B1 - Cell structure and transport

• Eukaryotic cells


  Animal and plant cells, have a cell membrane, cytoplasm, and genetic material enclosed in a nucleus
• Prokaryotes


  Bacteria, single-celled living organisms, genetic material not enclosed in a nucleus
• Bacteria are 1-2 orders of magnitude smaller than eukaryotes, 0.2-2.0 μm in length
• Bacterial cell structure


  • Cytoplasm
  • Cell membrane
  • Cell wall (not made of cellulose)
  • Genetic material not enclosed in a nucleus
  • May have plasmids
  • May have flagella
  • May have slime capsule
• Bacteria can cause diseases in humans, animals and plants, and can decompose and destroy stored food
• Many bacteria have little or no effect on other organisms and are very useful
• Orders of magnitude


  Used to make approximate comparisons between numbers or objects, if one number is about 10 times bigger than another, it is an order of magnitude bigger
• If the bigger number divided by the smaller number is less than 10, then they are the same order of magnitude
• If the bigger number divided by the smaller number is around 10, then it is 10' or an order of magnitude bigger
• If the bigger number divided by the smaller number is around 100, then it is 10² or two orders of magnitude bigger
• Orders of magnitude


  Used to make approximate comparisons between numbers or objects. If one number is about 10 times bigger than another, it is an order of magnitude bigger.
• Order of magnitude


  Shown using powers of 10. If one cell or organelle is 10 times bigger than another, it is an order of magnitude bigger or 10¹. If it is approximately 100 times bigger it is two orders of magnitude bigger or 10².
• Calculating orders of magnitude

  1. If the bigger number divided by the smaller number is less than 10, then they are the same order of magnitude.

  2. If the bigger number divided by the smaller number is around 10, then it is 10¹ or an order of magnitude bigger.

  3. If the bigger number divided by the smaller number is around 100, then it is two orders of magnitude or 10² bigger.
• Relative sizes of different cells and organisms

  • Giant redwood (100m)
  • Minke whale (10m)
  • Human (1m)
  • Human eye (10cm)
  • Apple (10cm)
  • Electron microscope (1cm)
  • Wasp (1mm)
  • Ant (1mm)
  • Light microscope (100μm)
  • Eukaryotic cells (10μm)
  • Prokaryotic cells (1μm)
  • Viruses (100nm)
  • DNA (10nm)
  • Small molecule (1nm)
• Eukaryotic cells all have a cell membrane, cytoplasm, and genetic material enclosed in a nucleus
• Prokaryotic cells consist of cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not in a distinct nucleus. It forms a single DNA loop. Prokaryotes may contain one or more extra small rings of DNA called plasmids
• Bacteria are all prokaryotes
• As an organism develops, cells differentiate to form different types of specialised cells
• Most types of animal cells differentiate at an early stage of development, whereas many types of plant cells retain the ability to differentiate throughout life
• As a cell differentiates, it gets different sub-cellular structures that enable it to carry out a particular function
• Some specialised cells, such as egg and sperm cells, work individually. Others are adapted to work as part of a tissue, an organ, or a whole organism
• Nerve cells


  • Lots of dendrites to make connections to other nerve cells
  • An axon that carries the nerve impulse from one place to another
  • They can be very long - the axon of a nerve cell in a blue whale can be up to 25 m long
  • The nerve endings or synapses are adapted to pass the impulses to another cell or between a nerve cell and a muscle in the body using special transmitter chemicals
  • They contain lots of mitochondria to provide the energy needed to make the transmitter chemicals
• Striated muscle cells


  • They contain special proteins that slide over each other making the fibres contract
  • They contain many mitochondria to transfer the energy needed for the chemical reactions that take place as the cells contract and relax
  • They can store glycogen, a chemical that can be broken down and used in cellular respiration by the mitochondria to transfer the energy needed for the fibres to contract
• Sperm cells


  • A long tail whips from side to side to help move the sperm through water or the female reproductive system
  • The middle section is full of mitochondria, which transfer the energy needed for the tail to work
  • The acrosome stores digestive enzymes for breaking down the outer layers of the egg
  • A large nucleus contains the genetic information to be passed on
• Cone cells are specialised nerve cells in the eye. They contain a chemical that changes in coloured light. As a result of the change, an impulse is sent along another nerve cell to the brain. Cone cells usually contain many mitochondria
• Root hair cells are always relatively close to the xylem tissue. The xylem tissue carries water and mineral ions up into the rest of the plant
• Root hair cells


  • They greatly increase the surface area available for water to move into the cell
  • They have a large permanent vacuole that speeds up the movement of water by osmosis from the soil across the root hair cell
  • They have many mitochondria that transfer the energy needed for the active transport of mineral ions into the root hair cells
• Photosynthetic cells


  • They contain specialised green structures called chloroplasts containing chlorophyll that trap the light needed for photosynthesis
  • They are usually positioned in continuous layers in the leaves and outer layers of the stem of a plant so they absorb as much light as possible
  • They have a large permanent vacuole that helps keep the cell rigid as a result of osmosis
• Xylem cells


  • The xylem cells are alive when they are first formed but a special chemical called lignin builds up in spirals in the cell walls. The cells die and form long hollow tubes that allow water and mineral ions to move easily through them, from one end of the plant to the other
  • The spirals and rings of lignin in the xylem cells make them very strong and help them withstand the pressure of water moving up the plant. They also help support the plant stem
• Phloem cells


  • The cell walls between the cells break down to form special sieve plates. These allow water carrying dissolved food to move freely up and down the tubes to where it is needed
  • Phloem cells lose a lot of their internal structures but they are supported by companion cells that help to keep them alive. The mitochondria of the companion cells transfer the energy needed to move dissolved food up and down the plant in phloem
• A cell within the trunk of a tree cannot carry out photosynthesis
• Diffusion is the spreading out of the particles of a gas, or of any substance in solution (a solute)
• The net movement of particles during diffusion is from an area of higher concentration to an area of lower concentration
• Diffusion takes place because of the random movement of the particles (molecules or ions)
• Diffusion


  The spreading out of the particles of a gas, or of any substance in solution (a solute), resulting in the net movement of particles from an area of higher concentration to an area of lower concentration
• Cells need to take in substances such as glucose and oxygen for respiration, and get rid of waste products and chemicals needed elsewhere in the body
• Diffusion


  One of the main ways in which substances move into and out of cells across the cell membrane
• Concentration gradient


  The difference in concentration between two areas
• The greater the difference in concentration


  The faster the rate of diffusion
• Increase in temperature


  Increases the rate of diffusion