4. Cell Membranes & Transport Flashcards
State the components of the phospholipid bilayer [5]
- Phospholipids
- Cholesterol
- Extrinsic and Intrinsic proteins
- Glycolipids
- Glycoproteins
Describe the fluid mosaic model of membranes [2]
- Fluid - individual phospholipids can move laterally or flip between monolayers, membrane is flexible
- Mosaic - Extrinsic and intrinsic proteins of different sizes and shapes form a pattern.
State the role of phospholipids in the phospholipid bilayer [3]
- form the basic structure
- hydrophilic heads face outwards and hydrophobic tails face inwards on either side of the bilayer
- These interactions allow lipid soluble molecules to cross but prevent water soluble molecules from crossing
Describe cholesterol in the phospholipid bilayer [3]
- Polar (has a hydrophilic heads + hydrophobic tails)
- fit between phospholipids
- head at membrane surface
Describe the function of cholesterol in the phospholipid bilayer
- Mechanical stability (without, membranes quickly break and cells burst open)
- In higher temps it helps stabilise cells, otherwise membranes could become too fluid
- can increase fluidity in cold temps and prevents it becoming too rigid
State the role of glycolipids in cell membranes
Involved in cell signalling and cell recognition
Outline the functions of the extrinsic (peripheral proteins) [3]
- Binding sites/receptors for hormones and drugs
- Help cells adhere to each other
- Involved in cell signalling
State the role of glycoproteins in cell membranes
Serve as recognition sites for chemicals
Outline the functions of intrinsic (integral) proteins in membranes [3]
- Electron carriers (respiration/photosynthesis)
- Channel proteins (facilitated diffusion)
- Carrier proteins (facilitated diffusion/active transport)
How are cholesterol, glycolipids and glycoproteins arranged in the bilayer [2]
- Cholesterol is dispersed in the membrane alongside the phospholipids
- Glycolipids and glycoproteins extend from either lipid or protein components within the membrane
Describe the functions of the cell surface membrane [3]
- Isolates cytoplasm from extracellular environment
- Selectively permeable to regulate transport of substances
- Involved in cell signalling/cell recognition
Outline the process of cell signalling [5]
Specific stimulus (e.g. light)
→ sender cell manufactures chemical to be sent
→ molecules (e.g. ligands) released by exocytosis and transported through bloodstream to target cell
→ binds to complementary receptor
→ effector cell stimulated
Define osmosis [2]
- diffusion of water across a partially permeable membrane from an area of higher water potential to an area of lower water potential
- This occurs until an equilibrium
What is water potential (Ψ)? [2]
- Pressure created by water molecules, measures in kPa
- The greater the concentration of water in a membrane, the more kinetic energy the system has, and therefore the greater the water potential.
How does osmosis affect plant and animal cells? [4]
Osmosis into cell:
- Plant - protoplast pushes against cell wall, turgid
- Animal - lysis
Osmosis out of cell:
- Plant - protoplast pulls away from cell wall, flaccid
- Animal - crenation
Define simple diffusion [2]
- Net spreading out of particles from an area of higher concentration to an area of lower concentration, down their concentration gradient
- passive process requiring no energy (atp)
Define facilitated diffusion
The net movement of substances from a higher concentration to a lower one (down a conc gradient) through transport proteins without the use of energy
Explain how channel proteins work [3]
- Form selective pores in phospholipid bilayer
- allow polar and charged molecules to pass through
- some channel proteins may be gated, opening or closing depending on the binding of a specific molecule or ion
Explain how carrier proteins work [2]
- Specific shape for the molecule they transport
- binds to complementary molecule, conformational change passes molecule to other side of membrane
Define active transport
Active movement of substances from a low conc to a high conc (against conc gradient) with use of energy in the form of ATP
Outline the process of active transport in cell membranes [5]
- Molecule binds to carrier protein with complementary shape
- ATP binds to separate binding site on carrier protein
- Carrier protein changes shape, moving molecules to the other side of the membrane
- Molecules released via ATP hydrolysis
- Carrier protein changes back to original shape
Define exocytosis
The bulk transport of substances out of a cell via vesicle that fuses with the plasma membrane using energy in the form of ATP
Define endocytosis
The bulk uptake of substances into a cell by invagination of the membrane to form a vesicle trapping the substances inside the cell with the use of energy in the form of ATP
Name 5 factors that affects the rate of diffusion [5]
- Temp
- Diffusion distance
- surface area
- size of molecule
- difference in conc (how steep the conc gradient is)
