Cell Transport and Membranes: Overview, Properties, and Functions

1. Lesson Overview 7.3 Cell Transport

2. Cell Membranes • All cells contain a cell membrane that ______________ what enters and leaves the cell and also _______________________________________the cell.

3. Cell Membranes • The cell membranes is a double-layered sheet called a ______________________, which gives cell membranes a flexible structure and forms a strong barrier between the cell and its surroundings.

4. The Properties of Lipids • Many lipids have oily ____________________ attached to __________________________ that interact strongly with water. • The fatty acid portions of such a lipid are ______________, or “water-hating,” while the phosphate end of the molecule is ____________________________, or “water-loving.”

5. The Properties of Lipids • When such lipids are mixed with water, their ________________________cluster together while their ________________________are attracted to water. A ________________________is the result.

6. The Properties of Lipids • The __________________of lipids in a bilayer are exposed to water, while the _________________form an oily layer inside the membrane from which water is excluded.

7. The Fluid Mosaic Model • Most cell membranes contain _____________ molecules that are embedded in the lipid bilayer. ________________________ molecules are attached to many of these proteins.

8. The Fluid Mosaic Model • Because the proteins embedded in the lipid bilayer can move around and “float” among the lipids, and because so many different kinds of molecules make up the cell membrane, scientists describe the cell membrane as a “_______________.”

9. The Fluid Mosaic Model • Some of the __________ form channels and pumps that help to move material across the cell membrane. • Many of the ___________________ molecules act like chemical identification cards, allowing individual cells to identify one another.

10. The Fluid Mosaic Model • Although many substances can cross biological membranes, some are ____________or too ______________________to cross the lipid bilayer. • If a substance is able to cross a membrane, the membrane is said to be ______________________to it. • A membrane is _______________________to substances that cannot pass across it. • Most biological membranes are ________________________, meaning that some substances can pass across them and others cannot. Selectively permeable membranes are also called semipermeable membranes.

11. Passive Transport • Every living cell exists in a _____________________environment. • One of the most important functions of the cell membrane is to keep the cell’s internal conditions relatively ___________. It does this by regulating the movement of molecules from one side of the membrane to the other side.

12. Diffusion • The cytoplasm of a cell is a ________________ of many different substances dissolved in water. • In any solution, __________ particles tend to move from an area where they are _____________________ to an area where they are _______________________. • The process by which particles move from an area of high concentration to an area of lower concentration is known as __________________. • Diffusion is the driving force behind the movement of many substances across the cell membrane.

13. Diffusion • Suppose a substance is present in ________________concentrations on either side of a cell membrane.

14. Diffusion • If the substance can cross the cell membrane, its particles will tend to move toward the area where it is ______ concentrated until it is evenly distributed.

15. Diffusion • At that point, the concentration of the substance on both sides of the • cell membrane is the same, and __________________ is reached.

16. Diffusion • Even when equilibrium is reached, particles of a solution will_______________________________across the membrane in both directions. • Because almost equal numbers of particles move in each direction, there _______________________in the concentration on either side.

17. Diffusion • Diffusion depends upon ________________________. Substances diffuse across membranes without requiring the cell to use additional energy. • The movement of materials across the cell membrane without using cellular energy is called ___________________.

18. Facilitated Diffusion • Cell membranes have ____________ that act as carriers, or channels, making it easy for certain molecules to cross. • Molecules that cannot directly diffuse across the membrane pass through special protein channels in a process known as ____________________________. • Hundreds of different proteins have been found that allow particular substances to cross cell membranes. • The movement of molecules by facilitated diffusion does not require any additional use of the cell’s _____________.

19. Osmosis: An Example of Facilitated Diffusion • The inside of a cell’s lipid bilayer is hydrophobic—or “____________.” Because of this, water molecules have a tough time passing through the cell membrane. • Many cells contain water channel proteins, known as ______________, that allow water to pass right through them. Without aquaporins, water would diffuse in and out of cells very slowly. • The movement of water through cell membranes by facilitated diffusion is an extremely important biological process—the process of __________________.

20. Osmosis: An Example of Facilitated Diffusion • ____________________ is the diffusion of water through a selectively permeable membrane. • Osmosis involves the movement of __________ molecules from an area of ___________ concentration to an area of ______________ concentration.

21. How Osmosis Works • 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.

22. 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 __________ on the right, where more of the solution is made of sugar.

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

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

25. How Osmosis Works • The more concentrated sugar solution at the start of the experiment was __________________, or “above strength,” compared to the dilute sugar solution. • The dilute sugar solution was _____________, or “below strength.”

26. Osmotic Pressure • For organisms to survive, they must have a way to _______________ the intake and loss of water. • The net movement of water out of or into a cell exerts a force known as _______________________________.

27. Osmotic Pressure • Because the cell is filled with salts, sugars, proteins, and other molecules, it is almost always ________________ to fresh water. • As a result, water tends to move quickly into a cell surrounded by fresh water, causing it to ________. Eventually, the cell may _______.

28. Osmotic Pressure • In plants, the movement of water into the cell causes the _____________________ 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.

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

30. Osmotic Pressure • In a _________________solution, water rushes out of the cell, causing animal cells to _________and plant cell vacuoles to_________.

31. Osmotic Pressure • Other cells, including those of plants and bacteria, that come into contact with fresh water are surrounded by tough _____________that prevent the cells from expanding, even under tremendous ________________.

32. Osmotic Pressure • Notice how the plant cell holds its shape in __________________ solution, while the animal red blood cell does not. • However, the increased osmotic pressure makes such cells extremely vulnerable to injuries to their cell walls.

33. Active Transport • Cells sometimes must move materials ________________ a concentration difference. • The movement of material against a concentration difference is known as __________________. Active transport requires ______________.

34. Active Transport • The active transport of small molecules or ions across a cell membrane is generally carried out by ______________, or _______________ that are found in the membrane itself.

35. Active Transport • Larger molecules and clumps of material can also be actively transported across the cell membrane by processes known as ______________ and ___________________. • The transport of these larger materials sometimes involves changes in the _________ of the cell membrane.

36. Molecular Transport • Small molecules and ions are carried across membranes by __________________ in the membrane that act like _____________. • Many cells use such proteins to move __________________________________________________ions across cell membranes. • Changes in protein _________ seem to play an important role in the pumping process.

37. Bulk Transport • ____________molecules and even solid clumps of material may be transported by movements of the cell membrane known as ______________________. • Bulk transport can take several forms, depending on the size and shape of the material moved into or out of the cell.

38. Endocytosis • ___________________is the process of taking material into the cell by means of infoldings, or pockets, of the cell membrane. • The pocket that results breaks loose from the outer portion of the cell membrane and forms a ______________________________________________ within the cytoplasm.

39. Endocytosis • Large molecules, clumps of food, and even whole cells can be taken up by _________________. • Two examples of endocytosisare _______________ and _____________.

40. Endocytosis • In ___________________, extensions of cytoplasm surround a particle and package it within a food vacuole. The cell then engulfs it. • _________________use this method for taking in food. • Engulfing material in this way requires a considerable amount of _________ and, therefore, is a form of _____________________.

41. Endocytosis • In ________________, cells take up ____________ from the surrounding environment by forming tiny pockets along the cell membrane. • The pockets fill with liquid and pinch off to form __________within the cell.

42. Exocytosis • Many cells also release large amounts of material from the cell, a process known as _________________. • During exocytosis, the membrane of the vacuole surrounding the material fuses with the cell membrane, forcing the contents __________________.