2.1.5 membranes

Question 1

what is active transport

Answer 1

the movement of particles from an area of low concentration to an area of high concentration, up the concentration gradient across a partially permeable membrane. this requires energy released from respiration

Question 2

what is diffusion

Answer 2

the movement of particles from an area of high concentration to an area of low concentration, down the concentration gradient across a partially permeable membrane.

Question 3

what is osmosis

Answer 3

the movement of water from an area of high water potential to an area of low water potential, down the concentration gradient across a partially permeable membrane

Question 4

what is the difference between simple and facilitated diffusion

Answer 4

• facilitated diffusion is faster than simple
• facilitated diffusion requires channel proteins to let through the molecules, simple does not
• simple diffusion only lets through small and non-polar molecules, whereas facilitated diffusion lets through larger, polar molecules
• simple diffusion is only limited by the concentration gradient, whereas facilitated is limited by number of proteins as well

Question 5

what molecules does simple diffusion let through

Answer 5

• vitamins A and D,
• small polar molecules (e.g. water),
• steroid hormones (oestrogen, testosterone),
• oxygen and carbon dioxide molecules

Question 6

what molecules does facilitated diffusion let through

Answer 6

• polar molecules e.g. Na+ or Cl- ions
• large molecules e.g. glucose (also polar)

Question 7

what factors affect diffusion

Answer 7

• pressure
• temperature
• steepness of concentration gradient
• surface area of membrane
• properties of the molecules (larger molecules are slower and require more energy to move, polar molecules require facilitated diffusion)

Question 8

differences between active transport and diffusion

Answer 8

• active transport requires specific carrier proteins in the membrane, diffusion does not
• active transport requires energy released from respiration, diffusion does not
• active transport is an active process, diffusion is a passive process

Question 9

what is bulk transport and what are the two main types

Answer 9

• the movement of large molecules or molecules of large quantities in or out of a cell. this requires energy released from respiration
• endocytosis
• exocytosis

Question 10

what is endocytosis

Answer 10

the process by which the cell surface membrane engulfs material, forming a vesicle around it. this requires energy released from respiration

Question 11

what happens in endocytosis

Answer 11

• the unwanted particles are invaginated by the membrane (surrounded)
• a vesicle is formed (the membrane breaks off to create an enclosed sac, or endocytic vescicle)
• the membrane reattaches to itself to reform a sealed cell

Question 12

what is endocytosis used for

Answer 12

• urea, lactic acid (to prevent toxic build up)
• phagocytosis (immune response to pathogens)
• pinocytosis (taking in extracellular fluids)

Question 13

what is exocytosis

Answer 13

the process by which materials are removed from or transported out of cells by isolating them in a vesicle. this requires energy released from respiration

Question 14

what happens in exocytosis

Answer 14

• a secretory vesicle is formed around the particles inside the cell by the golgi body
• the vesicle is carried to the membrane and fuses to it
• the unwanted particles are then slid over the membrane through the layers, forcing them out of the cell

Question 15

what is exocytosis used for

Answer 15

• enzymes (e.g. protease to prevent proteins which the cell is made of being broken down)

Question 16

what is water potential

Answer 16

the measure of the quantity of water compared to solute, measured as the pressure created by water molecules. it can be determined by the number of ‘free’ water molecules in comparison to water molecules which are bound to the solute
- water potential is represented by the symbol ‘Ψ’

Question 17

how is water potential measured

Answer 17

• in kPa (KiloPascals)
• pure water has a water potential 0kPa, the highest possible value
• all water with a water potential lower than pure water has a negative value of kPa

Question 18

what is the name of the membrane around a cell and its contents

Answer 18

the protoplast

Question 19

what is the name of the membrane around a vacuole in plant cells

Answer 19

the tonoplast

Question 20

what is a hypertonic solution

Answer 20

a solution where water potential is higher inside the cell than outside

Question 21

what is an isotonic solution

Answer 21

a solution where water potential is equal in and out of the cell

Question 22

what is a hypotonic solution

Answer 22

a solution where water potential is lower inside the cell than outside

Question 23

what happens to plant cells in hypertonic solutions

Answer 23

plasmolysis- water leaves the cell by osmosis, causing the protoplast to begin to peel back from the cell wall, the cell wall maintains cell structure however the protoplast will become flaccid
- at an extreme level, a plant cell can become completely plasmolysed, where the protoplast detaches from the wall entirely

Question 24

what happens to animal cells in hypertonic solutions

Answer 24

water leaves the cell, causing it to shrivel and become crenated

Question 25

what happens to cells in isotonic solutions

Answer 25

water enters and leaves the cell at the same rate

Question 26

what happens to plant cells in hypotonic solutions

Answer 26

water enters the cell via osmosis, causing the protoplast to become turgid, however the cell wall will maintain structure

Question 27

what happens to animal cells in hypotonic solutions

Answer 27

water enters the cell via osmosis, causing the cell to swell and become haemolysed, this can cause the cell to burst

Question 28

what is a phospholipid

Answer 28

• a form of lipid with a phosphate and glycerol head and a tail made of saturated and unsaturated fatty acids
• the head of a phospholipid is hydrophilic, and the tail is hydrophobic, making them polar

Question 29

how are the hydrophobic tails of phospholipids protected

Answer 29

they form a phospholipid bilayer, where the tails are on the inside and the heads outside

Question 30

how are phospholipid bilayers drawn

Answer 30

can be represented in a fluid mosaic model- this is because the bilayer is not in a set structure, and its components can move around when necessary

Question 31

what are glycolipids and what is their role

Answer 31

carbohydrate chains attached to lipids (i.e. phospholipids) which form specific shapes. Act as antigens in order to be recognised as non-foreign cells

Question 32

what are extrinsic proteins and what is their role

Answer 32

proteins which are located on the surfaces of the membrane. they maintain the structure and are hydrophilic, so attract water to the cell

Question 33

what are intrinsic proteins and what is their role

Answer 33

proteins which go through the entire membrane layer. they let larger and polar molecules into the cell through facilitated diffusion

Question 34

what are the three types of intrinsic protein and what are their functions

Answer 34

channel proteins- allow diffusion of polar molecules (ions, water) by forming pores in the membrane
carrier proteins- used for active transport as the contain ATP (energy source)
gated channel proteins- only allow molecules through in certain conditions (have specific binding sites for different particles and only open their ‘gate’ in certain temperatures, voltages, pHs etc.)

Question 35

what is a glycoprotein and what are their roles

Answer 35

a carbohydrate chain bonded to an extrinsic protein. they act as receptors for cell signaling, neurotransmitters, peptide hormones and cell adhesion

Question 36

what is the role of cholesterol in the cell membrane

Answer 36

• lipids break down in warm conditions, so cholesterol helps to maintain structure in the membrane when there is an increase in temperature by holding together the phospholipids. this prevents the membrane becoming too permeable
• in cold conditions the membrane solidifies and becomes less permeable, so cholesterol creates space in between the phospholipids to maintain membrane fluidity and permeability
• although cholesterol attaches to the phospholipid tails, they are polar molecules so work with the heads too

Question 37

what is cell signalling and what is it needed for

Answer 37

the process by which cells communicate with one another. it is needed in multicellular organisms to coordinate and control our bodies and responses

Question 38

what are examples of cell signalling

Answer 38

• neurotransmission along a synapse, movement of a nerve impulse
• hormones
• cell recognition
• cell adhesion
• cell growth/replication

Question 39

what is the process of cell signaling

Answer 39

• the sending cell releases ligands (signal molecules)(this can be from the same cell as the receptor, for instance in cell growth and replication)
• the ligands reach the target cell and bind to the receptor (made of glycolipids or glycoproteins and are complementary to the ligand)
• transduction takes place, where a reception is transferred into a (usually chemical) signal
• this triggers a change to something in the cell, usually in gene expression

Question 40

what are some uses of cell signalling

Answer 40

• beta blockers (block receptors in heart muscle cells to prevent increased heartrate/palpitations, can be used to help treat anxiety)
• botox (a bacterial toxin from clostridium botulinum, which binds to receptors in muscle cells to prevent contraction, essentially paralysing the muscle)

Question 41

what are some examples of cell signalling going wrong

Answer 41

• pathogens e.g. viruses (enter cell by binding to receptors and also prevent cells from carrying out functions in order to destroy it or multiply in it)
• HIV (binds to the CD4 receptor on T-cells to prevent immune response and attack immune system)
• autoimmune diseases (immune cells don’t recognise body cell receptors and therefore attack them)
• tumours/cancer (cell growth/replication ligands are over- released, causing disfunction and uncontrollable and rapid replication)
• type 2 diabetes (cells no longer respond to receiving insulin hormone ligands)