Biology GCSE (Year 11)

Cells, Plants, Energy.


Animal & Plant Cells (Structures in Common)

Animals and plant cells have some structures in common; they have:

  • a nucleus to control the cell's activities
  • cytoplasm where many chemical reactions take place
  • a cell membrane that controls the movement of materials 
  • mitochondria where energy is released during aerobic respiration
  • ribosomes where protiens are made (synthesised)

Plant cells also have:

  • a ridgid cell wall for support
  • chloroplasts that contain chlorophyll for photosynthesis
  • a permanent vacuole containing cell sap

REMEMBER: The cell membrane can control the movement of the materials into and out of the cell. This cell wall in plants, does not do this. It is there for support.

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Specialised Cells

When an egg is fertilised it begins to grow and develop.

At first there is a growing ball of cells. Then as the organism gets bigger some of the cells change and become specialised.

There are many different specialised cells, e.g.

  • Some cells in plants may become xylem or root hair cells.
  • Some cells in animals will develop into nerve or sperm cells.

Key Points:

  • As organisms develop, some of their cells become specialised to carry out particular jobs. This is called 'differentiation'.
  • Differentation happens much earlier in the development of animals than it does in plants.
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Osmosis is the movement of water.

Osmosis is a special case of diffusion involving a partially permeable membrane.

Just like diffusion, the movement of molecules is random and requires no energy from the cell.

Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution to a more concentrated solution. No solute molecules can move across the membrane. The cell membrane is partially permeable

Water is needed to support cells and because chemical reactions take place in solution

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Carbon Dioxide + Water (+ Light Energy) -> Glucose + Oxygen

  • The carbon dioxide is taken in by the leaves, and the water by the roots.
  • The chlorophyll traps the energy needed for photosynthesis.
  • In photosynthesis the sugar glucose (a carbohydrate) is made. Oxygen is given off as a waste gas.

Key Points:

  • Photosynthesis can only be carried out by green plants.
  • Chlorophyll traps the suns energy.
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Limiting Factors

  • Limiting Factor: The factor that controls the rate because it is in shortest supply.
  • A lack of light would slow down the rate of photosynthesis as light provides the energy for the process. Chlorophyll traps the light. Even on sunny days, light may be limited on the floor of a wood or rain forest.
  • If it is cold, then enzymes do not work effectively and this will slow down the rate.
  • If there is too little carbon dioxide, then the rate will slow down. Carbon dioxide may be limited in an enclosed space, e.g. in a greenhouse on a sunny day or in a rapidly photosynthesising rain forest.
  • If certain things are in short supply, they will slow down the rate of photosynthesis. Plant growers need to know this, otherwise they could waste money.
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Minerals in Plants

  • Plant roots take up mineral salts for healthy growth.
  • Nitrates are taken from the soil for producing amino acids. These are used to make proteins for growth. A plant that doesn't take up enough nitrate (is nitrate deficient) will have a stunted growth.
  • Plants also take up magnesium ions that are essential to produce chlorophyll. If the plant is deficient in chlorophyll it will have yellow leaves.
  • Nite/stunt + yellow/mag
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Pyramids of Biomass

Biomass if the mass of living material in plants and animals.

A pyramid of biomass represents the mass of the organisms at each stage in the food chain. It may be more accurate than a pyramid of numbers.

For example, one bush may have many insects feeding on it but the mass of the bush is far greater than the mass of the insects.

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Energy Losses

Not  all of the food eaten can be digested, so energy is lost in faeces (waste).

Some of the energy is used for respiration, which releases energy for living processes. This includes movement, so the more something moves the more energy it uses and the less is available for growth.

In animals that need to keep a constant temperature, energy from the previous stage of the food chain is used simply to keep the animal at the correct temperature. (e.g. 37 degrees in humans).

For a whole range of reasons there is energy loss between each stage of a food chain. This means that not all of the energy taken in by an organism results in the growth of that organism.

Energy is never really 'lost'. What we mean here is that all of the energy in one stage of the food chain does not result in the growth of organisms in the next stage.

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Energy in Food Production

The shorter the food chain, the less energy will be lost. It is therefore more efficient for us to eat plants than it is to eat animals.

We can artificially produce meat more efficiently by:

  • Preventing the animal from moving so it doesn't waste energy on movement.

This is seen as cruelty by many people and is controversial.

  • Keeping the animal at a warmer temperature so it doesn't use as much energy from food to keep itself at a constant temperature.
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Detritus feeders (such as some types of worm) may start the process of decay by eating dead animals or plants and producing waste materials. Decay organisms then break down the waste and dead plants and animals.

Decay organisms are micro-organisms (bacteria and fungi). Decay is faster if it is warm and wet.

All of the materials from the waste and dead organisms are recycled.

All organisms take up nutrients. If they didn't eventually release them the nutrients would run out

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The Carbon Cycle

Photosynthesis removes carbon dioxide from the atmosphere.

Green plants as well as animals respire. This returns carbon dioxide to the atmosphere.

Animals eat green plants and build the carbon into their bodies. When plants or animals die (or produce waste) micro-organisms release the carbon dioxide back into the atmosphere through respiration. 

A stable community recycles all of the nutrients it takes up.

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Enzyme Structure

Enzymes are biological catalysts - they speed up reactions.

Enzymes are large proteins and each has a particular shape. This shape has an area where other molecules can fit in. This area is called the 'active site'.

Too high a temperature will change the enzyme shape, and it will no longer work. We say it has been destroyed or denatured.

Enzymes can catalyse the build up of small molecules into large molecules or the break down of large molecules into small molecules.

Enzymes lower the amount of energy necessary for a reaction to take place - the 'activation' energy.

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Factors affecting enzyme action

Reactions take place faster when it is warmer.

At the higher temperature the molecules move around more quickly - so collide with each other more often and with more energy.

If the temperature gets too hot the enzyme stops working. This is because the active site changes shape and the enzyme becomes denatured.

Enzymes work best in certain acidic or alkaline conditions (pH). If the pH is too acidic or alkaline for the enzyme, then the active site could change shape. The enzyme would stop working.

*Enzymes are very sensitive to both temperature and pH*

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Aerobic Respiration

Glucose + Oxygen -> Carbon Dioxide + Water [+ Energy]

Aerobic respiration is the release of energy from food when oxygen is available.

The process mostly takes place in mitochondria.

The energy released is used to:

  • Build larger molecules from smaller ones
  • Enable muscle contraction in animals 
  • Maintain a constant body temperature in mammals and birds
  • Build sugars, nitrates and other nutrients in plants into amino acids and then proteins.

Plants and animals both respire - plants do not just photosynthesise. 

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Enzymes in Digestion

Digestion involves the breakdown of large, insoluble molecules into smaller soluble molecules.

  • Amylase is produced by the salivary glands, the pancreas and the small intestine.  Amylase catalyses the digestion of starch into sugars in the mouth and small intestine.
  • Protease is produced by the stomach, the pancreas and the small intestine. Protease catalyses the breakdown of proteins into amino acids in the stomach and small intestine.
  • Lipase is produced by the pancreas and small intestine. Lipase catalyses the breakdown of lipids (fats and oils) to fatty acids and glycerol.

Key Points:

  • Without enzymes digestion would be too slow.
  • There are specific conditions in different parts of the gut that help enzymes to work effectively. 
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Speeding up Digestion

  • Protease enzymes in the stomach work best in acid conditions. Glands in the stomach wall produce hydrochloric acid to create very acidic conditions.
  • Amylase and lipase work in the small intestine. They work best when the conditions are slightly alkaline.
  • The liver produces bile that is stored in the gall bladder. Bile is squirted into the small intestine and neutralises the stomach acid. It makes the conditions slightly alkaline.
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Making use of enzymes

Biological washing powders contain enzymes that digest food stains. They work at lower temperatures than ordinary washing powders so can save us money.

We also use:

  • Protease enzymes to pre-digest proteins in some baby foods.
  • Isomerases to convert glucose into fructose. Fructose is much sweeter, so less is needed in foods. The foods, therefore, are not so fattening.
  • Carbohydrates to convert starch into sugar syrup for use in foods.
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Controlling Internal Conditions

Homoeostasis: The processes in your body that help to maintain a constant internal environment are known as homoeostasis.

  • Carbon dioxide is a waste product of respiration, it is excreted through the lungs.
  • Some of the amino acids we take in are not used. They are converted into the urea by the liver and excreted by the kidneys in the urine. Urine can be stored in the bladder.
  • The water and ion content of cells must be carefully controlled. If they are not, then too much or too little water may move in and out of cells by osmosis.

Key Points:

  • We must remove the waste products produced through chemical reactions from the body.
  • There are other factors we must keep within certain limits, e.g water and ion content of the cells.
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Controlling Body Temperature

The thermoregulatory centre of the brain and receptors in the skin detect change in temperature. The thermoregulatory centre controls the body's response to a change in internal temperature.

If the core temperature rises:

  • Blood vessels near the surface of the skin dilate allowing more blood to flow through the skin capillaries. Heat is lost by radiation.
  • Sweat glands produce more sweat. This evaporates from the skin's surface. The energy required for it to evaporate comes from the skin's surface. So we cool down.

If the core temperature falls:

  • Blood vessels near the surface of the skin constrict and less blood flows through the skin capillaries. Less heat is radiated. 
  • We 'shiver'. Muscles contract quickly. This requires respiration and some of the energy produced is released as heat.
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Controlling Blood Sugar

The pancreas monitors and controls the level of sugar in our blood.

If there is too much sugar in our blood the pancreas produces the hormone insulin that results in the excess sugar being stored in the liver as glycogen. If insulin is not produced the blood sugar level may become fatally high.

If the pancreas is not producing enough insulin this is known as diabetes. It can sometimes be controlled by diet or the person may need insulin injections.

The level of sugar in the blood must be kept at the correct level - hormones help our bodies to do this.

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Cell division and growth

Cell division is necessary for the growth of an organism, or for repair if tissues are damaged.

Mitosis results in two identical cells being produced from the original cell.

A copy of each chromosome is made before the cell divides and one of each chromosome goes to each new cell.

Key Points:

  • Body cells need to divide to produce new cells for growth or repair.
  • Mitosis is the type of cell division that produces identical new cells.
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Stem Cells

Stem cells are unspecialised. They can develop (differentiate) into many different types of specialised cell. Stem cells are found in the embryo and in adult bone marrow.

Many embryonic stem cells that we carry research out on are from aborted embryos, or are 'spare' embryos from fertility treatment. This results in ethical issues and much debate, as it can be argued that you are destroying life to obtain these stem cells for research.

The use of stem cells from adult bone marrow is still limited by the number of different types of specialised cell we can develop them into.

Key Points:

  • Stem cells are not specialised but can differentiate into many different types of cell when required.
  • There are ethical issues surrounding the use of stem cells.
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Cell Division in Sexual Reproduction

Cells in reproductive organs, e.g. testes and ovaries, divide to form sex cells (gametes).

Before division, a copy of each chromosome is made. The cell now divides twice to form four gametes (sex cells). This type of cell division is called meiosis.

Each gamete has only one chromosome from the original pair. All of the cells are different from each other and the parent cell.

Sexual reproduction results in variation as the sex cells (gametes) from each parent fuse. So half the genetic information comes from the father and half from the mother.

Key Points:

  • Sex cells are produced by meiosis.
  • Four cells are produced from each parent cell. They are all different.
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From Mendel to DNA

Gregor Mendel was a monk who worked out how many characteristics were inherited. His ideas were not accepted for many years.

Genes are short lengths of DNA (deoxyribonucleic acid), which make up chromosomes and control our characteristics.

Genes code for combination of specific amino acids, which make up proteins.

Key Points:

  • Gregor Mendel worked out how characteristics are inherited.
  • Genes make up the chromosomes, which control our characteristics.
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Inheritance in Action

  • Human beings have 23 pairs of chromosomes, one pair are the sex chromosomes. Females are ** and males are XY.
  • Genes controlling the same characteristics are called alleles.
  • If an allele 'masks' the effect of another it is said to be 'dominant'. The allele where the effect is 'masked' is said to be 'recessive'.
  • Alleles control the development of characteristics.
  • Some alleles are dominant and some are recessive.  
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Inherited Conditions in Humans

Huntington's disease is a disorder of the nervous system. It is caused by a dominant allele, so even if one parent has the disease it can be inherited by a child.

Cystic fibrosis is a disorder of cell membranes. It is causes by a recessive allele so parents may be carriers (Cc). Only if both parents are either carriers or have the disorder does the child inherit it.

Embryos can be screened to see if they carry alleles for one of these or other genetic disorders.

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useful :)

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