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Biology Paper 1 > 3.2.3 Transport across membranes > Flashcards

3.2.3 Transport across membranes Flashcards

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1
Q

What are the functions of cell membranes ?

A
  • Control what substances can enter and exit
  • Form a barrier between cell/organelle and the external environment
    • separation of organelles so metabolic reactions can occur independantlyn (isolation of enzymes that may damage cells)
    • form internal transport systems (rough endoplasmic reticulum)
    • form surfaces for reactions (ribosomes and protein synthesis on the rough endoplasmic reticulum)
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2
Q

What is the basic structure of all cell membranes ?

A

Fluid-mosaic model
- Phospolipid molecules form a continuous bilayer (with some proteins and carbohydrates throughout)

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3
Q

Why the the membrane known as a fluid-mosaic ?

A

Fluid - phospholipids always move so shape is always shifting
Mosaic - scattered pattern produced by the proteins looks like a mosaic

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4
Q

Give the structure and function of a phospholipid

A

Hydrophillic heads point outwards and hydrophobic hydrocarbon tails point inwards
- Forms a barrier as the **centre of the bilayer is hydrophobic **so doesn’t allow water soluble substances to pass through

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5
Q

Define intrinsic (integral) protein

Give some examples

A

embedded in the cell membrane and control transport of molecules and ions across the membrane

Carrier and channel proteins

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6
Q

Define extrinsic (peripheral) proteins

Give some examples

A

on the outer or inner surface of the membrane and provide mechanical support and act as receptors

Receptors, antigens (These can connect to carbohydrates to make glycoproteins)

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7
Q

Give the function of channel proteins

A

enable water soluble ions to diffuse across the membrane

in order to do simple diffusion substances must be LIPID SOLUBLE

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8
Q

Give the function of carrier proteins

A

enable large molecules (amino acids, glucose) and ions to diffuse across the membrane

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9
Q

Give the five general functions of proteins

A
  1. provide structural support
  2. act as channels to transport water soluble substances across the membrane
  3. allow active transport across the membrane via carrier proteins
  4. cell surface receptors for recognition of other cells and hormones
  5. help cells adhere
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10
Q

Give the structure and function of cholesterol

A
  • fits between phospholipids molecules and binds to the hydrocarbon tails
  • causes phosholipids to pack close together which restricts movement - less fluid
  • so reduces lateral movement of the membrane, even at increased temperatures - This also prevents water and ion leakage

Present in ALL cell membranes (except prokaryotic)
-important in cells which arent supported by other cells - eg red blood cells

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11
Q

What two factors can increase membrane fluidity ?

A
  1. increase in unsaturated fatty acid chains - means chains are less tightly packed so intermolecular forces are weaker
  2. higher temperatures - molecules have more energy and move more freely which increases membrane fluidity
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12
Q

Give the structure and function of glycoproteins

A
  • Proteins with a polysaccharide attached
    1. form recognition sites for hormones and neurotransmitters
    2. form cell antigens for cell-cell recognition (allows body to recognise its own vs foreign cells in immuse response)
    3. involved in cell adhesion (carbohydrate forms a bond with water molecules around other receptors) - This allows cells to attach and form tissues
    4. increase membrane stability as glycoproteins can form hydrogen bonds with water molecules
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13
Q

Give the structure and function of glycolipids

A
  • Lipids with a polysaccharide chain attached
    1. same receptor functions as a glycoprotein
    2. increase **membrane stability **by forming hydrogen bonds with water molecules
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14
Q

What substances are able to pass through the semi-permeable phospholipid bilayer ?

A
  • lipid-soluble substances (hormones, vitamins)
  • very small molecules - gases (oxygen and carbon dioxide)
  • uncharged molecules
  • water as it is very small

substances such as glucose and amino acids are too large to pass through the phospholipid bilayer

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15
Q

What substances can’t pass through the phospholipid bilayer?

A
  • not lipid soluble
  • too large to pass through the channels
  • same charge as the charge on the channel proteins (causes repulsion)
  • polar (electrically charged) so can’t pass the non-polar hydrophobic tails
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16
Q

Define diffusion

A

net movement of molecules or ions from an area of high concentration to an area of low concentration till an equilibrium is achieved
- down the concentration gradient
- through a partially permeable membrane

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17
Q

Why is diffusion a passive process ?

A

particles are in constant random motion so are able to distribute themselves until an equilibrium is reached

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18
Q

What is simple diffusion ?

A

when molecules diffuse directly through the cell membrane

19
Q

What is facilitated diffusion ?

A

diffusion of large or polar molecules with the assistance of** highly specific membrane proteins**

20
Q

What is the role of carrier proteins and how do they do this ?

A

movement of **large molecules **
1. molecule binds to the protein
2. this causes the protein to change shape
3. so it can release the molecule on one side while closing off the other to prevent unwanted molecules passing

21
Q

What is the role of channel proteins and how do they do this ?

A

movement of charged ions (allows then to avoid the non-polar central layer)
1. water filled pores
2. they are gated so can open and close when required to control ion exhange

22
Q

What five factors affect rate of diffusion ?

A
  1. temperature - more kinetic energy = increased diffusion
  2. concentration gradient - increased gradient means more random movement across the membrane so increased diffusion
  3. surface area - increased SA means more molecules can diffuse at a given time, increasing rate of difffusion
  4. membrane thickness - thinner = shorter diffusion distance
  5. number of channel/carrier proteins
  6. size of molecule - large molecules move more slowly as more energy is required to move them
23
Q

Define osmosis

A

diffusion of water molecules from an area of high water potential (dilute solution) to an area of low water potential (concentration solution)
- across a partially permeable membrane
- down the concentration gradient

24
Q

Define water potential

What are the units ?

A

the tendancy of water molecules to diffuse out of a solution (due to the pressure created by the water molecules)

Kilopascals

25
Q

What is the water potential of pure water, what happens if you add solute ?

A

Water potential = zero
- adding solute **decreases the water potential **so it takes on a negative value

26
Q

Define isotonic

A

same water potential inside and outside of the cell (No net movement)

27
Q

Describe the mechanism for osmosis

A
  • solute and water molecules are in** random motion** due to kinetic energy
  • selectively permeable membrane allows water molecules through
  • water molecules diffuse from an area of high to low water potential
  • osmosis continues until dynamic equilibrium is achieved (No net movement)
28
Q

In a plant cell describe what happens in an external solution of:
- higher water potential
- lower water potential
- Equal water potential

Define protoplast
Define plasmolysis

A

Higher
- water enters the cell via osmosis
- the Protoplast swells
- cell becomes** turgid and supports the cell (Inelastic cell wall prevents bursting)
Lower
- water leaves the cell
- the Protoplast shrinks
- cell is plasmolysed **
Equal
- no net movement of water (incipient plasmolysis)

Protoplast - elastic living part of the cell
Plasmolysis - contraction of the protoplast of the PLANT CELL due to loss of water

29
Q

In an animal cell describe what happens in an external solution of:
* higher water potential
* lower water potential
* equal water potential

Define cytolysis

A

Higher
-water enters cell (due to increased solute in cell)
-cell swells and may burst (lyse)
- as there is no inelastic cell wall to stop bursting
Lower
- water leaves cell
- cell shrivels
Equal
- no net movement of water
- normal cells

when a cell bursts due to an osmotic imbalance

30
Q

What five factors affect the rate of osmosis ?

A
  1. Water potential gradient
    - increased gradient = increased rate
  2. Thickness of exchange surface
    - thinner = faster rate
  3. Surface area
    - larger = faster rate
  4. Presence of aquaporins
  5. surface area
31
Q

Define active transport

A

the movement of molecules or ions in or out of the cell from a region of low to high concentration (against the concentration gradient)
- using energy from respiration (ATP and carrier proteins)

32
Q

Describe the use of ATP in active transport

A
  • Energy is needed as molecules are** moving against the concentration gradient**
  • hydrolysis reaction** releases energy from ATP**
  • this makes the carrier molecules lose shape
  • for the** transfer of molecules across the membrane**
33
Q

Describe the use of carrier proteins in active transport

A
  • act as pumps to help transport materials against the concentration gradient, across the membrane
  • specific tertiary structure - specific binding sites mean they will only transport some substances
34
Q

Define direct active transport

A

directly uses chemical energy from ATP to transport molecules

35
Q

Describe the process of direct active transport

A
  1. molecule or ion** binds to receptor site **on the carrier protein
  2. inside the cell ATP binds to the protein - splits into ADP and a phosphate (energy)
    3.** protein molecule changes shape **and molecule/ion is released on the other side of the membrane
  3. phosphate is released so protein reverts to original shape (phosphate recombines with ADP)
36
Q

Define co-transport
Define co-transporter

A

Co-transport: coupled movement of substances across a cell membrane via a carrier protein
Co-transporter: type of carrier protein which binds to two molecules at the same time

37
Q

Describe the first stage of the co-transport of glucose

A
  1. sodium ions are transported out of the epithelial into the blood
  2. via active transport
  3. in the sodium potassium pump
38
Q

Describe the second stage of the co-transport of glucose

A
  • sodium ions diffuse from the lumen into the epithelial cell
  • down the **concentration gradient **
  • via sodium-glucose transporter proteins
39
Q

Describe the third stage of the co-transport of glucose

A
  • glucose is carried into the cell with sodium
  • via the sodium glucose transporter proteins
  • this causes the glucose concentration in the cell to increase
40
Q

Descrive the fourth stage of the co-transport of glucose

A

* Glucose diffuses into the blood (due to the increased concentration in the cell)
* down the concentration gradient
* via a protein channel
* This is facilitated diffusion

41
Q

What four factors affect the rate of active transport ?

A
  1. Speed of carrier proteins
  2. Number of carrier proteins
  3. Rate of respiration and availability of ATP
42
Q

How does temperature affect membrane fluidity ?

A
  • increased temperature
  • gives the phospholipids more kinetic energy so membrane is more fluid
  • increasing the permeability of the membrane
43
Q

How does solvent concentration affect membrane permeability?

A
  • stronger concentration means the lipid bilayer is more easily dissolved
  • increasing the permeability as the membrane begins to break down
44
Q

How does pH affect membrane fluidity ?

A
  • pH affects protein structure and charge distribution
  • too high/too low affects the membrane permeability

Biology Paper 1 (15 decks)

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