transport across cell membranes

• Cell membrane structure

  A) phospholipids

  B) proteins

  
• The cell membrane is selectively permeable - This means that some molecules freely cross the cell membrane, whilst other substances cannot.
• Passive transport involves the movement of molecules from HIGH concentration to a LOW concentration DOWN a concentration gradient. This process does NOT require energy.
• Diffusion is the movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient. This process does not require energy, so diffusion is an example of passive transport.
• Osmosis is the movement of water from an area of high water concentration to an area of low water concentration down a water concentration gradient across a selectively permeable membrane.
• Glucose, oxygen and amino acids all move into the cell because the concentration outside the cell is higher than inside. By diffusion, these molecules enter the cell.
• Carbon dioxide and waste all leave the cell, because the concentration is higher inside the cell than outside the cell. These leave the cell by diffusion.
• Diffusion is important as it allows living cells to gain oxygen and glucose but also allows carbon dioxide and waste to leave the cell.

  
• when a plant cell is placed in a solution with a higher water concentration, more water enters the cell and the plant cell becomes turgid, unlike animal cells plant cells have a cell wall to stop them form bursting

  
• when a plant cell is placed in a solution with a lower water concentration, water moves out of the cell and the cell becomes plazmolysed

  
• when an animal cell is placed in a solution with a high water concentration, such as pure water, water moves into the cell and the cell bursts

  
• when an animal cell is placed in a solution with a lower water concentration, like a strong salt solution, water moves out of the cell and it will shrink

  
• Active transport is the movement of molecules from an area of LOW concentration to an area of HIGH concentration against the concentration gradient.

  This process requires energy.
• an example of active transport is proteins moving molecules across cell membranes