Cell Membranes Flashcards
Roles of membranes (5)
- separating cell contents from outside environment
- separating cell components from cytoplasm
- cell recognition and signalling
- holding the components of some metabolic pathways in place
- regulating the transport of materials into or out of cells
Plasma membranes are….
Partially permeable barriers!!
Cholesterol
Provides mechanical stability. Steroid molecule fits between fatty acid tails, making barrier more complete so water molecules and ions can’t pass easily through the membrane
Phospholipid bilayer structure
A glycerol molecule with 2 fatty acid chains attached and a phosphate group. The phosphate group (the head) is hydrophilic, and the fatty acid chains (tail) is hydrophobic. The heads face outwards, and the tails inwards, forming a bilayer. The molecules can move freely, as a fluid. As the hydrophilic head can’t pass through the hydrophobic tail easily, the membrane has some stability, even though the molecules aren’t bonded together. 7-10nm thick
Phospholipid bilayer function
Basic structural component of all biological membranes. Hydrophobic layer (tails) creates a barrier to many molecules, and separates cell contents from outside world.
Membrane proteins
Channel proteins: Allow movement of substances across a membrane that are too large/polar to move through on their own.
Carrier proteins: Actively move substances across a membrane, using ATP, e.g. pumping ions into a cell in roots
Glycoproteins and glycolipids
Involved in cell signalling and in antigen detection. Are antigens them selves: recognition of a cell as self/non-self. Glycoproteins can also bind cells together in tissues - cell adhesion. Receptor sites for hormones/other drugs. Glycoproteins attach to water molecules to stabali
Membranes and temperature
Increasing temperature increases kinetic energy of the molecules. This means they vibrate more, making the membrane permeable. Therefore, more species can exit/enter the cell, which perhaps couldn’t before. The climate an organism lives in will effect the components of the membrane so it still has the required stability (e.g. cholesterol).
Cell signalling
The communication between cells which allows an effective coordination of a response. Cells communicate with one another by signals. Many molecules act as signals - some signal during processes taking place inside cells; others signal from one cell to others. Cytokines are an example of cell signals.
Membrane bound receptors
Sites where hormones and drugs can bind. Hormones are chemical messengers released into the blood from specific tissues. They bind onto receptors on target cells (any cell with the correct receptor), triggering a response (e.g. increased numbers of glucose channels in plasma membrane when insulin is released, so cells take up more glucose, lowering blood sugar levels). The receptors have a complementary shape to the chemical messenger.
Diffusion
Form of passive transport. Net movement of molecules from an area of high concentration to and area of low concentration, down a concentration gradient.
Rate of diffusion
The rate of diffusion depends on the concentration gradient; the kinetic energy of the molecules ; the surface area to diffuse across; the thickness of the membrane they must cross and the size of the molecule
Diffusion across membranes
Fat soluble molecules (e.g. steroid hormones), small molecules and ions (e.g. CO2,O2,H2O) can pass straight through the membrane. Large/charged molecules can’t pass directly through the membrane, and so they pass through channel proteins in the membrane surface - facilitated diffusion
Active transport
Movement of molecules/ions across a membrane, using ATP to drive protein pumps. Against a concentration gradient. Occurs through some carrier proteins, carrying specific molecules at a faster rate than diffusion, against a concentration gradient.
Process of active transport
Carrier protein changes shape so molecule fits into the protein on one side of the membrane only. This also forces the molecule to leave the protein on the other side of the membrane
