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