Chapter 1 • Lesson 2



Chapter 1 • Lesson 2 objectives 1.1.1,1.2.1,4.2.2

Homeostasis and Cell Transport

Key Terms • homeostasis • pH • buffer • diffusion • passive transport • equilibrium • selectively permeable 1 • osmosis • concentration gradient • hypertonic • hypotonic • isotonic • facilitated diffusion • active transport • observation • inference

Getting the Idea

Recall that every cell is enclosed in a plasma membrane. The plasma membrane defines the cell by separating it from its environment. The plasma membrane also regulates which materials enter and leave the cell. In this way, the plasma membrane serves as a means of transport and helps maintain homeostasis.

Homeostasis

Homeostasis is the maintenance of stable internal conditions in a cell or organism despite changes in its environment. The stable internal state is also called homeostasis. This balance allows cells to survive and function properly.

Many life processes involve biochemical reactions. Examples of such reactions include energy storage and release, and the synthesis of new molecules such as proteins. As you will read in later lessons, these reactions require certain conditions. Therefore, in order to carry out their life processes, cells must maintain homeostasis. Some of the conditions a cell must regulate to maintain homeostasis include pH, glucose levels, water balance, and temperature.

The pH of a solution describes how acidic or basic the solution is. Cells need to maintain a fairly constant pH because pH affects the rates of many biochemical reactions. Cells generally maintain pH by using buffers. A buffer is a substance that tends to maintain a stable pH. Buffers are mixtures of acids and bases. Adding either an acid or a base to a buffered solution has little effect on the pH of the solution.

The Plasma Membrane and Homeostasis

A plasma membrane is usually made up of two layers of lipids, oily or waxy organic molecules that tend to repel water. This structure is known as a lipid bilayer. As shown below, most cell membranes contain protein molecules within the lipid bilayer. Many of these proteins are attached to carbohydrates.

Controlling the movement of materials into and out of a cell is essential for the cell's survival. For example, most cells need to take in glucose and oxygen to generate energy.

Cells must also get rid of waste materials, such as carbon dioxide and excess water. The plasma membrane helps maintain homeostasis by regulating which materials enter and leave the cell.

Diffusion

One way materials enter and leave a cell is by diffusion. Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. In other words, the particles travel from areas where they are crowded to areas where they are less crowded.

Recall from Lesson 1 that cellular respiration is the process by which living things obtain energy from food. A sugar called glucose is the food that cells use for cellular respiration. Therefore, cells must maintain a proper glucose balance. Diffusion enables cells to maintain this balance.

Particles in solutions, such as those inside and outside a plasma membrane, are in constant motion. They constantly collide with one another and tend to spread out randomly. Diffusion depends on the random movements of particles, so it does not require a cell to use energy. The movement of materials into or out of the cell without the expenditure of energy is called passive transport.

Particles diffuse into or out of a cell until their concentration is the same on both sides of the plasma membrane. When particles reach this state of equal concentration or balance, the system is at equilibrium. Once equilibrium is reached, particles continue to diffuse across the cell membrane in both directions. However, the same numbers of particles move in each direction, so the concentration does not change.

Osmosis

Not all particles can diffuse across a plasma membrane. The plasma membrane is a highly selective barrier. Only certain substances can move through it. A membrane is permeable to substances that can pass through it and impermeable to those that cannot. Plasma membranes are selectively permeable—that is, only some kinds of particles can pass through them.

Water balance is essential for maintaining homeostasis. Living cells are made up mostly of water. Water is an excellent solvent. A solvent is a substance in which other substances, or solutes, dissolve to form a solution. Many different compounds dissolve in water. In fact, cytoplasm is made up mostly of substances dissolved in water. For these reasons, water may be the most important substance that passes through the plasma membrane.

Water molecules pass through selectively permeable membranes by a type of diffusion known as osmosis. Osmosis is the movement of water molecules into a concentrated solution from a less concentrated solution—either into or out of the cell. The diagram illustrates the movement of water molecules across a membrane by osmosis.

[pic]

A difference in concentration on opposite sides of a plasma membrane makes osmosis possible. This difference is called the concentration gradient. When there is a concentration gradient between a cell and its surroundings, the outside solution is either hypertonic or hypotonic.

In a hypertonic solution, the concentration of solute outside the cell is higher than the concentration in the cytoplasm. When a cell is placed in a hypertonic solution, water moves out of the cell. This movement of water causes the cell to shrivel.

In a hypotonic solution, the concentration of solute in the cytoplasm is higher than the concentration outside the cell. When a cell is placed in a hypotonic solution, water moves into the cell. This causes the cell to swell and possibly burst.

Solutions with equal concentrations on both sides of a membrane are isotonic. As shown below, a cell is in a balanced environment when placed in an isotonic solution. Equal amounts of water move into and out of the cell.

[pic]

You have just read how diffusion and osmosis help a cell maintain an appropriate balance of glucose and water. These processes are vital to maintaining homeostasis in individual cells. It is important to remember that both processes are also vital to maintaining the balance of glucose, water, and other substances throughout the organism.

Facilitated Diffusion

Some molecules move through the plasma membrane by facilitated diffusion (also called facilitated transport). Facilitated diffusion is the movement of substances across a plasma membrane with the aid of protein molecules embedded in the membrane. Facilitated diffusion helps specific molecules pass through the plasma membrane. Like the processes described above, facilitated diffusion does not require a cell to use energy.

Recall that protein molecules are embedded in the lipid bilayer. Some of these molecules are referred to as protein channels. Each protein channel helps a specific type of molecule enter or leave the cell. Protein channels in red blood cells, for example, carry only glucose. Although the protein channel helps glucose move across the plasma membrane, the process is still diffusion. It occurs only if there is a difference in concentration on the two sides of the plasma membrane.

[pic]

Active Transport

The movement of materials across a plasma membrane by diffusion and osmosis does not use energy because the materials are moving from an area of higher concentration to an area of lower concentration. In some cases, the cell needs to move material from an area of low concentration to an area of high concentration. Moving material against the concentration gradient requires energy and is called active transport.

In active transport, carrier proteins in the membrane move particles across the membrane. A substance being transported binds to a specific carrier protein. Unlike a protein channel, a carrier protein does not provide an open pathway through the membrane. Instead, the arrangement of atoms in the molecule changes to allow a substance to move through the membrane. One example of active transport is the sodium-potassium pump. Sodium ions (Na+) are pumped out of the cell, and potassium ions (K+) are pumped into the cell, by specific carrier proteins. The cell uses energy in the form of ATP to move these ions. You will learn about ATP in Lesson 7.

Temperature and Homeostasis

Temperature is important to many of the chemical reactions that sustain life. Maintaining a fairly stable internal temperature is an important part of maintaining homeostasis. Organisms use different means to accomplish this.

The temperature of many single-celled and multicellular organisms is largely dependent on the external environment. Controlling body temperature often relies on the organism's ability to move from a location that is too hot or too cold to an environment with a more suitable temperature. Animals such as snakes, for example, often move into an area of sunlight to increase their body temperature. They move away from sunlight to decrease their temperature. Other animals may burrow into the ground when the external environment becomes too warm. Many single-celled organisms also use movement to help regulate temperature. You will learn about some of the structures these organisms use for movement in Lesson 8.

Birds and mammals control their temperature internally—inside their bodies. In these animals, temperature regulation involves several body systems working together. Body temperature in humans and other mammals is controlled through coordinated activities of the endocrine, nervous, circulatory, muscular, and integumentary systems.

Focus on Inquiry

The goal of science is to ask questions about the natural world and to seek answers and explanations. Scientists carry out many different kinds of investigations, but they all have some things in common. For example, all scientists make observations or study observations made by others. A scientific observation is information gathered with the senses or measured with a tool. All scientists also make inferences from observations. An inference is an explanation based on knowledge and prior experience.

See Gummy bear Activity

Potatoes are nutritious because their cells contain starch, an important type of carbohydrate that the body can use for energy. Sugars are also carbohydrates. In this activity, you will observe potato pieces placed in water and in a solution of sugar and water. Then you will use your observations to make inferences.

You will need these materials: two identical transparent cups, an uncooked potato, a paring knife, a spoon, and a container of sugar. As in all investigations, you must follow safety rules, such as using sharp tools with care.

Fill both cups with tap water to the same level. While stirring, add sugar to one cup until the water has a hazy appearance. Peel the potato, and cut two small cubes of the same size. Each side of the cubes should measure 2-3 centimeters. Add one cube to each cup. Over several hours, observe any changes that occur in each cup. Record your observations below.

Use your knowledge of cell transport to make inferences from your observations. Explain what you think took place in each cup to cause any changes you observed.

Cell Membrane Worksheet

I. Cell Membrane Coloring: Color each structure a different color. Indicate the structure color by coloring the circle next to the structure name with the corresponding color.

[pic]

O A: Phospholipid

O A1: Hydrophilic phosphate head

O A2: Hydrophobic fatty acid tails

(you may simply trace the lines in the picture; DO NOT shade the entire middle section of the bilayer)

O B: Receptor protein

O C: Glycoprotein: protein with a carbohydrate attached; often used to identify cells

O D: Carbohydrate chain

O E: Cholesterol

O F: Peripheral protein

O G: Channel protein (also an example of an integral protein)

O H: Active transport (Sodium/Potassium pump)

II. Cell Membrane Protein Functions: Match the following descriptions with the

correct type of protein Choices may be used once, more than once or not at all.

a. Adhesion protein

b. Enzyme

c. Receptor

d. Recognition

e. Channel

f. Gated-channel

g. Pumps (Active Transporters)

1. _____ helps form the adhering junctions between cells

2. _____ transports ions against the concentration gradient

3. _____ binds to signal molecules like hormones

4. _____ plays an important role in many metabolic reactions including cellular respiration

5. _____ helps anchor cells to the extracellular matrix

6. _____ allows molecules like water to flow through freely from high to low concentration

7. _____ allows molecules to flow from high to low concentration but requires the binding

of a substrate to open the passageway

8. _____ helps identify the cell; includes MHC

Laboratory: Observing Osmosis in Gummy Bears

Purpose: To investigate the movement of water into and out of a Gummy Bear (a gelatin polymer).

Problem: Where is the concentration of H2O molecules highest, tap water, distilled water, salt water or gummy bears?

Background Information:

Gummy Bears are made of gelatin, starch, and sugar. Gelatin is a polymer (huge molecule made of many repeating units) that forms large three-dimensional matrices which give structural support to jellies and jams, and lots of other things that you use every day. (A matrix is like a complex cage)

Molecules are in constant motion, and tend to move from areas of higher concentrations to lesser concentrations. Diffusion is defined as the movement of molecules from an area of high concentration to an area of low concentration.

The diffusion of water molecules through a selectively permeable membrane is known as OSMOSIS. Selectively permeable means that some molecules can move through a membrane while others cannot.

Movement through membranes is called transport. Diffusion and osmosis are forms of PASSIVE TRANSPORT; this means that they do not need energy to move from areas of high concentration to areas of low concentration .

ACTIVE TRANSPORT requires energy to transport molecules from low concentration to high concentration.

OSMOSIS is the movement (transport) of water (small dots) through a selectively permeable membrane from an area of high concentration to an area of low concentration.

Vocabulary:

1. Concentration

2. Diffusion

3. Osmosis

4. Membrane

5. Selectively Permeable

6. Selectively Permeable Membrane

7. Transport

8. Active Transport

9. Passive Transport

10. Polymer: A huge molecule made of many repeating units.

11. Matrix: A complex three dimensional a structure similar to a cage with many compartments.

Hypotheses: Circle your choices to create your hypotheses

1. If the H2O concentration in tap water is (higher, lower) than the H2O concentration in a Gummy Bear, then Gummy Bears placed in tap water will (increase, decrease, remain the same) size. Circle your answer.

2. If the H2O concentration in distilled water is (higher , lower) than the concentration in a Gummy Bear, then Gummy Bears placed in distilled water will (increase, decrease, remain the same) size. Circle your answer.

Materials: for pairs of students

• 2- 50 or 100 ml. beakers

• Masking tape

• permanent marker

• 2 - plastic forks or small sieves

• Waxed paper or paper plates

• 2 - Gummy Bears (different colors)

• distilled water

• tap water

• saturated salt solution (6 oz per cup)

• 2 - centimeter rulers

Safety Considerations:

There are no safety hazards with materials used in this experiment. The filters may become moldy after storage for several days or more. If that occurs, they should be disposed of in the trash.

Procedure:

1. Obtain two beakers, two different colored Gummy Bears and a ruler.

2. Label your beaker with a piece of masking tape (folded over)

3. Write your name and class period using a permanent marker.

4. Label one Beaker "TAP WATER" and the other "DISTILLED) WATER".

5. Measure your bear (in cm) from top to bottom (length) and from side to side (width) and from front to back (height).

6. Record the dimensions in centimeters in the data table. Use decimals.

7. Find the mass of each bear. Record the mass in the data table in grams. Again, use decimals.

8. Place the bears in the beakers.

9. Cover one with distilled water. The bear should be completely covered - cup about half full.

10. Cover the other tap water. The bear should be completely covered - cup about half full.

11. Place the beakers on the counter away from direct sunlight.

12. Let them sit overnight.

13. On the next lab day, gently pour the water over a screen into a sink.

14. Catch each bear using a sieve, plastic fork or screen. Place on paper towel or waxed paper.

15. Measure the length, width, and height. Record.

16. Blot dry by placing bear on a paper towel.

17. BE CAREFUL not to break the bears, they are very fragile.

18. Find the mass of wax paper or screen used. Then place the dried bear on the paper or screen.

19. Carefully find the mass of the bears. Record

20. Place the bears back into their correct cups.

21. Cover the bears with saturated salt solution. The bear should be completely covered -cup about half full. Let them sit overnight.

22. The following day, find the dimensions of the bears and record. Find the mass of the bears and record.

23. Calculate the volumes (L x W * H).

Round your answers to the nearest hundredth

Data Table for Volume:

|Dimensions |Gummy | 1 (color: ) |Gummy Bear | 2 (color: ) |

| |Bear | | | |

| | | | | |

| |Initial |Final |

| |Before Soaking |After tap water |

| | | |

| |(day 1) |(day 2) |

| | | |

| |Initial |Final |

|Tap Water Bear | | |

|Distilled Water Bear | | |

Make a BAR graph of the percent of change in mass.

• Title the graph (what is being compared?)

• Label the axis.

• Place a scale on the vertical axis for percent change.

• Place the data for both bears on the same graph.

• Include a Key if needed.

• If you have a negative value for a percent change, start the vertical axis at

a negative number. (For example: -50, -25, 0, 25, 50, 75,100, etc.)

• An example of the horizontal axis is below (note : A is a symbol used to represent "change"):

[pic]

[pic]

[pic]

Analysis questions:

Answer the following using complete sentences. Be sure to restate the question in your answer!

3. What happened to the bears when placed in distilled water? Why?

4. What happened to the bears when placed in tap water? Why?

5. What happened to the bears when placed in salt water? Why?

6. What do you think would have happened to the bears if, after the last day, they were again placed in distilled water?

Conclusions:

Write a paragraph which explains the results of this experiment using the concept of osmosis. Think about how much swelling can occur (equilibrium) Include your specific data to explain your results and support conclusions.

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