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2.1.5 cell surface membrane Flashcards

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1
Q

functions of cell surface membrane

A
  • Partially permeable barriers between the cell and it’s outside environment, between organelles and the cytoplasm & within organelles.
  • Controls which substances enter and leave the cells.
  • Membranes allow recognition by other cells e.g. cells of the immune system.
  • Sites of cell communication (‘cell signalling’).
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2
Q

functions of cell surface membranes within cells

A
  • Separate the contents from the cytoplasm (act as a barrier)
  • Allowing selected molecules to move in and out of the cell
  • Allowing a cell to change shape
  • Isolating organelles from the rest of the cytoplasm, allowing cellular processes to occur separately.
  • A site for biochemical reactions e.g. respiration
  • Can form vesicles to transport substances e.g. golgi apparatus
  • Provide attachment sites for enzymes.
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3
Q

explain cell signalling

A
  • Cell signalling is how cells communicate with each other.
  • Cells need to communicate with each other to control processes inside the body and respond to changes in the environment. E.g. low blood glucose levels.
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4
Q

How do cells communicate with each other using messenger molecules?

A

1) One cell releases a messenger molecule (e.g. a hormone - insulin)
2) This molecule travels (e.g. in the bloodstream) to another target cell.
3) This messenger molecule binds to a receptor (called a membrane bound receptor with a complementary shape to the messenger molecule) on the cell surface membrane of the target cell.
4) A response is triggered e.g. uptake of glucose in the liver

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5
Q

what are receptors

A
  • Proteins in the cell surface membrane (e.g. glycoproteins act as receptors for the messenger molecules e.g. drug/hormone.
  • They are called ‘membrane bound receptors’.
  • Membrane bound receptors have a specific shape so only messenger molecules with a complementary shape can bind to them.
  • A cell that can respond to a particular messenger molecule is called a ‘target cell’.
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6
Q

explain the fluid mosaic model

A

The fluid mosaic model describes cell membranes as ‘fluid’ because:
- The phospholipids and proteins can move around via diffusion
- The phospholipids mainly move sideways, within their own layers
- The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
The fluid mosaic model describes cell membranes as ‘mosaics’ because the scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

  • Based on the ‘fluid mosaic model’ of Singer and Nicholson (1972).
  • The fluid mosaic model also helps to explain:
    o Passive and active movement between cells and their surroundings
    o Cell-to-cell interactions
    o Cell signalling
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7
Q

lipids and water

A
  • These two substances do not mix.
  • Water is a polar molecule (the oxygen end is slightly negative and the hydrogen end slightly positive).
  • Fats are non-polar and do not form hydrogen bonds with water.
  • Fats are said to be hydrophobic and lie on the surface of the water to reduce the surface area in contact between the fat and the water
  • When exposed to water, phospholipids form one of two structures: a micelle or a bilayer.
  • In each structure, the hydrophilic heads face the water, and the hydrophobic tails point inwards away from the water
  • This behaviour is key to the role that phospholipids play in membranes.
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8
Q

phospholipids

A
  • a molecule with a hydrophilic phosphate head (attracts water and polar) and a hydrophobic (non-polar and repels water) fatty acid tail.
  • many phospholipids form a phospholipid bilayer - the heads face out towards the water on either site of the membrane.
  • The centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances through it – it acts as a barrier to these dissolved substances
  • But fat-soluble substances can dissolve the bilayer and pass directly through the membrane
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9
Q

cholesterol

A
  • a steroid molecule that fits between fatty acid tails and provides mechanical stability.
  • makes membrane less fluid and more rigid to stabilise the membrane
  • affects fluidity and can reduce permeability to polar/charged molecules e.g. water and ions
  • The more cholesterol, the less fluid and the less permeable the membrane.
  • Cholesterol is also important in keeping membranes stable at normal body temperature – without it, cells would burst open.
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10
Q

glycolipids

A
  • where phospholipid molecules have a carbohydrate attached
  • they act as cell markers or antigens.
  • they act as receptor molecules
  • they facilitate cellular recognition e.g. in the immune response
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11
Q

intrinsic proteins

A

those which span the entire membrane
functions:
1. act as channels
2. transporters
3. receptors
4. enzymes
e.g. channel, carrier, glycoproteins
span the whole width of the membrane.

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12
Q

extrinsic proteins

A

peripheral proteins confined to the inner or outer surface of the membrane.

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13
Q

channel proteins

A
  • provide a hydrophilic channel
  • allows passive movement of polar molecules and ions
  • down a concentration gradient through membranes (i.e. diffusion of oxygen)
  • different channel proteins facilitate the diffusion of different charged particles.
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14
Q

carrier proteins

A
  • Binds specific molecules and transport these molecules and ions across the membrane by active transport and facilitated diffusion e.g sodium ion.
  • moves large molecules (including polar molecules and ions) into or out of the cell.
  • Down their concentration gradient
  • Different carrier proteins facilitate the diffusion of different molecules.
  • Carrier proteins change shape when a specific molecule binds.
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15
Q

glycoproteins

A
  • where intrinsic protein molecules in the membrane have a carbohydrate chain attached.
  • acts as a receptors in cell signalling - when a molecule binds to the protein, a chemical reaction is triggered inside the cell.
  • Stabilise the membrane by forming hydrogen bonds with surrounding water molecules.
  • They are also sites where drugs, hormones and antibodies bind – cell recognition.
  • They act as receptors for cell signalling.
  • Cell adhesion.
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16
Q

enzymes and coenzymes

A
  • some reactions take place in membranes thereby requiring enzymes
  • e.g. some reactions of respiration take place in the membrane of the cristae of the mitochondria
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17
Q

Role of membrane-bound receptors as sites where hormones and drugs can bind

A

Membrane bound receptors have a specific shape so only messenger molecules with a complementary shape can bind to them

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18
Q

factors affecting membrane structure: temperature

A

when temperature increases, phospholipids would have more kinetic energy and therefore the membrane’s fluidity increases and loses its structure. The loss of structure increases the permeability of the membrane

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19
Q

factors affecting membrane structure: solvents

A

non-polar solvents can dissolve membranes. Less concentrated solutions of alcohols for instance, cannot dissolve membranes but still cause damage by disrupting the membrane. When the membrane is disrupted, it becomes more fluid and more permeable.

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20
Q

factors affecting fluidity

A

The length of fatty acid side chains – the longer chains, the lower the fluidity
The proportion of saturated fatty acids – the higher the proportion of saturated fats, the lower the fluidity
The steroid content – the higher the steroid content, the lower the fluidity

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21
Q

why is the membrane fluid?

A
  • All lipids can move sideways (laterally) within the membrane and exchange position with each other.
  • This gives the membrane fluidity.
  • This is essential for some processes within the cell e.g. phagocytosis.
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22
Q

concentration gradient

A
  • The difference in concentration of particles between 2 areas is called a CONCENTRATION GRADIENT.
  • Concentration gradient is either steep or shallow.
  • If concentration gradient is steep rate of diffusion is faster.
  • If concentration gradient is shallow rate of diffusion is slower.
  • The direction of diffusion (from high to low concentration) is said to be ‘down’ or ‘with’ the concentration gradient.
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23
Q

5 ways in which substances enter and leave the cells

A

1) Simple diffusion (Passive process)
2) Facilitated diffusion (Passive process)
3) Osmosis (Passive process)
4) Active Transport (Active – require ATP energy)
5) Bulk transport (endocytosis and exocytosis) (Active – require ATP energy)

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24
Q

simple diffusion

A
  • The net (overall) random movement of particles (atoms, molecules, ions) from an region of high concentration to an region of lower concentration (down a concentration gradient).
  • Due to the random movement and collisions of particles (which have kinetic energy)
  • A passive process which means that no (metabolic – ATP) energy is needed
  • Continues until there is a concentration equilibrium between both sides.
  • E.g. Oxygen molecules diffuse into cells by simple diffusion.
  • Small, non polar molecules e.g. oxygen and carbon dioxide can diffuse easily through the cell surface membrane.
  • This is because they are very small and can pass through spaces between the phospholipids.
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25
Q

facilitated diffusion

A
  • Facilitated diffusion is the ‘passive movement of molecules down a concentration gradient (high concentration to low concentration) across a membrane, and involves special carrier & channel proteins in the membrane’.
  • Some larger polar molecules e.g. amino acids and glucose, or ions such as sodium ions (Na+ ) and chloride ions (Cl- ) cannot simply diffuse directly through the phospholipid layer of cell surface membrane.
  • These substances can only cross the phospholipid bilayer with the help of certain proteins.
  • This form of diffusion is known as facilitated diffusion
  • There are two types of proteins that enable facilitated diffusion:
  • Channel proteins * Carrier proteins
  • They are highly specific (they only allow one type of molecule or ion to pass through).
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26
Q

channel proteins in facilitated diffusion

A
  • Channel proteins are water-filled pores.
  • They allow charged substances (eg. ions) to diffuse through the cell membrane.
  • The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore.
  • This allows the channel protein to control the exchange of ions.
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27
Q

carrier proteins in facilitated diffusion

A
  • Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes.
  • This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape.
  • The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane.
  • Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule).
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28
Q

facilitated diffusion depends on…

A
  1. The concentration gradient – the higher the concentration gradient, the faster the rate of facilitated diffusion. As equilibrium is reached, the rate of facilitated diffusion will level off.
  2. The number of channel or carrier proteins – once all the proteins in a membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concentration gradient. So the greater the number of channel or carrier proteins in the cell membrane, the faster the rate of facilitated diffusion.
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29
Q

factors affecting rate of diffusion: difference in concentrations

A
  • The more particles there are in one area compared to another area the steeper the concentration gradients
  • Therefore the faster the rate of diffusion
  • This is due to more random frequent collisions of the particles
30
Q

factors affecting rate of diffusion: temperature

A
  • The higher the temp, the more kinetic energy the particles will have
  • This means there will be more frequent collisions between the particles as the particles are moving at higher speeds.
  • Rate of diffusion therefore increases
31
Q

factors affecting rate of diffusion: surface area of the membrane

A
  • The larger the surface area of the membrane, the more area there is for particles to diffuse across
  • Therefore, the rate of diffusion is faster
32
Q

factors affecting rate of diffusion: thickness of the exchange surface/membrane

A
  • The thinner the exchange surface the shorter the diffusion pathway for molecules to pass through the membrane
  • Therefore the rate of diffusion increases.
33
Q

factors affecting rate of diffusion: SA:V

A
  • SA:V changes with the size of the cell
  • As the size of a cell increases so does the surface area and the volume
  • But the volume of the cell increases faster than the surface area.
  • When volume is too large relative to the surface area of the cell, diffusion cannot occur at a high enough rate to supply raw materials to the entire volume of the cell.
34
Q

How are organs specialised to ensure diffusion is efficient?

A

1) Have a large surface area – increases the rate of diffusion.
2) A membrane that is thin, this provides a short diffusion pathway.
3) Animals have an efficient blood supply surrounding the exchange surface.
4) Large SA:V ratio. There is more surface area relative to the volume inside

35
Q

adaptations of the intestines

A
  1. The inside of the small intestine is covered in millions of villi
  2. They increase the surface area so that difested food is absorbed more quickly into the blood
  3. They have a single layer of surface cells, very thin walls and a very good blood supply to assist quick absorption.
36
Q

adaptations of the alveoli

A
  1. They are very thin – walls are only one cell thick – to make gas exchange more efficient
  2. There is a very efficient bloody supply network of capillaries around the alveoli
  3. The lungs and the alveoli have a large combined surface area, allowing fast exchange of gases
  4. The walls are moist, encouraging gas molecules to easily dissolve.
37
Q

adaptations for gas exchange in fish

A
  • Fish have gills which are made up of many gill filaments – this gives a large surface area for gas exchange
  • The gills are covered in tiny structures called lamellae which increase the surface area even more
  • The lamellae are surrounded by lots of blood capillaries to speed up the rate of diffusion
  • Counter current exchange system
  • They also have a thin surface layer of cells to provide a short diffusion pathway for the gases.
38
Q

roots of a plant

A
  • Plant roots anchor the plant into the soil and absorb water and mineral ions from the soil
  • Plant roots have extensions called root hair cells. These cells give the plant a larger surface area to absorb more water and mineral ions from the soil
  • Root hair cells are thin which provides a short diffusion pathway for water and mineral ions to travel into the root.
39
Q

gas exchange in leaves

A
  • Leaves absorb CO2 gas from the atmosphere and release O2 gas and water vapour in the process of photosynthesis
  • Leaves are very thin. This provides short diffusion pathways for a faster rate of diffusion of gases
  • Leaves are usually broad and flat. This gives a larger surface area so more gases can diffuse into and out of the leaf.
  • Leaves have tiny pores called stomata. This is where gases are exchanged.
40
Q

diffusion across the synapse of nerve cells

A
  • A synapse is a junction between 2 neurones across which electrical signals must pass
  • Neurotransmitter molecules diffuse from vesicles towards the neurotransmitter receptors moving from an area of high concentration to low concentration.
41
Q

active transport

A

the movement of molecules or ions into or out of a cell from an area of low concentration to an area of high concentration against a concentration gradient across a partially permeable membrane.
requires ATP
requires carrier protein - which acts as a pump and is complementary in shape to the molecule they transport

42
Q

why is active transport important?

A
  • The reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules
  • The absorption of some products of digestion from the digestive tract.
  • The loading of sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant
  • The loading of inorganic ions from the soil into the root hairs.
43
Q

bulk transport

A

larger quantities of materials into or out of cells is also possible
Examples of these larger quantities of materials that might need to cross the membrane include:
- Large molecules such as proteins or polysaccharides
- Parts of cells
- Whole cells e.g. bacteria
Bulk transport into cells = endocytosis
Bulk transport out of cells = exocytosis
These two processes require energy and are therefore forms of active transport

44
Q

endocytosis

A

the process by which the cell surface membrane engulfs material, forming a small sac around it.

45
Q

endocytosis: phagocytosis

A
  • This is the bulk intake of solid material by a cell
  • Cells that specialise in this process are called phagocytes
  • The vacuoles formed are called phagocytic vacuoles
  • An example is the engulfing of bacteria by phagocytic white blood cells
46
Q

endocytosis: pinocytosis

A
  • This is the bulk intake of liquids
  • If the vacuole (or vesicle) that is formed is extremely small then the process is called micropinocytosis.
47
Q

exocytosis

A

the process by which materials are removed from, or transported out of cells
- The substances to be released (such as enzymes, hormones or cell wall building materials) are packaged into secretory vesicles formed from the Golgi body
- These vesicles then travel to the cell surface membrane
- Here they fuse with the cell membrane and release their contents outside of the cell
- An example is the secretion of digestive enzymes from pancreatic cells

48
Q

water potential

A

This is a measure of the tendency of water molecules to move across a partially permeable membrane. The symbol used for water potential is the Greek letter psi, Ψ

49
Q

osmosis

A
  • Osmosis is the net random movement of water molecules from an region of high water potential to a region of low water potential (= down a water potential gradient) across a partially permeable membrane
  • Osmosis is a passive process (no ATP energy is required).
  • Pure water has a water potential of zero. Ψ = 0
  • The movement of water by osmosis, into or out of a cell changes the hydrostatic pressure of the cell.
50
Q

factors affecting rate of osmosis

A

Water potential gradient - The higher the water potential gradient, the faster the rate of osmosis. As osmosis occurs, the difference in water potential either side of the membrane decreases so the rate of osmosis levels off over time.
Thickness of the exchange surface - The thinner the exchange surface the shorter the pathway the water molecules have to travel. This increases the rate of osmosis.
Surface area of the exchange surface - The greater the surface area of the partially permeable membrane the more area water molecule have to cross the membrane.

51
Q

hypotonic solution

A

A solution with more water in it than solute, compared to another solution.

52
Q

hypertonic solution

A

A solution with more solute in it than water, compared to another solution.

53
Q

isotonic solution

A

Two solutions/cells that have the same water potential

54
Q

what happens in an animal cell placed in a hypotonic solution?

A
  • The outside environment of the cell has a higher water potential
  • Water enters the cells by osmosis down the water potential gradient via the partially permeable membrane
  • This increases the hydrostatic pressure inside the cell
  • Cell surface membrane cannot withstand the pressure increase
  • The animal cell bursts
  • This is called cytolysis
55
Q

what happens in a plant cell placed in a hypotonic solution?

A
  • The outside environment of the cell has higher water potential
  • Water enters cells by osmosis down the water potential gradient via the partially permeable membrane
  • This increases the turgor pressure inside the cell
  • As the turgor pressure increases, it resists the entry of further water
  • Cell surface membrane is pushed against the cell wall, increasing the volume of the cell
  • The cell is turgid
56
Q

what happens in an animal cell placed in a hypertonic solution?

A
  • The outside environment of the cell has a lower water potential
  • Water leaves the cell by osmosis down the water potential gradient via the partially permeable membrane
  • This decreases in hydrostatic pressure inside the cell
  • The animal cell shrinks and shrivels
  • The cell is crenated
57
Q

what happens in a plant cell placed in a hypertonic solution?

A
  • The outside environment of the cells has a lower water potential
  • Water leaves the cell by osmosis down the water potential gradient via the partially permeable membrane
  • This decreases the turgor pressure inside the cell
  • Leads to reduction in volume of the cytoplasm and the cell surface membrane pulls away from the cell wall due to the pressure change
  • The cell is plasmolysed
58
Q

structure and function of squamous epithelium tissue

A
  • single layer of flat cells lining a surface
  • found in many places e.g. alveoli in the lungs
59
Q

structure and function of ciliated epithelium tissue

A
  • ciliated epithelium is a layer of cells covered in cilia
  • found on surfaces where things need to be moved e.g. the trachea where the cilia waft mucus along
60
Q

structure and function of cartilage

A
  • type of connective tissue found in the joints
  • shapes and supports the ears, nose and windpipe
  • formed when cells called chondroblasts secrete an extracellular matrix which they become trapped inside
61
Q

structure and function of muscle tissue

A
  • muscle tissue is made up of bundles of elongated cells called muscle fibres
  • 3 different types of muscle tissue
    1. smooth (found in lining of stomach wall)
    2. cardiac (found in heart)
    3. skeletal (which is used to move)
  • all have slightly different structure
62
Q

structure and function of xylem tissue (vascular tissue)

A
  • xylem is a plant tissue with two jobs:
    1. it transports water around the plant
    2. it supports the plant
  • it contains hollow xylem vessel cells, which are dead, and living parenchyma cells
63
Q

structure and function of phloem tissue (vascular tissue)

A
  • phloem tissue transports sugars around the plant
  • arranged in tubes and is made up of sieve cells, companion cells and ordinary plant cells
  • each sieve cell has end walls with holes in them, so that sap can move easily through them. end walls are called sieve plates
64
Q

how are erythrocytes (red blood cells) specialised for its particular function?

A
  • biconcave, flattened shaped disks
  • increases surface area to volume ratio
  • no nuclei and few organelles
  • increases surface area for the protein haemoglobin (the molecule that carries oxygen)
  • flexible - can fit through very narrow capillaries
65
Q

how are neutrophils specialised for its particular function?

A
  • type of white blood cells - defend the body against disease
  • flexible shape allows them to engulf foreign particles or pathogens
  • many lysosomes in its cytoplasm which contain digestive enzymes to break down the engulfed particles
66
Q

how are squamous cells specialised for its particular function?

A
  • cover the surface of organs
  • cells are joined by interlinking cell membranes and a membrane at their base
  • squamous epithelia are very thin to allow efficient diffusion of gases
67
Q

how are ciliated epithelial cells specialised for its particular function?

A
  • cover the surface of organs
  • cells are joined by interlinking cell membranes and a membrane at their base
  • ciliated epithelia have cilia that beat to move particles away
68
Q

how are sperm cell specialised for its particular function?

A
  • male sex cells have flagellum (tail) so they can swim to the eff (female sex cell)
  • have lots of mitochondria to provide the energy to swim
  • the acrosome contains digestive enzymes to enable the sperm to penetrate the surface of the egg
69
Q

how are palisade cells specialised for its particular function?

A
  • palisade mesophyll cells in leaves do most of the photosynthesis
  • they contain many chloroplasts so they can absorb a lot of sunlight
  • the walls are thin so carbon dioxide can easily diffuse into the cell
70
Q

how are root hair cells specialised for its particular function?

A
  • root hair cells absorb water and mineral ions from the soil
  • they have a large surface area for absorption and a thin, permeable cell wall, for entry of water and ions
  • the cytoplasm contains extra mitochondria to provide the energy needed for active transport
71
Q

how are guard cells specialised for its particular function?

A
  • guard cells are found in pairs with a gap between them to form a stoma
  • this is one of the tiny pores in the surface of the leaf used for gas exchange
  • in the light, guard cells take up water and become turgid
  • their thin outer walls and thickened inner walls force them to bend outwards, opening the stomata
  • this allows the leaf to exchange gases for photosynthesis

biology (10 decks)

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