that controls cellular activities. Eukaryotic cells have their genetic material enclosed within a membrane-bound nucleus, while prokaryotic cells lack a nucleus, with DNA present in the cytoplasm. Prokaryotic cells are generally smaller and simpler, lacking membrane-bound organelles and nuclei, and evolved before eukaryotic cells; only archaea and bacteria are prokaryotes. Eukaryotic cells are larger, more complex, and contain membrane-bound organelles; they are found THE STRUCTURE OF PLANT CELLS THE STRUCTURE OF PLANT CELLS The plasma membrane serves as the outer boundary of the cell, confining its contents to an internal compartment and regulating the flow of materials in and out. Acting as a selective barrier, it allows some substances to pass freely while controlling or blocking the passage of others, maintaining a distinct internal environment. THE STRUCTURE OF PLANT CELLS The nucleus serves as the cell's control center, housing DNA, which directs cellular activities. DNA in the nucleus is continually used to guide cell functions and is replicated during cell division. THE STRUCTURE OF PLANT CELLS The nucleus is enclosed by a double membrane, the nuclear envelope, with selective pores that regulate the entry and exit of materials. Inside the nucleus, DNA combines with proteins to form chromatin, which condenses into chromosomes during cell division. Nucleoli within the nucleus are responsible for making and assembling ribosome subunits, essential for protein synthesis. THE STRUCTURE OF PLANT CELLS Chloroplasts, found in algal and plant cells, convert light energy into chemical energy through photosynthesis. Chloroplasts are a type of plastid surrounded by a double membrane, containing enzymes and chlorophyll necessary for photosynthesis. During photosynthesis, chloroplasts use light energy to convert carbon dioxide and water into carbohydrates like glucose. THE STRUCTURE OF PLANT CELLS Chloroplasts are typically disc-shaped in plant cells and come in various shapes in algae, with their interior containing thylakoids stacked into grana, embedded in a jellylike stroma where photosynthesis occurs. Chloroplasts contain small amounts of DNA and ribosomes, suggesting they evolved from free- living ancestors. Plant cells also contain leucoplasts, colorless plastids that store starch, oils, or proteins, commonly found in seeds, roots, and modified stems for food storage. Leucoplasts can transform into chloroplasts when exposed to light, synthesizing chlorophyll, as seen when potato tubers are exposed to light. Chromoplasts, another type of plastid, contain pigments that provide yellow, orange, and red THE STRUCTURE OF PLANT CELLS Mitochondria are the powerhouses of eukaryotic cells, converting the chemical energy from food molecules into ATP through cellular respiration. Cellular respiration occurs in mitochondria, where fuel molecules are broken down into carbon dioxide and water, releasing energy stored in ATP. Mitochondria are bounded by a double membrane, with the inner membrane forming folds called cristae, which house enzymes for cellular respiration. The matrix, the fluid inside the inner mitochondrial membrane, also contains enzymes involved in respiration. Mitochondria contain small amounts of DNA and ribosomes, indicating a possible evolutionary link to free-living ancestors. THE STRUCTURE OF PLANT CELLS Ribosomes are small organelles responsible for protein synthesis, using DNA instructions to join amino acids in precise sequences. Ribosomes consist of two subunits made of RNA and protein, and are found in the nucleus, plastids, mitochondria, and cytoplasm. In the cytoplasm, ribosomes are either free or attached to the endoplasmic reticulum (ER), where they are involved in protein production. The endoplasmic reticulum (ER) is an extensive network of membranes and is continuous with both the plasma membrane and the nuclear envelope. Rough ER, with attached ribosomes, is a site of protein synthesis, while smooth ER is involved in lipid synthesis. The ER also plays a role in synthesizing membranes for various organelles, including the nuclear envelope and Golgi apparatu THE STRUCTURE OF PLANT CELLS The Golgi body, or dictyosome, processes and packages proteins and polysaccharides, consisting of flattened sacs surrounded by membranes. Vesicles formed at the edges of the Golgi body transport materials to the plasma membrane, other organelles, or outside the cell. The collective term for all Golgi bodies in a cell is the Golgi apparatus, which is responsible for exporting and processing cellular materials. In plant cells, the Golgi apparatus produces and transports polysaccharides for the cell wall and collects materials for vacuoles. Proteins synthesized by ribosomes on the rough ER are transported in vesicles to the Golgi body, where they are modified and packaged for export. Vesicles from the Golgi body fuse with the plasma membrane, releasing their contents outside the cell. THE STRUCTURE OF PLANT CELLS A vacuole is a membrane-bound sac containing water, dissolved salts, ions, pigments, and waste products, commonly found in plant cells and some protists. In mature plant cells, the vacuole can occupy up to 90% of the cell's volume and is surrounded by a membrane called the tonoplast. Vacuoles help maintain plant cell shape by making the cell turgid, which provides rigidity, especially in non-woody plants. Vacuoles also serve as storage for excess materials like calcium ions and pigments such as anthocyanins, which contribute to plant coloration. Waste products and malformed proteins may enter the vacuole for disassembly or accumulate as crystals visible under a microscope. THE STRUCTURE OF PLANT CELLS Plant cells have a rigid cell wall outside the plasma membrane for support and protection, allowing the passage of water and dissolved materials. The combined strength of cell walls enables plants, like trees, to stand tall without collapsing. While animal cells lack cell walls, many organisms like prokaryotes, algae, and fungi have cell walls, differing in composition from plant cell walls. Plant cell walls are mostly made of cellulose, a polysaccharide composed of linked glucose molecules, produced inside the cell and transported via Golgi apparatus vesicles. THE STRUCTURE OF PLANT CELLS Cellulose fibers in the plant cell wall are held together by other polysaccharides like pectin, which helps cement cells together. Growing plant cells secrete a primary cell wall that stretches as the cell grows, and upon maturation, secondary walls rich in lignin may form for additional strength. Plant cells communicate through plasmodesmata, tiny channels that connect adjacent cells and allow the passage of molecules and ions between them. PLANT CELLS AND ANIMAL CELLS ARE MORE ALIKE THAN DIFFERENT Both plant and animal cells are eukaryotic, sharing many fundamental structures. Both types of cells have plasma membranes, nuclei, mitochondria, ribosomes, ER, the Golgi apparatus, and a cytoskeleton. Plant cells differ from animal cells by possessing cell walls, plastids, and large vacuoles. Animal cells have centrioles, which aid in cell division, and lysosomes, which are involved in digestion. Plant cells lack centrioles and lysosomes, while animal cells lack cell walls, plastids, and prominent vacuoles. BIOLOGICAL MEMBRANES The fluid mosaic model describes the structure of cell membranes, depicting them as a bilayer of lipid molecules. Proteins are embedded within this lipid bilayer, resembling a mosaic pattern. The membrane is fluid, allowing lipids and proteins to move laterally within the layer. Membrane lipids, especially phospholipids, have a polar, hydrophilic "head" that is water-attracted and a nonpolar, hydrophobic "tail" that is water-repellent. Phospholipids spontaneously form a bilayer, with hydrophilic heads facing the watery inside and outside of the cell and hydrophobic tails facing inward, away from water. BIOLOGICAL MEMBRANES Cell membranes perform several essential functions beyond merely acting as barriers. Selective Permeability: Membranes regulate the passage of materials, allowing some substances to enter or exit while blocking others. For instance, the lipid bilayer prevents ions and polar molecules like Na⁺ and Cl⁻ from passing through unaided. This regulation helps maintain homeostasis, ensuring a stable internal environment by controlling the influx of nutrients and gases such as oxygen and carbon dioxide. BIOLOGICAL MEMBRANES Cell membranes perform several essential functions beyond merely acting as barriers. Communication and Signal Reception: Membranes play a role in cell communication by receiving signals from the external environment. Chemical messengers, like hormones, bind to receptors on the membrane and trigger responses within the cell, allowing it to adapt to its surroundings. Energy and Enzymatic Activity: Membranes support crucial processes like energy production. In mitochondria, membranes are necessary for ATP synthesis, while in chloroplasts, thylakoid membranes are vital for capturing solar energy. Additionally, membranes in organelles like the ER provide sites for enzymatic reaction BIOLOGICAL MEMBRANES Simple Diffusion: During simple diffusion, atoms and molecules move from areas of higher concentration to areas of lower concentration along a concentration gradient. Membrane Structure: The cell membrane consists of a fluid bilayer of phospholipids with embedded membrane proteins. The hydrophilic heads face outward toward the watery surroundings, while the hydrophobic tails point inward, avoiding water. BIOLOGICAL MEMBRANES Random Motion: Diffusion occurs due to the constant random motion of atoms and molecules, which collide and rebound in different directions, eventually becoming uniformly distributed. Importance of Diffusion: Diffusion is crucial for cellular functions, enabling the movement of essential materials like oxygen, carbon dioxide, and water in and out of cells. However, larger or more polar molecules cannot pass through the membrane by diffusion alone. BIOLOGICAL MEMBRANES Osmosis: Osmosis is a type of diffusion specifically involving the movement of water through a selectively permeable membrane. It occurs from an area with a higher concentration of water (lower solute concentration) to an area with a lower concentration of water (higher solute concentration). Solute and Solvent: In biological systems, the solutes are substances dissolved in water, and water itself is referred to as the solvent. BIOLOGICAL MEMBRANES Isotonic Solutions: When a cell is placed in an isotonic solution, the solute concentration inside and outside the cell is equal. Water moves in both directions across the plasma membrane at the same rate, maintaining equilibrium. Hypertonic Solutions: In a hypertonic solution, where the solute concentration outside the cell is higher than inside, water moves out of the cell. This causes the cell to shrink as the cytoplasm loses water. An example would be placing a cell in a concentrated sugar solution. Hypotonic Solutions: A hypotonic solution has a lower solute concentration outside the cell than inside. Water moves into the cell, causing it to swell. For instance, BIOLOGICAL MEMBRANES Plant Roots: Roots are typically exposed to soil water, which is hypotonic compared to the cytoplasm of the root cells. This allows water to move into the plant, supporting its hydration and nutrient uptake. As water moves into root cells, their cell walls provide structural support, enabling them to withstand the pressure caused by the incoming water. This pressure is referred to as turgor pressure. Turgor pressure is the internal force of water pushing against the cell wall. As the turgor pressure builds, an equilibrium is eventually reached. At this point, the turgor pressure forces water molecules out of the cell at the same rate that water molecules enter through SOME SUBSTANCES CROSS MEMBRANES BY FACILITATED DIFFUSION OR ACTIVE TRANSPORT Facilitated diffusion: Materials move from high to low concentration through carrier proteins in the membrane. This enhances the diffusion of certain substances, such as ions, but only in the direction of the concentration gradient (high to low). Facilitated diffusion does not require energy. Active transport: Materials move from a lower concentration to a higher concentration (against the concentration gradient) with the assistance of carrier proteins. This process requires energy, usually supplied by ATP. SOME SUBSTANCES CROSS MEMBRANES BY FACILITATED DIFFUSION OR ACTIVE TRANSPORT Active transport allows cells to accumulate certain substances, like potassium ions (K+), which are required in higher concentrations within the cell. Substances such as Na+ and H+ ions, found in higher concentrations outside the cell, can enter via diffusion or facilitated diffusion but are expelled from the cell through active transport to maintain balance.