Biology

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Eukaryotes (animal and plant cells)

Plant and animal cells (eukaryotic cells) have genetic material enclosed in a nucleus.

Most animal cells have the following parts:

A nucleus, which controls the activities of the cell.

Cytoplasm, in which most of the chemical reactions take place.

A cell membrane, which controls the passage of substances into and out of the cell.

Mitochondria, which is where aerobic respiration takes place.

Ribosomes, which are where protein synthesis occurs.

In addition to the parts found in animal cells, plant cells often have:

Chloroplasts which absorb light to make food by photosynthesis.

A permanent vacuole filled with cell sap.

A cell wall made of cellulose, which strengthens the cell.

Prokaryotes

Bacterial cells (prokaryotic cells) are much smaller in comparison. They have a cytoplasm and a cell membrane surrounded by a cell wall. The genetic material is not enclosed in a nucleus. It is a single DNA loop and there may be one or more small rings of DNA called plasmids.

Microscopy

An electron microscope has much higher magnification and resolving power than a light microscope. This means that it can be used to study cells in much finer detail. This has enabled biologists to see and understand many more sub-cellular structures.

Magnification = Objective lens x Eye piece lens

Magnification= Image size ÷ actual size

Cell differentiation and specialisation

Cells may be specialised to carry out a particular function. Examples are sperm cells, nerve cells and muscle cells in animals, and root hair cells, xylem and phloem cells in plants.

Chromosomes

The nucleus of a cell contains chromosomes made of DNA molecules. Each chromosome carries a large number of genes.

In body cells the chromosomes are normally found in pairs.

Mitosis

Body cells divide by mitosis to produce additional cells during growth or to produce replacement cells.

Body cells divided by mitosis in a series of stages called the cell cycle (knowledge of the cycles is not required). During the cell cycle the genetic material is doubled and then divided into two identical cells.

Before a cell can divide it needs to grow and increase the number of organelles such as ribosomes and mitochondria. The DNA replicates to form two copies of each chromosome. One set is pulled to each end of the cell and the nucleus divides. Finally the cell membrane divide to form two identical cells.

Cell differentiation and stem cells in plants

As an organism develops cells differentiate to form different types of cells. Most types of plant cells retain the ability to differentiate throughout life. As a cell differentiates it acquires different sub –cellular structures to enable it to carry out a certain function. It is specialised.

Stem cells from meristems in plants can be used to produce clones of plants quickly and economically. This is important for plant research, for example to propagate rare species and in agriculture industry to produce large numbers of identical crop plans with special features such as disease resistance.

8. Cell differentiation and stem cells in humans

Cells from human embryos and adult bone marrow, called adult stem cells can be made to differentiate into many different types of human cells e.g. nerve cells.

In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient’s body so may be used for medical treatment.

Treatment with stem cells, from embryos or adult cell cloning, differentiated into specific cells may be able to help conditions such as diabetes and paralysis.

The use of stem cells has potential risks such as transfer of viral infection, and some people have ethical or religious objections.

Diffusion

Substances can move into and out of cells across membranes by diffusion.

Definition of diffusion and factors affecting rate.

Oxygen, carbon dioxide and urea passes through cell membranes by diffusion.

Single celled organisms have a bigger surface area to volume ratio than multicellular organisms, so transfer sufficient substances across their surface.

Multicellular organisms require specialised organ systems to exchange sufficient substances.

Factors affecting the effectiveness of an exchange surface.

Osmosis

Water may move across cell membranes by osmosis.

Osmosis is the movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane.

Active Transport

Some dissolved substances move across cell membranes via active transport.

Active transport involves the movement of a substance against a concentration gradient and requires energy from respiration.

Active transport allows minerals to be absorbed into plant root hairs from very dilute solutions in the soil.

Plants require ions for healthy growth. It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for respiration.

Organisational Hierarchy

Describe the functions of the digestive system to digest and absorb foods.

Identify the positions of the main organs on a diagram of the digestive system.

Know that food molecules must be small and soluble in order to be absorbed into the blood.

Describe the functions of the organs in the system

Explain how the small intestine is adapted for its function.

Enzymes

Define the terms ‘catalyst’ and ‘enzyme’.

Describe the properties of enzymes.

Use the lock and key theory and collision theory to explain enzyme action.

Explain why enzymes are specific and are denatured by high temperatures and extremes of pH.

Calculate the rate of enzyme controlled reactions. Interpret the results from enzyme controlled reactions.

Interpret graphs to determine the optimum temperature or pH for an enzyme.

Digestion

Explain why foods need to be digested into small, soluble molecules.

Describe the three types of enzymes involved in digestion, including the names of the substrates, products and where the enzymes are produced.

Explain how bile helps in the digestion of fats.

Plant tissues and organs

Describe the role of xylem, phloem and root hair cells and explain how they are adapted for their functions.

Define the terms ‘transpiration’ and ‘translocation’.

Xylem tissue transports water and dissolved ions. The flow of water from the roots to leaves is called the transpiration stream.

Xylem tissue is composed of hollow tubes strengthened with lignin.

Phloem tissue transports dissolved sugars from the leaves to other parts of the plant. The movement of food through phloem is called translocation.

Phloem cells have pores in their end walls for movement of cell sap.

Plant tissues and organs 2

Describe the organs that make up the plant transport system.

Describe the role of xylem, phloem and root hair cells and explain how they are adapted for their functions.

Define the terms ‘transpiration’ and ‘translocation’.

Xylem tissue transports water and dissolved ions. The flow of water from the roots to leaves is called the transpiration stream.

Xylem tissue is composed of hollow tubes strengthened with lignin.

Phloem tissue transports dissolved sugars from the leaves to other parts of the plant. The movement of food through phloem is called translocation.

Phloem cells have pores in their end walls for movement of cell sap.

Photosynthetic Reaction

Write the word and symbol equation for photosynthesis.

Explain why photosynthesis is important for the survival of other organisms.

Investigate the need for light, carbon dioxide and chlorophyll to make glucose.

Explain why plants should be de-starched before photosynthesis experiments and describe how this is done.

Describe experiments to show that plants produce oxygen in the light.

The rate of photosynthesis may be limited by:

low temperature

shortage of CO2,

shortage of light shortage of chlorophyll.

State factors that can limit the rate of photosynthesis.

Interpret data showing how factors affect the rate of photosynthesis.

Interpret graphs to decide which factor is limiting the rate.

Explain how conditions in greenhouses can be controlled to optimise the growth of plants.

Relate limiting factors to the cost effectiveness of adding heat, light or carbon dioxide to greenhouses.

Evaluate the benefits of artificially manipulating the environment in which plants are grown

List ways in which glucose is used by a plant (used for respiration, converted into starch for storage, used to produce fats and oils for storage or cellulose to strengthen cell walls, used to produce amino acids for protein synthesis

Describe functions of fats, oils, cellulose, starch and proteins in a plant).

Explain how plants obtain nitrate ions through active transport and what they are needed for.

Aerobic respiration

Respiration on cells can take place aerobically (using oxygen) or anaerobically (without oxygen) to transfer energy.

Reactions which transfer energy to the environment are exothermic reactions.

Organisms need energy for; chemical reactions to build larger molecules, movement, keeping warm.

The symbol and word equations for aerobic respiration.

Anaerobic respiration in animals

The energy transferred supplies all the energy needed for living processes. As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration.

The symbol and word equations for anaerobic respiration.

If insufficient oxygen is supplied anaerobic respiration takes place in muscles. The incomplete oxidation of glucose causes a build up of lactic acid and creates and oxygen debt.

During long periods of vigorous activity muscles become fatigued and stop contracting efficiently. One cause of muscle fatigue is the build up of acid in the muscles. The oxygen debt must be repaid once exercise stops causing deep breathing for some time.

Response to exercise

During exercise the human body reacts to the increased demand for energy.

The heart rate, breathing rate and breathing volume increase during exercise to supply muscles with more oxygenated blood. This is needed for the increased cellular respiration to release more energy to meet the demand.

If insufficient oxygen is supplied anaerobic respiration takes place in muscles. The incomplete oxidation of glucose causes a build up of lactic acid and creates and oxygen debt.

During long periods of vigorous activity muscles become fatigued and stop contracting efficiently. One cause of muscle fatigue is the build up of acid in the muscles. The oxygen debt must be repaid once exercise stops causing deep breathing for some time.

Anaerobic respiration in plants and microorganisms

Anaerobic respiration in plant cells and in some microorganisms produces ethanol and carbon dioxide.

Anaerobic respiration in yeast cells is called fermentation and is important in the manufacture of bread and alcoholic drinks.

Homeostasis

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes.

Control of blood glucose concentration, control of body temperature, and water level in the human body are examples of homeostasis.

An organism maintains homeostasis by monitoring its internal conditions and responding appropriately when these conditions deviate from their optimal state.

The Nervous System

The nervous system enable humans to react to their surroundings.

Information from receptors passes along cells (neurons) as electrical impulses to the central nervous system (CNS), the brain or spinal cord. The CNS coordinates the response of effectors which may be muscles contracting glands secreting hormones.

Reflex actions are automatic and rapid: they do not involve the conscious part of the brain.

Human Endocrine System

The endocrine system is composed of glands which secrete hormones directly into the blood stream. Hormones are large chemical molecules where it produces an effect. Compared to the nervous system the effects are slower but act for longer.

The pituitary gland in the brain is a ‘master gland’ which secretes several hormones into the blood in response to body conditions. These hormones in turn act on other glands to stimulate hormones to be released to bring about effects.

Students should know the position of the pituitary, pancreas, thyroid, adrenal, ovary and testes in the body.

Blood Glucose Concentration.

Blood glucose is monitored and controlled by the pancreas.

If the blood glucose concentration is too high, the pancreas produces the hormone insulin that causes glucose to move from the blood into the cells. In liver and muscle cells excess glucose is converted to glycogen.

Type 1 diabetes is a disorder in which the pancreas fails to produce insulin. It is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections.

In Type 2 diabetes the body cells no longer respond to insulin produced by the pancreas. A carbohydrate controlled diet and an exercise regime are common treatments. Obesity is a risk factor.

If blood glucose concentration is too low, the pancreas produces glucagon that causes glycogen to be converted into glucose and released into the blood.

Hormones in human reproduction

During puberty reproductive hormones cause secondary sex characteristics to develop.

Oestrogen is the main female reproductive hormone produced in the ovary. At puberty eggs begin to mature and one is released approximately every 28 days. This is called ovulation.

Testosterone is the main reproductive hormone produced by the testes and it stimulates sperm production.

Several hormones are involved in the cycle of a woman; Follicle Stimulating Hormone (FSH) causes the maturation of an egg and stimulates the ovaries to produce oestrogen, Luteinising Hormone (LH) stimulates the release of an egg, Oestrogen is secreted by the ovaries and inhibits FSH production and promotes LH production, Progesterone is secreted by the empty follicle and inhibits FSH and LH and maintains the uterus wall.

Contraception

Fertility can be controlled by a variety of hormonal and non-hormonal methods of contraception.

These include; oral contraceptives that contain hormones to inhibit FSH production so no eggs mature, injection or implant or skin patch of slow release progesterone to inhibit the maturation and release of eggs for a number of months, barrier methods such as condoms and diaphragms which prevent the sperm reaching the egg., intrauterine devices which prevent the implantation of an embryo or release a hormone, spermicidal agents which kill or disable sperm, abstaining from intercourse when an egg may be in the oviduct, surgical methods of male and female sterilization.

Use of hormones to treat fertility

The use of hormones to treat fertility include; giving FSH and LH in a fertility drug to a woman whose own level of FSH is too low to stimulate eggs to mature. She may then become pregnant in the normal way.

IVF treatment involves giving a mother FSH and LH to stimulate maturation of eggs. The eggs are collected from the mother and fertilised by sperm from the father. The fertilised eggs develop into embryos. At the stage when they are tiny balls of cells, one or two embryos are inserted into the mother’s uterus.

Although fertility treatment can give couples the chance to have a baby of their own it is emotionally and physical stressful. Success rates are low and it can lead to multiple births which are a risk to both mother and child.

Sexual and asexual reproduction

There are 2 types of reproduction; sexual and asexual.

Sexual reproduction involves the joining of male and female gametes; sperm and egg cells in animals, pollen and ovule cells in flowering plants. In sexual reproduction there is mixing of genetic information which leads to variety in offspring. The formation of gametes involves meiosis.

Asexual reproduction involves only one parent and no fusion of gametes. There is no mixing of genetic information. This leads to genetically identical offspring (clones). Only mitosis is involved.

Meiosis

Cells in reproductive organs divide by meiosis to form gametes.

When a cell divides to form gametes; copies of the genetic information are made, the cell divides twice to from four gametes, each with a single set of chromosomes, all gametes are genetically different to each other.

Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis to grow and as the embryo develops it begins to differentiate.