Transport Across Cell Membranes

model answer definition:

facilitated diffusion

• net movement of large polar water soluble substances

• down a concentration gradient

• using channel and carrier proteins

• passive process

model answer definition:

simple diffusion

• net movement of non polar or very small substances

• down concentration gradient

• across phospholipid bilayer 

• passive- doesn’t require energy from ATP 

model answer definition 

osmosis 

• water moves 

• from an area of high to low water potential 

• through aquaporins (water channels)

• passive- doesn't require energy from ATP

model answer definition 

active transport 

• movement of molecules through carrier proteins

• against concentration gradient

• requiring hydrolysis of ATP

what units are water potential measured in

kPa

what is the water potential of pure water?

0 kPa = maximum

what is a hypotonic solution?

A hypotonic solution has a higher water potential (less negative Ψ) than the cell’s cytoplasm

explain the movement of water in and out of an animal and plant cell in a hypotonic solution 

animal cell:

• water moves into the cell via osmosis

• from a region of higher water potential outside the cell to a region of lower water potential in the cell 

• cell swells and lyses 

plant cell:

• cell swells and becomes turgid 

• cell wall prevents lysis due to inward pressure it exerts 

explain the movement of water in and out of an animal and plant cell in a hypertonic solution 

• higher water potential in cell and lower water potential outside cell

• so water moves out of the cell via osmosis

animal: cell crenates

plant cell: cell becomes flaccid

what is a hypertonic solution?

solution has a lower water potential (more negative Ψ) than the cell’s cytoplasm

What is the evidence that active transport is an energy consuming process?

• Occurs against a concentration gradient

• Requires energy from ATP

• Stops when respiration inhibited / when ATP production stops

• Cells that carry out active transport have many mitochondria

describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport 

1. complementary substance binds to specific carrier protein

2. ATP binds, hydrolysed into ADP and Pi, releasing energy 

3. carrier protein changes shape, releasing substance on the side of higher concentration 

4. Pi released→ protein returns to original shape 

explain the role of carrier and channel proteins in facilitated diffusion

• shape / charge of protein determines which substances move

• channel proteins 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, conformational change in shape releases molecule to transport substance

give an example of 2 hydrophobic (non polar) molecules

1. gases (O₂ and CO₂)

2. lipids (fatty acids)

give an example of 3 hydrophilic (polar) molecules

1. sugars (glucose)

2. amino acids

3. ions

why is bilayer described as a fluid mosaic model

fluid- molecules are free to move laterally

mosaic- contains both phospholipids and proteins

describe the arrangement of the components of a cell membrane

• phospholipids form a bilayer - fatty acid tails face inwards, phosphate heads fact outwards

• proteins

intrinsic proteins span bilayer

extrinsic proteins on surface of membrane

• glycolipids and glycoproteins found on exterior surface

• cholesterol- bonds to phospholipid hydrophobic fatty acid tails

explain the arrangement of phospholipids in cell membrane

1. bilayer with water present on either side

2. hydrophobic fatty acid tails repel water, so point away from water into interior

3. hydrophilic phosphate heads attract water so point towards water on either side of bilayer

explain the role of cholesterol in cell membrane

1. restricts movement of other molecules making membrane

2. so decreases fluidity

define endocytosis

way of moving large molecules into cell

define exocytosis

way of moving large molecules out of cell

suggest how cell membranes are adapted for other functions

1. phospholipid bilayer is fluid→ membrane can bend for vesicle formation / phagocytosis

2. glycoproteins / glycolipids act as receptors / antigens → involved in cell signalling / recognition

explain the limitations imposed by the nature of the phospholipid bilayer

• restricts movement of polar, water soluble, larger substances 

• due to hydrophobic fatty acid tails in interior of bilayer 

describe how movement across membranes occurs by co transport

1. two different substances bind to and move simultaneously via a co-transporter protein

2. movement of one substance against its concentration gradient is often coupled with the movement of another down its conc gradient

give an example that illustrates co transport 

absorption of sodium ions and glucose by cells lining ileum 

describe an example that illustrates co transport

1. Na⁺ actively transported from epithelial cells lining ileum to blood by sodium potassium pump, using ATP

• establishing a concentration gradient of Na⁺ (lower in the cell, higher in ileum)

2. Na⁺ enters epithelial cell down its concentration gradient with glucose abasing its concentration gradient 

• via co transporter protein 

3. glucose moves down concentration gradient into blood via facilitated diffusion 

what is the process of the movement of Na⁺ and glucose into cell from ileum considered as 

• secondary active transport 

• does not directly require ATP but relies on ion conc grad established by primary active transport 

explain the adaptations of an epithelial cell for rapid transport

1. villi and microvilli→ increase SA for absorption

2. many mitochondria→ ATP for active transport

3. wall is 1 cell thick→ short diffusion distance

4. rich blood supply→ maintain concentration gradient

5. increase in number of proteins channels and carriers

describe how substances move across the cell surface membrane by facilitated diffusion (3)

1. water soluble polar molecules move down a concentration gradient

2. using channel and carrier proteins

3. each carrier / channel protein is specific to a substance

how does a straight line through the origin on a graph of concentration of molecule vs rate of uptake by cells show that monoglycerides are lipid soluble molecules

1. rate of uptake is directly proportional to concentration of monoglyceride molecule

2. indicating molecule diffuses directly through phospholipid bilayer

3. via simple diffusion

4. no other limiting factors e.g. number of carrier / channel proteins available

describe how surface area, number of channel or carrier proteins and differences in gradients of conc or water potential affect the rate of movement across cell membranes

• increasing SA of membrane increases rate of movement

• increasing number of channel / carrier proteins increases rate of facilitated diffusion/ active transport

• increasing concentration grad increases rate of facilitated diffusion - until number of channel / career proteins becomes a limiting factor as they’re all saturated

• increasing water potential grad increases rate of osmosis

explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes

• cell membranes folded e.g. microvilli in ileum → increase in SA

• more protein channels/ carrier proteins → for facilitated diffusion

• large number of mitochondria→ make more ATP by aerobic respiration for active transport

state Ficks law

rate of diffusion is directly proportional to both the surface area and concentration gradient but inversely proportional to thickness of membrane

how does increasing concentration of a water soluble, polar molecule affect rate of diffusion

1. at the beginning, conc of molecule is limiting factor, and so as conc of molecule increases, rate of diffusion also increases

2. as time progresses, molecule is passing through channels at its maximum rate, rate is limited by number of channel / carrier proteins

describe the effects of unsaturated and saturated fatty acid tails on the fluidity of cell surface membrane

saturated tail- contains single bonds only between carbons in hydrocarbon chain

unsaturated tails- contains at least one carbon carbon double bond in hydrocarbon chain

• meaning that tail becomes kinked

• therefore increasing fluidity as phospholipids cant pack as close together