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Multicellular Organisms – Facts to Learn for National 5 Biology
A. Cells, tissues and organs
1. Cells of one type combine together to form tissues. E.g. muscle cells combine together to
form muscle tissue. Different tissues combine together to form organs. E.g. Muscle tissue,
nerve tissue and blood tissue are all found in the heart. Different organs combine together to
form systems. E.g. the heart combines with arteries, veins and capillaries to form the
circulatory system of the body.
2. Specialisation of cells leads to the formation of a variety of tissues and organs in both plants
and animals. The cells of organs are specialised for different functions. E.g. muscle cells
allow the heart to contract while red blood cells carry oxygen to the contracting muscle
cells.
B. Stem cells and meristems
3. Stem cells in animals can divide and have the potential to develop into different types of
cells. Stem cells are involved in growth and repair.
4. Embryo stem cells have the potential to develop into all the cell types in the body. This
means that they could be used to treat conditions such as diabetes and Alzheimer’s disease in
the future. However, there are ethical issues associated with their use. E.g. should a human
embryo be destroyed to provide cells for research. Adult stem cells can only develop into a
few cell types but have already been successfully used in treatments such as providing bone
marrow transplants and growing skin grafts. There are less ethical issues surrounding their
use.
5. Meristems are the sites of production of non-specialised cells in plants and it is here that
mitosis (cell division) occurs in a plant. Meristems are found at the root and shoot tip. The
cells produced by cell division in meristems have the potential to become any plant cell.
Cell division within meristems leads to plant growth.
C. Control and Communication
6. Internal communication is required for survival of a multicellular organism. Cells in
multicellular organisms work together and not independently. In animals communication
within the body is brought about by the nervous system and hormones.
7. Advanced animals such as man have a central nervous system (CNS) consisting of a brain
and spinal nerve cord. The CNS processes the information from our senses and triggers an
appropriate response. Sensory neurons (nerve cells) carry messages in the form of
electrical impulses from receptors in sense organs into the CNS. E.g. optic nerves carry
impulses from light receptors in our eyes into the brain. Motor neurons then carry electrical
impulses from the CNS to muscles allowing a rapid response to occur e.g. catching a ball.
8. Reflex actions are rapid, protective actions which involve an arrangement of neurons
called a reflex arc. The reflex arc consists of a sensory, relay and motor neuron. Relay
neurons are in the CNS, often the spinal cord, and they connect sensory neurons to motor
neurons. This means that a response can occur to a stimulus almost instantly. E.g. when
receptors in the skin below the knee are touched, the impulse travels to the spinal cord and
back to the lower leg muscles in a fraction of a second, causing the lower leg to kick out.
This example of a reflex action is called the knee-jerk response.
9. Neurons do not actually touch each other. Instead a tiny gap called a synapse exists between
them. In order for an electrical impulse to pass between two neurons, chemicals (called
neurotransmitters) are released into the synapse.
10. Hormones are the body’s chemical messengers. They are released into the blood stream
from endocrine glands around the body. Target tissues have cells with receptor proteins
for specific hormones so different hormones target different tissues. E.g. adrenaline
targets the heart while insulin targets the liver.
11. The hormones insulin and glucagon regulate blood glucose concentration. Too much
glucose in the blood stream can be dangerous as it raises blood pressure (due to osmosis).
When too much glucose enters the blood stream (e.g. after eating a meal) the pancreas
detects this and releases insulin into the blood. The insulin travels to the liver where it
attaches to insulin receptors, causing the liver to absorb excess glucose and convert it into
glycogen, which is insoluble and easily stored. When too little glucose is in the blood
stream (e.g. while exercising) the pancreas detects this and releases glucagon into the blood.
The glucagon travels to the liver where it attaches to glucagon receptors, causing the liver
to convert glycogen into glucose, which is released into the blood.
12. People suffering from diabetes are unable to control their blood glucose concentration
which rises to high levels. Type 1 diabetics have a pancreas which no longer produces
insulin and so they have to take regular insulin injections. Type 2 diabetics no longer have
enough insulin receptors on their liver, so it does not absorb enough glucose from the
blood stream. Type 2 diabetics are advised to diet and exercise to control their blood sugar
levels. The number of cases of Type 2 diabetes is increasing in Scotland.
D. Reproduction
13. Sex cells are called gametes and they have half the number of chromosomes compared to
other cells. Sex cells are said to be haploid while body cells are said to be diploid. The
nuclei of haploid gametes fuse during fertilisation forming a diploid zygote. This zygote
then divides many times forming millions of diploid cells that make up the organism.
14. The gametes of animals are sperm and ova (eggs). Sperm are produced in the testes of the
male while ova are produced in the ovaries of the female. Sperm are much smaller than
ova, containing less of a food store in their cytoplasm. Sperm have a tail which enables
them to swim towards an ovum. In humans, sperm are released by the penis into the vagina
and fertilisation occurs in the oviduct, before the fetus develops in the uterus.
15. The gametes of plants are pollen and ovules. These are produced in specialised organs
within the flower. The male sex organ is called the stamen and pollen grains are produced
in a structure at its tip called the anther. The female sex organ (the carpel) produces ovules
in a structure at its base called the ovary. Pollen grains are carried by insects or the wind
from the anther of one plant to the stigma of another. The stigma sits above the ovary and it
is either sticky to pick up pollen from insects or hairy to catch pollen blown by the wind.
16. The pollen grain grows a pollen tube down from the stigma into the ovary. The male
nucleus travels down this tube and fertilises the female nucleus in the ovule. The fertilised
ovule develops into the seed while the ovary develops into the fruit. The function of the
fruit is to disperse the seeds so it can be juicy for animals to eat (cherry) or dry and winglike for the wind to pick up (sycamore).
E. Variation and Inheritance
17. There are two types of variation which exists between members of a species.
Characteristics which show discrete variation can be observed and fall into distinct
groups e.g. fingerprint type or ability to tongue-roll. Characteristics which show
continuous variation need to be measured and provide a range of readings e.g. height or
weight.
18. Sexual reproduction provides variation within species as it combines genes from separate
parents. Most features are polygenic, being caused by more than one gene, and show
continuous variation. However, some features are caused by a single gene and it is these
that show discrete variation.
19. Genes exist in two or more forms called alleles. Individuals inherit one allele from each
parent and the combination of alleles that they get is called their genotype. One allele is
normally dominant over the other allele, which is said to be recessive. Heterozygous
individuals have different alleles while homozygous individuals have the same alleles
making up their genotype. The physical appearance of the genotype in an individual is
called their phenotype.
20. Family trees and pedigree charts can be used to identify the genotypes and phenotypes of
individuals in the P1 (parents), F1 (children) and the F2 (grandchildren) generations. The
chances of individuals inheriting a condition or being a carrier of a recessive allele can be
determined using a punnet square. Capital letters are used to represent dominant alleles
while lower-case letters are used to represent recessive alleles. Once identified, carrier
individuals of medical conditions can be given genetic counselling.
F. The need for transport
21. The larger an organism gets the smaller its surface area to volume ratio becomes. This
means that multicellular organisms cannot absorb enough food and oxygen across their
skin to supply all their body cells. Instead they require a transport system.
22. Plant’s require water for photosynthesis. Water enters a plant through the root hairs (by
osmosis) and passes across the root into xylem vessels in the centre of the root. It then
passes up the xylem in the plant stem into the xylem in the veins of leaves. The mesophyll
cells inside the leaf use some of this water for photosynthesis. The upper tightly-packed
palisade mesophyll cells are where most photosynthesis occurs. The lower loosely-packed
spongy mesophyll cells allow gases to move easily within the leaf. Much of the water
passes through air spaces in the leaf reaching stomata (pores) in the epidermis (leaf
skin). The stomata are surrounded by a pair of guard cells which absorb water (in
daylight) swell up and open the pore. Water then evaporates out of the leaf through the
stomata. This loss of water by plants is called transpiration.
23. Xylem vessels also transport minerals (such as nitrates and phosphates) dissolved in the
water they carry. Xylem cells are surrounded by bands of lignin which strengthens the cell
wall and allows it to withstand the pressure changes as water moves through the plant.
24. Sugar is transported up and down a plant in phloem cells. The phloem cells have pores in
their end walls to let dissolved sugar through. Due to this they are called phloem sieve tube
cells. The phloem cells are alive and controlled by nearby companion cells.
25. In mammals, nutrients, oxygen and carbon dioxide are transported in the blood. Nutrients
and carbon dioxide are carried in the plasma while oxygen is carried by red blood cells.
Red blood cells are biconcave in shape to provide a large surface area for absorption of
oxygen. They also contain the pigment haemoglobin which carries oxygen forming
oxyhaemoglobin. Red blood cells have no nucleus allowing more space for haemoglobin
inside the small cell.
26. The heart has four chambers inside it. The upper chambers are called atria and they
collect blood, while the lower chambers are called ventricles and they pump blood.
Deoxygenated blood from the body enters the right atrium through the main vein of the
body, the vena cava. It then passes into the right ventricle which contracts and pumps the
blood along the pulmonary arteries to the lungs where it picks up oxygen. The
oxygenated blood returns to the left atrium of the heart along pulmonary veins. The
blood passes to the left ventricle which pumps it out of the heart into the main artery of
the body, the aorta which carries it around the body.
27. Inside the heart are four valves which prevent the blood flowing backwards. Valves are
found between each atrium and ventricle and also where blood enters the pulmonary artery
and the aorta. Coronary arteries supply the heart muscle with oxygen so that the heart
muscle cells can generate energy to keep contracting and allow the heart to pump blood.
28. There are three types of blood vessels in the body – arteries, veins and capillaries.
Arteries have thick muscular walls, a narrow central channel and carry blood under high
pressure. Veins carry blood under low pressure; have thinner walls and a wide central
channel. Veins also contain valves to prevent the backflow of blood. Capillaries form
networks at organs and tissues. They are thin-walled and have a large surface area
which allows exchange of materials with body cells.
29. Gases are exchanged with the blood stream in the lungs. Air enters through the mouth or
nose before passing down the windpipe. Rings of cartilage strengthen the walls of the
windpipe keeping it open. On the inside of the windpipe is a layer of mucus (slime) which
traps dirt and microorganisms that have been inhaled. Small hairs called cilia beat
continuously removing the trapped materials up and out of the lungs. Two tubes called
bronchi branch off the windpipe entering each lung. These then branch into numerous
smaller tubes called bronchioles which run through the lungs ending in alveoli (air sacs).
30. Alveoli are where oxygen and carbon dioxide are exchanged with the blood. Alveoli have
a large surface area, a good blood supply and thin walls to allow the diffusion
(movement) of gases. Oxygen moves into red blood cells from the alveoli while carbon
dioxide moves from the blood plasma into the alveoli. This is why exhaled air contains
more carbon dioxide and less oxygen compared to inhaled air.
31. Food moves through the digestive system by the process of peristalsis. Muscles behind the
food contract while muscles around it relax. This slowly squeezes the food along the
intestines.
32. The products of digestion pass into the blood stream in the small intestine. This contains
numerous finger-like villi which provide a large surface area for the absorption of
digested food. Each villus has a thin wall surrounding a capillary network which
provides a good blood supply. Glucose and amino acids pass into the blood here and are
carried around the body dissolved in the blood plasma. The products of fat digestion,
fatty acids and glycerol, are absorbed into a tube, in the centre of the villus, called the
lacteal. This is part of another transport system (the lymphatic system) which carries these
products to the blood stream.
G. Effects of lifestyle choices on animal transport and exchange systems
33. The lifestyle choices individuals make can affect their physical and mental health. Having
a diet which is high in fat can lead to obesity. It can also result in fat being deposited
inside blood vessels which can narrow arteries and so increase blood pressure. Blood
clots can form and if these occur in the coronary arteries the individual will suffer a
heart attack. If the clots occur in the arteries supplying the brain with blood the individual
will have a stroke. When an individual has a stressful lifestyle or does not take regular
exercise the chances of having a heart attack or stroke is increased.
34. Other lifestyle choices which can affect the blood system include a high-salt diet which
can raise blood pressure. A diet rich in sugar can lead to Type 2 diabetes. Smoking
tobacco increases the chances of blood clots as carbon monoxide in the smoke attaches to
red blood cells making them sticky. The carbon monoxide also reduces the amount of
oxygen that red blood cells can carry so smokers become breathless. The tar in cigarette
smoke also damages the lungs increasing the chances of developing lung cancer.
Regularly drinking too much alcohol can eventually destroy the liver.
35. Lack of iron in the diet means that haemoglobin cannot be made and can lead to anaemia
where the individual develops a pale skin and is constantly tired.
36. Exposure to certain environmental factors can affect the health of individuals. These
damaging factors include radiation (e.g. ultraviolet rays), heavy metals (e.g. lead) and
pollution.
37. Heredity plays a part in the incidence of some conditions. Certain individuals will contain
a combination of alleles which make them more likely to get a condition e.g. diabetes.