B2.1 - supplying the cell

• Diffusion

  • Diffusion is the net (overall) movement of particles from an area of higher concentration to an area of lower concentration.
  • If something moves from an area of higher concentration to an area of lower concentration it is said to have moved down its concentration gradient.
  • Diffusion happens in both liquids and gases that's because the particles in these substances are free to move about randomly.
• Transport processes used by living organisms for exchange

  • Diffusion
  • Osmosis
  • Active transport
• Osmosis

  • Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.
  • A partially permeable membrane is just one with very small holes in it. Only tiny molecules (like water) can pass through them, and bigger molecules (e.g. sucrose) can't.
  • A cell membrane is a partially permeable membrane.
• Water potential:

  • Water potential tells you how concentrated a solution is.
  • You can talk about osmosis in terms of water potential - water potential is the potential (likelihood) of water molecules to diffuse out of or into a solution.
  • If a solution has a high water potential, then it has a high concentration of water molecules. If it has a low water potential, then it has a low concentration of water molecules.
  • So, you can say that osmosis is the diffusion of water molecules across a partially permeable membrane down a water potential gradient.
• Pure water has the highest water potential of any solution
• Osmosis in animal cells

  • Animal cells don't have cell walls so the results of osmosis can be severe - e.g. cells can burst if they're surrounded by a solution with a higher water potential than them.
• Active transport

  • Active transport is the movement of particles across a membrane against a concentration gradient (i.e. from an area of lower to an area of higher concentration) using ATP released during respiration.
  • Plants use active transport to obtain the minerals they need from the soil.
• Active transport in the digestive system:


  1. When there's a higher concentration of nutrients in the gut they diffuse naturally into the blood.
  2. BUT sometimes there's a lower concentration of nutrients in the gut than in the blood.
  3. Active transport allows nutrients to be taken into the blood, despite the fact that the concentration gradient is the wrong way. This is essential to stop us starving. But active transport needs ATP from respiration to make it work.
• Cell cycle

  The cells of your body divide to produce more cells, so your body can grow and replace damaged cells. Cells grow and divide over and over again this is called the cell cycle. Cell division doesn't just happen in humans - animals and plants do it too.
• Parts of the cell cycle

  1. cell growth
  2. DNA replication
  3. More cell growth
  4. Movement of chromosomes
• Cell specialisation:

  • Differentiation is the process by which a cell changes to become specialised for its job.
  • In most animal cells, the ability to differentiate is lost at an early stage, but lots of plant cells don't ever lose this ability.
  • Having specialised cells is important it allows organisms to work more efficiently.
  • Most cells are specialised to carry out a particular job.
• Stem cells

  Undifferentiated cells capable of dividing by mitosis to become new cells, which then differentiate.
• Types of stem cells

  • Embryonic stem cells
  • Adult stem cells
• Embryonic stem cells

  • Embryonic stem cells are found in early human embryos.
  • They have the potential to turn into any kind of cell at all. This makes sense as all the different types of cells found in a human being have to come from those few cells in the early embryo.
  • This means stem cells are really important for the growth and development of organisms.
• Adult stem cells

  • Adults also have stem cells, but they're only found in certain places, like bone marrow.
  • These aren't as versatile as embryonic stem cells - they can't turn into any cell type at all, only certain ones from the tissue they originally came from.
  • In animals, adult stem cells are used to replace damaged cells, e.g. to make new skin or blood cells.
• Cell cycle:

  A) grows

  B) replicates

  C) DNA

  D) growing

  E) mitosis

  F) starts

  
• DNA replication (synthesis):

  A) DNA

  B) splits

  C) bases

  D) free-floating

  E) nucleotides

  F) bases

  G) DNA

  H) cross links

  I) nucleotide

  J) DNA

  K) DNA

  L) identical

  
• Mitosis- when a cell reproduces itself by splitting to form two identical offspring


  1. Cell has two copies of DNA spread out in long strings

  2. DNA forms x-shaped chromosomes. Each 'arm' of a chromosome is an exact copy of the other

  3. Chromosomes line up in the centre of the cell and cell fibres pull them apart, the two arms of each chromosome go opposite ends of the cell

  4. Membranes form around each of the sets of chromosomes. These become the nuclei of the two new cells.

  
• Cytokinesis:

  The cell cytoplasm and membrane divides to produce two genetically identical daughter cells. Each new cell has a copy of each of the chromosomes.
• Specialised cells for a particular job:

  Palisade leaf cells do most of the photosynthesis in plants, so they have many chloroplasts. Their tall shape means they have a lot of surface area exposed down the side for absorbing CO2 from the air in the leaf, and their thin shape means that you can fit loads of them in at the top of a leaf, so they're nearer the light.

  
• Specialised cells for a particular job:

  The function of sperm is to get the male DNA to the female DNA during reproduction. Sperm have long tails and streamlined heads to help them swim, they contain lots of mitochondria to provide them with energy, and they have enzymes in their heads to digest through the egg cell membrane.

  A) mitochondria

  B) DNA

  C) enzymes

  
• In multicellular organisms, specialised cells are grouped together to form tissues - groups of cells working together to perform a particular function. Different tissues work together to form organs. Different organs make up an organ system.
• Meristems:


  In plants, the only cells that divide by mitosis are found in plant tissues called meristems.

  Meristem tissue is found in the areas of a plant that are growing - such as the roots and shoots.

  Meristems produce unspecialised cells that are able to divide and form any cell type in the plant - they act like embryonic stem cells. But unlike human stem cells, these cells can divide to generate any type of cell for as long as the plant lives.

  The unspecialised cells can become specialised and form tissues like xylem and phloem.
• Cell membranes:

  • They hold the cell together but they let stuff in and out as well.
  • Only very small molecules can diffuse through cell membranes though - things like glucose, amino acids, water and oxygen. Big molecules like starch and proteins can't fit through the membrane.

  
• Particles move through the cell membrane from where there's a higher concentration (more of them) to where there's a lower concentration (not such a lot of them).

  They're only moving about randomly, so they go both ways - but if there are a lot more particles on one side of the membrane, there's a net (overall) movement from that side.
• Osmosis:

  Water molecules actually pass both ways through the membrane during osmosis. This happens because water molecules move about randomly all the time. But because there are more water molecules on one side than on the other, there's a steady net flow of water into the region with fewer water molecules, e.g. into the sucrose solution. This means the sucrose solution gets more dilute. The water acts like it's trying to even up the concentration either side of the membrane.

  A) net

  
• Osmosis in plants:

  Increased water potential in the soil means that all the plant cells draw water in by osmosis until they become turgid (plump and swollen). The contents of the cell push against the cell wall this is called turgor pressure. Turgor pressure helps support the plant tissues. If there's no water in the soil, a plant starts to wilt (droop). This is because the cells become flaccid they start to lose water. The plant doesn't totally lose its shape though, because the inelastic cell wall keeps things in position. It just droops a bit.

  A) turgid

  B) flaccid

  
• Measuring water potential in a potato:

  1. Select same size potato pieces.
  2. Measure the mass of each potato piece.
  3. Place each of the potato pieces into a test tube containing sucrose solution of different concentrations.
  4. Leave to sit, then remeasure the mass of each of the potato pieces.
  5. Calculate the change in mass of each of the potato pieces.
  6. then plot a graph
• Potato experiment

  The only thing that you should change in this experiment is the sucrose

  solution concentration. Everything else (e.g. the volume of solution, the size of the potato cylinders, the time the experiment runs for, etc.) must be kept the same or your results won't be valid.
• Results of potato experiment:

  Potato cylinders in solutions with a higher water potential than the cylinders will have drawn in water by osmosis - they'll be a bit longer and their mass will have increased.

  Potato cylinders in solutions with a lower water potential than the cylinders will have lost water - they'll have shrunk a bit and their mass will have decreased.

  A) pure

  B) concentrated