Biology revision – understanding ourselves

B1

Both parents are heterozygous for the faulty allele
Cystic fibrosis is an inherited disorder caused by a recessive allele. This genetic diagram shows the possible outcomes when both parents are heterozygous for the faulty allele. There is a one in four chance of the offspring being homozygous for the faulty allele, and so having cystic fibrosis.

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Fit people are able to carry out physical activities more effectively than unfit people. Their pulse rate is likely to return to normal more quickly after exercise.

But being fit is not the same as being healthy. Healthy people are free from disease and infection: they may or may not be fit as well. It is possible to be fit but unhealthy, or healthy but unfit.

High blood pressure can cause: • kidney damage • burst blood vessels • damage to the brain, including strokes
Low blood pressure can cause: • fainting • dizziness • poor blood circulation

During hard exercise when anaerobic respiration occurs with aerobic respiration, an oxygen debt builds up. This is because glucose is not broken down completely to form carbon dioxide and water. Some of it is broken down to form lactic acid.

During exercise, the muscle cells respire more than they do at rest. This means: • oxygen and glucose must be delivered to them more quickly • waste carbon dioxide must be removed more quickly
This is achieved by increasing the breathing rate and heart rate. The increase in heart rate can be detected by measuring the pulse rate.

Fit for life

Anaerobic respiration:
Glucose ’! lactic acid (+ energy)
Much less energy is released by anaerobic Glucose → lactic acid (+ energy)
Much less energy is released by anaerobic respiration than by aerobic respiration.

Aerobic respiration:
Glucose + oxygen → carbon dioxide + water (+ energy)
C6H12O6 + 6O2 → 6CO2 + 6H2O (+ energy)

During exercise, the breathing rate and heart rate increase. During hard exercise an oxygen debt may build up.

Blood pressure is affected by:

• Diet, stress, exercise • body mass, alcohol consumption

Respiration releases energy for cells from glucose. This can be aerobic respiration, which needs oxygen, or anaerobic respiration, which does not.

The blood in the arteries is under pressure because of the contractions of the heart muscles. This allows the blood to reach all parts of the body.

Blood pressure is measured in millimetres of mercury, mmHg. There are two measurements: • systolic pressure - the higher measurement when the heart beats, pushing blood through the arteries, and • diastolic pressure - the lower measurement when the heart rests between beats

In the stomach, stomach acid helps the enzymes to work. Bile is an alkaline substance produced by the liver and stored in the gall bladder. It is secreted into the small intestine, where it emulsifies fats. This is important, because it provides a larger surface area in which the lipases can work.

A balanced diet will also include nutrients that do not provide energy, such as: • minerals, like iron, to make haemoglobin • vitamins, such as vitamin C, which prevents a disease called scurvy • fibre, which prevents constipation • water

Overweight people may suffer from health problems, including: • diabetes - an illness in which the body is unable to control the amount of sugar in the blood • arthritis - an illness in which the joints become worn, inflamed and painful • heart disease • breast cancer

Carbohydrates are made from simple sugars such as glucose and have a high energy source. (Carbohydrases)

Fats are made from fatty acids and glycerol. They are also one of our energy sources. (Lipases)

Proteins are made from amino acids + help our bodies grow and repair. (Proteases)

The substances which our body needs cannot be absorbed into the blood until they have been broken down into small, soluble chemicals. This is called chemical digestion, and it works with the help of enzymes. Once the food is completely digested, its molecules are small enough to pass through the wall of the small intestine by diffusion.

What’s for lunch? Keeping healthy

Food is moved through the digestive system by the contractions of two sets of muscles in the walls of the gut. One set runs along the gut, while the other set circles it. Their wave-like contractions create a squeezing action, moving down the gut.

We must bite off small pieces and chew them up into even smaller parts, before swallowing them. When the food gets into our stomach it is broken down further as it is squeezed and moved around by the stomach’s muscular walls. This is physical digestion.

People in developing countries may not get enough protein in their diet. This is called protein deficiency. It can lead to a disease called kwashiorkor, which causes a swollen abdomen. RDA in g = 0.75 × body mass in kg

The Body Mass Index (BMI) is a guide to whether someone is underweight, normal weight or overweight. You can calculate the BMI using this formula:

BMI = mass in kg ÷ (height in m)2

Vaccination involves putting a small amount of an inactive form of a pathogen, or dead pathogen, into the body. Vaccines can contain: • live pathogens treated to make them harmless • harmless fragments of the pathogen • toxins produced by pathogens • dead pathogens

Keeping healthy

White blood cells can: • engulf pathogens and destroy them • produce antibodies to destroy pathogens • produce antitoxins that neutralise the toxins released by pathogens

Hydrochloric acid in the stomach kills harmful microorganisms that might be in the food or drink that we swallow.

If microorganisms get into the body through a cut in the skin, the most important thing to do is close the wound quickly so that no more microorganisms can enter. A scab does just that. The blood contains tiny structures called platelets, and a protein called fibrin.

Bacteria are microscopic organisms that come in many shapes and sizes. But even the largest ones are only 10 micrometres long - 1 micrometre = 1 millionth of a metre. Bacteria cause diseases such as cholera.
Viruses are many times smaller than bacteria. They consist of a fragment of genetic material inside a protective protein coat. Viruses cause diseases such as influenza
Fungi include moulds and mushrooms. Microscopic fungi can cause diseases such as athlete’s foot.
Protozoa are single-celled organisms. Food contaminated with protozoa can cause infections such as amoebic dysentery, of which severe diarrhoea is a symptom. Some protozoa are parasites. These organisms live on, or inside, another organism and cause it harm. Malaria is a disease caused by protozoa that live in the blood. It is passed to a person by an insect vector, the mosquito.

The skin covers the whole body. It protects the body from physical damage, microbe infection and dehydration. Its dry, dead outer cells are difficult for microbes to penetrate, and the sebaceous glands produce oils which help kill microbes.

Pathogens are microorganisms that cause disease. The body has several defence mechanisms to prevent pathogens from entering the body and reproducing there. The immune system can destroy pathogens that manage to enter the body. They include microorganisms such as bacteria, viruses, fungi and protozoa.

If you become infected again with the same pathogen, these white blood cells reproduce very rapidly and the pathogen is destroyed. This is active immunity. Sometimes you may be treated for infection by an injection of certain antibodies from someone else. This is passive immunity.

Over time, bacteria can become resistant to certain antibiotics. MRSA is methicillin-resistant Staphylococcus aureus. It is very dangerous because it is resistant to most antibiotics. To slow down or stop the development of other strains of resistant bacteria, we should: avoid the unnecessary use of antibiotics and always complete the full course.

The respiratory system is protected in several ways. Nasal hairs keep out dust and larger microorganisms. Sticky mucus traps dust and microbes, which are then carried away by cilia - tiny hairs on the cells that line the respiratory system.

Someone with long-sightedness can see distant objects clearly, but cannot focus properly on near objects. This is because the lens focuses the sharpest image behind the retina, instead of on it. Long-sightedness is called hypermetropia. This defect is often age-related, and due to a loss of elasticity in the lens. It is corrected by putting a convex lens in front of the eye.

Someone with short-sightedness can see near objects clearly, but cannot focus properly on distant objects. This is because the lens focuses the sharpest image in front of the retina, instead of on it. Short-sightedness is called myopia. This is caused by the eyeball being elongated, so that the distance between the lens and the retina is too great. It can be corrected by placing a concave lens in front of the eye.

Light is focused onto the retina by refraction at the cornea, and by the lens. The lens changes shape to make the fine adjustments needed to produce a sharp image. This is called accommodation. As we get older, accommodation becomes slower and weaker.

Colour blindness is an inherited condition. The retina contains cells that are sensitive to red, green or blue light. People with colour blindness have a lack of receptors, or defects in them. People with red-green colour blindness, for example, have difficulty distinguishing shades of red and green.

Because our eyes sit side by side, each eye captures a slightly different view. This is called stereoscopic vision. When signals from the two eyes reach the brain, they are superimposed and processed into a single picture with depth. As a result, we get a 3D picture and are able to judge distances well.
Most birds and lizards have monocular vision - their eyes are on each side of their head. This gives them a greater field of view, which is useful for spotting predators. However, they have poor depth perception.

Light passes through the eyeball to the retina. It is refracted by the cornea and lens, so that the light is brought to a focus on the retina.

Cornea- Front part of the tough outer coat, the sclera. It is convex and transparent. Refracts light - bends it as it enters the eye

Iris-Pigmented - decides the colour of your eyes - so light cannot pass through. Its muscles contract and relax to alter the size of its central hole or pupil. Controls how much light enters the pupil

Lens-Transparent, bi-convex, flexible disc behind the iris attached by the suspensory ligaments to the ciliary muscles. Focuses light onto the retina

Retina -The lining of the back of eye containing two types of photoreceptor cells - rods - sensitive to dim light and black and white - and cones - sensitive to colour. A small area called the fovea in the middle of the retina has many more cones than rods. Contains the light receptors

Optic nerve Bundle of sensory neurones at back of eye. Carries impulses from the eye to the brain

The eye

The nervous system consists of the central nervous system - brain and spinal cord -and peripheral nerves. Nerve impulses travel through the axons of neurones - nerve cells. Reflex actions are fast, automatic and protective responses.

The central nervous system - CNS - in humans consists of the brain and spinal cord. When a receptor is stimulated, it sends a signal along the nerve cells - neurones - to the brain. The brain then co-ordinates the response.

The sense organs contain receptors that are sensitive to stimuli. Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus. Skin- touch, pressure, pain and temperature Tongue – chemicals in food Nose – chemicals in the air Eyes – light Ears – sound, position of head and balance

Keeping in touch

Motor neurones carry nerve impulses to effectors such as the muscle. The nerve impulse is carried din the axon. The sheath acts as an insulator - rather like the plastic coating of an electrical wire. Neurones are adapted to their function by being long, so that they communicate with distant parts of the body. They also have branched endings called dendrites. These connect with many other neurones.
The gap between two neurones is called a synapse. The nerve impulse passes across this gap through chemicals that diffuse across the gap. The electrical signal travels along an axon and triggers the release of chemical transmitters from the nerve ending of the first neurone. These diffuse across the gap and make the second neurone re-transmit the electrical signal.

The main parts of the nervous system are the central nervous system - the CNS - the brain and spinal cord - and the peripheral nervous system. Nerve impulses are mainly electrical and carried in nerve cells, or neurones.

Hand on candle –stimulus (heat)

Pain receptor stimulated

Signal passed along sensory neurone

Signal passed along relay neurone

Signal passed along motor neurone

Effector contracts (muscle – response)

When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. This is a reflex action. Reflex actions are rapid and happen without us thinking. • receptor detects a stimulus - change in the environment • sensory neuron sends signal to relay neuron • motor neuron sends signal to effector • effector produces a response
The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action.

Drugs are chemicals that can alter the way the body works. There are different types of drugs, and these have different effects on the body. Tobacco and alcohol are legal recreational drugs which have potentially serious effects on the body. Illegal drugs are classified on a scale from Class A - the most dangerous - to Class C - the least dangerous.

The alcohol in alcoholic drinks is called ethanol. It is a depressant. This means that it slows down signals in the nerves and brain.
There are legal limits to the level of alcohol that drivers can have in the body. This is because alcohol impairs the ability of people to control their vehicles properly.
Alcohol has short-term effects such as sleepiness and impaired judgment, balance and muscle control. This leads to blurred vision and slurred speech. Vasodilatation occurs - blood vessels in the skin carry more blood - leading to heat loss.

The long-term effects of alcohol include damage to the liver and brain. The liver removes alcohol from the bloodstream, as it is a toxic chemical. Over time, alcohol consumption can lead to liver damage - cirrhosis.

Carcinogens are substances that cause cancer. Tobacco smoke contains many carcinogens, including tar. Smoking increases the risk of lung cancer, mouth cancer and throat cancer.

|class |examples |Possession penalty |Dealing penalty |
|Class A |ecstasy, LSD, heroin, |Up to 7 years in prison|Up to life in prison or|
| |cocaine, crack, magic |or an unlimited fine |an unlimited fine |
| |mushrooms, injected | | |
| |amphetamines | | |
|Class B |amphetamines |Up to 5 years in prison|Up to 14 years in |
| | |or an unlimited fine |prison or an unlimited |
| | | |fine |
|Class C |cannabis, |Up to 2 years in prison|Up to 14 years in |
| |tranquilisers, |or an unlimited fine |prison or an unlimited |
| |painkillers | |fine, |

Carbon monoxide combines with the haemoglobin in red blood cells and so reduces the ability of the blood to carry oxygen. This puts extra strain on the circulatory system, and can cause an increased risk of heart disease and strokes.
Smoking during pregnancy is very dangerous. It reduces the amount of oxygen available to the growing fetus. This leads to an increased risk of • miscarriage and premature birth • low weight of babies at birth

Drugs and you

About 114,000 people die every year as a result of smoking-related illnesses. All cigarettes sold now carry a prominent health warning.
Cigarettes contain about 4,000 different chemicals, many of which are harmful to the body. Nicotine is the addictive substance in tobacco smoke. It reaches the brain within 20 seconds and creates a dependency so that smokers become addicted.

|type of drug |effect on the body |example |
|depressant |slows down brain activity |alcohol, solvents, temazepam |
|hallucinogen |alters what we hear and see |cannabis, LSD |
|painkiller |blocks nerve impulses |aspirin, heroin |
|performance enhancer |improves muscle development |anabolic steroids |
|stimulant |increases brain activity |nicotine, caffeine, ecstasy |

Conditions in the body are controlled, to provide a constant internal environment. This is homeostasis. The conditions that must be controlled include body temperature, water content, carbon dioxide level, and blood sugar level. The nervous system and hormones are responsible for homeostasis.
Hormones are chemicals secreted by glands. They travel through the bloodstream and affect target organs. Sexual development, the menstrual cycle and fertility in women, and blood sugar levels, are all controlled by hormones. Blood sugar levels are controlled to provide cells with a constant supply of energy.

Body temperature (37°C) is controlled by: • controlling blood flow to the skin • sweating • shivering
It is the temperature at which enzymes work best.

The body’s water content is controlled to protect cells by stopping too much water from entering or leaving them. Water content is controlled by water loss from: • the lungs when we exhale • the skin by sweating • the body, in urine produced by the kidneys

Oestrogen, which causes the repair of the uterus wall
Progesterone, which maintains the uterus wall

• high temperatures can cause dehydration, heat stroke, and death if untreated • low temperatures can cause hypothermia, and death if untreated

The bloodstream transports hormones from the glands to the target organs. Bodily reactions to hormones are usually slower and longer lasting than nervous reactions.

Staying in balance

'The pill' is an oral contraceptive that greatly reduces the chances of mature eggs being produced. The pill contains oestrogen, or oestrogen and progesterone. These hormones prevent eggs from maturing in the ovaries.
Some women have difficulty becoming pregnant because they do not produce enough of a hormone called FSH, which is needed for eggs to mature. Fertility drugs contain FSH, which stimulates eggs to mature in the ovary.

|Brain |Contains sensitive area – hypothalamus which controls pituitary |
| |gland |
|Pituitary gland |Produces hormones important for controlling growth, menstrual |
| |cycle and lactation. Also controls glands |
|Thyroid gland |Produces thyroxine which controls metabolism rate |
|Pancreas |Produces insulin to control blood sugar levels |
|Testes |Produces testosterone which stimulates sperm production |
|Ovaries |Produces oestrogen and progesterone which helps maintain |
| |menstrual cycle |
|Adrenal gland |Produces adrenaline which prepares body for rapid activity by |
| |increasing heart rate and level of sugar in blood. Diverts blood|
| |to muscles and brain |

Human enzymes usually work best at 37ºC, which is human body temperature. This can be measured in several places, including the ear, finger, mouth and anus.
There are various ways to measure body temperature, including using a clinical thermometer, heat-sensitive strips, digital probes or thermal imaging cameras.

The body’s temperature is monitored by the brain. If you are too hot or too cold, the brain sends nerve impulses to the skin, which has three ways to either increase or decrease heat loss from the body’s surface: 1. Hairs on the skin trap more warm air if they are standing up, and less if they are lying flat. Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss. 2. If the body is too hot, glands under the skin secrete sweat onto the surface of the skin, to increase heat loss by evaporation. Sweat secretion stops when body temperature returns to normal. 3. Blood vessels supplying blood to the skin can swell or dilate - called vasodilation. This causes more heat to be carried by the blood to the skin, where it can be lost to the air. Blood vessels can shrink down again - called vasoconstriction. This reduces heat loss through the skin once the body’s temperature has returned to normal.

Glucose is a sugar needed by cells for respiration. It is important that the concentration of glucose in the blood is maintained at a constant level. Insulin, a hormone secreted by the pancreas, controls blood sugar levels in the body. Diabetes is a disorder in which the blood glucose levels remain too high. It can be treated by carefully maintaining a certain diet or injecting insulin. The extra insulin allows the glucose to be taken up by the liver and other tissues, so cells get the glucose they need and blood sugar levels stay normal.

|Glucose level |Effect on pancreas |Effect on liver |Effect of glucose |
| | | |level |
|Too high |insulin secreted into the |liver converts glucose into |Goes up |
| |blood |glycogen | |
|Too low |insulin not secreted into |liver does not convert glucose |Goes down |
| |the blood |into glycogen | |

These happen to both boys and girls: • pubic hair grows
Boys:
• voice breaks - gets deeper • hair grows on face and body • body becomes more muscular • testes and penis get bigger • testes start to produce sperm cells
Girls:
• hips get wider • breasts develop • ovaries start to release egg cells - periods start
.

DNA molecules contain chemicals called bases. The four bases in DNA are labelled A, T, C and G.
Only some of the genes are used in any individual cell, with the rest being switched off. Genes code for the production for proteins. Some of these are enzymes which control how the cell functions.

The nucleus of a cell contains chromosomes, which carry genetic information in a long molecule called DNA. Genes are regions of DNA that carry the code to control a particular cell activity or the production of a particular protein.
Asexual reproduction produces genetically identical individuals, but sexual reproduction produces unique individuals.

The nucleus of a cell contains chromosomes that carry genetic information. Most body cells have the same number of chromosomes, but this varies between species. Human body cells, for example, each have 23 matching pairs of chromosomes.
Chromosomes are made from long, coiled molecules of DNA. A gene is a shorter region of DNA that carries the genetic code for a particular characteristic or cell activity.

Asexual reproduction only needs one parent. All the offspring are genetically identical to each other, and their parent. They are clones.

Some characteristics are caused by the environment, some by inherited factors, while some are caused by both environment and inherited factors.
Male mammals carry XY sex chromosomes - female mammals carry XX sex chromosomes. Inherited disorders are caused by faulty genes on these chromosomes. Different versions of a gene are called alleles, and these alleles can be dominant or recessive.
Inherited characteristics: • the shape of the earlobes • eye colour • nose shape

Sexual reproduction needs two parents. Each parent produces sex cells, called gametes: • male gametes are called sperm • female gametes are called eggs
Each gamete contains half the number of chromosomes that a body cell has. At fertilisation, male and female gametes fuse or join together. The new cell contains the full number of chromosomes.
Half of its genes have come from the father, and half have come from the mother. The genetic material from both parents has combined to form a unique individual.

Gene control

In humans and other mammals, gender is controlled by a pair of sex chromosomes. Females have two X chromosomes, XX. Males have one X and one Y chromosome, XY.

Who am I?

The language we use, or whether we have scars, are environmental characteristics.

Some characteristics - including intelligence, body mass and height - are the result of both environmental and inherited factors. But there is debate about the relative importance of the two types of factor in some human characteristics, such as intelligence, health and sporting ability.

Genetic variation can be caused by: • the formation of gametes - sex cells • fertilisation - the joining of gametes • mutation
A mutation is a change to the genetic code in a gene. Mutations can happen spontaneously. They can also be caused by radiation and certain chemicals. Mutations may be beneficial, but most are harmful.
Inherited disorders such as red-green colour blindness, sickle cell anaemia and cystic fibrosis are caused by faulty genes.

Genetic diagrams are used to show the possible outcomes of a particular cross. A dominant allele is shown by a capital letter and a recessive allele by a lower case letter.

Alleles are different versions of the same gene. An allele can be dominant or recessive. Individuals, meanwhile, can be homozygous or heterozygous: • individuals who are homozygous for a certain gene carry two copies of the same allele • individuals who are heterozygous for a certain gene carry two different alleles
A recessive characteristic will only be shown if an individual is homozygous for the recessive allele. A dominant characteristic will be shown even if an individual is heterozygous for the dominant allele.