Cell Membrane Structure and Function

• All prokaryotic and eukaryotic cells are surrounded by a cell/plasma membrane.
• Cell/plasma membrane is a semipermeable membrane as it regulates substances that go into and out of the cell.
• The cell/plasma membrane is responsible for controlling how, when, and how much of the materials can enter and leave the cell.

Structure of the Membrane

• The cell membrane has a sandwich phospholipid bilayer.
• The bilayer consists of a hydrophobic region of tails (water-fearing) that faces in, and a hydrophilic (water-loving) region of heads that faces out.
• Hydrophilic heads act as bread.
• The greasy hydrophobic butter of lipid tails act as the filling.
• The listed structure is critical to the permeability of the cell membrane since it helps filter what will enter and exit the cell

Cell Membrane as a Fluid Mosaic

• The cell membrane is described as a fluid mosaic.
• It is considered fluid because the membrane exhibits properties that resemble a fluid in which lipids and proteins can move relative to each other.
• It is described as a mosaic for having diverse protein, cholesterol, and carbohydrate molecules embedded in its structure.

Proteins

• Proteins are found on the surface of the cell membrane, such as receptors, that transmit messages to the cell's interior.
• The inner surface also has some proteins that attach themselves to the internal support structure to provide cell shape.
• Some proteins are called transport proteins because they create tunnels, which selectively allow certain substances to enter and exit the cell.

Cholesterol Molecules

• Cholesterol molecules prevent fatty acid tails of the phospholipid layer from sticking to each other, which contributes to the fluidity of the cell membrane.

Carbohydrates

• Carbohydrates are attached to proteins to define the cell's unique characteristics and identify chemical signals.
• The phospholipid layer contains other molecules that float constantly moving, forming the basis of the fluid mosaic model of the cell membrane.

Transport of Molecules Across Cell Membrane

• There are three important characteristics of molecules that affect their ability to cross the cell membrane:
  • Size (macromolecules or micromolecules)
  • Charge (nonpolar or polar molecules)
  • Solubility (lipid-soluble molecules or not)
• Small, non-polar lipid-soluble molecules can easily cross the cell membrane.
• Large and polar molecules have difficulty crossing the cell membrane.
• Molecules found outside or inside the cell can cross the cell membrane in three ways:
  • Passive Transport
  • Active Transport
  • Bulk Transport
• The difference between passive and active transport mechanisms lies in the amount of a particular substance in its origin and its destination.

Concentration Gradient

• The concentration gradient is the difference between the amount of molecules present at two regions, which determines the movement of molecules.

Passive Transport

• Molecules move along the concentration gradient, from high to low concentration.

Active Transport

• Molecules move against the concentration gradient, from low to high concentration, through the use of energy.
• Energy is required for active transport to occur to move against the concentration gradient.
• Active transport is similar to swimming against the water current, while passive transport is simply swimming with the water current.

Simple Diffusion

• Simple diffusion: This is a natural process where molecules from the air move from a point of greater concentration to a point of lesser concentration to attain equilibrium
• Some small molecules, such as oxygen and carbon dioxide, move across the cell membrane easily via diffusion.
• Molecular oxygen and carbon dioxide can breeze straight into the cell membrane without being regulated, which is necessary since most organisms respire in the presence of oxygen.
• Diffusion occurs because molecules are in constant motion where the molecules tend to bump with each other.
• The more molecules in an area, the higher probability of collision among each other that causes them to be pushed away.
• Over time, molecules will begin to spread and will spread evenly through the area.
• Though the particles will continue to move randomly, there is no change in the concentration anymore.
• Dynamic equilibrium is the condition of continuous movement where there is no overall change in concentration.
• Diffusion can also occur in a liquid medium. An example of this is the spreading of powdered orange juice in a glass of water. The powdered orange juice is the solute, while water is the solvent.
• The diffusions of a solute within the solvent is affected by several factors:
  • Temperature: Higher temperature results in a faster movement of molecules, thus, faster rate of diffusion.
  • Pressure: Adding pressure like stirring results in faster rate of diffusion
  • Solvent Density: The higher the density of the solvent, the slower the diffusion
  • Concentration Gradient: The higher the concentration gradient, the faster the rate of diffusion
  • Solute Solubility: The more similar two substances are with each other, such as polar solute in polar solvents or nonpolar solute in nonpolar solvent, the faster the diffusion rate

Osmosis

• Water molecules move from a point of higher concentration to a point of lower concentration across the cell membrane.
• Since cells cannot function without adequate water, most cellular processes depend on osmosis.
• Osmosis is a regular process that happens in all living cells

Water Potential

• Water is the main compound inside the cytoplasm.
• Water solution is found inside and outside of the cell, separated by the semipermeable cell membrane
• The concentration of water on both sides of the cell membrane differs because of the presence of different solutes in and out of the cell.
• Water potential is the term used to describe the movement of water molecules as they undergo osmosis and measures the difference between the force that pushes water molecules and the force exerted by the membrane.
• Water solution with less solute (more diluted solution) has higher water potential, as compared to a water solution with more solute (more concentrated solution)
• Water potential gradient is established when two solutions of different water potentials are separated by a semipermeable membrane, thus allowing osmosis
• Osmotic pressure is the difference in the level of the two solutions after osmosis is measured and is the force that moves water molecules through the semipermeable membrane

Tonicity

• Tonicity refers to the strength of a solution in relation to osmosis
• When comparing two different solutions, such as that of a solution in the cytoplasm and that outside the cell, the terms isotonic, hypotonic, and hypertonic solutions are used.
• These terms are used only in relation to a pair of solutions.

Isotonic Solution

• When a cell is placed in a solution with the same concentration of water and other solutes, such as ions, phosphates, sugars, and other substances, as its cytoplasm, the cell is in an isotonic condition.
• Prefix iso means “the same"
• Tonicity refers to the strength of the solution
• In an isotonic solution, osmosis does not occur since there is no concentration gradient present
• Most cells in the body are in isotonic solution

Hypotonic Solution

• A cell placed in a solution with more water outside the cell than inside its cytoplasm is in a hypotonic condition
• Hypo comes from a Greek word, which means “under” or “beneath”
• In extreme situations, as osmosis continues, osmotic pressure builds up inside the cell, causing the cell to burst for its inability to withstand pressure
• Plants have a different response. When they are placed in a hypotonic solution, water molecules enter the vacuoles, causing it to expand, pushing the cytoplasm toward the cell wall.
• Plant cells' vacuoles engorged with water become firmer instead of bursting
• The pressure exerted by the water molecule is referred to as turgor pressure, and the phenomenon is called turgidity

Hypertonic Solution

• A hypertonic solution causes cells to shrink and lose their shape as a result of water loss
• Hyper comes from a Greek word, which means “above”
• This solution contains less water concentration as compared to that in the cytoplasm
• The result is the net movement of water molecules from cytoplasm to the external environment of the cell
• The removal of water molecules in the cytoplasm of animal cells causes the cell to become deformed
• In plant cells, the removal of water molecules causes the cell membrane to be pulled away from the cell wall and the cytoplasm shrinks, while the cell wall remains intact

Facilitated Diffusion

• Aside from essential gases and water molecules, cells need a supply of certain ions, sugar, and other small molecules to perform cellular functions. However, they need special membrane proteins to help them move through the cell membrane.

Membrane Transport Proteins

• Membrane transport proteins serve as vehicles in transporting molecules across the cell membrane

Facilitated Diffusion

• Facilitated diffusion is the movement of molecules across the cell membrane with the help of membrane transport proteins

Properties of Facilitated Diffusion

• A concentration gradient is required because it cannot transport molecules from low to high concentration
• Energy is not needed
• Transport proteins are specific to the type of molecules they can transport across the membrane
• The rate of transport reaches a maximum when all members transport proteins are used up (saturation)
• Membrane transport proteins are sensitive to inhibitors that can cause them not to function
• Two types of membrane transport proteins involved in facilitated diffusion:
  • Channel Proteins
  • Carrier Proteins

Channel Proteins

• Channel proteins contain tunnels or openings that serve as passageways of molecules

Carrier Proteins

• Carrier proteins undergo temporary binding to the molecule, resulting in a conformational change that moves the molecule through the membrane
• Amino acids and glucose use carrier proteins to cross the cell membrane

Aquaporins

• Water molecules can easily diffuse into the cell due to its small size, however, water molecules are polar, which makes its diffusion across the membrane very slow.
• Peter Agre, a recipient of the Nobel Prize in Chemistry in 2003, discovered water protein channels called aquaporins that allow water molecules to rapidly diffuse into and out of the cells found in areas with high water influx
• The role of aquaporins is most noticeable in the kidney where about 150 to 200 liters of water are reabsorbed from urine each day
• Water and facilitated diffusion of other substances do not require additional input of energy when they move from an area of high concentration to low concentration.
• All of these transport mechanisms are also known as passive transport mechanisms.

Active Transport

• Active transport involves the movement of molecules against the concentration gradient across the cell membrane that requires energy
• It also involves carrier proteins, which are often called pumps
• An example of a carrier protein used in active transport is the sodium-potassium pump that transports sodium and potassium ions into and out of nerve and muscle cells.
• Active transport is needed in nerve and muscle cells because potassium ions tend to move toward the cell, while sodium ions constantly move out of the cell despite its high concentration outside.
• Specifically, the pump transports three sodium ions out of the cell while moving two potassium ions into the cell.
• Because the level of sodium is high on the outside of the cell, it leads to a concentration gradient. As this process requires a lot of energy, cells that undergo such a process contain a lot of mitochondria.

Properties of Active Transport

• Energy is needed in the form of ATP
• Transport proteins are highly specific to the type of molecules they can transport across the membrane
• The rate of transport reaches a maximum when all membrane transport proteins are being used (saturation)
• Membrane transport proteins are sensitive to inhibitions that can cause them not to function

Bulk or Vesicular Transport

• Endocytosis is the process by which large molecules enter the cell through:
  • The cell membrane bends inward or invaginates and forms a vesicle containing the macromolecule to be transported.
• There are three types of endocytosis depending on the kind of macromolecule that needs to be transported:
  • Phagocytosis
  • Pinocytosis
  • Receptor-mediated endocytosis

Phagocytosis

• Phagocytosis, or "cell eating," is the process by which cells take in large particles or solids through infolding of the cell membrane to form endocytic vesicles.
• It starts with the formation and extension of the membrane as pseudopodia (false feet), which surrounds and engulfs the macromolecule, packaging them in a membrane-bound vacuole
• Is exhibited by white blood cells to capture and kill the invading bacteria
• When endocytic vesicle fuses with lysosomes, digestion occurs

Pinocytosis

• Pinocytosis, or "cell drinking,” is the process of taking in fluids into the cell by invagination of the cell membrane
• Any solute of small particles in the fluid will be moved into the cell. Cells lining the intestines as well as plant root cells use pinocytosis to ingest liquid substances

Receptor-Mediated Endocytosis

• Receptor-mediated endocytosis is very specific
• Plasma membrane becomes indented and forms a pit
• The pit lined with receptor proteins picks specific molecules from the surroundings
• The pit then closes and pinches off to form a vesicle, which eventually carries the molecules inside the cytoplasm
• Metabolites, hormones, and other proteins enter through this process

Exocytosis

• Exocytosis refers to materials for export, like proteins produced in the ribosomes and packaged in the Golgi apparatus, are secreted out of the cell.
• This is the opposite of endocytosis
• Macromolecules to be transported are carried by vesicles to the cell membrane
• The membrane surrounding the vesicle that fuses with the cell membrane breaks off
• As a consequence, the macromolecule is released out of the cell
• Insulin-secreting cells, waste materials, and secretion of hormones are done via exocytosis