Biology and Health Sciences

Key unit competency

To be able to analyse and interpret the process of active transport and its significance to living organisms.

Learning objectives

After studying this topic, I should be able to:

• Define active transport.

• Compare passive and active transport.

• Appreciate the importance of active transport.

• Describe how carrier proteins move particles across membrane during active transport.

Introduction

Study the pictures A and B below.

What is going in the pictures? Which is easier, climbing uphill or going downhill? Why is that the case? How do actions in the pictures relate to active transport? Think about how things are transported in our bodies. Are there special things that help in the transportation process? What do they do?

4.1  Active transport and its importance 

The types of movement of molecules and ions that we have considered in Unit 3 involved molecules moving down or along a concentration gradient. These movements are passive hence name passive transport. Molecules and ions can also move from an area of low concentration to that of higher concentration. They are said to move against a concentration gradient. Such a process requires the use of energy and is called active transport.

Discussion corner

1. You will be provided with Manila paper, marker pens and cello tape.

2.  Prepare a table on the Manila paper.

3.  Discuss and write the differences between passive and active transport.

4.  Share your findings with the rest of the class.

The table below shows the difference between passive and active transport.

Active transport is the movement of particles through the cell membrane from a region of low concentration to a region of high concentration using energy. This process requires the expenditure of energy since particles move against a concentration gradient.

The role of proteins in active transport

The movement of substances across a membrane by active transport requires carrier protein molecules. They pick up molecules of a substance from one side of the cell membrane and transport them across.

Carrier protein molecules have binding sites for the molecules. The molecules to be transported bind to the carrier protein using energy. The carrier protein changes shape and the particles are transported to the other side of the membrane; and they diffuse away from the carrier protein. 

Activity 4.1 Demonstrating active transport

You will be provided with charts and animated pictures involving active transport.

1.  Observe the charts and animations carefully.

2.  Record what you see in the pictures.

3. Discuss with your partner the importance of the process involved.

Study questions

(a) What is the role of active transport in organisms?

(b) Which factors affect active transport?

In active transport, the cell must use its own energy to move the molecules against a concentration gradient. Such energy is supplied by the cell’s mitochondria. Therefore active transport takes place only in living cells.

Role of active transport in living organisms

1.  Through active transport, root hair cells in plants absorb mineral salts and ions from the surrounding soil. This takes place even when the concentration of these mineral salts is already higher in the cells than in the soil.

2.  The cells lining the human small intestines continue to absorb food molecules by active transport even when the concentration of these molecules is higher in the cells than in the intestinal lumen.

3.  Nerve cells need sodium ions and potassium ions to function. The concentration of sodium ions outside a nerve cell is higher than the concentration on the inside. The concentration of potassium ions on the outside is lower than on the inside. The nerve cells maintain these concentrations in this way by active transport.

4.  When urine is first formed in the kidney, it contains useful substances like glucose in addition to the waste substances. The useful substances are reabsorbed into the blood by active transport.

5.  Active transport is involved in accumulation of substances in the body of some organisms to offset osmotic imbalances in arid and saline environments.

Factors that affect active transport

Any factor that affects energy production also affects active transport.

a. Concentration of oxygen

At higher oxygen concentration, the cells produce more energy. This leads to rapid active transport by the protein carriers.

b. Concentration of glucose

The amount of energy produced by a cell depends on the amount of glucose available. When more glucose is available, the cells produce more energy. This leads to rapid active transport.

c. Temperature

High temperatures can destroy (denature) the protein carriers in the cell membrane. This stops active transport. Active transport works best under normal temperature which in human beings is 37ºC.

d. Enzyme inhibitors

An enzyme is a substance which speeds up reactions taking place in the cell. Enzyme inhibitors are chemical substances which ‘poison’ or make an enzyme inactive. If enzymes involved in the release of energy during respiration are affected, there will be lack of energy. This will prevent active transport from taking place.

e. pH

Abnormal changes in pH in the environment of the cell can alter the structure of the protein carriers. This will reduce their ability to transport molecules across the cell membrane.

Self-evaluation Test 4.1

1.  Absorption of mineral salts from the soil to the root hair requires energy. Which of these describe the process used?

2. Which one is a function of the proteins in the cell membrane?

3.  The root of a certain plant was treated with a respiratory poison. 

4.2 Endocytosis and Exocytosis

The movement of macromolecules such as proteins or polysaccharides into or out of the cell is called bulk transport. There are two types of bulk transport, exocytosis and endocytosis, and both require the expenditure of energy.

Activity 4.2: Investigating the process of endocytosis and exocytosis

You will be provided with charts, computer simulations and animations.

1. Use the charts, computer simulations and animations to illustrate the process of endocytosis and exocytosis.

2.  Discuss with your group members the process involved.

3.  Share your findings with the rest of the class.

Study questions

(a) Differentiate between phagocytosis, exocytosis and pinocytosis.

(b) How important are these processes in a cell.

Endocytosis

This is the in folding of the cell surface membrane to form a vesicle or vacuole. A portion of the membrane wraps around the particle and fuses. Thereafter the particle is surrounded by the plasma membrane. The portion of the membrane containing the particle then pinches off leaving the plasma.

Endocytosis involves the bulk uptake of materials through the plasma membrane. It takes place in two ways: phagocytosis and pinocytosis.

(i) Phagocytosis

Phagocytosis means ‘cell eating.’  This is a process by which plasma membrane extend to form a depression. The depression encloses solid foreign particles. The food vacuole formed moves towards the interior of the cell. The process is selective and specific to solid materials engulfed. An example is white blood cells that engulf bacteria in our bodies.

(ii) Pinocytosis

Pinocytosis means ‘cell drinking.’ This process is similar to phagocytosis only that the cells take in materials in liquid form.  In this case molecules dissolved in liquid are taken up by formation of smaller vesicles compared to phagocytic vesicles.

Exocytosis

This is the process by which materials are removed from the cells. Materials removed include solid particles, undigested remains from food vacuoles among others.  The molecules or cell secretions accumulate in secretory vesicles, which then moves and fuses with the plasma membrane.  

The contents of the vesicle are then expelled out of the cell.

Examples of exocytosis:

Self-evaluation Test 4.2

1. State one difference and one similarity between endocytosis and active transport.

2.  A white blood cell ingests solid particles by forming vesicles. This describes which process?

3.  Given these characteristics:

Unit summary

• Active transport involves the use of energy to move molecules across the cell membrane. During active transport, molecules are moved against a concentration gradient.

• Factors affecting active transport include: concentration of oxygen and glucose, temperature, enzyme inhibitors and pH.

• Protein carrier molecules, which are part of the cell membrane, are involved in active transport.

• Endocytosis is the process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane, and bringing it into the cell.

• Exocytosis is a form of active transport in which a cell transports molecules out of the cell by expelling them through the plasma membrane.