Cell Membranes & Transport Flashcards
(27 cards)
Describe the basic structure of all cell membranes.
- phospholipid bilayer, embedded w proteins, carbs (glycoproteins + glycolipids) + cholesterol, allowing membrane to be partially permeable
Describe the fluid-mosaic model of membrane structure.
- fluid bc phospholipids + proteins can move around, giving membrane a flexible structure
- mosaic bc proteins embedded in phospholipid bilayer vary in shape, size + pattern
What are the diff components of membranes?
- phospholipids
- proteins
- cholesterol
- glycoproteins
- glycolipids
Describe the function of phospholipids in membranes.
- allows lipid-soluble substances + v small molecules to enter + leave cell
- prevents water soluble (polar) substances + large molecules entering + leaving cell
- makes membrane flexible + self-sealing
Describe the function of proteins in membranes.
- peripheral proteins (don’t extend completely across membrane): provide mechanical support, or combine w carbs to form glycoproteins, which act as receptors, along w glycolipids, in cell recognition
- integral proteins (extend completely across membrane): either protein carriers or channel proteins which transport molecules across membrane
Describe the function of cholesterol in membranes.
- restricts lateral movement of other molecules making up membrane
- this causes membrane to be less fluid at high T°Cs, + prevents water + dissolved ions leaking out cell
What ways can substances be exchanged across membranes?
- simple diffusion
- facilitated diffusion
- osmosis
- active transport
- co-transport
What is simple diffusion?
- passive, net movement of molecules, from an area of higher conc to an area of lower conc until equilibrium is reached
- molecules move bc of KE, enabling them to constantly move in fluids + must be lipid soluble + small
What is facilitated diffusion?
- passive, net movement of molecules, from an area of higher conc to an area of lower conc, w help of carrier proteins + channel proteins
Describe how carrier proteins transport substances across membranes.
- a molecule (e.g. glucose) binds to carrier protein, causing it to change shape, which enables molecule to be released to other side of membrane
Describe how channel proteins transport substances across membranes.
- they’re tubes filled w water, enabling water soluble ions to pass through membrane
- selective, bc only open when certain ions bind to protein
What is osmosis?
- movement of water molecules from an area of higher water potential, to an area of lower water potential, across a partially permeable membrane
What is the water potential of pure water?
- 0kPa, meaning when solutes are dissolved in water, water potential becomes neg
What are isotonic, hypotonic + hypertonic solutions?
- isotonic: when water potential of solution is same as in cell
- hypotonic: when water potential of solution is more pos. (higher) than in cell
- hypertonic: when water potential of solution is more neg. (lower) than in cell
Describe what happens when an animal cell is placed in an isotonic, hypotonic or hypertonic solution.
- isotonic: no change
- hypotonic: water moves into cell by osmosis, causing cell to burst
- hypertonic: water moves out of cell by osmosis, causing cell to shrink + shrivel
Describe what happens when a plant cell is placed in an isotonic, hypotonic or hypertonic solution.
- isotonic: no change
- hypotonic: water moves into cell by osmosis, causing cell to become turgid
- hypertonic: water moves out of cell by osmosis, causing protoplast to shrink + cell to shrivel + be plasmolysed
What is active transport?
- movement of molecules against a conc gradient, from an area of lower conc to an area of higher conc, using ATP + carrier proteins
Describe how substances are actively transported across membrane.
- molecule binds to complementary receptor on carrier protein
- ATP then binds to protein on inside of membrane, + is hydrolysed into ADP + Pi
- this causes carrier protein to change shape + release molecule to other side of membrane
- the Pi molecule is then released from protein, returning protein to its original shape
What is co-transport?
- when a molecule is transported across membrane, along w another molecule, in same direction through same carrier protein
How may cells be adapted for rapid transport across their internal or external membranes?
- by inc SA of membrane
- by inc NO° of carrier proteins in membrane
- by inc NO° of protein channels in membrane
Explain how SA affects the rate of movement across cell membranes.
- greater SA = greater NO° of molecules crossing at any moment so faster rate
Explain how NO° of carrier proteins + protein channels affect the rate of movement across cell membranes.
- greater NO° of carrier proteins + protein channels = faster rate
Explain how differences in conc gradients/water potential affect the rate of movement across cell membranes.
- greater diff. in conc gradient/water potential = faster rate
Describe a method used to investigate water potential of a plant tissue.
- use a cork borer to cut 5 potato cylinders (same length/mass) + blot dry to remove excess water
- measure + record initial mass, then place into 10cm3 sucrose solutions of diff. water potentials
- leave cylinders in solution for 30 mins, then remove + dry to remove excess solution
- measure + record final length + mass of each potato cylinder
- calculate % change in mass for each potato cylinder
