LECTURE 3 – Plant Cell Structure and Functions

• All cells have several basic features in common 

  • A plasma membrane bounds them 
  • They contain a semifluid substance called the cytosol 
  • They contain chromosomes 
  • They all have ribosomes
• Prokaryotic cells

  • Do not contain a nucleus 
  • Have their DNA located in a region called the nucleoid
• • Pili: attachment structures on the surface of some prokaryotes 
• • Nucleoid: region where the cell’s DNA is located (not enclosed by a membrane) 
• • Ribosomes: organelles that synthesize proteins 
• Plasma membrane: membrane enclosing the cytoplasm
• • Cell wall: rigid structure outside the plasma membrane 
• Capsule: jelly-like outer coating of many prokaryotes
• • Flagella: locomotion organelles of some bacteria
• Eukaryotic cells

  • Contain a true nucleus, bounded by a membranous nuclear envelope 
  • They are generally quite a bit bigger than prokaryotic cells
• Nucleus 

  • Genetic Library of the Cell 
  • It contains most of the genes in the eukaryotic cell 
  • genetic instructions are housed in the nucleus and carried out by the ribosomes
• The nuclear envelope 0.25 µm – Encloses the nucleus, separating its contents from the cytoplasm
• Endosymbiosis theory

  • explanation of how eukaryotic cells obtained organelles such as chloroplasts and mitochondria, but it does not explain how the genetic material in the nucleus became surrounded by a nuclear envelope.
• Ribosomes 

  • Protein Factories in the Cell 
  • Are particles made of ribosomal RNA and protein 
  • Carry out protein synthesis
• Endoplasmic Reticulum  

  • Biosynthetic Factory 
  • Accounts for more than half the total membrane in many eukaryotic cells
  • There are two distinct regions of ER
• There are two distinct regions of ER

  • Smooth ER, which lacks ribosomes 
  • Rough ER, which contains ribosomes
• The smooth ER 

  • Synthesizes lipids 
  • Metabolizes carbohydrates 
  • Stores calcium 
  • Detoxifies poison
• The rough ER 

  • Has bound ribosomes 
  • Produces proteins and membranes, which are distributed by transport vesicles
• The Golgi Apparatus 

  • Shipping and Receiving Center  
  • Receives many of the transport vesicles produced in the rough ER  
  • Modification of the products of the rough ER  
  • Manufacture of certain macromolecules 
• the golgi apparatus consists of flattened membranous sacs called cisternae
• • Central vacuoles – Are found in plant cells – Hold reserves of important organic compounds and water
• • Contractile vacuoles – pump excess water out of protist cells 
• Food vacuoles – Are formed by phagocytosis
• Mitochondria  

  • Are the sites of cellular respiration 
• Chloroplasts  

  • Found only in plants, are the sites of photosynthesis
• Mitochondria 

  • Chemical Energy Conversion 
  • Are found in nearly all eukaryotic cells
• mitochondria

  • enclosed by two membranes  
  • A smooth outer membrane  
  • An inner membrane folded into cristae
• Peroxisome 

  • Produce hydrogen peroxide and convert it to water
• Cytoskeleton 

  • is a network of fibers that organizes structures and activities in the cell 
  • Found in the cytoplasm 
  • Gives mechanical support to the cell
• Cell Wall 

  • Is an extracellular structure of plant cells that distinguishes them from animal cells 
  • Are made of cellulose fibers embedded in other polysaccharides and protein 
  • May have multiple layers
• Plasmodesmata 

  • Are channels that perforate plant cell walls
• Plasma membrane 

  • Functions as a selective barrier 
  • Allows sufficient passage of nutrients and waste 
  • It supports and protects the cell 
  • Also helps maintain homeostasis
• Simple Diffusion – materials move along a concentration gradient
• Isotonic 

  • When a cell is placed in a solution with a solute concentration equal to that inside the cell, water molecules diffuse through the plasma membrane equally in both directions 
  • have a solute concentration equal to that in the cell 
  • No net movement of water
• Hypotonic 

  • When a cell is placed in a solution with a solute concentration lower than that within the cell, the solution is said to be hypotonic to the cell. 
  • Water flows into the cell from the surrounding solution 
  • Plant cell become turgid/swells
• Hypertonic 

  • When a cell is placed in a solution with a solute concentration higher than that within the cell, the solution is hypertonic to the cell. 
  • In such situation, water flows out of the cell and into the surrounding solution, and the cytoplasm shrinks 
  • Plant cell shrinks/lysis
• Passive Transport: Diffusion

  • Diffusion is the movement of substance from a region of higher concentration to a region of lower concentration
• Active Transport

  • The assisted movement of a substance from a lower concentration to a higher concentration of that substance. 
  • Substances during active transport move against concentration gradient, thus requiring energy (ATP) and carrier proteins 
• Requirement for Active Transport 

  1. Energy (ATP) 
  2. Carrier proteins Against Concentration Gradient
• • A smaller cell – Has a higher surface to volume ratio, which facilitates the exchange of materials into and out of the cell