1.1 - Introduction to cells

• Cell theory states that:

  • All living things are composed of cells (or cell products)
  • The cell is the smallest unit of life
  • Cells only arise from pre-existing cells
• Cells only arise from pre-existing cells:

  • Cells multiply through division
  • All life evolved from simpler ancestors
  • Mitosis results in genetically identical diploid daughter cells
  • Meiosis generates haploid gametes (sex cells)
• Striated muscle

  • Challenges the idea that a cell has one nucleus.
  • Muscle cells have more than one nucleus per cell.
  • Muscle Cells called fibres can be very long (300mm).
  • They are surrounded by a single plasma membrane but they are multi-nucleated (many nuclei).
  • This does not conform to the standard view of a small single nuclei within a cell.
• Aseptate fungal hyphae

  • Challenges the idea that a cell is a single unit.
  • Fungal hyphae are again very large with many nuclei and a continuous cytoplasm.
  • The tubular system of hyphae form dense networks called mycelium.
  • Like muscle cells they are multi-nucleated.
  • They have cell walls composed of chitin.
  • The cytoplasm is continuous along the hyphae with no end cell wall or membrane.
• Giant algae (Acetabularia)

  • Acetabularia is a single-celled organism that challenges both the idea that cells must be simple in structure and small in size.
  • Gigantic in size (5 – 100mm).
  • Complex in form, it consists of three anatomical parts:

  1. Bottom rhizoid (that resembles a set of short roots)
  2. Long stalk
  3. Top umbrella of branches that may fuse into a cap.

  • The single nucleus is located in the rhizoid.
• Metabolism - the web of all the enzyme-catalysed reactions in a cell or organism, e.g. respiration

  Response - Living things can respond to and interact with the environment

  Homeostasis - The maintenance and regulation of internal cell conditions, e.g. water and pH

  Growth - Living things can grow or change size / shape

  Excretion – the removal of metabolic waste

  Reproduction - Living things produce offspring, either sexually or asexually

  Nutrition – feeding by either the synthesis of organic molecules (e.g. photosynthesis) or the absorption of organic matter
• Metabolism in Paramecium occurs mostly in the cytoplasm.
• The response of Paramecium involves the wave action of the cilia moving the paramecium in response to changes in the environment.
• Homeostasis in Paramecium involves the contractile vacuole filling up with water and expelling it through the plasma membrane to manage the water content.
• Growth in Paramecium occurs after consuming and assimilating biomass from food, until the paramecium divides.
• The nucleus in Paramecium can divide to support cell division by mitosis.
• Excretion in Paramecium is controlled by the plasma membrane, controlling the entry and exit of substances.
• Nutrition in Paramecium involves food vacuoles containing organisms the paramecium has consumed.
• Nutrition in Chlorella involves photosynthesis happening inside the chloroplasts to provide the algae with food
• Magnification=Image size/Actual size
• Benefits of Large SA:Vol ratio:

  • Shorter Diffusion pathways
  • Concentration gradients are easier to generate
• Disadvantages of large SA:Vol ratio:

  • Small, warm blooded animals lose heat very quickly and need to eat constantly
• Desert plants would lose water quickly with flat leaves

  • They minimise SA:Vol ratio to conserve water
  • CAM plants change their metabolism to save water
• Cells and tissues specialised for gas or material exchange will increase their surface area to optimise the transfer of materials, e.g. microvilli in the small intestine
• The cell must consequently divide in order to restore a viable SA:Vol ratio and survive.
• Cells divide as two small cells are more efficient than one large cell, allowing for cell differentiation, specialised functions and more complex multicellular life.
• Cells compartmentalise, using membranes to carry out metabolic processes, such as in organelles like mitochondrion, maximising the surface area for reactions.
• Some organs, like the intestines, fold up, making absorption of food molecules more efficient.
• Alveoli in the lungs are thin membranes that maximise the surface for gas exchange.
• Roots are long and branched, with root hairs on the cells, to maximise the surface area for water uptake.
• • All (diploid) cells of an individual organisms share an identical genome
  • Each cell contains the entire set of genetic instructions for that organism
  • BUT not all genes are expressed (activated) in all cells
  • In (totipotent) embryonic stem cells the entire genome is active
  • Newly formed cells receive signals which deactivate (or more rarely activate) genes, e.g. a skin cell does not need to be able to produce haemoglobin (the pigment in red blood cells that carries oxygen)
• • In humans 220 distinct highly specialised cell types have been recognised
  • All specialised cells and the organs constructed from them have developed as a result of differentiation
• Totipotent:

  Can differentiate into any type of cell.
• Pluripotent:

  Can differentiate into many

  types of cell.
• Multipotent:

  Can differentiate into a few closely-related types of cell.
• Unipotent:

  Can regenerate but can only differentiate into their associated cell type

  (e.g. liver stem cells can only make liver cells).  
• Stargardt's macular dystrophy:

  • Affects 1 in 10000 children
  • Recessive genetic condition
  • Mutation causes an active transport protein on photoreceptor cells to malfunction
  • The photoreceptor cells degenerate
  • Results in production of a dysfunctional protein that cannot perform energy transport
  • Causes progressive, and loss of central vision
• Stargardt's macular dystrophy treatment:

  • Embryonic stem cells are treated to divide and differentiate to become retinal cells
  • Retinal cells are injected into retina
  • Retinal cells attach to retina and become functional
  • Central vision improves as a result of more functional retinal cells
• Leukemia:

  • Cancer of the blood or bone marrow
  • Results in abnormally high levels of poorly functioning white blood cells
• Leukemia treatment:

  • Hematopoetic stem cells are harvested from bone marrow, peripheral blood or umbilical cord blood
  • Chemotherapy and radiotherapy used to destroy diseased white blood cells
  • New white blood cells need to be replaced with healthy cells
  • HSCs are transplanted back into the bone marrow
  • HSCs differentiate to form new healthy white blood cells
• Use of a patient's own HSCs mean there is far less risk of immune rejection than with a traditional bone marrow transplant
• Embryo stem cells:

  • Can be obtained from excess embryos generated from IVF programs
  • Only obtained by destruction of embryo
  • Almost unlimited growth potential
  • Higher risk of tumour development
  • Can differentiate into any cell type
  • Less chance of genetic damage than adult cells
  • Compatibility - not genetically identical to patient
• Cord blood stem cells:

  • Limited quantities but easily obtained and stored
  • Umbilical cord discarded anways
  • Reduced growth potential
  • Lower risk of tumour development
  • Limited capacity to differentiate
  • Less chance of genetic damage than adult cells
  • Fully compatible with patient as stem cells are genetically identical
• Adult stem cells:

  • Difficult to obtain - very few and buried deep in tissues
  • Adult patient permission to extract cells
  • Reduced growth potential
  • Lower risk of tumour development
  • Limited capacity to differentiate
  • Genetic damage can occur through mutations through life of adult
  • Fully compatible