Cell Biology

• Chromosomes

  Molecules of DNA found in the nucleus of cells, each carrying a large number of genes
• Genes

  Sections of DNA in chromosomes that encode (produce) many different proteins
• What are the 2 types of cells?

  Prokaryotic and eukaryotic.
• Properties of Prokaryotes

  • Plasmids — one or more small rings of DNA, which can replicate (have copies made) and move between cells so that genetic information can be shared.
  • Single DNA loop — Prokaryotic cells do not have a nucleus (where DNA is stored). Instead, most of their genetic material is stored in a single DNA loop in the cytoplasm
  • No mitochondria or chloroplasts — Prokaryotic cells do NOT contain mitochondria (where respiration takes place) or chloroplasts (where photosynthesis takes place)
• Properties of Eukaryotes

  • Cell membrane — The cell membrane separates the interior of the cell from the environment outside. The cell membrane is selectively permeable.
  • Cytoplasm — The cytoplasm is a jelly-like fluid located outside of the nucleus, and is where most of the cell's chemical reactions take place.
  • Nucleus — The nucleus contains the genetic material, in the form of chromosomes.
• Sub-cellular structures missing from prokaryotic cells that are found in eukaryotic cells:

  1. Nucleus
  2. Mitochondria
  3. Chloroplasts
• All eukaryotic cells contain a cell membrane, cytoplasm and a nucleus. Prokaryotic cells do NOT have a nucleus. Instead, their genetic material is stored in a single DNA loop in the cytoplasm.
• The cell cycle is the series of events involved in cell growth and cell division. It involves mitosis (a type of cell division), which allows for the growth, repair and asexual reproduction of cells
• Stages of the cell cycle

  • Initial growth stage
  • Mitosis
  • Cell Division
• Initial growth stage

  • Extra ribosomes, mitochondria and other sub-cellular structures are produced.
  • The cell’s chromosomes (which are made of DNA) are replicated (copied) so that there are two sets of the cell’s chromosomes.
• Mitosis

  • The two sets of chromosomes are pulled to opposite ends of the cell.
  • Then, the nucleus divides into two
• Cell division

  • The cytoplasm and cell membranes are divided resulting in the production of two identical cells.
• Mitosis

  • Mitosis ensures that both daughter cells have the same chromosomes as each other and the parent cell.
  • This is important for processes that require identical cells to be produced.
  • For example, the growth and repair of tissues and asexual reproduction.
• Stem Cells

  Stem cells are undifferentiated cells that have not yet specialised to perform a specific function. They can create more stem cells or differentiate (become another type of body cell) to perform a function
• Where can stem cells be found?

  • Embryos
  • Bone marrow
  • Plant meristems
• Stem cells in embryos:

  • The stem cells in embryos can differentiate into most cell types, to produce all of the cell types that will make up the organism.
• Stem cells in bone marrow:

  • In human adults, stem cells can be found in bone marrow (as well as other tissues and organs).
  • Adult stem cells differentiate into fewer cell types than stem cells in embryos. They are used to replace dying cells and damaged tissues.
• Stem cells in plant meristem:

  • Plant stem cells are found in the meristem tissue.
  • Plant stem cells can differentiate into all types of plant cell throughout the life of the plant. This allows plants to grow for their whole lives.
• Uses of stem cells

  • Stem cell treatment — Stem cells may be able to replace damaged cells in the body.
  • For example, stem cells may replace the damaged cells that cause diabetes or paralysis.
• Uses of stem cells

  • Plant clones — Plant stem cells can help us quickly and cheaply produce cloned plants.
  • This could help to protect rare plant species from extinction and create large populations of plants with special features, e.g disease resistance.
• Uses of stem cells

  • Therapeutic cloning — a process that produces an embryo with genes that are identical to the patient's.
  • Stem cells taken from this embryo will have the same DNA as the patient. This means that the patient's body will not reject the stem cells or body cells made from the embryo's stem cells.
• Disadvantages of stem cells

  • Viral infections — Stem cell transplantation could transfer viral infections.
  • Ethical beliefs — Some people disagree with using stem cells on ethical or religious grounds:
  • They believe that life begins at conception, which means that the embryo is alive.
  • Therefore, they view the use of embryonic stem cells as 'killing' an embryo.
• Diffusion

  Diffusion is the random movement of particles through a gas or liquid from high concentration to a low concentration.
• Factors affecting Diffusion

  • Concentration gradient — This is linked to the difference in concentration between two areas.
  • The bigger the difference in concentration between two areas, the greater the concentration gradient and the faster the rate of diffusion.
  • Temperature — The higher the temperature, the faster the rate of diffusion
  • Membrane surface area — The larger the surface area of the membrane that a substance is diffusing through (e.g. the membrane around a cell), the faster the rate of diffusion
• Surface Area to Volume ratio

  The higher the surface area to volume ratio, the higher the rate of diffusion
• Exchange Surfaces

  Exchange surfaces are surfaces that are adapted to maximise the efficiency of gas and solute (a substance dissolved in a liquid) exchange across them
• Adaptations of Exchange Surfaces

  • Large surface area — This allows more of a substance to diffuse at the same time
  • Thin membrane — reduces the diffusion distance
  • Ventilation — In animals, if a gas is exchanged, the surface is ventilated to maintain a high concentration gradient and increase the rate of exchange
• Adaptations of Exchange surfaces

  • Blood supply — Where substances are exchanged through blood in animals, exchange surfaces are densely packed with blood vessels:
  • These blood vessels replenish the blood supply to maintain a high concentration gradient by bringing in new blood as diffusion starts to even out the concentrations.
• Examples of Exchange Surfaces

  • Small intestine — adapted for exchanging nutrients between digested food in the small intestine and the blood.
  • Lungs — adapted for exchanging carbon dioxide and oxygen between the blood and air.
  • Gills (in fish) — adapted for exchanging oxygen that is dissolved in water, with the carbon dioxide in a fish's bloodstream
  • Roots — adapted for taking up both water and minerals from the soil around them
  • Leaves — adapted for exchanging carbon dioxide and oxygen between the leaves and the surrounding air
• Osmosis

  Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution (high concentration of water) to a concentrated solution (low concentration of water).
• Partially permeable membrane

  A partially permeable membrane allows water through, but won't let larger molecules dissolved in water pass through
• Water movement

  • Water will move to make the concentrations the same on both sides of the membrane: 
  • When there are lots of water molecules (in a dilute solution) on one side of a partially permeable membrane, but not many on the other side (in a concentrated solution), water will move from the dilute to the concentrated solution.
• Net movement of water

  • It is important to remember that water molecules will move through the membrane in both directions, but the net (overall) movement of water will be from the dilute solution to a concentrated solution.
• Active Transport


  • Sugar absorption in human gut — Active transport allows sugar molecules, which are needed for cell respiration, to be absorbed from the gut and into the blood from the gut, even when the sugar concentration of the blood is higher
• Active Transport

  • Mineral absorption in plants — Active transport in the root hairs of plants means that plants are able to absorb mineral ions that are necessary for healthy growth, even though the concentration of minerals is usually lower in the soil than in the root hair.
• Active Transport

  • Active transport is the net movement of particles against a concentration gradient. 
  • This means that energy is needed for active transport to happen.
• Cell differentiation

  Cell differentiation is the process where a cell develops new sub-cellular structures (structures inside a cell) to let it perform a specific function. When this happens, the cell becomes specialised.
• Cell differentiation

  Embryos

  • Cell differentiation happens during an organism’s development. 
  • Organisms start as one cell. 
  • These cells divide to form embryos that differentiate (specialise) to produce cells that can perform all of the body's functions.
• Cell differentiation

  Plants

  • Many plant cells keep their ability to differentiate throughout their life. 
  • Because of this, plants are always able to create new tissues (matter that animals and plants are made from).
• Cell differentiation

  Adult animals

  • Cell differentiation is rare in mature (adult) animals. 
  • Their cells mostly divide (one cell splits to create two cells) in order to replace cells and repair tissues. 
  • New tissues are rarely created.