Cells and Their Environment

Cell Membrane Phospholipids Phospholipids Lipid Bilayer Lipid Bilayer Fluid Mosaic Fluid Mosaic Integral Proteins Integral Proteins

Phospholipid

Transportation How do substances move into and out of cells? How do substances move into and out of cells? Passive Transport and Active Transport Passive Transport and Active Transport

Passive Transport Movement across the cell membrane that does not require energy Movement across the cell membrane that does not require energy

Some Important Terms Concentration gradient – a difference in the concentration of a substance across a space Concentration gradient – a difference in the concentration of a substance across a space Equilibrium – concentration of a substance is equal throughout space Equilibrium – concentration of a substance is equal throughout space

Diffusion Movement of substance from area of high concentration to low concentration (down its concentration gradient)–resulting in equilibrium Movement of substance from area of high concentration to low concentration (down its concentration gradient)–resulting in equilibrium

Example of Diffusion Small and nonpolar molecules diffusing across the cell membrane down their concentration gradient Small and nonpolar molecules diffusing across the cell membrane down their concentration gradient This is the simplest type of passive transport This is the simplest type of passive transport

Osmosis The diffusion The diffusion of water Movement of a substance down its concentration gradient

A simple rule to remember is: Salt is a solute, when it is concentrated inside or outside the cell, it will draw the water in its direction. This is also why you get thirsty after eating something salty.

3 Possibilities for the direction of water movement in a cell 1. Water moves out 2. Water moves in 3. No net water movement

The word "HYPER" means more, in this case there are more solute (salt) molecules outside the cell, which causes the water to be sucked in that direction. In plant cells, the central vacuole loses water and the cells shrink, causing wilting. In animal cells, the cells also shrink. In both cases, the cell may die. Water Moves Out – Hypertonic Solution

The word "HYPO" means less, in this case there are less solute (salt) molecules outside the cell, since salt sucks, water will move into the cell. The cell will gain water and grow larger. In plant cells, the central vacuoles will fill and the plant becomes stiff and rigid, the cell wall keeps the plant from bursting In animal cells, the cell may be in danger of bursting, organelles called CONTRACTILE VACUOLES will pump water out of the cell to prevent this. Water Moves In – Hypotonic Solution

If the concentration of solute (salt) is equal on both sides, the water will move back in forth but it won't have any result on the overall amount of water on either side. "ISO" means the same Water moves in & out at the same rate!

Extra This is why it is dangerous to drink sea water - its a myth that drinking sea water will cause you to go insane, but people marooned at sea will speed up dehydration (and death) by drinking sea water. This is also why "salting fields" was a common tactic during war, it would kill the crops in the field, thus causing food shortages.

Transport Proteins Provide polar passageways through which ions and polar molecules can move across the cell membrane Provide polar passageways through which ions and polar molecules can move across the cell membrane Each channel is specific for a substance – selectivity – the cell has the Control! Each channel is specific for a substance – selectivity – the cell has the Control!

How can ions get into a cell?

Even though we have added a third party…. This is still a form of passive transport This is still a form of passive transport Why? Why? There is no use of energy by the cell required because the ions are moving down their concentration gradient There is no use of energy by the cell required because the ions are moving down their concentration gradient

Facilitated Diffusion Passive transport….why?

Active Transport How do substances enter a cell when they are working against their concentration gradient? How do substances enter a cell when they are working against their concentration gradient? Active transport - Requires the cell to use energy from ATP Active transport - Requires the cell to use energy from ATP

Carrier Proteins as “Pumps”

Why do we like the Na-K Pump? 1. Na ions continuously diffuse into the cell membrane through ion channels (why is this a problem) and the pump prevents these ions from accumulating 2. Maintains the concentration gradient of Na ions and K ions across the cell membrane

What about the large molecules (proteins and polysaccharides)? Endocytosis – the movement of a substance into a cell by a vesicle Endocytosis – the movement of a substance into a cell by a vesicle

Endocytosis - terms Phagocytosis – cell eating Phagocytosis – cell eating Phagocytosis –Membrane sinks in and captures solid particles for transport into the cell –Examples:  Solid particles often include: bacteria, cell debris, or food Pinocytosis – cell drinking Pinocytosis – cell drinking –Cell brings in a liquid

Endocytosis - comments Phagocytosis and pinocytosis are not selective Phagocytosis and pinocytosis are not selective –Membrane sinks inward and captures whatever particles/fluid present. –Vesicle forms and merges with the Golgi body…

Receptor Mediated Endocytosis Receptor Mediated Endocytosis is a highly specific form of endocytosis. Receptor Mediated Endocytosis is a highly specific form of endocytosis. Receptor Mediated Endocytosis Receptor Mediated Endocytosis –Receptor proteins on the outside of the cell bind specific substances and bring them into the cell by endocytosis

Receptor Mediated Endocytosis 1.Receptor proteins on PM bind specific substances (vitamins, hormones..) 2.Membrane sinks in and forms a pit –Called a coated pit 3.Pit pinches closed to form a vesicle around bound substances  Cytoskeleton aids in pulling in the membrane and vesicle formation

Example Coated vesicle Coated pit Specific molecule Coat protein Coated pit Material bound to receptor proteins Plasma membrane

How to move large substances out of the cell? Exocytosis – the movement of a substance by a vesicle to the outside of a cell (used to export proteins modified by the Golgi) Exocytosis – the movement of a substance by a vesicle to the outside of a cell (used to export proteins modified by the Golgi)

How cells communicate Release signal molecules that carry information to nearby cells and throughout the body Release signal molecules that carry information to nearby cells and throughout the body Ex. hormones Ex. hormones

What else is in the cell membrane? Receptor Proteins - proteins in the cell membrane that bind these signal molecules and then “read the message” Receptor Proteins - proteins in the cell membrane that bind these signal molecules and then “read the message” Marker Proteins – proteins that identify cell type. Marker Proteins – proteins that identify cell type.

Cells