Biology - TRANSPORT ACROSS MEMBRANES

Which part of a phospholipid is hydrophobic?

tail

What is the role of cholesterol in the plasma membrane?

add strength, reduce lateral movement, stop leakage

Describe the importance of a partially permeable membrane

only allows certain molecules through - regulates movement of solutes

What does the fluid mosaic model describe?

the phospholipid bilayer is interspersed with proteins (like tiles in a mosaic) and constantly moves (fluid)

The centre of the bilayer is hydrophobic, so which type of molecules can diffuse through ?

small and non polar (and water as it’s so small)

how would Cl- ions move across a plasma membrane?

through a channel protein

What is a glycoprotein?

Proteins with a carbohydrate attached - usually protrude from the membrane.

If high temps are applied to the plasma membrane, what happens?

channel/carrier proteins denature, so membrane becomes more permeable

What is diffusion?

The net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient.

is diffusion active or passive ?

passive

Give an example of a molecule that can use simple diffusion to pass through the membrane

small and non polar eg oxygen, carbon dioxide

Describe the role of a carrier protein

Used to move large molecules across the membrane. When ion/molecule attaches to protein, it changes shape and releases molecules on other side of membrane.

What is facilitated diffusion?

Movement of particles from a high to low concentration, down the concentration gradient using a membrane protein

which types of molecule would be transported using facilitated diffusion across the membrane?

large, polar, water-soluble (eg glucose, amino acids)

Describe the role of a channel protein

acts as a pore in the membrane to allow larger/polar molecules to diffuse across

A graph showing the uptake of glucose by GLUT1 carrier proteins, shows that the rate increases as the number of glucose molecules increases. However, the graph plateaus. Why?

The system reaches equilibrium - there are only so many carrier proteins - if most of these are in use then the rate of glucose uptake would slow.

How can the gradient of a straight line or tangent be calculated?

change in y/change in x

Is facilitated diffusion passive or active?

Passive - no energy needed

What is osmosis?

The net movement of water molecules through a partially permeable membrane, from an area of high to low concentration OR from an area of high water potential (less negative) to an area of lower water potential (more negative).

What is the water potential of pure water?

0

If more of a solute (eg salt) is added to a beaker of water, what would happen to the water potential of the solution?

More solute = less water molecules so lower WP (less negative)

Describe the term isotonic

2 solutions are isotonic if they have the same WP , so no net movement of water occurs.

Pieces of potato with a WP of -350kPa are put into a solution with a WP of -275kPa. Explain what will happen to the potato.

Potato gains mass (has a lower WP than the solution) so water moves from solution into the potato

What is active transport?

The movement of particles from an area of lower concentration to higher concentration against a concentration gradient requiring energy from ATP.

How does ATP release energy to a cell?

ATP undergoes hydrolysis to form ADP + Pi, releasing energy.

What are co-transporters?

A type of carrier protein , which binds 2 molecules at a time and uses the gradient of one molecule to move the other against it’s conc. gradient.

Will the rate of active transport increase or decrease if the number of carrier proteins in the membrane is increased?

Increase

State 2 molecules moved by co-transport

glucose+sodium and amino acids+ sodium

What is a glycolipid?

A lipid with a carbohydrate molecule attached.

State 4 functions of membranes at the surface of cells

• Compartmentalisation

• Regulate transport of material into and out of cells (endo/exocytosis)

• Has surface antigens so immune system recognises cell as ‘self’

• Contains receptors for chemical signals (hormones, drugs, etc)

State 3 functions of membranes WITHIN cells

• compartmentalisation

• chemical reactions in the mitochondria and chloroplast

• vesicle production

Explain the orientation of phospholipids in the bilayer

Hydrophilic phosphate heads orientated outwards (towards water). Hydrophobic fatty acid tails orientated inwards (away from water)

Describe the function of glycoproteins and glycolipids in the phospholipid bilayer

As markers or antigens, cell signalling, cell communication, cell adhesion

Describe the structure of a phospholipid

Glycerol backbone, one hydrophilic phosphate head, 2 hydrophobic fatty acid tails

Describe the effects of increasing temp on membrane structure

As temp increases, phospholipids have more KE, carrier and channel proteins may become denatured, affecting membrane transport.

What is a channel protein?

A protein which creates a fluid filled pore in the cell membrane through which ions and small polar molecules can pass.

What is a carrier protein?

A protein which changes shape to allow larger molecules to pass through the membrane. In facilitated diffusion this requires no energy, in active transport this requires ATP.

Describe the movement of water through the phospholipid bilayer

simple diffusion, aquaporins

State 5 factors that affect the rate of simple diffusion

temp, surface area, diffusion distance, size of molecule, conc gradient

Describe how carrier proteins are used in active transport

Molecule binds to specific site in carrier protein, ATP binds to separate binding site, carrier protein changes shape (conformational change) and transports molecule across membrane.

Define bulk transport and give 2 examples

The movement of large molecules that are too big to pass across the plasma membrane. Endocytosis brings large molecules INTO the cell, enclosed in a vesicle. Exocytosis transports large molecules OUT of cells.

Describe the role of ATP in bulk transport

ATP is required to provide energy for the movement of vesicles along microtubules of cytoskeleton and to fuse vesicle membrane and plasma membrane together

What is the main difference between active transport and facilitated diffusion?

Active transport requires ATP, facilitated diffusion is passive.

What is water potential?

The tendency of water molecules to move from one region to another.

Describe what would happen to a RBC placed in a solution with a more negative WP than that of its cytoplasm

water would move OUT , down a WP gradient , shrinking to crenation

What would happen to a liver cell placed in a solution with a more positive WP than its own cytoplasm

water would move IN , down a WP gradient, swelling the cell and causing it to burst (cytolysis) as no cell wall

What would happen to a root hair cell placed in a solution with a lower WP than its own cytoplasm/vacuole

Water would move out down a WP gradient causing the cell to become flaccid then plasmolysed

What would happen to a guard cell placed in a solution of less negative WP than its own cytoplasm

Water would move IN down a WP gradient , causing the cell to swell and become turgid

Define hypotonic solution

lower solute concentration outside cell

Define hypertonic solution

higher solute concentration outside cell