transport across cell membranes

• The cell membrane controls what substances can enter and exit a cell.
• The membrane is made of two main parts: phospholipids and proteins
• The membrane is described as selectively permeable.   

  This means that only small soluble molecules can pass across the membrane whereas large, insoluble molecules cannot. 
• small soluble molecules that can pass through the membrane

  glucose

  amino acids

  carbon dioxide

  oxygen

  water
• large insoluble molecules that cant pass through the membrane 


  starch

  proteins

  fat
• The membrane creates different conditions inside and outside of the cell. This means there can be more or less molecules of a substance inside or outside the cell. This forms a concentration gradient and affects how molecules move in and out of cells.
• The greater the concentration gradient 

  (the bigger the difference in concentration on either side of the membrane) 

  the faster the movement of molecules and the more molecules will move.
• passive transport - the movement of substances from an area of high concentration to an area of low concentration requiring no energy
• active transport - movement of molecules from a low concentration to a high concentration through a membrane
• passive transport

  molecules move down a concentration gradient

  from a high concentration to a low concentration

  passive transport requires no energy
• diffusion is the exact same as passive transport
• diffusion 


  Molecules move from a high concentration to a low concentration down a concentration gradient
• Diffusion is important to cells because

  • They can gain the raw materials for respiration 
  • They can remove harmful waste products
• active transport

  the movement of molecules and ions

  molecules and ions move against a concentration gradient

  molecules and ions move from a low to a high concentration

  active transport requires energy (ATP)
• when plant cells are places in a low water concentration

  Water leaves the cell from a higher water concentration inside the cell to a lower water concentration outside the cell. 

  • The mass of the cell decreases. 
  • The cell becomes plasmolysed and keeps its shape because of the cell wall.
• what happens when a plant cell is placed in a low water concentration solution?


  they become Plasmolysis
• what happens when a plant cell is places in a high water concentration?

  they become Turgid
• when plant cells are places in a high water concentration

  • Water enters the cell from a higher water concentration outside the cell to a lower water concentration inside the cell. 
  • The mass of the cell increases. 
  • The cell becomes turgid but doesn’t burst because of the cell wall.
• what happens when animal cells are places in a high water concentration?

  they burst
• what happens when animal cells are placed in a low water concentration?

  they shrink
• when animal cells are placed in high water concentrations

  • Water enters the cell from a higher water concentration outside the cell to a lower water concentration inside the cell. 
  • The cell swells and bursts because animal cells do not have a cell wall to keep their shape
• when animal cells are placed in a low water concentration

  • Water leaves the cell from a higher water concentration inside the cell to a lower water concentration outside the cell. 
  • The cell shrinks because animal cells do not have a cell wall to keep their shape. 
• Osmosis is the movement of water from a high water concentration to a low water concentration through a selectively permeable membrane
• osmosis

  water moves from a high water concentration to low water concentration

  water moves down a concentration gradient

  this does not require energy