Structure of plants

• Autotrophs and heterotrophs require organic and inorganic substances
• Organic substances include: glucose, amino acids, glycerol, fatty acids, vitamins, nucleotides
• Inorganic substances: water, chloride ions, phosphates, sodium
• Most autotrophic organisms are plants
• Plants can be either vascular (possess a transport system) or nonvascular (do not possess a transport system)
• Plants have specialised cell that form tissues and then form organs whose role is to carry out certain functions. These roles include:

  • Transport of substances
  • Photosynthesis
  • Reproduction
  • Gas exchange
• Plant tissue is categorised based on the composition of 3 types of cells:

  • Parenchyma cells: present in photosynthesis, organ repair, storage of starch. Found in stems, leaves and fruit.
  • Collenchyma cells: Support and structure. Found in the epidermis of stem and leaves.
  • Sclerenchyma cells: Dead cells with heavy, thickened walls, with ligin which provides structure and support to the plant.
• Plant organ systems are made out of 3 types of tissue:

  
  

  • Dermal tissue
  • Vascular tissue
  • Ground tissue
• Plants have 2 major organ systems:

  • The shoot system
  • The root system
• The shoot system includes all structures above the ground
• The root system includes all structures below the ground
• The plant body consists of a connected shoot system and root system
• The shoot system collects carbon dioxide, oxygen and light and produces sugars. It also disperses pollen and seeds.
• The root system collects water and nutrients from the soil, and provides anchorage to ground or substrate.
• Vascular tissues link all plant parts. They are called xylem and phloem.
• Xylem transport water and minerals.
• Phloem transport manufactured foods. e.g. glucose
• All plants rely on fluid pressure within their cells (tugor) to give support to their less rigid structures. These include leaves and flowers.
• There are two major root systems:

  • Tap root system
  • Fibrous root system
• The tap root system has a main root from which lateral roots emerge. They may penetrated deep into soil and sometimes act as storage organs that swell up as food reserves are deposited. For example, carrots, beetroot and parsnips.
• Fibrous root systems form a network of roots close to the soil surface and may spread widely to anchor the plant, helping to bind the soli and prevent erosion.
• Some plants form cluster roots which are groups of tiny roots to increase surface area for uptake of mineral ions in nutrient poor soils. E.g. banksias
• External root structure: The root of most plants grow down into soil, their growing point protected by a root cap.

  
  

  Behind the growing point is the region of root hairs which provide a large surface areas to soak up water and minerals.
• External root structure

  • Epidermis: the protective outer layer of roots (usually lacks cuticle)
  • In young roots, the epidermis is covered with a slimy coating or sheath called mucigel.
• Cortex: In the root of many plants, the large cells of the cortex act as a storage area for excess materials. There are air spaces between the cells for the circulation of gases.
• Vascular tissue: In the roots, the vascular tissue forms a cylinder in the centre which consists of xylem and phloem, sometimes called stele
• Sunlight and Carbon dioxide: Obtained from water (if water plants) or the air.

  The specialised structure for obtaining light and CO2 in most plants is the leaf (where most photosynthesis occurs)
• Leaf

  • External structures: Lamina, petiole (joins leaf to stem), network of veins
  • Internal structure: Specialised tissues for photosynthesis
• External leaf structure

  • Arrangement: usually arrayed along the stem of plants in a way to absorb as much sunlight as possible, usually angled so the sunlight hits the top of the leaf
  • Shape: most are broad, flat and thin which gives a large surface area
• Internal leaf structure

  • Cuticle: leaves secrete a waxy substance that forms an outer layer or cuticle. This is to maintain the shape of the leaf and provide protection
  • In terrestrial plants, it plays an important role in reducing water loss by evaporation
  • May be thin or absent in aquatic plants
• Internal leaf structure

  • Epidermis: forms a protective layer o fcells on the upper and lower surfaces of the leaf. It i transparent so that sunlight can easily penetrate
• Internal leaf structure

  • Stomates: pores in the leaf that open and close
  • When open, permit the exchange of gases between the leaf and environment
  • Water is lost via evaporation
• Internal leaf structure

  
  

  There are 2 types of mesophyll cells in the centre of the leaf:

  • Palisade
  • Spongy mesophyll
• Internal leaf structure

  
  

  Palisade mesophyll:

  • Mostly found in one or two rows below the upper epidermis
  • Regularly arranged, elongated cells packed with green chloroplasts
  • In these cells in where most of the plant's photosynthesis occurs
• Internal leaf structure

  
  

  Spongy mesophyll:

  • Situated between the palisade cells and the lower epidermis
  • Contains fewer chloroplasts than palisade
  • Irregularly arranged with very large spaces between them
  • This arrangement enables vapour to mover easily between cells and stomata
• Internal leaf structure

  
  

  Veins:

  • Tubes of vascular tissue (bundles) consisting of xylem and phloem cells
  • Transport material to and from the leaf
  • Forms a branching network throughout the leaf: this gives rigidity to the leaf, maintains its shape and ensures every leaf cell is close to a vein
• Xylem vessels consist of dead cells, thickened, whose crosswalls have broken down, forming a continuous system of tubes
• Phloem consists of long columns of living sieve tube cells with perforated end walls so that cytoplasm is continuous from one cell to the next

  • lying alongside the sieve tube cells are companion cells and other supporting tissue
  • Organic materials in the phloem are transported upwards and downwards within the plant. This movement is called translocation