MOVING THROUGH MEMBRANES

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MOVING THROUGH MEMBRANES

• Basic Ideas of Biology
• Unit 1

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Issue One Membrane Structure
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Membranes are important to cell structure and
function.

• The plasma membrane
• surrounds the cells cytoplasm
• separates the cell from its environment
• regulates what moves in and out of the cell

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Internal membranes surround and form part of the
structure of many of the organelles of eukaryotic
cells

• Nucleus
• Endoplasmic reticulum
• Mitochondria and chloroplasts
• Golgi bodies
• Lysosomes

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Phospholipids form the framework of a cells
membranes.

• A phospholipid molecule consists of two parts
• a hydrophilic head, which contains phosphorus.
• a hydrophobic tail, composed of two fatty acid
  molecules

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H2O
H2O
H2O
H2O
Membrane
H2O
H2O
H2O
H2O
H2O
Water makes up a major part of the cells
internal and external surroundings.
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The framework of the membrane is a phospholipid
bilayer.

• The tails project inward--away from the water.
• The hydrophilic heads are in contact with the
  water.

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Phospholipid molecules arrange themselves in two
layers facing each other.
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
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Proteins are also an important part of membrane
structure.

• Some proteins penetrate the entire phospholipid
  framework others do not.

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Carbohydrate molecules attached to the outer
surface help the membrane recognize various
substances.
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Issue TwoMethods of Transport
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Diffusion is the net movement of molecules from a
region where they are highly concentrated to a
region where they are less highly concentrated.
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Diffusion stops when molecules are evenly
distributed.
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Diffusion can occur through membranes.
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Osmosis is the diffusion of water through a
semipermeable membrane.
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Solvent Molecules

• Think of an example of a substance that could be
  a solvent.
• Think of an example of a substance that could be
  a solute.

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Cells are about 90 water.
90 water 10 solute

• What would happen to a cell placed in pure water?

100 water
90 water 10 solute

• What would happen to a cell placed in a 20 salt
  solution?

80 water
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Transport proteins are embedded in cell membranes

• Facilitated diffusion is a passive method that
  uses transport proteins.

• Molecules move along a concentration gradient.
• No energy is required.

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Active transport uses transport proteins to move
some types of molecules against a concentration
gradient.

• Active transport requires the cell to use energy.

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Exocytosis and endocytosis also require energy.
These processes involve rearrangement of the
membrane.
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Review Substances pass through membranes in
several different ways

• Diffusion
• Osmosis
• Facilitated Diffusion
• Active Transport
• Exocytosis and Endocytosis

Forms of Passive Transport do not require energy

Require energy as ATP.