Cell Transport Mechanisms

Question 1

What does the cell surface membrane act as?

Answer 1

The gatekeeper to the cell, controlling the transport of materials in and out of the cell

Question 2

Why is there a need for transport in cells?

Answer 2

• cells require a supply of chemicals such as glucose and oxygen for cellular respiration. These must be transported from outside the organism into the cells
• respiration produces the toxic waste product carbon dioxide. This and other waste products need to be removed from the cells before they cause damage to them

Question 3

What is in the fluid mosaic model of a cell membrane?

Answer 3

• glycoproteins: often act as antigens, important for cell recognition or as receptors for hormones or neurotransmitters
• peripheral proteins: may be enzymes, can be involved in regulating transport
• membrane bilayer: the fluid bilayer of phospholipids and other polar lipids. The polar lipids face the inside and outside of the cell with a hydrophobic lipid core. This layer contains the integral proteins
• integral proteins: the main transport system of tne membrane forming either permanent pores or other transport mechanisms such as carrier proteins or active pumps involving enzymes

Question 4

What does passive transport take result of?

Answer 4

Concentration, pressure or electrochemical gradients

Question 5

What does acrive transport involve?

Answer 5

Moving substances into or out of the cell by using adenosine triphosphate (ATP) produced during cellular respiration

Question 6

What are the three main types ot passive transport in cells?

Answer 6

• Diffusion - the movement of particles in a liquid or gas down a concentration gradient. They move from an area where they are at relatively high concentration to an area where they are at a relatively low concentration as a result of random movements
• facilitated diffusion - diffusion that takes place through carrier proteins or protein channels. The protein-lined pores of the cell membrane make facilitated diffusion possible
• osmosis - a specialised form of diffusion that involves the movement of solvent molecules (usually water) down a concentration gradient through a partially permeable membrane

Question 7

What are the three active transport mechanisms?

Answer 7

• endocytosis - the movement of large molecules into cells through vessel formation. The fluid nature of the cell membrane makes it possible to form vesicles
• exocytosis - the movement of large molecules out of cells through vesicle formation
• active transport - the movement of substances across the membrane of cells directly using ATP. The proteins in the membrane act as carriers or enzymes making ATP energy available to move ions or molecules through the membrane

Question 8

What affects the transport of substances into and out of cells?

Answer 8

• the properties of the membrane
• the properties of the molecules to be transported:
- size of the molecule is important to how it is transported through cell membranes
- its solubility in lipids and water is important to how it is transported through cell membranes
- the presence or absence of charge on a molecule also affects how it is transported

Question 9

Why do molecules diffuse down a concentration gradient?

Answer 9

Because of the random motion of molecules due to the energy they have. If you have a large number of molecules tightly packed together random motion will result in their spreading out and eventually reaching a uniform distribution.

Question 10

Why can oxygen and carbon dioxide move into cells by diffusion alone.

Answer 10

Because the membrane is no barrier

Question 11

What does each type of channel protein allow?

Answer 11

One particular type of molecule or ion through dependent on its shape and charge

Question 12

What are gated channels?

Answer 12

Channels that open only when a specific molecule is present or there’s an electrical charge across the membrane such as during the passage of nerve impulses along neurones

Question 13

How does facilitated diffusion with carrier molecules take place?

Answer 13

• the carriers will be found on the outside surface of the membrane when a substance is to be moved into the cell
• the carriers will be found on the inside for transport out of the cell
• the protein carriers are specific for a particular molecules or groups of molecules, depending on the shape of the protein carrier and the substance to be carried.
• once a carrier has picked up a molecule it changes shape, moves the molecule through the membrane within the carrier, and then releases the molecule
• the movement through the membrane takes place because the membrane changes its shape when it is carrying something.
• the process can only take place down a concentration gradient

Question 14

What does osmosis involve?

Answer 14

The movement of water from a region of high concentration of water molecules to a region of lower concentration across a partially permeable membrane such as the cell surface membrane or nuclear membrane

Question 15

What is the osmotic concentration?

Answer 15

A measure of the concentration of the solutes in a solution that have an osmotic effect

Question 16

What does the osmotic concentration concern?

Answer 16

Only the solutes in a solution that have an osmotic effect. Many large insoluble molecules found in the cytoplasm such as starch and lipids do not affect the movement of water so are ignored when considering osmotic concentration. Only soluble particles are considered, including the big plasma proteins such as albumin and fibrinogen

Question 17

What would happen in animals if water moved continuously into cells from a dilute external solution?

Answer 17

The cells would swell up and burst

Question 18

How do you model osmosis in cells?

Answer 18

You use an artificial membrane that is permeable to some molecules such as water and insoluble to others such as sucrose and measure the net movement. You can see the absence or presence of sucrose by carrying out Benedicts test for non-reducing sugars

Question 19

What are isotonic, hypotonic and hypertonic solutions?

Answer 19

• in an isotonic solution the osmotic concentration of the solutes in the solution is the same as that in the cells
• in a hypotonic solution the osmotic concentration of solutes in the solution is lower than that in the cytoplasm of the cells
• in a hypertonic solution, the osmotic concentration of solutes in the solution is higher than thatnin the cytoplasm

Question 20

What happens when too much water moves out of an animal cell?

Answer 20

The cells shrivel as the concentrated cytoplasm loses its internal structure and the chemical reactions that normally take place in the cell stop working

Question 21

How does the cell wall in plant cells protect the cell from bursting?

Answer 21

• as the cytoplasm swells and presses on the cell walls, it generates hydrostatic pressure.
• The inward pressure of the cell wall on the cytoplasm increases until it cancels out the tendency for water molecules to move in
• the inward pressure is called the pressure potential
• when the osmotic force moving water jnto the plant cell is balanced by the pressure potential forcing it out, the plant cell is rigid in a state of turgor
• most plant cells are turgid most of the time as the rigid structure supports the stems and leaves of the plant

Question 22

What happens if plant cells are put in a solution which is just slighrly hypertonic?

Answer 22

Water moves out of the cell by osmosis and turgour is lost. The cell membrane begins to pull away from the cell wall as the protoplasm shrinks. This is called incupient plasmolysis

Question 23

How do we measure incipient plasmolysis?

Answer 23

Using serial dilutions, looking dor the point at which 50% of the cells are plasmolysed and 50% are not. This is the concentration that is equivalent to the solute potential of the cell sap

Question 24

What happens if the cell is placed in a hypertonic solution?

Answer 24

So much water will leave the cell that the vacuole will he reduced and the protoplasm will shrink away from the cell walls. The cells suffer plasmolysis

Question 25

Why does the size and shape of plant cells not change much whether they are fully turgid or plasmolysed?

Answer 25

Because of the cell wall. They do not swell or burst, or become very small. Its only the contents that change

Question 26

Why is most of the work done on osmosis done on plant cells?

Answer 26

* they are bigger and easier to see with the light microscope * the changes are easier to see than in animal cells

Question 27

What is water potential?

Answer 27

* A measure of the potential for water to move out of a solution by osmosis. * Pure water has the highest water potential because water molecules will always move from pure water into any solution on the other side of a partially permeable membrane. * The maximum water potential is always given as zero. As a result all other solutions will have a lower water potential than pure water. As a result the water potential for any solution will be negative * water always moves from an area of high water potential to an area of low water potential

Question 28

What are the two variables that interact inside a plant cell?

Answer 28

The turgour pressure exerted by the cell wall and the osmotic potential of the cell contents

Question 29

What is turgor pressure?

Answer 29

* the hydrostatic pressure generated as the cell contents push against the cell wall of a plant cell as a result of the movement of water into the cell * turgor pressure will rise until the osmotic force pulling water into the cell is balanced by the turgor pressure opposing further entry of water. At this point turgor remains constant * Turgor pressure usually has a positive value and can be measured directly in an individual cell using a pressure probe

Question 30

What is osmotic potential?

Answer 30

The potential of water to move across a partially permeable membrane from a hypotonic solution to a hypertonic solution. Pure water has the highest osmotic potenti. Osmotic potential is always negative

Question 31

What combination gives us the water potential of the cell?

Answer 31

The combination of the osmotic potential of the protoplasm and the turgour pressure exerted by the protopladm against the cell walls

Question 32

How can you work out the water potential of a cell?

Answer 32

Water potential of cell = turgor pressure + osmotic potential

Question 33

Why is the water potential of cells simply the osmotic potential of the cytoplasm in animal cells?

Answer 33

Because there is no cell wall to exert a hydrostatic pressure

Question 34

What does active transport involve?

Answer 34

A carrier protein which often spans the whole membrane. It may be specific, picking up only one type of ion or molecule, or it may work for several relatively similar substances that have to competes with each other for a place on the carrier

Question 35

How do the carrier molecules get enough energy to transport the substance across the membrane?

Answer 35

* Cells that carry out a lot of active transport generally have a lot of mitochondria to supply the ATP they need. * The enzyme ATPase catalyses the hydrolysis otf ATP breaking one bond and forming two more to provide the energy needed by the carrier system in the membrane or to release the transported substances and return the system to normal

Question 36

Active transport is a one-way substance for each specific substance. What does this mean?

Answer 36

An active transport system moves substances only in the direction required by the cells

Question 37

In active transport the movement of a substance is often linked with that of another particle. How does this work with the sodium ion?

Answer 37

The sodium pump actively moves potassium ions into the cell and sodium ions out

Question 38

How does the cell surface membrane provide control over what moves into or out of the cell?

Answer 38

Through a combination of diffusion, facilitated diffusion and active transport

Question 39

Much of the evidenc for active transpirt comes from linking together active transport and ATP together showing that without ATP active transport cannot take place. How does the evidence show this?

Answer 39

* active transport takes place only in living, respiring cells * the rate of active transport depends on temperature and oxygen concentration. These affect the rate of respiration and so the rate of production of ATP * many cells that are known to carry out a lot of active transport contain very large numbers of mitochondria - the sute of aerobic cellular respiration and ATP production * poisons that stop respiration or prevent ATPase from working also stop active transport.

Question 40

What is phagocytosis?

Answer 40

The active process when a cell engulfs something relatively large such as a bacterium and encloses it in a vesicle.

Question 41

What is pinocytosis?

Answer 41

The active process by which cells take tiny amounts of extracellular fluid into the vesicles

Question 42

What is exocytosis?

Answer 42

The term for the emptying of a membrane-bound vesicle at the surface of the cell or elsewhere. E.g. in cells producing hormones vesicles containing the hormone fuse with the cell surface membrane to release their contents

Question 43

What is cyanide?

Answer 43

A metabolic poison that stops mitochondria working