Unit 1.1.2 - Membranes - SICM

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Biology AS Level: OCR Syllabus: Unit 1.1.2

Membranes Cell Membranes -

all cells are delineated by a membrane

-

play a crucial role in almost all cellular activity

-

separate the cell from its surroundings

-

regulates the movement of substances in and out of the cell o (i.e. a selective barrier – controls concentration variation of molecules in cell)

Internal membranes -

enclose: o nucleus o mitochondria o chloroplasts etc. o (i.e. compartmentalise special metabolic activities)

Analysis of extracts of cell membranes (a)

made of lipid

(b)

associated with proteins which contribute to their function i. selective permeability ii. trans-membrane transport iii. structural integrity

The cell membrane is a structure common to both animal and plant cells. There used to be a theory of the structure as shown on the following page. However, this was later not accepted and a new theory was created which is now accepted to be correct.

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Davson Danielli (1930) model – unit membrane Original model globular proteins 2nm

phospholipid bilayer 3.5nm

The problem seen in this model was that it did not explain how the substances got through the membrane. (a) (b) (c)

hydrophilic bond 2nm Total: 7.5 nm

7.5 nm thick not seen under light microscope forms anchorage for component protein

Phospholipids (a) (b) (c)

Composed of fatty acid chains attracted to glycerol have polar heads (hydrophilic) non polar tails (hydrophobic – do not mix with water)

If a thin layer of polar lipids is spread on the surface of water, the molecules orientate into a monolayer.

water

polar heads

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Singer and Nicholson – Fluid Mosaic Model – 1972 – model now accepted cholesterol glycoprotein

protein

phospholipidic tail carrier protein

head pore protein • “proteins flowing in a sea of lipids” • fluid mosaic model – i.e. membrane is not static • both proteins and lipids have considerable freedom of movement: mainly lateral Even with an electron microscope it is not possible to see he molecular structure of a cell membrane. Thus it is necessary to construct a model to explain its various properties. The matrix of the membrane is composed of a bilayer of phospholipids and cholesterol molecules. Proteins are embedded in the bilayer. (a) some penetrate only part of the way through, while others penetrate all the way through (b)

Lipids

functions: i. structure ii. transport iii. act as enzymes -

composition similar to olive oil variation affects fluidity and permeability i. unsaturated lipids have curled tails to prevent close packing and to make membrane structure more open and fluid ii. cholesterol is important in regulating fluidity

The structure is asymmetric as there are different proteins in either half of the bilayer. Carbohydrates close t exterior forming “glycocalyx” – receptor molecule / cell recognition Proteins are involved in the transport of molecules across membrane specific receptors for hormones or act as enzymes. Evidence for the structure is proposed from:

(a)

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freeze fracture studies

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(b) (c)

isotopic labelling of proteins electron microscope studies

Movement through membranes (a) (b) (c) (d) (e) (a)

diffusion facilitated diffusion active transport osmosis exocytosis / endocytosis Diffusion

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“the net movement of uncharged molecules (e.g. CO2, O2, urea) from a region where they are in high concentration to a region where they are in low concentration”

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i.e. DOWN THE CONCENTRATION GRADIENT

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it occurs wherever a concentration gradient exists and would continue until the diffusing substance is evenly distributed

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e.g. O2 through cell membrane of plant and animal cells or released from photosynthesising chloroplasts.

Rate of diffusion depends on… -

concentration gradient

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the distance – shorter the distance, the faster the rate:

rate

α

1 distance 2

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area – the larger the surface area, the greater the rate of diffusion

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nature of the structure – the greater the number/size of pores, the greater the rate of diffusion

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the size and nature of the diffusing molecule – the smaller the molecule, the greater the rate of diffusion. e.g. fat soluble molecule – diffuses fast

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size of charge on ion – positive (+) ions move in more readily than negative (-)ions. Ions of a higher charge are attracted more into the cell

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Fick’s law of diffusion rate of diffusion

surface area × concentration difference

α

thickness of membrane

Routes of diffusion (a)

(b) -

-

CO2, O2, H2O(even though it is polar), urea, ethanol (i.e. small uncharged molecules)

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the smaller / the more fat soluble the molecule, the faster the rate (e.g. O2, CO2, urea, ethanol – very rapid)

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molecules squeeze between polar phospholipids heads – they dissolve on the lipid on one side and emerge from the other O2

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Thus, K+, Na+, Cl-, HCO3- and glucose cannot cross in this way. They have to be aided by proteins.

Glucose, amino acids and ions travel through in different ways: through water filled pores in the channel proteins

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pores are selective in determining which substance will move across (by size of pore)

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pores may be gated – open / closed by nerves.

Conclusion Diffusion: down a concentration gradient no energy needed Page 5 of 12

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(b)


Facilitated Diffusion -

for substances (e.g. glucose) the rate of diffusion is speeded up by the presence of protein carriers in the membrane

rate of reaction glucose concentration limiting rate

number of carrier molecules limits rate concentration of glucose

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down a concentration gradient (i.e. from high conc. → low conc.)

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there are 2 types of protein membrane for facilitated diffusion: i.

specific carrier proteins

forming a “gate” allowing solute to go through (e.g. to transport glucose)

carrier protein for glucose – permease glucose binds to permease on one side of the membrane and is released from the other carrier protein specific to particular molecules

the binding of the solute molecule to the carrier protein alters the conformation of the carrier so that its position in the membrane changes and the solute molecule is discharged to the other side of the membrane ii.

ion channels

protein pores that open / close to control the passage of selected ions (e.g. Na+, K+ Page 6 of 12

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hydrophilic channel allows solutes through Summary – facilitated diffusion: (a) Carrier proteins (b) (c) Active Transport

No energy

(c)

down a gradient

-

the movement of substances across a membrane using energy (usually A.T.P.)

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occurs against a concentration gradient – involves a specific carrier protein

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typically, A.T.P. is hydrolysed and the binding of the phosphate group to the carrier molecule changes the protein’s confirmation in such a way that the solute is delivered / transferred across the membrane.

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the specific carrier protein also acts as ATPase – releasing the energy for A.T.P.

Key facts -

factors that affect the rate of respiration (and therefore A.T.P. production) will affect the rate of active transport

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in cells where active transport is particularly important, there are large numbers of mitochondria (to produce A.T.P.)

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active transport is important in many aspects of physiology. E.g.:o uptake of products of digestion o uptake of mineral ions from the soil o nerve impulses o reabsorption in kidney tubules

Summary -

AGAINST concentration gradient uses energy (A.T.P.) specific carrier protein

More on active transport… -

there are 3 methods of carrier proteins:

(a)

Uniport

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where a single substance is transported across a membrane (i.e. in one direction) (e.g. calcium pumps in the muscles)

(b)

Symport

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where 2 substances are transported in the same direction (e.g. glucose/sodium pumps)

(c)

Antiport

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where 2 substances are transported in opposite directions at the same time (e.g. sodium/potassium pumps - nerve impulse) Page 7 of 12

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Osmosis uniport

(d)

symport

antiport

“the movement of water from a region of high water potential to a region of low water potential through a differentially permeable memrane.” lets molecules through pores at different rates e.g. sucrose (big) = 0 m.p.h water (small) = fast!

Water potential (φ) this is a measure of the free energy of water molecules. Molecules of water are in a constant state of random motion. This is greatest in pure water – water potential = 0. When a solute is added (i.e. a solution is formed) the solute molecules slow down the water molecules so that they have less energy – the solution therefore has a lower water potential than pure water. The amount by which the solute particles lower the water potential is called the solute potential. All solutions have a lower value than 0.

Osmosis therefore can be redefined: “The movement of water from a region area of less negative water potential to a region of more negative water potential through a differentially permeable membrane.” (i)

animal cells a. in pure water i.

water moves in by osmosis from a higher water potential to a lower water potential.

ii.

the cell expands and bursts.

b. in concentrated sugar solution Page 8 of 12

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i.

(ii)

water moves out of the cell by osmosis and the cell shrivels.

plant cells a. in pure water i.

water moves into the vacuole through the tonoplast, which increases in size, forcing the cytoplasm against the cell wall.

ii.

the cell is turgid and no more water enters due to the presence of the cell wall

iii.

as there is now no net movement of water into the cell, the water potential inside the cell must be the same as outside the cell

iv.

as pure water surrounds the cell, the water potential within the cell is equal to 0.

v.

inside the cell, there is till a solute potential trying to draw the water into the cell – but the effect is counteracted due to a force called the pressure potential due to the cell wall.

vi.

Thus: water potential = solute potential + pressure potential

b. in concentrated sugar solution i.

water leaves the cell due to osmosis as the water potential is higher inside the cell

ii.

the cell contents shrink away from the cell wall (i.e. the cytoplasmic membrane loses contact with the cell wall)

iii.

the cell is called plasmolysed

Incipient plasmolysis -

the cell wall will not be pressed upon by the cytoplasm (i.e. cell membrane has just begun to shrink from the cell wall)

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thus the pressure potential = 0 Page 9 of 12

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therefore: water potential = solute potential

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Endocytosis and Exocytosis Endocytosis -

moving large quantities of material into a cell stuff which is too big for a membrane

object

membrane buckles inward

membrane surrounds object

object in vesicle

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(e.g. phagocytosis)

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this is called pinocytosis when liquid (which would normally be water in living organisms).

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when the object is in the cell, the lysosomes (which contain enzymes) go to the object to digest it

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the buckling of the membrane is caused by protein receptors. Exocytosis object to be removed

fuses with membrane

released

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(e.g. releasing enzymes: e.g. pancreatic cells or wall in the digestive system)

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endocytosis would cause the membrane to be smaller, but this des not happen: therefore, exocytosis and endocytosis must equal out. Cell Signalling

Cell signalling is an important part of how the cell responds to the environment around it. This therefore ensures that cells respond correctly for homeostasis. Cell signalling can be between cells within an organism or between cells of different organisms. The way in which cell signalling works is via receptors on the surface of the cell. These receptors respond to external signals including responding to hormones.

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Syllabus Checklist (a)

outline the roles of membranes within cells and at the surface of cells;

(b)

state that plasma (cell surface) membranes are partially permeable barriers;

(c)

describe, with the aid of diagrams, the fluid mosaic model of membrane structure (HSW1);

(d)

describe the roles of the components of the cell membrane; phospholipids, cholesterol, glycolipids, proteins and glycoproteins;

(e)

outline the effect of changing temperature on membrane structure and permeability;

(f)

explain the term cell signaling;

(g)

explain the role of membrane-bound receptors as sites where hormones and drugs can bind;

(h)

explain what is meant by passive transport (diffusion and facilitated diffusion including the role of membrane proteins), active transport, endocytosis and exocytosis;

(i)

explain what is meant by osmosis, in terms of water potential. (No calculations of water potential will be required);

(j)

recognise and explain the effects that solutions of different water potentials can have upon plant and animal cells (HSW3).

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