Plants

• Plant cell structure


  • Cell wall - outer membrane that offers structural support, made of polysaccharide cellulose
  • Middle lamella - outermost layer that acts like glue to stick adjacent plant cells together, provides stability
  • Plasmodesmata - narrow threads of cytoplasm that connect the cytoplasm of neighbouring plant cells, allow substance transport and cell-to-cell communication
  • Pits - very thin regions of cell wall that facilitate transport of substances between cells
• Chloroplasts


  • Larger than mitochondria, surrounded by a double-membrane
  • Contain membrane-bound compartments called thylakoids that contain chlorophyll stacks called grana
  • Grana are joined by lamellae (thin, flat thylakoid membranes)
  • Site of photosynthesis, with some processes happening in thylakoid membranes and others in stroma
  • Contain small circular DNA and ribosomes used to synthesise proteins needed for chloroplast replication and photosynthesis

  
• Amyloplasts


  • Small, membrane-bound organelles that store starch granules
  • Responsible for storing starch in plants and converting it back to glucose when needed
• Vacuole and tonoplast


  • Vacuole is a sac surrounded by the tonoplast (selectively permeable membrane)
  • Cell sap is a mixture of water, minerals, waste, enzymes
  • Vacuoles keep cells turgid to prevent wilting, break down and isolate unwanted chemicals, and control what can enter and leave
• Xylem vessels


  • Transport dissolved minerals and water around plants
  • Provide structural support
  • Cell walls contain lignin to withstand pressure of moving water column
  • Formed by dead cells without cytoplasm or organelles to slow water flow
  • Have pits in walls to allow lateral movement of water and minerals
• Sclerenchyma fibres


  • Provide support, not involved in transport
  • Formed from long, hollow tubes with end walls, unlike xylem
  • Highly lignified with more cellulose than other plant cells
• Phloem


  • Transport organic compounds, particularly sucrose, up and down the plant (translocation)
  • Made of living cells including sieve tube elements (main conducting cells) and companion cells (control metabolism of sieve tubes)
• Types of plant tissue


  • Vascular tissue (xylem and phloem for transport)
  • Epidermis (single layer of protective cells on plant surface)
  • Ground tissue (provides support, stores nutrients, aids photosynthesis)

  
• Plant cells perform a variety of functions and need water and inorganic ions to do so
• Water


  Important component of photosynthesis, transport medium for minerals, maintains cell turgidity, regulates temperature
• Magnesium ions


  Important for production of chlorophyll, provide green colour and support photosynthesis
• Nitrate ions


  Essential for synthesis of DNA, proteins, chlorophyll, and enzymes, important for plant growth, fruit and seed production
• Calcium ions


  Important cell components, important for proper growth
• Water pathways - transpiration


  1. Cellulose cell walls are permeable to water, helping movement

  2. Many cells joined by plasmodesmata, allowing osmosis and diffusion

  3. Mass flow system - large volume of water moving provides push to continue movement, also helps transport ions
• Transpiration


  Linked to cohesion-tension theory - water moves up through xylem vessels due to capillary action and evaporation from leaves
• Translocation (mass flow theory)


  1. Highest water concentration in xylem causes water to osmose into phloem where there is high sugar concentration

  2. Water helps transport sugars across phloem towards sink

  3. Sugars are lost/absorbed at sink, increasing water potential to return to xylem

  4. Hydrogen ions move out of cell using ATP, then co-transporters bring them back in with sucrose

  5. Sucrose is actively transported into phloem sieve tubes at source, hydrogen ions actively transported out of companion cells to increase concentration gradient and trigger more sucrose uptake
• Starch


  • Storage polysaccharide of plants, stored as granules in plastids and amyloplasts
  • Consists of long chains of glucose monomers, takes longer to digest than glucose
• Types of starch


  • Amylose (10-30% of starch) - unbranched helix-shaped with 1,4 glycosidic bonds
  • Amylopectin (70-90% of starch) - highly branched with 1,4 and 1,6 glycosidic bonds
• Phloem transport


  1. Sucrose enters phloem

  2. Sucrose moves down phloem

  3. Sucrose exits phloem

  4. Process repeats
• Starch is a storage polysaccharide of plants

  
• Starch


  • Stored as granules in plastids (e.g. chloroplasts) and amyloplasts
  • Consists of thousands of glucose monomers
  • Two groups: amylose and amylopectin
• Amylose


  Unbranched helix-shaped with 1,4 glycosidic bonds between α-glucose molecules

  
• Amylopectin


  1,4 glycosidic bonds between α-glucose molecules but also form 1,6 glycosidic bonds creating branched molecule

  
• Starch


  • Compact so large quantities can be stored
  • Insoluble so no osmotic effect
  • Stored in seeds as glucose source
• Cellulose


  • Polymer of β-glucose joined by 1,4-glycosidic bonds
  • Consecutive β-glucose molecules rotated 180°
  • Provides structural support to plants
• Cellulose has high tensile strength allowing cell walls to withstand turgor pressure
• Sclerenchyma fibres and xylem vessels are made of long, hollow plant cells

  
• Plant fibres


  • Strength due to arrangement of microfibrils in mesh-like pattern
  • Strength due to secondary thickening of cell wall with lignin
• Modern drug testing


  1. Computer modelling

  2. Human tissue testing

  3. Animal testing

  4. Clinical trials (Phase 1, 2, 3)
• Placebos and double blind studies are used in drug testing
• Plants produce chemical and physical defences to stop other organisms consuming or attacking them
• Plant antimicrobials


  • Aspirin (from willow bark)
  • Morphine and codeine (from opium poppies)
  • Antibiotics from garlic
• Binary fission


  Allows bacterial cells to divide into new cells quickly
• Conditions for bacterial growth


  • Moist environment
  • High temperature
  • Neutral pH
• Conditions not ideal for bacterial growth


  • Dry
  • High or low pH
  • Cold temperature
  • Salty