Cells

Question 1

Cell Membranes-basics

Answer 1

-Cell surface membranes surround cells, they are partially permeable.
-Substances move across by diffusion, osmosis and active transport.
-Membranes around organelles divide the cell into different compartments, they are also partially permeable.

Question 2

Membrane Structure

Answer 2

-Composed of lipids, proteins and carbohydrates.
-The fluid mosaic model was suggested in 1972, phospholipid molecules form a continuous double layer (called a bilayer), which is fluid because they move constantly.
-Cholesterol are also present within the bilayer.

Question 3

Proteins and Lipids

Answer 3

-Proteins are scattered throughout the bilayer-these include:
-Channel proteins which allow charged ions(K+) to pass through.
-Carrier proteins which allow large molecules (glucose) to pass through.
-Receptor proteins allow the cell to detect chemicals released from the other cells, the chemicals signal to the cell to respond in some way.
-Glycoproteins-have a carbohydrate attached.
-Glycolipids-lipids that have a carbohydrate attached.

Question 4

Phospholipids

Answer 4

-Phospholipids consist of a hydrophilic head and hydrophobic tail.
-The heads face outwards towards the water and the tail inwards.
-The centre doesn’t allow water soluble substances (like ions and polar molecules) to diffuse through.
-Small non polar substances can diffuse through (e.g. oxygen).

Question 5

Cholesterol

Answer 5

-Type of lipid that gives the membrane stability.
-Present in all cell membranes (except bacterial).
-Cholesterol fits between the phospholipids and binds to the hydrophobic tail, causing them to pack together.
-This makes the membrane less fluid and more rigid.

Question 6

Describe the movement of proteins within the bilayer.

Answer 6

Some are fixed and some can move sideways.

Question 7

Explain why a cell membrane is an effective barrier against water soluble substances.

Answer 7

The centre of the bilipid layer is hydrophobic so doesn’t allow water soluble substances to pass through.

Question 8

How does the cell surface membrane control what enters and leaves the cell?

Answer 8

Channel and carrier proteins.

Question 9

Describe the role of cholesterol in a cell membrane.

Answer 9

Cell stability -stops phospholipid being so fluid.

Question 10

Beetroot cells
-Investigating cell membranes permeability-required practical.

Answer 10

-In beetroot cells the vacuole has a high concentration of purple belatin.
-Working membranes will keep the pigment inside the cell.
-Damaged membranes will leak purple pigment.
-The more damaged the membrane the more pigment will leak out -the higher the permeability of the membrane.

Question 11

Factors that will affect permeability of the cell membrane.

Answer 11

Temperature -increasing temperature will denature proteins-increasing permeability.
Solvents (alcohol/acetone) -increasing concentration of a solvent dissolves the lipids in the cell membrane causing it to lose its structure.

Question 12

Diffusion

Answer 12

-Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
-Particles diffuse both ways but the net movement is to the area of lower concentration, until the particles are evenly distributed (reaches an equilibrium).
-Particles diffuse down the concentration gradient.
-It is a passive process (no energy needed).
-Diffusion can happen across cell membranes.
-When molecules diffuse directly through the membrane this is simple diffusion.

Question 13

Factors affecting rate of diffusion.

Answer 13

-The concentration gradient -the higher it is, the faster the rate of diffusion. Diffusion will slow over time as the difference in gradient decreases.
-Thickness of exchange surface -the thinner the exchange surface. The faster the rate of diffusion.
-The surface area -the larger the surface area the faster the rate of diffusion. Some specialised cells have microvilli (epithelial cells) these are projections formed by the cell surface membrane folding.

Question 14

Facilitated diffusion

Answer 14

-Large molecules and charged particles diffuse through carrier proteins or channel proteins in the cell membrane -this is called facilitated diffusion.
-Facilitated diffusion moves particles down the concentration gradient (from higher to lower) and is also a passive process.

Question 15

Carrier proteins

Answer 15

-Carrier proteins move large molecules, examples include:
-amino acids, glucose
-different carrier proteins facilitate the diffusion of different molecules.
-They work by:
-a large molecule attaches to a carrier protein in the membrane
-the protein then changes shape
-this releases the molecule on the opposite side
-the molecules are moved against the concentration gradient.

Question 16

Channel proteins

Answer 16

-Channel proteins form pores in the membrane for charged particles to diffuse through.
-Different channel proteins facilitate the diffusion of different charged particles.

Question 17

What type of molecule must be present in the cell membrane for facilitated diffusion of chloride ions to take place?

Answer 17

Channel proteins

Question 18

Explain why the simple diffusion of chloride ions would be too extremely slow.

Answer 18

Ions are water soluble and the centre of the phospholipid bilayer is hydrophobic.

Question 19

The chloride ions in the cell are not immediately used up. Describe and explain what will happen to the rate of facilitated diffusion of the chloride ions over time.

Answer 19

Rate will slow down/level as diffusion progresses, the concentration gradient of the chloride ions will decrease.

Question 20

What is net movement?

Answer 20

Number of molecules moving in one direction minus the number of molecules moving in the other direction (overall movement).

Question 21

What does hypertonic mean?

Answer 21

Higher concentration of solutes-lower water potential.
Solutions with lower water potential compared with inside the cell.

Question 22

What does hypotonic mean?

Answer 22

Lower concentration of solutes- higher water potential.
Solutions with a higher water potential compared with inside a cell.

Question 23

What is osmosis?

Answer 23

Osmosis is the diffusion of water molecules across a partially permeable membrane from an area of higher water potential (higher concentration of water molecules) to an area of low water potential (lower concentration of water molecules).

Question 24

Water potential

Answer 24

-Water potential is the likelihood of water molecules to diffuse out or into a solution.
-Pure water has a water potential ( trident symbol/kPa) of zero.
-Adding solutes to pure water lowers its water potential (making it negative), therefore the lower the water potential the stronger the concentration of solutes in the solution.

Question 25

What does isotonic mean?

Answer 25

If two solutions have the same water potential

Question 26

What happens to the cell in an isotonic solution?

Answer 26

Cell stays the same.

Question 27

What happens to the cell in a hypertonic solution?

Answer 27

Cell shrinks.

Question 28

What happens to the cell in a hypotonic solution?

Answer 28

Cell swells

Question 29

Factors that affect the rate of osmosis.

Answer 29

-Water potential gradient -the higher the gradient the faster the rate. As osmosis takes place the difference in water potential decreases so the rate levels off over time.
-The thickness of the exchange surface -the thinner the exchange surface the faster the rate.
-The surface area of the exchange surface -the larger the surface area the faster the rate.

Question 30

What is active transport?

Answer 30

Active transport uses energy to move molecules and ions across plasma membranes, usually across a concentration gradient, and it involves carrier and co-transporter proteins.

Question 31

What are three differences between facilitated diffusion and active transport?

Answer 31

1) active transport usually moves solutes from low to high concentrations, in facilitated diffusion they always move from high to low concentrations.

2) active transport requires energy (ATP), facilitated diffusion doesn’t.

3) active transport does not use channel proteins, facilitated diffusion does.

Question 32

ATP

Answer 32

-ATP (adenosine triphosphate) is a molecule produced by respiration.
-It is a common source of energy in the cell-important for active transport.
-ATP undergoes a hydrolysis reaction (water breaks the bond) splitting ADP and Pi (inorganic phosphate). This releases energy so the solutes can be transported.

Question 33

Co-transporter

Answer 33

-Co-transporter are a type of carrier protein and they bind two molecules at a time.
-The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient.

Question 34

Co-transporter and the absorption of glucose.

Answer 34

-Glucose is absorbed into the bloodstream in the small intestine.
-In the ileum (final part of a mammals small intestine) the concentration of glucose is lower so is absorbed by co-transport.

Question 35

Absorption of glucose.

Answer 35

Step 1: Sodium ions are actively transported out of epithelial cells into the blood via the sodium-potassium pump.
This creates a concentration gradient - higher concentration of sodium ions in the lumen of the ileum than inside cells.

Step 2: Sodium ions diffuse through the lumen into epithelial cells down their concentration gradient via the sodium-glucose co-transporter proteins.
The co-transporter carries glucose into the cell with the sodium. As a result the concentration of house inside the cell increases.

Step 3: Glucose diffuses out of cell, into the blood, down its concentration gradient through a protein channel by facilitated diffusion.

Question 36

Factors affecting rate of absorption of glucose.

Answer 36

-The speed of individual carrier proteins - the faster they work the faster the rate.
-The number of carrier proteins present - the more proteins there are, the faster the rate.
-The rate of respiration in the cell and the availability of ATP.