transport in cells

Question 1

Passive transport

Answer 1

During passive transport, substances move according to their own natural tendency without an input of energy from the cell. No ATP is required.

Question 2

Simple diffusion definition

Answer 2

Simple diffusion is the net movement of a substance from an area of higher concentration to an area of lower concentration ie. Down its concentration gradient, until equilibrium is reached

Question 3

Simple diffusion

Answer 3

• It is caused by the constant random motion of all atoms and molecules. This motion is caused by the kinetic energy of the molecules
• diffusion results in equilibrium

Question 4

Factors affecting rate of diffusion ( concentration gradient )

Answer 4

The greater the difference in concentration between the two regions (steeper concentration gradient), the greater the rate of diffusion. for rapid diffusion, a constant supply of substance for diffusion needs to reach the cell surface and substances which have crossed the cell membrane need to be transported away.

Question 5

factors affecting rate of diffusion (distance over which diffusion occurs)

Answer 5

the shorter the distance over which diffusion occurs, the greater the rate of diffusion across it

Question 6

factors affecting rate of diffusion (SA:V over which diffusion occurs)

Answer 6

the larger the SA:V, the greater the rate of diffusion.
eg: hair root cell

Question 7

Facilitated diffusion definition

Answer 7

Facilitated diffusion is the diffusion of ions and water-soluble molecules across the cell membrane using two types of transport proteins, channel proteins or carrier proteins, down a concentration gradient

Question 8

Characteristics of facilitated diffusion

Answer 8

• A passive process - No energy required
• once all transport proteins are in use, rate of diffusion cannot be increased further

Question 9

Channel proteins

Answer 9

• Provides a hydrophilic channel across the membranes for ions or hydrophilic molecules to enter or leave the cell
• specific for the molecules that can move across the membrane

Question 10

Carrier proteins

Answer 10

• diffusing substances bind temporarily to the carrier protein, causing it to change shape to transport the substances across the membrane
• changes in shape may be triggered by the binding and release of the particles

Question 11

Osmosis definition

Answer 11

Osmosis is the net movement of water molecules from a solution of higher water potential to a solution of lower water potential, through a partially permeable membrane

Question 12

Osmosis - water potential

Answer 12

• Water potential is a measure of the tendency of water molecules to more from a region to another
• water molecules always move from a solution of higher water potential to a solution of lower water potential, down a water potential gradient
• the more concentrated the solution is, the lower its water potential

Question 13

Answering technique- osmosis

Answer 13

Compare the water potential: solution A has higher water potential than solution B
Direction of water molecules: more water molecules move from A to B
Process: by osmosis
Conclude: hence, the water level of B rises while A falls

Question 14

Effect of osmosis on animal cell

Answer 14

Placed in solution of higher water potential: water enters, cell swells and may burst (lysis)
Placed in solution of same water potential: no net movement of water, cell normal size
Placed in solution of lower water potential: water leaves, cell shrinks and crenates

Question 15

Effect of osmosis on plant cell

Answer 15

Placed in solution of higher water potential: water enters, cell swells a bit and becomes turgid
Placed in solution of same water potential: no net movement of water, cell normal size
Placed in solution of lower water potential: water leaves, cytoplasm shrinks and cell becomes plasmolysed

Question 16

Tugor pressure - plant cells when placed in dilute solution

Answer 16

• Diluted solution has higher water potential than cell sap
• water enters plant cell by osmosis
• vacuole smells, pushes cytoplasm and cell membrane against cell wall
• cells become turgid
• cells do not lyse because of the rigid cell wall
• pressure exerted by the vacuole on the cell wall is known as turgor pressure
• this is important for the plant as it keeps it firm and upright, and leaves can spread out more.it also helps in the opening of the stomata. This increases the rate of photosynthesis

Question 17

Tugor pressure-plant cells when placed in concentrated solution

Answer 17

• Concentrated solution has lower water potential than cell sap
• water leaves the cells by osmosis
• vacuole decrease in volume and size and the cytoplasm and cell membrane pull away from the cell wall
• cell becomes plasmolysed. This withdrawal of the cell membrane from the cell wall is called plasmolysis. it is reversible of full plasmolysis is not reached
• the space between the cell wall and call membrane will be filled with the surrounding solution
• plant starts to wilt as turgor pressure in the plant cells decreases
• full plasmolysis is reached when the cell membrane has completely withdrawn from the cell wall. This process is irreversible

Question 18

Adaptations of organisms due to osmosis

Answer 18

• Unicellular organisms (protozoans) in freshwater habitats are in an environment of higher water potential than itself. They survive by constantly expelling water using contractile vacuoles to prevent swelling and lysis.
• Organisms in marine environments are in an environment of lower water potential than their cells. They must actively pump ions into their cells to reduce their water potential and so reduce water loss by osmosis.
• Young non-woody plants rely on cell turgor for their support, and without enough water they wilt. Plants take up water through their root hair cells by osmosis, and must actively pump ions into the root hair cells to lower water potential as compared to the soil so that water does not leave by osmosis. This is particularly difficult for plants rooted in high salinity.

Question 19

Active transport

Answer 19

• Definition: active transport is the movement of ions or molecules across a cell membrane from a region of lower concentration to a region of higher concentration ie. Against a concentration gradient, with the expenditure of energy from ATP molecules by the cell
• active transport allows a cell to maintain its internal concentration of solutes regardless of the concentration of the solutes in its surroundings
• it requires the use of specific carrier proteins which require energy

Question 20

Importance of active transport

Answer 20

• it allows cells to take up nutrients/ ions even when their concentration outside the cells are lower than inside of the cells
• it enables cells to get rid of waste products when their concentrations outside the cell are higher than those inside the cells

Question 21

Bulk transport across cell membrane

Answer 21

Bulk transport is the transport of materials into or out of a cell by enclosing it within a vesicle
- it allows large molecules, such as proteins and polysaccharides or large quantities of molecules to enter or leave a cell without passing through the membrane
- It is an active process as it requires the expenditure of energy by the cell

Question 22

Endocytosis

Answer 22

• Endocytosis occurs when the substances are taken into the cells by inward folding of the cell membrane to form vesicles that separate from the membrane
• ATP is used to provide energy during this process

Question 23

Phagocytosis

Answer 23

• An example of endocytosis
• the process of engulfing or ingesting foreign particles
• a key feature of phagocytosis is the formation of pseudopodia, which are temporary protrusions of the cytoplasm
• the vesicles formed after phagocytosis fuse with lysosomes. The hydrolytic enzymes in lysosomes digest/ breaks down the engulfed foreign particles

Question 24

Exocytosis

Answer 24

• exocytosis occurs when substances in secretory vesicles move towards and fuse with the cell membrane and release the contents of the vesicle into the fluid around the cell.
• ATP is used to provide energy during this process