Cell Boundaries/Active and Passive Transport Chapter 7, Section 3

Cell Boundaries All cells are surrounded by a thin, flexible barrier known as the cell membrane. Many cells also produce a strong supporting layer around the membrane known as a cell wall. Copyright Pearson Prentice Hall

Cell Membrane: Lipids The cell membrane regulates what enters and leaves the cell and also provides protection and support. It is made of a lipid bilayer, or a double-sheet of lipids. These are often phospholipids, with an outer, polar phosphate “head” of each lipid, which is hydrophilic (water loving), and an inner, nonpolar fatty acid tail that is hydrophobic (water repellent). The lipid bilayer gives the cell membranes a flexible structure that forms a strong barrier between the cell and its surroundings.

Cell Membrane Cell Membrane Outside of cell Carbohydrate chains Proteins Cell membrane The cell membrane regulates what enters and leaves the cell. Inside of cell (cytoplasm) Protein channel Lipid bilayer Copyright Pearson Prentice Hall

Cell Membrane: Proteins and Carbohydrates Most cell membranes contain protein molecules inside the lipid bilayer. Carbohydrates are attached to many of these proteins. Proteins form channels and pumps to help move materials back and forth across the cell membrane. Many carbohydrates are like chemical identification cards that allow cells to identify one another.

Cell Wall Importance Cell walls are in plants, fungi, and most prokaryotes. Cell walls are outside cell membranes. Most cell walls have enough space to allow water, oxygen, carbon dioxide, and other substances through. The cell wall’s main function is to provide support and protection for the cell.

Cell Wall Construction Most cell walls are made from fibers of carbohydrates and proteins. These substances are made inside the cell and then used to construct wall. Plant cell walls are mostly made up of cellulose, a tough carbohydrate.

Cell Walls Cell Wall Cell walls are found in plants, algae, fungi, and many prokaryotes. Copyright Pearson Prentice Hall

Concept Review What are the similarities and differences between the function and composition of the cell wall and cell membrane? What is the main reason cell membranes are flexible? How do carbohydrates assist with the cell membrane’s tasks?

Diffusion and Osmosis Diffusion is when particles move from an area of higher concentration to an area of lower concentration. When the concentration of the solute is the same, equilibrium has been reached. Diffusion does not require energy. Osmosis is the diffusion of water through a selectively permeable membrane. These types of movement that require no energy are part of passive transport.

Diffusion Through Cell Boundaries Diffusion is the process by which molecules of a substance move from areas of higher concentration to areas of lower concentration. Diffusion does not require the cell to use energy. Copyright Pearson Prentice Hall

Osmosis Osmosis is the diffusion of water through a selectively permeable membrane. When the concentration of water is the same on both sides of the membrane, the solutions are isotonic. When there is less water on one side of the membrane, it is known as being in a hypertonic. The other side of the membrane is then hypotonic

Selectively permeable membrane Osmosis How Osmosis Works Dilute sugar solution (Water more concentrated) Concentrated sugar solution (Water less concentrated) Sugar molecules Osmosis is the diffusion of water through a selectively permeable membrane. In the first beaker, water is more concentrated on the right side of the membrane. As a result, the water diffuses (as shown in the second beaker) to the area of lower concentration. Movement of water Selectively permeable membrane Copyright Pearson Prentice Hall

Facilitated Diffusion Some materials are too large to fit through the membrane, so they need to be aided in being brought through

Facilitated Diffusion Glucose molecules Facilitated Diffusion During facilitated diffusion, molecules, such as glucose, that cannot diffuse across the cell membrane’s lipid bilayer on their own move through protein channels instead. Protein channel Copyright Pearson Prentice Hall

*** Active transport requires energy. Cells sometimes have to move molecules from lower to higher concentrations. When this happens, energy needs to be used, making it active transport. Proteins generally take care of this transport. *** Active transport requires energy.

Active Transport Active Transport Molecule to be carried Active transport of particles against a concentration difference requires transport proteins and energy. Copyright Pearson Prentice Hall

Concentration The mass of solute in a given volume of solution is known as concentration. A concentration gradient is the amount of the concentration of a substance that changes as you move from one area to another. If the solute is moving from an area of higher concentration to an area of lower concentration, it is moving with its concentration gradient. If moving from an area of lower to higher concentration, it is moving against the gradient.

Diffusion and Osmosis Review Diffusion and osmosis require no energy to occur. Because of this they are known as forms of passive transport.

Passing Through the Cell Membrane The following materials can easily pass through the cell membrane: Oxygen Carbon dioxide Amino acids Lipids Some materials require more help. They include: Glucose Protein Sodium Potassium

Facilitated Diffusion Some molecules pass through protein channels during facilitated diffusion No energy is required. The molecules move from an area of higher concentration to an area of lower concentration. Glucose is an example of a molecule that needs help in this way.

Active Transport Some molecules cannot pass through the membrane easily due to their polarity, which prevents them from going past the fatty acid tails of phospholipids. These molecule are then moved via active transport. Energy is required for this, since the molecules are being moved from an area of lower concentration to an area of higher concentration (against their concentration gradient of higher to lower). Two examples of molecules that do this are: Sodium (Na+) Potassium (K+)

Other Active Transport Endocytosis occurs when the cell membrane brings materials into the cell by forming pouches to bring it in. These materials include proteins. Phagocytosis is cell-eating, when cells bring materials in that are then digested by lysosomes. The materials eaten may be bacteria, as happens in white blood cells. Pinocytosis brings liquid in a similar fashion to endocytosis. Both phagocytosis and pinocytosis are examples of endocytosis. Exocytosis occurs when the cell sends materials outside of the cell in pouches. An example is proteins.

Endocytosis

Exocytosis