Cell Membranes And Transport

Question 1

What is the function of the cell membrane?

Answer 1

Controls the exchange of the materials such as nutrients and waste products between the cell and it’s environment, so acts as a selective barrier.
For communication between cells, by cell signaling. For example, they enable cells to receive hormone messages.
Separate cell components from the outside environment which is known as compartmentalisation.
Allows recognition of other external substances such as antigen.
Allows mobility in some organisms, e.g amoeba.

Question 2

What is the thickness of the cell membrane

Answer 2

These cell membrane is about 7nm thick, made of phospholipid bilayer.

Question 3

Why is the cell surface membrane illustrated as a fluid mosaic model?

Answer 3

Fluid: because both the phospholipids and proteins can move about by diffusion within their own monolayers. Some protein molecules can move also within the phospholipid bilayer.
Mosaic: describes the pattern produced by the scattered protein/glycoprotein molecules when the surface of the membrane is viewed from above.

Question 4

What are the factors affecting the fluidity of the cell membrane?

Answer 4

Some of the phospholipid tails are saturated and some are unsaturated. The more unsaturated they are, the more fluid the membrane. This is because the unsaturated fatty acid tails are bent and therefore fit together more loosely.
Fluidity is affected by the length of the phospholipid tail: the longer the tail the less fluid the membrane.
Temperature: The lower temperature the less fluidity of the membrane.

Question 5

How do some organisms which cannot regulate their own temperature use the fluidity of the cell membrane to survive in cold temperatures?

Answer 5

Some organisms which cannot regulate their own temperature, such as bacteria and yeasts, respond by increasing the proportion of unsaturated fatty acids in their membranes. The increased fluidity means the cells can survive colder temperatures.

Question 6

Which biomolecules exist in the membrane structure.

Answer 6

Lipids:
1.Phospholipids
2.glycolipids
3.cholesterol
Protein
1.protein
Carbohydrates:
1.glycolipids
2.glycoproteins

Question 7

Describe the structure of phospholipids

Answer 7

The basic structure of a cell membrane is phospholipid bilayer
The hydrophilic phosphate heads face outwards into the aqueous environment outside the cell.
Phosphate heads may contain a fatty substance known as choline.
The hydrocarbon tails of fatty acid face in words and create a hydrophobic interior.

Question 8

What are the functions of the phospholipids tails?

Answer 8

The tails are non-polar, so it is difficult for most polar molecules or ions to pass through membranes as they act as a barrier against water soluble molecules such as sugars amino acids and proteins.
Phospholipid tails maintain fluidity of the cell membrane which allows it to stretch under pressure and to reseal itself if it is disrupted as in cytosis.

Question 9

How do phospholipids function as signalling molecules.

Answer 9

Some phospholipids can be modified chemically to act as signalling molecules. For example they move about in the phospholipids bilayer and activate other molecules such as enzymes.
Alternatively they may be hydrolysed to release small water soluble, glycerol-related molecules and these diffuse through the cytoplasm and bind to specific receptors. This system releases Ca ions from ER to cause exocytosis of digestive enzymes in pancreatic cells.

Question 10

Describe the structure of cholesterol

Answer 10

A cholesterol molecule has a hydrophilic head and a hydrophobic tail and fits neatly between phospholipid molecules.
It is made of four linked hydrocarbon rings.
Animal cell membrane has as much cholesterol as phospholipids.
Cholesterol is less common in plant cell membrane and absent from most prokaryotes.

Question 11

What are the functions of cholesterol

Answer 11

Regulates Fluidity
Provides mechanical stability
Controls the exchange of molecules across the membrane

Question 12

How does cholesterol regulate fluidity?

Answer 12

At low temperature cholesterol increases the fluidity of the membrane to prevent it from becoming too rigid. This is because it prevents close packing of phospholipid tails.
Interaction between phospholipid and cholesterol molecules also helps to stabilize cells at higher temperatures when the membrane becomes otherwise too fluid.

Question 13

How does cholesterol provide the cell membrane with mechanical stability?

Answer 13

Without cholestrol molecules the cell membrane could break and the cell could burst open.

Question 14

Cholesterol control the exchange of molecules across the membrane ❓

Answer 14

As the hydrophobic regions of cholesterol molecules help to prevent ions and polar molecules from passing through the membrane. This is particularly important in the myelin sheath (made up of many layers of cell surface membrane) around the nerve cells, where leakage of ions would slow down nerve impulses.

Question 15

What are the two protein structures?

Answer 15

Proteins may be intrinsic or extrinsic proteins.

Question 16

Describe intrinsic (integral) proteins.

Answer 16

Proteins that are found inbedded within the membrane are called intrinsic proteins or integral proteins. Intrinsic proteins may be found in the inner layer, the outer layer or most commonly commonly spanning the whole membrane in which case they are known as trans membrane proteins.

Question 17

What are the hydrophobic regions which cross the membrane in transmembrane proteins made up of?

Answer 17

In trans membrane protein the hydrophobic region which cross the membrane is often made up of one or more alpha – helix chains.

Question 18

How does intrinsic proteins help form the fluid mosaic model?

Answer 18

Some intrinsic proteins are fixed in the membrane while others are floating and moving forming the fluid mosaic model.

Question 19

How are intrinsic proteins arranged ❓

Answer 19

Are arranged with their hydrophilic parts (R groups of amino acids) of their chains on the outer surface of the membrane, while hydrophobic parts embedded within the membrane among hydrophobic tales of phospholipids.
The hydrophilic parts of protein are repelled by hydrophobic interior of the membrane, and therefore face into the aqueous environment inside or outside of the cell or line hydrophilic pores which pass through the membrane.

Question 20

Describe extrinsic (peripheral) proteins:

Answer 20

These are found on the inner or outer surface of the membrane. Extrinsic proteins might be bound to intrinsic proteins phospholipids or other molecules in the cell.

Question 21

What are the functions of channel proteins and carrier proteins in a membrane?

Answer 21

They are involved in the selective transport of polar molecules and ions across the membrane. Many proteins act as transfer proteins as they provide hydrophillic channels for passageways for ions or polar molecules.

Question 22

What is the function of enzymes in a membrane

Answer 22

Some membrane proteins are enzymes, for example the digestive enzymes found in the cell surface membrane of the cells lining the small intestine. These catalyze the hydrolysis of molecules such as disaccharides.

Question 23

How do proteins in the cell membrane function as energy transfer.

Answer 23

Proteins present on the membrane of chloroplast and mitochondria are involved in the process of photosynthesis and respiration respectively.

Question 24

How do proteins function as cell-shape proteins?

Answer 24

Some proteins on the inside of the cell surface membrane are attached to a system of protein filaments inside the cell, known as the cytoskeleton. These proteins help to maintain and decide the shape of the cell. They may also be involved in changes of the cell shape when cells move.

Question 25

What are the three different functions of receptor proteins?

Answer 25

Signaling receptors
Endocytosis receptors
Cell adhesion receptors

Question 26

List the functions of proteins found in the membrane

Answer 26

Channel proteins or carrier proteins
Enzymes
Energy transfer
Cell shape proteins
Receptor proteins

Question 27

Explain how signaling receptor proteins function.

Answer 27

Proteins have very specific shapes. This makes them ideal as receptor molecules for chemical signaling between cells. For example, hormones are chemical messengers only binding to specific target cells which have the correct receptor sites, e.g. glucagon receptors in the liver cell. Cells that do not have glucagon receptors are not affected by glucagon. Another example is neurotransmitters that are chemicals that cross synapses allowing nerve impulses to pass from one cell to another.

Question 28

What is the function of receptor molecules?

Answer 28

The carbohydrate chain helps glycoproteins and glycolipids to act as receptor molecules. The function of receptor molecules is to bind with particular substances at the cell surface membrane. Different cells have different receptors depending on their function.

Question 29

How do proteins function as endocytosis receptors?

Answer 29

Proteins bind to molecules that are part of the structure to be engulfed by the cell surface membrane.

Question 30

How do proteins function as cell adhesion receptors?

Answer 30

Proteins are involved in binding cells to other cells and tissues and organs of animals.

Question 31

Describe the structure of carbohydrate molecules in the cell surface membrane.

Answer 31

Some proteins and lipids have short branching carbohydrate chains like antennae (glycocalyx) forming glycoproteins and glycolipids respectively. Both are found on the external surface of the membrane only. In animal cells the glycocalyx formed mainly from glycoprotein while in plant cells it is formed mainly in glycolipids.

Question 32

What are the functions of carbohydrates in cell membrane?

Answer 32

Stabilize membrane structure
Act as receptor molecules
Involved in cell-cell recognition and cell attachment
Are antigens

Question 33

Explain how carbohydrates help to stabilize the membrane?

Answer 33

Carbohydrates project out into the water fluid surrounding the cell where they form hydrogen bonds with water molecules and so help stabilize the membrane structure.

Question 34

Explain how carbohydrates function as receptor molecules?

Answer 34

Carbohydrates also act as receptor molecules for chemical signals by binding with particular substances such as hormones or neurotransmitters.

Question 35

Explain how carbohydrates can function in cell attachment or adhesion.

Answer 35

Glycolipids also have branching carbohydrate side chains and are involved in cell to cell recognition. Glycoproteins are also involved in sticking the correct cells together in tissues (cell attachment or adhesion)

Question 36

Explain how carbohydrates function in cell to cell recognition?

Answer 36

One group of glycoproteins known as antigens are important in allowing cells to recognize each other each type of cell has its own type of antigen.

Question 37

What is the difference in units of thickness in which the cell wall and cell membrane are normally measured in?

Answer 37

Cell wall is measured in micrometers. (0.2)
Cell membrane is measured in nanometers. (7)

Question 38

What is the difference in location of the cell wall in the cell membrane?

Answer 38

Cell wall surround some cells but not all cells such as not animal cells and it only surrounds sounds whereas the cell membrane surrounds all cells and can be found inside the cells.

Question 39

What is the difference in the permeability of the cell membrane and the cell wall?

Answer 39

These cell wells is freely permeable whereas the cell membrane is partially / selectively permeable.

Question 40

What is the function and stiffness of the cell wall and cell membrane

Answer 40

The function of the cell wall is to provide mechanical strength whereas the function of the cell membranes to act as a selective barrier. The cell wall is rigid whereas the cell membrane is fluid.

Question 41

Explain the importance of exchange materials between cell and their environments.

Answer 41

1. Two obtain nutrients such as glucose or other useful materials such as oxygen for respiration.
2. To excrete waste substances such as carbon dioxide from respiration.
3. To secrete useful materials such as hormones and enzymes
4. To generate ionic gradients essential for nervous and muscular activity.
   To maintain a stable pH and ionic concentration within the cell for enzyme activity.

Question 42

Define diffusion

Answer 42

Diffusion is the net movement of molecules or ions of a substance from a region of high concentration to a region of its lower concentration down a concentration gradient, due to random motion of ions and molecules caused by the natural kinetic energy of molecules or ions. This includes the diffusion of small non-polar molecules (such as oxygen and carbon dioxide) through the plasma membrane, as well as diffusion of fat-soluble molecules (such as vitamin A) through the plasma membrane.

Question 43

Is diffusion a passive or active process?

Answer 43

Diffusion is a passive process in which molecules or ions tend to reach an equilibrium situation, where they are evenly spread in each volume of space.

Question 44

What are the factors affecting the rate of diffusion?

Answer 44

The steepness of the concentration gradient
Temperature
The surface area
The nature of a molecules
The distance

Question 45

Explain the effect of the steepness of the concentration gradient towards the rate of diffusion.

Answer 45

The steepness of the concentration gradient is the difference in the concentration of the substance on the two sides of the surface membrane. The greater the difference in concentration, the greater the difference in the number of molecules passing in two directions and hence faster the net rate of diffusion.

Question 46

Explain the effect of temperature on the rate of diffusion

Answer 46

The higher the temperature the more the kinetic energy of molecules or ions, so faster random motion and diffusion takes place in higher rates.

Question 47

Explain the effect of surface area towards the rate of diffusion?

Answer 47

The greater the surface area, then the more molecules or ions can cross it at any one moment, and therefore the faster the diffusion can occur. The surface area can be increased by folding membranes such as cristae in mitochondria and microvilli in the lining epithelium of the small intestine. The larger the cell the smaller the surface area in relation to its volume.

Question 48

Explain the effect of the nature of the molecules on the rate of diffusion.

Answer 48

Size: large molecules require more energy to get them moving than small ones do, so substances with large molcules tend to diffuse more slowly than ones with small molecules.
Polarity and charge:Non polar molecules diffuse more easily through cell membranes than polar ones, as they are soluble in the non polar phospholipid tails.
Solubility in the phospholipid bilayer: Fat soluble molecules diffuse faster than fat insoluble ones.

Question 49

Give examples of molcecules which can pass through the phospholipid bilayer.

Answer 49

Glycerol, steroid hormones and alcohol becuase they are non polar, soluble in phospholipid tails.
Oxygen and carbon dioxide can pass through the phospholipid membrane easily by diffusion as they are uncharged, non polar molecules.
Water molecules despite being very polar, can diffuse rapidly across the phospholipid bilayer because they are small enough.

Question 50

Explain the effect of distance on the rate of diffusion.

Answer 50

The rate of diffusion decreases rapidly with distance. Diffusion is therefore only effective over short distances, that is why cells are small in size.

Question 51

What will happen if phospholipids are mixed with water?

Answer 51

If phospholipids are mixed with water they form either:
Ball-like structures called micelles
Sheet like structures called bilayers

Question 52

Describe the structure of a micelle

Answer 52

In a micelle all the hydrophilic heads face outwards into the water. They shield the hydrophobic tails from the water. In the middle of the ball the tails point in towards each other getting a hydrophobic environment.

Question 53

What is cell signaling?

Answer 53

The molecular mechanisms by which cells detect and respond to external stimuli including communication between cells is called cell signaling.

Question 54

Why do living organisms need cell signaling?

Answer 54

All cells and organisms must be able to respond appropriately to their environments. This is made possible by means of signaling pathways which coordinate the activities of cells even if they are large distances apart in the same body.

Question 55

What are the two types of cell signaling pathways?

Answer 55

Signaling pathways can be electrical for example the nervous system or chemical for example the hormone system in animals. they involve a wide range of molecules such as neurotransmitters and hormones.

Question 56

What do the three main stages of the first part of the signaling pathway involve?

Answer 56

A stimulus causes cells to secrete a ligand.
The ligand is transported to the target cells. Signaling molecules are usually relatively small for easy transport. In the case of hormones the transport system is the blood system.
The ligand binds to cell surface receptors on the target cells. The receptors are protein molecules located in the cell surface membrane.

Question 57

Define a ligand.

Answer 57

A biological molecule which binds specifically to another molecule, such as a cell surface membrane receptor during cell signaling.

Question 58

What occurs after the ligand binds to the cell surface receptors on the target cells?

Answer 58

The cell surface receptor is a specific shape and recognizes the ligand. Only cells with this receptor can recognize the ligand. The ligand brings about a change in the shape of the receptor. The receptor spans the membrane, so the message is passed to the inside of the cell. Changing the shape of the receptor allows it to interact with the next component of the signaling pathways, so the message gets transmitted. Conversion of the original signal to a message that is then transmitted is called transduction.

Question 59

Define transduction.

Answer 59

Transduction occurs during cell signaling and is the process of converting a signal from one method of transmission to another.

Question 60

What is the next component in the signaling pathway after transduction?

Answer 60

The next component in the signaling pathway is often a ‘G protein’ which acts as a switch to bring about the release of a ‘second messenger’. The second messenger is a small molecule which diffuses through the cell relaying the message.

Question 61

What happens after a second messenger molecule is released?

Answer 61

The stimulation of one receptor molecule results in many second messenger molecules being made in response. This represents an amplification of the original signal, a key feature of signaling. The second messenger typically activates an enzyme, which in turn activates further enzymes, increasing the amplification at each stage. Finally enzymes are produced which bring about the required change in cell metabolism.

Question 62

What is a signaling Cascade?

Answer 62

The sequence of events triggered by the G protein is called a signaling cascade.

Question 63

Apart from second messengers what are the other basic ways in which a receptor can alter the activity of a cell?

Answer 63

Apart from second messengers there are three other basic ways in which a receptor can alter the activity of a cell:

Opening an ion channel, resulting in a change of membrane potential.
Acting directly as a membrane-bound enzyme
Acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane.

Question 64

What are the changes which occur in signaling systems when the molecules are hydrophobic?

Answer 64

Some signalling molecules are hydrophobic for example the steroid hormones such as oestrogen. Hydrophobic signaling molecules can diffuse directly across the cell surface membrane and bind to receptors in the cytoplasm or nucleus.

Question 65

Is cell to cell contact a mechanism of cell signaling?

Answer 65

Direct cell to cell contact is another mechanism of signaling. This occurs when lymphocytes detect foreign antigens on other cells.

Question 66

What are the ways substances can move across the membrane?

Answer 66

The five basic mechanisms by which exchange between the cell membrane occurs are:
Diffusion
Osmosis
facilitated diffusion
Active transport
Bulk transport

Question 67

Describe the relationship between surface area in volume ratio as the size of a three-dimensional object increases.

Answer 67

The surface area to volume ratio decreases as the size of any three-dimensional objects increases.

Question 68

What is the main method by which molecules move about inside cells?

Answer 68

Cells rely on diffusion because it is the main method by which molecules move about inside cells.

Question 69

What is the diameter of prokaryotic and eukaryotic cells?

Answer 69

Most eukaryotic cells are no larger than about 50 micrometers in diameter and prokaryotic cells are even smaller.

Question 70

Does volume or the surface area increase more rapidly?

Answer 70

The larger the cell the smaller its surface area in relation to its volume. volume increases much more rapidly than surface area size increases. This has important implications for cells.

Question 71

Why can microscopic organisms exchange all of the substances they require directly through the cell surface membrane?

Answer 71

In microscopic organisms such as amoeba organisms can exchange All the substances it needs directly through the cell surface membrane. There are two reasons for this. Firstly microscopic organisms such as an amoeba have a relatively low rate or respiration. That’s because there are not very active organisms. Secondly the surface area of the cell membrane of amoeba is relatively large compared to the volume of the cell. Scientists call this the cell surface area to volume ratio.

Question 72

How do we calculate the surface area of a cube?

Answer 72

First calculate the area of the square lxl. Then multiply the area of the square by the number of sides. In a cube there are six sides.

Question 73

How do we calculate the volume of a cube?

Answer 73

The volume is the length multiplied by the height multiplied by the depth.

Question 74

Define facilitated diffusion

Answer 74

Facilitated diffusion is a diffusion of a substance either large polar molecules such as glucose amino acids or ions such as Na+ or Cl- through a transport protein (channel protein or carrier protein) in a cell membrane. The protein provides hydrophilic areas that allow the molecule or ion to pass through the membrane, which would otherwise be less permeable to it.

Question 75

What are the two types of transport proteins involved in facilitated diffusion?

Answer 75

There are two types of transport proteins involved in facilitated diffusion: channel proteins and carrier proteins. Each channel protein or carrier protein is highly specific only allowing one type of molecule or ion to pass through it.

Question 76

Describe the structure of channel proteins

Answer 76

Channel proteins have a fixed shape. They have water-filled pores. Part of the protein molecule is hydrophilic on the inside and can move to close or open the pore like a gate. Channel proteins allow the passage of molecules or ions down concentration gradients. Channel proteins have no binding sites and have high transport rates.

Question 77

Examples of channel proteins.

Answer 77

Gated proteins can be found in nerve cell surface membranes. One allows entry of sodium ions which happens during the production of an action potential. Another protein channel allows the exit of potassium ions during the recovery phase (repolarization). Some gated channels require energy in the form of ATP to operate the gate.

Question 78

Describe the structure of carrier proteins.

Answer 78

Carrier proteins can flip between two shapes. As a result the binding site is alternatively open to one side of the membrane then the other. This allows the molecule or ion to pass through the membrane. Some carrier proteins change shape spontaneously. These are the ones that allow facilitated diffusion. Carrier proteins allow the passage down the concentration gradients and against the concentration gradients in the presence of ATP molecules on the binding sites. Carrier proteins have binding sites and low transport rates.

Question 79

What are the factors which affect the rate of diffusion through channel and carrier proteins?

Answer 79

The number of channel or carrier proteins there are in the membrane.
Whether the protein channels are open or not
The surface area of the membrane

Question 80

What controls the direction of the diffusion through channel and carrier proteins?

Answer 80

If molecules are diffusing across a membrane, the direction of movement depends on their relative concentration on each side of the membrane. They move down concentration gradients from a higher to a lower concentration.

Question 81

What is cystic fibrosis caused by?

Answer 81

The disease cystic fibrosis is caused by a defect in the channel protein that should be present in the cell surface membrane of a certain cell including those lining the lungs. The protein normally allows chloride ions to move out of the cell. If the channel protein is not correctly positioned in the membrane or if it does not open the chloride channel as and when should then the chloride ions cannot move out.

Question 82

Define osmosis

Answer 82

Osmosis is the net diffusion of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Question 83

What is water potential?

Answer 83

Water potential is a measure of the tendency of water to move from one place to another. Water moves from a solution with higher water potential to one with lower water potential. Water potential is decreased by the addition of solute and increase by the application of pressure. The symbol for water potential is ψ (psi). ψ=Ψs+Ψp.

Question 84

What substance contains the highest water potential?

Answer 84

Pure/distilled water has the highest water potential which is 0.

Question 85

What is water potential measured by?

Answer 85

Water potential can be measured in pressure units called kilopascals (kPa). Since all solutions have a lower water potential than pure water the water potential of all solutions must be negative and the units will be -kPa. A diluted solution will have a less negative value than a concentrated solution. For example a solution with a water potential of -10 kPa has a higher water potential than a solution with a water potential of-20 kPa.

Question 86

What is solute potential?

Answer 86

Solute potential Ψs is the number of solute molecules that lower the water potential. Solute potential is always negative.

Question 87

What is pressure potential?

Answer 87

Pressure potential(Ψp) also called turgor potential is the contribution made by pressure to water potential. Pressure potential can be positive or negative. The pressure potential makes water less negative and therefore positive.

Question 88

What is an isotonic solution?

Answer 88

Isotonic solutions are two solutions that have the same concentration of solute with the same water potential. There is no net movement. Ψp=0 so Ψ=Ψs

Question 89

What is a hypotonic solution?

Answer 89

A hypotonic solution is one of two solutions that has a lower concentration of the solute. Therefore it has less negative water potential.

Question 90

What is a hypertonic solution?

Answer 90

Hypertonic solution is one of two solutions that has a higher concentration of solute. More negative water potential.

Question 91

What is the effect of osmosis on animal cells?

Answer 91

If the water potential of the solutions surrounding the cell is too high, the cell swells and bursts. If it is too low the cell shrinks. this shows why it is important to maintain a constant water potential inside the bodies of animals.

Question 92

What is protoplast?

Answer 92

Protoplast is the living contents of a plant cell, including the cell surface membrane but excluding the cell wall.

Question 93

What is the effect of having a cell wall on osmosis in plant cells?

Answer 93

Unlike animal cells, plant cells are surrounded by cell walls, which are very strong and rigid. The cell wall resist the expansion of the protoplast and therefore pressure rapidly starts to build up inside the cell. This pressure increases the water potential of the cell until the water potential inside the cell is equal to the water potential outside of the cell and equilibrium is reached.

Question 94

What is meant by a turgid cell?

Answer 94

When your plant cell is fully inflated with water it is described as turgid.

Question 95

Define plasmolysis.

Answer 95

Plasmolysis is a loss of water from a plant or prokaryotic cell to the point where the protoplast shrinks away from the cell wall.

Question 96

Define incipient plasmolysis.

Answer 96

Incipient plasmolysis is the point at which plasmolysis is about to occur when a plant cell or a prokaryote cell is losing water. At this point the protoplast is exerting no pressure on the cell wall.

Question 97

How can the changes of water potential in a plant cell be observed?

Answer 97

The changes in water potential in a cell can easily be observed with a light microscope using strips of epidermis peeled from rhubarb petioles or from the swollen storage leaves of onion bulbs. The epidermal strip could be placed in a range of sucrose solutions of different concentrations to find out which cause plasmolysis.

Question 98

Define active transports

Answer 98

Active transport is the energy consuming transport the molecules or ions through carrier protein, from a region of low concentration so origin of high concentration against the concentration gradients. The energy is provided by ATP produced by cellular respiration.

Question 99

What is the ATP in active transport used for?

Answer 99

The energy in active transport is used to make the carrier protein change its 3D shape, transferring the molecules or ions across the membrane.

Question 100

How can active transport be stopped?

Answer 100

Some poisons like cyanide can stop active uptake by stopping the production of ATP.

Question 101

Give an example of a carrier protein used for active transports.

Answer 101

The sodium potassium pump is an example of carrier protein used for active transports. It is found in the cell surface membrane of all animal cells. In most cells they run all the time and it is estimated that they consume 30% of the cells energy. They use 70% in nerve cells.

Question 102

What is the role of the sodium potassium pump?

Answer 102

The role of the NA+ k +pump is to pump three sodium ions out of the cell at the same time as allowing two potassium ions into the cell for each ATP molecule used. Both ions are positively charged so the net result is that the inside of the cell becomes more negative than the outside. A potential difference is created across the membrane.

Question 103

In which direction does active transport occur?

Answer 103

Active transport can occur either into or out of the cell.

Question 104

What is the role of the receptor found on the sodium potassium pump?

Answer 104

The sodium potassium pump has a receptor side for the ATP on its inner surface of the cell surface membrane. The receptor site acts as an ATPase enzyme in bringing about the hydrolysis of ATP to ADP (Adenosine diphosphate) and phosphate to release energy.

Question 105

What is the need for bulk transport?

Answer 105

Sometimes cells need to transport materials across their cell surface membranes on a much larger scale. The materials include large molecules such as proteins or polysaccharides, parts of cells or even whole cells. As a result mechanisms have evolved for the bulk transport of large quantities of materials into and out of cells.

Question 106

Define endocytosis

Answer 106

Endocytosis is about movement of liquids (pinocytosis) or solids (phagocytosis) into a cell, by the unfolding of the cell surface membrane to form vesicles containing the substance. Endocytosis is an active process requiring ATP.

Question 107

Define exocytosis.

Answer 107

Exocytosis is the bug movement of liquids or solids out of a cell, by the fusion of vesicles containing the substance with the cell surface membrane, exocytosis is an active process requiring ATP.

Question 108

What is a phagocyte?

Answer 108

A phagocyte is a type of cell that ingests and destroys pathogens of damaged body cells by the process of phagocytosis. some phagocytes are white blood cells.

Question 109

What is phagocytosis?

Answer 109

Phagocytosis or cell eating is the bulk uptake of solid material. Cells specializing in this are called phagocytes. The process is called phagocytosis and the vacuoles are called phagocytic vacuoles. An example is the engulfing of bacteria by white blood cells.

Question 110

What is pinocytosis?

Answer 110

Pinocytosis or cell drinking is the bulk uptake of liquid. The vacuoles or vesicles formed are often extremely small in which case the process is called micropinocytosis.

Question 111

Give an example of exocytosis

Answer 111

Exocytosis for example can happen in the secretion of digestive enzymes from cells of the pancreas. Secretory vesicles from the Golgi apparatus carry the enzymes to the cell surface and release their contents. Plant cells use exocytosis to get their cell wall building material to the outside of the cell surface membrane.