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Transport Across Cell Membranes Flashcards

1
Q

Why is the fluid mosaic model called so?

A

Fluid:
- individual PL molecules can move relative to one another
- so the membrane has a flexible structure as a whole
- that is constantly in movement and changing shape
Mosaic:
- The various proteins/glycoproteins etc embedded in the PLB vary in size, pattern and shape like the stones and tiles of a mosaic
- therefore it looks like a mosaic when viewed from above

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2
Q

Structure of the PL Bilayer

A
  • Phospholipid molecules have a hydrophilic head and a hydrophobic tail
  • So they arrange with their tails pointing inwards
  • And heads pointing outwards into the aqueous environment inside and outside of the cell
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3
Q

Function of the PL Bilayer

A
  • Allows entry/exit of lipid-soluble substances (ie non polar, hydrophobic or not charged)
  • Prevents entry/exit of water-soluble substances (ie polar, hydrophilic or charged)
  • Make the membrane fluid and self sealing
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4
Q

Function of proteins in the CSM

A
  • Provide structural support
  • Act as channels for water-soluble substances
  • Act as cell-surface receptors for hormones/identifying cells
  • Allow active transport and co-transport
  • Allow cells to adhere to each other to form tissues
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5
Q

Glycoproteins - what are they and what is their function in the CSM?

A
  • Extrinsic proteins in the CSM with a saccharide chain attached
  • Act as recognition sites and cell-surface receptors
  • Allow cells to adhere to each other to form tissues
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6
Q

Glycolipids - what are they and what is their function in the CSM?

A
  • Phospholipids/lipids with a saccharide chain attached
  • Act as recognition sites and cell surface receptors
  • Allow cells to adhere to each other to form tissues
  • ALSO PROVIDE STABILITY to the CSM
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7
Q

Role of Cholesterol in the CSM of animal cells

A
  • It is very hydrophobic therefore it binds to hydrophobic FA tails of the PL molecules (squeezes them together and restricts their movement)
  • Makes the cell more rigid and less fluid at higher temperatures
  • Prevents lateral movement of molecules in the bilayer
  • Prevents leakage of water and dissolved mineral ions from the cell
  • Ultimately provides stability and allows animal cells to maintain their shape
  • Plant cells don’t have it - they have a cell wall and other sterols to do the same job
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8
Q

Why do most molecules not freely diffuse across the CSM?

A
  • Molecule might be, polar or not lipid soluble so can’t pass through the PLB (protein channels are numbered - this makes it harder to find a site it can pass through)
  • Molecule might be too large to get through a channel/carrier protein
  • Molecule might be of the same charge as the surface of the proteins so it is repelled and can’t pass through the PLB either as it is charged
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9
Q

Difference between arrangement of extrinsic and intrinsic proteins in the PLB

A

INTRINSIC: Embedded completely in the membrane. Held in place by hydrophobic interactions between hydrophobic R groups on the surface of the protein and hydrophobic FA tails - they are amphipathic
- Arrangement depends on hydrophobic/hydrophilic regions
EXTRINSIC: Never span across the whole membrane - only outer or inner membrane. Bound loosely to the hydrophobic tails by weak hydrophobic interactions

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10
Q

How does a channel protein work? Why can it do this?

A
  • It is a water-filled hydrophilic tunnel that allows diffusion of polar molecules and ions down a conc. gradient
  • Can do this as hydrophobic R groups point outwards and hydrophilic ones point inwards to the aqueous environment (they are amphipathic)
  • They only work for PASSIVE transport
  • Very specific for a particular ion/molecule
  • Some can open/close to control the exchange of ions
  • Some are always open
  • Some open only in response to a chemical trigger
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11
Q

Difference between functions of intrinsic and extrinsic proteins

A

INTRINSIC:
- Usually for transporting polar/hydrophilic/water-soluble substances across the PLB
EXTRINSIC:
- Usually act as cell-surface receptors and recognition sites OR are enzymes (eg ATPase or dipeptidases)
BOTH provide structural support, however.

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12
Q

How does a carrier protein work?

A

1) Specific molecule/ion binds to the binding site
2) Tertiary structure of the carrier protein changes, so protein changes shape
3) Molecule/ion moves across the membrane and re-emerges on the other side
4) Molecule/ion is released
- Transport can be both passive and active
- They have an ATP binding site for active transport
- They allow large molecules (eg glucose/AAs) and specific ions through

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13
Q

Simple diffusion definition

A
  • The passive net movement of particles
  • Down a concentration gradient
  • Until dynamic equilibrium is reached
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14
Q

What types of molecules need a carrier/channel protein for diffusion?

A
  • Ions or charged molecules
  • Large molecules eg glucose/AAs
  • Water
  • Other polar/water soluble molecules
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15
Q

Compare the rate of diffusion of molecules that can diffuse across the phospholipid bilayer by simple diffusion

A

FASTEST
- Small, non-polar eg O2 or CO2
- Large, lipid-soluble eg steroid hormones or FAs
- Small polar molecules eg water or ethanol
SLOWEST

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16
Q

How are alveoli, nephrons, hepatocytes and villi adapted for exchange of substances?

A
  • Thin (short diffusion pathway)
  • Good blood supply (steep conc. grad.)
  • Large SA
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17
Q

Where does the energy required for facilitated diffusion come from?

A

The kinetic energy of the molecules themselves

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18
Q

Difference between simple and facilitated diffusion?

A

Facilitated = across a channel/carrier protein
Simple = across the PLB

19
Q

State Fick’s Law

A

Rate of Diffusion = (concentration gradient x surface area)/Diffusion distance

20
Q

What are the 5 main factors that affect the RoD?

A
  • Temperature
  • Surface area
  • Concentration gradient
  • Diffusion distance
  • Number of channel and carrier proteins (because more ions/charged molecules can diffuse across)
21
Q

How do particle size and charge affect the RoD? Explain.

A

Size - generally smaller particles diffuse faster than larger ones
Charge - uncharged particles diffuse faster across as there is a larger surface area for this to happen (95% of the area of the CSM is PLB)

22
Q

How does conc. gradient affect the RoD?

A
  • The greater the CG, the greater the RoD
  • This relationship is directly proportional provided the other factors remain the same
23
Q

How does temperature affect the RoD? Why does this usually not affect birds/mammals?

A
  • The greater the temp the greater the diffusion distance
  • Because particles have more KE so they move faster
  • Not an issue for mammals/birds as they are endotherms so maintain a constant body temp.
24
Q

How does surface area increase the RoD?

A
  • There is more space for diffusion of particles
25
How does water diffuse across the CSM?
- Through an aquaporin - a type of channel protein - Can technically also diffuse across the CSM but this is slow
26
Define Osmosis
- The passive net movement of water molecules - From an area of higher to lower water potential - Through a partially permeable membrane
27
What is water potential?
- When you have a membrane, water molecules will collide with both sides of the membrane - This exerts a pressure - This is water potential - Water potential is a measure of pressure so its units are kPa
28
What is the water potential of pure water?
- 0 kPa - this is the highest possible water potential - Therefore the lower the WP, the more negative the WP value is
29
What are 3 other definitions of osmosis? Use the words solute, negative and hypertonic
○ From an area of a less negative water potential to more negative water potential through a PPM ○ From an area of low solute potential to high solute potential through a PPM ○ From a more hypotonic to a more hypertonic region through a PPM
30
What happens if an animal cell is placed in a hypotonic solution?
- Water moves into cell - Hydrostatic pressure inside the cell increases - CSM can’t withstand significant HP increases so it won’t be able to function - If enough H2O molecules move inside the cell the CSM will rupture and the cell will burst (cytolysis)
31
What happens if an animal/plant cell is placed inside an isotonic solution?
- Water potential is the same inside and outside of the cell so there is no net movement of water molecules and osmosis is not taking place. - Although water molecules still move in both directions, the rate of movement inwards is the same as the rate of movement outwards.
32
What happens if an animal cell is placed in a hypertonic solution?
- Water moves to the outside of the cell (lower water potential) by osmosis from the cell (higher water potential) to equalise the water potential - Cell becomes CRENATED - it shrivels up and the CSM cannot function
33
What is the protoplast?
the space between the tonoplast (single membrane around the LPV) and the cell wall of a plant cell
34
Why do plant cells need a cell wall?
They cannot regulate the water potential of the liquid around their cells like animals can
35
What happens if a plant cell is placed in a hypotonic solution?
- Water moves into cell (lower water potential) from outside (higher water potential) to equalise water potential - The hydrostatic pressure inside the cell increases - The protoplast pushes outwards against the cell wall - this creates an internal pressure known as turgor pressure. - The cellulose cell wall is very strong so rather than bursting, the plant cell becomes firm - it becomes TURGID. - At a certain point, the turgor pressure prevents any more water by entering via osmosis.
36
What happens if a plant cell is put into a hypertonic solution?
- Water moves to the outside of the cell (lower water potential) from the cell (higher water potential) to equalise water potential - Protoplast pulls away from the cell wall, including the cell membrane which therefore won’t be able to function. - The cell becomes PLASMOLYSED. When this happens, the space created between the cell wall and the cell membrane becomes filled with the solution that surrounds the cell.
37
Define direct active transport
- The active net movement of particles - Against a concentration gradient - Through a carrier protein - Using ATP/energy from respiration
38
Describe 4 processes where active transport is used in living organisms
- Reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules - Absorption of some products of digestion from the digestive tract - Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant - Loading inorganic ions from the soil into root hairs
39
Define co-transport
- the coupled movement of substances across a cell membrane via a carrier protein - using a combination of facilitated diffusion and active transport
40
Outline the process of the co-transport of glucose in the intestine epithelial cells
- Na+ ions move out of the cell into the bloodstream by active transport - Now there is a concentration gradient between the epithelial cell and the lumen of the ileum - So Na+ ions move from the lumen of the ileum into the epithelial cell via facilitated diffusion through the sodium-glucose co-transporter protein - Glucose travels with it by facilitated diffusion through the same co-transporter protein - Then glucose diffuses out of the epithelial cell into the bloodstream via facilitated diffusion via its own concentration gradient
41
What other molecule can be absorbed in this same way via co transport?
Amino acids
42
Describe the process of endocytosis, give an example and why is it done?
- A cell surrounds a substance with a section of its CSM - The membrane pinches off to form a vesicle inside the cell that contains the substance - eg phagocytosis - this is done when the plasma molecule/substance is too large to diffuse through a carrier/channel protein
43
Describe the process of exocytosis
- Substances produced by the cell are enclosed within vesicles - These vesicles then pinch off from sacs of the Golgi iand move to the plasma membrane - Vesicles fuse with the plasma membrane and release their contents outside of the cell
44
Name the factors that affect the fluidity of the CSM

Biology AS (13 decks)

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