Cell Membranes GateKeepers of the Cell Membranes and
Cell Membranes Gate-Keepers of the Cell
Membranes and Transport To remain alive all living things must Øexchange gases (usually CO 2 and O 2), Øtake in water, minerals, and food, Øand eliminate wastes at the cellular level this requires that molecules move through the membrane that surrounds the cell.
Cell Membrane Structure • Bilayer of various types of lipids and proteins • Lipids have the glycerol replaced by phosphorous group • Hydrophobic tails align in the middle • Hydrophyllic phosphorous ends are on the exterior and interior of the cell • This structure limits passage based on Polarity and Size
• Random movement of the components of the membrane allow small openings • This allows small polar molecules such as water to easily pass through
There are 2 ways molecules move across the cell membrane: • Passive Transport – requires no use of cell energy • Active Transport – uses cell energy
Passive Transport The principle means of passive transport is diffusion. Diffusion • the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. • dependent on the motion of the molecules and continues until the system in which the molecules are found reaches a state of equilibrium
• Molecules in solution move randomly due to kinetic energy colliding with each other and with the molecules of the solvent. • This constant random motion causes them to move from higher to lower concentrations, or “down their concentration gradient”. • This happens spontaneously without any use of cell energy. • Once equilibrium is reached motion continues and maintains a dynamic equilibrium
• concentration = mass of solute / volume of solvent
• When a substance can diffuse across a membrane the membrane is said to be permeable to that substance. • Most membranes are selectively permeable, meaning that some substances can diffuse while others can not. • Because the cell membrane is selectively permeable only small – medium size non-polar molecules and water readily diffuse in and out of the cell.
Osmosis • the diffusion of water across a selectively permeable membrane. • isotonic, – Solutions of equal concentration • Hypertonic – Solution of greater strength • Hypotonic, – Solution of lesser strength
The key to remember about osmosis is that water flows from the solution with the lower solute concentration into the solution with higher solute concentration
• Cell membranes are completely permeable to water; therefore, the environment the cell is exposed to can have a dramatic effect on the cell • Osmosis exerts a pressure on the hypertonic side of a cell, called osmotic pressure. • This pressure almost squeezes the water from the hypotonic side to the hypertonic side.
RECAP Diffusion & Osmosis
Facilitated Diffusion • Facilitated diffusion is a process whereby a substance passes through a membrane with the aid of an intermediary or a facilitator. • The facilitator is an integral membrane protein that spans the width of the membrane. • The force that drives the molecule from one side of the membrane to the other is the force of diffusion • It is similar to simple diffusion in the sense that it does not require use of cell energy
• The cell membrane is impermeable to many molecules and ions which must cross. – i. e. Glucose, Na+1, K+1, Cl-1 • Must be "facilitated" by proteins • These proteins span the membrane and provide an alternative route or bypass • are very specific with respect to the kinds of molecules or ions that will be transported. – i. e. a Na+ transport protein transports Na+ ions much better than it will transport K+ ions.
Active Transport • Process by which materials are moved across the cell membrane against the concentration gradient. • Requires metabolic energy • Uses transport proteins or “pumps” integrated in the membrane • May involve the processes of exocytosis and endocytosis for very large materials.
Why pump against the concentration Gradient? • This happens a lot in neurons. Na+ and K+ are pumped against their concentration gradients The membrane proteins are constantly pumping ions in and out to get the membrane of the neuron ready to transmit electrical impulses. • Ca+ is pumped in and out of muscle cells to conduct electrical current aiding in movement. • H+ is pumped to the space between layers of the mitochondria membrane to aid in formation of ATP.
• Endocytosis and Exocytosis are processes which require considerable cell energy and so are also classified as means of active transport. • Phagocytosis and pinocytosis are 2 types of endocytosis
Diversity of Cells • The many differences among organisms is entirely dependent on how their cells specialize. • Despite their many differences all organisms – are made of cells – carry out the same metabolic chemical processes of life
Unicellular • exhibit all characteristics of life • outnumber multicellular organisms on earth
Multicellular • Cell Specialization - Cells develop in different ways base on the different functions they are intended to perform. Cells specialize to form --Tissues specialized tissues work together to form Organs organs with different functions work together to form Organ Systems
• Any given tissue (specialized set of cells) can only do its job if all other tissues are doing theirs. All tissues organs and organ systems of an organism are dependent on all others to maintain homeostasis of the organism and thus survive themselves.
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