GENERAL BIOLOGY 1 Course code: BLY 101

Credit units: 3 Instructor: Olukemi Aromolaran (PhD) Office: Room 36, College of Agriculture, Engineering and Science Office hours: 8am – 5pm

Recommended text

• Taylor, D.J., Green, N.P.O and Stout, G.W. (2010). Biological Sciences. Cambridge University Press, 10th Edition, South Africa.

2

PASSIVE AND ACTIVE TRANSPORT

• An Introduction

- Living cell needs to exchange materials in and out of the cell membrane for its proper functioning.

- This is carried out by the membrane transport system.

• Why do cell transport materials across its membrane?

- To obtain nutrients

- To excrete waste substances

- To secrete useful substances

- To generate the ionic gradient essential for nervous and muscular activity

- To maintain a suitable pH and ionic concentration within the cell for enzymatic activity.

• Mechanisms of cell membrane transport:

- Passive transport (diffusion – simple and facilitated; osmosis)

- Active transport

- Bulk transport (endocytosis and exocytosis)

4

• Passive transport

Diffusion

- The movement of molecules/ions from a region of their high concentration to a region of their low concentration down a diffusion gradient e.g. respiratory gases, water molecules.

- This process is passive.

- It does not require energy and happens spontaneously e.g. oxygen

diffuses from the lungs into the blood while at the same time carbon

dioxide diffuses in the opposite direction.

5

Factors that affects the rate of diffusion:

1. Surface area – the greater the surface area of a membrane through which diffusion is taking place, the greater the rate of diffusion e.g. Microvilli increases surface area for absorption purposes.

2. Distance – rate of diffusion decreases with distance. Diffusion is effective at short distance e.g. an amino acid molecule can travel a few micrometer in several seconds, but would take several days to diffuse a few centimeters.

3. Membrane thickness – molecules/ions can cross a thin membrane more rapidly than thick membrane.

6

Fick’s law summarizes the factors affecting diffusion rate across a cell membrane.

Fick’s law:

Rate of diffusion is proportional to difference in concentration across the membrane and surface area of membrane and inversely proportional to the thickness of the membrane.

Fick’s law = Difference in concentration across membrane X Surface area of membrane

Membrane thickness

Diffusion can be grouped into: (a) Simple diffusion

(b) Facilitated diffusion

7

Simple diffusion

- Molecules move from region of higher concentration to region of lower concentration until equilibrium is reached.

- Molecules diffuses through the pores of the membrane without expending energy.

- Molecules must be small and nonpolar in order to pass through the membrane.

Facilitated diffusion

- This is simple diffusion with the aid of (facilitated by) special transport proteins called channel protein and carrier proteins.

- These proteins contain channels/pores to transport ions/ molecules that cannot be transported through simple diffusion.

- The channel proteins have a fixed shape. Carrier proteins undergo rapid change in shape, up to 100 cycle per second.

- The protein channels are glycoproteins and they are specific in their action. For example, GLU T4 is a glucose transporter found in fat and skeletal muscles. When there is need to take in glucose from the blood into the cell, Insulin hormone send signal to GLU T4 to insert into the membrane of the cell, so that glucose can be taken into the cell. In Type II diabetes mellitus disease condition, cell do not respond very well to the presence of insulin, therefore, GLU T4 is not inserted into the membrane, thus glucose can not get into the cell (suffering in plenty!).

8

9

Simple diffusion Facilitated diffusion

Diffusion Image source: https://www.bing.com/th?id=OIP.m5_xmdCN5z4TX8-Um4jCLAHaFu&w=241&h=183&c=7&o=5&pid=1.7

B C A

Osmosis

- Movement of a solvent (usually water) through a semi permeable membrane.

- Only water molecule moves until equilibrium is reached.

For example in red blood cells,

- when the concentration of solution outside the cell is lower than inside –

hypotonic solution, water enters the cell by osmosis and the cell will burst and

disperse it contents.

- when the concentration of solution outside the cell is higher than inside –

hypertonic solution, water leaves the cell by osmosis and the cell will shrink.

- when the concentration of solution outside the cell is the same with inside –

isotonic solution, no net movement of water occurs. The cell is normal.

10

11

Direction of water movement

Image source: https://www.bing.com/th?id=OIP.7ef8vdOZetEqVmmb2TtmfgHaFj&w=237&h=178&c=7&o=5&pid=1.7

• Active transport

- Sometimes, the body needs to move molecules against their gradient. Thus, they require energy.

- Active transport is the energy-consuming transport of molecules or ions across a membrane against a concentration gradient. Energy is required to move molecules against its gradients.

- Movement is only in one direction, unlike diffusion which is reversible.

- The energy needed for active transport is derived from hydrolysis of ATP (adenosine triphosphate), which is an energy carrier made during respiration. Active transport is not possible without respiration.

- The major ions in and around the cells are sodium ions (Na+), potassium ions (K+) and chloride ions (Cl-).

12

Concentration of Na+, K+ and Cl- in human red blood cells and their environment

13

Arrows shows the direction of movement of ions across the cell membrane (against the concentration gradients)

Ion Blood plasma (mM)

Red blood cell (mM)

Na+ 144 15

K+ 5 150

Cl- 111 73

14

Active transport of sodium and potassium across the cell membrane through the Na-K ATPase pump (For every 2 K+ taken into the cell, 3 Na+ are removed)

Image source: http://slideplayer.com/slide/9306389/28/images/36/Active+Transport-example.jpg

- Sodium is actively pumped out of the cell and potassium is actively pump into the cell.

- Active transport is made possible by carrier proteins, which are situated in the cell membrane.

- These carrier proteins need energy to carry out their functions, unlike facilitated diffusion.

- Transport that directly use ATP for energy is called Primary active transport.

- Sodium-potassium pump (Na+- K+) are found in the cell surface of animal cells to pump Na+ out of the cell and K + into the cell.

- More than a third of the ATP consumed by resting animal is used to pump sodium and potassium.

15

- The pump is a carrier protein.

- A potential difference is built up across the membrane, with the inside of the cell being negative because for every 2 K+ taken into the cell, 3 Na+ are removed.

- This tends to restrict the entry of negatively charged ions (anions), such as chloride (that is why the chloride concentration outside the cell is higher than inside the cell).

- Thus, positively charged ions are attracted into cells.

- Cells having cell wall, such as bacteria, fungi and plant do not need the pump.

16

- Proton/potassium exchanger (H + /K + ATPase) found in the stomach is another example of a location for ATP pump. This proton pump is responsible for the acidic environment of the stomach and this can cause acid reflux (resulting in ulcers). Omeprazole are prescribed to patients with ulcers to help reduce the acidity of the stomach.

Importance of Na-K pump

- to control the osmotic balance of animal cells (osmoregulation).

- for maintaining electrical activity in nerve and muscle cells.

- needed in driving active transport of some substances, such as sugars and amino acids.

- for pumping K+ into the cell, which is needed for protein synthesis, glycolysis, photosynthesis and other important activities.

When one type of molecule migrates from higher concentration to lower concentration and release energy, it is called Secondary active transport.

This released energy is used to transport other molecule from its lower concentration to higher concentration across cell membrane.

17

• Differences between passive and active transport

18

Passive transport Active transport

Do not require energy Require energy in form of ATP

Move molecules with the concentration gradient.

Move against concentration gradient.

It is a slow and bidirectional process

It is a rapid and unidirectional process

It is use to maintain equilibrium within and outside the cell

To carry substances through the cell membrane

Passive transport are usually diffusion, osmosis

Types include proton pump, sodium-potassium pump