Cell Membranes_ also known as Plasma membrane
SC.912.L.14.2 Relate structure to function for the components of plant and animal cells. Explain the role of cell membranes as a highly selective barrier (passive and active transport).
STRUCTURE:
- Double membrane, composed of phospholipids, (Phospholipid bilayer. Each phospholipid is composed of a hydrophilic head and a hydrophobic tail.
- Contains embedded proteins, some of the proteins span the membrane, other do not, some are in the inside surface.
- The hydrophilic region (water-loving) is composed of the collection of all the phospholipid heads. They compose the outside surface, and the internal surface.
- It has a hydrophobic region (water-fearing) towards the middle portion of the membrane, where the hydrophobic, nonpolar phopholipid tails are.The tails of each phospholipid face each other.
FUNCTIONS:
- The semipermeable membrane is a structure surrounding every cell, that separates the internal environment of the cell from the external environment.
- It regulates the movement of molecules into, and out of the cell, thus maintaining a steady internal environment, also known as homeostasis.
- What can pass freely? Water and small, non-charged molecules, like carbon dioxide, oxygen, and alcohol.
- What needs assistance to cross the membrane? Ions and polar molecules, such as glucose and amino acids can cross the membrane but too slowly, thus requiring assistance via carrier proteins- AKA protein chanels. These carrier proteins are specific to the molecules they help transport.
- Macromolecules are another type of molecule that can't pass the membrane freely due to their large size. They move through the formation of vesicles. These processes are called: endocytosis, and exocytosis. Click here for animation
Types of transport across the semipermeable membrane
We can categorize the type of transport based on the requirement of energy (ATP) or not to be able to move such particle:
Passive Transport (No ATP Required)
- Passive transport is diffusion of a substance across a membrane with no energy investment.
- Simple Diffusion: simple movement from regions of high concentration to regions of low concentration.
- Osmosis: Specifically the diffusion of water across the semipermeable membrane from an area of low solute to into a region of higher solute concentration.
- Facilitated Diffusion: Movement of molecules down the concentration gradient through a transport protein.These proteins are highly specific. Example: Glucose and aminoacids can move via transport proteins, also Aquaporin facilitates the movement of water across the membrane.
Picture from Wikipedia
The blue dots represent the solute molecules driving the concentration gradient. If the membrane is impermeable to the solute, but permeable to water, then water moves in the direction that tends to equalize the solute concentrations on the two sides. As a result the water column rises in the area that was holding the higher concentration of molecules at the beginning. At this point one can say that the concentration is the same in both sides, or that it has reached equilibrium.
Observe how osmosis occurs under different conditions:
Active Transport (ATP required)
- Active transport uses energy to move solutes against their concentration gradients.
Examples:
- Sodium/Potassium Pump and cotransport
SAMPLE QUESTION 1:
Cells need to bring in molecules to carry out cellular processes. Often, this requires moving the molecules across the cell membrane against the concentration gradient. How do these molecules get into the cell?
A. passive transport by diffusion
B. active transport using ATP
C. passive transport by osmosis
D. phagocytosis
SAMPLE QUESTION 2:
A person with swollen gums rinses his mouth with warm salt water, and the swelling decreases. Which of the following has occurred?
A. The swollen gums have absorbed the saltwater solution.
B. The saltwater solution lowers the temperature of the water in the gums.
C. The salt in the solution has moved against the concentration gradient.
D. The water in the gums has moved out due to the high concentration of salt in the solution.
Reasoning: The salt water solution is hypertonic to the cells of the gums. It contains a higher concentration of solute (salt) and a lower concentration of water. Water from inside the cells of the gums moves down the concentration gradient and out of the cells.
SAMPLE QUESTION 3:
The cell membrane of the red blood cell will allow water, oxygen, and carbon dioxide to pass through. Because other substances are blocked from entering, this membrane is called
A. perforated
B. semi-permeable
C. non-conductive
D. permeable
Cells need to bring in molecules to carry out cellular processes. Often, this requires moving the molecules across the cell membrane against the concentration gradient. How do these molecules get into the cell?
A. passive transport by diffusion
B. active transport using ATP
C. passive transport by osmosis
D. phagocytosis
SAMPLE QUESTION 2:
A person with swollen gums rinses his mouth with warm salt water, and the swelling decreases. Which of the following has occurred?
A. The swollen gums have absorbed the saltwater solution.
B. The saltwater solution lowers the temperature of the water in the gums.
C. The salt in the solution has moved against the concentration gradient.
D. The water in the gums has moved out due to the high concentration of salt in the solution.
Reasoning: The salt water solution is hypertonic to the cells of the gums. It contains a higher concentration of solute (salt) and a lower concentration of water. Water from inside the cells of the gums moves down the concentration gradient and out of the cells.
SAMPLE QUESTION 3:
The cell membrane of the red blood cell will allow water, oxygen, and carbon dioxide to pass through. Because other substances are blocked from entering, this membrane is called
A. perforated
B. semi-permeable
C. non-conductive
D. permeable