moving through membranes basic ideas of biology unit 1
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MOVING THROUGH MEMBRANESMOVING THROUGH MEMBRANES
Basic Ideas of Biology
Unit 1
Issue One: Membrane Structure
Membranes are important to cell structure and function.
The plasma membrane
– surrounds the cell’s cytoplasm
– separates the cell from its environment
– regulates what moves in and out of the cell
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
Phospholipids form the framework of a cell’s membranes.
A phospholipid molecule consists of two parts:
– a hydrophilic head, which contains phosphorus.
– a hydrophobic tail, composed of two fatty acid molecules
H2O
H2O H2OH2O
H2O H2O H2O H2O
H2O
Water makes up a major part of the cell’s internal and external surroundings.
Membrane
The tails project inward--away from the water.
The hydrophilic heads are in contact with the water.
The framework of the membrane is a phospholipid bilayer.
Phospholipid molecules arrange themselves in two layers facing each other.
H2O
H2O H2OH2O
H2O H2O H2O H2O
H2O
Proteins are also an important part of membrane structure.
Some proteins penetrate the entire phospholipid framework; others do not.
Carbohydrate molecules attached to the outer surface help the membrane recognize various substances.
Issue Two:Methods of Transport
Concentration Gradient
Diffusion is the net movement of molecules from a region where they are highly concentrated to a region where they are less highly concentrated.
Diffusion stops when molecules are evenly distributed.
Diffusion can occur through membranes.
Osmosis is the diffusion of water through a semipermeable membrane.
Solute molecules
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.
What would happen to a cell placed in pure water?
90% water
10% solute 100% water
What would happen to a cell placed in a 20% salt solution?
90% water
10% solute
Cell’s are about 90% water.
80% water
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.
Active transport uses transport proteins to move some types of molecules against a concentration gradient.
Active transport requires the cell to use energy.
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:
DiffusionOsmosisFacilitated
Diffusion Active TransportExocytosis and
Endocytosis
}Forms of Passive Transport;do not require energy
}Require energy as ATP.
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