Absorption

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• Absorption secretion of materials

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Plant animal cells in solutions of different
strength.
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• Absorption and secretion of materials
• Cell Walls
• The cell wall is a non-living layer composed
  mainly of cellulose a fibrous criss-crossing
  carbohydrate.
• Cell walls allow free movement of most substances
  into or out of the cell and provide a continuous
  water-conducting path around the plant.
• The cell membrane is selectively permeable !

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• The molecules of liquids and gases move about
  freely all the time.
• Molecules move from a region of high
  concentration of that substance to a region of
  low concentration of that substance.
• Diffusion continues until the concentration
  becomes equal.

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Diffusion
Factors affecting the rate of diffusion across
membranes include
Diffusion is the net movement of molecules down a
concentrated gradient.
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Osmosis in animal cells red blood cells
Movement of water into or out of red blood cells
by osmosis in solutions of different concentration
Red cell bursts
Red cell shrinks
Red cell remains normal
In (hypotonic) pure water or dilute solution
In a (hypertonic) more concentrated solution
In a (isotonic) solution with the same
concentration as the red cell
Low concentration of solute molecules, high
concentration of water molecules
High concentration of solute molecules, low
concentration of water molecules
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• Hypotonic contains more water
• Isotonic contains equal water
• Hypertonic contains less water

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Lipids
Simple lipids (Triglycerides)
The most common forms of lipids are triglycerides
(fats and oils)
These are made by the combination of 3 fatty acid
molecules with 1 glycerol molecule.
GLYCEROL
FATTY ACID
FATTY ACID
FATTY ACID
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Lipids
Phospholipids
The third fatty acid has been replaced by a
phosphate group.
Phosphate groups are hydrophilic.
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Lipids
Phospholipids and membranes
A cell membrane is made up of a double
phospholipid layer
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Membrane structure
When mixed with water, phospholipid molecules
spontaneously assemble to form membrane-like
structures.
Their polar heads point outwards towards the
surrounding charged water molecules, and their
non-polar tails point inwards.
Under certain conditions they form bilayers, the
basis of cell membranes
DETAILED VIEW OF BILAYER
PHOSPHOLIPID MOLECULE
HYDROPHOBIC TAIL
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Membrane structure
Features of the fluid mosaic model
The double line seen at very high power is
thought to be the 2 phospholipid layers. The
bilayer is about 7 nm wide.
Membranes also contain proteins and the model of
membranes accepted at present is called the
fluid mosaic model.
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Membrane structure
Features of the fluid mosaic model
Most protein molecules are mobile, moving around
freely. Others are fixed like islands to
structures in the membrane and do not move
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• Membrane structure the fluid mosaic model
• Phospholipids consist of a phosphate group head
  and two fatty acid tails.
• The phosphate head is hydrophilic and the tail is
  hydrophobic in water the molecules spontaneously
  arrange themselves into membrane like structures.
• Under certain conditions they form bilayers.
• Membranes also contain various proteins, set in
  the phospholipid layer in a mosaic pattern.

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Torrance p.8 Questions 1 3
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Transport across the plasma membrane
Active transport
Active transport is the pumping of ions across
membranes against a diffusion gradient.
ATP is required to change the shape of the
protein and move the ion or molecule across.
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Transport across the plasma membrane
- Endocytosis
Bulk transport
Diffusion, osmosis and active transport refer to
the movement of individual particles across
membranes
Ingestion
Release of microbial debris
Mechanisms also exist for the bulk transport of
materials in and out of cells (endo- and
exocytosis).
Destruction of microbe
Stages in phagocytosis of a bacterium by a white
blood cell
Phagocytosis or cell eating. The bulk uptake
of solid materials. Cells which do this are
phagocytes, e.g. some white blood cells
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Transport across the plasma membrane
- Exocytosis
Bulk transport
Exocytosis is the reverse of endocytosis
It happens, for example, in the secretion of
digestive enzymes from the pancreas
Secretory vesicles from the Golgi body carry the
enzymes to the cell surface and release them to
the outside of the cell
Diagram of Golgi apparatus secretion
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Exocytosis
EM of pancreatic acinar cell secreting protein
Golgi apparatus
Secretory vesicle containing secretory product,
e.g. enzyme
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Torrance pp. 15/16 Questions 1 3Do either 1.
a) or 1. b) and either 3. a) or 3. b)
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• ATP and energy release

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Aerobic respiration
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• ATP and energy release
• In respiration, glucose is broken down to release
  a large amount of energy.
• This energy must be transferred to the point in
  the cell where it is needed.
• Adenosine triphosphate (ATP) is the molecule
  which acts as an energy carrier in all living
  cells.

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Structure of ATP
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energy released
ATP
ADP Pi
energy required
(high energy state)
(Low energy state)
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• Turnover of ATP
• The total ATP in the body would be used up, at
  rest, in about 90 seconds.
• Therefore, ATP is constantly being reformed from
  ADP using energy released during the breakdown of
  glucose.
• The net result is a constant quantity of ATP in
  the body about 50g.

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Torrance pp. 22/23 All questions