Movement In And Out Of Cells

Question 1

Why is the transport of substances important?

Answer 1

To:
a) Supply cells with oxygen for respiration and raw materials for anabolism (synthesis of biological molecules)
b) Regulate the pH and solute concentration for maintaining a stable internal environment for enzymes to function optimally
c) Excrete toxic waste substances
d) Secrete useful substances for cell activities

Question 2

Define diffusion

Answer 2

Diffusion is the random movement of ions or molecules from a region where they are at higher concentration to a region of lower concentration.

Question 3

Which molecules can diffuse easily in and out of the membrane?

Answer 3

Very small and uncharged molecules like oxygen and carbon dioxide, Hydrophobic substances (lipid-soluble) e.g. steroids, can also diffuse through.

Question 4

Does water freely diffuse across the plasma membrane?

Answer 4

Even though water is an extremely small, its polar therefore it does not move across the cell membrane by simple diffusion.

Question 5

Which factors affect the rate of diffusion?

Answer 5

• The concentration gradient
  This refers to the relative concentration on either side of the membrane or between two points. The greater the difference between the points, the faster the rate of diffusion and if the difference is less, the slower the diffusion rate.
• Temperature
  When increased, temperature causes an increased rate of diffusion because the particles acquire increased kinetic energy which causes increased speed of movement hence increased rate of diffusion.
  At low temperatures, the kinetic energy is very low and the speed of movement by particles is equally very low.
• Surface area
  The larger the surface area over which the molecules are exposed, the faster the rate of diffusion.
• Distance over which diffusion takes place This is the distance over which the molecules are to travel i.e. the surface thickness across which the molecules move. The greater the distance the lower the rate of diffusion
• Size and nature of diffusing molecules. The smaller the size of the diffusing particles, the faster they diffuse i.e. smaller particles move very fats while the large ones will move slowly.
• Permeability
  The more porous a surface is, the greater the number of particles that diffuse through it hence the greater the rate of diffusion

Question 6

What is the significance of diffusion?

Answer 6

a) It’s a means by gaseous exchange occurs in plants and animals e.g. in plants diffusion of gases occur through the stomata and in animals, in gills of fish, , the skin and buccal cavity of amphibians alveoli of reptiles, mammals and birds.

b) Absorption of certain digested food materials e.g. glucose in the ileum.

c) A means of exchange of materials between blood in capillaries and the tissues

d) During formation of the nerve impulse, sodium ions diffuse into the nerve cells facilitating generation of nerve impulses and ensures transmission of nerve impulses from one neurone to another i.e. diffusion facilitates synaptic transmission

e) It ensures excretion of waste products e.g. ammonia in fresh water fishes

f) It’s the main means of transportation of materials within the cell’s cytoplasm e.g. in unicellular organisms

g) Absorption of mineral salts by plants from the soil is effected by diffusion as one of the mechanisms

Question 7

Give adaptations of tissues to maximize the rate of diffusion

Answer 7

1. The lungs are ventilated by the respiratory tract (trachea, bronchus, bronchioles) which maintain a steep concentration gradient between the lung alveoli and blood in the capillaries.
2. Respiratory surfaces like the lung alveoli and intestine epithelial lining possess a rich supply of blood vessels which transport away the diffusing materials hence maintaining a steep gradient which sustains the fast diffusion
3. Diffusion surfaces e.g. lung alveoli and intestines (ileum) are covered by a thin epithelium lining which reduces the distance over which diffusion takes place.
4. The epithelial lining covering the alveoli and rumen of the ileum is very permeable to allow molecules to travel across them
5. In lungs there are numerous alveoli and in the ileum infoldings known as villi and microvilli which is coupled with a very long ileum also increases the surface area along which particles move into cells hence increase the rate of diffusion.
6. Flattened body e.g. platyhelminthes (flatworms) which increases the surface area for movement of materials by diffusion
7. Some organisms are of small size e.g. unicellular organisms which increases the surface area to volume ratio of the surface that permits increased rate of diffusion

Question 8

Define facilitated diffusion

Answer 8

This refers to the transport of molecules and ions across a membrane by specific transport proteins, carrier and channel proteins, found within the membrane in favour of the concentration gradient (difference) of ions.

Question 9

True or false
Facilitated diffusion is a faster form of movement than simple diffusion

Answer 9

True

Question 10

What are the specialized channels for water in plant and animal cells called?

Answer 10

Aquaporins

Question 11

What are the differences between simple and facilitated diffusion?

Answer 11

Simple
- The rate of diffusion depends on the concentration gradient
- does not require special transport proteins
- Diffusion can occur in either direction
- Similar molecules diffuse at the same rate

Facilitated
- The rate of diffusion does not depend on the concentration gradient
- Occurs via special channels or carrier proteins
- Diffusion occurs in only one direction
- Specific molecules diffuse faster than others

Question 12

What is the similarity between simple and facilitated diffusion?

Answer 12

Both move molecules from a region of high concentration to a region of low concentration through a partially permeable membrane.

Question 13

Define active transport

Answer 13

This is the movement of molecules or ions across a cell membrane against their concentration gradient aided by the protein pump with specific binding sites, involving the expenditure of energy.

Question 14

Describe the mechanism of direct active transport in the Na+- K+ pump

Answer 14

• ATP is hydrolysed and the binding of the phosphate group to the protein pump changes the protein conformation.
• The protein pump actively transports three sodium ions (3 Na+) out of the cell for every two potassium ions (2K+) pumped against their concentration gradient into the cell.
• This generates a difference in ionic charge on the two sides of the membrane which is important for the transmission of nerve impulses.
• The Na+ gradient is also used in the coupled uptake of solutes such as glucose into the cells against its concentration gradient.

Question 15

Describe the mechanism of indirect active transport mechanism (secondary active transport/ co-transport) in the coupled uptake of glucose into cells lining the ileum in mammals.

Answer 15

• Here glucose and Na+ ions are absorbed into the cells.
• Sodium ions down a concentration gradient while the glucose molecules against the concentration gradient.
• In co-transport of Na+ and glucose, ATP is used by the protein pump to pump Na+ out of the cell creating a Na+ concentration gradient.
• The Na+ and glucose molecules then bind to trans- membrane protein (carrier protein), also called co- transport proteins/coupled transport proteins.

Question 16

What are the types of membrane proteins involved in active transport?

Answer 16

a) Uniport carriers. They carry (transport) a single ion or molecule in a single direction.

b) Simport carriers. They carry two substances in the same direction.

c) Antiport carriers. They carry two substances in opposite directions.

Question 17

What factors are required for active transport to take place?

Answer 17

i. Temperature
- Increase in temperature increases the rate of transport of substances by active transport, so long as the increase in not above the optimum.
- The increase in temperature makes respiratory enzymes more active, having their speeds of movement increased (kinetic energy) with that of substrate molecules which results into collisions of molecules at a faster rate thus forming enzyme substrate complexes that form products. In this case, ATP is required to power active transport.
- At very high temperatures, above the optimum, respiratory enzymes are denatured in the carrier proteins in the membrane. This reduces the rate of active transport.
- At very low temperatures, below the optimum, the respiratory enzymes together with the carrier proteins are inactive and this reduces the rate of active transport.

ii. Availability of oxygen
- Oxygen is required for aerobic respiration to generate ATP.
- Increase in oxygen concentration results into increased rates of active transport as more ATP molecules are available for the process.
- In circumstances of very little or no oxygen, the rate of active transport is reduced since in the case of anaerobic respiration, there’s very little or no ATP molecules available for active transport

iii. Concentration of respiratory substrates e.g. glucose
- If the concentration of respiratory substrate is increased, the rate of active transport also increases and if it is lowered, the rate of active transport lowers.
- This is because increase in the amount of the substrate increases the rate of ATP generation during respiration. If the amount of substrate is reduced, the rate of ATP generation is also lowered.

Question 18

What is the importance of active transport?

Answer 18

a) It is a means of absorption of food materials in the mammalian gut
b) It is the means of absorption of mineral salts by plant root hairs and the root epidermal cells of the peliferous layer
c) It facilitates the excretion of waste materials from the cells to the extracellular fluids against a concentration gradient e.g. excretion of urea
d) It is important in muscle contractions and relaxations where there’s active pumping in and out of calcium ions inside the cytoplasm (sarcoplasm) of the muscle.
e) It is used in the loading and unloading of materials in the plants phloem tissue which creates pressure differences in the
phloem tissue that maintain mass flow of materials.
f) Active transport is vital in transmission of nerve impulses along nerve cells where it creates a membrane action potential
using the potassium-sodium pumps.
g) It plays a part in the opening and closure of stomata where differential pumping of potassium ions between the guard cells and neighboring subsidiary cells lead to turgidity changes hence causing stomatal movements (opening/closure).

Question 19

How do metabolic poisons (inhibitors) inhibit the enzymes and carrier proteins required to bring about active transport?

Answer 19

• By changing the active sites/binding sites for the enzymes/carrier proteins for the molecules to be transported.
• The poisons also inhibit ATP synthesis hence cutting off the source of energy needed to affect the active transport.

Question 20

What is osmosis?

Answer 20

This is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

Or:

It may also be defined as the passive movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.

Question 21

What is a selectively permeable membrane?

Answer 21

A selectively permeable membrane is one that allows unrestricted passage of water molecules but no passage of solute molecules.

Question 22

How is a net flow of free water molecules maintained in osmosis ?

Answer 22

In the side with more solute molecules, water forms hydrogen bonds with solutes which are charged or polar forming a hydration shell around them in solution, making water molecules unfree and therefore cannot flow back across the membrane.

Question 23

Define water potential

Answer 23

This is the net tendency of any system to donate water to its surroundings.

Question 24

Why does a dilute solution have a higher water potential than a concentrated solution?

Answer 24

This is because the movement of the water molecules is restricted by the attraction between solute and water molecules i.e. there are fewer water molecules with a high kinetic energy to move across the membrane.

The greater the concentration of solutes, the more negative is the water potential.

Question 25

Define solute potential/ osmotic potential

Answer 25

• This is a change in water potential of a system in the presence of solute molecules
• This is the potential or force of attraction towards water molecules caused by dissolved substances (solutes) inside the solution.

Question 26

Define pressure potential

Answer 26

This is the pressure exerted on a fluid by its surrounding.

Or:
For plants; For plants therefore, pressure potential is the pressure exerted on the cell contents by the cell wall and cell membrane.

Question 27

True or false
At any one time, the water potential of a plant is the sum of the solute potential and pressure potential.

Answer 27

True

Question 28

What happens when water enters the cell by osmosis?

Answer 28

• The pressure of the cytosol builds up, pushing out against the cell membrane.
• This pressure is called hydrostatic pressure.
• In plant cells, this pressure builds up pushing the cell membrane against the cell wall.
• The cell wall begins to resist the swelling caused by the influx of water.
• The pressure that the cell wall develops is the pressure potential.

Question 29

Define osmotic pressure

Answer 29

Osmotic pressure is the pressure needed to stop osmotic flow.

Question 30

What occurs when a plant cell becomes turgid?

Answer 30

• When the external solution is hypotonic (dilute), the cell’s cytosol has a lower water potential, causing an influx of water into the cells.
• The water enters into the cells vacuole causing an internal hydrostatic pressure developed by the cell by osmosis.
• The pressure potential reaches its maximum when the cell wall is stretched to its maximum.
• At this point, the cell is described as a fully turgid or it has full turgor reached and the water potential at this point equals to 0 and no more water can enter the cell.

Question 31

What roles does turgor pressure play in plants?

Answer 31

• Turgor pressure plays part in supporting plants especially herbaceous plants by being filled with fully turgid cells tightly packed together.
• It is also responsible for holding leaves in flat and horizontal position.

Question 32

What occurs when a plant cell becomes flaccid?

Answer 32

• When a plant cell is immersed in a hypertonic (concentrated) solution, the cell decreases in volume as water moves out osmotically from its vacuole.
• The protoplast( nucleus, cytoplasm & cell membrane) shrinks, pulling away from the cell wall and leaving gaps between the cell wall and plasma membrane.
• A cell in this condition is said to be plasmolysed and the cell is flaccid.

Question 33

Define Plasmolysis

Answer 33

This is the shrinking of a plant cell’s protoplasm away from the cell wall leaving gaps between the cell wall and the plasma membrane.

Question 34

Define wilting

Answer 34

This is the drooping of plant leaves and stems as a result of transpiration rates being greater than rates of water uptake.

Question 35

What occurs at full turgor?

Answer 35

The cell cannot expand anymore and solute/ osmotic potential is exactly outbalanced by pressure potential

Question 36

What happens if a human red blood cell is placed in a hypertonic solution?

Answer 36

It will shrink and appear crinkled, this is called crenation.

Question 37

What happens if a human red blood cell is placed in a hypotonic solution?

Answer 37

It will swell and even burst and this is called haemolysis.

Question 38

What causes Haemolysis?

Answer 38

It is due to red blood cells lacking cellulose cell walls which would prevent red blood cells expansion and therefore stops bursting.

Question 39

What is the role of osmosis in living organisms?

Answer 39

1. It is the main form by which root hairs and piliferous layer cells on roots absorb water from the soil
2. Kidney nephrons (tubules) re-absorb water back into the blood stream via the blood capillaries osmotically leading to water conservation in the body hence bringing about osmoregulation
3. In herbaceous plants, osmosis brings about turgidity in plant cells due to presence of cell wall leading to provision of support and shape in a whole plant body.
4. Osmosis causes plant structures (organs) like leaves and flowers to determine their form for example holding the leaf in flat and horizontal position enabling it to trap maximum sunlight.
5. Osmosis bring about opening and closure of petals of flowers and osmosis bring about the opening and closure of stomata in plant leaves when the guard cells become turgid facilitating gaseous exchange in plants.

Question 40

Define cytosis

Answer 40

This is a form of active transport involving infoldings or folding of secretions into vesicles or vacuoles which can be moved.

Question 41

Define endocytosis

Answer 41

This is bulk transport of materials inside the cell.

Question 42

What are the three types of endocytosis?

Answer 42

• Phagocytosis
• Pinocytosis
• Receptor-mediated endocytosis

Question 43

Describe the process of phagocytosis (cellular eating)

Answer 43

• It involves the cell taking in large solid substances.
• Phagocytosis involves invagination of cell membrane surrounding of the organism or particle forming a phagocytotic vesicle or vacuole which pinches off the cell membrane and moves into the cytoplasm.
• Lysosomes fuse with vacuoles and release hydrolytic enzymes into the vacuole which break down the substances in the vacuole.
• The protein substances are absorbed into the surrounding cytoplasm across the lining of the vacuole.
• Any undigested material may be got rid of by the vesicles of vacuoles moving into the cell surface membrane and fusing with it.

Question 44

Describe the mechanism of phagocytotic killing by white blood cells

Answer 44

• White blood cells form cytoplasmic extensions to form pseudopodia which surround and engulf micro-organisms.
• Micro- organisms are completely surrounded by pseudopodia to form phagocytotic vesicles or phagosomes which pinch off the cell membrane into the cytoplasm.
• The phagosome fuses with the lysosome to form a phagolysosome. Inside the phagolysosome are microbes which are broken down by hydrolytic enzymes.

Question 45

Describe the process of pinocytosis

Answer 45

• It is similar to phagocytosis only that the infoldings forming the vesicles are much smaller.
• Liquid and large macro molecules such as proteins are taken in via small pinocytotic vesicles.
• The process is highly specific involving the binding of the molecules with corresponding receptor molecules in the plasma membrane.

Question 46

Describe receptor-mediated endocytosis

Answer 46

• This involves receptor molecules on a cell membrane which binds with specific substance from extracellular fluid as the receptor sites are filled, the surface falls inwards until the coated vesicles finally separates from the cell surface membrane.

Question 47

Describe exocytosis

Answer 47

This involves the vesicles or vacuoles moving to the cell membrane fusing with the releasing their contents to the outside of the cell.

• The vesicles are often derived from the Golgi apparatus which move along microtubules of the cytoskeleton of the plasma membrane.
• When the vesicles get into contact with the plasma membrane, the lipid molecules of the two bilayers rearrange and diffuse.
• The content of the vesicles spill to the outside of the cell and the vesicle membrane becomes part of the plasma

Question 48

What is the importance of cytosis?

Answer 48

1. Many secretory cells use exocytosis to release their excretory products outside themselves e.g. pancreatic cells manufacture insulin and secrete it into blood by exocytosis and many other hormones are secreted in this form by the gland cells
2. Exocytosis facilitates synaptic transmission during which neuro-transmitter substances like acetylcholine in synaptic vesicles of synaptic knobs fuse with the pre-synaptic membrane to release neuro transmitter substances into the synaptic cleft of the synapse.
3. Exocytosis delivers cell wall materials to the outside of the cell from the Golgi apparatus/body through vesicles which contain proteins and certain carbohydrates
4. Exocytosis leads to replenishment of the plasma membrane as the vesicle membrane become part of the plasma membrane become part of the plasma membrane after spilling/discharging their contents to the outside.

Question 49

What are the differences between wilting and plasmolysis?

Answer 49

Wilting
- The entire cell shrinks
- Water is lost from cells by evaporation

Plasmolysis
- The cell protoplasm shrinks away from the cell wall
- Water is lost from cells by osmosis