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Cell Physiology

CELL PHYSIOLOGY: In Primary Science, you learnt that molecules exist in three states of matter; solids, liquids, and gases. You have also learnt that the cell membrane controls the movement of molecules in and out of the cell.

In this lesson you will learn how these molecules move across the cell membrane and the factors affecting their movement. You will also learn the importance of the movement of these molecules to living organisms.

BACKGROUND INFORMATION

In Primary Science, you learnt that molecules exist in three states of matter; solids, liquids, and gases. You have also learnt that the cell membrane controls the movement of molecules in and out of the cell.
In this lesson you will learn how these molecules move across the cell membrane and the factors affecting their movement.

You will also learn the importance of the movement of these molecules to living organisms.

 

LESSON OBJECTIVES

By the end of this lesson you should be able to

:

  1. Define the term diffusion
  2. Demonstrate diffusion of substances in gaseous, liquid, and solid media.
  3. Describe four factors that affect the rate of diffusion.
  4. State four roles of diffusion in living organisms.

Cell Physiology

Molecules needed for cellular activities move into the cell across the cell membrane. Some of the manufactured materials pass out of the cell to be used for various activities. Cell physiology refers to to the cellular activities including the movement of materials into and out of the cell across the cell membrane. Materials taken in include water, oxygen, food substances, vitamins and minerals. The products of cellular activities which cells release to their environment include secretions and waste products. The processes through which this exchange occurs include diffusion, osmosis, active transport, phagocytosis and pinocytosis. In this lesson we will discuss the first three of these processes: diffusion, osmosis and active transport.

CELL PHYSIOLOGY

Diffusion

 

Diffusion

Diffusion occurs in gases, liquids, and solids as demonstrated in the following activities.

 

 

 

GAME
Activity 2


Q. Drag and drop the substances that can dissolve in water from the list below.
Basin with water is shown.
List of substances

  1. glucose
  2. chalk dust
  3. soil
  4. ash
  5. salt

Correct answer ' glucose, salt
Incorrect - try again

Activity
An animation of a scientist pouring cold water into two beakers labeled A and B. one beaker (B) heated until boiling. Crystals of equal mass of potassium permagnate are dropped using a glass tube are dropped to the two beakers A and B having cold and hot water simultaneously. A stop watch is then set to show how long it takes for the water to have a uniform purple colour.

Question: 1


In which beaker did it take the water to have a uniform purple colour faster?
Click either r A or B button.

Correct answer ' beaker B
Incorrect answer ' try again ' beaker A
Question: 2
Explain your answer in question 1 above. Type your answer in the box below.

 
Once student types his/her answer the correct explanation appears in word and print.

Diffussion in Gases

 

 

 

GAME

Factors Affecting the rate of Diffusion
Temperature

Increase in temperature increases the rate of movement of molecules hence increases the rate of diffusion while decrease in temperature decreases the rate of movement of molecules. 

As temperature increases the kinetic energy of the molecules increase making them to move faster and further from each other while drop in temperature decreases kinetic energy of molecules.               

ACTIVITY
 

Q. Explain what happened to the potassium permagnate crystals at the end of the experiment. Type your answer in the space below.

 

After the student types his answer the correct explanation appears in both print and voice. Voice says 'the potassium permagnate crystals dissolve in water and spread evenly until the whole water environment is coloured purple.'

DIFFUSION IN LIQUIDS

Potassium permanganate crystals dissolve in water and spread evenly until the whole water environment is coloured purple. Rate of diffusion in liquids is slower than in gases.

 

FACTORS AFFECTING RATE OF DIFFUSION

Temperature:

Activity
 

An animation of a scientist pouring cold water into two beakers labeled A and B. one beaker (B) heated until boiling. Crystals of equal mass of potassium permagnate are dropped using a glass tube to the two beakers A and B having cold and hot water simultaneously. A stop watch is then set to show how long it takes for the water to have a uniform purple colour.

Question: 1
In which beaker did it take the water to have a uniform purple colour faster?
Click either r A or B button.

Correct answer ' beaker B
Incorrect answer ' try again ' beaker A
Question: 2
Explain your answer in question 1 above. Type your answer in the box below.

 
Once student types his/her answer the correct explanation appears in word and print.


Temperature
 

Increase in temperature increases the rate of movement of molecules hence increases the rate of diffusion while decrease in temperature decreases the rate of movement of molecules.

As temperature increases, the kinetic energy of the molecules increases, causing them to move faster and further from each other while drop in temperature decreases kinetic energy of molecules. This makes molecules to move more slowly and closer to each other.

Surface area to volume ratio

Activity
Video shooting:  A scientist obtains two large blocks of Copper II sulphate crystals. One of the crystals is crushed into fine powder. They are then dropped simultaneously into two beakers containing warm water (beaker A has large particles while beaker B has fine/powdery crystals).


The solutions in the beakers are stirred gently for about 1 minute. (Use a stop watch to time.)
Q1. In which beaker did the crystals take a shorter time to dissolve and diffuse evenly? Click the correct answer.

  • Beaker A
  • Beaker B


Q2. Explain your observations in the above experiment. Type your answer in the space below.

 

Surface Area to Volume Ratio

Small particles expose a large surface area to volume ratio to their surrounding compared to large particles. As a result more molecules from small particles dissolve and diffuse compared to the molecules of the large particles.

 

 

Diffusion Gradient

Activity
A video shooting of dissolving potassium permagnate in water of different volumes.
Put equal volumes of potassium permagnate crystals in two beakers. In beaker A add a little water (10ml), and in beaker B add much water (20ml). Let to stand for 5 minutes observe the colour of water in both beakers.

Questions

  1. In which beaker was the colour more intense.

Beaker A                     Beaker B
Click the correct answer.
Explain what you have observed in the demonstration. Type your answer in the box below.

 

Diffusion Gradient

Diffusion gradient refers to the diffrence in concentration between two regions.

The greater the difference in concentration of a substance between two regions the faster the rate of movement of those molecules from the region of higher concentration to the region of lower concentration, until an even concentration or equilibrium is achieved.

 

 

Size of Molecules

Small and light molecules diffuse faster than large and heavy molecules.Gas molecules diffuse faster than liquid molecules while liquid molecules diffuse faster than solid molecules.

 

 

 


 

 

 

Importance of diffussion.
Diffusion is important to living organisms for the following reasons:

Absorption of digested food nutrients from the small intestine into the blood stream

Gaseous exchange

Removal of metabolic wastes from cells

Transport of synthesised food from the leaves to the rest of the plant body

Absorption of food nutrients by cells.

 

BACKGROUND INFORMATION

In Primary Science you learnt that mixtures of solids

and liquids can be separated by sieving and filtering

methods. A filter paper and a piece of clothing were used

to separate soil and water. You have also learnt about

the movement of substances into and out of the cell.

In this lesson you will learn how water molecules move across

the cell membrane, factors influencing their movement and

importance of their movement to living organisms.

Activity
Q. Why did the soil particles remain trapped on the piece of cloth while water passed through?
Type your answer in the space below.

 

LESSON OBJECTIVES
 

By the end of this lesson you should be able to:

  1. Explain the term osmosis
  2. Describe two factors which affect the rate of osmosis.
  3. Explain four roles of osmosis in living organisms

QUIZ - GAME
 

  1. Explain the meaning of the following terms. Type in your answers.
    1. Osmosis
    2. Osmotic potential
    3. Plasmolysis
    4. Haemolysis
    5. Turgor pressure
    6. Osmotic pressure
  1. What would happen to the rate of osmosis in a living organism if
  1. The temperature was increased to 400C.
  2. The temperature was raised above 400C.
  3. State four roles of osmosis in living organisms.

a)  Osmosis is the movement of water molecules from a dilute solution to a concentrated solution through a semi permeable membrane. It can also be looked at as the movement of water molecules from a region of high concentration to a region of low concentration of water molecules through a semi permeable membrane
b) This is the ability of a concentrated solution to draw in water molecule by osmosis when separated from a dilute solution by a semi permeable membrane.

Answers

  1. This is the shrinkage of the protoplasm away from the cell wall due to loss of water by osmosis.
  2. This is the bursting of red blood cells when placed in a placed in   a hypotonic solution due to excessive uptake of water by osmosis.
  3. This is the pressure that is exerted on the cell wall by the protoplasm as a result of the plant cell absorbing water by osmosis.
  4. This is the pressure requires to stop flow of water molecules into a solution by osmosis.
  5. a) The rate of osmosis will increase.b) Osmosis will stop because the cell membrane will have been destroyed.

Introduction

The illustration shown indicates the process of osmosis across a semipermeable membrane. The small particles represent water molecules while the large ones represent salt molecules. The semipermeable membrane is a visking tubing.

Osmosis is the movement of water molecules from a region of high concentration of water molecules to region of low concentration of water molecules through a semi permeable membrane.

Osmosis can also be defined as the movement of water molecules from a dilute solution to a concentrated solution through a semi permeable membrane. A semi permeable membrane is a membrane that allows some substances to pass through but not others. It has pores which allow substances made up of small particles to pass through but not the large ones. The illustration shows osmosis of water molecules across a semi permeable membrane.The smallest molecules are water molecules while the largest molecules are salt molecules. The semi permeable membrane used is a visking tube.


                      

Osmosis across a cell membrane


 

Activity
Q. From the illustration of dirty water using a piece of cloth suggest the meaning of a semi permeable membrane.

 

Activity
The list below contains three semi permeable membranes. Identify them by clicking the button beside each.

  • Polythene  bag
  • Cows bladder
  • Visking tubing
  • Filter paper
  • Piece of cloth
  • Cell membrane

Factors Affecting the Rate of Osmosis

The rate of osmosis is affected by concentration gradient, temperature and the thikness of the membrane across which the process is taking place.

Concentration Gradient

This refers to the difference in concentration between adjacent regions. In the figure below the solution in visking tubing A is more concentrated compared to that in visking tubing B.

Water molecules from the beaker move into visking tubing A at a faster rate than in visking tubing B. This is because there is a higher osmotic gradient between the water in the beaker and the solution in visking tubing A compared to the difference in concentration between the water in the beaker and visking tubing B.

The concentrated solution in visking tubing A is referred to as hypertonic solution while the distilled water in the beaker is referred to as a hypotonic solution.

Hypotonic and Hypertonic solutions

A hypertonic solution is a more concentrated solution than the surounding. It has a lot of solute molecules and a few solvent molecules.
A hypotonic solution is a dilute solution, compared to the surounding. It has a lot of solvent molecules and few or no solute molecules.
Isotonic solutions refer to two solutions of equal concentration.

NB:Plant and animal cells gain water when put in a hypotonic solution and lose water when put in a hypertonic solutions.

GAME

Q1. What happens to the water molecule when the temperature increases up to 400c?
Click on the suggested answer below

  • Molecules move across the membrane at a faster rate.
  • Molecules move across membrane
  • Molecules move across membrane at slow  rate
  • No change in rate of movement of water molecules

Temperature

Molecules of water in a container vibrate at a slow rate when the temperature is low.The rate of osmosis is equally low. As temperature is increased the rate of osmosis increases due to increase in movement of solvent molecules.

Further increase in temperature, up to 40 degrees Centigrade causes water molecules to move faster but they do not cross the semi permeable membrane. This is becausethe plasma membrane is protein in nature. Temperatures of 40*C and above denature the semi-permeable membrane and therefore stop osmosis from taking place.

 

 

 

 

 

 

GAME

Q2. Explain the observation in the set up.
Type your answer below.

 

After student types answer the following print and voice appears.
'The solution in Visking tubing A is less concentrated compared to that in Visking tube B. Water molecules from the beaker moves into Visking tubing B at a faster rate than in Visking tubing A, this is because there is a higher osmotic gradient between the water in the beaker and the solution in the Visking tubing B compared to the difference in concentration between the water in the beaker and Visking tubing A.'

Thickness of membrane

 

 

 

 

 

 

Most membranes in the body are thin to ensure that molecules move across at a faster rate.

Molecules take a longer period of time to cross thicker membranes.

Water Relations in Plants

A plant cell placed in hypotonic solution gains water. This ability of a concentrated solution to draw in water by osmosis is called osmotic potential. As the cell draws in water it develops an osmotic pressure. This is the pressure required to stop flow of water into a solution across a semi-permeable membrane

Turgor Pressure and Wall Pressure

When a plant cell is placed in a hypotonic solution (dilute solution) it gains water by osmosis. The cell sap increases in volume, pushing the protoplasm against the cell wall. The resultant internal pressure pushing the protoplasm against the cell wall is called turgor pressure. When the cell wall is being stretched out ward it develops a resistant pressure on the outside. This is known as wall pressure. Wall pressure is the pressure which acts against the cell protoplasm to oppose the turgor pressure. When the cell wall can no longer stretch then the cell is said to be fully turgid. When the cell is fully turgid the wall pressure is equal to the turgor pressure.

 

Illustration of Turgor Pressure and Wall Pressure in a Plant Cell

Plant Cells in Hypertonic Solution

A cell is placed in a more concentrated or hypertonic solution will loose water molecules to that solution by osmosis. The cell protoplasm detaches itself from the cell wall. This is called plasmolysis. During drought, plants lose water by transpiration at a faster rate than the root hair cells are absorbing water from the soil. This leads to the cells loosing turgidity and the tissues become flaccid. This leads to wilting of the plant.

Water Relations in Animal Cells

When an animal cell such as a red blood cell is placed in a hypotonic solution it gains water by osmosis. Volume of the cytoplasm increases, creating an outward pressure on the cell membrane.

The cell swells and eventually bursts because the cell membrane cannot resist the outward pressure. The bursting of red blood cells when placed in a hypotonic solution is called haemolysis.

When placed in hypertonic solution the cells lose water and shrink or become crenated.

Animal Cell in Hypotonic and Hypertonic solutions

 

 

 

 

Animal cell in hypertonic and hypotonic solution

When an animal cell such as red blood cell is placed in hypertonic solution it looses water by osmosis to the surrounding solution. This makes the cell to shrink such that the cell membrane appears wrinkled. This is known as crenation.

A hypotonic solution surrounding an animal cell causes that cell to absorb water by osmosis. This may lead to swelling and eventually bursting of the cell. This condition is known as haemolysis.

Animation of an animal cell subjected to hypotonic and hypertonic solutions.

Role of Osmosis in Living Organisms
Osmosis is important to living organisms for the following reasons:

Support: Plant cells take in water by osmosis and become turgid. This makes the plant tissues to become firm and rigid hence providing support. This support is important in seedlings, leaves and herbaceous plants which are less woody.

Absorption of water from soil: The root hair cells have a higher concentration of salts compared to the solution in soil . This provides a suitable osmotic gradient for water to enter the root hair cells by osmosis. Water moves from the cell within the plant by osmosis.

Structures associated with absorption of water from the soil

iii. Opening and closing of the stomata: During the day the guard cells have a higher concentration of sugars than the surrounding epidermal cells. This makes water to enter the guard cells by osmosis, subsequently making them turgid. The stomata opens up. At night the concentration of sugars in the guard cells is lower than the surrounding epidermal cells. This makes the guard cells to lose water by osmosis and become flaccid. The stomata closes up. The illustration shows an open and cosed stomata.

iv. Osmoregulation: Movement of water in and out of the blood balances the concentration of fluids in the blood and tissue fluid.

Summary of osmosis
In this lesson you have learnt the following:

  • Osmosis is the movement of water molecules from a dilute solution to a concentrated solution across a semi permeable membrane.
  • Rate of osmosis is affected by temperature and the osmotic gradient between solutions separated by a semi permeable membrane.
  • Osmosis is important to living organisms since it absorbs water from the soil and the stomata to open and close it and also enables herbaceous plants, seedlings and leaves to have support.

    BACKGROUND INFORMATION


    You have learnt that molecules move from where they are highly concentrated to where their concentration is low by osmosis and diffusion.

    In certain circumstances, molecules move from where they are less concentrated, through a semi-permeable membrane, to where they are highly concentrated. This movement which occurs against the concentration gradient requires energy. The process involved in such movement is known as ACTIVE TRANSPORT.

    Activity

    An animation of movement of a stone up and down the slope.

    Direction of movement
     

    Q1. in which of the two illustrations is energy required to move the stone?

    • A
    • B

    Click the correct answer.
    Correct answer: B

    LESSON OBJECTIVES

    By the end of the lesson you should be able to

    :
    1. Explain the term active transport
    2. Describe five factors which affect active transport.
    3. State five roles of active transport in living organisms.

    QUIZ-GAME


    Type in your answers in the space provided
    Q1. Explain two uses of active transport in: 

    1. Plants

    i)
    ii)

    1. Humans

    i)
    ii)

    Q2. Explain how the following factors affect the rate of active transport.

    1. Temperature
    2. Oxygen concentration
    3. Enzyme inhibitors

    Q3.The illustration below shows two physiological processes occurring in a cell.
     

    1. Identify the two physiological processes.
    Give two differences between processes (a) and (b).

    Active Transport

    Sometimes solute molecules move from where they are less concentrated to where they are more concentrated across a semi-permeable membrane. They are then said to move against a concentration gradient. The process is known as active transport and energy is used.

    This can be compared to the energy required to roll a stone uphill. Active transport is unlike diffusion and osmosis where materials move along a concentration gradient and energy is not required. This is equivalent to a stone rolling downhill.

    ACTIVITY
    Q2. Explain your answer in one above
    Type your answer in the box below

                                                                                                             
    The correct answer appears after student types the answer.
    Correct answer energy in requires to move stone B up the slope against force of gravity.
    From the illustration above define Active transport. Type your answer in the box below.
     

    Correct answer. Active transport is the movement of substances across a semi permeable membrane against concentration gradient. The process requires energy. The energy is generated within the cell. Active transport takes place only in living cells.

     

    Factors Affecting Active Transport

    Any factor that affects energy production affects rate of active transport. Factors affecting rate of active transport include the following: Oxygen concentration, Temperature, Enzyme inhibitors, pH, Co-factors and Co-enzymes.

    Oxygen Concentration

    Oxygen is used to break down food to release energy. When the concentration of oxygen is low, less energy is produced; hence rate of active transport is slow. The higher the oxygen concentration the more energy is released and therefore the rate of active transport is increased.

    Temperature
    Increase in temperature up to optimum levels increases rate of chemical reactions that release energy in the cell. Increase in energy increases the rate of active transport.Temperatures above optimum levels denature enzymes that speed up chemical reactions. This results in low energy production and therefore rate of active transport is slowed down.Low temperatures inactivate enzymes hence less energy is produced. This slows down active transport.

    Enzyme inhibitors

    Enzyme inhibitors are substances that slow down the rate of enzyme activity. Presence of enzyme inhibitors slows down the rate of active transport. These block the enzyme active sites which makes it hard for the enzymes to bind and react with the substrates.

    pH

    This is the acidity or alkalinity of a solution. Some enzymes function best in acidic, alkaline or neutral pH. If the pH of a chemical reaction is altered, enzyme activity will be slowed down or stopped. This will slow down or stop energy production. Consequently active transport will be slowed down or may stop.

     

    Effect of pH on Enzymes A and B.

     

    Co-factors and Co-Enzymes
    Co- factors and Co-enzymes are substances that activate enzymes. Their presence increases the rate of chemical reactions leading to more energy production. This increases rate of active transport.

    Role of Active Transport in Living Organisms
    Active transport is used by various organisms for different purposes such as:

    1. Absorption of digested food from the alimentary canal into blood stream.
    2. Absorption of mineral salts from soil by roots.
    3. Excretion of metabolic waste products from body cells.
    4. Reabsorption of sugars and some salts by the kidney.
    5. Accumulation of substances into the body to offset osmotic imbalance in the body especially for organisms in arid and saline environments.

    Summary

    In this lesson you have learnt that: 1.Active transport is a physiological process that controls the movement of important substances into and out of the organism, across cell membranes.
    2. This is an energy-consuming proces, occurring only in living cells.
    3. Active transport is affected by;
    Temperature
    pH change
    Oxygen concentration
    Enzyme inhibitors
    Co-factors and coenzymes

  •  



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