NOTES: Movement of Substances across the Plasma Membrane

•Living cells carry out the following life processes:

–absorb water and nutrients (minerals, ions,glucose, amino acids) from the surroundings.

–Excrete waste products (urea, uric acid)

–Exchange respiratory gases during resp.

•These processes are important to a cell because they help the cell to:

–maintain the optimum pH value and concentration gradient of ions that are suitable for the activities of the cell (muscle and nerve activities)

–obtain nutrients for metabolism

–eliminate toxic waste products from the cell

–secrete useful substances (hormones,enzymes)

•The movement of substances in and out of the cell occurs across the plasma membrane.

•The plasma membrane regulates the exchange of substances between the content of the cell and the external environment.

The structure of the plasma membrane

•Plasma membrane, according to the fluid mosaic model (S. Singer and G. Nicolson, 1972) are composed mainly of phospholipids and proteins.

•Each phospolipid molecule consist of two parts:

–a polar head that gives it a hydrophilic property (attracted to water)

–a pair of non-polar fatty acid tails  that give it a hydrophobic property (repelled by water)

•At the plasma membrane, phospholipids are arranged in a double layer, called phospholipid bilayer.

•

•According to this model,

–the phospholipid, proteins and other components of the membrane are not rigid or static, but form a dynamic and fluid structure.

Phospholipid Bilayer

–The protein molecules float about freely in the phospholipid bilayer.

•The protein and phospholipids are free to move sideway within the membrane, and this causes the membrane to have a fluid characteristic.

•The various proteins built into the plasma membrane form a mosaic pattern

Fig. A model of the plasma membrane (fluid- mosaic  structure)

Fluid Mosaic Model

•Cell-cell recognition proteins 

- identify type of cell and identify a cell as “self” versus foreign

–Most are glycoproteins

•-Carbohydrate chains vary between species, individuals, and even between cell types in a given individual.

•-Glycolipids also play a role in cell recognition

Fluid Mosaic Model of the PM

•A membrane is a mosaic

–-Proteins and other molecules are embedded in a framework of phospholipids

•

•A membrane is fluid

–-Most protein and phospholipid molecules can move laterally

The permeability of the Plasma Membrane

•-The plasma membrane is selectively permeable/ semi- permeable/partially permeable.

•-This means that certain substances can move across the membrane freely while  others cannot.

•The selective permeability of the plasma membrane depends on the:

–- the size of molecules or ions, or

–- the polarity of the molecules.

•The phospholipid bilayer is permeable to:

–-non-polar molecules (lipid-soluble)

–-small, uncharged molecules

•The phospholipid bilayer is not permeable to:

–-charged ions

–-polar molecules (lipid-insoluble)

•Due to hydrophobic core of the bilayer. The plasma membrane is only permeable to:

–- small hydrophobic solutes such as fatty acids, glycerol, steroids and all the fat soluble vitamins (A,D,E,K).

–- non- polar molecules such as oxygen and carbon dioxide

–- water as it is small enough to slide between the phospholipid bilayer or move through the pores.

•- Pore proteins allow small water-soluble molecules and ions to pass through the membrane.

•- Charged and large molecules such as glucose, amino acids, water soluble vitamins and polysaccharides cannot pass across the plasma membrane and therefore they are transported by either facilitated diffusion or active transport (aided by carrier proteins)

•There are two ways of transporting substances or solutes across the plasma membrane:

–-Passive transport (without the involvement of chemical energy)

–- Active transport (use energy and carrier proteins)

Diffusion

•-Movement of molecules from an area of high concentration to an area of low concentration.

•

•-Movement from one side of a membrane to another, un-facilitated

•A. Passive Transport

•i) Simple Diffusion

•lipid-soluble molecules, gases and water.

•not control by cell.

•movement of the molecules from a region of higher concentration to a region of lower concentration.

•Factors affecting the rate of diffusion are temperature, size of molecules/ions, diffusion gradient, surface area and diffusion medium.

•example: diffusion of oxygen and carbon dioxide at the alveolus

Osmosis

•Osmosis – diffusion of water across a selectively permeable membrane

•- Water moves from an area of high water concentration (dilute solution) to an area of low water concentration(concentrated solution).

– Is energy required ?

•Type of Solution

•-Hypotonic

•-Isotonic

•-Hypertonic

•1) Hypotonic

•Solute concentration in the external solution is lesser than solute concentration inside the cell.

•Water concentration outside the cell is higher than the water concentration inside the cell.

•2) Isotonic

•Solute concentration in the external solution is equal to the solute concentration inside the cell.

•Water concentration inside and outside of the cell is the same.

•3) Hypertonic

•Solute concentration in the external solution is greater than solute concentration inside the cell.

•Water concentration outside the cell is lower than the water concentration inside the cell.

Osmosis and Animal Cells

Osmosis and Plant Cells

Osmosis Summary

•When a cell is placed in a Hypotonic solution:

–-Cell gains water through osmosis

–-Animal cell lyses; plant cell becomes turgid (firm)

–

–

•When a cell is placed a Hypertonic solution:

–-Cell loses water through osmosis

–-Animal cell shrivels; plant cell plasmolyzes

•Facilitated Diffusion:

•very specific: glucose, nucleic acids, amino acids and mineral ions.

•control by cell.

•transport of molecules (only certain molecules) across the outer membrane of living cell aided by a  carrier protein within the cell membrane.

•normally take place from a region with higher concentration of molecules to a region of lower concentration.

•example: absorption of digested food in the villus

•Process of Active Transport

•very specific: minerals ions and amino acids.

•control by cell.

•This process needs carrier proteins and energy (due to against concentration gradient) from a region of lower concentration to a region of higher concentration).

•Cell must expend energy that derived from ATP (adenosine triphosphate)

•example: human nerve cells (sodium ions are constantly transport out of the cell) / ions intake by root hairs of a plant

Active Transport

•Active transport proteins move substances across the PM against their concentration gradient.

–Requires energy (ATP)

–Active transport proteins are highly selective

–Active transport is needed for proper functioning of nerves and muscles

•Application

•Food is soaked in a concentrated salt solution to prevent bacteria and fungus to survive.

•Chemical fertiliser (dissolved ions) increases solute concentration (decrease water molecules) in soil. Therefore, water leaves from the cell sap of the plant which result the plant wither.

CHAPTER 3 : MOVEMENT OF SUBSTANCES ACROSS PLASMA MEMBRANE

Activity 3.1

1.     Q :What are the substances required by cells?

        A : Oxygen, amino acids, glucose, minerals ( Na⁺, K⁺ ), water, vitamins. 

2.     Q : What are the substances that need to be eliminated from cells?

        A :  Carbon dioxide, urea.

3.     Q : Explain the necessity for the movement of  substances across the plasma  membrane.

        A : (i)      To maintain the concentration of substances inside and outside the cell

              (ii)     To maintain constant internal environment ( homeostasis ), to ensure that the cells can function effectively.

Activity 3.2

2.             Discuss he following questions with you groups members:

a)   Q: How does diffusion occur?

A:  The movement of substances or molecules from a region of higher   concentration to a region of lower concentration.

b)  Q:What is the differences between simple diffusion and osmosis?A:In osmosis the movement of substances are across the plasma membrane , while in simple diffusion , plasma membrane is non required.

c)   Q:What are the substances that can be transported across the plasma membrane by facilitated diffusion?

A:Water –soluble molecules /not soluble in lipids  such as nucleic acids, ions, amino acids and glucose        .

d) Q:Explain how active transport occurs?

    Answers 

i)  The concentration of sodium ion is higher on the outside of the cell.

ii)         The sodium ion approach the carrier protein to bind to one of the    binding sites and the other binding site attach to ATP.

iii)                   Energy from ATP is used by carrier protein and cause it to change its shape and allows sodium ions to bind to the active site..

iv)                   This causes the carrier protein to release the sodium ions outside the cell.

Activity 3.3

2. (a)       Q : How a gaseous exchange occurs in the alveoli and blood capillaries?

A : The concentration of oxygen in alveoli is higher than in the blood capillaries.   Therefore, oxygen diffuses across the surface of the alveolus and blood capillaries into the blood.

(b)  Q : How food is absorbed in the villi of the small intestine ? 

  

Types of transport	Substances	Explaination
Simple diffusion	Water-soluble and lipids-soluble molecules	Move into the villi through the plasma membrane
Facilitated diffusion	Glucose, amino acids	Glucose, amino acids are transported via facilitated diffusion
Active transport	Ions ( Na+, K+ )	The ions are transported via active transport

    ( c )      

Water	Ions
-       uptake by roots -       the soil has a higher water concentration compared to the cell sap of the root hair -       therefore, water diffuses into the vacuoles of root hair by osmosis	-       uptake of mineral salts by root hair -       the concentration of ions and mineral salts in the cell sap is higher compared to the soil -       these substances are transported into the cells via active transport.

ASSESMENT ( CHAPTER 3 )

OBJECTIVE QUESTIONS

1. C                2. D                3. D                4. B                5. D

Assessment Chapter 3

1.     Figure 2 shows the fluid-mosaic model of plasma membranes.

a)         Name the structure labeled W, X, Y and Z in figure 2.

        W:  Carrier protein  X:  Pore protein      Y:  Non-polar tail   Z: Polar head

b)   What are the characteristics of plasma membranes based on the model in     Figure 2?

Plasma membrane made up of phospholipid bilayer and proteins that are not rigid or static but form dynamic and flexible structure.  Membrane has fluid characteristic  with the protein and phospholipid molecules free to move within the membrane. It acts as a barrier to isolates the two sides of membrane. The plasma membrane is selectively permeable.

c)   State one molecule that can pass through structure X.

      Calcium ion/water molecule/small soluble water molecule

d)  Explain how molecules pass through structure W.

W is carrier protein which are specific as it  can only combine with certain molecules . The carrier protein functions by binding to the molecules. It changes shape to allow the molecules to pass through to the other side of plasma membrane.

1.   Figure 3 shows the process of osmosis.

The beaker is separated into 2 compartments by a semi-permeable membrane that is only permeable to water molecules.

    Compartment A contains sucrose and water molecules while compartment B contains only water molecules.

    (a)   Name the process that take place in the beaker.

           The process is osmosis.

    (b)   Explain the process that occurs in the beaker.

        There is a concentration gradient occurs between the two compartments. Therefore, water molecules from Compartment B which has higher water concentration diffuse through a semi-permeable membrane to Compartment A which has lower water concentration 

(c ) Predict what would happen to:

(i)    the concentration of solution in Compartment A, and

The concentration of solution becomes lower after the system has reached a dynamic equilibrium

(ii)   the level of solutions in both compartments

The level of solution in Compartment A is higher than in Compartment B

after the system has reached a dynamic equilibrium.

SECTION C

1.     Explain what you understand about the term ‘semi-permeable’.

This means that some substances can move across the membrane freely while others cannot.

        2.     Explain how facilitated transport is different from active transport.

FACILITATED DIFFUSION

-       going down concentration gradient

-       facilitated by carrier proteins

-       movement of glucose and amino acids in the villi

   ACTIVE TRANSPORT

-       against the concentration gradient

-       occurs in living cells

-       cellular energy

( ATP ) is required

-       the process results in the accumulation of or elimination of substances from the cell

-        movement of mineral salts and ions

    3.     Explain :

  (i)    How gaseous exchange occurs in alveoli and blood capillaries.

- The concentration of oxygen in alveoli is higher than in the blood capillaries.   Therefore, oxygen diffuses across the surface of the alveolus and blood capillaries into the blood.

(ii)   the uptake of ions by the root hairs of a plant.

-       uptake of mineral salts by root hair -       the concentration of ions and mineral salts in the cell sap is higher compared to the soil these substances are transported into the cells via active transport.

            

Take note

Activity 2.2 (page 20) and  Activity 2.3 (page 21) [TEXTBOOK] 

I will paste the answers in class 4C tomorrow. Thank you. 

CHAPTER 2 : CELL STRUCTURE AND CELL ORGANISATION

Textbook

Focus Practise 2.1 (page 22)

1. Why is the use of the electron microscope important in the study of cell biology ?

      Electron microscope enables scientist to see the cellular components of a cell in

greater details.  Some of the organelles can only be seen under an electron microscope.

2. Why is it important to study cellular components and their functions?

Living organisms are made of basic units forming the building blocks of all living organisms.

3. Which organelle is found abundantly in a cell that secretes protein products?

Ribosome, Rough ER, Golgi apparatus.

Ribosomes – synthesis of proteins

Rough ER – transport of protein from one part of the cell to another

Golgi apparatus – processing, packaging and transporting proteins, phospholipids  

                             and glycoproteins

4. Match the following cellular components to their functions.

Cellular component	Function
A.Vacuole	An organelle that contains digestive enzymes ( C )
B.Smooth endoplasmic       reticulum	An organelle that traps light energy during photosynthesis ( D )
C. Lysosome	A fluid-filled sac which contains a complex mixture of sugars, mineral salts, pigments and waste products ( A )
D. Chloroplast	An organelle which is involved in the synthesis of lipids ( B )

Focus Practice 2.2 (page 35)

1. What is the function of contractile vacuoles in Amoeba sp.?

 Contractile vacuoles of unicellular organisms such as Amoeba  sp. help regulate water balance by pumping out excess water that diffuses into the cell.

2. Explain the process of phagocytosis in Amoeba sp.

 The process of phagocytosis involves;

·         Amoeba sp. approaches the food particles

·         Two pseudopodium extend out and enclose the food particles

·         The food particles are packaged in a food vacuole, which fuses with a lysosome containing hydrochloric enzymes called lysozyme

·         The food particles are digested by the lysozyme. The resulting nutrients diffuse into the cytoplasm and are assimilated

·         Undigested material is left behind when the Amoeba sp. moves away

3. What is the difference between a tissue and an organ?

A tissue is a group of specialised cells with a common structure and function, where as an organ is formed by two or more types of tissues working together to perform particular functions.

4. What are the major systems in humans?

The major systems in humans are;

·         Nervous system

·         Skeletal system

·         Circulatory system

·         Digestive system

·         Respiratory system

·         Excretory system

·         Reproductive systems

·         Muscular system

·         Integumentary system

·         Endocrine system

·         Lymphatic system

5. Name the systems that are involved in regulating the concentrations of oxygen and carbon dioxide in the bloodstream.

The systems that are involved in regulating the concentrations of oxygen and carbon dioxide in the bloodstream are cirulatory system and respiration system.

Focus Practice 2.3 (page 35)

1. List all the different types of tissues that make up the connective tissue. Briefly describe the function of each type of tissue named.

	Type of tissues	Function
1	Cartilage	·         Provides support to the nose, ears, and covers the ends of bones at joints. ·         Also forms dics between the vetebrate, this enables them to act as cushions to absorb pressure
2	Bone	·         Provide protection to organs in the body and supports the body
3	Blood cells	·         Has regulating, tranporting and protective functions. ·         Transports nutrients and oxygen to cells and removes carbon dioxide and waste products from the cells
4	Adipose tissues	·        Acts as an energy reserve ·        Provides insulation an protection

2. What are the different tissues that make up a leaf? Give the function of each type tissue in the leaf.

	Type of tissues	Function
1	Epidermal tissues	·         The cuticle on the epidermal on epidermal tisssue minimies water loss through evaporation ·         Protects the plant from mechanical injury ·         Prevents invasion of diseases caused by microorganisms
2	Ground tissues	·         Produces food by photosynthesis and stores food produced ·         Provides support and strengthens the plant
3	Vascular tissues	·         Tranports water and mineral salts from roots to the stems and leaves by the xylem tissues ·         Transports dissolved nutrients such as glucose from the leaves to the roots and stems by phloem tissues ·         Xylem also provide support to the plants

3. Discuss the process taking place in Figure 2.12. Predict the outcome if the cell does not have the Golgi apparatus.

          Process : Protein synthesis

-                   Synthesis of proteins takes place in the ribosomes.Ribosomes use information carried by the chromosomes to make these proteins.

-                   The rough ER transports proteins made by the ribosomes.

-                   Proteins are enclosed in vesicles before the proteins are secreted from the rough ER.

-                   Trnsports vesicle carrying proteins from the ER fuse with the membrane of the Golgi apparatus and empty their contents into the membranous space.

-                   These proteins are modified as tehy pass through the Golgi apparatus.

-                   The Golgi apparatus will then sort these products and package them into vesicles.

-                   Vesicles containing these products bud off form the Golgi membrane and travel them to other parts of the cells or to the plasma membrane.

-                   These vesicles will then fuse with the plasma membrane before releasing their contents outside of the cells.

·                     Prediction :

-                   The apparatus functions as a processing, packaging and transporting centre of carbohydrates, proteins phospholipids and glycoproteins cannot occur.

Activity 2.5 (page 33)

1.                  Why do cells need to maintain an optimal internal environment ?

   

 So that cells can function efficiently.

2.                  State the factors that affect the internal environment of an organism. Explain using an example.

    

            2 factors that affect the internal environment – physical factors and chemical compositions

            Physical factors – example : temperature, blood pressure, osmotic pressure

            Chemical factor – example : salt, sugar content, pH

- The metabolism of the body is affected by the changes in the chemical factors such as glucose and oxygen levels and physical factors such as temperature and osmotic pressure.

3.                  Why is it important for an animal to maintain relatively constant conditions in its 

            internal environment ?

     

The ability of organisms to maintain a stable environment enables them to live in a wider range of habitat. For example, mammals have highly developed internal  mechanisms to keep the body temperature around 37˚C despite considerable   variations in the external temperature.

4.                  Do you think a similar regulatory system exists in plants ? Explain why.

            In plants, the need for a regulatory system is lesser because

·         most plant cells are dead at functional maturity, and therefore chemical reactions 

            do not take place in these cells.

·         plant cells are not bathed in fluid but surrounded by air.

·         plant cells do not lyse in a hypotonic environment because they have cell walls.

·         water movement in plants is aided by natural factors such as transpirational pull.   

Assessment (page 39)

SECTION B

1. Figure 1 shows a unicellular organism.

a)      Explain how the organism in Figure 1 moves.

  

Amoeba sp. Moves with the help of pseudopodium (false-foot). Pseudopodium is an extension of the cytoplasm. The rest of the cytoplasm slowly flows into this extension, hence moving the organism along. Pseudopodium can be extended out in any directions.

b)      Name the structure labeled X.  State the main function of  X

Nucleus – controls and regulates all the activity of the cell

- contains the heredity factors responsible for the traits of the animals and  plants  

c)      Describe how the organism feeds.

Amoeba sp. feeds by a process called phagocytosis. It simply move round the food particles and enclose the food which is then packaged in a food vacuole. The fod vacuole fuses with a lysosome and the food is digested by hydrolytic enzyme called lysozyme. The resulting nutrients are absorbed into the cytoplasm. (refer figure 2.3, page 23)

d)      (i)  How does the organism reproduce ?

Amoeba sp. reproduce by binary fission. Its nucleus divides to form daughter     nuclei. The cytoplasm then divide and 2 daughter amoeba are form.

           ( ii ) Draw diagrams to show the reproduction process of the organism and label    

                   them

                 

       Refer figure 2.4 page 23

e)      Explain why the structure of the unicellular organism is more complex than the cells in the human body.

Unicellular organisms carry out all vital functions within a single cell. It feed, respire, excrete, move, respond to stimuli, reproduce and grow within its cell. Their survival depends largely on its cellular components which include the organelles. It does not have organ or system so the structure of the unicellular organism is more complex than cells in human body.

2. Figure 2 shows four different types of organelles.

Use the letters P, Q, R and S to match the following functions to the organelles.

  (a)

Function	Organelle
The site where cellular respiration occurs and energy is generated.	P
The organelle that captures light energy to make food	S
The organelle that functions as the processing, packaging and transporting centre of carbohydrates, proteins, phospholipids and glycoproteins.	R
A\series of membranes in the cytoplasm through which substances can be transported	Q

(b)    What organelles are present in plant cells but not in animal cells ?

S

(c)     Why is organelle P present in large numbers in the sperm cells of humans and flight muscle cells of birds ?

Sperm cells of human and flight muscle of birds have large number of mitochondria because the sperm cells require energy to propel through the uterus towards the fallopian tubes so that fertilization can take place, the flight muscle cells of birds contract and relax to enable movement and flight.

SECTION C

1. Figure 3 shows two different types of cells, labeled A and B

(a)    Which is a plant cell?  Give one reason for your answer.

B is a plant cell.

Reason:

-cell B have cell wall but cell A do not/

-cell B have fixed shape but cell A do not/

-cell B have chloroplast but cell A do not/

-cell B have large central vacuole but cell A do not have or small vacuole

(b)    Describe briefly how to prepare a slide of plant cells.

i)              Obtain a scale leaf from an onion bulb as shown in figure 2.1 (a) (Refer pg 9).

ii)            Using a pair of forceps, peel off the transparent epidermis from the inner surface of the onion scale leaf.

iii)          Put a drop of water onto the middle of the slide and place the epidermis on the drop of water (figure 2.1 (b) ). (Refer pg 9)

iv)          With the help of a mounting needle, cover the specimen with a cover slip at a 45° angle (figure 2.1 (c) ) (Refer pf 9)

v)            Add a drop of iodine solution onto one side of the cover slip.  Place a filter paper at the opposite end of the cover slip to allow the iodine solution to spread through the epidermis (figure 2.1 (d) ) (Refer pg 9)

vi)          Use a piece of filter paper to absorb the excess iodine solution.

vii)        Examine the slide under a light microscope using the low power objective lens and then the high power objective lens.

viii)      Draw the epidermis and label the cell structures which can be observed.  Record the magnification used.

(c)    By giving appropriate examples, describes how different organelles work together to enable the cell to function in an orderly manner.

i)        Synthesis of proteins, such as enzymes, takes place in the ribosomes.  Ribosomes use information carried by the chromosomes to make these proteins.

ii)      The rough endoplasmic reticulum transports protein made by the ribosomes.  Proteins that depart from the rough endoplasmic reticulum are enclosed in vesicles.  These proteins can be transported from one part of the cell to another. 

iii)    These protein are modified as they pass through the Golgi apparatus.  The golgi apparatus will then sort these products and package them into vesicles. 

iv)    Vesicles containing these products bud off from the Golgi membrane and travel to other parts of the cell or to the plasma membrane.  These vesicles will then fuse with the plasma membrane before releasing their contents outside the cell.

2. Describe briefly how cells are organized in a multicellular organism. 

Figure 4 (a) shows cells found on the lining of the trachea and figure 4 (b)  shows a type of vascular tissue found in plants.

(a)    Cell organisation in multicellular organization be summarized as follows :

Cells ------> tissues ------> organs ------> system ------> multicellular organism

·         Cells -  basic unit of life

·         Tissues – similar cells performing a particular function

·         Organ – A group of organs

·         Organisms – Made up of all the systems that work together.  The function of all the systems must be coordinated to work together for the survival of the organisms.

   

(b)   Describe how the cells in Figure 4 (a)  and 4 (b)  differ from a typical animal and plant cells respectively.

The epithelial tissue that live the trachea consists of elongated cells with hair like projection called cilia.  They also screate mucus.  The mucus traps dust particles while the cilia sweep the impurities away from the lungs.

The phloem consist of sieve tube and companion cells.  The sieve tube have pores of both end, so that it can transport organic nutrients from the leaves to storage organs or growing parts of the plants.

 (c)

An organ is made up of many types of tissues.  The function of an organ is therefore related to the types of tissues that make up the organ.

Explain the validity of the above statement using suitable diagrams.

• Stomach is made up of different tissues. 

 

                Each type of tissue performs a particular function. 

               Outer layer of stomach is made up of epithelial tissue which forms a protective layer.

  Inner layer of stomach is made up of epithelial glands to secrete enzymes and mucus.

 Smooth muscle tissue found in abundance in the stomach helps in digestion of food.

 Contraction and relaxation of the muscle tissues churn and mix the food.

 Connective tissue in the stomach such as collagen fibres gives elasticity to the stomach.

 Connective tissues such as blood supply oxygen to the stomach.