Topic 1

• Cell theory


  • 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
• Specialist cells


  • RED BLOOD CELL: large surface area for O2 to pass through, contains haemoglobin + no nucleus
  • STRIATED MUSCLE CELL: multiple nuclei, longer and larger than typical cell
• Striated Muscle fibres


  • Muscle cells fuse to form fibres that may be very long (>300mm) 🡪 may have multiple nuclei despite being surrounded by a single, continuous plasma membrane 🡪 challenges the idea that cells always function as autonomous units
• Aseptate fungal hyphae


  • May have filamentous structures called hyphae, separated into cells by internal walls called septa 🡪 some fungi aren't portioned by septa + have continuous cytoplasm along the length of hyphae
• Giant Algae


  • Certain species of unicellular algae may grow to very large sizes 🡪 challenges the idea that larger organisms are always made of many microscopic cells
• Functions of life


  • METABOLISM: undertake essential chemical reactions
  • REPRODUCTION: produce offspring, sexually or asexually
  • SENSITIVITY: responsive to internal and external stimuli
  • HOMEOSTASIS: maintain a stable internal environment
  • EXCRETION: exhibit the removal of waste products
  • NUTRITION: exchange materials + gases with the environment
  • GROWTH: move and change shape or size.
• Paramecium


  • Metabolism: food particles are enclosed within small vacuoles that contain enzymes for digestion
  • Reproduction: Divide asexually (fission) although horizontal gene transfer can occur via conjugation
  • Sensitivity: Paramecia are surrounded by small hairs called cilia which allow it to move
  • Homeostasis: Essential gases enter (e.g. O2) and exit (e.g. CO2) the cell via diffusion
  • Excretion: Solid wastes are removed via an anal pore, while liquid wastes are pumped out via contractile vacoules
  • Nutrition: Paramecia engulf food via a specialised membranous feeding groove called a cytostome
• Scenedesmus


  • Metabolism: Chlorophyll pigments allow organic molecules to be produced via photosynthesis
  • Reproduction: Daughter cells form as non-motile autospores via the internal asexual division of the parent cell
  • Sensitivity: Scenedesmus may exist as unicells or form colonies for protection
  • Excretion/nutrition: Scenedesmus exchange gases and other essential materials via diffusion
• Tissue

  It consists of a group of structurally and functionally similar cells and their intercellular material.
• Organ


  A collection of tissues that structurally form a functional unit specialized to perform a particular function e.g. heart, liver
• Multicellular organism


  Composed of more than one cell, with groups of cells differentiating to take on specialized functions.
• Specialist cells


  Specialist cells have a specific role to perform. Each specialised cell has a different job to do. They have special features that allow them to do these jobs.
• Differentiation


  The formation of a specialist cell from a stem cell. This action is controlled by genes that are switched on in the nucleus.
• Stem cells


  Stem cells are undifferentiated cells. They undergo something called differentiation where certain genes are switched on in a pluripotent cell (stem cell). The stem cell then changes into a specialist cell. The process is irreversible and only happens in multicellular organisms.
• Uses of stem cells

  • Alzheimer's
  • Ligament reconstruction
  • Spinal injuries
• Stargardt's


  • Stargardt's disease is a disease of the eye. It causes recessive genetic mutation causing photoreceptor cells to malfunction. Stem cells are injected into the retina. Cells attach to the retina and become functional.
• Leukaemia


  • Leukaemia is a type of cancer of the blood or bone marrow. Patients undergo chemotherapy and radiotherapy to get rid of diseased cells then stem cells (take from bone marrow or umbilical cord, patient or donor) are transplanted into marrow.
• Embryonic stem cells


  • Almost unlimited growth potential
  • Differentiate into any type in the body
  • Less chance of genetic damage
  • Are risk of becoming tumour cells
  • Likely to be genetically different to the patient
  • Removal of cells from embryo kills it, ethical dilemma
• Umbilical cord stem cells

  • Easily obtained and stored
  • The umbilical cord is discarded regardless
  • Commercial collection and storage are available
  • Limited capacity to differentiate
  • Limited quantity of stem cells from one umbilical cord
• Adult stem cells


  • Fully compatible with the adult's tissue, no rejection problems
  • Removal of stem cells does not kill the adult
  • Less chance of malignant tumours
  • Difficult to obtain
  • Less growth potential
  • Limited capacity to differentiate into different cells
• Eukaryotic cells


  These types of cells contain a large, obvious nucleus. They include cells of plants, animals, fungi, and protist (e.g. algae. The surrounding cytoplasm contains many different membranous organelles.
• Animal Cell


  A type of eukaryotic cell that lacks a cell wall and has a true, membrane-bound nucleus along with other cellular organelles.
• Plant Cell


  Plant cells are eukaryotic cells with a true nucleus along with specialized structures called organelles that carry out certain specific functions.
• Prokaryotic cells


  These cells contain no true nucleus and their cytoplasm does not have the organelles of eukaryotes. They are bacterial cells.
• Binary fission


  1. Cell elongates + DNA is replicated

  2. Cell wall + membrane begin to divide

  3. Cross-wall forms completely around the divided DNA

  4. Cells separate
• Magnification


  Magnification is the ability to make small objects seem larger.
• Resolution


  The ability to tell that two objects that are very close together are distinct objects rather than just one. The amount of detail that can be seen.
• Light microscope


  Light passes through the specimen from below. Regions that are denser in the specimen let less light pass through, thus creating the image.
• Scanning electron microscope


  A beam of electrons is directed to the specimen. Electrons bounce off the surface of the specimen creating a 3D image on the computer screen. This must be done in a vacuum (no living specimens)
• Transmission electron microscope


  A beam of electrons is sent to a thin specimen. The electrons pass through the specimen creating an image of inside the cell on the computer screen. This must be done in a vacuum (no living specimens)
• Phospholipids


  Phospholipid is a molecule with two fatty acid tails. It has a hydrophobic end and hydrophilic end.
• Phospholipid bilayer


  When mixed with water the tails on the phospholipids repel from the water as they are more attracted to each other, but the heads are attracted to the water. This forms two layers of phospholipids this is called a phospholipid bilayer.
• Role of cholesterol


  Cholesterol is amphipathic (attracted to both polar and non-polar molecules). This means that it can interact with both the polar phosphate heads and the non-polar fatty acid tails. It acts as a buffer of membrane fluidity, so that the fluidity is always at a good level for the cells.
• Davson and Danielli


  • 1935: A model whereby two layers of protein flanked a central phospholipid bilayer. Dark segments seen under EM were wrongly identified as two protein layers. Proved wrong with fluorescent antibody tagging and freeze etching.
• Singer and Nicolson


  • 1972: Purposed proteins were embedded within the liquid bilayer rather than sperate layers. Known as the fluid-mosaic model, remains the preferred model today.
• Functions of membrane proteins

  • Junctions – Serve to connect and join two cells together
  • Enzymes – Fixing to membranes localises metabolic pathways
  • Transport – Responsible for facilitated diffusion and active transport
  • Recognition – May function as markers for cellular identification
  • Anchorage – Attachment points for cytoskeleton and extracellular matrix
  • Transduction – Function as receptors for peptide hormones
• Structure of membrane proteins

  • Non-polar amino acids associate directly with the lipid bilayer
  • Polar amino acids are located internally and face aqueous solutions.
• Diffusion


  Movement of particles from higher to lower concentration through the phospholipid bilayer. Movement is passive (i.e. no direct energy needed).
• Osmosis


  The diffusion of water molecules across a partially permeable membrane, from lower to higher solute concentration. Movement is passive.
• Passive transport


  No direct energy needed.