Unit 1.3 - Cell membranes and transport Flashcards Preview

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What is the importance of the hydrophobic tails in the phospholipid bilayer?

Tails point towards each other, to the centre of the membrane.


What is the cell membrane made of?

phospholipids and proteins


1) What is the importance of the inner layer of the phospholipid?

2) What is the importance of the outer layer of the phospholipid?

1) Hydrophilic heads points towards the cell and interacts with the water in the cytoplasm.

2) Hydrophilic heads point outwards, interacting with water surrounding the cell.


What are biological membranes?

Phospholipid bilayers which contain protein molecules.


What does the phospholipid component of a membrane allow?

Allows lipid soluble molecules across, but no water-soluble molecules.


What is the function of cholesterol?

Stabilises the membrane.


1) Where are extrinsic proteins found?

2) What are their functions?

1) Either surface of bilayer

2) Structural support, form recognition sites by identifying cells and receptor sites for hormone attachment.


1) Where are intrinsic proteins found?

2) What are their functions?

1) Extend across both layers of the phospholipid bilayer.

2) Act as channel and carrier proteins for the active or passive transport of molecules.


Who was the fluid mosaic model proposed by?

Singer and Nicolson in 1972.


Fluid-Mosaic Model

1) What is meant by the term fluid?

2) What is meant by the term mosaic?

1) Individual phospholipid molecules are free to move within a layer relative to one another.

2) Proteins embedded in bilayer vary in shape, size and distribution among the phospholipids.


What do plant and animal cell membranes contain?

Glycoproteins, glycolipids and sterols.


1) What is the sterol in animal cell membranes?

2) Where does it occur?

3) What effect does this have on the membrane?

1) Cholesterol

2) Between phospholipid molecules

3) More stable at high temperatures and more fluid at low temperatures.



How does small molecules move across the cell membrane?

e.g. oxygen, moves between phospholipid molecules and diffuse across the membrane.



How does lipid-soluble molecules move across the cell membrane?

e.g. vitamin A, dissolves in phospholipid and diffuses across the membrane. Layer is hydrophobic so lipid-soluble move more easily than water-soluble.



1) How does water soluble molecules move across the cell membrane?

2) How does this effect the cell-surface membrane?

1) e.g. glucose and ions, can't readily diffuse through phospholipids, so pass through intrinsic protein molecules, which form water-filled channels across the membrane.

2) Selectively-permeable to water and some solutes.


What is the size of the phospholipid bilayer?



What is simple diffusion?

Diffusion of small lipid soluble molecules which is a passive transport.


How does the concentration gradient affect the rate of diffusion?

Greater difference in the concentration of, more molecules diffuse in a given time.


How does the thickness of the exchange surface affect the rate of diffusion?

Thinner membrane, more molecules diffuse in a given time.


How does the surface area of the membrane affect the rate of diffusion?

Larger area, more molecules diffuse across in a given time.


How does the size of a diffusing molecule affect the rate of diffusion?

Smaller molecules diffuse faster than larger ones.


How does the nature of diffusing molecules affect the rate of diffusion?

Lipid-soluble molecules diffuse faster than water-soluble molecules.


How does temperature affect the rate of diffusion?

Increased temperature, increases rate as molecules/ions gain kinetic energy.


What is the relationship between rate of uptake and oxygen concentration?

Directly proportional - external conc. oxygen around a root increases, conc. gradient becomes steeper.


Why does facilitated diffusion occur?

Ions and molecules can't pass through membrane because they're insoluble in the phospholipid bilayer. Facilitated makes it easier for these molecules to diffuse.


How does facilitated diffusion (water-soluble molecules) occur?

Passive process and uses transport protein molecules.


1) What are channel proteins?

2) What is their function?

1) Molecules with pores with a hydrophilic lining allowing polar molecules to pass through.

2) Channels can be opened or closed to regulate the needs of the cell.


1) What do carrier proteins allow?

2) How does this occur?

1) Diffusion of larger polar molecules (sugars, amino acids) across the membrane.

2) Molecule attaches to binding site, carrier protein changes shape and releases the molecule on other side of membrane before changing back to its original shape.


What effect do carrier and channel proteins have on the rate of diffusion?

Increase rate down a concentration gradient without ATP from respiration.


State 6 differences between channel and carrier proteins.

Channel proteins:

Pore, only transport down a concentration gradient, solute doesn't bind to transport protein, don't change shape and has rapid transport.


What is the limiting factor between amino acid concentration and the rate of uptake?

There is a plateau. No increase in rate of uptake because all of the carrier proteins are occupied.


What is active transport? (contains mitochondria)

Ions and molecules are moved from an area of lower to an area of higher concentration against the concentration gradient using ATP.


1) Where does active transport occur?

2) What is rate limited by?

1) Through intrinsic carrier proteins spanning the membrane.

2) Number and availability of carrier proteins.


State 4 processes involving active transport.

Muscle contraction, nerve impulse transmission, reabsorption of glucose in the kidney and mineral uptake into plant root hairs.


What does an active transport graph show?

Maximum rate of transport can still be reached when the carrier proteins are saturated.


1) What is the rate of active transport reduced by?

2) How does it cause this?

1) Respiratory inhibitor - cynanide

2) Prevent aerobic respiration and the production of ATP in the mitochondria.


What is co-transport?

A type of facilitated diffusion that brings molecules and ions into cells together on the same transport protein molecule.


What is sodium-glucose co-transport used for?

Absorbing glucose and sodium ions across membranes and into the blood in the ileum and kidney nephron.


Describe the process of co-transport (6).

1) Glucose molecule & 2 sodium ions outside bind to a carrier protein in cell membrane.

2) Faciliated diffusion - protein changes shape & deposits them inside the cell.

3) Both separately diffuse through the cytoplasm to opposite membranes.

4) Glucose passes into blood capillaries via facilitated diffusion.

5) Sodium ions carried out of epithelial cell by active transport.

6) Concentration remains low, so more sodium ions move in from intestinal lumen, bringing glucose in on same carrier molecule.


What is the water potential of pure water?



What is the relationship between solute concentration and water potential?

Higher concentration, more strongly water molecules are pulled in and so the lower the water potential.


What type of energy does a dilute solution contain?

High potential energy because they are free to move.


What is the water potential of a strong glucose solution?



What is the relationship between solute potential and the concentration of the solute?

Higher concentration solution has a lower, more negative solute potential.


1) How does a plant cell become turgid by osmosis?

2) What is the pressure known as?

1) Water entering cell by osmosis expands the vacuole and pushes the cytoplasm against the cell wall. It can only expand a little and so pressure outwards builds up making the cell turgid.

2) Pressure potential


What two opposing forces are plant cells under the influence of?

Solute potential - higher concentration, less likely water is to move out.

Pressure potential - increases tendency of water to move out.


How do you calculate water potential?

water potential = solute potential + pressure potential


1) How can you tell if a solution is hypotonic?

2) Where does the water go?

1) Water potential of external solution is higher than solution inside the cell.

2) Water flows into the cell.


1) How can you tell if a solution is hypertonic?

2) Where does the water go?

1) Water potential of external solution is lower than solution inside the cell.

2) Water flows out of the cell.


1) How can you tell if a solution is isotonic?

2) Where does the water go?

1) Cell has the same water potential as the surrounding solution.

2) No net water movement.


1) Describe the process of plasmolysis.

2) What happens to the plant cell?

1) Hypertonic loose water by osmosis. Vacuole shrinks and cytoplasm draws away from the cell wall.

2) Flaccid - causing plant to wilt.


1) Describe the process of incipient plasmolysis.

2) How does this effect the pressure potential?

1) External concentration is high enough that the cell has lost enough water that its membrane pulls away from the cell wall.

2) Cell wall doesn't exert any pressure on cytoplasm so pressure potential is 0.


What is the water and pressure potential in a turgid cell?

w= 0
p= -s


What is water and pressure potential at incipient plasmolysis?

w= s


What is water and pressure potential when a cell is plasmolysed?

w is greater than s of external solution


Turgid cell

1) How does a plant cell become turgid?

1) Hypotonic solution takes in water until prevented by opposing pressure from cell wall. As water enters, contents expand and push out on cell wall increasing pressure potential.


Why does water potential = solute potential in an animal cell?

No cell wall so pressure potential doesn't need to be considered.


1) What is haemolysis?

2) What happens to the red blood cells when they are placed in a concentrated salt solution (hypertonic)?

1)Red blood cells are in distilled water (hypotonic), water enters by osmosis and they burst due to no cell wall.

2) Water leaves the cells and they shrink becoming crenated.


1) What can be used to investigate the permeability of membranes?

2) What is the rate that betacyanin diffuses out of the vacuole affected by?

1) Beetroot, has vacuoles containing red pigment called betacyanin.

2) Temperature and presence of organic solvents.


When beetroot discs are cut with a borer and immersed in a solution of 70% ethanol at 15°C, red pigment leaks out into the ethanol turning it red.

Explain why this leakage of pigment occurs.

Ethanol dissolves phospholipids which creates holes in the membrane.


When the beetroot experiment was repeated at 30°C, the time taken for the ethanol to turn red decreased. Explain why.

Increased temperature, increases kinetic energy of the dye molecules which increases the rate of diffusion of dye across membrane.



Passive movement of molecules or ions form a region of high concentration to a region of lower concentration (down a concentration gradient).


Faciliated diffusion

Passive transfer of molecules or ions down a concentration gradient, across a membrane, by channel or carrier proteins.


Active transport

Movement of molecules or ions across a membrane against a concentration gradient, using energy from hydrolysis of ATP made by respiration.



Transport mechanism in which facilitated diffusion brings molecules and ions, such as glucose and sodium ions, across the cell membrane together into a cell.



Net passive diffusion of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential.


Water potential

Tendency for water to move into or out of a system.


Solute potential

Measure of the osmotic strength of a solution.



Plant cell holds as much water as possible. Further entry of water is prevented as the cell wall can't expand any further.


Pressure potential

Hydrostatic pressure exerted by the cell contents of the cell wall. It's equal and opposite to the pressure exerted by the cell wall on the cell contents.


Hypertonic solution

Lower water potential relative to the solution inside the cell, due to solutes.


Hypotonic solution

Higher water potential relative to the solution inside the cell, due to absence of solutes.


Isotonic solution

Same water potential relative to the solution inside the cell.



Retraction of the cytoplasm and the cell membrane from the cell wall as a cell loses water by osmosis.


Incipient plasmolysis

Cell membrane and cytoplasm are partially detached from the cell wall due to insufficient water to make cell turgid.



Active process of the cell membrane engulfing material, bringing it into the cell in a vesicle.



Active process of a vesicle fusing with the cell membrane, releasing the molecules it contains.



Active process of the cell membrane engulfing large particles, bringing them into the cell in a vesicle.



Active process of cell membrane engulfing droplets of fluid, bringing them into the cell in a vesicle.


When does endocytosis (into cell) occur?

Material is engulfed by extensions of the cell membrane and cytoplasm, making a vesicle.


State 2 types of endocytosis.

Phagocytosis and Pinocytosis.


1) What happens during phagocytosis (uptake when can't be taken by diffusion or active transport)?

2) Where are the products absorbed?

1) Granulocytes engulf bacteria, lysosome fuses with vesicle formed and enzymes digest the cells.

2) Into the cytoplasm.


What is pinocytosis?

Uptake of liquid by small vesicles.


1) What is exocytosis?

2) What is often secreted by this process?

1) Substances leave the cell, having been transported through cytoplasm in vesicle, which fuses with cell membrane.

2) Digestive enzymes.


What do endocytosis and exocytosis have in common?

Cell membrane needs to change shape so they're active processes using ATP by respiration.


What is essential for bulk transport to occur?

Property of fluidity of the cell membrane.