Sara Roizen Essay Portion of Test Membranes are important structural features of cells. (a) Describe how membrane structure is related to the transport of materials across a membrane. (b) Describe the role of membranes in the synthesis of ATP in either respiration or photosynthesis. A. The plasma membrane controls the passage into and out of the cell it surrounds. Membranes exhibit selective permeability, which allows some substances to cross it more easily than others. Phospholipids form membranes. Phospholipids are amphipathic which means it has both a hydrophilic region and a hydrophobic region. Most of the proteins found within membranes have both hydrophobic and hydrophilic regions. The fluid mosaic model is an example model of a membrane in which the membrane is a fluid structure which has various proteins embedded in or attached to a bilayer of phospholipids. This phospholipid bilayer is able to exist because the hydrophobic tails or the phospholipids take shelter from the water while exposing the hydrophilic heads to the water. Nonpolar molecules such as carbon dioxide, oxygen, and hydrocarbons, are hydrophobic and can dissolve in the membrane’s lipid bilayer and cross it easily without the aid of membrane proteins. The hydrophobic interior of the membrane impedes the direct passage of polar molecules and ions which are hydrophilic, through the membrane. Thus polar molecules such as glucose, other sugars, and even water (which is only extremely small polar) pass through the lipid bilayer slowly. It is very difficult for a charged atom and its surrounding shell of water to penetrate the hydrophobic interior of the membrane.
concentration. This type of transport that works like a water pump requires energy input froma source like ATP.
Passive transport does the exact opposite. With a desire to reach equilibrium, particles movefrom where there is a high-concentration to a low-concentration. When equilibrium is reached(when there are equal amounts of the compound on both sides of the membrane), transportdoes not stop, but continues at an equal rate in both directions.
This flow of particles that occurs to reach equilibrium is analogous to the movement of particles of a scent when a bottle of perfume is sprayed. The particles flow away from eachother to spread out evenly across a room.
The role of many proteins that reside in the membranes is tofunction as enzymes and provide a channel for active transport.Like those of an anabolic reaction, the enzymes require energy toaccomplish their task.
This is seen in the sodium potassium pump that keeps a lowconcentration of sodium ions (Na
) and high concentration of potassium ions (K
) within the cell. It pumps three sodium ionsout of the cell for every two potassium ions transported in. Thispump in the plasma membrane is necessary in the nervoussystem by maintaining the resting potential.
After some chemical reactions, the wasteproducts must be moved out of the cellagainst the concentration gradient.Active transport is required to pumpthese compounds out of the cell usingenergy. This occurs through exocytosiswhere the waste products aretransported in small sacs with structuressimilar to that of the plasma membrane’sphospholipids bilayer.
Endocytosis is used to take things intothe cell through a process similar toexocytosis. It uses the same type of sacsused in exocytosis that are made out of sections of the plasma membrane.14. When food particles too large to be taken in through passive transport require entrance to thecell, a type of endocytosis occurs. This is called phagocytosis and is a very common methodof eating in single-cell organisms like amoebas. The plasma membrane engulfs the pieces of food and creates a small sac made of its phospholipid bilayer totransport it through the cell.
Smaller particles, commonly liquids, enter the cell through thesame method as phagocytosis, but on a smaller scale. The activetransport of small particles such as liquids into the cell ispinocytosis. These particles may not be able to enter the cellbecause of their charge.
In all of the above types of active transport like endocytosis andexocytosis, the sacs the particles travel by are actually smallpieces of the plasma membrane. In exocytosis, a section of themembrane is actually replaced as the waste products are releasedfrom the cell. Endocytosis removes some phospholipids from the plasma membrane thatpermits them to enter the selective permeable membrane.