Transport across membranes Flashcards
What is diffusion?
- Net movement of particles from high to low concentration
- Moves down a concentration gradient until equilibrium is reached
- A passive process- does’nt require energy
What molecules can diffuse through the lipid bilayer?
- Lipid-soluble molecules (e.g., steroids).
- Non-polar molecules (e.g., oxygen).
- Small polar molecules (e.g., carbon dioxide).
What molecules cannot diffuse through the lipid bilayer?
- Large polar molecules- eg. ions
- Rejected by the hydrophobic fatty acid tails
How does temperature affect diffusion?
- Increasing temperature increases the kinetic energy of molecules
- Speeds up diffusion
What is Fick’s Law?
- Rate of diffusion is proportional to the concentration gradient
- As time progresses, the concentration gradient decreases
- Diffusion rate slows down until equilibrium is reached
What is facilitated diffusion?
- Movement of larger molecules through hydrophilic channels in channel proteins
- Moves from high to low concentration down the gradient
- Passive process
Why does facilitated diffusion reach a maximum rate?
- All carrier proteins become saturated- at a higher temperature
- Rate plateaus despite increasing concentration
How can you identify diffusion vs. facilitated diffusion on a graph?
- Diffusion: Linear increase with concentration gradient.
- Facilitated diffusion: Increases initially then plateaus due to protein saturation
What is osmosis?
- Net movement of water molecules from high to low water potential
- Moves down the water potential gradient through a partially permeable membrane
- Passive process
What is water potential (Ψ)?
- Measure of water’s ability to leave a solution.
- Measured in kilopascals (kPa).
- Pure water has the highest water potential (Ψ = 0 kPa).
- Solutes lower water potential (more negative Ψ).
In which direction does water move in osmosis?
Always moves from high water potential to low water potential
What happens when plant cells are placed in pure water?
- Water enters the cell by osmosis
- Vacuole enlarges and pushes against the cell wall
- The cell becomes turgid
What happens when plant cells are placed in a concentrated salt solution?
- Water leaves the cell by osmosis.
- The cytoplasm and membrane shrink away from the cell wall
- The cell becomes plasmolysed
What is active transport?
- Movement of molecules against the concentration gradient- low to high
- Requires energy from ATP
- Uses carrier proteins in the plasma membrane
How does ATP provide energy for active transport?
- ATP is broken down
- Releases ADP + Pi + energy.
- Energy changes the shape of the carrier protein which allows molecules to be transported
What is endocytosis?
- Process where large molecules are taken into the cell
- Active process- requires energy
- Plasma membrane pulls inwards and forms a vesicle
What is phagocytosis?
- A form of endocytosis
- Lysosomes fuse with the vesicle and release hydrolytic enzymes
- Large molecules are digested
How do root hair cells take up mineral ions?
- Ions are transported against their concentration gradient
- Requires carrier proteins and energy from respiration
Why do plants take up fewer minerals from waterlogged soils?
- Water fills air spaces in the soil
- Reduces oxygen availability
- Less oxygen = less respiration = less ATP for active transport
