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OCR Biology > 2.5 - Biological Membranes > Flashcards

2.5 - Biological Membranes Flashcards

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1
Q

What is compartmentalisation?

A

The formation of separate membrane-bound areas in a cell

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2
Q

Why is compartmentalisation important?

A

Metabolism requires many different, and often incompatible, reactions.

Containing reactions in separate parts of the cell allows specific conditions required for cellular reactions, such as chemical gradients, to be maintained, and protects vital cell components.

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3
Q

What are the roles of membranes?

A
  • partially permeable barriers between the cell and its environment, organelles and the cytoplasm and within organelles
  • sites of chemical reactions
  • sites of cell communication (cell signalling)
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4
Q

What is the fluid mosaic model?

A
  • the best representation of the membrane structure based on the evidence which is currently available (may change/be updated)
  • states membrane is made of a phospholipid bilayer, where phospholipids are free to move within the layer relative to each other (fluid), giving membrane flexibility.
  • proteins are randomly embedded in the membrane which vary in shape, size and position (like a mosaic)
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5
Q

What is the plasma membrane?

A

The cell surface membrane which separates the cell from its external environment

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6
Q

What is the phospholipid bilayer?

A
  • two layers of phospholipids
  • hydrophilic phosphate heads turn to face the water, meaning they form the inner and outer surface of the membrane.
  • hydrophobic fatty acid tails turn towards each other (away from water), meaning they are on the inside of the membrane.
  • these interactions occur due to the aqueous environment cells are usually found in as well as the aqueous cytoplasm.
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7
Q

What are the components of the plasma cell membrane?

A
  • Phospholipids
  • cholesterol
  • glycolipids
  • glycoproteins
  • extrinsic proteins
  • intrinsic proteins
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8
Q

What are membrane proteins?

A

Extrinsic and intrinsic proteins

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9
Q

What are intrinsic proteins?

A
  • aka integral proteins
  • transmembrane proteins that are embedded through both layers of the membrane
  • have amino acids with hydrophobic R groups on their external surfaces which interact with the hydrophobic core of the membrane, keeping them in place
  • include channel + carrier proteins, involved in transport across the membrane, and glycoproteins
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10
Q

What are channel proteins?

A
  • intrinsic proteins
  • provide a hydrophilic channel
  • allows passive movement of polar molecules and ions down a concentration gradient through membranes
  • held in position between the hydrophobic core of membrane and hydrophobic R groups on the outside of the proteins
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11
Q

What are carrier proteins?

A
  • intrinsic proteins
  • have an important role in both passive and active transport into cells.
  • often involves shape of protein changing
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12
Q

What are glycoproteins?

A
  • intrinsic proteins
  • embedded in the cell surface membrane with carbohydrate chains attached to them
  • carbohydrate can vary in shape and length
  • play a role in cell adhesion (when cells join together to form tight junctions in certain tissues)
  • act as receptors for cell signalling
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13
Q

What is a glycolipid?

A
  • similar to glycoproteins
  • lipids with an attached carbohydrate chain.
  • called cell markers or antigens and can be recognised by the cells of the immune system as self (belonging to organism) or non-self (cells belonging to another organism)
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14
Q

What is cell signalling?

A
  • aka cell communications
  • when chemicals bind to the receptor (glycoprotein), it elicits a response from the cell.
  • this may be a direct response or a trigger for a series of events inside the cell
  • eg receptors for peptide hormones eg insulin and glucagon
  • some drugs act by binding to cell receptors.
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15
Q

What are extrinsic proteins?

A
  • aka peripheral proteins
  • present in one side of the bilayer
  • normally have hydrophilic R groups on their outer surfaces and interact either the polar heads of the phospholipids, or with intrinsic proteins.
  • can be present in either layer and may move between them.
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16
Q

What is cholesterol?

A
  • a lipid with a hydrophilic and hydrophobic end like a phospholipid
  • 4 ring carbon structure
  • regulates membrane fluidity
  • molecules are positioned between phospholipids in a membrane bilayer, with hydrophilic end interacting its the heads and hydrophobic ends interacting with the tails, pulling them together.
  • this adds stability to the membranes without making them too rigid.
  • cholesterol also prevents the membrane from becoming too solid by stopping phospholipids from grouping too closely and crystallising.
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17
Q

How do membranes act as a site of chemical reactions?

A
  • proteins in the membranes forming organelles or present within organelles, have to be in particular positions for chemical reactions to take place.
  • eg electron carriers and enzyme ATP synthase must be in the correct position within the cristae of mitochondria for ATP production.
  • the enzymes of photosynthesis are found on the membrane stacks within the chloroplast.
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18
Q

What happened of membranes lose their structure?

A
  • they lose control of the passage of different substances into and out of cells and organelles.
  • cell processes are interrupted.
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19
Q

What factors affect membrane structure?

A
  • temperature
  • solvents
  • cholesterol level
  • saturated v unsaturated fatty acids
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20
Q

How does temperature affect the membrane?

A
  • at temps below 0 degrees, phospholipids have little KE so are packed tightly together, decreasing permeability.
  • HOWEVER colder temperatures can lead to proteins deforming and ice crystals puncturing the membrane, increasing permeability.
  • for temps above 0 degrees onwards membrane permeability increases
  • phospholipids have so much KE that they lose bilayer structure
  • water expands - hydrostatic pressure from inside increases
  • proteins deform, tend to allow anything through. Membrane more permeable.
  • graph in a U shape

lol Aalia is a poop

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21
Q

How to fatty acids affect membrane permeability

A
  • saturated fatty acids = straighter. Can pack together closer, membrane less fluid. Membrane is less permeable.
  • unsaturated fatty acids = bent. Pack further apart. Membrane more fluid with more gaps. Membrane more permeable.
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22
Q

How does cholesterol affect membrane permeability?

A

At low temps, cholesterol disrupts close packing of phospholipid molecules, increases fluidity, membrane more permeable.

At high temps, cholesterol constrains motion of phospholipids. Decreases fluidity, membrane less permeable.

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23
Q

How do solvents affect membrane permeability?

A
  • water is a polar solvent and is important in the formation of the phospholipid bilayer due to hydrophilic/phobic interactions
  • less polar solvents eg alcohol or non polar eg benzene dissolve phospholipids, breaking up membranes.
  • this increases permeability
  • some solvents increase permeability more than others.
  • anything that dissolves lipids is toxic to living things (why alcohol wipes are used - kills bacteria in wounds).
24
Q

How does ethanol affect the membrane?

A
  • dissolves phospholipids
  • Ethanol affects bonding in the membrane proteins leading to desaturation, causing gaps in membrane
  • cholesterol is soluble in ethanol so once it is removed, larger gaps.
  • increases permeability for larger molecules.
25
Q

What is diffusion?

A
  • The net movement of particles from a region oh higher concentration to a region of lower concentration.
  • it is a passive process and will continue until a concentration equilibrium is reached
26
Q

Why does diffusion happen?

A
  • The particles in a liquid/ gas have kinetic energy and are constantly moving.
  • there movement is random and an uneven distribution of particles will eventually become even.
27
Q

What is equilibrium?

A
  • when there is an equal concentration of particles on both sides of a membrane
  • doesn’t mean particles stop moving, just that movement is equal - no net movement.
28
Q

What effect does diffusion distance have on diffusion?

A
  • particles move at high speeds and are constantly colliding, which slows down their overall movement.
  • these means that diffusion over short distances is fast, and over long distances is slow (as more collisions take place)
29
Q

What factors affect the rate of diffusion?

A
  • temperature: the higher the temp, the higher the rate if diffusion as particles have more KE and move at higher speeds
  • concentration difference: the greater the difference in concentration between two regions the faster the rate of diffusion as the overall movement from the higher conc to lower conc will be larger.
30
Q

What is a concentration gradient?

A
  • The concentration gradient between two regions.
  • goes from am area of high concentration to an area of low concentration
  • diffusion takes place DOWN a concentration gradient (high to low)
  • active transport takes place UP a concentration gradient (low to high)
31
Q

What is simple diffusion?

A

Diffusion in the abs envy of a barrier or membrane

32
Q

What does partially permeable mean?

A

Allows some substances to pass through but acts as a barrier to others.

33
Q

Which substances can and cannot pass through a membrane by diffusion?

A
  • small, non-polar molecules: can diffuse through easily (eg O2, CO2, N2)
  • small, polar molecules: most can pass, some can’t (eg NH3, H2O, glycerol)
  • large, polar molecules: majority cant get through (eg glucose, sucrose)
  • ions: cant get through (eg K+, Na+, Cl-, amino acids with charge)
34
Q

What cant polar/charges molevcules diffuse through the phospholipid bilayer?

A

The hydrophobic interior of the membrane (fatty acids) repel substances with a positive or negative charge.

Ions cannot easily pass through

Polar molecules eg water can pass through but at a very slow rate.

35
Q

Which factors affect the rate of diffusion across a membrane?

A
  • temperature: the higher the temp, the higher the rate if diffusion as particles have more KE and move at higher speeds
  • concentration difference: the greater the difference in concentration between two regions the faster the rate of diffusion as the overall movement from the higher conc to lower conc will be larger.
  • surface area: the larger the area of exchange surface, the higher the rate of diffusion
  • thickness of membrane: the thinner the exchange surface, the higher the rate of diffusion
36
Q

What is facilitated diffusion?

A
  • the net movement of molecules or ions through a transmembrane protein (channel/carrier protein) down a concentration gradient
  • polar molecules and ions are often transported this way
  • may involve carrier proteins, which change shape when specific molecules bind.
  • diffusion occurs down a concentration gradient.
  • passive process
37
Q

What factors affect the rate of facilitated diffusion?

A
  • temperature: the higher the temp, the higher the rate if diffusion as particles have more KE and move at higher speeds
  • concentration difference: the greater the difference in concentration between two regions the faster the rate of diffusion as the overall movement from the higher conc to lower conc will be larger.
  • surface area: the larger the area of exchange surface, the higher the rate of diffusion
  • thickness of membrane: the thinner the exchange surface, the higher the rate of diffusion
  • no of channel proteins present - the more protein channels, the higher the rate of diffusion
38
Q

What is active transport?

A
  • the movement of particles into or out of a cell against a concentration gradient (area of low concentration to an area of high concentration)
  • active process - requires energy in the form of ATP
39
Q

How do carrier proteins work?

A

1) molecule/ion needing to be transported binds to receptors in the channel of the carrier protein
2) ATP binds to the carrier protein and is hydrolysed into ADP and phosphate, releasing energy.
3) the binding of the phosphate changes the shape of the carrier protein and causes it to open up on the other side
4) the molecule/ion is released into the other side of the membrane
5) the phosphate molecule is released from the carrier protein and binds to the ADP again to form ATP
6) the carrier protein returns to its original shape

The process is selective - specific proteins are transported by specific carrier proteins.

40
Q

What is bulk transport?

A
  • form of active transport
  • large molecules eg hormones, enzymes and bacteria cells are too large to move across channel or carrier proteins, so are moved into and out of the cell by bulk transport
41
Q

What is endocytosis?

A
  • bulk transport of material INTO cells

- two types: phagocytosis for solids and liquids, pinocytosis for liquids (same process for both)

42
Q

What is the process of endocytosis?

A
  • cell surface membrane INVAGINATES (bends inwards) when it comes into contact with material to be transported
  • the membrane enfolds the material until the membrane fuses, forming a vesicle
  • the vesicle pinches off and moves into the cytoplasm to transfer material for further processing.

Eg vesicle containing bacteria moves to lysosomes where bacteria is digested by enzymes

43
Q

What is exocytosis and what is the process?

A
  • the bulk movement of material OUT of cells (reverse of endocytosis)
  • vesicles, usually formed by Golgi apparatus, move towards and fuse with the plasma membrane. Contents of vesicle are then released out of cell
44
Q

What is energy used for in endocytosis/exocytosis?

A

energy in the form of ATP is used for:

  • movement of vesicles along the cytoskeleton
  • changing of shape of cells to engulf material
  • fusion of cell membranes as vesicles form or when vesicles fuse with the cell surface membrane.
45
Q

What is osmosis?

A
  • Diffusion of water across a partially permeable membrane from a region of high water potential to a region of low water potential down a water potential gradient.
46
Q

What is water potential?

A
  • The pressure exerted bye water molecules as they collide with a membrane or container.
  • Measured in pascals or kilopascals.
  • Symbol of water potential is Greek letter psi
  • it is the potential (likelihood) of water molecules moving across a membrane
  • pure water has a water potential of 0 kPa at standard conditions. This is the highest value possible - addition of solutes results in water potential below zero
  • all values of water potential are negative
  • the more concentrated the solution, the more negative the water potential (lower water potential)
47
Q

How does water potential affect osmosis?

A
  • solutions with different concentrations have different water potentials
  • if they are separated by a partially permeable membrane, water molecules can move between the solutions but the solutes usually cannot.
  • there will be a net movement of war we from the solution with the higher water potential (less concentration) to the solution with lower water potential (more concentrated).
  • this will continue until tight water potential is equal on birth sides of the membrane (equilibrium)
48
Q

What effect does osmosis have on plant and animal cells?

A
  • the diffusion of water inti a solution leads to an increase in volume of the solution and vice versa.
  • if the solution is in a closed system eg cell, this result is in an increase in pressure - HYDROSTATIC PRESSURE (units = kPa)
  • at a cellular level, this pressure is relatively large and potentially damaging.
49
Q

What does hypotonic mean?

A
  • Solution on the outside is less concentrated than the solution on the inside of the membrane, so there is a higher water potential outside.
  • Net movement of water into cell
50
Q

What does isotonic mean?

A
  • environmental solution has the same concentration, and therefore same water potential, as the inside of the cell.
  • no net movement of water
51
Q

What does hypertonic mean?

A
  • Environmental solution has a higher concentration, and therefore lower water potential, than the cell.
  • net movement of water out of cell
52
Q

What happens if you place an animal cell in a hypotonic solution?

A
  • water potential of solution is higher compared to cell
  • net movement of water = into cells
  • cell swells and may burst - CYTOLYSIS

This is because animal cells have a thin cell surface membrane and no cell wall. The plasma membrane cannot stretch much and cannot withstand increased pressure, so it bursts

53
Q

What happens if you place an animal cell in an isotonic solution?

A
  • concentration and weathered potential of external solution and cell solution is the same
  • water constantly enters and leaves the cell at equal rates = no net movement
  • no change in state of cell
54
Q

What happens if you place an animal cell in a hypertonic solution?

A
  • water potential of external solution is lower than the cell solution
  • net movement of water out of cell
  • cell shrinks = CRENATION

This is because the loss of water causes a reduction ion volume of the cell and the cell-surface membrane to ‘pucker’

55
Q

What happens when you place a plant cell in a hypotonic solution?

A
  • water potential of external solution and cell solution are equal
  • water constantly enters and leaves cell, but at equal rates - no net movement of water
  • no change in cell
56
Q

What happens when you place a plant cell in a hypertonic solution?

A
  • water potential of external solution is lower than water potential of cell solution
  • net movement of water out of cell
  • cell contents shrink - become PLASMOLYSED

This is because water is lost from cells via osmosis , reducing volume.
This eventually pulls the cell-surface membrane away from the cell wall

57
Q

What happens when you place a plant cell in a hypotonic solution?

A
  • the water potential of external solution is higher than cell solution
  • net movement of water into cell
  • cell swells and becomes turgid

unlike animal cells, plants are able to control the flow the fluid around them
plants have strong cellulose walls surrounding the cell surface membrane.
when water enters the cell via osmosis, the increased hydrostatic pressure pushes the membrane against the rigid cell walls. This pressure is called turgor pressure.
as the pressure increases it resists the entrance of further water and the cell is said to be turgid.

OCR Biology (25 decks)

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