5. Cell Membrane Transport Flashcards
(16 cards)
Cell Membrane Structure
Primarily made up of phospholipids, cholesterol, and proteins—that move freely and fluidly
Phospholipid
Has a polar phosphate head (hydrophillic) and a hydrophobic fatty acid with a kink in the tail due to the double bond. Arranged in a bilayer, with hydrophabic tails inwards and hydrophillic heads outwards to cytoplasm or extracellular fluid.
Components of the Plasma Membrane
Function of the Cell Membrane
- Barrier:Cell membrane separaes the interior of the cell from the external environment, consisting of a semi-permeable lipid bilayer.
- Controls Movement: The membrane also regulates the transport of material entering and exiting the cell. The membrane is semi permeable, therefore ions and large molecules cannot enter wihtout assistance.
- Cell Signalling: Hormones bind to complementary receptors
- Cell Recognition:cells can be recognised as specific cells with specialised functions
Selective permeability
Polar and charged molecules have much more trouble crossing the membrane due to the hydrophobic core. E.g Water molecules cannot cross the membrane rapidly but can pass due to their small size.
While small ions are small enough to move through the membrane, their charge prevents them from doing so. Therefore ions like sodium, potassium, calcium, and chloride cannot cross membranes by simple diffusion as well as larger charged molecules, like sugars and amino acids, also need help from proteins to efficiently cross the membrane.
Diffusion
Movement of molecules from regions of high concentration to low concentration. Molecules will diffuse across the cell membrane
Facilitated Diffusion
Passive transport where molecules diffuse across the plasma membrane along the concentration gradient with assistance from membrane proteins. These molecules require assistance to move through the hydrophobic core of the membrane because they are charged or polar.
Facilitated transport proteins shield these molecules from the hydrophobic core of the membrane. The two types of facilitated transport proteins are channel proteins and carrier proteins.
Channel Proteins
Hydrophillic tunnels that span the membrane, allowing specific molecules to pass through diffusion. These channels are selective, only one type of molecule may pass through. Passage through a channel protein allows polar and charged compounds to avoid the hydrophobic core of the membrane.
Aquaporins
Aquaporins are channel proteins that allow water to cross the membrane very quickly, and they play important roles in plant cells, red blood cells, and parts of the kidney (where they minimize the amount of water lost as urine).
Carrier Proteins
Carrier proteins are typically selective and will change shape in response to binding of their target molecule, with the shape change moving the molecule to the opposite side of the membrane. Provides hydrophilic molecules with a way to move down an existing concentration gradient
What affects diffusion?
Membrane Permeability
Active Transport
Active transport is the transport of pumping molecules and ions across membranes that requires the use of ATP energy to move against the concentration gradient.
Active Transport v Facilitated Diffusion
Active transport uses carrier proteins, not channel proteins. Three main active transport methods - ion pumps, exocytosis and endocytosis.These carrier proteins are different than the ones seen in facilitated diffusion, as they need ATP in order to change shape. Channel proteins are not used in active transport because substances can only move through them along the concentration gradient.
Osmosis
Osmosis is a form of passive transport when water molecules move from low solute and high water to high solute andlow water concentration across a membrane that is not permeable to the solute
Endocytosis and Exocytosis
Endocytosis is the process by which substances are engulfed into the cell. The membrane folds over the substance making a vesicle which pinches off and moves the substance into the cytosol. Exocytosis is the reverse; the process by which substances are released from the cell. These processes allow larger molecules that cannot diffuse through the lipid bilayer to cross the membrane.
both require atp
Tonicity
