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5.8: Active Transport and Homeostasis - Biology

5.8: Active Transport and Homeostasis - Biology


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Like Pushing a Humvee Uphill

You can tell by their faces that these airmen are expending a lot of energy trying to push this Humvee up a slope. The men are participating in a competition that tests their brute strength against that of other teams. The Humvee weighs about 13,000 pounds, so it takes every ounce of energy they can muster to move it uphill against the force of gravity. Transport of some substances across a plasma membrane is a little like pushing a Humvee uphill — it can't be done without adding energy.

What Is Active Transport?

Some substances can pass into or out of a cell across the plasma membrane without any energy required because they are moving from an area of higher concentration to an area of lower concentration. This type of transport is called passive transport as you learned in the last section. Other substances require energy to cross a plasma membrane often because they are moving from an area of lower concentration to an area of higher concentration. This type of transport is called active transport. The energy for active transport comes from the energy-carrying molecule called ATP (adenosine triphosphate). Active transport may also require transport proteins, such as carrier proteins, which are embedded in the plasma membrane. Two types of active transport are pump and vesicle transport.

Pump

Two pump mechanisms (primary and secondary active transports) exist for the transport of small-molecular weight material and macromolecules. The primary active transport moves ions across a membrane and creates a difference in charge across that membrane. The primary active transport system uses ATP to move a substance, such as an ion, into the cell, and often at the same time, a second substance is moved out of the cell. The sodium-potassium pump is a mechanism of active transport that moves sodium ions out of the cell and potassium ions into the cells — in all the trillions of cells in the body! Both ions are moved from areas of lower to higher concentration, so energy is needed for this "uphill" process. The energy is provided by ATP. The sodium-potassium pump also requires carrier proteins. Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane. Figure (PageIndex{2}) shows in greater detail how the sodium-potassium pump works and the specific roles played by carrier proteins in this process.

To appreciate the importance of the sodium-potassium pump, you need to know more about the roles of sodium and potassium in the body. Both are essential dietary minerals, meaning you have to obtain them in the foods you eat. Both sodium and potassium are also electrolytes, meaning that they dissociate into ions (charged particles) in solution, which allows them to conduct electricity. Normal body functions require a very narrow range of concentrations of sodium and potassium ions in body fluids, both inside and outside of cells.

  • Sodium is the principal ion in the fluid outside of cells. Normal sodium concentrations are about 10 times higher outside than inside of cells.
  • Potassium is the principal ion in the fluid inside of cells. Normal potassium concentrations are about 30 times higher inside than outside of cells.

These differences in concentration create an electrical gradient across the cell membrane, called the membrane potential. the secondary active transport describes the movement of material using the energy of the electrochemical gradient established by the primary active transport. Using the energy of the electrochemical gradient created by the primary active transport system, other substances such as amino acids and glucose can be brought into the cell through membrane channels. ATP itself is formed through secondary active transport using a hydrogen ion gradient in the mitochondrion. Tightly controlling the membrane potential is critical for vital body functions, including the transmission of nerve impulses and the contraction of muscles. A large percentage of the body's energy goes to maintaining this potential across the membranes of its trillions of cells with the sodium-potassium pump.

Vesicle Transport

Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell. Very large molecules cross the plasma membrane with a different sort of help, called vesicle transport. Vesicle transport requires energy, so it is also a form of active transport. There are two types of vesicle transport: endocytosis and exocytosis.

Endocytosis

Endocytosis is a type of vesicle transport that moves a substance into the cell. The plasma membrane completely engulfs the substance, a vesicle pinches off from the membrane, and the vesicle carries the substance into the cell. It is used by all cells of the body because most substances important to them are polar and consist of big molecules, and thus cannot pass through the hydrophobic plasma membrane. When an entire cell or other solid particle is engulfed, the process is called phagocytosis. When fluid is engulfed, the process is called pinocytosis. When the content is taken in specifically with the help of receptors on the plasma membrane, it is called receptor-mediated endocytosis.

A targeted variation of endocytosis employs binding proteins in the plasma membrane that are specific for certain substances. The particles bind to the proteins and the plasma membrane invaginates, bringing the substance and the proteins into the cell. If passage across the membrane of the target of receptor-mediated endocytosis is ineffective, it will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration. Some human diseases are caused by a failure of receptor-mediated endocytosis. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by receptor-mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood because their cells cannot clear the chemical from their blood.

Exocytosis

Exocytosis is a type of vesicle transport that moves a substance out of the cell. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then, the vesicle membrane fuses with the cell membrane, and the substance is released outside the cell.

Homeostasis and Cell Function

For a cell to function normally, a stable state must be maintained inside the cell. For example, the concentration of salts, nutrients, and other substances must be kept within a certain range. The process of maintaining stable conditions inside a cell (or an entire organism) is homeostasis. Homeostasis requires constant adjustments because conditions are always changing both inside and outside the cell. The processes described in this and previous lessons play important roles in homeostasis. By moving substances into and out of cells, they keep conditions within normal ranges inside the cells and the organism as a whole.

Feature:Feature: My Human Body

Maintaining the proper balance of sodium and potassium in body fluids by active transport is necessary for life itself, so it's no surprise that getting the right balance of sodium and potassium in the diet is important for good health. Imbalances may increase the risk of high blood pressure, heart disease, diabetes, and other disorders.

If you are like the majority of Americans, sodium and potassium are out of balance in your diet. You are likely to consume too much sodium and too little potassium. Follow these guidelines to help ensure that these minerals are in balance in the foods you eat:

  • Total sodium intake should be less than 2300 mg/day. Most salt in the diet is found in processed foods or added with a salt shaker. Stop adding salt and start checking food labels for sodium content. Foods considered low in sodium have less than 140 mg/serving (or 5% daily value).
  • Total potassium intake should be 4700 mg/day. It's easy to add potassium to the diet by choosing the right foods, and there are plenty of choices. Most fruits and vegetables are high in potassium, but especially potatoes, bananas, oranges, apricots, plums, leafy greens, tomatoes, lima beans, and avocado. Other foods with substantial amounts of potassium are fish, meat, poultry, and whole grains.

Review

  1. Define active transport.
  2. What is the sodium-potassium pump? Why is it so important?
  3. Name two types of vesicle transport. Which type moves substances out of the cell?
  4. What are the similarities and differences between phagocytosis and pinocytosis?
  5. The sodium-potassium pump is a:
    1. Phospholipid
    2. Protein
    3. Carbohydrate
    4. Ion
  6. What is the functional significance of the shape change of the carrier protein in the sodium-potassium pump after the sodium ions bind?
  7. A potentially deadly poison derived from plants called ouabain blocks the sodium-potassium pump and prevents it from working. What do you think this does to the sodium and potassium balance in cells? Explain your answer.
  8. True or False. The sodium-potassium pump uses one protein to pump both sodium and potassium.
  9. True or False. Vesicles are made of the nuclear membrane.
  10. What is cn electrical gradient across the cell membrane is called?
  11. Chemical signaling molecules called neurotransmitters are released from nerve cells (neurons) through vesicles. This is an example of:
    1. Pinocytosis
    2. Phagocytosis
    3. Endocytosis
    4. Exocytosis
  12. The energy for active transport comes from
    1. ATP
    2. RNA
    3. Carrier proteins
    4. Sodium ions
  13. Transport proteins that move substances into and out of a cell are located in which structure?

Active Transport

Active transport is the process of transferring substances into, out of, and between cells, using energy. In some cases, the movement of substances can be accomplished by passive transport, which uses no energy. However, the cell often needs to transport materials against their concentration gradient. In these cases, active transport is required.


Passive Transport in Cells: Simple and Facilitated Diffusion and Osmosis

Simple Diffusion

Diffusion is the movement of a substance across a membrane, due to a difference in concentration, without any help from other molecules. The substance simply moves from the side of the membrane where it is more concentrated to the side where it is less concentrated. Figure below shows how diffusion works. Substances that can squeeze between the lipid molecules in the plasma membrane by simple diffusion are generally very small, hydrophobic molecules, such as molecules of oxygen and carbon dioxide.

Take a look at this video that shows diffusion across a membrane:

Here’s a Video recap worksheet to fill out and put in your biology notebook:

Osmosis

Osmosis is a special type of diffusion — the diffusion of water molecules across a membrane. Like other molecules, water moves from an area of higher concentration to an area of lower concentration. Water moves in or out of a cell until its concentration is the same on both sides of the plasma membrane.

Khan Academy: Diffusion and Osmosis:

Here is an animation that shows how osmosis works:

Facilitated Diffusion

Water and many other substances cannot simply diffuse across a membrane. Hydrophilic molecules, charged ions, and relatively large molecules such as glucose all need help with diffusion. The help comes from special proteins in the membrane known as transport proteins. Diffusion with the help of transport proteins is called facilitated diffusion. There are several types of transport proteins, including channel proteins and carrier proteins. Both are shown in Figure below.

  • Channel proteins form pores, or tiny holes, in the membrane. This allows water molecules and small ions to pass through the membrane without coming into contact with the hydrophobic tails of the lipid molecules in the interior of the membrane. They are like tunnels that only allow certain molecules to go through. “Sorry buddy, you can’t use this channel, it’s only for these fellows over here. You’ll have to go to the carrier protein and he’ll let you through.”
  • Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane. Don’t they look like crocodile mouths picking up a piece of stuff from the outside of the cell and then spitting it into the inside? Carrier crocodile proteins. Ok, that’s not really their name, but it might help you remember.

Active Transport

Active transport occurs when energy is needed for a substance to move across a plasma membrane. Energy is needed because the substance is moving from an area of lower concentration to an area of higher concentration. This is a little like moving a ball uphill it can’t be done without adding energy. The energy for active transport comes from the energy-carrying molecule called ATP. Like passive transport, active transport may also involve transport proteins. You can watch a video of active transport here:

Sodium-Potassium Pump

An example of active transport is the sodium-potassium pump. When this pump is in operation, sodium ions are pumped out of the cell, and potassium ions are pumped into the cell. Both ions move from areas of lower to higher concentration, so ATP is needed to provide energy for this “uphill” process. Figure below explains in more detail how this type of active transport occurs.

Vesicle Transport

Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell. Very large molecules cross the plasma membrane with a different sort of help, called vesicle transport. Vesicle transport requires energy, so it is also a form of active transport. There are two types of vesicle transport: endocytosis and exocytosis. Both types are shown in Figure below and described below.

  • Endocytosis is the type of vesicle transport that moves a substance into the cell. The plasma membrane completely engulfs the substance, a vesicle pinches off from the membrane, and the vesicle carries the substance into the cell. When an entire cell is engulfed, the process is called phagocytosis. When fluid is engulfed, the process is called pinocytosis.
  • Exocytosis is the type of vesicle transport that moves a substance out of the cell. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then, the vesicle membrane fuses with the cell membrane, and the substance is released outside the cell.

Homeostasis and Cell Function

For a cell to function normally, a stable state must be maintained inside the cell. For example, the concentration of salts, nutrients, and other substances must be kept within a certain range. The process of maintaining stable conditions inside a cell (or an entire organism) is homeostasis. Homeostasis requires constant adjustments, because conditions are always changing both inside and outside the cell. The processes described in this lesson play important roles in homeostasis. By moving substances into and out of cells, they keep conditions within normal ranges inside the cells and the organism as a whole.

Lesson Summary

  • A major role of the plasma membrane is transporting substances into and out of the cell. There are two major types of cell transport: passive transport and active transport.
  • Passive transport requires no energy. It occurs when substances move from areas of higher to lower concentration. Types of passive transport include simple diffusion, osmosis, and facilitated diffusion.
  • Active transport requires energy from the cell. It occurs when substances move from areas of lower to higher concentration or when very large molecules are transported. Types of active transport include ion pumps, such as the sodium-potassium pump, and vesicle transport, which includes endocytosis and exocytosis.
  • Cell transport helps cells maintain homeostasis by keeping conditions within normal ranges inside all of an organism’s cells.

Lesson Review Questions

Recall

1. What is osmosis? What type of transport is it?

2. Describe the roles of transport proteins in cell transport.

3. What is the sodium-potassium pump?

4. Name two types of vesicle transport. Which type moves substances out of the cell?

Apply Concepts

5. Assume a molecule must cross the plasma membrane into a cell. The molecule is a very large protein. How will it be transported into the cell? Explain your answer.

6. The drawing below shows the fluid inside and outside a cell. The dots represent molecules of a substance needed by the cell. The molecules are very small and hydrophobic. What type of transport will move the molecules into the cell?

Think Critically

7. Compare and contrast simple diffusion and facilitated diffusion. For each type of diffusion, give an example of a molecule that is transported that way.

8. Explain how cell transport helps an organism maintain homeostasis.

Points to Consider

All cells share some of the same structures and basic functions, but cells also vary.


Endocytosis

In addition to moving small ions and molecules through the membrane against their concentration gradients, cells also need to remove and take in larger molecules and particles. Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that the uptake and release of large particles by the cell requires energy. A very large particle, however, cannot pass directly through the membrane, even with energy supplied by the cell.

Endocytosis is a type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis, but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created vacuole that is formed from the plasma membrane.

Figure 7 Three variations of endocytosis are shown. (a) In one form of endocytosis, phagocytosis, the cell membrane surrounds the particle and pinches off to form an intracellular vacuole. (b) In another type of endocytosis, pinocytosis, the cell membrane surrounds a small volume of fluid and pinches off, forming a vesicle. (c) In receptor-mediated endocytosis, uptake of substances by the cell is targeted to a single type of substance that binds at the receptor on the external cell membrane. (credit: modification of work by Mariana Ruiz Villarreal)

Phagocytosis is the process by which large particles, such as cells, are taken in by a cell. For example, when microorganisms invade the human body, a type of white blood cell called a neutrophil removes the invader through this process, surrounding and engulfing the microorganism, which is then destroyed by the neutrophil (Figure 7a).

A variation of endocytosis is called pinocytosis. This literally means “cell drinking” and was named at a time when the assumption was that the cell was purposefully taking in extracellular fluid. In reality, this process takes in solutes that the cell needs from the extracellular fluid (Figure 7b).

A targeted variation of endocytosis employs binding proteins in the plasma membrane that are specific for certain substances (Figure 7c). The particles bind to the proteins and the plasma membrane invaginates, bringing the substance and the proteins into the cell. If passage across the membrane of the target of receptor-mediated endocytosis is ineffective, it will not be removed from the tissue fluids or blood. Instead, it will stay in those fluids and increase in concentration.

Some human diseases are caused by a failure of receptor-mediated endocytosis. For example, the form of cholesterol termed low-density lipoprotein or LDL (also referred to as “bad” cholesterol) is removed from the blood by receptor mediated endocytosis. In the human genetic disease familial hypercholesterolemia, the LDL receptors are defective or missing entirely. People with this condition have life-threatening levels of cholesterol in their blood, because their cells cannot clear the chemical from their blood.


Homeostasis & Transport

I. Hypertonic Solution
1. Solute concentration outside the cell is higher (less water)
2. Water diffuses out of the cell until equilibrium is reached
3. Cells will shrink & die if too much water is lost
4. Plant cells become flaccid (wilt) called plasmolysis

J. Hypotonic Solution
1. Solute concentration greater
inside the cell (less water)
2. Water moves into the cell until equilibrium is reached
3. Animal cells swell & burst (lyse) if they take in too much water
4. Cytolysis is the bursting of cells
5. Plant cells become turgid due to water pressing outward against cell wall
6. Turgor pressure in plant cells helps them keep their shape
7. Plant cells do best in hypotonic solutions

K. Isotonic Solutions
1. Concentration of solutes same inside & outside the cell
2. Water moves into & out of cell at an equal rate so there is no net movement of water
3. Animal cells do best in isotonic solutions

IV. How Cells Deal With Osmosis

A. The cells of animals on land are usually in isotonic environment (equilibrium)

B. Freshwater organisms live in hypotonic environments so water constantly moves into their cells

C. Unicellular freshwater organisms use energy to pump out excess water by contractile vacuoles

D. Plant cell walls prevent plant cells from bursting in hypotonic environments

E. Some marine organisms can pump out excess salt

A. Faster than simple diffusion

B. Considered passive transport because extra energy not used

C. Occurs down a concentration gradient

D. Involves carrier proteins embedded in a cell’s membrane to help move across certain solutes such as glucose

E. Carrier molecules change shape when solute attaches to them

F. Change in carrier protein shape helps move solute across the membrane

G. Channel proteins in the cell membrane form tunnels across the membrane to move materials

H. Channel proteins may always be open or have gates that open & close to control the movement of materials called gated channels

I. Gates open & close in response to concentration inside & outside the cell

A. Requires the use of ATP or energy

B. Moves materials against their concentration gradient from an area of lower to higher concentration

C. May also involve membrane proteins

D. Used to move ions such as Na+, Ca+, and K+ across the cell membrane

E. Sodium-Potassium pump moves 3 Na+ out for every 2 K+ into the cell
1. Causes a difference in charge inside and outside the cell
2. Difference in charge is called membrane potential

F. Ion pumps help muscle & nerve cells work

G. Plants use active transport to help roots absorb nutrients from the soil (plant nutrients are more concentrated inside the root than outside)

A. Moves large, complex molecules such as proteins across the cell membrane

B. Large molecules, food, or fluid droplets are packaged in membrane-bound sacs called vesicles

C. Endocytosis moves large particles into a cell

D. Phagocytosis is one type of endocytosis
1. Cell membrane extends out forming pseudopods (fingerlike projections) that surround the particle
2. Membrane pouch encloses the material & pinches off inside the cell making a vesicle
3. Vesicle can fuse with lysosomes (digestive organelles) or release their contents in the cytoplasm
4. Used by ameba to feed & white blood cells to kill bacteria
5. Known as “cell eating”

E. Pinocytosis is another type of endocytosis
1. Cell membrane surrounds fluid droplets
2. Fluids taken into membrane-bound vesicle
3. Known as “cell drinking”

F. Exocytosis is used to remove large products from the cell such as wastes, mucus, & cell products

G. Proteins made by ribosomes in a cell are packaged into transport vesicles by the Golgi Apparatus


Section Summary

The combined gradient that affects an ion includes its concentration gradient and its electrical gradient. Living cells need certain substances in concentrations greater than they exist in the extracellular space. Moving substances up their electrochemical gradients requires energy from the cell. Active transport uses energy stored in ATP to fuel the transport. Active transport of small molecular-size material uses integral proteins in the cell membrane to move the material—these proteins are analogous to pumps. Some pumps, which carry out primary active transport, couple directly with ATP to drive their action. In secondary transport, energy from primary transport can be used to move another substance into the cell and up its concentration gradient.

Endocytosis methods require the direct use of ATP to fuel the transport of large particles such as macromolecules parts of cells or whole cells can be engulfed by other cells in a process called phagocytosis. In phagocytosis, a portion of the membrane invaginates and flows around the particle, eventually pinching off and leaving the particle wholly enclosed by an envelope of plasma membrane. Vacuoles are broken down by the cell, with the particles used as food or dispatched in some other way. Pinocytosis is a similar process on a smaller scale. The cell expels waste and other particles through the reverse process, exocytosis. Wastes are moved outside the cell, pushing a membranous vesicle to the plasma membrane, allowing the vesicle to fuse with the membrane and incorporating itself into the membrane structure, releasing its contents to the exterior of the cell.


Watch the video: 21. Μενδελική Κληρονομικότητα -εισαγωγή στον 1ο Νόμο Mendel 1. 5ο κεφ - Βιολογία Γ λυκείου (July 2022).


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