Chapter 3B: Cell Structure and Function3.4 The Cell Membrane

Using the models and information from the textbook answer the following questions. There is more information about the plasma membrane at the beginnings of section 3.3. Chapter 2 has a short section on the molecules that make up cell or plasma membranes (located almost at the end of the information on Lipids, just before Proteins).

What is the most predominant molecule making up the plasma membrane?

How many layers of phospholipids make up the membrane?

Why are membranes called a “phospholipid bilayer”? (the two previous questions may help you answer this)

Phospholipids are drawn with a circle “head” end and two dangling “tails”. Do the heads or the tails line the outside surfaces of a membrane?

Do the heads or the tails create a hydrophobic environment in the interior of the membrane (not inside the cell, but sandwiched in between the two layers of phospholipids)?

Which type of molecule is embedded throughout the phospholipid bilayer? (all of the blue molecules in the diagram)

Glycoproteins and glycolipids have what type of molecule attached to them?

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Phospholipids

True-False: If the statement is true, write a T in the blank. If the statement is false, correct it by changing the underlined word(s) and writing the correct word(s) in the answer blank. The first two are done for you as an example.

__T___________ Cell or plasma membranes define the boundary of the cell.

__Some_______ The plasma membrane prevents all substances from crossing into or out of a cell

_____________ Plasma membranes are rigid

_____________ The plasma membrane carries markers that allow cells to isolate one another

_____________ The plasma membrane carries receptors for specific hormones or neurotransmitters

_____________ Some viruses exploit receptors to gain entry to a cell

Fluid Mosaic Model

_____ 1. Mosaic quality A. Regulates the fluidity of the membrane

_____ 2. Fluid quality B. Always found on the exterior surface of cells

_____ 3. Cholesterol C. Components of the membrane are able to flow and change positions

_____ 4. Phospholipids D. Mixed composition of phospholipids, cholesterol, proteins, and carbohydrates

_____ 5. Hydrophobic E. Make up the second major chemical component

_____ 6. Hydrophilic F. Hydrophilic ends and hydrophobic fatty acid tails

_____ 7. Proteins G. Serve as channels or pumps to move material into or out of the cell

_____ 8. Carbohydrates H. The surfaces of the membrane

_____ 9. Integral Proteins I. The interior of the membrane

Note: plasma membrane, cell membrane, phospholipid bilayer, lipid bilayer are all terms that can refer to the structure surrounding a cell. This type of membrane is also used to make up several cellular structures such as the endoplasmic reticulum and surround organelles like a mitochondrion.

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3.5 Passive Transport(Use the figure, the textbook, and information provided in this section to answer the questions.)

Diffusion is a passive process of the movement of molecules from an area of high concentration to an area of low concentration.

In panel 1 of the figure which side of the plasma membrane (extracellular or cytoplasm) has the highest concentration of blue molecules?

Which side has the lowest concentration?

The different concentrations on the extracellular side compared to the cytoplasm side creates what type of gradient?

Using a graph to represent the concentration of two regions is a good way to visualize a concentration gradient. Notice the slope of the line. If you placed a hot wheels car in the middle of the line, which way would it go?

Going down the line is referred to as “going down the concentration gradient.” Molecules move in response to their concentration gradient the same way as the hot wheels car. They are going to go “down”: in other words from an area of high concentration to an area of low concentration.

By panel 3, does the cytoplasm have the highest, lowest, or equal concentration compared to the extracellular fluid?

Over time, which general direction do the blue molecules move? Towards the extracellular fluid or toward the cytoplasm?

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extracellular Cytoplasm0

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Selective Permeability

In panel 2 the blue molecules are moving through the phospholipid molecules from one side of the membrane to the other.

Plasma membranes are selectively permeable allowing some substances to cross while preventing others from crossing. Size and chemical properties of each molecule determines whether the molecules can move through the membrane.

Size: molecules need to be small enough to move between the phospholipids.

Chemical: the tails of the lipids create a hydrophobic region inside the membrane. Non-polar (hydrophobic) molecules can cross, but polar molecules cannot, except for water. Anything with a charge, like ions will also be blocked from passing through.

Based on the information in section 3.5 titled “Selective Permeability” and the above information, indicate for each of the following molecules whether they can cross a plasma membrane (type “yes” or “no”)

_____ Amino acids _____ Oxygen

_____ lipid-soluble material _____ Water

_____ Polar molecules _____ Vitamin C

_____ Vitamin A _____ Sodium ions

_____ Simple sugars

For each of the following factors indicate which condition will speed up diffusion.Extent of the concentration gradient

Mass of the molecules diffusing

Temperature

Solvent density

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Facilitated Transport (also called facilitated diffusion)

Fill in the table based on the figure and the information in the textbook.

What is the difference between simple diffusion and facilitated transport?

What types of molecules can use facilitated transport that could not simply diffuse over the membrane?

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DiffusionFacilitated Transport

Direction? (high to low or low to high concentration)

Pass directly over the membrane or presence of a channel protein?

Energy required? (presence of ATP indicates energy used)

Diffusion

Facilitated Transport

Osmosis

Fill in the table based on the figure and information in the textbook. The water concentration is the amount of water per amount of solute (small blue circles).

Comparing the two beakers, what has happened to the water from beaker 1 to beaker 2?

What did the water have to cross through to get to the left side of the beaker?

Do the solutes move from the left to the right side of the beaker?

What direction is the concentration gradient for water in the figure, left to right or right to left? (High water concentration to low water concentration.)

Compare what happens to the concentration of the solute (amount of solute/amount of water)Beaker 1 left side to beaker 2 left side

Beaker 1 right side to beaker 2 right side

Beaker 1 left side to beaker 1 left side

Beaker 2 left side to beaker 2 left side

What ultimately happens to the concentration of the solute from beaker 1 to beaker 2?

Based on the example in the figure and the information in the text, what is the definition of osmosis?

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Beaker 1, left side

Beaker 1, right side

Beaker 2, left side

Beaker 2, right side

Number of solutes

Level of water

Water concentration(high, low, or equal)

Tonicity

Define each of the following:Tonicity

Osmolarity

Hypotonic

Hypertonic

Isotonic

Examine figures 3.22 and 3.23. Describe for each of the following what would happen to the cells in response to osmosis. In other words indicate whether water would be leaving or entering the cell and what would happen to the cell (shrivel, stay the same, swell up).

1. A Paramecium (a single-celled organism) is hypertonic compared to the pond it lives in.

2. The fluid part of human blood has salts, proteins and other substances in equal concentrations to red blood cells.

3. A diatom living in the ocean is hypotonic compared to the marine waters.

4. A plant is trying to grow in soil that is high in salts. Normally the root cells easily take up water, what will happen in the salty soil?

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3.6 Active Transport

What three things are strikingly different in the diagram about active transport compared to diffusion and facilitated transport?

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Imagine placing the hot wheels car on the concentration gradient slope in the graph. If you wanted it to go “up” the gradient, what would you have to do?

An energy input like a “push” is required. For molecules going “up” or “against” the concentration gradient, energy is required. What is the usual form of the energy required?

What are some of the molecules that are moved “up” the concentration gradient by cells?

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Extracellular Intracellular05

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_____ Electrochemical gradient A. Energy used to actively pump ions or molecules against the concentration gradient

_____ Example of primary active transport B. A targeted variation of endocytosis using binding protiens

_____ Carrier proteins C. Sodium-potassium pump

_____ ATP D. A form of active transport where particles exit the cell from a membranous envelope (vesicle) fusing with the plasma

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_____ Secondary active transport E. Arises due to the combined effects of concentration and electrical gradients

_____ Endocytosis F. Actively pump against electrochemical gradients

_____ Phagocytosis G. A specialized form of endocytosis for bring large particles into a cell

_____ Pinocytosis H. Moves substances such as amino acids and glucose

_____ receptor-mediated endocytosis I. A form of active transport that involves the cell membrane invaginating forming a pocket around the target particle

_____ exocytosis J. A specialized form of endocytosis taking in solutes that the cell needs

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