B2.2 - The challenges of size

• As organisms increase in size, their surface area to volume ratio decreases
• Transport systems in animals

  • Blood and circulatory system
• Transport systems in plants

  • Xylem
  • Phloem
• Exchange surfaces

  • Have a large surface area to increase the rate of transport
  • Have a barrier that is as thin as possible to provide a short diffusion path
  • In animals, have a large network of blood vessels to reduce the distance of exchange and maintain concentration gradients
  • In animals, have gas exchange surfaces that are well ventilated to maintain concentration gradients
• Diffusion distance

  The smaller the distance molecules have to travel, the faster transport will occur
• Concentration gradient

  The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
• Temperature

  The higher the temperature, the faster molecules move as they have more energy, resulting in a faster rate of movement across membranes
• Substances organisms must take in

  • Oxygen
  • Water
  • Dissolved food molecules
  • Mineral ions
  • Carbon dioxide (for plants)
• Waste substances organisms must remove

  • Urea
  • Carbon dioxide
• Circulatory system

  A network of blood vessels connected to the heart and lungs, where gas exchange occurs
• Double circulatory system

  The heart has two circuits - the pulmonary circuit pumping blood to the lungs, and the systemic circulation pumping blood to the body
• Mammals (and birds) have a double circulatory system, but not all animals do - fish don't
• Benefits of a double circulatory system

  • Blood travelling through the small capillaries in the lungs loses a lot of pressure which reduces the speed at which it can flow
  • By returning oxygenated blood to the heart from the lungs, the pressure can be raised before sending it to the body, meaning cells can be supplied with oxygenated blood more quickly
  • This is important for mammals as they require a lot of oxygen
• Capillaries

  • Widen to veins as they move away from the organ to carry deoxygenated blood back towards the heart
• Double circulatory system

  Two circuits joined together, with a pump (the heart) and valves that maintain a one-way flow of blood around the body
• Parts of the double circulatory system

  • Right side of the heart pumps blood to the lungs for gas exchange (pulmonary circuit)
  • Left side of the heart pumps blood under high pressure to the body (systemic circulation)
• Mammals (and birds) have a double circulatory system, but fish don't
• Benefits of a double circulatory system

  • Blood travelling through the small capillaries in the lungs loses a lot of pressure which reduces the speed at which it can flow
  • By returning oxygenated blood to the heart from the lungs, the pressure can be raised before sending it to the body, meaning cells can be supplied with oxygenated blood more quickly
  • This is important for mammals as they require a lot of oxygen to maintain their body temperature
• Heart

  A pumping organ that ensures blood continuously flows around the body
• Parts of the heart

  • Right side receives deoxygenated blood from the body and pumps it to the lungs
  • Left side receives oxygenated blood from the lungs and pumps it to the body
  • Chambers at the top are the atria
  • Chambers at the bottom are the ventricles
  • Valves prevent blood flowing backwards
• Cardiac muscle

  • Never gets tired
  • Contains far more mitochondria than skeletal muscle to provide lots of ATP for energy
  • Beats continuously so needs a constant supply of oxygen (and glucose) for aerobic respiration
  • Has its own blood supply via the coronary arteries which branch off from the aorta
• Pathway of blood through the heart

  1. Deoxygenated blood (from the body) enters the heart via the vena cava, emptying into the right atrium

  2. Oxygenated blood (from the lungs) returns to the heart via the pulmonary vein, emptying into the left atrium

  3. The atria contract, pushing the blood into the ventricles

  4. The right ventricle contracts, forcing blood through the pulmonary artery to the nearby lungs where gas exchange occurs (and the blood becomes oxygenated)

  5. At the same time, the left ventricle contracts, forcing the oxygenated blood through the aorta, out of the heart

  6. The oxygenated blood flows through arteries around the body delivering oxygen to organs and tissues

  7. Once the oxygen has been used up, deoxygenated blood returns to the heart through veins
• Types of blood vessels

  • Arteries - carry blood away from the heart
  • Veins - carry blood towards the heart
  • Capillaries - involved in the exchange of gas of materials with tissues
• Arteries

  • Carry blood at high pressure away from the heart
  • Carry oxygenated blood (except the pulmonary artery)
  • Have a narrow lumen
  • Have thick muscular walls compared to the size of the lumen
  • The strong muscular walls contain elastic fibers to allow them to stretch and spring back
  • Blood flows through at a fast speed
• Veins

  • Carry blood at low pressure towards the heart
  • Have thin walls as the blood is at a lower pressure
  • Have a larger lumen than arteries
  • Contain valves
  • Blood flows through at a slow speed
• Capillaries

  • Very small - too small to be seen with the naked eye
  • Carry blood at low pressure within tissues
  • Have permeable walls that are one cell thick
  • Supply oxygenated blood and nutrients to tissues
  • Take away waste and deoxygenated blood
  • Speed of blood flow is slow
• Red blood cells

  Specialised cells that carry oxygen from the lungs to cells throughout the body
• Structure of red blood cells

  • Small with a 'biconcave disk' shape, which gives them a large surface area to volume ratio to maximise diffusion of oxygen in and out
  • Full of hemoglobin, a protein that binds to oxygen to form oxyhemoglobin
  • Have no nucleus, which allows more space for hemoglobin to be packed in
  • Small size and flexibility allow them to pass through capillaries
• Blood plasma

  A straw-coloured liquid in which the other components of the blood are suspended within
• Substances transported in blood plasma

  • Red blood cells, white blood cells, and platelets
  • Water
  • Carbon dioxide - the waste product of cellular respiration
  • Digested food and mineral ions - absorbed from the small intestine and delivered to cells around the body
  • Urea - the waste substance produced in the breakdown of proteins by the liver
  • Hormones - chemical messengers released into the blood from the endocrine organs (glands) and delivered to target tissues/organs of the body
  • Antibodies - which are special proteins to help fight infection
  • Heat energy - created in respiration (an exothermic reaction), heat energy is transferred to cooler parts of the body or to the skin where heat can be lost
• Plant roots

  • Adapted to maximise absorption of water and mineral ions from the surrounding soil
  • Many epidermal cells have 'hair-like' extensions that project into the soil to increase the surface area for absorption
• Root hair cells

  • Adapted for the efficient uptake of water (by osmosis) and mineral ions (by active transport)
  • Long and thin so they can penetrate between soil particles and absorb water and minerals from the soil
  • Increase the surface area to volume ratio, which increases the uptake of water and mineral ions
  • Have a low water potential due to the high proportion of dissolved minerals and sugars in the cytoplasm
• Transpiration

  The evaporation and diffusion of water from the surfaces of the plant
• Transpiration stream

  • Water travels up the xylem from the roots into the leaves of the plant to replace the water that has been lost due to transpiration
  • Movement in the xylem only takes place in one direction - from roots to leaves
• Role of stomata

  • Allow for gas exchange
  • Formed by two kidney-shaped guard cells which open and close the stomatal pore
  • Open when the availability of water is high, allowing gases to diffuse in and out of the leaf and causing water loss via transpiration
  • Close when less water is available, reducing water loss via transpiration
  • Sensitive to light and open in the day and close during the night, allowing water to be conserved whilst no photosynthesis is occurring
• Translocation

  The transport of the soluble products of photosynthesis (mainly sucrose) in the plant
• Direction of translocation

  • Early spring: sucrose is transported from sources in the root to sinks in the leaves
  • Summer: sucrose is transported from sources in the leaves to sinks in the roots (where it can be used or stored as starch)
• Xylem

  • Transports water and minerals from the roots to the stem and leaves in the transpiration stream
  • Composed of dead cells joined end to end with no end walls between them, creating a hollow tube
  • Cells are strengthened by a woody material called lignin which makes the cell walls strong, waterproof, and stiff
• Phloem

  • Transports nutrients like dissolved sugars (e.g., sucrose) and amino acids both up and down the stem
  • Made of columns of living cells called sieve-tube elements, which are joined end to end and contain sieve plates to allow easy flow of sugars and amino acids
  • Sieve-tube elements are supported by companion cells that carry out the living functions for them
• Environmental factors affecting transpiration rate

  • Air movement
  • Humidity
  • Temperature
  • Light intensity