B3 Tissues, Organs & Organ Systems

• Cells are the basic building blocks that make up all living organisms. These cells are usually specialised to carry out a specific function. These specialised cells form tissues, which form organs, which form organ systems.
• Similar Cells are Organised into tissues. A tissue is a group of similar cells that work together to carry out a specific function.
• Tissues are organised into organs. An organ is a group of different tissues that work together to perform a specific function.
• Organs are organised into organ systems. An organ system is a group of organs working together to perform a particular function.
• Lungs:

  The air that you breathe in goes through the trachea. This splits into two tubes called bronchi, one going to each lung. The bronchi split into progressively smaller tubes called bronchioles. The bronchioles finally end up at small bags called alveoli where gas exchange takes place.
• Lungs:

  The lungs contain millions and millions of little air sacs called alveoli, surrounded by a network of blood capillaries. The blood passing next to the alveoli has just returned to the lungs from the rest of the body, so it contains lots of carbon dioxide and very little oxygen. Oxygen diffuses out of the alveolus into the blood. Carbon dioxide diffuses out of the blood into the alveolus to be breathed out.
• Lungs:

  When the blood reaches body cells, oxygen is released from the red blood cells and diffuses into the body cells. At the same time, carbon dioxide diffuses out of the body cells into the blood.
• Plants are made of organs: stems, roots and leaves.
• Plant organs are made out of tissues such as:

  • Epidermal tissue - covers the whole plant
  • Palisade mesophyll tissue - where most photosynthesis happens
  • Spongy mesophyll tissue - contains big air spaces in the leaf to allow gases to diffuse in and out of cells
  • Xylem and phloem - transport things like water, mineral ions and food around the plant
  • Meristem tissue - found at the growing tips of shoots and roots and can differentiate into lots of different types of plant cells.
• The Leaf is an Organ made up of Several Types of Tissue:

  • Epidermal tissues covered with a waxy cuticle to reduce water loss by evaporation
  • Upper epidermis is transparent so light can pass through to the palisade layer
  • The palisade layer has lots of chloroplasts so they are where the leaf gets the most light.
  • The xylem and phloem form a network of vascular bundles, which deliver water and other nutrients to the entire leaf and take away glucose produced by photosynthesis. they also help support the structure.
  • The tissues of leaves are also adapted for efficient gas exchange.
• Phloem Tubes Transport Food:

  • Made of columns of elongated living cells with small pores in the end walls to allow cell sap to flow through.
  • They transport food substances made in the leaves to the rest of the plant for immediate use or storage.
  • The transport goes in both directions.
  • This process is called translocation.
• Xylem Tubes Take Water Up:

  • Made of dead cells joined end to end with no walls between them. Strengthened with lignin.
  • Carry water and mineral ions from the roots to the stem and leaves.
  • Movement of water from the roots, through the xylem and out of the leaves is called the transpiration stream.
• Transpiration is the Loss of Water from the Plant:

  • Caused by evaporation and diffusion of water from a plant’s surface.
  • Creates a slight shortage of water in the leaf so more water is drawn up from the rest of the plant through the xylem vessels to replace it.
  • This means more water is drawn up from the roots and so there is a constant transpiration stream of water.
• Transpiration Rate is affected by:

  • Light Intensity
  • Temperature - When it's warm, the water particles have more energy to evaporate and diffuse out of the stomata.
  • Air Flow - If airflow around a leaf is poor, the water vapour just surrounds the leaf and doesn’t move away. This means there’s a high concentration of water particles outside the leaf as well as inside it so diffusion doesn’t happen.
  • Humidity - If the air is humid, there is a lot of water in it already so there is not a large concentration gradient.
• Guard Cells are adapted to open and close stomata:

  • Kidney shape which opens and closes the stomata.
  • When the plant has lots of water, the guard cells fill with it and go plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesis.
  • When a plant is short of water, the guard cells lose water and become flaccid making the stomata close. This helps stop too much water vapour from escaping.
• Guard Cells are adapted to open and close stomata:

  • Thin outer walls and thickened inner walls make the opening and closing work.
  • Sensitive to light and close at night to save water without losing out on photosynthesis.
  • More stomata on the undersides of leaves than on the top. The lower surface is shaded and cooler so less water is lost.
• Humans have a double circulatory system - two circuits joined together.

  In the first one, the right ventricle pumps deoxygenated blood to the lungs to take in oxygen. The blood then returns to the heart.

  In the second one, the left ventricle pumps oxygenated blood around all the other organs of the body. The blood gives up its oxygen at the body cells and the deoxygenated blood returns to the heart.
• The circulatory system is made up of the heart, blood vessels and blood.
• The heart also has valves to make sure that blood flows in the right direction.
• Arteries - carry blood away from the heart:

  • The heart pumps out blood at high pressure so artery walls are strong and elastic.
  • The walls are thick compared to the size of the lumen.
  • They contain thick layers of muscle to make them strong and elastic fibres to allow them to stretch and spring back.
• Capillaries - involved in the exchange of materials at the tissues:

  • They have permeable walls so substances can diffuse in and out
  • Their walls are only one cell thick. This increases the rate of diffusion by reducing the distance.
• The heart contracts to pump blood around the body. It is a pumping organ. It contracts its chambers to force blood into areas e.g. into the pulmonary artery.
• Veins - carry the blood to the heart:

  • Walls don’t need to be as thick as the blood is at lower pressure.
  • Large lumen to help the blood flow despite the lower pressure.
  • Valves to keep the blood flowing in the right direction.
• Red Blood Cells:

  • Job is to carry oxygen from the lungs to all the cells in the body.
  • Biconcave disc shape so it has a large surface area for absorbing oxygen.
  • Don’t have a nucleus so they have more room for oxygen.
  • Contains haemoglobin, which binds to oxygen in the lungs to become oxyhaemoglobin. The reverse happens in the cells.
• White Blood cells:

  • Job is to defend against infection.
  • Performs phagocytosis and produces antibodies to fight microorganisms as well as antitoxins to neutralise the toxins produced by a microorganism.
  • They do have a nucleus.
• Platelets:

  • Job is to help blood clot.
  • Small fragments of cells with no nucleus.
  • They help the blood to clot at a wound to stop blood pouring out and microorganisms from getting in.
• Plasma:

  • Job is to carry everything in the blood.
  • It carries:
  • Red and white blood cells and platelets
  • Nutrients like glucose and amino acids (soluble products of digestion).
  • Carbon dioxide from organs to lungs
  • Urea from liver to kidneys
  • Hormones
  • Proteins
  • Antibodies and antitoxins