Module 2 Section 5: Cell Membrane Flashcards

1
Q

Function of the cell surface membrane

A

They are the barrier between the cell and its environment, controlling which enter and leave the cell
They’re partially permeable - they let some molecules through but not others
Substances can move across the plasma membrane by diffusion, osmosis or active transport
They allow recognition by other cells, e.g. the cells of the immune system
They allow cell communication (sometimes called cell signalling)

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2
Q

Function of membrane within cells

A

Membranes around organelles divide the cell into different compartments - they act as a barrier between the organelle and cytoplasm
This makes functions more efficient e.g. the substance needed for respiration (like enzymes) are kept together inside mitochondria
They can form vesicles to transport substances between different areas of the cell
Control which substances enter and leave the organelle, e.g. RNA leaves the nucleus via the nuclear membrane.
They are also partially permeable

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3
Q

Functions of membranes within organelles

A

Membranes within organelles - act as barriers between the membrane contents and the rest of the organelle e.g. thylakoid membranes in chloroplasts
Membranes within cells can be the site of chemical reactions e.g. the inner membrane of a mitochondrion contains enzymes needed for respiration

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4
Q

Structure of cell membranes

A

Structure of all membranes is basically the same:
Composed of lipids (mainly phospholipids), proteins and carbohydrates (usually attached to proteins or lipids)

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5
Q

What is the fluid mosaic model

A

1972 fluid mosaic model was proposed
Phospholipid molecules form a continuous bilayer
This bilayer is ‘fluid’ because the phospholipids are constantly moving
Cholesterol molecules are present within the bilayer
Protein molecules are scattered through the bilayer, like tiles in a mosaic
Some proteins have a polysaccharide (carbohydrate) chain attached - glycoproteins
Some lipids also have a polysaccharide chain attached - called glycolipids

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6
Q

How thick is the phospholipid bilayer

A

Approximately 7nm thick

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7
Q

Structure of phospholipids

A

Phospholipids molecules have a ‘head’ and ‘tails’
The head is hydrophilic - it attracts water
The tail is hydrophobic - it repels water

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8
Q

Structure of the phospholipid bilayer

A

The molecules automatically arrange themselves into a bilayer
The heads face out towards the water on either side of the membrane
The centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble (like ions) through it - it acts as a barrier to these dissolved substances

But fat-soluble substances e.g. fat-soluble vitamins, can dissolve in the bilayer and pass directly through the membrane

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9
Q

Function of cholesterol

A

Cholesterol is a type of lipid
It’s present in all cell membranes (except bacterial membrane)
Cholesterol molecules fit between the phospholipids
They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together
This makes the membrane less fluid and more rigid

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10
Q

How do proteins control what enters and leaves the cell

A

Some proteins form channels in the membrane - these allow small or charged particles through
Other protein (called carrier proteins) transport molecules and ions across the membrane by active transport and facilitated diffusion
Proteins also act as receptors for molecules (e.g. hormones) in cell signalling
When a molecule binds to the protein, a chemical reaction is triggered inside a cell

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11
Q

Functions of glycolipids and glycoproteins

A

Glycolipids and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecules
They’re also sites where drugs, hormones and antibodies bonds
They act as receptors for cell signalling
They’re also antigens - cell surface molecules involved in the immune responses

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12
Q

What is cell signalling

A

This is how cell communicate with eachother
This is needed to control processes inside the body and to respond to changes in the environment

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13
Q

How do cell communicate with eachother using messenger molecules

A

One cell releases a messenger molecule ( e.g. a hormone )
This molecule travels ( e.g. in the blood ) to another cell
The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

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14
Q

How do cell membrane receptors help cell signalling

A

Proteins in the cell membrane act as receptors for messenger molecules which are called ‘membrane-bound receptors’
Receptor proteins have specific shapes - only messenger molecules with a complementary shape can bind to them
Different cells have different types of receptors - they respond to different messenger molecules
A cell that responds to a particular messenger molecule is called a target cell

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15
Q

What is an example of cell membrane receptors doing their job

A

Glucagon is a hormone that’s released when there isn’t enough glucose in the blood
It binds to receptors on liver cells, causing the liver cells to break down stores of glycogen to glucose

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16
Q

How to investigate the permeability of cell membranes

A

Cut five equal sizes pieces of beetroot and rinse them to remove any pigment released during cutting
Place the five pieces in five different test tubes, each with 5cm3 of water
Place each test tube in a water bath at a different temperature, e.g. 10°C, 20°C, 30°C, 40°C, 50°C for the same length of time
Remove the pieces of beetroot from the tubes, leaving just the coloured liquid
Use a colorimeter to pass light through the liquid and measure the absorbance of light. The higher the permeability of the membrane, the more pigment is released, so the higher absorbance of the liquid

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17
Q

What do temperatures below 0°C do to the membrane permeability

A

Phospholipids don’t have much energy, can’t move very much
They’re packed closely together and the membrane is rigid
However, channel and carrier proteins in the membrane deform, increasing the permeability of the membrane
Ice crystals may form and pierce the membrane making it highly permeable when it thaws

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18
Q

What do temperatures between 0 and 45°C do to the membrane permeability

A

The phospholipids can move around and aren’t packed as tightly together - the membrane is partially permeable
As the temperature increases the phospholipids move more because they have more energy
This increases the permeability of the membrane

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19
Q

What do temperatures above 45°C do to the membrane permeability

A

The phospholipid bilayer starts to melt (break down)
Membrane becomes more permeable
Water inside the cell expands, putting pressure on the membrane
Channel proteins and carrier proteins deform so they can’t control what enters or leaves the cell
Increases the permeability of the membrane

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20
Q

How does changing the solvent affect membrane permeability

A

Surrounding cells in a solvent (such as ethanol) increases permeability of their cell membranes
This is because solvents dissolve the lipids in the cell membrane, so the membrane loses its structure
Some solvents increase cell permeability more than others, e.g. ethanol increases cell permeability more than methanol

21
Q

How to investigate effects of different solvents on the permeability of the cell membrane

A

Can investigate this by doing an experiment using beetroot cylinders for different solvents and measure the concentration of the colours of the solution using a colorimeter

22
Q

What does increasing the concentration of the solvent do to effect membrane permeability

A

Increasing the concentration of the solvent will increase membrane permeability

23
Q

What is diffusion

A

Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration
Molecules will diffuse both ways, but the net movement will be to the area of lower concentration.
This continues until particles are evenly distributed throughout the liquid of gas
Diffusion is a passive process - no energy is needed for it to happen

24
Q

What is a concentration gradient

A

A path from an area of higher concentration to an area of lower concentration
Particles diffuse down a concentration gradient

25
What particles diffuse though cell membranes
Small, non polar molecules such as oxygen and carbon dioxide are able to diffuse easily through spaces between phospholipids Water is also small enough to fit between phospholipids, so it’s able to diffuse across plasma membranes even though it’s polar The diffusion of water molecules like this is called osmosis
26
What does the rate of diffusion depend on
The concentration gradient: the higher it is, the faster the rate of diffusion The thickness of exchange surface: the thinner the exchange surface (the shorter the distance the particles have to travel), the faster the rate of diffusion Surface area: larger the surface area the faster the rate of diffusion Temperature: warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster
27
How to investigate diffusion in model cells
Make up agar jelly with phenolphthalein and sodium hydroxide so they turn pink Fill a beaker with hydrochloric acid and cut out the agar into cubes using a scalpel and put them into the beaker Leave the cubes and they will eventually turn colourless as the acid diffuses into the jelly and neutralises the sodium hydroxide to turn the indicator colourless
28
How to investigate factors effecting rate of diffusion using model agar cells
Surface area: Cut jelly into different sized cubes and measure SA : V ratio Time how long it takes each cube to go colourless when placed into the same concentration of hydrochloric acid Largest SA : V goes colourless first Concentration gradient: Prepare different concentrations of hydrochloric acid Put equal sized cubes of agar jelly in each test tube and time how long it takes each to go colourless Highest concentration turns colourless first Temperature: Prepare several boiling tubes with same concentration of hydrochloric acid Put tubes into water baths or varying temperatures Put equal sized cubes of agar jelly into boiling tubes and time how it takes each one to go colourless Highest temperature goes colourless first
29
What is facilitated diffusion
Larger molecules (e.g. amino acids, glucose), ions and polar molecules don’t diffuse directly through phospholipid bilayer of cell membrane Instead, they diffuse through carrier proteins or channel proteins in the cell membrane (facilitated diffusion) - membranes with protein channels are selectively permeable Facilitated diffusion moves particles down concentration gradient Passive process
30
What are carrier proteins
Move large molecules into or out of the cell, down concentration gradient Different carrier proteins facilitate the diffusion of different molecules
31
What are channel proteins and what is their function
Channel proteins form pores in the membrane for ions to diffuse though ( down concentration gradient ) Different channel proteins facilitate the diffusion of different particles
32
What is active transport
Active transport uses energy to move molecules and ions across plasma membranes, against a concentration gradient This process involves carrier proteins
33
Process of active transport
Similar to facilitated diffusion Molecule attaches to carrier protein, the molecule changes shape and this moves the molecule across the membrane, releasing it on the other side Energy is used from ATP being hydrolysed to form ADP and P and releasing energy to move the solute against its concentration gradient, the phosphate group remains bound to the protein ( phosphorylating it ) Phosphate molecule releases from the carrier protein and recombines with ADP to form ATP The carrier protein returns to its original shape
34
Why do cells need endocytosis
Some molecules are too large to enter a cell by carrier proteins e.g. proteins, lipids and some carbohydrates
35
How does endocytosis work
A cell can surround a substance with a section of it’s plasma membrane Membrane pinches off to form a vesicle inside the cell containing the ingested substance - this is endocytosis This process uses ATP for energy
36
What is exocytosis
Some substances produced by the cell (e.g. digestive enzymes, hormones, lipids) need to be release from the cell - this is done by exocytosis
37
How does exocytosis work
Vesicles containing substances such as enzymes, hormones or lipids pinch off from the sacs of the Golgi apparatus and move towards the plasma membrane The vesicles fuse with the plasma membrane and release their contents outside the cell Some substances ( like membrane proteins ) aren’t released outside the cell - instead they are inserted straight into the plasma membrane Exocytosis uses ATP as an energy source
38
What is osmosis
The diffusion of water molecules across a partially permeable membrane Happens down a concentration gradient This means that water molecules move from an area of higher water potential ( higher concentration of water molecules ) to an area of lower water potential ( lower concentration of water molecules )
39
What is water potential
The potential (likelihood) of water molecules to diffuse out of or into a solution Pure water has the highest water potential All solutions have a lower water potential than pure water
40
What are the three types of solutions depending on water potential
Hypotonic solution: solution with a higher water potential than the cell ( cell has lower WP ) Isotonic solution: solution with the same water potential as the cell Hypertonic solution: solution with a lower water potential than the cell ( cell has high WP )
41
What happens in an animal cell when it is a hypotonic solution
The outside environment of the cell has a higher water potential Water enters the cell by osmosis ( high WP to low WP ) via the partially permeable membrane This increases the hydrostatic pressure inside the cell Cell surface membrane cannot withstand pressure Cell bursts by cytolysis
42
What happens in an animal cell when it is a isotonic solution
Water molecules pass into and out of the cell in equal amounts No net movement Cell stays the same
43
What happens in an animal cell when it is a hypertonic solution
The outside environment of the cell has a lower water potential Water exits the cell by osmosis ( high WP to low WP ) via the partially permeable membrane This decreases the hydrostatic pressure inside the cell The cell crinkles up as the water exits This is called crenation
44
What happens in a plant cell when it is a hypotonic solution
The outside environment of the cell has a higher water potential Water enters the cell by osmosis ( high WP to low WP ) via the partially permeable membrane This increases the hydrostatic pressure inside the cell The cell wall provides strength to the cell and stops it from bursting The plant cell swells up Vacuole swells The vacuole and cytoplasm push against the cell wall It is now turgid
45
What happens in a plant cell when it is a isotonic solution
Water molecules move into and out of the cell in equal amounts No net movement Plant cell stays the same
46
What happens in a plant cell when it is a hypertonic solution
The outside environment of the cell has a lower water potential Water exits the cell by osmosis ( high WP to low WP ) via the partially permeable membrane This decreases the hydrostatic pressure inside the cell The cell membrane shrinks away from the cell wall The cell becomes flaccid ( limp ) this is called plasmolysis
47
What cells specifically use endocytosis
Some cells take in much larger substances by endocytosis such as white blood cells ( mainly phagocytes) which use endocytosis to take in microorganisms and dead cells so they can destroy them
48
How do carrier proteins perform their function
- First, a large molecule attaches to a carrier protein in the membrane - Then protein changes shape - This releases the molecule on the opposite side of the membrane
49
Why can’t glucose simply diffuse through phospholipid bilayer
Glucose is soluble in water and phospholipids act as a barrier to water soluble substances Also a large molecule