Download Skill Builder _6B homeostasis

Skill Builder # 6 B: Homeostasis and Cell Transport –Active Transport
Cell membranes help cells maintain homeostasis by controlling what substances may enter
and leave the cell. Some substances can cross the cell membrane without any energy use
by the cell. The movement of such substances is called passive transport and involves
substances moving from an area of high concentration to an area of low concentration.
However, in some cases, cells must move substances up the concentration gradient (from
low to high), which is known as active transport. Active transport requires the cell to
expend energy.
II. ACTIVE TRANSPORT
Active transport is the movement of a substance against its concentration gradient (from
low to high concentration). In cells, this is usually concerned with accumulating high
concentrations of molecules that the cell needs, such as ions, glucose, and amino acids.
Active transport uses energy, unlike passive transport, which does not use any type of
energy. Active transport is a good example of a process for which cells require ATP.
Examples of active transport include the uptake of glucose in the intestines in humans,
sodium/potassium pump (see figure below) in cell membranes (especially nerve cells) and
the uptake of mineral ions into root hair cells of plants.
Sodium potassium pump
Figure 1. This carrier protein shown is located in
the cell membrane. It transports both Na+ and K+ across the membrane against the
concentration gradient of each (low to high). In nerve cells, the difference in electrical charge
between the inside and outside of the cell, allows the conduction of electrical impulses to
take place.
Endocytosis
Endocytosis is a form of active transport. The cell must expend energy to make it happen.
Phagocytosis is a special form of endocytosis in which large particles such as microorganisms
and dead cells are ingested via large endocytic vesicles called phagosomes. In protozoa (tiny,
one-celled organisms), phagocytosis is a form of feeding: large particles taken up into vesicles
end up in lysosomes, and the products of the subsequent digestive processes pass into
the cytosol to be utilized as food. However, few cells in multicellular organisms are able to
ingest such large particles efficiently. In the gut of animals, for example, the breakdown of the
particles of food is extracellular and their hydrolysis products are imported into cells.
Pinocytosis, another form of endocytosis, is the transport of solutes or fluids into the cell.
Phagocytosis is important in most animals for purposes other than nutrition, and it is mainly
carried out by specialized cells—so-called professional phagocytes. In mammals, three classes
of white blood cells act as professional phagocytes—macrophages, neutrophils, and dendritic
cells. These cells all defend us against infection by ingesting invading microorganisms.
Macrophages also have an important role in scavenging senescent (aging) cells and cells that
have died by apoptosis (self-destruction).
Figure 2. Phagocytosis and
Exocytosis
Exocytosis
Exocytosis is essentially the reverse of endocytosis. Cells often use exocytosis to release
large molecules to the outside. An example of a large molecule that might need to leave the
cell is a protein. Proteins that are produced for export from the cell are made on ribosomes
and packaged into vesicles by the Golgi apparatus. The vesicles move to the edge of the
cell, fuse with the cell membrane, and then open up and release the proteins outside the
cell.
Homeostasis
Transport mechanisms are an integral part of the process of homeostasis.
Homeostasis refers to stability, balance, or equilibrium within a cell or the body. It is an organism’s
ability to keep a constant internal environment. Homeostasis is an important characteristic of living
things. Keeping a stable internal environment requires constant adjustments as conditions change
inside and outside the cell. The adjusting of systems within a cell is called homeostatic regulation.
Because the internal and external environments of a cell are constantly changing, adjustments must
be made continuously to stay at or near the set point (the normal level or range). Homeostasis can be
thought of as a dynamic equilibrium rather than a constant, unchanging state.
Feedback Regulation Loops
The endocrine system plays an important role in homeostasis because hormones regulate the activity
of body cells. The release of hormones into the blood is controlled by a stimulus. For example, the
stimulus either causes an increase or a decrease in the amount of hormone secreted. Then, the
response to a stimulus changes the internal conditions and may itself become a new stimulus. This
self-adjusting mechanism is called feedback regulation.
Feedback regulation occurs when the response to a stimulus has an effect of some kind on the
original stimulus. The type of response determines what the feedback is called. Negative
feedback occurs when the response to a stimulus reduces the original stimulus. Positive
feedback occurs when the response to a stimulus increases the original stimulus.
Thermoregulation: A Negative Feedback Loop
Negative feedback is the most common feedback loop in biological systems. The system acts to
reverse the direction of change. Since this tends to keep things constant, it allows the maintenance of
homeostatic balance. For instance, when the concentration of carbon dioxide in the human body
increases, the lungs are signaled to increase their activity and exhale more carbon dioxide, (your
breathing rate increases). Thermoregulation is another example of negative feedback. When body
temperature rises, receptors in the skin and the hypothalamus sense the temperature change. The
temperature change (stimulus) triggers a command from the brain. This command, causes a response
(the skin makes sweat and blood vessels near the skin surface dilate), which helps decrease body
temperature. Figure 3 shows how the response to a stimulus reduces the original stimulus in another
of the body’s negative feedback mechanisms in the context of blood glucose levels.
Figure 3: Control of blood glucose level is an example of negative feedback. Blood glucose
concentration rises after a meal (the stimulus). The hormone insulin is released by the pancreas, and it
speeds up the transport of glucose from the blood and into selected tissues (the response). Blood
glucose concentrations then decrease, which then decreases the original stimulus. The secretion of
insulin into the blood is then decreased.
Main examples of homeostasis in mammals are as follows:
• The regulation of the amounts of water and minerals in the body. This is known as osmoregulation.
This happens primarily in the kidneys.
• The removal of metabolic waste. This is known as excretion. This is done by the excretory organs
such as the kidneys and lungs.
• The regulation of body temperature. This is mainly done by the skin.
• The regulation of blood glucose level. This is mainly done by the liver and the insulin and glucagon
secreted by the pancreas in the body.
Name_______________________________________Pd_______________Date________________
6B Homeostasis and Cell Transport- Active Transport Questions
1. What two characteristics identify active
transport?___________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
2. What are two examples of active transport in
organisms?__________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
3. In the case of a sodium potassium pump, what does the electrical charge differential allow a nerve cell to
do?________________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
4. Give one example of phagocytosis being used for feeding and one example of phagocytosis being used for
defense.____________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
5. White blood cells called macrophages have an important role. What is
it?_________________________________________________________________________________________
___________________________________________________________________________________________
6. Give an example of how a cell might use
exocytosis.__________________________________________________________________________________
___________________________________________________________________________________________
7. Explain the meaning of
homeostasis?________________________________________________________________________________
___________________________________________________________________________________________
8. Explain negative feedback using blood glucose level as an
example.____________________________________________________________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
9. Give an example of another body system that depends on homeostatic regulation._______________________