2.5 Biological Membranes Flashcards
What is the role of membranes within cells?
Membranes around organelles separate them from the cytoplasm, providing them with discrete entity to be able to carry out its function
e. g
- mitochondria (inner membrane cristae) provides large surface area for aerobic respiration
- chloroplasts (thylakoid membrane) house chlorophyll
What is the role of membranes at the surface of cells?
separates cell’s components from external environment
regulates transport of materials in and out of cells
has antigens so the organism’s immune system can recognise and not attack the cell
cell signalling - has receptors, hormones may bind to receptors
site of chemical reactions
What is the fluid mosaic model?
A model which proposes that the fabric of the membrane consists of a phospholipid bilayer with proteins floating in it, making up a mosaic pattern
The ‘fluid’ part represents how some parts of the membrane can move freely
What are the components of the fluid mosaic model?
Phospholipids - hydrophilic towards outside and hydrophobic facing inwards
Transport proteins:
Channel proteins (intrinsic - completely embedded) allow the movement of molecules that are too large to pass through a tube-like structure
Carrier proteins (extrinsic - partially embedded) use energy in the form of ATP to actively move substances across the membrane, by changing their shape…
…enzymes, antigens or receptor sites for cell signalling may attach to carrier protein
Cholesterol maintains fluidity and stability of the membrane
What is simple diffusion?
Movement of small molecules from an area of high concentration to an area of low concentration, without involving ATP
When the molecules have moved down their concentration gradient, they are still moving randomly but they are evenly dispersed so there is no net diffusion
Why is water a special case for diffusion?
Water molecules are polar so are insoluble in the non-polar phospholipid bilayer
However, in membranes where a very high rate of water is required, there may be special water channel proteins (aquaporins) to allow water molecules to pass
How is a concentration gradient maintained?
Many molecules entering the cells and then passing into the organelles and used for metabolic reactions
e. g
- oxygen entering respiring cells then diffusing into mitochondria for aerobic respiration
- carbon dioxide diffuses into the palisade mesophyll cells and then diffuses into chloroplasts
What are the factors affecting the rate of simple diffusion?
Temperature - kinetic energy of molecules increases so rate of diffusion increases
Diffusion distance - the thinner the membrane, the faster the rate of diffusion
Surface area - more diffusion across a larger surface area (cells specialised for absorption have extensions to plasma membrane called microvilli which increase SA)
Size of diffusing molecule - the smaller the ion or molecule, e faster the rate of diffusion
Concentration gradient - the steeper the gradient, the faster the rate of diffusion to the side of low concentration
What is facilitated diffusion?
A process by which molecules are transported down the concentration gradient
across a partially permeable plasma membranes
with the help of integral membrane proteins
and does not involve ATP
How does facilitated diffusion work?
A concentration gradient exists where ions and polar molecules can diffuse into the membrane but this is repealed by hydrophobic tails of the lipid bilayer
Facilitated diffusion uses integral membrane proteins to transport these molecules
Channel proteins - form hydrophilic passage through membrane for ions and polar molecules to pass, can be open at all times or gated by biological signal
Carrier proteins - bind to a molecule and then alter shape to bring it in or out of cell
What is osmosis?
The movement of water molecules, down their water potential gradient, across a partially permeable membrane
What is water potential?
The tendency of water molecules to move from one area to another
Pure water has highest water potential - 0kPa
The more solute molecules, the lower the water potential - becomes more negative
When water potential on both sides of the membrane become equal, there is no net osmosis although molecules will keep moving
What is cytolysis?
If water potential of solution is higher than inside an animal cell, it swells and bursts
What is crenation?
If surrounding solution’s water potential is lower than inside an cell, the water leaves the cell by osmosis cell and shrivels and becomes crenated
What is turgidity?
If the surrounding solution of a plant cell is of a higher water potential, water enters the cell by osmosis and it swells and pushes against the cell wall, becoming turgid
