Bio unit 2

• The intestine has increased surface area in the form of folds, ridges, and a large intestine that allows for absorption of nutrients.
• Amphibians:

  • Inactive amphibians use moist skin for gas exchange
  • Active amphibians use lungs
  • Surface is highly folded to increase surface area
• Reptiles:

  • Reptilian skin impermeable to gases and can't be used as a respiratory surface
  • More efficient lungs than amphibians
  • Gas exchange exclusively in the lungs
  • Reptiles have ribs but no diaphragm
  • Ventilation aided by movement of the ribs by the intercostal muscles
• Birds:

  • Warm-blooded
  • High respiratory rate for efficient gas exchange
  • Lungs small and compact made of numerous branching air tubes (bronchi)
  • Gas exchange occurs in the smallest air tubes (parabronchi) with extensive blood capillary network
  • Ventilation of lung brought about by movement of the ribs
  • Flight - action of wing muscles ventilates the lungs
• Insects:

  • Terrestrial, live on land
  • Water evaporates from the body surface causing dehydration
  • Gas exchange occurs through spiracles running along the side of the body
  • Spiracles lead into a system of branched, chitin-lined air tubes (tracheae)
  • Ventilation of tracheal system through compression and expansion of the abdomen
  • Gas exchange takes place at the end of tracheoles
  • Muscle fibres connected to tracheoles never exceed 20 micrometres in diameter for rapid diffusion
  • Fluid levels in tracheoles decrease during flight for more surface area for gas exchange
• Human respiratory system:

  • Lungs enclosed in the thorax
  • Trachea transports air to bronchi, then bronchioles, and finally alveoli for gas exchange
  • Lungs not muscular, need ventilation mechanism
  • Ventilation by negative pressure breathing
  • Gas exchange occurs in alveoli with extensive capillary network
  • Surfactant prevents alveoli collapse
• Plants:

  • Main gas exchange surface is the leaf
  • Leaf blade thin and flat with large surface area
  • Waxy cuticle reduces water loss
  • Stomata allow gas exchange
  • Adaptations for photosynthesis include large surface area, orientation towards the sun, and transparent cuticle and epidermis
• Roots:

  • Water and ions absorbed mainly through root hair cells
  • Functions of main root tissues: epidermis, cortex parenchyma, endodermis, pericycle, xylem, phloem, cambium
  • Mycorrhizae association increases surface area for absorption
• Stem:

  • Functions of stem tissues: cuticle, epidermis, collenchyma, cortex parenchyma, pith parenchyma, sclerenchyma, xylem, phloem, cambium
  • Xylem transports water and ions, phloem transports products of photosynthesis
• Xylem:

  • Transports water and ions from roots to the rest of the plant
  • Contains xylem vessels, tracheids, and xylem parenchyma
  • Water enters and leaves xylem vessels through pits
• Phloem:

  • Transports organic molecules in plants
  • Contains sieve tubes and companion cells
  • Sieve tubes have little cytoplasm and few organelles
  • Companion cells support sieve tubes and aid in transport
• Phloem sieve tube cells contain numerous mitochondria, rough ER, and a large dense nucleus
• Companion cells and sieve tube elements are connected via plasmodesmata
• End walls of phloem sieve tube cells are perforated by small pores called sieve plates
• Strands of cytoplasm pass through these pores from one phloem sieve tube element to the next
• Vascular bundles contain xylem, phloem, cambium, and other cells
• Cambium is a meristematic tissue that can keep dividing by mitosis
• Xylem cells include vessels and tracheids, while phloem cells include sieve tube elements and companion cells
• Xylem cells are dead, while phloem cells are alive
• Xylem cell walls are thick and contain lignin, while phloem cell walls are thin and contain cellulose
• Xylem cell walls are impermeable, while phloem cell walls are permeable
• Xylem cells have no cytoplasm, while phloem cells have cytoplasmic strands
• Xylem transports water and mineral ions, while phloem transports products of photosynthesis like sucrose and amino acids
• Direction of transport in xylem is from roots upwards, while in phloem it is to and from sites of photosynthesis/storage to growing regions and sites of storage
• Water uptake by roots can occur through apoplast, symplast, and vacuolar routes
• Ions are absorbed by plant cells through active transport and co-transport mechanisms
• Root pressure can push water up the xylem against gravity
• Cohesion-tension theory explains water movement in xylem
• Transpiration is the main force that pulls water into roots and up stems
• Factors affecting rate of water uptake in plants include temperature, humidity, wind speed, and light intensity
• Hydrophytic adaptations include floating leaves and stems for plants living in water
• Adaptations of hydrophytes include:

  • Floating leaves and stems
  • Large air-filled cavities in leaves and/or stems enable plants to float and act as reservoirs of oxygen/carbon dioxide
  • Increased leaf surface area for gas exchange and photosynthesis
  • Lack of protective tissues like waxy cuticles to reduce water loss
  • Roots are usually reduced in size and act mainly to anchor the plant
• Translocation is the process of moving the products of photosynthesis from where they are made or stored to other parts of the plant

  • Carried out by the phloem
  • Mechanism of phloem transport is not well understood
  • Mass flow hypothesis (pressure flow hypothesis) proposed by Ernst Munch in 1930
• Details of how scientists believe the mass flow hypothesis works in plants:

  • Glucose produced during photosynthesis is converted into sucrose
  • Sucrose is passed into phloem sieve tubes through various routes
  • Increased sucrose concentration in sieve tubes reduces Ψ of sieve tube contents
  • Water moves into sieve tubes from xylem through osmosis
  • Roots and growth points act as sinks where sucrose is unloaded and converted into glucose/starch/other carbohydrate
• Alternative theories to explain translocation:

  • Electro-osmosis: sieve plates become charged due to movement of water and ions, attracting/repelling substances
  • Cytoplasmic streaming: strands of cytoplasm move within cells and in different directions
  • Protein contraction/peristalsis: protein microtubules in sieve tubes contract and push cytoplasm along in different directions
• The lifecycle of a potato plant:

  • Leaves act as source in summer
  • Potato tubers act as sink
  • Buds grow in autumn/winter and become sink
  • Shoots grow in spring, tubers are source of energy/nutrients until leaves develop for photosynthesis
  • All parts of plant are sink during summer growth
  • Tubers act as source at night
• Investigating translocation:

  • Ringing experiment: removing bark and phloem from stem affects transport
  • Use of radio-isotopes: exposing leaves to radioactive CO2 to track transport
  • Use of aphids: feeding on phloem contents to analyze composition
• Circulatory system in animals:

  • Closed circulation system: blood travels through vessels with heart as pump
  • Open circulation system: blood bathes all cells and organs, no red blood cells to transport oxygen
  • Single circulation: blood passes through heart once in each circulation, found in fish
  • Double circulation: blood passes through heart twice in one circulation, found in humans and mammals
• Blood vessels and heart structure:

  • Arteries take blood away from the heart
  • Veins take blood into the heart
  • Capillaries are site of gas exchange and tissue fluid formation
  • Arteries have thick tunica externa, muscle, and elastic tissue to carry blood at high pressure
  • Arterioles constrict and dilate to control blood flow to capillaries
  • Capillaries consist of a single layer of endothelial cells
• Capillaries:

  • Consist of a single layer of endothelial cells and are a tissue rather than an organ
  • Site of gas exchange with a short diffusion path
  • Capillary beds have a massive surface area for diffusion
  • Pressure is lowered as blood passes through capillaries due to greater cross-sectional area compared to arterioles
  • Capillaries are narrow, leading to greater resistance and slower blood flow, allowing more time for gas exchange
  • Red blood cells have to bend to squeeze through capillaries