 Cell Membrane and Transport sec 7.3 ( )

Cell Membrane Structure  The Fluid Mosaic Model of membrane structure describes the organization of cell membranes. › Phospholipids drift and move like a fluid. › The bilayer is a mosaic mixture of phospholipids, various proteins, and other molecules.

What does “polar” mean? A) Even distribution of electrons (charge) B) Uneven distribution of electrons (charge) C) No distribution of electrons (charge) D) Total distribution of electrons (charge)

Polar “Head”: hydrophilic (likes water) Non-Polar “Tails”: hydrophobic (hates water) Bilayer Phospholipid Bilayer -two layers of phospholipids One phospholipid molecule

 Cholesterol molecules embedded in the membrane keep the membrane from being too fluid or too solid. Membrane Bilayer

 Bilayer (double layer) arrangement of phospholipids makes the cell membrane selectively permeable.  The cell membrane regulates movement into and out of the cell. Membrane Bilayer

Selective Permeability  Selectively Permeable – some substances can pass across and others cannot

Which of the following is a function of the cell membrane? A) breaks down lipids, carbohydrates, and proteins from foods B) stores water, salt, proteins, and carbohydrates C) keeps the cell wall in place D) regulates the movement of materials into and out of the cell

Cell Transport Passive Transport  Does not require energy Active Transport  Requires energy

Passive Transport  The process by which particles move from an area of high concentration to an area of lower concentration is known as diffusion.  Diffusion is a form of passive transport

Diffusion  Diffusion is the driving force behind the movement of many substances across the cell membrane.

Diffusion

Diffusion occurs because: A) molecules are attracted to one another. B) molecules constantly move and collide with each other. C) cellular energy forces molecules to collide with each other. D) cellular energy pumps molecules across the cell membrane.

› facilitated diffusion: Molecules pass through special protein channels because they can’t go directly across the membrane › Facilitated diffusion is another type of passive transport › High to low concentration › No energy needed Facilitated Diffusion

Osmosis  Osmosis : the diffusion of water through a selectively permeable membrane.  from higher concentration to lower concentration.  Solute: the stuff dissolved in water

How Osmosis Works (just listen to this part)  In the experimental setup below, the barrier is permeable to water but not to sugar.  This means that water molecules can pass through the barrier, but the solute, sugar, cannot (too big).

How Osmosis Works  There are more sugar molecules on the right side of the barrier than on the left side.  Therefore, the concentration of water is lower on the right, where more of the solution is made of sugar.

How Osmosis Works  There is a net movement of water into the compartment containing the concentrated sugar solution.  Water will tend to move across the barrier until equilibrium is reached. At that point, the concentrations of water and sugar will be the same on both sides.

How Osmosis Works isotonic, which means “same strength.”  When the concentration is the same on both sides of the membrane, the two solutions will be isotonic, which means “same strength.”

How Osmosis Works hypertonic, or “above strength,  The more concentrated sugar solution at the start of the experiment was hypertonic, or “above strength,” compared to the dilute sugar solution. hypotonic, or “below  The dilute sugar solution was hypotonic, or “below strength.”

Osmotic Pressure see videovideo  Because the cell is filled with salts, sugars, proteins, and other molecules, it is almost always hypertonic to fresh water. Fresh water is hypotonic to cells.  As a result, water tends to move quickly into a cell surrounded by fresh water, causing it to swell. Eventually, the cell may burst.  Water moves from hypotonic to hypertonic

Osmotic Pressure  In plants, the movement of water into the cell causes the central vacuole to swell, pushing cell contents out against the cell wall.  Since most cells in large organisms do not come in contact with fresh water, they are not in danger of bursting.

Osmotic Pressure  These cells are bathed in fluids, such as blood, that are isotonic and have concentrations of dissolved materials roughly equal to those in the cells.  Cells placed in an isotonic solution neither gain nor lose water.

Osmotic Pressure  In a hypertonic solution, water rushes out of the cell, causing animal cells to shrink and plant cell vacuoles to collapse.

Osmotic Pressure  Other cells, including those of plants and bacteria, that come into contact with fresh water are surrounded by tough cell walls that prevent the cells from expanding, even under tremendous osmotic pressure.  Notice how the plant cell holds its shape in hypotonic solution, while the animal red blood cell does not.

The diffusion of water across a selectively permeable membrane is called: A) osmotic pressure. B) osmosis. C) pinocytosis. D) active transport.

An animal cell that is surrounded by fresh water will burst because the osmotic pressure causes a.water to move into the cell. b.water to move out of the cell. c.solutes to move into the cell. d.solutes to move out of the cell.

Active Transport  The movement of material against a concentration difference (gradient) is known as active transport.  Active transport requires energy.

Protein Pumps  The active transport of small molecules or ions across a cell membrane is generally carried out by transport proteins, or protein “pumps,” that are found in the membrane itself.

Active Transport  Small molecules and ions are carried across membranes by proteins in the membrane that act like pumps.  Many cells use such proteins to move calcium, potassium, and sodium ions across cell membranes.

Active Transport  Larger molecules and clumps of material can also be actively transported across the cell membrane by processes known as endocytosis and exocytosis.  The transport of these larger materials sometimes involves changes in the shape of the cell membrane.

Why are cells so small?  Food, oxygen, and water enter a cell through the cell membrane. Waste products leave in the same way. The rate at which this exchange takes place depends on the surface area of a cell. The rate at which food and oxygen are used up and waste products are produced depends on the cell’s volume.

The cell membrane contains channels and pumps that help move materials from one side to the other. What are these channels and pumps made of? a.carbohydrates b.lipids c.bilipids d.proteins

Compared to small cells, large cells have more trouble A) dividing. B) producing daughter cells. C) storing needed materials and waste products. D) moving needed materials in and waste products out.