Cell organisation

Living processes in unicellular organisms

• Unicellular organisms (organisms which consist a single cell).

• They are structurally simple, but able to perform all vital functions and living processes within a cell.

• They can feed, respire, excrete, move, sensitive to external and internal conditions (respond to stimuli), able to reproduce and grow.

• they does not have any systems, so the survival depends largely on their structure and cellular components.

• Example of unicellular organisms: protozoa (Amoeba sp and Paramecium sp)

Amoeba sp

Habitat • live in freshwater lakes, ponds and damp soil• Most are free living, but some are parasitic.General characteristics• Enclosed by a plasma membrane• Changes its shape constantly as it meets obstacles and

responds to stimuli.• Ectoplasm (the gel-like outer part of the cytoplasm),

endoplasm (the inner part of the cytoplasm)• Moves and feeds by using pseudopodia.

Living processes1. Locomotion• Moves by cytoplasmic projection (extending

pseudopodia or ‘false feet’ and anchoring the tips to the ground)

• Followed by the flow of cytoplasm into the pseudopodia.

• This form of locomotion is known as ameboid movement.

2. Feeding • Feed on microscopic organisms such as bacteria

and diatoms.• Engulfs food by phagocytosis.• Nutrition in Amoeba sp:a) Amoeba sp approaches the food particles.b) Two pseudopodia extend out and enclose the

food particles.c) The food particles are packaged in a food

vacuole, which fuses with a lysosome containing hydrolytic enzymes called lysozyme.

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

(e) Undigested material is left behind when the Amoeba sp moves away.

3. Respiration • Exchange of oxygen and carbon dioxide occurs

through the plasma membrane by diffusion.

4. Excretion • Waste products such as carbon dioxide and

ammonia are eliminated by diffusion.• The contractile vacuole is involved in

osmoregulation.• As Amoeba sp lives in freshwater environments ,

water diffuses into the cell and fills the contractile vacuole.

• When the vacuole is filled to its maximum size, it contracts to expel its contents from time to time.

5. Responses to stimuli• reacts by retreating from adverse stimuli such as

bright light and acidic solutions.• Favorable stimuli such as contact with food,

cause it to move toward the stimuli. 6. Reproduction • Reproduces asexually by binary fission and by

forming spores.• When food is abundant, Amoeba sp reproduces

by binary fission.• When food is scarce and the condition is dry,

Amoeba sp forms spores.

Binary fission in Amoeba sp• Amoeba sp divides after it has grown to a certain

size.• The pseudopodia are pulled in and the nucleus

divides.• The cytoplasm begin to divide when the nucleus

has split.• Two daughter amoebae are formed.7. Growth• Grow by synthesizing new cytoplasm.

Paramecium sp

Habitat• Found abundantly in freshwater ponds rich in

decaying organic matter.General characteristics • The surface is covered by numerous rows of cilia

(short hair-like outgrowths which cover the entire outer surface of the Paramecium sp).

• cilia are involved in locomotion and feeding.

Living processes1. Locomotion • Moves by means of the rhythmic beating of the

cilia.• This action enable the Paramecium sp to move

forward while rotating and spiralling along its axis.

2. Feeding • Feed on microscopic organisms.

• Nutrition in Paramecium sp:a) the rhythmic beating of cilia along a mouth-like

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

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

c) The food particles in the food vacuoles are digested by hydrolytic enzymes from the cytoplasm.

d) Nutrients from the digested food then dissolve and diffuse into the cytoplasm and are used to generate energy.

e) The undigested contents are released through the anal pore.

3. Respiration • Gaseous exchange occurs by diffusion through

the entire plasma membrane.

4. Excretion • Waste products are expelled from the cell by

diffusion.• Control of water balance in the cell

(osmoregulation) is aided by two contractile vacuoles (one at the anterior and the other at the posterior end).

• This is necessary because Paramecium sp constantly takes in water from the hypotonic environment by osmosis.

• Contractile vacuoles periodically expand, filling with water, and then contract to expel their contents to the exterior of the cell.

5. Responses to stimuli• Has cilia which function as sensory structures.• It is sensitive to chemicals, oxygen, carbon

dioxide, pH and physical stimuli such as light, contact and temperature.

• Moves in the opposite direction when it comes into contact with adverse stimuli.

6. Reproduction • Reproduce asexually as well as sexually.

• Has two nuclei. macronucleus control cellular metabolism of the cell and for asexual reproduction (binary fission, occur in favorable environment). micronucleus is required for sexual reproduction (conjugation, occur in unfavorable environment).

• Binary fission in Paramecium sp: Macronucleus and micronucleus divide. The cytoplasm divides when the nuclei have

split. Two daughter Paramecium sp are formed.

7. Growth• Grow to a certain size by synthesizing new

cytoplasm.

Cell specialisation in multicellular organisms

• Multicellular organisms (organisms that consist more than one cell).

• Cell specialisation (a process of change and adaptation that a cell undergoes to give it special structures and specific functions).

• Human and other multicellular organisms begin life as a single cell known as a zygote (result of fertilisation between an ovum and a sperm.

• Then, zygote divides into two identical cells divide to become four cells, and so on until it forms a ball of cells (embryo).

• These cells grow, change shape and adapt themselves to carry out specific functions.

• They differentiate and become specialised in order to perform specific tasks more efficiently.

• The different levels of cell organisation as below:A group of cells (similar in structure, perform a

specialised function) tissue.Different types of tissues organs.

Several organs system.All systems multicellular organism.

• Without cell specialisation, organism will not be able to survive itself, given the complexity and the millions of cells found in its body.

• Organisation into cells, tissues, organs and systems is essential to multicellular organisms because:

a group of cells or tissues are able to carry out a specific function more efficiently compared to a single cell.

the division of labour among cells enables the organisms to carry out the various tasks and functions in an orderly and systematic manner.

this enables them to achieve a higher growth rate.

organism are able to adapt and survive in diverse habitats and environments.

all living processes can be carried out more efficiently.

A few types of human cells

1. Nerve cells• Have long, thin fibres called axons which

conduct nerve impulses throughout the body.2. Muscle cells• Are usually long with multiple nuclei and

contain protein fibres. These fibres can contract to produce movement.

3. Red blood cells• Are shaped like a biconcave disc and lack a

nucleus. This increase their efficiency in transporting oxygen.

4. White blood cells• Can change their shape easily to move through

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

5. Sperm cells• Have long tails 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.

6. Epithelial cells with simple glands• Found in the intestines.• Function: secrete mucus• The epithelial layer is highly folded with the

secretory cells arranged compactly to increase the surface area for mucus secretion.

tissues

• Four major types of tissues in animals:1. Epithelial tissues Consist of one or more layers of cells. The cells of an epithelium are tightly

interconnected, with little space between them. They form a continuous layer over body surfaces

(skin and mouth area), inner lining of cavities (digestive tract and lungs), and also form glands (exocrine and endocrine glands).

Functions:a) Epithelial cells carry out functions associated

with protection, secretion and absorption.

b) At surface of the skin, act as a protective barrier against infections, mechanical injuries, chemicals, dehydration and regulate body temperature.

c) The epithelial cells (alveoli of the lungs and the walls of blood capillaries) are thin, flattened and arranged in a single layer (allow the exchange of gases between the alveoli and the blood in the capillaries to take place efficiently).

d) In the lining of the small intestine, absorb nutrients after digestion is completed.

e) Undergo modification to form mucus-secreting goblet cells which secrete mucus into the digestive tract.

f) Certain epithelial tissues are modified to form glands in the skin (sweat glands and sebaceous/oil-secreting glands).

g) The epithelial tissues that lines the trachea consist of elongated cells with hair-like projections called cilia. They secrete mucus (trap dust particles while the cilia sweep the impurities away from the lungs).

2. Muscle tissuesThe most abundant tissues in most animals.Composed of long cells called muscle fibres.Three types of muscle tissues:a) Smooth muscles (along the walls of the

digestive tract, blood vessels, bladder and reproductive tract).

b) Skeletal muscles (attached to the bones of the skeletal).

c) Cardiac muscles (form the contractile wall of the heart).

Function:a) Smooth muscles • responsible for the involuntary actions of the

body. Example: when the smooth muscles of the intestine contract, food is moved along the digestive tract.

• Responsible for the churning action of the stomach and the constriction of arteries.

• Contract more slowly than skeletal muscles but remain contracted for a longer period of time.

b) Skeletal muscles• Responsible for the voluntary movements of the

body. Contractions of skeletal muscles produce movements of various body parts.

c) Cardiac muscles• Contract to pump blood to all parts of the body.

Contractions are involuntary.

3. Nerve tissuesComposed of neurones, or nerve cells.Each neurone consists of a cell body and nerve

fibres called dendrites and axons.

Three types of neurones:a) Afferent neuronesb) Efferent neuronesc) Interneurones

Function: a) Neurones are specialised to transmit signals

called nerve impulses over long distances.

b) Nerve tissues control and coordinate all activities of the body.

4. Connective tissuesConsist of various types of cells and fibres

separated by an extracellular matrix.The connective tissues which underlines

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

Examples: 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.

tendons and ligaments: formed when the collagen fibres are densely packed.

cartilage: strong yet flexible connective tissues.

Bones: consist of cells embedded in a matrix of collagen hardened by mineral deposits such as calcium. This combination makes the bones harder than cartilage.

Blood: consists of red and white blood cells and cell fragments called platelets, suspended in a fluid called blood plasma. Blood cells are manufactured in the bone marrow, located at the ends of long bones.

Lymph: consists mostly of fluid which leaks out of blood capillaries.

Adipose tissues (formed by fat cells): tightly packed. Found in the dermis of the skin.

Functions:a) Bind structures together, provide support and

protection to other tissues.b) Tendon: attach muscles to bones.c) Ligaments: attach bones to bones.d) Cartilage: provides support to nose, ears, and

the ends of bones at joints. Also forms discs between the vertebrae. This enables them to act as cushions to absorb pressure.

e) Bones: provide protection to organs in the body and supports the body.

f) blood: has regulating, transporting and protective functions. transports nutrients and oxygen to cells and removes carbon dioxide and waste products from the cells. Distribute heat throughout the body and contains regulatory substances (hormones and enzymes). Red blood cells transport oxygen, white blood cells assist in fighting infections, platelets aid in blood clotting.

g) Adipose tissues: store energy and insulate the body.

Organs

• Formed by two or more types of tissues (not individual tissues) working together to perform particular functions.

• Examples: heart, skin, lungs, kidneys, eyes and ears.

• The heart consists of cardiac muscle, connective tissue, nerve tissue and epithelial tissues. These tissues work together to pump blood to all parts of the body.

Skin

• Covers the entire body, and acts as a barrier against in infection, physical trauma and water loss.

• The largest organ of the body.• Is organ because it consists of various types of

tissues combined together to perform specific functions.

• Divided into two main layers (epidermis and dermis).

• Epidermis: the outermost, thinner layer of the skin. Mostly made up of dead epithelial cells which continually slough off.

epithelial cells at the basal layer of the

epidermis undergo continuous cell division.

• Dermis: composed of connective tissue, nerve tissue, epithelial tissue and muscle tissue.

Connective tissue contain elastic fibres and collagen fibres. Elastic fibres give elasticity to the skin, allow the skin to return to its original shape after being stretched.

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.

Epithelial cells: form glands such as hair follicles (produce hair), sweat glands (secrete sweat), oil glands (secrete sebum). Sebum is an oily substance that lubricates the hair and the skin.

Smooth muscle (hair erector muscle) is attached to the hair follicle. when contracted, it causes the hair to stand on end.

systems

• Each system consists of several organs that cooperate to carry out a living process (digestion of food).

• 11 major systems in humans:1. Nervous system Major organs: brain, spinal cord, nerves

Main functions: detects stimuli, formulates responses, transmits nerve impulses and integrates the activities of systems. Controls physiological processes in conjunction with the endocrine system.

2. Skeletal systemMajor organs: bones, cartilage, tendons,

ligamentsMajor functions: supports the body, provides

sites for attachment of muscles, and protection for internal organs.

3. Circulatory systemMajor organs: heart, blood vessels, bloodMain functions: delivers nutrients, respiratory

gases and hormones to body cells. Transports waste products to excretory organs.

4. Digestive systemMajor organs: mouth, pharynx, esophagus,

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

Main functions: ingests and digests food, absorbs nutrients for use by the body and eliminates undigested material.

5. Respiratory systemMajor organs: nose, trachea, lungsMain functions: provides a surface area for gases

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

6. Excretory systemMajor organs: kidneys, ureters, urinary bladder,

urethra, skin, lungs.Main functions: removes metabolic waste

products (urea and carbon dioxide). Regulates the osmotic balance of the blood.

7. Reproductive systemMajor organs: male (testes, seminal vesicles,

penis), female (ovaries, Fallopian tubes, uterus, cervix, vagina)

Main functions: male (produces sperms and male hormones), female (produces ova, nurtures developing foetuses, and produces female hormones.

8. Muscular systemMajor organs: skeletal muscles, smooth muscles

and cardiac musclesMain functions: contraction of muscles enables

body movements.9. Integumentary system Major organs: skinMain functions: physically separates the body

from the external environment. Protects the body against mechanical injuries, microbial infection and dehydration.

10. Endocrine systemMajor organs: glands (pituitary, thyroid, adrenal

and pancreas) secrete hormones.Main functions: coordinates body activities in

conjunction with the nervous system.11. Lymphatic systemMajor organs: lymph vessels, lymph nodes, bone

marrow, thymus.Main functions: defends the body against

infections. Returns tissues fluid to the blood.

• An organ may belong to more than one system. Example: pancreas secretes hormones and pancreatic juice. Pancreas is a part of the endocrine system and the digestive system.

• All systems combine to form a multicellular organism.

Cell organisation in plants

• Plant cells also undergo cell specialisation.• plant cells undergo differentiation and become

specialised in structure and function:1. Mesophyll palisade cells Consist of elongated, cylindrical cells, arranged

vertically and close to one another, situated just below the upper epidermis. This allow as much light as possible to reach the chloroplasts for photosynthesis.

Contain a large number of chloroplasts for maximum absorption of sunlight.

2. Xylem vesselsNo cytoplasm, hence, water can pass freely.The end walls of the cells are perforated, and the

cells form a continuous vessel or tube which allows water and mineral salts to be transported in the plant.

3. Guard cellsSpecialised epidermal cells at the lower

epidermis which are kidney-shaped when turgid.Control the size of the stoma by changing their

shape.

4. Root hairsHave a long extension which increases the

surface area for absorption of minerals and water from the soil.

• 2 main types of tissues (meristematic tissues and permanent tissues) form plant organs (leaves, stems and roots).

1. Meristematic tissuesConsists of small cells which have thin walls,

large nuclei, dense cytoplasm and no vacuoles.Comprise young and actively dividing cells which

have not undergone differentiation.

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

Lateral meristems/cambia are found in the peripheral areas of stems and roots. They are responsible for secondary growth which adds girth to the plants.

2. Permanent tissuesConsist of more mature cells that are either

undergoing differentiation or have already undergone differentiation.

3 types of permanent tissues:a) Epidermal tissue Characteristicsi. Forms the epidermis (the outermost layer that

covers the stems, leaves and roots of young plants). This tissues also covers the flowers, fruits and seeds.

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

iii. Most are flat and have large vacuoles.

Functions i. The cuticle helps prevent the loss of excessive

water through evaporation, protects the plant from mechanical injury and prevents invasion by disease-causing microorganisms.

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

iii. In leaves, the lower epidermis contains specialised cells (guard cells) which surround the stomata.

b) Ground tissueForms the bulk of a plant.3 types of ground tissues:

1) Parenchyma tissue Characteristicsi. Consists of thin-walled cells which are loosely

arranged with spaces between them. ii. cells have a large vacuole.iii. The least specialised cells and can be found in

all the organs of a plant.

Functionsi. Parenchyma cells store products of

photosynthesis (sugar and starch). ii. The palisade mesophyll cells and spongy

mesophyll cells in leaves are specialised parenchyma cells which contain chloroplasts and carry out photosynthesis.

iii. Gives support and shape to plants.

2) Collenchyma tissue Characteristicsi. Consists of elongated, polygonal cells with

unevenly thickened cell walls (thickened by cellulose and pectin. Strong and flexible).

Functionsi. an important source of support in herbaceous

plants, young stems, leaf stalks and petioles.

3) Sclerenchyma tissue Characteristicsi. Sclerenchyma cells are much more rigid than

collenchyma cells because they have cell walls which are uniformly thickened by lignin.

ii. Most of the cells are dead at maturity. Functionsi. Supports and strengthens the plant body and

provides protection to the plant.

c) Vascular tissueVascular tissues (are continuous throughout the

plant) are involved in the transport of substances between the roots and shoots.

2 types of vascular tissues:1) Xylem tissue characteristicsi. Xylem consists of tracheids and xylem vessels.

these are long tubes joined together end to end, stretching from the roots right up to the shoots.

ii. The cell walls of the xylem are thickened with lignin which prevents food substances from entering the cells.

iii. Dies upon reaching maturity.iv. When the cytoplasm disintegrates, a hollow tube

is left behind. This allows water and minerals to flow easily through the xylem vessels.

Functionsi. Xylem conducts water and minerals from the

roots to the leaves.ii. It provides support and mechanical strength to

the plant.

2) Phloem tissue Characteristicsi. Consists of parenchyma cells, sclereids, sieve

tubes and companion cells.ii. The conducting cells of phloem are the sieve

tubes (have pores at both ends, called sieve plates. Obtain nutrients and energy from the adjacent companion cells).

Functionsi. Phloem transports organic nutrients

(carbohydrates and amino acids) downwards from the leaves to storage organs and growing parts of plants.

Organs and systems in plants

• Plant organs: leaves, stems, roots and flowers.• Systems in plants are not specialised as those of

animals. Plants have fewer organs as compared to animals.

• Flowering plants (angiosperms) consist of 2 main systems:

1. Root system Is the absorptive system of a plant.The roots are highly branched to absorb water

and minerals from the soil and transport them to the main vascular system (xylem).

the roots anchor the plant to the ground.

2. Shoot systemConsists of organs (stems, leaves, buds, flowers

and fruits).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.

Leaves are made of ground tissue, epidermal tissue, mesophyll tissue and vascular tissue.

They form the main photosynthetic system (synthesises organic compounds required for plant growth).

The products of photosynthesis are transported via the vascular system (phloem) to other parts of plants.

Reproductive system of plants consists of flowers which produce fruits after fertilisation has taken place.

The internal environment of multicellular organisms

• Although multicellular organism lives in external environment, the cells of its body live within internal environment (consists of the interstitial fluid and blood plasma).

• interstitial fluid fills the spaces between the cells and constantly bathes the cells.

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

The necessity for maintaining optimal internal environment

• In order for cells of the body to function optimally, the physical factors and the chemical composition within the internal environment must be maintained at relatively constant level regardless of the conditions outside the cells.

• Physical factors: temperature, blood pressure, osmotic.

• Chemical factors: salt and sugar contents, pH.

• Homeostasis is the maintenance of a relatively constant internal environment for the optimal functions of cells.

• In humans and animals, mechanism of homeostasis maintain the physical factors and chemical factors of the internal environment at steady state with minimal fluctuations.

In homeostatic control mechanism, the change in internal environment

detectedreceptor

send messagecontrol centre

processes information effector (bring about a counter response that reverse the

change back to original condition)

• in plants, the need for regulatory system is lesser because:

a) Most plant cells are dead at functional maturity, so chemical reactions do not take place in these cells.

b) Plant cells are not bathed in fluid but surrounded by air.

c) Plant cells do not lyse in hypotonic environment because they have cell walls.

d) Water movement in plants is aided by natural factors (transpirational pull).

The importance of regulating optimal internal environment

• Ensure optimal cellular activities at times when the external environment (temperature of environment) changes.

• Enable organisms to live in a wider range of habitats.

• Metabolic activities and physiological processes can still continue even though the external environment fluctuates substantially.

• Organisms can control the metabolic rate according to their needs because any internal fluctuations can be corrected through the homeostatic control system.

• Ensure a more efficient and optimal metabolism in the cell.

• A stable internal environment allows enzymes to function at optimal rate.

The involvement of various system in maintaining optimal internal environment

• Physical factors/chemical factors of the internal environment:

1. Body temperature Systems involved Integumentary system (skin and sweat glands). Nervous system Circulatory system Muscular system Endocrine system

Types of regulationRegulate the balance of heat loss and heat gain

to maintain stable body temperature of 37oC.When the temperature of the body rises above

the normal value,Thermoreceptors in the skin detect the change in

temperature.The nervous system conveys the information to

the hypothalamus.Then, the information is conveyed to effectors.

The effectors involved are:a) Sweat glands: secrete more sweat to cool the

body through evaporation.b) Blood vessels: dilate to increase heat loss to the

environment.c) Hair erector muscles: relax so as to flatten hair

to reduce insulation.d) Thyroid gland: less stimulated, the secretion of

thyroxine is reduced and this result in a lower basal metabolic rate.

Overall result: lowering of the body temperature to the normal level.

On cold day, the opposite mechanism occur.

2. Oxygen and carbon dioxide levels Systems involvedRespiratory system Circulatory systemNervous system Types of regulationRegulate the concentration of oxygen and

carbon dioxide in the bloodstream.

circulatory system transports oxygen from the lung to the cells. Carbon dioxide produced during respiration diffuse into the bloodstream and is transported back to the lungs.

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

3. Blood glucose level Systems involvedEndocrine system (pancreas)Circulatory systemDigestive system (liver) Types of regulationRegulate blood glucose level at a set point of 90

mg per 100 cm3.

Pancreas secrete insulin when the level of glucose in the blood rises above the set point.

Circulatory system transport insulin to the liver.Insulin stimulate liver cells to convert glucose

into glycogen. Blood glucose level drops.When the glucose level falls below the set point,

pancreas secrete glucagon (promote the breakdown of glycogen and the release of glucose into the blood).

4. Blood osmotic pressure Systems involvedNervous system Endocrine systemExcretory systemCirculatory system Types of regulationRegulate the amount of water and dissolved

substances (mineral salts) in the tissue fluid and blood.

5. pH Systems involvedRespiratory systemCirculatory systemExcretory system (kidneys) Types of regulationpH of blood and tissue 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

• Cell: the basic unit of all living organisms and is capable of functioning on its own.

• Cellular components of the cell are completely dependent upon one another for the cell to function at optimal level.

• The cell as a whole will not be able to function properly if any of its cellular components loses its ability to function normally. Optimal cellular function is the result of cellular order within the cell.

• Systems of organisms are able to function efficiently because the cellular components of cells work together and cooperate harmoniously with each other.

• This ensure the survival of the organism.