7-3 Cell Boundaries
All cells are surrounded by a thin, flexible barrier ◦ cell membrane. Many cells also produce a strong supporting layer
around the membrane known as a ◦ cell wall.
Cell Membrane
Cell wall
• Cell Membrane – regulates what enters and leaves the cell – provides protection and support.
Outside of cell
Cell membrane
Inside of cell (cytoplasm)
Protein channel
Proteins
Lipid bilayer
Carbohydrate chains
• cell membranes is a double-layered sheet called a lipid bilayer. – Gives a flexible structure – forms a barrier between the cell and surroundings.
Lipid bilayer
• contain protein molecules embedded. – Some have carbohydrate molecules attached to them.
Protein channel
Proteins
Carbohydrate chains
• Cell Wall – provides support and protection for the cell. – found in plants, algae, fungi, and many
prokaryotes. – lies outside the cell membrane. – porous enough to allow water, oxygen, carbon
dioxide, and other substances to pass through
• Diffusion Through Cell Boundaries • Cell are in a liquid environment. • The cell membrane
– regulates movement of dissolved molecules from one side of the membrane to the other side.
– Measuring Concentration • Solution -mixture of two or more substances. • Solutes – The stuff dissolved. • Concentration - mass of solute in a given volume of
solution, (mass/volume).
– Diffusion • Diffusion - Particles in a solution move from an area of
high concentrated to an area of low concentration • Equilibrium - concentration is the same throughout
• Osmosis – diffusion of water through a selectively permeable
membrane.
Movement of water
Dilute sugar solution (Water more concentrated)
Concentrated sugar solution (Water less concentrated)
Sugar molecules
Selectively permeable membrane
• Water tends to diffuse from a highly concentrated region to a less concentrated region. – compare two solutions, the more concentrated solution is hypertonic
(“above strength”). – more dilute solution is hypotonic (“below strength”). – concentrations of solutions are the same on both sides of a membrane, the
solutions are isotonic (”same strength”).
– Osmotic Pressure • pressure osmosis exerts on the hypertonic side of a
selectively permeable membrane.
• Facilitated Diffusion • Cell membranes have protein channels that act as
carriers – making it easy for certain molecules to cross.
• facilitated diffusion. – the movement of specific molecules across cell membranes
through protein channels
• Hundreds of different protein channels have been found that allow particular substances to cross different membranes.
Protein channel
Glucose molecules
• facilitated diffusion – Fast – Specific – still diffusion.
• Passive transport – does not require energy • only occur if there is differences in concentration on
one side of a cell membrane as compared to the other side.
• Active Transport • movement of materials in the opposite direction from which the
materials would normally move – against a concentration difference
• Active transport requires energy. – Molecular Transport
• small molecules and ions are carried across membranes by proteins in the membrane.
• Energy is used – helps cells to concentrate substances in a particular location
Endocytosis and Exocytosis ◦ Endocytosis process of taking large material into the cell by means of
infoldings, or pockets, of the cell membrane. pocket breaks loose and forms a vacuole within the cytoplasm. Examples
Phagocytosis extensions of cytoplasm surround a particle and package it
within a food vacuole. The cell then engulfs it. requires a considerable amount of energy
Pinocytosis tiny pockets form along the cell membrane, fill with liquid, and
pinch off to form vacuoles within the cell
– Exocytosis • Release of large amounts of material from the cell • membrane of the vacuole surrounding the material
fuses with the cell membrane, forcing the contents out of the cell.