Chapter 2 Cell Structure and Cell Organisation

Cell Structure and Function

1. All living things are made up of basic units called cells.2. All cells share certain general characteristics.3. Protoplasm is the living component of a cell which consists

of cytoplasm and nucleus.4. Nucleoplasm is the content of the nucleus. The part of the

protoplasm surrounding the nucleus is called the cytoplasm.

5. Plasma membrane, a thin layer around the protoplasm. 6. Cell wall the outer boundary in the plant cells which

surrounds completely the plasma membrane.7. Cytoplasm contains a variety of tiny structures called

organelles.(a) Organelles are cellular components that perform specific

functions within the cell.(b) Many of these organelles are enclosed by their own

membranes.8. E.g. of organelles include the nucleus, mitochondria,

chloroplasts, vacuoles, ribosomes and Golgi apparatus.

Animal Cell

Plant Cell

Organelles Function

Nucleus- spherical with a double

membrane- contain nucleolus,

chromosomes, nucleoplasm and nuclear membrane

Controls all cell activity

Mitochondria- rod-shaped with a double

membrane- outer membrane is smooth

and regular while the inner membrane is folded to form cristae

Site of energy synthesis

Vacuole- cavities filled with cell sap

surrounded by a semi-permeable membrane called the tonoplast

Keeps cell sap

Chloroplast- disc-shaped organelles with a

double membrane- consists of an orderly arrangement

of grana with the stroma. Each granum contains chlorophyll

Site for photosynthesis

Golgi apparatus- vacuolar region

surrounded by a complex meshwork of vesicles budding off at its end

Completes the synthesis of glycoproteins, hormones and polysaccharides

Ribosomes- small particles consisting of RNA

(ribonucleic acid)- exists freely in the cytoplasm or on

the surface of the endoplasmic reticulum

Synthesis protein

Endoplasmic reticulum- a system of membrane-

enclosed tubules closely packed together and continuous with the nuclear membrane

- rough ER has ribosomes while the smooth ER does not have ribosomes

Transport of protein and lipids

Lysosomes- membrane-bound vesicles

found in animal cells

Destroy worn out organelles within the cell

Centrioles- consists of two cylindrical body

structures arranged at right angles to one another

- found only in animal cells

Involve in cell division

Non-Organelles Function

Plasma membrane- thin , semi- permeable membrane

Controls movement of substances in and out of the cell

Cell wall- thick layer outside the plasma membrane- made up of cellulose and fully permeable

Gives shape and support to the cell

Cytoplasm- jelly-like substance that

contains water and mineral salts

- contains organelles

Medium for reactions in the cell

Cell Structure

The density of organelles in specific cells

1. Different body cells perform different functions. For examples, nerve cells send nerve impulses and red blood cells transport oxygen.

2. The density of certain organelles found in the cell is correlated to the specific functions of the cell.

3. Those organelles which are found in a high density in specific cells are as follows.

Types of cells functions Organelles found abundantly or which occur in high density

Sperm cells Requires energy to propel through the uterus towards the Fallopian tubes, so that fertilisation can take place.

Mitochondria Energy in the

form of ATP is generated during cellular respiration when food substances are oxidised in the mitochondria.

ATP is an instant source of energy for these cells.

Muscle cells (flight muscle cells in insects and birds)

Contract and relax to enable movement and flight.

Meristematic cells (cells in the meristems of plants)

Requires large amount of energy during active cell division to produce new cells.

Palisade mesophyll cells

Absorb sunlight during photosynthesis.

Chloroplasts

Pancreatic cells Synthesise and secrete enzymes and hormones.

Rough endoplasmic reticulum and Golgi apparatusCells in the

salivary glandsSynthesise and secrete enzymes.

Intestinal and Secrete digestive

stomach epithelium

enzymes.

Interstitial cells in the tests and adrenal glands

Synthesise and secrete steroids and hormones.

Smooth endoplasmic reticulum

Liver cells Carbohydrate metabolism and hormones.

Goblet cells in the intestinal epithelium and respiratory tract

Secrete mucus Golgi apparatus

Cells in the root cap

Secrete a slimy lubricant that helps the movement of roots between soil particles.

Cell organisationLiving processes in unicellular organisms

1. Unicellular organisms are organisms consist of a single cell.2. Even though they are structurally simple, they are able to

perform all important functions and living processes within a cell.

3. Unicellular organisms can feed, respire, excrete, and move. They are sensitive to external and internal environments (respond to stimuli), and able to reproduce and grow.

4. A unicellular organism survives by depending largely on its structure and cellular components as it does not have any systems.

5. Amoeba sp. and Paramecium sp. are the examples of protozoa (singular, protozoan) of unicellular organisms.

6. Amoeba sp. and Paramecium sp. are capable of performing all the functions and living processes that the specialised cells in a multicellular organism perform.

Amoeba sp.1. Many species live in freshwater lakes and ponds and

are also abundant in damp soil.2. Most species of Amoeba are free-living while others

are parasitic.3. The cytoplasm is enclosed by a plasma membrane.4. It changes its shape constantly as it meets

obstacles and responds to stimuli.5. Ectoplasm is the gel-like part of the outer

cytoplasm while the inner part is called endoplasm.

6. Pseudopodia (singular, pseudopodium) are Amoeba sp. moving and feeding locomotion (ameboid movement).

Comparison between the structure of animal and plant cells

Take Note!

Mitochondria are found in large numbers in the midpiece of a sperm cell.

Similarities in animal and plant cellsBoth have a nucleus, cytoplasm, a plasma membrane, Golgi apparatus, mitochondria, endoplasmic reticulum and ribosomes.

Differences between animal cells and plant cells

Animal Cell Feature Plant Cell

Usually smaller Size Usually bigger

Irregular Shape RegularNone Cell Wall Present

None Chloroplast Present in green plants

Usually none or very small

Vacuole Present

Carbohydrate is stored in the form of glycogen

Food Storage

Carbohydrate is stored in the form of starch

Have centrioles Centrioles Do not have centrioles

7. It moves by cytoplasmic projection, extending its pseudopodia or called ‘false feet’ towards the direction it wants to move.

8. This is followed by the flow of cytoplasm into the pseudopodia.

Amoeba sp.

9. Amoeba sp. practices simple diffusion process where it exchange oxygen and carbon dioxide occurs through the plasma membrane. [SPM ‘05/P1]

10. Amoeba sp. responses to stimuli by moving towards them. It is called favourable stimuli (e.g. food).

11. While responds to adverse stimuli (e.g. bright light, acidic condition) by moving away from them.

12. Amoeba sp. grows by synthesising new cytoplasm.

Feeding

1. Amoeba sp. feeds on microscopic organisms such as bacteria and diatoms.

2. Phagocytosis is the method of engulfing food. Diagram below shows how Ameoba sp. feeds.(a) Amoeba sp. moves towards the food particles.(b) The two pseudopodia extend out and enclose the

food particles.(c) Then, the food particles are packaged in a food

vacuole which fuses with a lysozyme, a lysosome containing a hydrolytic enzyme.

(d) Lysozyme digests the food particles and the resulting nutrients diffuse into the cytoplasm and are assimilated.

(e) The undigested material is left behind as the Ameoba sp. moves away.

Feeding in Amoeba sp.

Excretion

1. Waste products such as carbon dioxide and ammonia are excreted by diffusion.

2. The contractile vacuole is involved in osmoregulation.3. Water diffuses into the cell and fills the contractile

vacuole as the Ameoba sp. lives in freshwater environment.

4. The vacuole contracts to remove its contents from time to time when the vacuole is filled to its maximum size.

Reproduction

1. Amoeba sp. reproduces by asexually where binary fission takes by forming spores.

2. It reproduces in a conducive environment where there is a lot of food.

3. When there is not enough food and the condition is dry, the environment become non-conducive for reproduction. Amoeba sp. forms spores and the spores are germinated when the environment become conducive again.

Binary Fission in Amoeba sp.Paramecium sp.

1. Paramecium sp. found abundantly in freshwater ponds rich in decaying organic matter.

2. It has a fixed slipper-like shape and the surface is covered by numerous rows of cilia.

3. Cilia are the short hair-like outgrowths.4. The cilia are involved in locomotion and feeding.5. Paramecium sp. has two nuclei. The macronucleus and

the micronucleus. The macronucleus controls cellular metabolism of the cell and is also necessary for asexual reproduction while the micronucleus is required for sexual reproduction.

Paramecium sp.

6. Paramecium sp. moves by means of the regular beating of the cilia (singular, cilium).

7. This type of locomotion enables the Paramecium sp. to move forward while rotating and spiralling along its axis.

Locomotion in Paramecium sp.

8. Cilia functions as sensory structures. It responses to stimuli.

9. It is sensitive to chemicals, oxygen and carbon dioxide, and physical stimuli such as light, contact and temperature.

10. Cilia moves in the opposite direction when it comes into contact with a negative stimulus.

11. Paramecium sp. respire through diffusion of gaseous exchange entire the plasma membrane of the organism.

12. Paramecium sp. grows to a certain size by synthesising new cytoplasm.

Feeding

1. Alike Amoeba sp., Paramecium sp. also feeds on microscopic organisms.

2. Nutrition in Pramecium sp.:a) The regular beating of cilia along a mouth- like

opening (oral groove) sweeps water and suspended food particles into cytostome.

b) In cytostome, food vacuoles containing the food particles are formed.

c) The food particles is then digested by hydrolytic enzymes from the cytoplasm.

d) Nutrients from the digested food are dissolved and diffused into the cytoplasm and are used to generate energy.

e) The undigested food is released through the anal pore.

Feeding in Paramecium sp.

Excretion [SPM ‘07/P1] [SPM ‘08/P2]

1. Waste products such as carbon dioxide are excreted from the cell by simple diffusion.

2. Osmoregulation (control of water balance in the cell) is aided by two contractile vacuoles, one at the anterior end and the other at the posterior end.

3. Water constantly enters the cell from the hypotonic environment by osmosis. Therefore, excretion is necessary.

a) Excess water in the cell enters the contractile vacuole by osmosis.

b) The contractile vacuole enlarges.c) The contractile vacuole contracts and eliminates

excess water to the surroundings.4. The contractile vacuoles periodically expand as it is

filled with water, and then contract to eliminate their contents to the exterior of the cell.

Reproduction

1. Paramecium sp. reproduces asexually as well as sexually.

2. Paramecium sp. reproduces rapidly by binary fission in a favourable environment. Sexual reproduction is known as conjugation, takes place when the environment is not favourable.

Binary fission in Paramecium sp.

Cell specialisation in multicellular organisms

1. Organisms that consist of more than other one cell are called multicellular organisms.

2. Zygote, a single cell that humans and other multicellular organisms begin life which result of fertilisation between an ovum and a sperm.

3. The zygote is divided into two identical cells, then divide into four cells, and so on until eventually it forms a ball of cells known as an embryo.

4. These cells grow, change shape and adapt themselves to carry out specific functions. It is called differentiation.

5. Cells differentiate and become more specialised in order to perform specific tasks more efficiently.

6. (a) tissues are the cells which have become specialised to carry out particular tasks.(b) a tissue is a group of cells which are similar in structure and perform a specific function. An example is the muscle tissue.

7. (a) organs are formed from group of different types of tissues.(b) an organ is a group of different tissues that work together to carry out a function.(c) a system is a group of several organs combined together. For examples, digestive system or the respiratory system. An organism is made up of various systems.

8. The different levels of cell organisation is as follows:[SPM ‘07/P1] [SPM ‘08/P2]

The necessity for cell specialisation and cell organisation

1. Cell specification is a process of change and adaptation that a cell undergoes to give its special structures and specific functions.

2. The different types of cells work together to perform the various living processes in the multicellular organism.

3. Organisation is essential to multicellular organism because:(a) A group of cells or tissues are able to carry out

a specific function more efficiently compared to a single cell.

(b) The division of work among cells enables the organisms to carry out different tasks and functions orderly and systematically.

(c) Enables them tp achieve a higher growth rate.(d) Organisms are able to adapt and survive in

diverse habitats and environments.

Nerve cells Have long, thin fibres called axons which carry

nerve impulses throughout the body.

Muscle cells Usually long with multiple nuclei and contain

protein fibres. These fibres can contract to produce movement.

Red blood cells Biconcave discs like in shape and do not have

nuclei. This increases the surface area of the cell and allows oxygen to diffuse into the whole cell at a faster rate.

White blood cells Can change their shape easily to move through the

walls of blood vessels and migrate to the sites of injuries to fight infections.

Sperm cells Sperm cells have a long tail and a high density of

mitochondria which allow them to swim towards the ovum. The nucleus contains one set of chromosomes from the male parent.

Epithelial cells

Simple glands are found in the intestines which are to secrete mucus. The epithelial layer is highly folded with the secretory cells arranged compactly to increase the surface area for mucus secretion.

Cell organisation in humans and animals

Tissues1. Types of tissues in animals:

i. Epithelial tissuesii. Muscle tissues

iii. Connective tissuesiv. Nerve tissues

Epithelial tissues- Consists of one or more layers of cells.- Tightly interconnected, with little space between

them.- They form a continuous layer over body surfaces

(e.g. the skin and mouth area), and inner lining of cavities (digestive tract and lungs).

- Some epithelial tissues also form glands (exocrine and endocrine glands)

- Carry out functions associated with protection, secretion and absorption.

Epithelial tissues Adaptations and functions(a) The epithelial tissues

at the surface of the skin and linings of the mouth and oesophagus

- Forms a protective barrier against infections, mechanical injuries, chemicals and dehydration.

- They also regulate body temperature.

(b) The epithelial tissues at the lining of body cavities, heart, blood vessels and lungs

- Lines the alveoli of the lungs and form the walls of blood capillaries. These cells are thin, flattened and arranged in a single layer.

- This adaptation allows the exchange of gases between the alveoli and the blood in the capillaries to take place efficiently.

(c) The epithelial tissue at the lining of the small intestine

- Absorb nutrients after digestion is completed.

- May undergo modification to form mucus-secreting goblet cells which secrete mucus into the digestive tract.

(d) The epithelial tissue at the lining of glands, ducts and kidney tubules

- Certain epithelial tissues are modified to form glands in the skin, e.g. sweat glands and sebaceous glands.

(e) The epithelial tissue at the lining of the trachea

- Lines the trachea consists of elongated cells with hair-like projections called cilia. They also secrete mucus.

- The mucus traps dust particles while the cilia sweep the impurities away from the lungs

Muscle tissues

- The most abundant tissue in most animals.- Made up of long cells called muscle fibres.- Three types of muscle tissues: smooth muscles,

skeleton muscles and cardiac muscles.

Muscle tissues Functions(a) Smooth muscles- Found along the walls

of the digestive tract, blood vessels, bladder and reproductive tract

- Responsible for the involuntary actions of the body. E.g. smooth muscles contract as food is moved along the digestive tract.

- For the churning action of the stomach and the constriction of the arteries.

- Smooth muscles contract more slowly than skeleton muscles but remain contacted for a longer period of time.

(b) Skeleton muscles- Attached to the bones

of the skeleton

- Voluntary movement of the body.

- Contractions of skeleton muscles produce movements of various body parts.

(c) Cardiac muscles- Form the contractile

wall of the heart

- Contract to pump blood to all parts of the body.

- Contractions of cardiac muscles are involuntary

Nerve tissues- Composed of neurones or nerve cells.- Each neuron consists of a cell body and nerve fibres

called dendrites and axons.- Types of neurons: afferent neurones, efferent

neurones and neurones interneurons.- Neurones are specialised to transmit signal called

nerve impulses over long distances.- Nerve tissues control and coordinate activities of

the body.

Connective tissues [SPM ‘05/P1]- Consists of various types of cells and fibres

separated by an extracellular matrix.- The connective tissue which underlies epithelial

tissues consists of a network of collagen, capillaries and spaces filled with fluid.

- E.g. tendons, ligaments, cartilage, bones, blood, lymph and adipose tissue.

- Connective tissues, with the exception of blood and lymph, are interwoven with fibrous strands called collagen.

Cartilage tissue- Form tendons and ligaments when the collagen

fibres are densely packed.- Tendons attach muscles to bones.- Ligaments attach bones to bones.- Cartilage is a strong yet flexible connective tissue

which provides support to the nose, ears, and covers the ends of bones at joints.

- Forms discs between vertebrae and enables them to act as cushions to absorb pressure.

Bones- Consists of cells located deep in a matrix of collagen

hardened by mineral deposits such as calcium.- This combination makes the bones harder than

cartilage.- Provide protection to organs in the body and

support the body.

Blood - Consists of red and white blood cells and cell fragments

called platelets, suspended in a fluid called blood plasma.- Manufactured in the bone marrow, located at the ends of

long bones. [SPM ‘06/P1]- Blood regulates, transports and protects.- Transports nutrients and oxygen to cells and

removes carbon dioxide and waste products from the cells.

- Helps distribute heat throughout the body and contains regulatory substances such as hormones and enzymes.

- Red blood cells transport oxygen, white blood cells flight infections while platelets aid in blood clotting.

- Lymph consists mostly of fluid which diffuses out of blood capillaries.

Adipose tissue- Called as fat cells which are tightly packed.

They can be found in the dermis of the skin.- Adipose tissue stores energy and insulates the

body.

Organs1. Formed by two or more types of tissues working

together to perform particular functions.2. E.g. heart, connective tissue, nerve tissue and

epithelial tissue.3. If we take heart for example, it consists of cardiac

muscle, connective tissue, nerve tissue and epithelial tissue. There tissues work together to pump blood to all parts of the body.

The skin as an organ1. The skin covers the entire body, and protects it

against infections, physical trauma and water loss.

2. The largest organ of the body.3. Skin is an organ because it consists of various

types of tissues combined together to perform specific functions.

4. Can be divided into two main layers: (a) Epidermis: is the outermost, thinner layer

of the skin .it is mostly made up of dead epithelial cells which continuously slough off.

(b) The dermis is composed of connective tissue, nerve tissue, epithelial tissue and muscle tissue.

The cross section of the skin

1. The epithelial cells at the basal layer of the epidermis undergo continuous cell division.

2. The smooth muscle (hair erector muscle) is attached to the hair follicle. When the smooth muscle contracts, it causes the hair to stand on end.

3. Various nerve endings are scattered all over the dermis and epidermis. These nerve endings are receptors for pressure, temperature, touch and pain. They detect various stimuli and transmit nerve impulses to the nervous system.

4. Connective tissues contain elastic fibres and collagen fibres. The elastic fibres give elasticity to the skin,

allowing the skin to return to its original shape after being stretched.

Arterioles supply blood to the skin through a network of blood capillaries.

Lymphatic vessels collect interstitial fluid within the dermis.

5. Specialised epithelial cells in the skin form glands such as hair follicles which produce hair, sweat glands which secrete sweat and oil glands which secrete sebum. Sebum is an oily substance that lubricates the hair and the skin.

Systems1. Consists of several organs that work together to

carry out a living process such as the digestion of food.

2. A multicellular organism is made up of many systems.

3. An organ may belong to more than one system. E.g. pancreas secretes hormones and pancreatic juice.

4. The pancreas is a part of both the endocrine system and the digestive system.

5. All systems combine to form a multicellular organism. The functions must be coordinated for the survival of the organism. For instance, the nutrients which are absorbed by the digestive system and the oxygen which is obtained by the

respiratory system are transported by the circulatory system to all parts of the body.

Nervous system- The major organs or tissues are brain, spinal cord

and nerve,- Detects stimuli, formulates responses, transmits

nerve impulses and integrates the activities of other systems.

- Controls physiological processes together with the endocrine system.

Skeleton system- The major organs or tissues are bones, cartilage,

tendons and ligaments.- Supports the body, provides sites for attachment of

muscles, and protection for internal organs.

Circulatory system- The major organs or tissues are heart, blood

vessels, and blood.- Delivers nutrients, respiratory gases and hormones

to body cells- Transports waste products to excretory organs.

Digestive system- Major organs: mouth, pharynx, oesophagus,

stomach, liver, pancreas, small and large intestines, rectum, and anus.

- Ingests and digests food, absorbs nutrients for use by the body and eliminates undigested material.

Respiratory system- Major organs: nose, trachea, lungs- Provides a surface area for gaseous exchange

between the blood and the external environment.- Allows oxygen intake and carbon dioxide

elimination.

Excretory system- Major organs: kidneys, ureters, urinary bladder,

urethra, skin, lungs- Removes metabolic waste products such as urea

and carbon dioxide.- Regulates the osmotic balance of the blood.

Muscular system- Major organs or tissues are skeleton muscles,

smooth muscles and cardiac muscles.- Contraction of muscles enables body movements.

Reproductive systems- Major organs or tissues: male: testes, seminal

vesicles, penis- Male: produces sperms and male hormones- Female: ovaries, fallopian tubes, uterus, cervix,

vagina- Female: produces ova, nurtures developing

foetuses and produces female hormones.

Lymphatic system- Major organs: lymphatic vessels, lymph nodes,

bone marrow, thymus- Lymphatic system defends the body against

infections.- It returns excess tissue fluid to the blood.

Endocrine system- Major organs: glands, such as the pituitary, thyroid,

adrenal and pancreas, which secrete hormones.- Main function: Coordinates body activities together

with the nervous system.

Integumentary system- Major organs: skin- Physically separates the body from the external

environment.- Protects the body against mechanical injuries,

microbial infection and dehydration.

Cell organisation in plants1. Plants such as leaves, stems and roots are formed

from two main types of tissues:(a) The meristematic tissues(b) The permanent tissues

Meristematic tissues

1. Consist of small cells which have thin walls, large nuclei, dense cytoplasm and no vacuoles.

2. They consist of young and actively dividing cells which have not undergone differentiation.

3. Apical meristems are located at the tips of roots and the buds of shoots. The tissues undergo mitosis to increase the number of cells for plant growth.

4. Lateral meristems or cambia (singular, cambium) are found in the peripheral areas of stems and roots. They are responsible for secondary growth which makes it wider for support and strength in plants.

Permanent tissues

1. Permanent tissues consist of more mature cells that are either undergoing differentiation or have already undergone differentiation.

2. There are three types of permanent tissues:(a) Epidermal tissue(b) Ground tissue(c) Vascular tissue

3. Palisade mesophyll cells consist of elongated, cylindrical cells, arranged vertically and close to one another. It is situated below the upper epidermis.

4. This allows as much as light possible to reach the chloroplast for photosynthesis. These cells contain a large number of chloroplasts for maximum absorption of sunlight.

5. Xylem vessels have no cytoplasm, hence, water pass freely. The end walls of the cells have tiny holes, and the cells form a continuous vessels or tube which allows water and mineral salts to be transported in the plant.

6. Guard cells specialised epidermal cells at the lower epidermis which are kidney-shaped when turgid. They control the size of the stoma by changing their shape.

7. Root hairs have a long extension which increases the surface area for absorption of minerals and water from the soil.

Tissue Characteristics FunctionsEpidermal tissue [SPM ‘07/P1]

Forms the epidermis, the outermost layer that covers the stems, leaves and roots of young plants. This tissue also covers the flowers, fruits and seeds.

The walls of epidermal cells which are exposed to air are normally covered by a waxy, waterproof coating called the cuticle.

Most epidermal cells are flat and have large vacuoles.

The cuticle on the epidermal tissue(a) Prevents

the loss of excessive water through evaporation

(b) Protects the plant from mechanical injury

(c) Prevents invasion by disease-causing microorganism

In roots, some of the epidermal cells have long projections called root hairs. These specialised surface area for absorption of water and minerals

In leaves, the lower epidermis contains specialised cells, called guard cells, which surround the stomata

Ground tissue

(a) Parenchyma tissue

Parenchyma tissue consists of thin-walled cells which are loosely arranged with spaces between them

These cells are generally

Parenchyma cells store products of photosynthesis such as sugar and starch. E.g. the parenchyma tissue of sweet potatoes is packed with

round and have large vacuoles.

They are the least specialised cells and can be found in all the organs of a plant

starch.

The palisade mesophyll cells and spongy mesophyll cells in leaves are specialised parenchyma cells which contain chloroplasts and carry out photosynthesis

Parenchyma tissue also gives support and shape to plants

(b) Collenchyma tissue

Consists of elongated, polygonal cells with unevenly thickened cell walls

The cell walls are thickened by cellulose and pectin. They are strong and flexible

An important source of support in herbaceous plants, young stems, leaf stalks and petioles

(c) Sclerenchyma tissue

Much more rigid than collenchyma cells because they have cell walls which are uniformly thickened by lignin

Most of them are dead at maturity

Supports and strengthens the plant body and provides protection to the plant

Vascular tissueContinuous throughout the plant, are involved in the transport of substances between the roots and the shoots. There are two types of vascular tissues.

(a) Xylem tissue [SPM ‘06/P1]

Consists of tracheids and xylem vessels. These are long tubes joined together end to end, from the roots right up to the shoots

The cell walls of the xylem are thickened with lignin which prevents food

Xylem conducts water and minerals from the roots to the leaves

It provides support and mechanical strength to the plant

substances from entering the cells. Consequently, the xylem tissue dies upon reachin maturity

When the cytoplasm disintegrates, a hollow tube is left behind

This feature allows water and minerals to flow easily through the xylem vessels

(b) Phloem tissue

Consists of parenchyma cells, sclereids, sieve tubes and companion cells

The conducting cells of phloem are the sieve tubes which have pores at both ends, called sieve plates

Sieve tubes obtain nutrients and energy from the adjacent companion cells

Transports organic substances such as carbohydrates and amino acids from the leaves to storage organs and to growing parts of plants

Organs and systems in plants

1. Leaves, stems, roots and flowers are examples of plant organs.

2. (a) Systems in plants are not as specialised as those in animals. Plants have fewer organs compared to animals.(b) flowering plants (angiosperms) consist of two main systems: the root system and the shoot system.

3. (a) the root system is the absorption system of the plant.(b) the roots are highly branched to absorb water and mineral from the soil and transport them to the main vascular system (xylem).

(c) the roots also anchor the plant to the ground.4. The shoot system consists of organs such as stems,

leaves, buds, flowers and fruits.5. The stems and branches act as a support system for

the plant by holding the leaves in a position that facilitate maximum absorption of sunlight during photosynthesis. They also position the flowers for pollination.

6. (a) the leaves are made up of ground tissue, epidermal tissue, mesophyll tissue and vascular tissue.(b) they form the main photosynthetic system.(c) the photosynthetic system synthesises organic compounds required for plant growth.(d) the products of photosynthesis are transported via the vascular system (phloem) to other parts of plants.

7. The reproductive system of plants consists of flowers which produce fruits and seeds after fertilisation have taken place.

Regulating the internal environment

The internal environment of multicellular organisms [SPM ‘05/P1]

1. The cells of its body actually live within an internal environment although a multicellular organism lives in an eternal environment.

2. Internal environment refers to the conditions that exist within the body of an organism, particularly the composition of the interstitial fluid and blood plasma.

3. The interstitial fluid fills the spaces between the cells.

4. Nutrients and waste substances are exchanged between the interstitial fluid and the blood plasma contained in the blood capillaries.

The internal environment of multicellular organisms

The necessity for maintaining an optimum internal environment

1. In order for cells of the body to function optimally, the physical factors and the chemical factors within the internal the internal environment must be maintained at a relatively constant level even if the conditions outside the cell change.

2. (a) temperature, blood pressure and osmotic pressure are the physical factors affecting the internal environment.(b) salt and sugar contents, and pH are the chemical factors.

3. In mammals and birds, there is a mechanism which regulates the physical factors and chemical factors in the internal environment so that cells can function efficiently. This mechanism is known as homeostasis.

4. Homeostasis is the maintenance of a relatively constant internal environment for the optimum functions of cells.

5. Through the mechanism of homeostasis, the physical factors and chemical factors of the internal environment are kept in a steady state with minimal changes.

6. When the value of a physical factor or chemical factor increases, the homeostatic mechanism will be activated to reduce that value to the normal level.

7. When the value of the physical factor or chemical factor decreases, the homeostatic mechanism will be activated to raise it back to normal.

8. The changes in the external environment and also in the body’s metabolism causes the chemical and physical factors of the internal environment in an animal body are constantly changing.

9. Negative feedback mechanism is the mechanism which governs homeostasis. The control of body temperature at 37 0C is the example of a negative feedback response.

10. In the positive feedback mechanism, the stimulus activates as response that increases the initial change instead of reversing it to the original state.

11. The need for a regulatory system is lesser in plant because

i. Most plant cells are dead at maturity, and therefore, chemical reactions do not take place in these cells.

ii. Plant cells do not lyse in a hypotonic environment because they have cell walls.

iii. Water movement in plants is aided by natural factors such as transpirational pull.

The importance of regulating an optimum internal environment

1. The ability of organisms to maintain a stable internal environment

i. Enables organisms to live in a wider range of habitats.

ii. Enables metabolic activities and physiological processes to continue even though the external environment changes.

iii. Enables living organisms to control the metabolic rate according to their needs.

iv. Ensures a more efficient and optimum metabolism in the cell.

v. Allows enzymes to function at an optimum rate.

The involvement of various systems in maintaining an optimum internal environment

1. The various systems in the body function and interact with one another to maintain a stable internal environment.

Physical factors/chemical factors of the internal environment

Systems involved Types of regulation

Body temperature Integumentary system (skin and sweat glands)

Nervous system Circulatory

system Muscular

system Endocrine

system

Regulate heat loss and heat gain to maintain a stable body temperature of 37 0C.

Oxygen and carbon dioxide levels

Respiratory system

Circulatory system

Nervous system

Regulate the concentrations of oxygen and carbon dioxide in the bloodstream. The circulatory

system transports oxygen from the lungs to the cells. The carbon dioxide produced during respiration diffuses into the bloodstream and is transported back to the lungs.

The changes in the concentration of oxygen and carbon dioxide are detected by the nervous system.

Blood glucose level Endocrine system (pancreas)

Circulatory system

Digestive system (liver)

Regulate blood glucose level at a set point of 90 mg per 100 ml of blood.

Blood osmotic pressure

Nervous system Endocrine

system Excretory

system Circulatory

system

Regulate the amount of water and dissolved substances (mineral salts) in the interstitial fluid and blood.

pH Respiratory system

The pH of blood and interstitial

Circulatory system

Excretory system (kidneys)

fluid is maintained at a value of 7.4 by regulating the concentration of hydrogen ions (H+), hydroxyl ions (OH-) and hydrogen carbonate ions (HCO3

-).

Appreciating the Uniqueness of the Cell

1. A cell is the basic unit of all living organisms and capable of functioning on its own.

2. The cellular components of the cell are completely dependent upon one another for the cell to function at an optimum level.

3. The various cellular components of a cell carry out their functions in an orderly manner to ensure life processes are not disturbed.

4. The cell as a whole will not be able to function properly if any of its cellular components loses its ability to function normally.

5. All the systems of an organism are able to function efficiently because the cellular components of cells work together and cooperate harmoniously with one another.

6. Conclusion, an optimum cellular function is the result of cellular order within the cell. This ensures the survival of the organism as a whole.