#biologys2

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Section 3:A - Reproduction: Humans

Definitions:

Sexual reproduction: two parents contribute; offspring are genetically different to parents & each other 

Asexual reproduction: one ‘parent’ organism involved; offspring are genetically identical to parent & each other 

Fertilisation: fusion of a male & female gamete to produce a zygote that undergoes cell division & develops into an embryo 

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Section 2:J - Coordination & response

Organisms are able to respond to changes in their environment. Homeostasis is the maintenance of a constant internal environment, examples are body water content and body temperature. 

A coordinated response requires a stimulus, a receptor and an effector

Stimulus = change in the internal/external environment

FLOWERING PLANTS 

Tropism = plant’s response to directional stimuli. Movement towards = positive tropism; movement away = negative tropism. 

Phototropism

Controlled by a hormone called auxin - produced by cells at the tip of the stem 

in LIGHT - auxin transported to shaded side of stem, causes cells behind the tip to elongate - so the tip bends towards the light 

thus, called postitive phototropism

Geotropism - response to gravity 

Shoots - show negative geotropism (grow up) 

Roots - positive geotropism (grow down) 

HUMANS

Two systems: nervous & endocrine 

Endocrine - made up of glands that secrete hormones. (stimulus > receptor > hormone > effector) 

Nervous - made up of brain and spinal cord. Is linked to sense organs via nerves

receptors detect stimuli

these are transduced into a signal (nerve impulses) 

conducted into CNS (central nervous system) via a sensory neurone 

transmits the nerve impulses to effectors via a motor neurone

Differences: endocrine is slower (hormones travel in bloodstream), whereas nerve impulses are much faster 

Reflex Arc

The Eye 

Photoreceptors in the retina: 

Rods - sensitive to light, work well in dim light; respond equally to all wavelengths of light 

Cones - only work in bright light; 3 types, one responds to green, one to red, one to blue

these cells react to produce impulses in sensory neurones & these are transmitted to brain via optic nerve

Image formation: light has to be focused through the lens to form an image on the retina 

Response to light intensity

Iris controls the amount of light entering the eye by changing the size of the pupil. (two muscles - circular and radial) 

Circular - contract to constrict the pupil and allow LESS light in (LIGHT)

Radial - contract to dilate the pupil and allow MORE light in (DARK)

Focusing on near/distant objects

Close object: 

lens becomes fatter, more convex to make light refract more 

muscles contract, ligaments become slack & lens bulges outwards - more convex

Distant object: 

lens becomes flatter, less convex to refract light less

muscles relax, pressure of vitreous humour flattens the lens & pulls sensory ligaments tight

Skin temperature regulation 

Vasodilation: body is too HOT. Vasoconstriction: body is too COLD

Sweating: when too HOT, glands under the skin secrete sweat and this increases heat loss by evaporation 

Hormones - sources, roles & effects 

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Section 2:I - Excretion

Organs of excretion = lungs, kidneys & skin 

Kidneys

Have 2 major functions: excretion & osmoregulation

EXCRETION: filter the blood to remove urea and other toxins, forming urine

Step 1: ULTRAFILTRATION 

blood enters the kidney through the renal artery and then enters the glomerulus 

high pressure forces liquid called glomerular filtrate through into the bowman’s capsule. Glomerular filtrate contains: water, glucose, amino acids, salts & urea

Step 2: SELECTIVE REABSORPTION 

as components travel through the nephron, some are reabsorbed into the blood as they are needed by the body 

water is reabsorbed from the collecting duct 

glucose is reabsorbed from the proximal convoluted tubule 

Step 3: URINE FORMATION 

excess water, salts and all the urea form urine 

passes through the ureter and is stored in the bladder 

Structure of a nephron

Structure of urinary system 

water/excess products are filtered from blood stream by kidneys ----> ureters carry urine to the bladder ----> urine leaves the body through the urethra

Osmoregulation

Osmoregulation is an example of homeostasis

Homeostasis = maintenance of a constant internal environment, within limits 

osmoregulation is the control of the body’s water content

too LITTLE water in the blood: 

hypothalamus detects imbalance

sends message to pituitary gland to release MORE ADH (ADH= Antidiuretic hormone)

collecting duct becomes MORE PERMEABLE, so MORE WATER is reabsorbed into the blood and a small volume of concentrated urine is produced

too MUCH water in the blood: 

hypothalamus detects imbalance 

sends message to pituitary gland to secrete LESS ADH 

collecting duct becomes LESS PERMEABLE. so LESS WATER is reabsorbed into the blood and a large volume of dilute urine is produced

Urine contains water, urea & salts 

Flowering plants

the origin of carbon dioxide and oxygen as waste products of metabolism and their loss from the stomata of a leaf

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Section 2:H - Transport: Humans

Composition of the blood 

Plasma, red blood cells, white blood cells, platelets 

Plasma: carries carbon dioxide, digested food, urea & hormones around the body; distributes heat around the body 

Red blood cells: carry oxygen around the body

Adaptations of red blood cells: 

Haemoglobin: contain this. Oxygen bonds to it.

No nucleus: maximise room for haemoglobin

Biconcave: dip in the middle increases the surface area & decreases distance for diffusion 

Flat: pass through narrow capillaries 

Immune system

Responds to disease using white blood cells. Two main ways: phagocytes & antibodies 

Phagocytes: ingest microorganisms such as bacteria 

Phagocytosis:

phagocyte changes shape, producing extensions of their cytoplasm

this surrounds and encloses the organism in a vacuole

once inside, the phagocyte secretes enzymes into the vacuole to break the microorganism down 

Lymphocytes: make chemicals called antibodies - soluble proteins that pass into plasma & destroy bacteria, by: 

causing bacteria to stick together so phagocytes can ingest them easily

causing bacterial cells to burst open 

neutralising poisons produced by pathogen 

Vaccination

Injecting a modified form of the pathogen, so memory cells to the pathogen are produced. Id the same pathogen infects the body later, the memory cells divide to produce antibody producing cells. The response is quicker, faster & bigger.

Platelets

Cell fragments, involved in blood clotting. 

when a platelet is damaged/exposed to air, it releases a chemical 

this converts soluble fibrinogen to insoluble fibrin threads

threads form a mesh across cut, in which red blood cells are trapped and this dries to form a clot 

Why > to minimise blood loss; prevents pathogens from entering bloodstream

The Heart

blood from vena cava flows into the right atrium & the valve is closed 

right atrium contracts forcing the blood through the atrioventricular valve and into the right ventricle

right ventricle contracts forcing the blood through the semi-lunar valve and into the pulmonary artery to be oxygenated at the lungs 

pulmonary vein fills the left atrium with blood 

left atrium contracts forcing blood into left ventricle 

left ventricle contracts, forcing blood out through the aorta

LEFT ventricle has thicker muscle wall as it has a higher pressure - to the body

Exercise & heart rate 

During exercise heart rate increases as the muscles require more energy by respiration, so they need more oxygen brought in the blood 

Adrenalin: produced in the adrenal gland (top of the kidneys); increases heart rate; diverts blood flow from the intestines to the muscles

Arteries, Veins & Capillaries

Arteries

carry blood from heart TO organs 

thick, strong walls to withstand pressure

small lumen 

have ability to recoil to help push blood along 

Veins

carry blood from organs TO heart 

thin walls

large lumen (^both to allow low pressure)

have valves to prevent backflow of blood 

Capillaries 

carry blood THROUGH organs 

exchange between blood in capillaries and cells 

very small - fit between cells 

walls one cell thick - allow substances to diffuse easily 

Circulation system 

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Section 2:H - Transport: Flowering Plants

Unicellular organisms have a large surface area to volume ratio, so they can absorb everything they need through the surface of the cell 

Multicellular organisms need transport systems because they have a very small surface area to volume ratio

Vascular bundle - Xylem & Phloem 

Vascular bundle composed of xylem and phloem vessels 

Phloem: composed of living cells joined end to end; transports sucrose and amino acids from the leaves to the rest of the plant 

Xylem: made of dead, lignified cells joined end to end; hollow; transports water and mineral salts from the roots to other parts of the plant

Root hair cells - absorbing water

mineral ions dissolved in water are actively transported into root hair cell

lowering the water potential inside the cell 

water moves into the cell by osmosis, raising the water potential above that of the cells in the cortex 

water moved into & across cortex by osmosis 

Transpiration 

The evaporation of water from the leaf via the stomata

Factors affecting the rate of transpiration: 

Humidity: humidity increase makes transpiration decrease. Increasing number of water molecules outside the leaf reduces concentration gradient

Wind speed: wind speed increases, the rate of transpiration increases. Water molecules are moved away from leaf, increasing the concentration gradient 

Temperature: increased temperature increases transpiration rate. Water molecules have more kinetic energy

Light intensity: increased light intensity, increased transpiration. Light causes stomata to open, wider stoma can allow faster diffusion of water vapour out of the leaf

EXPERIMENT - rate of transpiration 

The potometer

water is drawn up the system to replace water lost from the leaves in transpiration 

rate of movement of the bubble is a measure of the rate of transpiration

Apparatus can be put in conditions where temperature, wind speed & humidity can be varied 

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Section 2:G - Gas Exchange: Humans

Diffusion plays a role in gas exchange. Gases move across the walls of the alveoli; oxygen into the blood; carbon dioxide moves into the lungs 

Structure of the thorax 

Ventilation 

Breathing in: 

Diaphragm muscle contracts & flattens, pulling diaphragm down 

External intercostal muscles contract to pull the ribcage up & out

Volume increases inside thorax, decreasing pressure below that of atmospheric air

air is drawn in 

Breathing out: 

Diaphragm relaxes, pulling it up 

Internal intercostal muscles contract to pull the ribcage down & in

Volume decreases, pressure increases above that of atmospheric air 

air is expelled 

Alveoli

(an alveolus is in an alveoli)

Diffusion of oxygen and carbon dioxide takes place between the air in the lungs and the blood in the capillaries

Adaptations: 

Thin - gases can diffuse easily (alveolus is one cell thick & so is capillary wall) 

Many - large surface area through which much gas can diffuse at once

Moist - moist lining for gases to dissolve into 

Smoking - biological consequences

Tar - deposited in the lungs, decreasing their surface area; stains the lungs; can cause cancerous mutations 

Smoke - removes the cilia (tiny hairs) which keep the lungs clean 

Carbon monoxide - binds irreversibly to haemoglobin, reducing the ability of the blood to carry oxygen 

Nicotine - speeds heart rate; damages arteries, leading to coronary heart disease 

EXPERIMENT - exercise & breathing 

Rate of breathing: number of breaths per minute can be recorded during different amounts of exercise

Volume of gas: amount of gas produces for different amounts of exercise can be collected and measured 

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Section 2:G - Gas Exchange: Flowering Plants

Photosynthesis needs light to occur, so it does not happen at night. Respiration is carried out all the time.

Net Gas Exchange 

Structure of the leaf for gas exchange 

High stomata density: greater amount of gas can be exchanged 

Thin: shorter distance for diffusion = quicker 

Large surface area: greater surface area = more gas can be exchanged

Internal air spaces: diffusion of carbon dioxide and oxygen is faster in air

Role of Stomata

Stomata are located on the lower epidermis of the leaf. They allow carbon dioxide and oxygen to be exchanged between the leaf and the atmosphere. They are closed at night. 

EXPERIMENT - gas exchange & light

hydrogen carbonate indicator in 3 boiling tubes

place a leaf in two of them & leave the last one as a control 

place one in dark and one in light

hydrogen carbonate indicator is purple when carbon dioxide is absent; RED when carbon dioxide is LOW; yellow when carbon dioxide is high

dark one = yellow; light one = purple 

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Section 2:F - Respiration

Respiration is a chemical reaction that releases energy inside cells 

AEROBIC: 

glucose  +  oxygen ---->  carbon dioxide  +  water  +  lots of energy 

ANAEROBIC: 

1. animals

glucose  ----------> lactic acid  +  some energy 

2. plants & yeast

glucose -----------> ethanol  +  carbon dioxide 

Differences between aerobic & anaerobic: 

aerobic produces more energy 

anaerobic doesn’t use oxygen 

EXPERIMENT - evolution of heat 

boil one set of seeds (to kill them) and soak another in water 

place them in two ‘jars’, plugged with cotton wool & a thermometer

note the changes in temperature 

after several hours there should be a heat change in the living seeds

EXPERIMENT - evolution of carbon dioxide 

place an organism on a gauze platform above a solution of hydrogen carbonate 

hydrogen carbonate indicator turns from orange to yellow in the presence of carbon dioxide