Plant Cell: Structure and Function PDF

Plant Cell: Structure and Function PLANT CELL Importance: Defines the boundary of the cell. Controls the passage of materials into and out of the cell. Governs the interaction of the cell with other cells. Fluid Mosaic Model: A two-layered phospholipid membrane (phosphol...

Plant Cell: Structure and Function PLANT CELL Importance: Defines the boundary of the cell. Controls the passage of materials into and out of the cell. Governs the interaction of the cell with other cells. Fluid Mosaic Model: A two-layered phospholipid membrane (phospholipid bilayer) with anchored proteins. Membrane Lipids: Phospholipids: ○ Constitute 75% of the membrane lipids. ○ Occur as two layers (phospholipid bilayer). ○ Composition: Head: Glycerol and phosphate unit (hydrophilic; faces the fluid part of the cell). Tail: Two fatty acid chains (hydrophobic; oriented away from the fluid environment). Cholesterol: ○ Constitutes 20% of the membrane lipids. ○ Provides fluidity to the cell membrane. Glycolipids: ○ Constitute 5% of the membrane lipids. ○ Composed of 2 fatty acids + short carbohydrate chains. Membrane Proteins: Two kinds: ○ Integral proteins: Pass all the way through the cell membrane. ○ Peripheral proteins: Adhere to the surface of the cell membrane. Glycoproteins: Proteins + short carbohydrate chains. Functions of Membrane Proteins: Receptors: ○ Bind to chemical messengers such as hormones. ○ Binding to receptor results in a physiological change in the cell. ○ Important for cell communication. Enzymes: ○ Act as catalysts for chemical reactions. Anchor Proteins: ○ Physically link intracellular structures with extracellular structures. Transport Proteins: ○ Channel proteins: Allow the passage of water and solutes to enter or exit the cell. ○ Carrier proteins: Transport substances into and out of the cell by changing shape. ○ Protein pumps: Transport substances against their concentration gradient. Fluidity of Membranes: Permits movement of membrane pieces. Allows for vesicle formation and fusion. Important for: ○ Exocytosis: Release materials to the outside (secretion). ○ Endocytosis: Materials are taken inside the cell. Phagocytosis: Solid substances. Pinocytosis: Liquid substances. Compartmentalization: ○ Each compartment is specialized for a particular process, producing a particular substance (e.g., Cellular organelles). Permeability of the Membrane: Selectively permeable: ○ Hydrophobic substances, smaller molecules (e.g., water) may diffuse easily through the phospholipid bilayer. ○ Large hydrophilic substances (e.g., amino acids, glucose) and ions pass through membrane proteins by facilitated diffusion or active transport. Protoplasm: All substances inside the cell membrane except the vacuole. Includes the nucleus and the cytoplasm. Membranous Organelles: Surrounded by one or two bilayer lipid membranes. Includes the endomembrane system: ○ Set of membranes forming a single functional unit, connected directly or exchanging material through vesicle transport. ○ Works together to modify, package, and transport lipids and proteins. Nonmembranous Organelles: Lack membranes and are in direct contact with the cytosol. Nucleus: Spherical structure with a double-layered membrane. Outer membrane continuous with the rough endoplasmic reticulum. With nuclear pores for the passage of substances between the nucleus and the cytoplasm. Contains: Nucleoli(us): Involved in the synthesis and assembly of ribosomes. Chromatin: Tightly coiled DNA and histone proteins. Nucleoplasm: Fluid substance containing water, enzymes, and RNA. Endoplasmic Reticulum (ER): Extensive membranous network continuous with the nuclear membrane. Two kinds: ○ Rough ER: With ribosomes; synthesizes proteins for secretions. ○ Smooth ER: Lacks ribosomes; synthesizes membrane lipids. Golgi Apparatus (Dictyosomes): Consists of flattened membrane sacs which receive protein from ER or lipids from SER. Form vesicles that process, sort, and package proteins/lipids for transport to different parts of the cell (vacuole, lysosome, etc.) or for secretion. Microbodies: Spherical-shaped organelle surrounded by a single membrane. Contain enzymes that participate in biochemical reactions in the cell. Types of Microbodies: Glyoxysomes: ○ Involved in converting stored fats into sugars. ○ Important during the germination of fat-rich, oily seeds (e.g., peanut, sunflower, coconut). Peroxisomes: ○ Contain enzymes that degrade fatty acids and amino acids. ○ Detoxify harmful substances in the cell (e.g., Hydrogen peroxide). Central Vacuole: Composed of a single membrane called tonoplast. Functions: ○ Digestive: Breakdown of macromolecules. ○ Storage: Ions, sugars, amino acids, toxic waste. ○ Maintain cell turgidity: High ionic concentration generates high water potential. ○ Cell growth: As it expands, it forces the cell to grow rapidly. Mitochondria: Site of aerobic cell respiration (C6H12O6 + O2 -> CO2 + H2O + Energy (ATP)). The process that breaks down organic compounds in the presence of oxygen to yield energy (as ATP). With DNA and ribosomes distinct from those of the cell, capable of reproduction. Plastids:  Associated with pigments and storage products. With outer and inner membranes, distinct ribosomes, and circular DNA. Capable of reproduction. Types of Plastids: Proplastid: Small undifferentiated plastids in young developing cells. Chloroplast: Site of photosynthesis; converts solar energy to chemical energy in the form of sugars. Etioplast: Chloroplasts that have not been exposed to light. Chromoplast: Store pigments (e.g., carotenoids) found in flowers and fruits. Leucoplast: Include amyloplast, proteinoplast, and elaioplast. Ribosome: Consist of RNA and proteins, important for protein synthesis. Two structural parts: Large subunit and small subunit. Two forms: Free ribosomes (in cytosol) and bound ribosomes (attached to endoplasmic reticulum). Cytoskeleton: A network of different kinds of protein filaments that extend throughout the cytosol. Maintains cell shape and provides support. Anchors organelles and enzymes to specific regions of the cell. Contractility and movement (e.g., amoeboid movement). Intracellular transport: Provide tracks for vesicle and organelle movement. Cytoplasmic Streaming (Cyclosis): Moving currents of cytoplasm. Facilitate the transport of nutrients, enzymes, and other substances between the cell and its surroundings, and within the cell itself. Cell Wall: Composed of cellulose, hemicellulose, pectin substances, lignin, and proteins (structural or enzymatic). Three layers:  ○ Middle Lamella/Intercellular Layer: Primarily of pectin; binds individual cells to form tissues. ○ Primary Cell Wall: First true cell wall; composed of cellulose bound by hemicellulose; develops on newly formed cells. ○ Secondary Cell Wall: Formed on the inner surface of the primary wall; thicker; impregnated with lignin; increases strength, waterproofing, resistance to pathogens; develops in mature functional cells. Plasmodesmata: Channels through cell walls that connect the cytoplasms of adjacent cells. CHLOROPLASTS Outer Membrane: A semipermeable membrane that encloses the chloroplast, allowing certain molecules to pass in and out. Inner Membrane: Lies just inside the outer membrane and is also semi-permeable, forming a boundary between the chloroplast stroma and the intermembrane space. Intermembrane Space: The space between the outer and inner membranes. Stroma: The dense fluid inside the inner membrane, containing enzymes, DNA, and ribosomes. It is where the light-independent reactions of photosynthesis (Calvin cycle) occur. Thylakoid Membranes: Flattened, disk-like sacs within the stroma. They contain chlorophyll and other pigments necessary for the light-dependent reactions of photosynthesis. Thylakoid Grana: Stacks of thylakoid membranes. The arrangement of these stacks increases the surface area for light absorption. Lumen: The internal space of each thylakoid. Chlorophyll: The green pigment located in the thylakoid membranes that captures light energy. Plastid DNA: Circular DNA found in the stroma that is responsible for coding some of the proteins needed by the chloroplast. Ribosomes: Present in the stroma, these are responsible for protein synthesis within the chloroplast. THE NUCLEAR ENVELOPE Outer Nuclear Membrane: The outer membrane of the nuclear envelope, which is continuous with the endoplasmic reticulum (ER). It has ribosomes attached to it and is involved in various cellular processes. 1. Inner Nuclear Membrane: The inner membrane of the nuclear envelope, which lines the nuclear interior and is associated with the nuclear lamina. 2. Nuclear Lamina: A dense fibrillar network inside the inner nuclear membrane, composed of intermediate filament proteins (lamins). It provides structural support to the nuclear envelope and helps organize chromatin. 3. Nuclear Pores: Large protein complexes embedded in the nuclear envelope that regulate the transport of molecules between the nucleus and the cytoplasm. They allow the passage of RNA, proteins, and other macromolecules. 4. Perinuclear Space: The space between the outer and inner nuclear membranes, also known as the intermembrane space. It is continuous with the lumen of the endoplasmic reticulum. ENDOPLASMIC RETICULUM Rough Endoplasmic Reticulum (Rough ER): Ribosomes: Attached to the cytoplasmic side of the rough ER membrane, ribosomes are responsible for protein synthesis. The rough ER is named for its "rough" appearance due to these ribosomes. Cisternae: Flattened, membrane-bound sacs or tubules that make up the rough ER's structure. They provide a large surface area for protein synthesis and processing. Lumen: The internal space within the cisternae where newly synthesized proteins are processed and folded. Smooth Endoplasmic Reticulum (Smooth ER): Smooth ER Membrane: Lacks ribosomes, giving it a "smooth" appearance. It is involved in lipid synthesis and metabolism, as well as detoxification processes. Tubules: The smooth ER primarily consists of tubular structures, which help in the transport of lipids and other substances within the cell. Lumen: The internal space within the tubules, where lipid synthesis and various metabolic processes occur. ER Membrane: Both rough and smooth ER are bounded by a lipid bilayer membrane that separates the internal lumen from the cytoplasm. ER Vesicles: Small, membrane-bound sacs that transport proteins and lipids from the ER to other parts of the cell, such as the Golgi apparatus. GOLGI APPARATUS Cisternae: Flattened, membrane-bound sacs that make up the Golgi apparatus. They are stacked on top of each other and serve as the primary site for processing and modifying proteins and lipids. Cis Face: The side of the Golgi apparatus that is oriented towards the endoplasmic reticulum (ER). It is the entry face where vesicles containing newly synthesized proteins and lipids from the ER fuse with the Golgi. Trans Face: The side of the Golgi apparatus that faces the cell membrane. It is the exit face where processed proteins and lipids are packaged into vesicles for transport to their final destinations. Medial Cisternae: The middle layers of the Golgi apparatus, between the cis and trans faces, where further processing and modification of proteins and lipids occur. Golgi Vesicles: Small, membrane-bound sacs that bud off from the cisternae. These vesicles transport processed proteins and lipids from one part of the Golgi to another or to other cellular locations, including the cell membrane. Golgi Matrix: A network of proteins and lipids that helps to maintain the structure and organization of the Golgi apparatus. Enzymes: Located within the cisternae, these enzymes are responsible for the modification of proteins and lipids, such as adding carbohydrate groups (glycosylation) or phosphate groups (phosphorylation). MITOCHONDRIA Outer Membrane: The smooth, outermost lipid bilayer that encloses the mitochondrion. It is permeable to small molecules and ions. Inner Membrane: The membrane that lies within the outer membrane and is folded into structures called cristae. The inner membrane is less permeable and contains proteins involved in the electron transport chain and ATP synthesis. Cristae: The folds or invaginations of the inner membrane. They increase the surface area available for chemical reactions, particularly those involved in ATP production. Matrix: The gel-like fluid enclosed by the inner membrane. It contains a variety of enzymes, mitochondrial DNA, ribosomes, and other molecules necessary for the citric acid cycle (Krebs cycle) and other metabolic processes. Intermembrane Space: The space between the outer membrane and the inner membrane. It plays a role in the generation of the proton gradient used to produce ATP. Mitochondrial DNA: Circular DNA located in the matrix. It contains genes that encode some of the proteins needed by the mitochondria. Mitochondrial Ribosomes: Present in the matrix, these ribosomes are responsible for synthesizing some of the proteins encoded by mitochondrial DNA. ATP Synthase: An enzyme complex embedded in the inner membrane, it uses the proton gradient across the membrane to synthesize ATP from ADP and inorganic phosphate. CELL WALL PARTS 1. Primary Cell Wall: The flexible, outermost layer that is formed first and allows for cell growth. It is primarily composed of: ○ Cellulose: Long chains of glucose molecules that provide structural support. ○ Hemicellulose: Polysaccharides that help to bind cellulose fibers together. ○ Pectin: A gel-like substance that provides rigidity and adhesion between cells and helps to hold them together. 2. Secondary Cell Wall: Located inside the primary cell wall, it is thicker and more rigid. It is formed after the cell has stopped growing and is composed of: ○ Cellulose: Often in higher concentrations than in the primary cell wall. ○ Lignin: A complex polymer that strengthens the cell wall and makes it more rigid and waterproof. ○ Suberin: Found in some cell types, such as in cork cells, providing additional protection and water resistance. 3. Middle Lamella: The pectin-rich layer that acts as a glue to cement adjacent plant cells together. It is located between the primary cell walls of adjacent cells. 4. Plasmodesmata: Microscopic channels that traverse the cell wall, allowing for communication and the transport of nutrients, ions, and signaling molecules between adjacent plant cells. 5. Cuticle: In some plant cells, especially on the surface of leaves, a waxy or fatty layer called the cuticle covers the outer surface of the cell wall, providing a barrier to reduce water loss and protect against environmental damage.