Higher Biology

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Higher Biology

• Absorption Secretion

Mr G Davidson
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Absorption Secretion

• Body cells are isotonic with blood plasma, which
  means they have the same solute concentration
  (osmotic pressure).
• Any changes in the osmotic pressure of the plasma
  will distort and possibly damage cells as they
  either lose or gain water.

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Absorption Secretion

• Therefore the osmotic pressure of plasma must be
  kept within narrow limits.
• If we drink a large quantity of water, the
  osmotic pressure of the blood falls and the blood
  is now said to be hypotonic (higher water
  concentration).

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Hypotonic Solutions
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Absorption Secretion

• The body responds by producing large volumes of
  dilute urine.
• If water is lost from the body, e.g. sweating or
  a lot of salt is consumed, the blood will become
  hypertonic (lower water concentration).
• The body responds by producing small volumes of
  concentrated urine.

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Hypertonic Solutions
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Absorption Secretion

• Tonic - tension
• Hyper - over, beyond
• Hyperactive - Overactive
• Hypersonic - Beyond sound
• Hypo - under, below
• Hypothermia - Under temperature
• Hypodermic - Below the skin

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Water Relations of Cells

• Cells have a number of methods for taking in
  molecules and the membrane of the cell or wall of
  the cell has an important part to play.

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Water Relations of Cells

• Diffusion- The movement of molecules from an
  area of high concentration to an area of low
  concentration down the concentration gradient.
• Osmosis- The movement of water molecules from an
  area of higher water concentration to an area of
  lower water concentration across a selectively
  permeable membrane.

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Water Relations of Cells

• Active Transport- The uptake of molecules
  against the concentration gradient using energy
  from respiration.
• Phagocytosis- The engulfing of large molecules
  by invagination of the cell membrane.
• Pinocytosis- Similar to phagocytosis but for
  liquids.

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Plant Cells

• Plant cells are surrounded by a cell wall which
  is composed of cellulose fibres.
• The cell wall is laid down on top of the plasma
  membrane in two layers.
• The primary wall is formed first and consists of
  a random arrangement of cellulose fibres.

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Plant Cells

• The secondary wall consists of layers of closely
  packed fibres of cellulose laid down like a mesh,
  and it is laid down between the plasma membrane
  and primary wall.
• The spaces between the cellulose fibres are
  filled with water, allowing water to move from
  cell to cell easily.

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Plant Cells

• Cellulose is therefore said to be hydrophilic
  i.e. has an attraction for water.
• Because of the strong cell wall, plant cells are
  capable of withstanding pressure when water moves
  in by osmosis.

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Turgid Plant Cells
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Plasmolysed Plant Cells
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Plasma Membrane

• All living cells have a plasma membrane which is
  composed of protein and phospholipid (fat)
  molecules.
• The exact structure is still unclear but the most
  popular model is known as the Fluid Mosaic Model
  which suggests that the membrane has constantly
  mobile phospholipid molecules known as the
  Mosaic.

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Fluid Mosaic Model
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Plasma Membrane

• Because phospholipids are held by weak bonds,
  this allows them to change position which is
  necessary to allow the formation of pseudopodia.
• It is thought that oxygen and carbon dioxide
  diffuse freely through the phospholipid layer
  while water enters through the pores by osmosis.

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Plasma Membrane

• Sugars and amino acids diffuse across the
  membrane aided by carrier protein molecules.
• Some inorganic ions have to be pumped across by
  protein molecules using energy.

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Plasma Membrane

• This is active transport.
• Due to the selectively permeable nature of the
  plasma membrane, whenever a difference in
  concentration occurs on either side of the
  membrane, osmosis will occur.
• The absence of the strong cell wall in animal
  cells can lead to the membrane bursting if placed
  in a hypotonic solution.

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Plasma Membrane

• Some protein molecules of the membrane act as
  recognition molecules and can recognise
  antibodies.
• This plays an important part in immunity.
• They may recognise hormones and play a part in
  regulation.