Transport Across Cell Membranes Flashcards
Why is the cell surface membrane model said to be fluid and mosaic?
Molecules free to move laterally in phospholipid bilayer
Many components -phospholipids proteins glycoproteins and glycolipids
Describe the structure of a phospholipid
Hydrophilic head, which is a attracted to water
Hydrophobic tail which is repelled by water
Describe the function of phospholipids
Allows lipid soluble substances to enter/leave
Prevent water soluble substances entering/leaving
Makes membrane flexible and self healing
Describe extrinsic proteins
Occur on the surface of the bilayer and don’t extend across it
Describe the structure of intrinsic proteins
Channel for carrier proteins
Describe the function of extrinsic proteins
Provide structural support
Act as channel supporting water soluble substances
Allow active transport
What is the function of intrinsic proteins?
Cell surface receptors identify cells
Helps cell adhere together
Acts as receptors, e.g. hormones
Describe the structure of cholesterol
Very hydrophobic molecule that pulls fatty acid tails off the phospholipids together
What is the function of a cholesterol?
Reduces lateral movement of other molecules, including phospholipid
makes membrane decrease fluid at increase in temperature
prevents leakage of water and dissolved ions from cells
What is the structure of glycolipid?
Made up of carbohydrate bonded with a lipid the carbohydrate extends from the bilayer
What is the function of glycolipid?
Recognition sites, maintain stability, help cells attach to form tissues
What is the structure of glycoprotein?
Carbohydrates attached to an extrinsic protein
What is the function of a glycoprotein?
Recognition sites
maintain stability
allow cells to recognise one another
Explain the arrangement of phospholipids in a cell membrane
Bilayer with water present on either side
Hydrophobic fatty acid tails repelled from water so point away from water
Hydrophilic phosphate heads attracted to water so point to water
Explain the role of cholesterol in cell membranes
Restricts movement of other molecules making up membrane
So decreases fluidity and permeability and increases rigidity
Suggest how cell membranes are adapted for other functions
Phospholipid bilayer is fluid -membrane can bend for vesicle formation/phagocytosis
Glycoproteins/ glycolipids act as receptors/ antigens-involved in cell signalling/ recognition
Describe how movement across membranes occurs by simple diffusion
Lipid soluble(non polar) or very small substances for example oxygen
Move from an area of higher conc to an areas of lower con down a conc grad
Across phospholipid bilayer
Passive- doesn’t require energy from atp/ respiration (only Ek of substances)
What are the limitations imposed by the nature of the phospholipid bilayer?
Restricts movement of water soluble (polar) and larger substances eg sodium/ glucose
Due to hydrophobic fatty acid tails in interior of bilayer
Describe movement across membranes occurs by facilitated diffusion
Water soluble (polar) slightly larger substances
Moves down a concentration gradient
Three specific channel/carrier protein
Passive – doesn’t require energy from ATP/respiration
Explain the role of carrier and channel proteins in facilitated diffusion
Shape/charge of protein determines which substances move
Channel
protein facilitate diffusion of water soluble substances
-hydrophilic pore filled with water
-may be gated can open/close
Carrier proteins facilitate diffusion of slightly larger substances
– complementary substance attaches to binding site
-Protein changes shape to transport substances
Describe how movement across membranes occurs via osmosis
Water diffusers/move
From an area of high to no water potential/down the water potential gradient
Through a partially permeable membrane
Passive – doesn’t require energy from ATP/respiration (only kinetic energy of substances )
Describe how movement across membranes occurs by active transport
Substances move from an area of lower to higher concentration/against a conc grad
Requiring hydrolysis of ATP and specific carrier proteins
Describe the role of carrier proteins and the importance of hydrolysis of atp in active transport
Complementary substance binds to specific carrier protein
ATP binds, hydrolysed into ADP plus inorganic phosphate group releasing energy
carrier proteins change shape releasing substances on side of higher concentration
in organic phosphate group released – protein returns to original shape
Describe how movement across membranes occurs by active transport
Two different substances bind to and move simultaneously via a
co-transporter protein (type of carrier protein)
● Movement of one substance against its concentration gradient is often
coupled with the movement of another down its concentration gradient
How is the movement of sodium described to be ?
The movement of sodium can be considered indirect / secondary active transport, as it is reliant on a
concentration gradient established by active transport.
Describe how surface area, number of channel or carrier proteins and
differences in gradients of concentration or water potential affect the rate of
movement across cell membranes
● Increasing surface area of membrane increases rate of movement
● Increasing number of channel / carrier proteins increases rate of facilitated diffusion / active transport
● Increasing concentration gradient increases rate of simple / facilitated diffusion and osmosis
● Increasing concentration gradient increases rate of facilitated diffusion
○ Until number of channel / carrier proteins becomes a limiting factor as all in use / saturated
● Increasing water potential gradient increases rate of osmosis
Explain the adaptations of some specialised cells in relation to the rate of
transport across their internal and external membranes
Membrane folded eg. microvilli in ileum → increase in surface area
● More protein channels / carriers → for facilitated diffusion (or active transport - carrier proteins only)
● Large number of mitochondria → make more ATP by aerobic respiration for active transport
Describe how sodium ions and glucose by cells lining the ileum are absorbed
● Na+ actively transported from
epithelial cells to blood (by
Na
+/K+
pump)
● Establishing a conc. gradient
of Na
+ (higher in lumen than
epithelial cell)
2 ● Na+ enters epithelial cell down
its concentration gradient with
glucose against its
concentration gradient
● Via a co-transporter protein
3 ● Glucose moves down a conc.
gradient into blood via
facilitated diffusion
