Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins

Aside from carbohydrates, membranes are actually composed mainly of lipids and proteins with phospholipids being the most abundant lipids present. The reason phospholipids are able to create membranes is because of their amphipathic structure. As the most current accepted model for the arrangement of molecules in a membrane, the fluid mosaic model depicts how proteins are embedded in a bilayer of phospholipids. These molecules are held in place by
Unsaturated hydrocarbon tails of phospholipids enhance membrane fluidity.
Unsaturated hydrocarbon tails of phospholipids enhance membrane fluidity.
hydrophobic interactions. Most lipids move laterally in a membrane while some proteins never move at all. Such membrane fluidity is affected by temperature; a cooler temperature causes a membrane to switch from fluid to solid due to a closer packed arrangement of phospholipids. Asides from temperature, membrane fluidity is affected by the membrane's components as well. Unsaturated fatty acids, saturated fatty acids, and cholesterols all some factors that can alter fluidity.
A membrane's functions is primarily determined by proteins. One single membrane protein can be responsible for: transportation, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cycoskeleton and extracellular matrix. Two major membrane proteins include: integral proteins and peripheral proteins. Integral proteins pierce through the hydrophobic core of the lipid bilayer and spans the membrane. On the other hand, peripheral proteins are not embedded in the lipid bilayer whatsoever.

  • Selective Permeability- Regulation of the substances that enter the cell membrane
    • Ex: Due to selective permeability, some substances corss the plasma membrane more easily than others.
  • Amphipathic- Having both a hydrophilic and hydrophobic region
    • Ex: Most lipid membranes are amphipathic, such as a phospholipid.
  • Fluid Mosaic Model- The current model of a cell membrane structure
    • Ex: In the fluid mosaic model, the membrane is envisioned to be drifting laterally in a fluid bilayer of phospholipids.
Detailed fluid mosaic model of the structure of a plasma membrane inside an animal's cell
Detailed fluid mosaic model of the structure of a plasma membrane inside an animal's cell

  • Integral Proteins- Transmembrane proteins with hydrophobic regions
    • Ex: An integral protein's hydrophobic regions consists of nonpolar amino acids.
  • Peripheral Proteins- Proteins loosely bounded to the surface of a membrane or parts of an integral proteins
    • Ex: Peripheral proteins are never embedded in the lipid bilayer.
  • Glycolipids- Lipids with covalently bonded carbohydrates
    • Ex: Membrane carbohydates that covalently bond to lipids result in glycolipids.
  • Glycoproteins- A protein with one or more carbohydrates covalently attached to it
    • Ex: Most carbohydrates covalently bond to proteins, forming glycoproteins.


Concept 7.2: Membrane structure results in selective permeability
Small molecules and ions move across the membrane in both directions. However, because the cell membrane is selectively permeable, the cell can take in some substances while others are strictly excluded. Nonpolar molecules can cross the membrane on its own more easily than other molecules due to its hydrophobic ability to dissolve the lipid bilayer. For the hydrophilic substances that meed more assistance in avoiding contact with the lipid bilayer, transport proteins are there to help. Some transport proteins act as a tunnel for molecules to go through to enter the membrane; these transport proteins are called channel proteins. Thus, a membrane's selective permeability is dependent on the barrier of the lipid bilayer and the specific transport proteins built.
  • Transport Proteins- Transmembrane proteins that help certain substances cross the membrane
    • Ex: By passing through transport proteins, some hydrophilic substances can avoid contact with the lipid bilayer.
  • Aquaporins- A channel protein in the plasma membrane that specifically faciliates osmosis
    • Ex: One aquaporin can allow an entry of up to 3 billion water molecules per second.

Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investmentDuring diffusion, a substance goes from being more concentrated to being less concentrated. In other words, a substance diffuses down its own concentration gradient. No work is required to make this happen. Because the cell does not need to spend energy, the diffusion of a substance across a membrane is called passive transport. When water crosses the selectively permeable membrane, it diffuses from a high to a low concentration and balances both sides of the membrane until the solute concentrations are equal. This diffusion is called osmosis and is imperative to the life of organisms. Cell survival depends on the balance of cell tonicity. Three environments that the cell can be immersed in are: an isotonic environment, a hypertonic environment, or a hypotonic environment. Adaptations such as osmoregulation assists organisms that lack rigid cell walls. Altogether, diffusion is passive and receives help from transport proteins.
During osmosis, the concentrations of a substance reach equilibrium.
During osmosis, the concentrations of a substance reach equilibrium.

  • Diffusion- The movement of molecules of any substance so that they spread out evenly into the available space
    • Ex: Diffusion results in a substance going from a region where it is more concentrated to a region where it is less concentrated.
  • Concentration Gradient- A region along which the density of a chemical substance increases or decreases
    • Ex: A substance diffuses down its own concentration gradient, unaffected by other substances' concentration differences.
  • Passive Transport- The diffusion of a substance across a membrane
    • Ex: Passive transport does not require the cell to use energy.
  • Osmosis- The diffusion of water across a selectively permeable membrane
    • Ex: Osmosis took place if the sugar concentrations on both sides of the membrane are equalized.
  • Tonicity- The ability of a solution to cause a cell to gain or lose water
    • The tonicity of a solution depends on the concentration of solutes that cannot cross the membrane.
An animal cell's best sate is in an isotonic environment; a plant's cell is healthiest in a hypotonic environment.
An animal cell's best sate is in an isotonic environment; a plant's cell is healthiest in a hypotonic environment.

  • Isotonic- Solution surrounding a cell that has no effect on the passage of water into or out of the cell
    • If a cell's environment is isotonic, then there is no net movement of water across the membrane.
  • Hypertonic- Solution surrounding a cell that causes it to lose water
    • A cell will shrivel and probably die if located in a hypertonic solution.
  • Hypotonic- Solution surrounding a cell that causes it to take up water
    • Ex: In a hypotonic solution, a cell will swell and burst like an overfilled water balloon.
  • Osmoreregulation- The control of water balance
    • Ex: Animals without rigid cell walls that live in hypertonic or hypotonic solutions have special adaptations for osmoreregulation.
  • Turgid- Swollen and firm
    • Ex: A healthy state for most plant cells is when it is turgid.
  • Flaccid- Limp and lacking in stiffness or firmness
    • Ex: If there is no net tendency for water to enter, the cells become flaccid.
  • Plasmolysis- When the cytoplasm shrivels and the plasma membrane pulls away from the cell wall
    • Ex: Plasmolysis occurs when a cell loses water to a hypertonic environment.
  • Faciliated Diffusion- Spontaneous crossing of molecules or ions across a membrane
    • Ex: Faciliated diffusion requires the assistance of specific transmembrane transport proteins.
  • Ion Channels- Transmembrane protein channels that allow specific ions to flow across the membrane
    • Ex: Ions are able to cross the membrane down its concentration gradient because of ion channels.
  • Gated Channels- Transmembrane protein channels that opens or closes in response to a stimulus
    • Ex: Gated channels can respond to both an electrical or a chemical stimulus.

Concept 7.4: Active transport uses energy to move solutes against their gradients

Some transport proteins can move solutes across a membrane against its concentration gradient. This process, however, requires energy and is called active transport. One way ATP provides energy for active transport is by transferring its phosphate group directly to the transport protein. This can cause the protein's shape to change so that the solute can bound to it and cross the membrane. An example of a transport system that works this way is the sodium-potassium pump. Besides diffusing from its concentration gradient, an ion can diffuse down its electrochemical gradient too. Transport proteins like the electrogenic pump or proton pump generate the voltage gradient across the membrane. By doing so, stored energy is pumped and can be used for cellular work.
  • Active Transport- Movement of substance across a cell membrane that requires energy
    • Ex: The movement of solutes against a concentration gradient during active transport are done by carrier proteins.
  • Sodium-Potassium Pump- Transport protein that actively exchanges sodium for potassium for the cell
    • Ex: The sodium-potassium pump is found in the plasma membrane of animal cells.
  • Membrane Potential- The difference of voltage across a cell's membrane
    • Ex: A cell's membrane potential ranges from about -50 to -200 millivolts.
  • Electrochemical Gradient- Combination of a chemical force and an electrical force acting on an ion
    • Ex: An ion's electrochemical gradient is affected by the concentration difference of the chemical and eletrical force.
A cotransprorter is able to diffuse a substance down its electrochemical gradient into a cell.
A cotransprorter is able to diffuse a substance down its electrochemical gradient into a cell.

  • Electrogenic Pump- Transport protein that generates voltage across a membrane
    • Ex: The major electrogenic pump of animal cells is the sodium-potassium pump.
  • Proton Pump- Active transport protein that uses ATP to transport hydrogen ions out of a cell
    • Ex: The main electrogenic pump of plants, fungi, and bacteria is a proton pump.
  • Cotransport- The pairing of the "downhill" diffusion of one substance with the "uphill" transport of another against its concentration gradient
    • Ex: A single ATP-powered pump can indirectly drive the active transport of other solutes through the mechanism of cotransport.

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
3 types of endocytosis in animal cells
3 types of endocytosis in animal cells
Small solutes and water need to both enter and leave the cell; particles and large molecules need to cross the membrane in packagings too. They do so through endocytosis or exocytosis; both of which requires energy. Exocytosis is used by many secretory cells to export products. An example of this is when neurons release neurotransmitters that signal other neurons or muscle cells. On the other hand, endocytosis is used by cells to take in molecules.This process results in the forming of new vesicles from the plasma membrane. Phagocytosis, pinocytosis, and receptor-mediated endocytosis are three examples of endocytosis.

  • Exocytosis- When the cell secretes certain biological molecules
    • Ex: Exocytosis occurs through the fusion of vesicles to the plasma membrane.
  • Endocytosis- When the cell takes in biological molecules and particulate matter
    • Ex: Endocytosis occurs through the forming of new vesicles from the plasma membrane.
  • Phagocytosis- "Cellular eating"
    • Ex: In phagocytosis, cells consume large substances.
  • Pinocytosis- "Cellular drinking"
    • Ex: In pinocytosis, cells gulps down droplets of extracellular fluid.
  • Receptor-Mediated Endocytosis- The movement of specific molecules inside a cell; possible due to proteins in the membrane that contain receptor sites specific to the molecules being taken in
    • Ex: Cells are able to acquire bulk quantities of specific substances because of receptor-mediated endocytosis.
  • Ligands- Molecules that bind specifically to other molecules
    • Ex: Ligands tend to bind to a receptor site of other larger molecules.

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