Biology AQA GCSE - B2

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Animals and Plants

Cells are the smallest unit of life; human cells include a

nucleus, cytoplasm, cell membrane, mitochondria, ribosomes while plants have

additions of a cell wall, chloroplasts and a permanent vacuole.

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Nucleus – controls the cells activities

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Cytoplasm – where the most chemical reactions

take place

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Cell Membrane – controls what enters and leaves

the cell

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Mitochondria – where energy is released during

respiration

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Ribosomes – where protein synthesis happens

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Cell Wall – strengthens the plant’s cells

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Chloroplasts – absorbs light energy for food

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Permanent Vacuole – filled with cell sap

Enzymes are found in mitochondria, chloroplasts and

ribosomes to control the chemical reactions inside the cells. Some cells may be

specialised e.g. root hair cell, palisade cell, white blood cell, sperm cell

and nerve cell.

Tissues are groups of cells, organs are groups of tissues,

and a system is a group of organs all for a particular functions.

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Movement In and Out of Cells

For this to happen, a dissolved substance has to cross the

cell membrane. A solute is a particle in

a solution, while a solvent is the liquid in the solution. Both set of

molecules move randomly but only solutes can move in and out cells.

Diffusion is the spreading of the solute from a high

concentration (HC) to a low concentration (LC), the greater the difference in

concentration the fast the rate of diffusion. E.g. the oxygen required for

respiration uses diffusion.

Osmosis is the diffusion of water from a LC to a HC, if the

concentration is higher on one side the solute will travel in that direction.

In animal cells if there is a HC than the cytoplasm then the cell will shrink

and die, if there is a LC then the cell will burst and die. In plant cell, if

there is a HC compared to the cytoplasm the cell/plant will wilt but if there

is a LC then the cell will remain upright.

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Photosynthesis

Photosynthesis is the process of which light is used to make

glucose from carbon dioxide and water. Light is absorbed by chlorophyll, which

makes the plant look green through green wavelengths of light; this energy is

then used for photosynthesis. Light energy converts 6CO2 and 6H2O

into C6H12O6 and 6O2, oxygen is the

by-product. It happens in the leaves in the mesophyll cells that contain lots

of chloroplasts that contain chlorophyll. There are two types of mesophyll cell

they are palisade cells and spongy cells.

The limiting factors of photosynthesis are temperature,

light intensity and carbon dioxide. The lower the temperature is the slower

moving molecules so slower reactions happen. The less light the less energy to

power the reaction. The less carbon dioxide there is there is a shortage of

molecules available for the reaction.

In farming they manipulate the growth of plants with the

above conditions; they are usually in greenhouses or polythene tunnels. They

control the temperature with heaters and ventilation, they can increase the

carbon dioxide levels, they can control light using florescent lighting; this

make them grow faster with optimum conditions.

Plants use glucose as an insoluble starch for storage and it

is also used by the plant itself. The plant roots absorb mineral salts

including nitrates for healthy growth. Nitrates are used for making amino acids

for protein, if absent there is a growth stunt. Magnesium is used for

chlorophyll production; if absent the leaves grow yellow.

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Energy Flow in Ecosystems

Food Chains use radiation from the sun as an energy source

in most communities of living organisms. Green plants take in the solar energy

that reaches them, stored in substances that make up the plants’ cells. A food

chain represents relationships between organisms and the direction of energy

flow. A food web represents all feeding relationships between organisms in an

ecosystem.

At each stage of a food chain, less material and energy is

contained in the biomass of an organism. The amount of material and energy in

the biomass of an organism is reduced at each stage of the food chain. Some

materials and energy are lost in the waste products; most is lost to the

surroundings as heat energy. Respiration supplies the energy needed for living

processes. The energy loss is larger in mammals and birds as they have a

higher, constant temperature in comparison to the surroundings.

A pyramid of numbers is the number of organisms at each

trophic level in the food chain. A pyramid of biomass is the mass of the living

materials, these have a pyramid shape and each stage is less than the previous

stage.

To improve the energy transfers in farming, the farmers

reduce the number of stages in the food chain so there is less energy is lost

between the producers and humans. This requires more people to convert to a

vegetarian diet. This is necessary if developing countries are to sustain their

population growth. The efficiency of food production can be improved by

restricting movement of the animals and controlling the temperature of their

surroundings. This is battery farming and is thought of as unethical due to the

limit in movement, pain and discomfort and the unnatural environment.

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Recycling of Waste Materials

Living things remove materials in the environment for growth

and other processes, these materials return to the environment either as in

waste materials or when an organism dies and decays. Material decay is a result

of being digested by decomposers. The optimum conditions for material to decay

in warm, moist conditions; they are also more active where there is more oxygen.

The decay process release substances for the plants to grow in; a stable

community is balanced with the materials removed and returned. Materials are

constantly cycled.

The carbon cycle is the constant cycle of carbon in

ecosystem. CO2 is removed from the environment by photosynthesis via

green plants; the carbon is used to make carbohydrates, fats and proteins for

the body of the plants. Some of the CO2 is returned when green

plants respire. When green plants are eaten by animals and these animals are

eaten by animals, the carbon becomes fats and proteins in their bodies. When

animals respire come of the carbon becomes CO2 and is released into

the atmosphere. When plants or animals die, other animals or micro-organisms

feed on their bodies. By the time micro-organisms or detritus feeders have

broken down the waste products, all the energy from the green plants have been

transferred.

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Enzymes

Enzymes are biological catalysts; they increase the rate of

chemical reactions. Enzymes are protein molecules made up of long chains of

amino acids; these long chains are folded to produce a special shape to enable

other molecules to fit into the enzymes, this is vital for the enzymes’

functions. Only one type of substrate will fit into the active site of the

enzyme. In order for the chemical reaction to take place, energy is required,

called the activation energy. Enzymes reduce the activation energy.

With temperature increase the enzyme-controlled reaction

increases. The enzyme and substrates move faster so collide more often, the

fastest working enzymes are called the optimum. This is true up to 40°C, higher

than this and the enzyme structure will change. This results in the active site

becoming a different shape and the substrate no longer fits – denatured.

With pH the shape of the active site will also change if

goes over the optimum, a large change can stop the enzymes completely. It is

done by affecting the forces that hold the enzyme molecules together. Different

enzymes can work best at different pH values e.g. Stomach enzymes = acidic

conditions, Mouth enzymes = neutral conditions etc.

Enzymes in living cells catalyse processes of respiration,

photosynthesis and protein synthesis. Respiration is the process of

transferring energy from food molecules with or without oxygen. Respiration is

many series of small reactions, all which are controlled by enzymes. Most

respiration reactions happen in the mitochondria, the inner surface is highly

folded to increase the surface area for enzymes. The energy released during

respiration is used to build up larger molecules using smaller ones, enable

contractions of muscles (animals), maintain a steady body temperature (mammals

and birds), build up sugars, nitrates and other nutrients into amino acids to

make proteins (plants).

Some enzymes work outside the body cells, called

extracellular enzymes. The digestive enzymes are produced by specialist cells

in glands and in the lining of the gut, which is where they are secreted from

so they can come into contact with food molecules. They catalyse the digestion

of large molecules into smaller ones. Digestion is the process of food breaking

down into molecules that the body can absorb, this nutrition where the body is

taking in and using food. Digestion in the mouth – the food is chewed to create

a larger surface area for enzymes; the enzymes are from the saliva which h

contains amylase. Amylase digests starch into maltose, which enables swallowing

via the oesophagus. Digestion in the stomach – the food enters from the oesophagus;

the walls of our stomach produce juice containing pepsin (protease enzyme) to

digest protein into amino acids, hydrochloric acid to kill bacteria in our food

and mucus to protect the wall of our stomach from the acid and pepsin. The

stomach wall is muscular and churns our food which remains in our stomach for a

few hours. The food goes from the stomach into the small intestine. Digestion

and absorption in the small intestine – the small intestine completes the

digestion of food and absorb the soluble products of digestion in to the blood.

The three juices release are: Bile – produced by the liver, stored in the gall

bladder and released into the small intestine to neutralise the stomach acid

and to convert large fat droplets into small fat droplets for a larger surface

area, Pancreatic and Intestinal juice – both are released into the small

intestine and contain amylase (starch into sugar), protease (protein into amino

acids) and Lipase (fats into fatty acids and glycerol).

Enzymes are made use of when some micro-organisms can produce

enzymes that pass outside of cells. These enzymes have many uses in the home or

in industry. At home, biological detergents contain protein and fat digesting

enzymes so the clothes can be washed at lower temperatures and it is easier to

remove stains but if not rinsed fully protease will remain and digest the

protein of the skin which can lead to irritation, allergies and dermatitis. In

industry, protease is used to pre-digest the protein in baby foods, carbohydrase

is used to convert starch into sugar syrup, isomerase is used to convert

glucose syrup into fructose syrup so industrial reactions can take place at

lower temperatures which means it is cheaper to run the process but the enzymes

are sensitive to temperature and pH changes so they need to be carefully

monitored and controlled (expensive).

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Homeostasis

This is the controlling of internal conditions. Waste

products which have to be removed from the body include CO2 from

respiration, Urea from the liver from the breakdown of excess amino acids. Internal

conditions that are controlled include: Water content of the body, ion content

of the body, temperature and blood sugar levels. Osmoregulation is the control

of water and ion, if this is wrong then the water movement will damage the

cells, they enter our bodies through the food and drinks that we consume.

Sweating helps keep cool as more water is loss when it’s hot so more water has

to be taken to balance the loss, the water content is controlled in the

kidneys. Thermoregulation is the maintenance of body temperature, where all

enzymes work best at 37°C. The metabolism of cells release heat which warms the

animal’s body, the metabolism is the most efficient if the internal temperature

is maintained when the external conditions change. The brain is the thermoregulatory

centre where body temperature is monitored and controlled; it had receptors

sensitive to temperature of blood flowing through the brain and also the

receptors in the skin send impulses to the centre about the skin temperature.

The skin contains sweat glands and a network of capillaries

just under the surface. If the main body temperature is too high the

capillaries will dilate so there is more blood flowing through the capillaries

and more heat loss from the skin and the sweat glands release more sweat so the

body cools when it evaporates. If the core body temperature is too low then the

capillaries will constrict to reduce the blood flow in the body so there is a

minimum heat loss and the muscles will shiver as their contractions need respiration

which releases some heat energy.

The blood glucose concentration of the body is monitored and

controlled by the pancreas which produces insulin to allow glucose to move from

the blood to the cells. Food is eaten → Carbohydrates are digested into glucose

→ Glucose is absorbed into the blood → Blood glucose levels are raised → This is detected by the pancreas →

Insulin is released into the blood → Insulin reaches the liver → The glucose is

converted into glycogen → Glycogen is stored → Blood glucose levels return to

normal. This ensures that blood glucose never rises too rapidly and that the

glycogen can be broken down to maintain regularity of blood glucose levels.

Diabetes is a disease where a person can’t produce enough

insulin in the pancreas; this could be fatal after a meal as the blood glucose

levels can’t be controlled. In the 1920s, cures were tested on dogs then humans

by injecting insulin into the patients with diabetes to control the blood sugar

levels; insulin from pigs and cows were used to treat diabetic people, now

insulin is produced by micro-organisms with genetically modified cells that

contain human insulin and pancreas transplants are available but there aren’t

enough donors. In the future, stem cells may be used to cure diabetes; however

this requires human embryos which many people disagree with.

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Inheritance

In the nucleus of a human body there are 23 pairs of

chromosomes, 23 chromosomes from each parent which are made of a large molecule

– DNA. Deoxyribose Nucleic Acid (DNA) can replicate prior to mitosis or meiosis

and its code is used to synthesise proteins. A gene is a small section of DNA

and each one codes for a combination of amino acids for a specific protein

which determines our characteristics. Some characteristics are controlled by a

single gene and each different form is called an allele.

Each person has a unique DNA (excluding identical twins)

which can be found in blood, saliva and semen. Special techniques are used to

cut out DNA and then separate them according to lengths, repeated sequences can

be identified and each individual’s DNA produces a specific pattern. This can

be used to identify criminals and find the biological father of a child.

New body cells are produced when an animal is growing, to

repair damaged tissues and to replace tissues. Mitosis occurs when all the

chromosomes are replicates and the nucleus is divided into two, with the result

of genetically identical nuclei. Some cells undergo cell division multiple time

and some cells carry out their functions until they die. Cell differentiation

is when some genes are turned on and other are turned off but specialised cells

only carry out their roles. Asexual reproduction is an offspring by mitosis of

the parent cells, where they contain the same