2. Movement Of Substances Flashcards
State the fluid mosaic model of membrane structure.
There is a bilayer of phospholipids which are arranged with their hydrophobic tails facing inwards. the phospholipids move within the bilayer. A various immobile proteins are embedded in or attached to the bilayer of phospholipids.
Evidence for fluid mosaic model.
The model was supported by freeze fracture technique.
State the 3 functions of the cell membrane.
- The cell membrane is selectively permeable and it control the movement of substances moving in and out of the cell.
- The cell membrane forms a boundary between the contents of the cell and the external environment, ensuring that a constant internal environment within the cell is maintained.
- The cell membrane is responsible for cell-to-cell communication. Cell membranes are equipped with receptor proteins that receive chemical messenger molecules from other cells, allowing the cell to sense changes in the external environment and respond to them.
State and explain the 2 major populations of membrane proteins.
- Integral / intrinsic proteins are tightly bound within the bilayer.The hydrophobic amino acids will be in contact with the hydrophobic lipid bilayer, while the hydrophilic portions will be exposed to the aqueous medium on either side of the membrane.
- Peripheral / extrinsic proteins are loosely attached to the surface of the membrane, often to the exposed portions of the integral proteins.
State the 5 functions of membrane proteins.
i. Transport: Transport proteins are involved in the movement of specific molecules across the membrane by acting as channel or carrier proteins.
ii. Enzymes reaction: Some proteins are enzymes which have their active sites exposed to substances on either side of the membrane. In some cases, the enzymes are organised together as part of a metabolic pathway.
iii. Signal transduction: Some proteins act as receptors to receive chemical messenger, e.g. a hormone. Upon binding and receiving the signal, the protein will undergo a conformational change to relay the message to the inside of the cell.
iv. Cell to cell recognition: Cells are able to determine if other cells it encounters are alike or different from itself. Some proteins have identification tags exposed so that they can be specifically recognised by other cells.
v. Intercellular joining / adhesion: Some proteins may attach with other proteins of adjacent cells.
Define ‘diffusion’
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient.
State the 4 features of diffusion.
- It is a passive process – it does not require the expenditure of energy; it relies on the kinetic energy of the particles.
- It is a slow process.
- It will occur wherever a concentration gradient exists. An equilibrium is reached when the net concentration of particles on each side are equal. At this point, net movement stops.
- If there is more than one type of particles, diffusion of the particles will occur independently of one another.
What is facilitated diffusion?
Facilitated diffusion occurs when a substance is aided across a membrane by transport proteins. Charged particles (e.g. ions like Na+ , Ca2+) and relatively large polar molecules (e.g. amino acids, sugars, fatty acids and glycerol), cannot diffuse across the plasma membrane directly by diffusion because they are insoluble in lipids or are repelled by the hydrophobic region of the membrane.
How are the 2 types of transport proteins involved in facilitated diffusion?
- Channel proteins provide a hydrophilic passage for small polar molecules to flow very quickly from one side of the membrane to the other by shielding the molecules from the hydrophobic phospholipid bilayer.
- Carrier protein allows the ion or molecule that needs to be transported binds to a binding site on the carrier protein. This causes a change in the shape of the carrier protein such that a hydrophilic channel forms in the centre. Once the molecule reaches the other side of the membrane, the carrier protein releases the molecule and resumes its original shape.
Define ‘osmosis’.
Osmosis is the net movement of water molecules from a solution of higher water potential to a solution of lower water potential, down a concentration gradient, through a partially permeable membrane.
Channel protein on plasma membrane.
Aquaporins on the plasma membrane facilitate the movement of water molecules across the membranes of plant and animal cells.
Define ‘hypotonic’.
When solution B has higher water potential to solution A, solution B is hypotonic to solution A
Define ‘hypertonic’.
When solution B has lower water potential to solution A, solution B is hypertonic to solution C.
Define ‘isotonic’
When solution B has the same water potential as solution A, solution B is isotonic to solution A.
What happens when plant cell are placed in solution with higher water potential?
Cell sap has a lower water potential than its surrounding solution. Water enters cell by osmosis through selectively permeable cell membrane.Large central vacuole increase in size. Cytoplasm gets pushed against cell wall, resulting in turgor pressure. The cell is said to be turgid.
