Knowt
Organism Level Systems (GCSE OCR)
I. Definitions and Concepts
ADH: A hormone secreted by the pituitary gland that increases water reabsorption in the kidney (making the kidney tubules more permeable to water).
Adrenaline: A hormone released by the adrenal gland that increases heart rate and breathing rate (involved in fight/flight).
It also raises blood sugar levels by increasing the conversion of glycogen into glucose
Auxin: A plant hormone that is responsible for cell elongation.
Central Nervous System (CNS): The brain and the spinal cord.
Cerebellum: The region of the brain that controls unconscious functions such as posture, balance, and muscular movement.
Cerebral Cortex: The outer layer of the cerebrum.
Cerebrum: The highly folded region of the brain that is responsible for controlling voluntary actions such as learning, personality, and memory.
It is divided into the right and left hemispheres.
Ciliary Body: An extension of the iris.
It contains the ciliary muscle, which can contract or relax, allowing the eye to focus.
Color Blindness: A condition where a person has a defect in the receptors or a lack of receptors in the retina.
Colorception: Methods that are used to prevent pregnancy.
Cornea: The part of the eye that refracts light as it enters, focusing it onto the retina.
Dormancy: A period of time in which seeds hibernate.
This stops when they germinate.
Effector: A gland or muscle that produces a response to the stimulus to restore optimum conditions.
Endocrine Glands: A group of cells that are specialized in secreting chemicals (hormones) directly into the bloodstream.
Endocrine System: A chemical messenger system that releases hormones directly into the bloodstream to control metabolism, development, growth, and reproduction.
Ethane: A plant hormone that promotes fruit ripening.
Follicle Stimulating Hormone (FSH): A female reproductive hormone that is released by the pituitary gland.
It is responsible for the maturation of an egg in the ovary.
Geotropism: The growth response of a plant to gravity.
Germination: The process by which seeds develop into plants.
Gibberellins: Plant hormones that initiate germination and flowering.
Glucagon: A hormone produced by the pancreas that works with insulin to control blood sugar levels.
It increases blood glucose concentration by converting glycogen into glucose.
Herbicide: A type of pesticide used to kill unwanted plants (weeds).
Homeostasis: The maintenance of a stable internal environment in the body despite fluctuations in internal and external conditions.
Hormone: A chemical messenger secreted by the endocrine glands into the bloodstream and transported to receptors on target organs.
Hypothalamus: The part of the brain that is the regulation center for temperature and water balance of the body.
Infertility: The inability to reproduce after 12 months or more of unprotected sex.
Insulin: A hormone produced by the pancreas that controls the body’s blood sugar levels
It works to decrease glucose levels.
In Vitro Fertilization (IVF): The fertilization of an egg using sperm outside the body.
IVF is used when a couple is having difficulty conceiving.
Iris: The part of the eye that contracts or relaxes to control the amount of light entering the eye.
Kidney: The organ in the body that maintains water balance and produces urine.
Lens: A part of the eye that further refracts light to focus it onto the retina.
Long-sightedness (Hyperopia): A defect of the eye where distant objects appear out of focus due to the convergence of light rays in front of the retina.
Luteinizing Hormone (LH): A female reproductive hormone released by the pituitary gland that stimulates the release of an egg.
Medulla: The part of the brain responsible for non-voluntary movements such as breathing rate and heart rate.
Menstrual Cycle: The monthly cycle in women that involves the development of the uterus lining, ovulation, maintenance of the uterus lining, and shedding.
Motor Neuron: The neuron that transmits impulses from the relay neuron to the effector to produce a response.
Negative Feedback System: A system that works to reverse the initial stimulus.
Nephron: A kidney filtering unit.
Estrogen: A female sex hormone produced in the ovaries that regulates the menstrual cycle and controls the development of secondary sexual characteristics.
Optic Nerve: The nerve that carries impulses between the brain and the eye.
Osmosis: The net movement of water molecules from a region of high water concentration to a region of low water concentration across a partially permeable membrane.
Osmotic Lysis: When water moves into an animal cell, causing it to burst.
Osmotic Shrinking: When water moves out of an animal cell, causing the cell to shrink.
Parthenocarpic Fruit Development: The development of seedless fruit.
Phototropism: The growth response of a plant to unilateral light.
Pituitary Gland: The gland that stores and releases hormones that regulate many bodily functions.
Progesterone: The hormone that maintains the uterus lining during the later stages of the menstrual cycle and pregnancy.
Pupil: The round opening in the center of the iris. It changes size to let light into the eye.
Receptor: A cell or organ that recognizes the stimulus.
Reflex Arc: The pathway of neurons involved in a reflex action.
Relay Neuron: The neuron that transmits electrical impulses from the sensory neuron to the motor neuron.
Renal Artery: The blood vessel that provides the kidney with blood.
Renal Vein: The blood vessel that takes blood away from the kidney.
Retina: The layer at the back of the eye that contains light receptors and is sensitive to light.
Root Cuttings: A method of cloning plants in which a root is cut from a parent plant and replanted in compost.
Rooting Powder: A powder that contains auxin.
The cut root is dipped into this before being replanted (during root cuttings).
Sensory Neuron: The neuron that detects the stimulus and transmits the electrical impulse to the relay neuron located in the spinal cord.
Short-sightedness (Myopia): A defect of the eye where distant objects appear out of focus due to the convergence of light rays in front of the retina.
Suspensory Ligaments: Attach the lens to the ciliary muscle.
Synapse: The junction between two neurons (nerve cells).
Testosterone: The male reproductive hormone that controls sperm production and the development of secondary sexual characteristics.
Thyroxine: A hormone released by an endocrine gland (thyroid gland) that controls the metabolic rate and the rate of glucose uptake during respiration.
It also promotes growth.
Tissue Culture: A method of cloning plants where plants are grown in a growth medium containing many nutrients.
Type 1 Diabetes: A condition in which the pancreas fails to produce insulin, resulting in high blood sugar levels.
Type 2 Diabetes: A condition in which a person develops insulin resistance or doesn’t produce enough insulin.
Urine: A liquid produced by the kidneys to help maintain water balance.
It contains mineral ions, water, and urea.
Vasoconstriction: The constriction of blood vessels.
Vasodilation: The dilation of blood vessels.
Water Potential: A measure of the tendency of water to move from one area to another is
It is represented by the Ψ (Psi).
II. Coordination and Control: The Nervous System
1. How the Nervous System Coordinates Responses
The central nervous system (CNS) consists of the brain and the spinal cord.
It allows us to make sense of our surroundings and respond to them in order to survive.
Receptor cells convert a stimulus (such as a bright light) into an electrical impulse.
This electrical impulse travels along cells called sensory neurons to the central nervous system (CNS).
Here, the information is processed, and the appropriate response is coordinated, sending an electrical impulse to motor neurons to effectors.
The effectors carry out the response (this may be muscles contracting or glands secreting hormones).
2. Reflex Arc
The reflex arc is a subconscious response to dangerous stimuli, such as a hot surface.
Sometimes a swift response is needed and there is not enough time for it to go through the conscious portion of the brain, so the CNS is involved instead.
A stimulus is detected by receptors, such as thermoreceptors in fingertips, detecting heat.
Impulses are sent along a sensory neuron.
In the CNS, the impulse passes to a relay neuron.
Impulses are sent along a motor neuron.
The impulse reaches an effector, resulting in the appropriate response, such as a contraction of the biceps to move the arm away from the heat source.
3. The Eye
a. Structures of the Eye:
Cornea: the transparent outer part of the eye.
It refracts light to reach the retina.
Iris: the colored part of the eye that does not allow light to go through.
Controls how much light enters the eye.
In bright light, the circular muscles contract, and radial muscles relax to make the pupil smaller, avoiding damage to the retina.
In dim light, the circular muscles relax and the radial muscles contract to make the pupil larger, so more light can enter to create a better image.
Lens: a transparent, biconvex disc that attaches to ciliary muscles by the suspensory ligaments.
Focuses light onto the retina.
Retina: contains light receptors.
Contains rods (respond to dim light) and cones (respond to color).
Optic Nerve: Carries impulses between the eye and the brain.
b. Common Defects
Color Blindness: the inability to tell the difference between different colors due to the lack of or defects in the receptors in the retina.
It is an inherited condition, and the most common form is red-green color blindness.
Myopia (Short-sightedness): the ability to see near objects but not distant objects, due to the lens focusing the image in front of the retina.
They are treated by concave lenses in glasses.
Hyperopia (Long-sightedness): ability to see distant objects but not near objects, due to the lens focusing the image behind the retina.
They are treated by convex lenses in glasses.
4. The Brain
a. Structure of the Brain
Cerebrum
Functions: intelligence, vocabulary, personality, conscious thought.
Features:
The largest part of the brain divides into 2 cerebral hemispheres.
Each half processes the information it receives from the opposite side of the body.
The outside is made from gray matter (containing myelinated nerves) and the inside is made of white matter.
Cerebellum: Coordinates voluntary body movements and helps with balance.
Medulla: Control center for heart rate, blood pressure, and breathing rate.
Hypothalamus: Control temperature and water balance in the body through the hormonal system.
Pituitary gland: An important gland releasing hormones, such as growth hormone, into the blood.
b. Limitations in Investigating and Treating Brain Damage
There are ethical issues with investigating brain damage, as it is unethical to ask patients who may not be in the state to make an informed decision about whether they want to take part in a study.
The investigative study would require surgery, which has many risks in itself.
There is also a considerable amount that we have yet to learn about the brain and its functions, so it may be hard to interpret the findings of case studies.
There are limitations in treating brain damage because the central nervous system cells cannot regenerate and repair, and there may be areas that are very difficult or dangerous to access during surgery.
Drugs may also have difficulty penetrating the different membranes surrounding the brain.
III. Coordination and Control: The Endocrine System
1. Endocrine System
Hormones are chemical messengers that are secreted by endocrine glands into the bloodstream, where they are transported around the body to receptors on the target organ.
The pituitary gland, as mentioned before, is one of the most important glands.
Endocrine System | Nervous System | |
Type of Signal | Chemical | Electrical |
Transmitter | Hormones in Bloodstream | Nerve Cells |
Speed of Response | Slower | Very Fast |
Duration of Response | Long | Short |
a. Examples of Hormones
Adrenaline
Produced by the adrenal glands that sit on top of the kidneys.
Targets many organs, such as the heart and lungs.
Responsible for the ‘fight or flight’ response for survival: Increases heart rate, dilates pupils, makes hairs stand erect, increasing breathing rate.
Thyroxine
Produced by the thyroid gland in the neck.
Responsible for controlling metabolic rate, meaning it controls how quickly oxygen and food react to release energy. It is therefore responsible for growth.
Example of negative feedback:
If thyroxine levels are too low it stimulates the hypothalamus to produce a hormone called TRH and the pituitary then releases TSH (thyroid stimulating hormone) which causes the thyroid to produce more thyroxine.
When the levels are returned to normal, these hormones are inhibited to stop further increases.
2. Human Reproduction
Hormones are responsible for controlling puberty and the menstrual cycle.
a. Puberty
Hormones are responsible for the changes to the body that occur during puberty.
Testosterone: Produced by the testes, is responsible for the development of sperm and also secondary sexual characteristics in males, such as deepening voices and increased hair.
Estrogen: Produced by the ovaries, is responsible for secondary sexual characteristics in females, such as breast development and widening hips.
b. Menstrual Cycle
The menstrual cycle lasts 28 days and the egg is usually released on day 14 (ovulation).
Estrogen
Estrogen causes the thickening of the uterus in preparation for the implantation of an egg.
Levels peak on day 10 and then begin to fall.
FSH and LH
Follicle Stimulating Hormone causes maturation of the egg within the ovary.
Luteinizing Hormone stimulates the release of the egg during ovulation.
A decrease in estrogen causes LH and FSH to start increasing.
Ovulation (the release of an egg cell from one of the ovaries) occurs when LH and FSH levels peak.
Progesterone
Progesterone maintains the thick lining of the uterus.
Inhibits the release of LH and FSH.
The egg matures on day 14 and progesterone starts increasing after this until it reaches its peak 3 days later.
If the egg is not fertilized, progesterone levels fall and the uterus lining breaks down in a period that lasts for around 5 days.
Menopause occurs when a woman no longer has a menstrual cycle, often around 50–55 years of age.
c. Contraception
Hormonal Methods
Oral Contraceptives (The Pill)
Contains progesterone and estrogen, which inhibit the production of FSH so that eggs do not mature.
More than 99% effective if taken properly.
Side effects such as mood swings, and weight gain.
Contraceptive Implants
More than 99% effective.
Cause slow release of progesterone to prevent ovaries from releasing the egg thickens mucus in the cervix so sperm cannot swim through.
Will not need to remember to take it every day.
Non-Hormonal Methods
Physical Barrier Methods, such as condoms.
Easy to use and obtain and also protects from STIs.
However, condoms can rip.
Over 99% effective.
Can be made more effective with spermicidal agents, but some people can be allergic to it.
Vasectomy/Tubal Ligation
Sperm tubes/oviducts are cut to prevent gametes from becoming fertilized.
Almost 100% reliable.
However, can be irreversible.
Copper Intrauterine Device (IUD/The Coil)
A T-shaped device is implanted into the uterus.
Stops fertilized embryos implanting, copper kills sperm.
Can last for up to 10 years, over 99% effective.
Must be fitted by a doctor, small chance of ectopic pregnancy.
Abstinence
Not taking part in any sexual activity (vaginal, anal, or oral).
d. Fertility Treatments
Allow infertile couples the opportunity to have children.
Infertility can be caused by many issues, such as sperm quality or quantity problems or a lack of FSH to mature eggs.
However, fertility treatments are not guaranteed to work and if they do, they can increase the likelihood of twins, triplets, etc.
Fertility Drugs
FSH and LH are mainly used because they stimulate the maturation and release of the egg.
The woman can then become pregnant normally.
IVF
The mother is given FSH and LH to stimulate eggs to mature.
Eggs are collected and fertilized by the father’s sperm in a laboratory.
The fertilized eggs develop into embryos.
These embryos are then implanted into the mother’s uterus.
However, can be extremely expensive if it needs to be repeated.
3. Plant Hormones
Plants need hormones to coordinate and control growth.
They are needed for tropisms.
Examples of these include phototropism, the response to light, and gravitropism, or geotropism, the response to gravity.
Hormones move from the place they are made to where they are needed to produce the appropriate response.
a. Auxin
Most plants show positive phototropism because they grow towards the light source.
The plant is exposed to light on one side.
Auxin, a growth hormone, moves to the shaded side of the shoot.
Auxin stimulates cells to grow more here.
This means the shoot bends towards the light.
The plant receives more light, meaning photosynthesis can occur at a faster rate.
Most shoots show negative gravitropism as they grow away from gravity. If a shoot is horizontal:
Auxin moves to the lower side.
The cells of the shoot grow more on the side with most auxin, so it stimulates cells to grow more here.
This makes the shoot bend and grow away from the ground.
This is beneficial as light levels are likely to be higher further away from the ground.
Most roots show positive gravitropism as they grow towards gravity. If a root is horizontal:
Auxin moves to the lower side.
The cells of the root grow more on the side with less auxin, so it stimulates cells to grow on the upper side.
This makes the root bend and grows downwards.
This is beneficial as there are more likely to be increased levels of water and nutrients lower down, and it provides stability for the plant.
When the auxin distribution becomes equal on both sides, it grows straight in that direction.
You can investigate the effect of light or gravity on newly germinated seedlings by varying conditions.
Placing in a cardboard box and shining light from one side.
Attaching a petri dish containing the seedlings to a wall (effects of gravity).
Uses of Auxin:
As Weed Killers:
Many weeds are broad-leaved.
Weed killers, containing auxin, have been synthesized so they only affect broad-leaved plants.
The increased amount of auxin causes the cells to grow too rapidly.
This results in the weed dying.
As Rooting Powders:
Plants with desirable features are cloned to make more plants with the same feature.
One way to clone a plant is to take a cutting from the original plant.
Rooting powder containing auxin is applied to it, and it is placed in the ground.
Roots grow, and the new plant begins to grow very quickly.
To Promote Growth in Tissue Culture:
Another way to clone a plant is to use tissue culture.
Cells from the plant are taken are placed in a growth medium containing lots of nutrients.
Hormones such as auxin are added.
The cells begin to form roots and shoots.
b. Gibberellins
Gibberellins are another plant hormone important to stimulate seed germination, flowering, and shedding of leaves.
Uses:
Ending seed dormancy
In the brewing industry, the germination rate of barley seeds is increased to make malt.
Promoting flowering
Instead of requiring certain conditions such as longer days and low temperatures to flower, applying this hormone allows it to flower in any conditions and with bigger flowers.
Increasing fruit size
The seeds in fruit produce gibberellins to increase fruit size.
This means that seedless fruit is generally smaller.
Seedless fruit can be sprayed with gibberellins to increase their size.
c. Ethane
Ethane is involved in cell division and the ripening of fruits.
Uses:
Used in the food industry.
Fruit is picked when it is not ripe.
It is firm, which means that during transport it gets less bruised and damaged.
When it is needed to be sold, it is exposed to ethane and warmer temperatures.
Ethene is involved in controlling cell division and stimulates enzymes that result in fruit ripening.
This reduces wastage as more fruit is suitable to be sold, and it does not ripen too early.
IV. Maintaining Internal Environments
1. Controlling Temperature
a. Homeostasis
The maintenance of a constant internal environment.
Mechanisms are in place to keep optimum conditions despite internal and external changes.
This is needed for enzyme action and all cell functions.
b. Thermoregulatory Center
Monitors and controls body temperature.
Found in the brain.
Has receptors that monitor the temperature of the blood.
Has receptors in the skin that send impulses to the thermoregulatory center.
c. Human Body Temperature
37.5 degrees Celsius.
If it becomes too high:
Sweat (evaporates from the skin surface, resulting in increased energy transfer away from the body) is produced by sweat glands.
Vasodilation means more blood flows closer to the surface of the skin, resulting in increased energy transfer from the body.
If it decreases too much:
Sweating stops.
Skeletal muscles contract rapidly (shivering) to generate heat from respiration.
Hairs stand on end to create an insulating layer, trapping warm air.
Vasoconstriction means blood does not flow so close to the surface, resulting in less heat lost.
2. Controlling Blood Sugar Levels
The concentration of glucose in your blood needs to be kept within a certain limit because glucose is needed by cells for respiration.
It is controlled by the pancreas.
a. Insulin
Eating foods that contain carbohydrates increases the glucose levels in the blood.
If the glucose levels are too high, the pancreas produces the hormone insulin.
Insulin binds to cells in target organs (muscles and liver) causing:
Glucose moves from the blood into muscle cells for respiration.
Excess glucose is converted into glycogen, which is stored in the liver.
The blood glucose concentration is reduced.
b. Glucagon
Rigorous activity, e.g., exercise, uses glucose for respiration and therefore there is less in the blood.
If glucose levels decrease, the pancreas produces the hormone glucagon.
Glucagon binds to the liver cells, causing glycogen to be broken down into glucose.
Glucose is released into the blood, increasing the blood glucose concentration.
Your blood glucose concentration is kept constant by using these two hormones. They work in a negative feedback loop.
When blood glucose levels increase/decrease, a hormone is secreted to oppose the change.
The action of this hormone cannot occur continually because when the blood arrives at a certain glucose concentration the other hormone is produced, resulting in the opposite effect.
c. Diabetes
Diabetes is a disease where the body cannot control blood sugar levels properly.
Type 1 Diabetes: The pancreas cannot produce enough insulin.
Blood glucose levels can rise to a fatal amount.
Glucose is excreted with urine and lots of urine is produced, leaving the individual very thirsty.
It is treated with insulin injections at meal times, which results in glucose being taken up from the bloodstream.
It is also advised to limit the intake of simple carbohydrates, which contain lots of glucose.
Doctors are attempting to cure diabetes with pancreas and pancreatic cell transplants, and genetically engineering pancreatic cells from mice to make insulin.
Type 2 Diabetes: The body cells no longer respond to insulin.
Blood glucose levels can rise to a fatal amount.
Obesity is a risk factor for this disease.
Treatments include reducing the number of simple carbohydrates in the diet, losing weight, and increasing exercise.
There are also drugs to make insulin more effective on body cells, help the pancreas make more insulin, or reduce the amount of glucose absorbed from the gut.
3. Kidneys
a. Effects of Osmotic Changes to Body Fluids
If the water concentration of the blood increases, i.e., has a higher water potential than the cells, water will move into the cells causing them to expand. Eventually, this can lead to bursting (lysis).
If the water concentration of the blood decreases, i.e., has lower water potential than the cells, excess water will leave the cell causing shrinking.
b. Function of the Kidneys
The kidneys are very important in maintaining the balance of water and other substances in the body.
As blood moves through the body, it makes urine by:
Filtering out the waste products, such as water, ions, and urea (from amino acids), at high pressures.
Selectively reabsorbing useful substances such as glucose, ions, and water.
c. Structure of the Kidney
The inner part of the kidney is called the medulla and the outer part is called the cortex.
The ureter carries from the kidneys to the bladder to be excreted out of the body.
The kidney is supplied by the renal artery and a renal vein takes blood away.
Each kidney contains millions of kidney tubules or nephrons and these are made up of a glomerulus (ball of capillaries), a region for selective reabsorption of substances to occur, and a kidney tubule where water and salt are regulated.
d. ADH
Antidiuretic hormone (ADH) is a hormone involved in the control of the loss of water as urine.
It is released into the pancreas by the pituitary gland when a receptor in the brain detects that the blood is too concentrated.
It travels in the bloodstream to the kidney tubules.
An increased amount of ADH reaching the tubules increases their permeability to water, so more moves out of the tubule and back into the bloodstream.
This results in a smaller volume of more concentrated (yellow) urine and the blood becoming less concentrated as more water moves into it.
This is an example of a negative feedback loop, because if the concentration of the blood increases/decreases, more/less ADH is secreted to reverse this change.
In high temperatures, increased sweating can lead to dehydration.
This can lead to salt loss in sweat, meaning that the kidneys may try to compensate for the change by increasing salt retention.
The brain detects this and makes us feel that we are thirsty, so we drink more water to dilute the salts in our blood.