cell transport

introduction to cell transport

Cells are found in all different types of environments, and these environments are constantly changing. For example, one-celled organisms, like bacteria, can be found on your skin, in the ground, or in all different types of water. Therefore, cells need a way to protect themselves. This job is done by the cell membrane, which is also known as the plasma membrane.

controlling the cell contents

The cell membrane is semipermeable, or selectively permeable, which means that only some molecules can pass through the membrane. If the cell membrane were completely permeable, the inside of the cell would be the same as the outside of the cell. It would be impossible for the cell to maintain homeostasis. Homeostasis means maintaining a stable internal environment. For example, if your body cells have a temperature of 98.6 F, and it is freezing outside, your cells will maintain homeostasis if the temperature of the cells stays the same and does not drop with the outside temperature. How does the cell ensure it is semipermeable? How does the cell control what molecules enter and leave the cell? The composition of the cell membrane helps to control what can pass through it.

composition of the cell membrane

Molecules in the cell membrane allow it to be semipermeable. The membrane is made of a double layer of phospholipids (a "bilayer") and proteins ( Figure below). Recall that phospholipids, being lipids, do not mix with water. It is this quality that allows them to form the outside barrier of the cell. A single phospholipid molecule has two parts: 1. A polar head that is hydrophilic, or water-loving. 2. A fatty acid tail that is hydrophobic, or water-fearing. The cell membrane is made up of a phos- pholipid bilayer, two layers of phospho- lipid molecules. Notice the polar head group of the phospholipid is attached to the phosphate, and the tails are two fatty acid chains. The head group and tails are attached by a glycerol backbone. There is water found on both the inside and the outside of cells. Since hydrophilic means water-loving, and they want to be near water, the heads face the inside and outside of the cell where water is found. The water-fearing, hydrophobic tails face each other in the middle of the cell membrane, because water is not found in this space. The phospholipid bilayer allows the cell to stay intact in a water-based environment. An interesting quality of the plasma membrane is that it is very "fluid" and constantly moving, like a soap bubble. This fluid nature of the membrane is important in maintaining homeostasis. It allows the proteins in the membrane to float to areas where they are needed. Due to the composition of the cell membrane, small molecules such as oxygen and carbon dioxide can pass freely through the membrane, but other molecules, especially large molecules, cannot easily pass through the plasma membrane. These molecules need assistance to get across the membrane. That assistance will come in the form of transport proteins.


instructional diagrams

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the cell membrane and the plasma membrane are the same thing.

-->  a. true

b. false

the cell membrane is very fluid, so it is constantly moving, like a soap bubble.

-->  a. true

b. false

what part of the phospholipid is hydrophobic?

a) the head group

-->  b) the tail

c) the middle part

d) all of the phospholipid

in the phospholipid bilayer, the fatty acid chains face ____.

a) the outside of the cell

b) the inside of the cell

c) both the inside and outside of the cell

-->  d) the middle of the bilayer

how could large molecules cross the cell membrane?

a) they can pass freely through the membrane

-->  b) with the assistance of transport proteins

c) large molecules can not pass through the membrane

d) none of the above

what attaches the head group to the tails in a phospholipid molecule?

-->  a) a glycerol backbone

b) a bilayer sheet

c) a phosphate molecule

d) the phospholipid

one important feature of the semipermeable membrane is that

a) semipermeability keeps the cell intact.

-->  b) semipermeability helps the cell maintain homeostasis.

c) semipermeability makes the bilayer form.

d) all of the above

diagram questions

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