1.1The Importance of Having a Transport System in Some

Multicellular Organisms

Exchange of substances

occurs rapidly if…

The surface area of organisms in contact with the environment is

large

The volume of the organism is small

The total surface area/volume ratio of the

organism is large

The distance between the source of the

substances and the body cells is

near

A high concentration

gradient is maintained between the

source and the body cells

Unicellular

organism

• Obtain substances via diffusion through the cell surface

• The volume of the body is very small. The total surface area/volume ratio is very high

• Nutrients and oxygen can diffuse across the cell surface to reach the inside of the cell easily

• Waste products can be rapidly removed from the cell trough simple diffusion

Large

multicellul

ar organism

s diffusi

on alone canno

t ensure a

constant

supply of

oxygen and nutrients to the cell

because

•The total surface area/volume ratio of the organisma is too small

•The distance between the source of substances and the cells of the organisms is too far from the environment for a direct exchange to take place effectively

They overcom

e this

problem by

having a circulator

y system to

carry out

•The distribution of nutrients and oxygen throughout the body•The removal of waste products from the body

Total Surface Area to Volume

Ratio

1.2The Circulatory System

The Circulatory

system

Delivers nutrients and oxygen to cells

Carries waste products away from the cells

Protects the body from infections

blood

•Type of connective tissue made up of plasma,blood vessels and platelets•Blood acts as a medium of transport

heart

•Function as a muscular pump•Circulates the blood troughout the body

Blood vesse

ls

•Consists of arteries,capillaries and veins •Connected to the heart to deliver blood to all body tissues

3 Major Components

Blood- The medium of transport in humans and

animals

Transportation- Oxygen from the lungs to the cells of the whole body, carbon

dioxide from the cells to the lungs-nutrients,hormones and antibodies throughout the body

- waste products away from the cells to the organs of the excretory system

Regulation- the pH of body fluids

- the body temperature- the water content of cells

Protection- from excessive blood loss in an injury through the mechanism of

blood clotting. It helps to heal wounds-from diseases by helping to fight against infections,for example, white blood cells carry out phagocytosis and produces antibodies to

destroy pathogens which enter the body

Human Blood- made up 55 % plasma and 45% cellular components

Cellular Components

Erythrocytes (red blood cells)

Leucocytes(white blood cells)

Platelets

Plasma

Constituents

Water

Ions

Plasma proteins (albumin,fibrinogen,immunoglobulins)

Hormones

Dissolved substances

BLOOD CELLSWHITE BLOOD CELLS

(Leucocytes)• Colourless, nuclei, mitochondria• Irregular in shape• Make up less than 1% of the volume of blood• Most of them larger than red blood cells• To fight infections in various ways• Most activities of leucocytes take place in

the interstitial fluid outside the blood vessels• Can squeeze through the pores in the blood

capillaries• Fight the phatogens present in the interstitial

fluid• Classified as either granular or agranular

• Cell fragments from the bone marrow.• No nucleus.• Involved in blood clotting

RED BLOOD CELLS (Erythrocytes)• Like biconcave disc• Does not have nucleus• Is small• Large surface area to volume ratio for the rapid

diffusion of oxygen across its plasma membrane• An elastic membrane• Enables to squeeze easily into the thiniest blood

capillaries• A erythrocytes contains about 250 million

molecules of haemoglobin• Contains haemoglobin• Haemoglobin is an oxygen carrying protein

pigment which gives the RBC the colour red • Hb contains haem groups which contains iron. It

is the site of oxygen binding.• Each haemoglobin molecules can bind up to

four oxygen molecules.• Lifespan of erythrocyte is 120 days.• Destroyed in the liver and spleen.• Manufactured in the bone marrow

PLATELETS

HUMAN BLOOD VESSELS

Arteries• Blood vessels that carry

blood away from the heart• Transport blood quickly at

high pressure (due to heart’s pumping action).

• To withstand such a high pressure, the walls of the aorta must be thick.

• The muscle tissue enables the artery to constrict and dilate.

• Aorta is the main artery.• The blood pressure in the

human aorta is about 120 mm Hg.

• Arteries branch into smaller vessels called arterioles

   

Capillaries Veins

• Brings back blood to the heart

• Smooth muscle layer in veins are thinner than that in arteries.

• Large lumens and valves that maintain the one-way flow of blood.

• Thin walled vessels (one cell thick)• Allow rapid gaseous exchange between the blood and cells via diffusion.

CHARACTERISTIC ARTERIES CAPILLARIES VEINS

WALL THICK,MUSCULAR,ELASTIC

ONE-CELL THICK,NO MUSCLE/ELASTIC TISSUE

THIN,LESS MUSCULAR,LESS ELASTIC

LUMEN SMALL VERY SMALL LARGE

VALVE NO VALVE NO VALVE HAVE VALVES WHICH MAINTAIN THE ONE WAY FLOW OF BLOOD

BLOOD PRESSURE HIGH LOW VERY LOW

DIRECTION OF BLOOD FLOW

FROM THE HEART TO THE ORGANS

FROM ARTERIES TO VEINS

FROM ALL PARTS OF THE BODY TO THE HEART

BLOOD CONTENT OXYGENATED BLOOD EXCEPT PULMONARY ARTERY

OXYGENATED BLOOD AT THE ARTERIOLE ENDS AND DEOXYGENATED BLOOD AT THE VENULE ENDS

DEOXYGENATED BLOOD EXCEPT THE PULMONARY VEIN

FUNCTION TO TRANSPORT BLOOD QUICKLY AT HIGH PRESSURE FROM THE HEART TO THE TISSUES

ALLOW RAPID GASEOUS EXCHANGE BETWEEN THE BLOOD AND THE BODY CELLS BY DIFFUSION

ALLOW BLOOD FROM THE TISSUES TO RETURN TO THE HEART

Heart is situated between the two lungs in the thoracic cavity

Valves are present to allow blood to flow in

only one direction

The muscular wall of the left ventricle is thicker than the right because

the leftventricle needs to pump blood to all the

parts of the body

Four chambers two upper chambers (atria)

– receive blood returning to the heart

 b) two lower chambers (ventricle) – pump blood out

of the heart.

Semi lunar valves prevent blood from

flowing back into the ventricles when the

ventricle relax

Bicuspid and tricuspid valve

prevent blood from flowing back into

the atria

As blood fills the atria, the atria

contract and push the blood into the

two ventricles.

Oxygenated blood from the lungs enters the left atrium

via the pulmonary veins. Deoxygenated blood from the rest of the body enters

the right atrium via the vena cava

When the ventricles begin to contract, the bicuspid and

tricuspid valves are closed,and blood is pushed out through the semi-lunar valves into the pulmonary

arteries and theaorta.

Deoxygenated blood is pumped to the lungs

through the pulmonary arteries whileoxygenated

blood is pumped through the aorta to the rest of the body.

The first sound lub is caused by the closing of the bicuspid

and tricuspid valves.The second sound dub is

caused by the closing of the semi-lunar valves

How blood pressure is regulated1. Blood pressure:

a) Pressure exerted on the wall of the blood vessel. b) Force that drives blood through the arteries and capillaries.c) Highest in aorta and large arteries during systole (the contraction of ventricles when blood is pumped out of the aorta and pulmonary artery).d) 120 (systolic) /80 (diastolic)mmHge) e)Regulated by negative feedback mechanisms.

 

2. Baroreceptor (arch of aorta) and carotid arteries in the neck detect blood pressure and send impulses to the medulla oblongata (cardiovascular centre).

3. This brings the bp to normal value

BLOOD PRESSURE INCREASE DECREASE

IMPULSE SENT AT FASTER RATE SENT AT A SLOWER RATE

CARDIAC MUSCLE CONTRACTION WEAKER STRONGER

SMOOTH MUSCLE OF ARTERY RELAX CONTRACT

RESISTANCE OF BLOOD FLOW DECREASE INCREASE

BLOOD VESSELS WIDEN (VASODILATION) NARROW (VASOCONSTRICTION)

1.3 THE MECHANISM OF BLOOD CLOTTING

Wound in skin

Platelet gather StickyForms a temporary plug

in leaking vessel

Produce Trombokinase

Trombokinase

Ion Calcium

Prothrombin

Need Vitamin D

ThrombinFibrinogen

Fibrin

Forms the threads of the clot

Later

Harden (scab)

Mechanism of blood clotting

1 •Whenever an injury occurs a chain reaction is set off.

2 •Platelets gather at a site of the injury and become sticky

3 •Forming a temporary plug in the leaking vessel

4 •Prothrombin (non-active enzyme) need ion calcium to convert into thrombin.

5 •Thrombin converts soluble fibrinogen (plasma protein formed by the liver) into insoluble fibrin. Fibrin forms the threads of the clot

6 •A mesh-like network of fibrin traps red blood cells together, forming the blood clot, which later hardens into a scab.

THE LYMPHATIC SYSTEM

Part of the circulatory system & immune system.

A closed system consist of a network of lymphatic vessels, ducts & nodes.

No pump, the fluid (lymph) id squeeze along the lymphatic vessels by pressure associated

with movements of skeletal muscles, intestinal movements & breathing.

Thymus & spleen are important organs of lymphatic system.

FORMATION OF INTERSTITIAL FLUID

• Blood flows from arteries

• capillaries, there is a higher hydrostatic pressure at the arterial end of the capillaries

LYMPHATIC FLUIDOxygenated blood @ arterial end of the capillaries

high pressure

force water & dissolved substances out

capillary wall

interstitial spaces (spaces between cells)

interstitial fluid/ tissue fluid/ extracellular fluid.

Erythrocytes & plasma protein are too large to

pass through

remain in the capillary.

COMPOSITION OF INTERSTITIAL FLUID

Contains water + dissolved

oxygen + amino acids + glucose + fatty acids +

glycerol + vitamins + minerals + hormones.An adult has

~11 litres of interstitial fluid providing the cells of the body with

nutrients & a means of

waste removal.

At the venous end

of the capillary

90% absorbs by blood

consists of water,

minerals & waste

products.

The rest passes back into the BCS as lymph via

the LYMPHATIC SYSTEM.

THE IMPORTANCE OF INTERSTITIAL FLUID

Carries waste

products (CO2,

urea, etc.) into BCS.

Carries dissolved

substances (O2, amino

acids, glucose,

fatty acids, glycerol, vitamins,

minerals & hormones) to cells for

exchange.

THE FATE OF INTERSTITIAL FLUID

10% of interstitial fluid drains into

the lymphatic capillaries forming

lymph.

Move due to the

contraction &

relaxation of skeletal muscles backflow is prevented

by semilunar

valves.

Lymph in the right

lymphatic duct & the thoracic

duct flows into the

right & left subclavian veins of the

BCS, respectively.

THE STRUCTURE OF THE LYMPHATIC SYSTEM

Starts from the minute blind-ended tubes including lacteals (in the ileum) & lymphatic capillaries in other tissues.

Lymphatic capillaries form larger lymphatic vessels lymphatics.

Semilunar valves are present along the lymphatics.

Lymphatics from all parts of the body two large ducts the right lymphatic duct, the thoracic duct.

THE STRUCTURE OF THE LYMPHATIC SYSTEMSituated at

intervals along the lymphatics are the oval-

shaped structures lymph nodes

(neck, armpits & groin). Lymphocytes

are found in the lymph nodes produce

antibodies & play an

important role in the body’s

immune system.

Phagocytes in the lymph

nodes help to remove bacteria.

THE ROLE OF THE LYMPHATIC SYSTEM IN TRANSPORT

• Role of the lymphatic system in transport- carry excessive interstitial fluid back to the bloodstream-helps to maintain the balance of fluid in body-the process is crucial because water, nutrients and other molecules continously leak out of blood capillaries into the surrounding body tissues.

8) If interstitial fluid not return to the circulatory system, will cause-swollen of body tissues ( because too much fluid is retained)-Oedema ( condition of excessive accumulation of interstitial fluid in the spaces between the cells, cause by a blocked lymphatic vessel)

WHY DO WE NEED THE BODY’S DEFENCE

MECHANISM?

Transmitted by air,

contaminated food & water,

animal (vector) &

contaminated needles. Also by contact.

Pathogens are

bacteria, viruses & parasites

To defend the body against

disease-causing

microorganism

(pathogens)

BODY’S DEFENCE MECHANISM

NON-SPECIFIC SPECIFIC

1ST LINE 2ND LINE 3RD LINE

SKIN : sweat, sebum

MUCOUS MEMBRANES : secretion of mucus

Phagocytosis by phagocytes

Antibodies produced by lymphocytes

FIRST LINE of DEFENCEIf there is a cut, the blood clots quickly to prevent blood loss & entry of m/organisms

Tears secreted by tear gland & acidic sebum (sebaceous gland) – contain lysozymes which destroy some bacteria

Mucus (mucous membrane) in nasal cavity & trachea traps dust particles & microbial spores

The cilia (respiratory tract) sweep the trapped particles to the pharynx swallow into stomach (secretes HCl)

SECOND LINE of DEFENCE

The phagocytic white blood cell are attracted by chemicals produced at the sites of infection

and move to these sites.

Engulf & digest the pathogens

The soluble products are absorbed &

assimilated by the phagocytes

May also be destroyed by

toxins produced by the pathogens

Number of leucocytes increases to try to

destroy the pathogen & neutralise the toxin

THIRD LINE of MECHANISMIn higher group of animals have a more specific immune defence mechanism against pathogen IMMUNE SYSTEM

IMMUNE RESPONSE – interaction between antibody & antigen which result in the antigen being eliminated from the body

MECHANISM TO DESTROY ANTIGENSAGGLUTINATION : •the clumping of antigens easy targets for phagocytes to destroy

NEUTRALISATION •: toxin are made non-toxic by reaction with the antibodies

LYSIS : •lysin (antibodies) bind to antigens cause antigens/pathogens to rupture

OPSONISATION : •The binding of antibodies to antigens stimulate phagocytes (macrophage) to destroy the antigens.

•The state in which the body is resistant to infection by a disease-causing pathogens @ the ability of an animal or plant to resist infection by pathogens

IMMUNITY :

•: The process of inducing immunity by administering a vaccine

IMMUNISATION

•: A preparation of weakened, dead or non-virulent forms of a pathogen

VACCINE

Defence system

specificNon specific

1st line 2nd line3rd line

•Skin

•mucous membrane

Phagocyte

Phagocytosis

lymphocyte antibody

passive active

naturalartificial natural artificial

immunisation

immunity

Divided into

Divided into is

throughproduce

gives

Divided into

Divided into Divided into

are

Carry out

eg

Various types of immunity

ACTIVE IMMUNITY : when an individual’s own immune system produces its own antibodies to defend against specific antigens

Natural Active Immunity : after recovered from certain disease (examples : mumps, measles & chicken pox)

Artificial Active Immunity : can be established upon immunisation or vaccination. (mumps, rubella, measles & poliomyelitis)

Various types of immunity

PASSIVE IMMUNITY : when an individual is given the antibodies required to defend against the pathogen

Natural Passive Immunity : when antibodies produced by the mother are passed across the placenta to the foetus during development or in early infancy through

breast milk (breast feeding). Colostrum (1st formed milk) – rich in antibodies

Artificial Passive Immunity : ready-made antibody or serum is injected into the individual. Prepared from cows or horses. (rabies, hepatitis, tetanus & snakebites)

• 1.6 - APPRECIATING A HEALTHY • CARDIOVASCULAR SYSTEM

CARDIOVASCULAR DISEASE

The dysfunctional conditions of the

heart & blood vessels that supply

oxygen to vital organs (brain,

heart). The organs & tissues will die if oxygen does not

arrive.

CD is the biggest killer in Malaysia

Atherosclerosis, hypertension, thrombosis,

embolism, angina, arrhythmia, heart

attack, stroke

RISK FACTOR

S

High levels of blood cholesterol often associated with the diet rich in saturated fats

& cholesterol

Family history

(genetic)

Age – older people are

more at risk

Obesity

Cigarette smoking –

increase the risk of atherosclerosis

& thrombosis

Sedentary lifestyle – poor blood circulation

Suitable Ways to Maintain a Healthy Cardiovascular

System

Proper nutrition

Limit intake of high cholesterol foods (fatty meat, organ meat, lard,

butter, cheese, egg yolk, prawns, squids)

Limit total fat intake : reduce

deep fried & oily foods

Minimise sodium intake : raise blood pressure (table salt, soy sauce, MSG)

Eat more soluble-fibre foods (oats,

barley, legumes)

Eat more fruits & vegetables – rich in antioxidants (vit. A, C & E)

Include omega-3 fatty acids in diet – reduce the risk of cardiovascular diseases (mackerel,

tuna & salmon)

Healthy lifestyle

Do not smoke : nicotine constricts

blood vessels, carbon monoxide

decrease the oxygen intake.

Engage in moderate exercise

Rest & relax : have

sufficient sleep

Maintain a healthy

weight

NECESSITY

STRUCTURE OF YLEM IN RELATION

TO TRANSPORT

It transport water & mineral ions from roots to the upper parts of the plant

It also provide mechanical support

to the plant

In flowering plants, the xylem consists of

xylem vessels, tracheids,

parenchyma & fibres.The vessels are

elongated cells arranged end to end.

To allow water to flow in a continuous

column.

Large flowering plant @ angiosperms ,

conifer n ferns have a small TSA/V ratio n the substances have

to move a greater distance need vascular system

Unlike animals, plants are unable to

pump necessary substances through

great distances, sometimes requiring

the need to defy gravity.

To overcome, plants use a combination of

root pressure, capillary action n transpiration to provide enough

force to transport water to shoot

Vascular Tissue in Stem, Root & Leaves

• XYLEM – transport water & mineral salts

• PHLOEM – transport organic substances

• Vascular system is not involved in the transport of oxygen & carbon dioxide.

The lignified walls make the xylem vessels rigid to prevent them from collapsing under the large tension forces set up by the transpiration pull.

Tracheids do not have open ends to form a continuous hollow tube to pass water from cell to cell through pits.

STRUCTURE OF PHLOEM IN RELATION TO TRANSPORT

• Transports organic food substances (sucrose & amino acids) from leaves to various plant parts

• Consists mainly of sieve tubes & companion cells

• A sieve tube is a cylindrical tube made up of elongated living sieve tube cells.

• The cross-walls separating the sieve tube cells are perforated by small pores. The cross-walls with the pores look like a sieve & are called sieve plates.

• There are cytoplasmic connections between the sieve tube cells through the sieve pores. (allow the flow of dissolved food materials from one sieve tube cell to the next).

• Mature sieve tubes many cell organelles including nucleus degenerate.

• A thin layer of cytoplasm & some mitochondria are found lining the inside of the thin cellulose cell wall less resistance to the rapid flow of nutrient solution through the sieve tube cells.

• Companion cells are only found in flowering plant, not in conifers or ferns. Its adjacent & closely associated with the sieve tube cells.

• Each companion cells has a nucleus, dense cytoplasm & many mitochondria.

• Help to transport manufactured food from leaf cells into the sieve tubes.

• Many mitochondria to generate ATP needed for active transport of sucrose from companion cells into the sieve tubes.

• When a stem of a woody plant is ringed, the bark containing tissues external to the xylem (including phloem tissue) is removed

• Nutrient solution containing organic substances is prevented from being transported to the roots.

• Food materials would accumulate in the outer stem, above the ringed region of the plant.

• In early stage, no wilting because xylem vessels can still transport water & minerals from the roots to the upper parts of the plant.

• The plant eventually wilt & die because food synthesised in the leaves cannot be transported to the root cells can no longer absorb water & mineral salts from the soil solution.

TRANSLOCATION• The transport of dissolved

organic food substances by the phloem.

IMPORTANCE OF TRANSLOCATION• Ensures a plant’s survival• Organic food molecules can be

transported :– To the growing regions in the plant for growth &

development– To the plant cells for metabolism– Downwards from the leaves to the storage

organs such as the roots

TRANSPIRATION• The loss of water as water

vapour from a living plant to the atmosphere due to evaporation.

• Most of the water is lost through the stomata of leaves.

• A small amount is lost through the cuticle or through the lenticels in woody stem

• Three main sites of transpiration are :

– STOMATA – 90% of transpiration takes place through minute pores on leaves called stomata

– CUTICLE – 10% of transpiration takes place through the cuticle. The amount of water loss varies with the thickness of the waxy cuticle

– LENTICELS – small pores in the stem which allow gaseous exchange. Only small amounts of water are lost through the lenticels.

• Transpiration occurs mainly during the day when the stomata are opened.

• Heat from the sun causes the water to evaporate from the walls of the spongy mesophyll cells.

• The water vapour accumulates in the substomatal air chambers before diffusing from the plant through the open stomata

• Water move into the mesophyll cells by osmosis from the adjacent cell

• Water flows from the xylem vessels along a chain of cells to the outermost mesophyll cell.

• Water is pulled through the xylem vessels from the root to the leaves.

THE IMPORTANCE

OF TRANSPIRATI

ON

Creates a transpirational pull that pulls

water & dissolved mineral ions from

the root to the leaves.

Water supplied to the plant is used

for photosynthesis & cell

metabolism. Mineral ions is used for cell activities & growth &

development of the plant.

Water keeps the cell turgid give support to leaves & young stems

(herbaceous plants).

To keep the plant cool, an important effect particularly in hot condition.

Water evaporates from the plants to the atmosphere faster than it can be absorbed from

the soil, it is withdrawn from the plant cells flaccid – leading

to wilting

PATHWAY OF WATER FROM THE SOIL TO THE LEAVES

Water & mineral ions are absorbed by young roots with thin epidermis. (osmotic forces + active transport).The pressure of root hairs increases the surface area for absorption.

In the cells of the endodermis, mineral ions are actively pumped from these cells into the solution in the xylem vessels in the root. Solution in xylem more concentrated water enters xylem by osmosis.

GUTTATION• In small plants, root pressure can

push water all the way up the stem & out of special pores called hydathodes at the edges of leaves Guttation

• Occurs on cool humid morning when the air is too saturated for the water drops to evaporate from the leaves.

THE EXTERNAL CONDITIONS AFFECTING THE RATE OF TRANSPIRATION

• The external factors that affect the rate of evaporation.–Air movement–Temperature–Light intensity–Relative humidity

AIR MOVEMENT

Movement of air carries

away water molecules that have diffused

from the leaves.

The faster the air movement (wind speed)

the greater the rate of

transpiration.

When there is little air

movement/ in still air, water

vapour diffusing out of

the leaf accumulates around the stomata.

Decreases the concentration

gradient of water, the

transpiration rate reduces.

TEMPERATURE

A rise in temperature increases the

kinetic energy of water molecules & increase the

rate of evaporation

(transpiration).

Lowers the relative

humidity of the air increase

the rate of transpiration.

Temperature low, kinetic

energy of water molecules is decreased.

Relative humidity of the air is higher decrease the

rate of transpiration

LIGHT INTENSITY

The light intensity

increased, the rate of evaporatio

n increases.

Light not effects

evaporation but causes

the stomata to open increasing water loss from the

plant.

At night @ dark,

stomata close rate

of transpiration

decreases

RELATIVE HUMIDITY

High relative

humidity reduces

water loss.

The lower the relative humidity

of the surrounding atmosphere, the

faster will the water vapour

escape from the stomata.

The rate of evaporation depends on the difference in

concentration of water molecules in the sub-stomatal spaces & in the

surrounding atmosphere.

ROOT PRESSUREIf a stem is cut just above ground level, a considerable amount of sap will exude from the cut stump, showing that there is a force pushing water up the stem from the roots.

Its ROOT PRESSURE The result of an active process in which energy from ATP is used.

Root pressure alone is not strong enough to drive water to the tops of tall trees at the speeds achieved during a normal growing season.

COHESION & ADHESION OF WATER

COHESION = the force of attraction

between the same molecules

The cohesive forces between water molecules hold the

continuous columns of water together & when water

transpires from the leaf, the whole of the water column

moves up the xylem vessels.

ADHESION = the force of attraction between different

molecules

Water molecules tend to adhere to the walls of the xylem vessels.

The adhesive forces can support

a considerable mass of water.

Cohesion + adhesion = capillarity (the rise

of water in a capillary tube).

OPENING & CLOSING OF STOMATADicotyledonous leaves = on the lower surface

M

o

n

o

c

o

t

y

l

e

d

o

n

o

u

s

l

e

a

v

e

s

=

b

o

t

h

t

h

e

u

p

p

e

r

&

l

o

w

e

r

s

u

rf

a

c

e

s

.

Each stoma consists of a pair of kidney-shaped guard cells surrounding a stomatal pore.

The opening & closing of the stomata in the leaf controls the diffusion of CO2 & O2 into & out of a leaf as well as the rate of transpiration.

STOMATAL OPENINGDAY : photosynthesis occurs & CO2 level is low in the leaf.Potassium ions (K+) diffuse from surrounding epidermal subsidiary cells into the guard cells.

In many plant species, starch stored in the guard cell is converted into malate.

The accumulation of K+ & malate ions increase the osmotic pressure in the guard cell.

Water enters by osmosis from surrounding epidermal cells into the guard cells

Two guard cells become turgid & curve outward & the stoma opens.

STOMATAL CLOSINGNIGHT : no photosynthesis & CO2 level is high. Potassium ions move out of the guard cells into the epidermal cells.

Malate ions are converted to starch. The osmotic pressure in the guard cells decreases.

The stomata also close, when there is insufficient water supply & the transpiration rate is high.