AQA Biology B2, Unit 2, full Detailed Revision Notes

1. Unit 2 Biology
Cells Photosynthesis
Energy and
biomass
Nutrient
cycling
Enzymes
Homeostasis
Genes and cell
division
Genetics

2. Cells

3. Animal Cells
Nucleus: controls everything in the
cell, contains the information to make
new cells
Cytoplasm: A jelly like substance in
which chemical reactions in the cell
take place, the chemical reactions are
controlled by enzymes
Cell membrane: gives the cell its
shape and controls which substances
enter and leave the cell
Ribosomes: Where proteins are
made
Mitochondria: Releases energy from
respiration for the cell to use- found in
the cytoplasm, small sausage shaped
structures

4. Plant Cells
Cell wall: outside the cell
membrane, made of
cellulose and strengthens
the cell
Chloroplasts: found in the
cytoplasm. They absorb
light energy to make food
by photosynthesis
Vacuole: a large sac in the
centre of the cytoplasm. It
contains a watery fluid
called cell sap

5. Differences between plant and animal cells
Plant cells Animal cells
Have tough cell walls for
support
No cell wall
A large PERMENANT vacuole
containing cell sap
Some with small vacuoles,
however no cell sap
Chloroplasts, absorb sunlight
for photosynthesis
No chloroplasts
Box-like shape SHAPE VAIRES

6. Specialised cells
Leaf palisade cells- found in the
leaf of the plant, contain lots of
chloroplasts for photosynthesis:
Xylem cells- small
tubes that carry
water up the stem
Root hair cells- are
long and thin to
absorb water from
the soil

7. Nerve cells- are like wires to carry
messages around the body
Sperm cells have a tail to swim to the
egg
Red blood cells have a substance that
carries oxygen

8. Tissues and organs
Tissues are groups of cells that all do the same
job
Organs are made up of groups of tissues.
A number of organs working together make a
system.

9. Diffusion and osmosis
Diffusion is the movement of particles from a region of high
concentration to a region of lower concentration until they are
evenly spread out.
Osmosis is the diffusion of water molecules from a dilute
solution to a more concentrated one through a partially
permeable membrane.

11. What is the use of the:
• Nucleus?
• Cytoplasm?
• Cell membrane?
• Ribosomes?
• Cell wall?
• Chloroplasts?
• Vacuole?
Also state whether they are in Plant (P) cells/ Animal (A) cells or both
State the differences between animal and plant cells
What is the mitochondria and where is it found?
Name 3 cells found in the plant and their uses
Define:
• Tissues
• Organs
• Diffusion
• Osmosis
SLIDES 2 + 3
SLIDE 4
SLIDE 3
SLIDE 4
SLIDES 7 + 8

12. Describe the role of enzymes inside the cell
Suggest 2 factors that increase the rate of diffusion
Name the substance that moves across a partially permeable membrane during the
process of osmosis
Explain what cell specialism means
answers
CONTROL CHEMICAL REACTIONS
CONCENTRATION, TEMPERATURE
WATER
CELLS BEING CHANGED TO DO A SPECIFIC JOB; NERVE CELLS, SPERM CELLS, RED BLOOD
CELLS

13. PHOTOSYNTHESIS

14. What is photosynthesis?
• Photosynthesis is the process where plants make
their own food
• The equation =
Carbon dioxide + water glucose + oxygen
• Sunlight is absorbed by the green chlorophyll
which are found in chloroplasts
• Light energy is used to convert carbon dioxide
and water into glucose
Light energy + chlorophyll

15. Carbon dioxide + water glucose + oxygen
Light energy + chlorophyll
Glucose is used through respiration
to give plants their own energy
Excess glucose can be stored in the
leaves as insoluble starch
Water is absorbed through
the roots from the soil and
carried up the stem to the
leaves in the xylem vessels
Carbon dioxide enters the leaf by
diffusion through the stomata
Oxygen is released as a by product
of photosynthesis

16. Limiting factors
• Limiting factors decrease the rate of
photosynthesis, they include:
• Shortage of sunlight
• Shortage of CO2
• Low temperature
• The rate of photosynthesis would increase if
these factors increased

17. The products of photosynthesis
• The glucose produced can be used in respiration to release energy.
• This energy can be used to build large molecules from small ones:
• Sugars can be converted into insoluble starch,
stored in the roots
• Sugars can be changed into cellulose to form new
cell walls
• Sugars can combine with nitrates and other
nutrients to form amino acids- build proteins for
plant growth
• Sugars can be converted into lipids (fat and oils)
and stored in seeds

18. Healthy growth
Nitrate Magnesium
Nitrate is needed to produce
amino acids, which then
form proteins for the plant
Magnesium is needed to
produce chlorophyll- to be
used in photosynthesis to
absorb the sunlight
If Nitrate is lacking the
plant’s growth will be
stunted and older leaves will
urn yellow
Without magnesium the
leaves will turn yellow, due
to the lack of chlorophyll
which is green
Plants need mineral ions and nutrients to stay healthy. They are
found in the soil and absorbed through the plant’s roots.
Magnesium and nitrate are vital for a plant to remain healthy:

19. Leaves
• Leaves are well adapted to carry out
photosynthesis. They have:
• A large surface area- to absorb light rays
• A thin shape- to allow gases to diffuse in and
out of the leaf easily
• Chloroplasts- containing chlorophyll that
absorb light energy
• Veins- to carry substances to and from all of
the cells of the leaf

20. Leaf Structure
cuticle
Palisade mesophyll: Where
photosynthesis takes place, as lots of
chloroplasts
Spongy mesophyll: Lots of air space
between them, for gas exchange to
occur
Vein: contains xylem tubes and
phloem tubes
Xylem tubes: brings water and salt to
leaf through stem
Phloem tubes: takes dissolved foods
away
Cuticle: waterproof layer that also cuts water loss by evaporation
Upper epidermis: single layer of cells, no chloroplasts present, sunlight goes straight
through to palisade cells

22. Why do plants need the process photosynthesis?
How is sunlight absorbed?
Light energy is used for what?
Explain the job of chlorophyll during photosynthesis?
Write the equation for photosynthesis and explain each gases’ use
Name 3 limiting factors of photosynthesis
What are the 5 products and uses of photosynthesis?
What do plants need for healthy growth?
Name 2 gases that are needed and what would happen if they were lacking
Why are leaves well adapted for photosynthesis?
What do the following do in the leaf:-
Cuticle?
Spongy mesophyll?
Lower epidermis?
Upper epidermis?
Palisade layer?
Vein?

23. ENERGY AND
BIOMASS

24. Energy
The original source of energy for all communities of living
organisms is radiation from the Sun
Green plants capture this solar energy, transferring it into chemical
energy and stored in starch and other substances that make up plant cells
Only about 10% of the solar energy reaching the plant is transferred into
chemical energy by photosynthesis.
Most of the energy is:
• Reflected by the leaf
• Passes straight through the leaf
• Simply heats the leaf up

25. Biomass
• Biomass is the mass of living material, and as you go along the
food chain it gets less and less.
• A pyramid of biomass shows the mass of all the organisms at
each point in a food chain
We can use these biomass figures to draw a pyramid of biomass to scale
2500g 400g 20g
grass
snails
bird

26. Food production
• If there is less biomass at each stage in a food chain then
there must also be less material and less energy.
The efficiency of food production can be
improved by reducing the number of stages in
food chains
• If we want an energy efficient diet we should
eat from low down the food chain- eating the
plants rather than the plant eaters.

27. Energy losses
Energy consumed
30% lost as heat in respiration
10% for growth
60% lost in urine
and faeces
The cow is a mammal, and mammals must keep their body temperature
constant.
This results in a lot of energy being lost as heat to their surroundings.
Only 10% is used for growth
90% is used for:
• Lost in food that is uneaten
• Lost in faeces and urine
• Used in respiration

28. Ways to improve the efficiency of food
production
• Intensive animal farming involves taking steps
to reduce energy losses from food animals
• Animals such as poultry are kept indoors, in a
temperature controlled environment
• Their movement is also restricted to reduce
heat loss from respiration, and to ensure they
put weight on quickly

29. • Plants are treated with hormones
• These chemicals ensure that they ripen as they
appear on supermarket shelves, rather than on
the plant or during transport.
When discussing the positive and negative effects
of managing food production there has to be a
compromise between :
Maximising food production from the available
land, plants and animals and protecting the
environment from damage by pollution or over use
and treating animals as humanely as possibly

30. NUTRIENT
CYCLING

31. Decomposition
• Useful materials like carbon and nitrogen are
removed from the environment from living
things.
• Eventually they are released when dead
organisms are broken down (digested) by
microorganisms- we call these microorganisms
decomposers
• These decomposers are bacteria and fungi

32. The ideal conditions for
decomposition by microorganisms
are warm and moist
Many bacteria are also more active when there is plenty of oxygen.
Nutrients in environment
producers decomposers
consumers
breakdown
photosynthesis
feeding
breakdown

33. Decomposers
• Fungi and bacteria use enzymes to
digest their food
• These work in the same way as
enzymes do in your gut
• The soluble products are taken up by
the bacteria and fungi
• The decomposers absorb the food and
use it for growth and energy
• These bacteria and fungi may be
eaten by other organisms and so the
nutrients are passed on
• Decomposers food chain:
Dead leaves  fungus  beetle  frog
DEAD MATERIALS
BACTERIA AND FUNGI
(CARRY OUT
DECOMPOSITION)
GROWTH AND CELL DIVISION
NEW DECOMPOSER MATERIAL
RESPIRATION
RELEASE OF CARBON DIOXIDE
CYCLING
NUTRIENTS RETURNED TO
SOIL

34. The carbon cycle
• Carbon dioxide is removed from the atmosphere
by green plants for photosynthesis
• The carbon from the carbon dioxide is used to
make carbohydrates , fats and proteins which
make up the body of plants
• Some of the carbon dioxide is returned to the
atmosphere when green plants respire
• When green plants are eaten by animals and
these animals are eaten by other animals , some
of the carbon becomes part of the fat and
proteins which make up their bodies
• When animals respire some of this carbon
becomes carbon dioxide and is released into the
atmosphere
• When plants and animals die or release waste,
microorganisms feed on their bodies and the
waste
• Carbon is released into the atmosphere as
carbon dioxide when these microorganisms
respire

35. Photosynthesis and respiration
• Respiration and photosynthesis dominate the carbon
cycle
Photosynthesis takes carbon dioxide out of the
atmosphere and provides the input for carbon into
food chains
Carbon dioxide + water glucose + oxygen
Light energy + chlorophyll
Respiration, by animals,
plants and microbes
releases carbon dioxide
back into the atmosphere

36. ENZYMES

37. How Enzymes Work
• Enzymes are BIOLOGICAL CATALYSTS

38. Hotting up
• High temperatures can destroy the shape of
an enzyme – DENATURE
• The substrate can then no longer fit and the
reaction can no longer take place

39. Enzymes and respiration
• Enzymes inside living cells catalyse processes such as
respiration, photosynthesis and protein synthesis
• Aerobic respiration=
Glucose +oxygen  carbon dioxide + water + ENERGY
The energy released during respiration is used:
• To build larger molecules from small ones
• In animals, to make muscles contract
• In mammals + birds to maintain a constant body temp
• In plants, to build up sugars, nitrates and other
nutrients into amino acids= proteins

40. OR.. Woman Gone Mad!
Warmth
Growth
Movement

41. Enzymes and digestion
Enzyme Where found Substrate it acts
on
Product
Amylase Mouth Insoluble starch glucose
Protease Stomach, small
intestine
Proteins Amino acids
Lipase Small intestine Lipids Fatty acids/
glycerol

42. In the stomach and small intestine
• Hydrochloric acid is produced in the stomach,
gives best pH for protease to work at
• The liver produces bile, which is stored in the gall
bladder, before passing down the bile duct into
the small intestine.
• Bile neutralises the acid that was added to the
food in the stomach- providing alkaline
conditions for the enzyme in the small intestine
to work best.
• Bile also emulsifies.

43. Enzymes in industry
• Microorganisms produce enzymes that pass OUT
of cells.
• Biological detergents contain proteases and
lipases to digest protein and fat stains.
• In industry:
• Proteases: pre-digest the protein in baby foods
• Carbohydrases: convert starch into glucose
• Isomerase: convert glucose syrup into fructose
syrup (fructose is sweeter, therefore less is
needed, used in SLIMMING FOODS).

44. HOMEOSTASIS

45. What is homeostasis?
• Homeostasis means keeping conditions inside the
body constant
• Conditions such as temperature, water level,
sugar level, pH of the blood (ion content) and
carbon dioxide level.
• These constant conditions are controlled by
hormones
• Hormones are produced by glands and are
transported to their target organs in the
bloodstream

46. Diabetes
• Our blood glucose level is controlled by two hormones:
insulin and glucagon.
• Both of these hormones are released by the pancreas.
• Diabetics are unable to control their blood sugar level
properly. They can not make enough insulin and so the
concentration of glucose in their blood may become
dangerously high- ‘hypos’ may occur.
• This can make them tired and thirsty.
• Sometimes the blood sugar level can fall too low.
• The person can feel weak, irritable and confused- may even
faint.
• Diabetes can be treated by following a low glucose diet or
by injecting with exactly the right dose of insulin.

47. Controlling blood sugar
• It is the pancreas that monitors and controls blood glucose
level
• After a high carbohydrate meal, blood glucose will be too
high.
• The pancreas detects this and releases insulin into the blood
• Insulin causes the liver to convert glucose to insoluble
glycogen. The liver removes the glycogen from the blood and
stores it and so your blood sugar level returns to normal.
• After a lot of exercise your blood sugar will be too low. The
pancreas detects this and releases a different hormone,
glucagon, into the blood.
• Glucagon causes the liver to convert glycogen into glucose
and release it into the blood.
• So your blood sugar returns to normal

48. Controlling body temperature
• Your body temperature is controlled by
receptors in the brain- forming the
thermoregulatory centre.
• This part of the brain monitors the
temperature of the blood running through it.
• This is your core body temperature
• Also temperature receptors in the skin send
impulses to this centre, giving it information
about skin temperature

49. When it’s hot
• Blood vessels supplying the capillaries at your skin surface dilate,
so that your blood reaches the surface of your skin and more heat
is lost by radiation- so you look flushed
• Sweat glands in your skin release more sweat, which cools the
body as it evaporates
Getting colder
• Blood vessels supplying the capillaries at your skin surface
constrict, so that less blood reaches the surface of your skin and
less heat is lost by radiation- so you look pale
• Sweat glands stop making sweat
• Your muscles may start to shiver/ contract.
• These contractions need respiration which produces extra heat
energy and these warm your body.

50. Waste products
• Waste products must be removed from the body
because they are toxic.
They include:
• Carbon dioxide produced during respiration and
removed from the body by the lungs when we
breathe out;
• Urea produced in the liver by the breakdown of
excess amino acids – the process deamination.
Urea is filtered out of the blood by the kidney and
removed as urine, which is temporarily stored in
the bladder.

51. GENES
AND CELL
DIVISION

52. Cell division
• Inside the nucleus are thread like chromosomes
• These chromosomes carry genes along their length
• A gene codes for a certain characteristic e.g. eye colour
• Genes are made up of the chemical DNA
• In body cells, chromosomes are found in pairs
• Humans have 46 chromosomes which can be
separated into 23 identical pairs
• The only human cell that does not have 46
chromosomes is the gametes (sex cell)
• The sperm and the egg each have 23 chromosomes

53. Mitosis
• Mitosis is a type of cell division which makes all the body cells
except the gametes
• Before a cell can divide by mitosis, it must make a second set of
chromosomes
• Each chromosome makes a copy of itself, so when the cell divides
into two, each cell has a copy of each of the original chromosome
set
• Each of the new cells will have exactly the same genetic information
• Body cells divide by mitosis:
• To produce additional cells during growth
• To produce replacement cells if some are damaged
• During asexual reproduction to produce clones, that are genetically
identical to the parent

54. Meiosis
• Is a type of cell division which results in gametes
• Cells in reproductive organs divide to form
gametes
• Cells in the testes divide by meiosis to form
sperm
• Cells in the ovaries divide by meiosis to form eggs
• When a cell divides by meiosis:
• Copies of each of the chromosomes are made
• Then the cell divides twice to form four gametes,
each with a single set of chromosomes

55. Fertilisation
2 gametes fuse together
A sperm with 23 chromosomes joins with an egg
with 23 chromosomes making 46.

56. Variation in offspring
• The cells of an offspring produced by asexual
reproduction are formed by mitosis from
parental cells. Identical genes as the parent
• Offspring from sexual reproduction vary:
• meiosis= both parental cells mixed
• Obtain different alleles
• Different characteristics

57. Stem cells
• Most types of animal cells differentiate at an
early stage- specialised cells
• Cell division is usually for repair and
replacement

58. GENETICS

59. Chromosomes and alleles
• There are 46 chromosomes in human
body cells.
• Chromosomes are thread like structures and
are made up of long proteins called DNA.
• A gene is a length of DNA that codes for
protein, by combining with amino acids.
• Some genes have 2 different forms- alleles.
• Dominant alleles and recessive alleles

60. Hereditary diseases
• Cystic Fibrosis- disorder of the cell membrane-
caused by the recessive allele
• Huntington’s Disease- disorder of the nervous
system- caused by the dominant allele.
• Embryo Screening= taking cells from embryos
and analysing them for diseases/ genetic
disorders.

61. Genetic crosses
• Homozygous= both alleles are the same, e.g.
AA/ aa (think homosexual- the SAME sex)
• Heterozygous= different alleles e.g. Aa

62. DNA
• Is a molecule, containing coded information
that determines inherited characteristics.