Biology Re sit!!!

Enzymes are biological catalysts. They speed up chemical reactions in all living things, and allow them to occur more easily. They occur in plant cells and animal cells. Without them we would not be alive.

Enzymes are just chemical molecules, made up of proteins.

Each particular enzyme has a unique, 3-dimensional shape shared by all its molecules. Within this shape there is an area called the active site where the chemical reactions occur.

Some enzymes help to break down large molecules.

Others build up large molecules from small ones.

While many others help turn one molecule into another.

Probably the fastest enzyme known is called catalase. It breaks the chemical hydrogen peroxide down to water and oxygen. Catalase is found in all cells and protects them from this dangerous waste chemical.

Each type of enzyme has its own specific optimum condition under which it works best.

Enzymes work best when they have a high enough substrate concentration for the reaction they catalyse. If too little substrate is available the rate of the reaction is slowed and cannot increase any further.

The pH must be correct for each enzyme. If the conditions are too alkaline or acidic then the activity of the enzyme is affected. This happens because the enzyme's shape, especially the active site, is changed. It is denatured, and cannot hold the substrate molecule.

Temperature is a key factor too. If it is too cold the enzymes will move around too slowly to meet the substrate molecules, so the reaction rate is slowed. Likewise, if it is too warm they do not work properly either. This is because the extra heat energy shakes them around so much that the active sites change shape so, just like with pH, the enzyme molecules are denatured, and can't hold the substrate.

Enzymes control all kinds of reactions in all cells. For example, they help control; respiration, photosynthesis, and our digestion, amongst many others.

Protease and lipase enzymes are used in biological washing powders to remove those stubborn stains.

Enzymes are also used in making foods and drinks. The enzyme pectinase helps to break down the cells in fruit to release more of their juice.

Plants and animal cells share the same basic structural features, although plant cells have a few extra bits.

Animal Cells

Animal cells come in all kinds of shapes and sizes but have the same basic features.

The control centre is the nucleus; this contains all the genetic information for the cell and controls all its activities.

The cytoplasm is like a big soup of chemicals in which the reactions occur.

Then forming the outside of the cell is the cell membrane, which acts as a barrier and controls the transfer of materials into and out of the cell.

Plant Cells

Plant cells also come in a variety of forms but share similar features. In addition to the three basic features found in animal cells, plant cells have some useful extra ones.

Firstly they have a rigid cell wall made of fibres of cellulose (which we use to make paper!) that gives them shape and strength. The cell wall fits closely just outside the cell membrane like a plastic box with an inflated balloon stuffed inside.

Secondly they have a vacuole, which stores extra water and gives extra support to the cell by pressing hard against the cell wall.

Thirdly, most plant cells also contain small round structures called chloroplasts, which contain the green pigment chlorophyll, which is needed for photosynthesis.

Tissues, organs and organisms

A living plant or animal is called an organism and is made up of lots of cells all working together.

Some of these cells are all of the same type, collectively they are called a tissue. They all do the same job, for example connective tissue, which is used in animals to connect other tissues together - and stop us falling apart!

Next, some different tissues are grouped together to make up an organ such as the stomach or a leaf.

Finally, some organs work together to form an organ system such as the digestive system.

Characteristics of Life

All living things show 7 characteristics of life. All plants and animals are alive.

The 7 characteristics are:

• Movement
• Respiration
• Sensitivity
• Growth
• Reproduction
• Excretion
• Nutrition

Moving Molecules

Diffusion: Molecules move from a place of high concentration into a place of low concentration.

Osmosis: a special kind of diffusion, which is very important in Biology; it keeps us alive!!

Key Facts

1. Osmosis only involves water molecules- nothing else. Osmosis is water diffusion.
2. Osmosis occurs across a barrier such as a cell membrane which is 'selectively permeable', which means it only lets through small molecules like water.

Respiration is the reason that we need oxygen.

Respiration is the process of releasing energy from food. You could think of it as burning the fuel, glucose.

Glucose is the key molecule. It is made by plants in photosynthesis. In animals, although lots of different molecules are absorbed after digestion they are usually turned into glucose. In animals glucose is especially important as two organs can only use glucose as a fuel.

The two organs which use glucose as fuel are; the brain and the heart.

Respiration summarised in this equation:

glucose + oxygen > carbon dioxide + water + energy

Aerobic Respiration: involving oxygen

Anaerobic Respiration: without oxygen

Photosynthesis is the way that plants make their food using energy from sunlight.

What is the word equation for photosynthesis?

Carbon Dioxide + Water >>sunlight>> Sugar + Oxygen

Chlorophyll: green dye (or pigment) plants use to pick up the energy from the sunlight.

Plants make sugar and use some of it for energy to keep them alive (respiration) but they also use some for growth and repair by making fats and proteins.

Plants can use starch or glucose. Starch is insoluble (it does not dissolve in water) while glucose is soluble. This means that if starch is used, less water is required to keep its food stored.

Things which might effect how well plants can carry out photosynthesis. The amounts of water, carbon dioxide, sunlight and temperature.

Amount of water: effected by how much is taken up through the roots and how much is lost from the leaves. If less water is available in the leaf then photosynthesis will occur more slowly.

Carbon Dioxide: if there is less carbon dioxide around then photosynthesis will occur more slowly. There wont be enough of the fuel (substrate) to get the reaction to work

Sun: If there is less sun, which usually means it is cooler too, then there is less energy for photosynthesis and it occurs more slowly. So photosynthesis works best when it is warm and sunny - don't we all!

Homeostasis means keeping a constant internal environment. It is carried out around the whole body. Homeostasis reaches from every cell up to whole organs and systems.

If there was not a constant internal environment, our enzymes would not work properly. That would mean that nothing would operate correctly and we would die.

There are basically 6 things that are essential for health and that must be controlled:

1. Carbon Dioxide
2. Urea
3. Ions
4. Sugar
5. Water
6. Temperature

A cool way to remember these 6 things is by learning this...

• When (Water)
• Shall (Sugar)
• I (Ions)
• Clean (CO2)
• The (Temperature)
• Utensils (Urea)

Hypothalumus: monitors water, temperature and carbon dioxide content of blood.

Pituitary gland: secretes a number of hormones, a key one is ADH which is important in regulating the water content of the body.

Liver: helps to control glucose content of the body by storing it as glycogen. It is also involved in temperature regulation, acting as the body's furnace by increasing the rate of respiration when we are cold.

Lungs: involved by getting rid of carbon dioxide from the body.

Pancreas: involved in maintaining a constant amount of glucose in the body through the actions of glucagon and insulin.

Muscles: of the body can help to maintain a stable body temperature as muscular activity and shivering help to generate heat.

kidney: are involved in controlling the amount of water in the body.

Skin: is the largest organ and has a central role in maintaining a constant temperature.

Temperature control is important for the normal operation of enzymes and cells.

The brain has a key role in co-ordinating this function. Near the bottom of the brain is a place called the hypothalamus, which monitors a number of key things in the body, including temperature.

When the hypothalamus detects a change in the temperature of the blood it sends impulses down neurones to the skin.

The skin is described as the biggest organ in the body. An average person has about 2 square metres of skin.

The skin keeps our water in, has a layer of fat to keep us warm and is tough enough to keep out microbes that might cause disease. It is also a great place for nerve receptors.

In addition to all of this, the skin has some interesting mechanisms to help control temperature. It can alter blood flow, hair position and the amount of sweating.

When we get too hot the hypothalamus sends impulses to the skin which cause 3 things to happen:

1. Our hairs lie flat: so letting more heat out.
2. We sweat: the evaporation of this cools us down.
3. More blood goes through the skin: this acts like a radiator to radiate out heat.

If we get too cold the hypothalamus sends other impulses so that the reverse happens:

1. Our hairs stand up: this traps a layer of air which acts like an insulator.
2. We stop sweating: this stops the heat loss by evaporation.
3. Less blood goes through the skin: the skin will appear paler and colder.

The chemical messenger between the brain and the kidney is the hormone ADH, Anti-Diuretic Hormone.

The important parts of the process involve:

1. The hypothalamus in the brain, which detects the lower blood water content.
2. The pituitary gland at the base of the brain, which releases the hormone ADH.
3. The kidney, which reabsorbs the water.

In order to get back to the normal level of water in the blood we absorb more water from the digestive system, feel thirsty, and so drink more.

A similar sequence of events occurs when there is too much water in the body. This time, some of the details are reversed from what they were when there was too little water.

The genetic code is carried by an amazing molecule called Deoxyribonucleic acid, or DNA to its friends. DNA is an amazingly long and complicated molecule.

The DNA is found in the nucleus of all cells. It is formed into X-shaped bundles called chromosomes. In human cells, except for eggs and sperm, there are 46 chromosomes. These are divided into 23 pairs.

Each chromosome has the appearance of two knitted sausages tied together in the middle. The more scientific description would be that a chromosome is made up of two chromatids held together in the middle by a centromere. You choose which is easier to remember!

Mitosis

Mitosis is the process used during growth to make new cells within a plant or animal. It is also used during asexual reproduction, in which an individual can clone itself to produce identical offspring.

However our body often has to make new cells to replace damaged ones or as we grow. So human cells also go through mitosis in the same way as animal and plant cells but is for growth and repair.

The offspring cells have the same number of chromosomes as the parent cells, therefore they are diploid.

Meiosis

Meiosis is a slightly different process. It is used to create the gametes, these are the sperm or eggs, used in sexual reproduction. The offspring produced during sexual reproduction have characteristics, selected from those of the parents.

The main difference in meiosis as compared to mitosis is that the new cells have half the number of chromosomes as the diploid 'parent' cell. One chromosome comes from each homologous pair of chromosomes. So these offspring cells are haploid not diploid.

Homozygous and heterozygous

Chromosomes come in pairs. Each chromosome in a pair will have a gene at the same point on the chromosome. There can be more than one alternative form of the gene at that point. These alternative forms are called alleles.

Both chromosomes in a pair have one allele for the gene. If the two alleles are the same we say that the individual is 'homozygous' for that gene. It they are different the individual is 'heterozygous'.

Dominant and Recessive

HH = Homozygous dominant

hh = Homozygous recessive

Hh = Heterozygous