Specialised cells

• All multicellular organisms need specialised cells because not all the cells in the organism will have the same access to all the required resources. Without specialization, the organism would not act as one coordinated organism but as a collection of independent cells instead.
• Plant stem cells

  Located in the meristems of the plant. Their function is to differentiate into all types of different specialised plant cells including root hair cells and palisade mesophyll cells. The stem cells can differentiate into any type of plant cell at any time during the life of the plant. They help enable continuous growth of the plant throughout its life.
• Animal stem cells

  Help specific tissue with regeneration and repair. Rather than being able to differentiate into any type of animal cell, animal stem cells tend to be specific to the tissue so blood stem cells can only differentiate into blood cells.
• Function of root hair cells

  Absorb water from the soil for use in photosynthesis and other metabolic reactions. They also absorb minerals required by the plant e.g. magnesium, nitrogen, sulfur, phosphorus and potassium.
• Root hair cell

  • Long hair-like extension to increase surface area for absorption of water and mineral ions
  • Thin cell wall to ensure a short diffusion pathway for water and mineral ions
  • Large numbers of mitochondria needed for active transport of solutes into the cell sap to lower the water potential
• Membrane permeability of root hair cell

  The membrane is selectively permeable to allow water molecules and mineral ions to enter the cell easily. This is achieved by the presence of specific channels in the membrane which allow the molecules to diffuse into the cell from high concentrations (in the soil) to low concentrations (in the cell). This process is known as facilitated diffusion due to the use of the channel proteins in the membrane.
• Movement of water into root hair cell

  1. Water enters the cell sap by osmosis through the tonoplast membrane.

  2. Solutes e.g. NaCl moved by active transport into cell sap to lower its water potential and draw water in. Energy is needed for this process which is why there are a lot of mitochondria in root hair cells.
• Osmosis

  The movement of water through a partially permeable membrane from an area of high concentration of water to an area of low concentration of water.
• Water potential

  A measure of how easily can move in relation to pure water. Pure water has a water potential of zero. When solutes are added to water, it lowers the water potential to a negative value. The more solutes dissolved in the water, the lower the water potential. The lower the water potential the more readily water moves into the solution by osmosis.
• Root hair cells do not have chloroplasts as they are not needed and it gives more room for water storage and the many mitochondria needed.
• Function of palisade mesophyll cells

  Palisade mesophyll are the cells where most photosynthesis takes place. The purpose of photosynthesis is to capture the energy from sunlight to build molecules such as glucose from carbon dioxide and water.
• Adaptations of palisade mesophyll cells

  • Cell wall is clear to allow light in more easily for photosynthesis. They also keep the cell rigid, so the leaf is kept at right angles to sun.
  • Chloroplasts can move within the cell using the cytoskeleton so more will migrate towards the surface of the leaf during dull weather but move away to avoid damage if the sun is bright.
  • The cells are packed closely together to provide a continuous layer for photosynthesis.
  • Large vacuole helps keep the cell rigid by maintaining turgor pressure so the leaf remains at the optimal position in relation to the sun. The vacuole also pushes the chloroplasts closer to the edges to absorb more sunlight.
  • Palisade mesophyll cells are close to the surface of the leaf to allow a short diffusion pathway for carbon dioxide.
  • There are large number of chloroplasts in palisade mesophyll cells.
• Haploid nucleus

  The sperm and egg contain only half the number of chromosomes (23 in humans) and no pairs. This means that when the sperm fertilises the egg, the resulting zygote will have the correct number of chromosomes.
• Acrosome

  A specialized vesicle containing enzymes. The enzymes are used to break down the membrane of the egg to allow the sperm to fertilise the egg.
• Sperm cell

  • Mitochondria are arranged in a spiral pattern in the mid-section to provide the energy needed for the sperm to swim from cervix to fallopian tube to fertilise the egg.
  • The undulipodium (tail or flagellum) is required for mobility.
• Zona pellucida

  A special coat outside of the egg cell membrane made up of glycoprotein filaments. After one sperm has entered the egg, it becomes impermeable preventing other sperm from entering (polyspermy). After fertilisation, the zona pellucida protects the developing embryo until it is about to implant into the endometrium – this is called hatching.
• Corona radiata

  Consists of two or three layers of cells from the follicle that are attached to the zona pellucida. Its main function is to supply vital proteins to the egg cell.
• Egg cell

  • Contains a large amount of cytoplasm and associated organelles to provide nutrients that support the development of the developing zygote after fertilisation.
  • Large numbers of mitochondria are needed to provide the energy required after fertilisation.
• Red blood cells carry oxygen from the lungs to wherever it is needed in the body. The oxygen is needed for cellular respiration to provide the energy for the cells' needs.
• Red blood cells make up 45% of the blood by volume. There is estimated to be about 5 million red blood cells per ml of blood. The large numbers of red blood cells means that the blood can carry more oxygen.
• Red blood cell

  • Does not contain a nucleus, mitochondria, rough or smooth ER and mitochondria, allowing more room to pack in haemoglobin molecules.
  • Biconcave shape allows a large surface area to volume ratio so that more oxygen can be absorbed into the red blood cell.
  • The cell membrane is very thin to allow a short diffusion pathway for oxygen (or carbon dioxide) to enter the cell. It also gives the cell flexibility to allow it to squeeze through narrow capillaries.
• Haemoglobin

  A complicated protein made up of 4 sub-units each with an iron ion. Each haemoglobin can combine with up to four oxygen molecules. When it is combined with oxygen it is called oxyhaemoglobin. Carbon dioxide can also bind with the haemoglobin molecule to become carboxyhaemoglobin.
• Neutrophils

  The most common type of white blood cell. They are involved in non-specific response against infection. They destroy invading microorganisms but do not give long-term immunity.
• Neutrophils

  • Lobed nucleus takes up less room and is flexible allowing the neutrophil to be able to squeeze through small spaces such as through the capillary walls.
  • Migrate towards pathogens by chemotaxis – due to the chemicals they give out. They can squeeze out of the blood capillaries and enter the tissues.
  • Can engulf pathogens using the process of phagocytosis. The pathogens will end up inside the neutrophil inside a vacuole where it can be destroyed by the neutrophil.
  • Contain a lot of lysosomes – an organelle containing destructive enzymes. These are used to destroy the pathogens that have been engulfed during phagocytosis.
• T lymphocytes


  Made in the bone marrow but mature in the thymus gland. They are involved in specific immunity.
• B lymphocytes


  Made and mature in the bone marrow. They are also involved in specific immunity. They produce antibodies.
• How T lymphocytes work

  1. T lymphocytes detect cancerous cells or pathogens with the antigen receptor.

  2. Helper T cells release cytokines which regulate other white blood cells.

  3. Cytotoxic T cells release toxins which kill cells that are cancerous.

  4. T cells can produce memory cells which can respond more quickly to a particular pathogen if it encounters it again.
• How B lymphocytes work

  1. B lymphocytes work by producing antibodies which are specific to particular pathogens. Antibodies do not directly kill pathogens. They can cause pathogens to clump together because each antigen can combine with more than one pathogen. This makes them easier for T cells and other white blood cells to find and destroy. Antigens also bind to viruses and prevent them from entering the cells.

  2. When a specific infection is encountered, the specific B cell will divide rapidly into plasma cells which produce the antibodies and memory cells which speed the response to future infections by the same pathogen.

  
• Photosynthesis

  The process by which plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose or other sugars. It occurs in the chloroplasts of plant cells, and it requires carbon dioxide and water as raw materials. It produces glucose and oxygen.