Cell membranes and transport Flashcards
What are the membranes described as?
Fluid mosaic model
Describe an extrinsic protein:
On either surface of the bilayer. They provide structural support and form recognition sites by identifying cells and receptor sites for hormone attachment
Describe an intrinsic protein:
Extend across the bilayer, they include transport proteins which use active or passive transport to move molecules and ions across the cell membrane
Who proposed the fluid- mosaic model?
Singer and Nicolson in 1972
Why is the model of the membrane called the fluid mosaic model?
Fluid= the individual phospholipid molecules can move within the layer relative to one another
Mosaic= The proteins embedded in the bilayer vary in shape and size and in their distribution
Describe the permeability of the membrane to specific molecules and substances:
Small molecules e.g oxygen and carbon dioxide move between phospholipid molecules and diffuse across the membrane
Lipid soluble molecules e.g vitamin A dissolve in the phospholipid and diffuse across the membrane as its hydrophobic
Water soluble molecules e.g glucose, polar molecules or ions must pass through an intrinsic protein which form water filled channels, the cell surface membrane is selectively permeable
Why is cholesterol important within the membrane?
It will restrict the lateral movement of other molecules in the membrane. Also makes the membrane less fluid at high temperatures which prevents water and dissolved ions from leaking in or out of the cell.
What is the difference between a channel protein and a carrier protein?
Channel protein= are tubes filled with water to enable water soluble ions to diffuse
Carrier proteins= will bind to larger molecules such as glucose or amino acids and change shape to transport them to the other side of the membrane
Definition of Simple Diffusion:
Simple diffusion is an example of passive transport. It is the movement of molecules or ions from a high concentration to a region of lower concentration until they are equally distributed.
What affects the rate of simple diffusion?
The concentration gradient- the greater the difference in the concentration the more molecules will diffuse
The thickness of the exchange surface or distance to travel
The surface area of the membrane
The size of the diffusing molecule (smaller diffuse faster)
The nature of the diffusing molecule (Fat soluble molecules diffuse faster and so do non-polar molecules)
Temperature (temperature gives the ions/molecules more kinetic energy)
What does passive transport mean?
Down the concentration gradient
What is the definition of facilitated diffusion?
Facilitated diffusion allows insoluble molecules/ ions to move across the membrane via transport protein molecules. It is a passive process and therefore moves down the concentration gradient. However, the availability of the transport proteins limit the rate of diffusion.
Features of active transport:
Ions and molecules are moved from a lower to a higher concentration against the concentration gradient.
Requires energy from ATP. Anything that affects respiration will affect active transport.
Occurs through intrinsic protein
The rate is limited by how many carrier proteins are available
How does active transport work when passing a molecule through the membrane?
The molecule combines with a specific carrier protein on the outer membrane. ATP transfers a phosphate group to the carrier protein. The protein changes shape and carries the molecule across the membrane and into the cell. The molecule or ion is released into the cell. The phosphate ion is released from the carrier molecule and recombines with ADP back to ATP. Carrier protein returns to its original shape
What is water potential measured in?
Kpa(kilo pascals)
Definition of osmosis:
The movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane.
What is meant by an isotonic solution?
When the water potential is the same in the solution and the cell within the solution
What is meant by an hypotonic solution?
When the water potential of a solution is more positive (closer to zero) than the cell.
What is meant by hypertonic solution?
When the water potential of a solution is more negative than the cell.
What is Co-transport?
Type of active transport
Brings ions and molecules into the cell together on the same transport protein.
What word is used to describe a cell bursting in a hypotonic solution?
Haemolysis
What word is used to describe when a cell shrinks in a hypertonic solution?
Crenated
What is Endocytosis?
Occurs when material is engulfed by extensions of the cell membrane and cytoplasm, surrounding it, making a vesicle.
What are the two types of endocytosis? Explain them.
Phagocytosis= the uptake of solid material that is too large to be taken in by diffusion or active transport. Once a vesicle is formed, enzymes digest the contents. These products are absorbed into the cytoplasm.
Pinocytosis= The uptake of liquid by the same mechanism, although the vesicles are much smaller
