Section 2: Structures and functions in living organisms

Section 2; Structures and functions in living organisms 
a) Levels of organisation

2.1 describe the levels of organisation within organisms: organelles, cells,
tissues, organs and systems.

Living organisms are made up of cells. Some organisms only consist of a single-cell, and others are multicellular and contain lots of cells. There is an order of organisation for these cells.

In order of increasing complexity; 

organelle - cell - tissue - organ - organ system.

Organelle - These are tiny structures within cells. They can only be seen when using a                           powerful microscope as they are so small. They are cell structures that are                             specialised  to carry out a particular function or job. 

Cell -  Cells are also specialised to carry out a particular function, so their structures can                vary. An example of this are the red blood cells found in humans, that are specialised             in carrying oxygen.

Tissue -  A tissue is a group of cells with similar structures that work together to perfom a                     specific function. A group of tissues that also works together to perform a specific                 function would be an organ.

Organ - An organ is a group of tissues that work together to perform a function. An example              of an organ would be leaves on plants or lungs in mammals. 

Organ System - Group of organs with related functions. Organs work together in organ                                    systems, and each system does a different job. An example of an organ                               system is the digestive system that can be found in mammals. The digestive                          system is made up of organs such as; the stomach, intestines, pancreas and                         liver.

b) Cell structure

2.2 describe cell structures, including the nucleus, cytoplasm, cell membrane,

cell wall, chloroplast and vacuole

2.3 describe the functions of the nucleus, cytoplasm, cell membrane, cell wall,

chloroplast and vacuole

Animal and plant cells have certain structures in common, these are; 

Nucleus: an organelle which contains genetic material that controls it's activities. It is surrounded by the cell membrane. 

Cytoplasm: this is a gel-like substance where most of the cell's chemical reactions happen. It contains enzymes, which control these reactions. 

Cell membrane: this forms the outer surface of the cell and controls the substances that go in and out. 

Plant cells also have additional structures that they do not share with animal cells; 

Cell wall: This is a hard structure made of cellulose that surrounds the cell membrane. It supports the cell and strengthens it. 

Chloroplast: It is here that photosynthesis happens, which makes food for the plant. Chloroplasts contain a green substance called chlorophyll, which is used in photosynthesis. 

Vacuole - Both plant and animal cells can have these, however they are never permanent in animal cells, but they always are in plant cells. It is a large organelle that contains cell sap, which is a weak solution of sugars and salts that help to support the cell. 

2.4 compare the structures of plant and animal cells.

Differences between the two; 

• Plant cells have additional structures in their cells that animal cells do not. 
• These include: chloroplasts, cell wall, and vacuole. 
• However, some animal cells do have a vacuole, but these are never permanent like they are in plant cells.

Similarities between the two;

• Both have nuclei.
• Both have cytoplasm.
• Both have a cell membrane.

The Nature and Variety of Living Organisms (Section 1)

Characteristics of Living Organisms (Section 1a)

1.1 Understand that living organisms share the following characteristics: 

• they require nutrition 
• they respire 
• they excrete their waste 
• they respond to their surroundings 
• they move 
• they control their internal conditions 
• they reproduce 
• they grow and develop. 

All living organisms share the same eight basic characteristics. They must have all of these characteristics in order to count as being a living organism, and to therefore count as being alive. 

• They require nutrition. 

All living organisms need nutrients to provide them with energy for growth and tissue repair. These nutrients required from food include; fats, protein, vitamins, minerals and carbohydrates.

• They respire 

The process in which organisms release energy from their food.Nutrient molecules are broken down in chemical reactions, releasing energy.

• They excrete their waste 

Removal of toxic materials from the body. Waste products like carbon dioxide have to be removed from the body and this is done through a process known as excretion. 

• They respond to their surroundings 

Living organisms react to changes in their surroundings. An example of this is the way in which plants can grow towards a light source if light is scarce in the environment. 

• They move 

Living organisms move towards and away from things. 

• They can control their internal conditions (homeostasis)

This is the ability to control their internal conditions, such as temperature and water content, and keep this at the level needed in order for them to survive and function. 

• They reproduce 

Living organisms produce offspring and this allows for their species to survive. 

• They grow and develop

Living organisms grow and develop until they reach their adult form. 

The best way to remember these characteristics is through the acronym: Mrs Gren. 

Movement 

Respiration

Sensitivity

Growth

Reproduction

Excretion

Nutrition

The only characteristic not included in this acronym is homeostasis, the ability to control internal conditions. 

b) Variety of living organisms 

1.2 describe the common features shared by organisms within the following 
main groups: plants, animals, fungi, bacteria, protoctists and viruses, and 
for each group describe examples and their features as follows (details of life 

cycle and economic importance are not required)

Plants;

• Multicellular
• They have chloroplasts (can photosynthesise) 
• Their cells have cell walls (made of cellulose)
• Store carbohydrates as sucrose or starch. 
• Examples include; flowering plants like cereals (maize) and herbaceous legumes (peas and beans)

Maize.

Animals; 

• Multicellular
• Don't have chloroplasts (cannot photosynthesise)
• Their cells do not have cell walls.
• Most have nervous coordination. Can respond rapidly to changes in their environment. 
• They can usually move around from one place to another.
• Often store carbohydrates in the form of glycogen. 
• Examples; mammals and insects.

                                                       Typical housefly.

Fungi;

• Some are single-celled
• Others have body called mycelium (made up of thread like structures,) contain lots of nuclei. 
• They cannot photosynthesise.
• Their cell walls have cell walls (made of chitin.)
• Most are saprotrophic feeders.
• Can store carbohydrates as glycogen. 
• Examples; years and mucor.

Different examples of fungi. 

Protoctists; 

• Single-celled
• Microscopic
• Some have chloroplasts, similar to plant cells. 
• Others are closer to animal cells. 
• Examples: chlorella (plant-cell like) and amoeba (animal-cell like, lives in pond water.)

Chlorella

Bacteria:

• Also single-celled and microscopic.
• Do not have a nucleus.
• Have a circular chromosome of DNA.
• Only some can photosynthesise. 
• Most bacteria feed off other organisms, both living and dead. 
• Examples: Pneumoccoccus and Lactobacillus bulgaricus.

Diagram of cross-section of typical bacterium.

Viruses: 

• These are particles, not cells. 
• Smaller than bacteria. 
• Can only reproduce inside living cells.
• Organisms that rely on other organisms in order to survive = parasites. 
• They infect all types of living organisms. 
• Come in varying different shapes and sizes. 
• Have no cellular structure, have protein coat around genetic material. 
• Examples: HIV, influenza virus, tobacco mosaic virus. (Stops tobacco leaves from producing chloroplasts, discolours them.)

                                     Diagram of cross-section of typical flu virus. 

1.3 recall the term ‘pathogen’ and know that pathogens may be fungi, bacteria,

protoctists or viruses.

Pathogens are organisms that cause disease.They come in many different forms, including some; fungi, protoctists, bacteria and viruses. For example, plasmodium is a protoctist that causes malaria and influenza virus is a virus that causes flu and HIV.