2.4b Active Transport

23/09/2012 § Leave a comment

Sometimes cells work against the concentration gradient and pump particles out of its interior and out of the cell even if there is already a high concentration of that particle outside of the cell. The same happens vice versa (bringing substances inside the cell even if there’s already a high concentration of that substance inside of the cell) but less frequently. This is active transport, where the movement of substances across a membrane uses energy from ATP (adenosine triphosphate – the currency of energy in a cell) to transport substances from areas of higher concentration to areas of lower concentration. For this, cells use protein pumps (like channel proteins or transporter proteins, I’m assuming) that can only accept certain substances so that the entry and exit of all substances can be maintained.

The pumps work as such:

  • the particle enters the pump from the side of the membrane with the lower concentration
  • the particle binds to the site (which is specialized for that particle)
  • ATP is used to change the shape of the pump
  • the pump releases the particle into the side of the membrane with the higher concentration; pump returns to its original shape

Endocytosis and exocytosis are the processes that move substances into and out of the cell via active transport. Endocytosis (endo- meaning IN) uses vesicles made from phospholipids taken from the membrane itself to bring substances from outside of the cell to the inside of the cell. Exocytosis (exo- meaning OUT) uses vesicles made from the membranes of the cell’s Golgi apparatuses and are transported out of the cell. The below image illustrates both endo- and exocytosis.

Sometimes the cell produces substances to be used outside of the plasma membrane called extracellular components which we talked about last unit. Two examples are the plant cell wall and glycoproteins. For the cell wall, vesicles transfer cellulose fibers to be secreted outside of the membrane to create the wall. The wall, as we all know, helps to stabilize the plant’s shape, allows for high pressure without bursting, maintains turgor, and prevents an excess in water consumption. For the glycoproteins (in animals), cells release a thin layer of extracellular matrix called the basement membrane made of a protein (glycogen, I’d say) with a carbohydrate attached to it. These membranes can be found around blood capillaries and alveoli in the lungs and their main functions are to support the thin layers of the cells to prevent from tearing, and for cell-to-cell adhesion.

DATA BASED QUESTIONS

Page 35 :: DBQ

1. Describe the effect of reducing the oxygen concentration below 21.0% on the rate of phosphate absorption by roots. You should only use information from the table in your answer.

Reducing the oxygen concentration in the air bubbled through will start to decrease the phosphate absorption in the roots. From 21.0% to 2.1%, though the change in oxygen is drastic, the decrease in phosphate absorption is very small – only 0.01/µmol g-1 h-1. However, after 2.1%, as we keep reducing the oxygen percentage, phosphate absorption also starts to decrease more, e.g. at 0.9% (from 2.1%), phosphate is reduced from 0.32 to 0.27µmol g-1 h-1. 21.0% seems to be the margin or the limit that can affect when the phosphate absorption will really start to reduce.

2. Explain the effect of reducing the oxygen percentage from 21.0 to 0.1 on phosphate absorption. In your answer you should use as much biological understanding as possible of how cells absorb mineral ions.

The oxygen (and nitrogen) that is bubbled through the roots would have increased the concentration inside the root. In active transport, particles from outside the cell (in this case, the phosphate ions) would still be transported into the cell despite the higher concentration of particles inside. If the oxygen that is bubbled through the roots is reduced, then the concentration inside the root cell is not high enough to trigger active transport but if the oxygen bubbled through the roots increases, then the concentration inside the root will also increase, thus triggering reason for active transport, bringing in phosphate ions from outside the root cells to the inside of the cell.

3. Deduce, with a reason, whether the roots absorbed the phosphate by diffusion or active transport.

According to the graph, an increase in DNP concentration causes a decrease in phosphate absorption. If more DNP means less production of ATP, and ATP is required for active transport to bring in phosphate ions against a concentration gradient, then we can assume that phosphate is absorbed by active transport. The lack of ATP (caused by the increase of DNP) supports this deduction.

4. Discuss the conclusions that can be drawn from the data in the graph about the method of membrane transport used by the roots to absorb phosphate.

Well, the graph shows that the more DNP concentration added to the phosphate substance and roots causes less phosphate absorption. Since the root is absorbing the DNP and we know that DNP blocks production of ATP, then the root cells probably don’t have enough ATP to do active transport and bring in phosphate ions. This is supported by the fact that there is an immediate decrease of phosphate absorption as soon as the first additions of DNP are added to the solutions.

 

Page 37 :: Chapter 2 Questions

1. Figure 23 shows the structure of a membrane…

(a) I – phosphate heads that are hydrophilic, II – fatty acid tails that are hydrophobic

(b) III are integrated proteins that are fixed solidly into the membrane while IV are peripheral proteins that are more loosely attached to the membrane.

(c) V – glycoprotein, VI – cholesterol

(d) the thickness of a typical membrane is 10nm

2. Particles can enter or leave cells by diffusion, active transport, osmosis, endocytosis or exocytosis. Identify which method is used in each of these examples.

(a) osmosis

(b) endocytosis

(c) diffusion

(d) exocytosis

(e) active transport

3. In human secretory cells, for example in the lung and the pancreas, positively charged ions are pumped out, and chloride ions follow passively through chloride channels. Water also moves from the cells into the liquid that has been secreted. In the genetic disease cystic fibrosis, the chloride channels malfunction and too few ions move out of the cells. The liquid secreted by the cells becomes thick and viscous, with associated health problems.

(a) i – active transport, ii – (facilitated) diffusion, iii – osmosis

(b) Between the liquid and the water, the liquid is not hypertonic enough for the water to move into it, because there is a lack of chloride ions. This means the water can’t dilute or thin the liquid itself, which means that when the liquid is secreted, it will be thicker and more viscous than the normal secretion.

4. Table 6 shows the area of membranes in a rat liver cell.

(a) total area of membranes in the liver = 98,130 µm^2

(b) 1780 ÷ 98130 = 0.0181392 * 100% = 1.81%

(c) The inner mitochondrial membrane (39600 µm^2) is much thicker than the outer mitochondrial membrane (7470 µm^2). The inner membrane is a little more than five times thicker than the outer membrane (about 5.3 times larger). Also, the outer membrane is smoother while the inner membranes of the mitochondria are all wonky and folded and invaginated. 

(d) Since the rER and the mitochondria are such a large portion of all the membranes in the liver cell, two main activities of liver cells can be to produce proteins (via the rER) and to produce ATP or energy for the organism (mitochondria).

5.

(a) So I drew the sketch – it’s a little blurry but if you squint…?

(b) Cells receive and send messages via the proteins on their membrane. As we know, there are both integrated and peripheral proteins fixed or hanging loosely from the membrane. There are hormone binding sites and proteins used for cell-to-cell adhesion on the membrane. These proteins can receive particles that will inform the cell of the activity in its environment so that the cell can act accordingly. 

(c) Cells use endocytosis and exocytosis to transport all kinds of materials into and out of the cell. Both of these processes use vesicles to contain the materials while they are being transported. The processes are opposites of each other. In endocytosis, for example, materials are received outside of the membrane. The structure of the membrane draws a small portion towards the inside of the cell, bringing with it the material. This section is pinched off from the rest of the membrane and forms little circular pod – the vesicle! – that will transport the materials around the cell. In exocytosis, the opposite happens where membranes from within the cell transport the produced material to the membrane to be secreted outside of the cell. The ribosomes of the rER (rough endoplasmic reticulum) produce proteins or other material. Similarly to how vesicles were pinched off the plasma membrane for endocytosis, vesicles form from the rER to carry the proteins to the Golgi body, which meld with the vesicles because they also have a membrane. The Golgi body modifies the proteins and, with more budded vesicles, the proteins are carried to the plasma membrane to be released outside of the cell.

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