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Biology: Structure and function

Science Stage 7 Chapter Notes

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Biology: Structure and function
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In this chapter

  1. 1Animal Cell Structure and Function
  2. 2Plant Cell Structure and Function
  3. 3Specialised Cells and Differentiation
  4. 4Levels of Organisation
  5. 5Viruses: Living or Non-living?

1. Animal Cell Structure and Function

Animal cells are the basic building blocks of animal life. They are eukaryotic, meaning they have a true nucleus. Each part of the cell, called an organelle, has a specific job to do to keep the cell alive and functioning. Think of it like a tiny factory: the nucleus is the main office, the mitochondria are the power generators, and the cell membrane is the security gate.

Key term

Organelle: A specialised subunit within a cell that has a specific function.

Examiner insight

Examiners look for precise descriptions of organelle functions. For example, simply saying the cell membrane 'is a wall' is incorrect; you must specify that it 'controls the movement of substances into and out of the cell'.

Common pitfall

Stating that mitochondria 'produce' or 'create' energy. Energy cannot be created, only transferred. It is crucial to state that mitochondria 'release' energy from glucose during respiration.

Worked example 16 marks

The diagram shows a typical animal cell. Identify the parts labelled A, B, and C, and state the function of each part.

  1. 1

    Part A is the Nucleus. Its function is to control the activities of the cell and contain the genetic material (DNA).

  2. 2

    Part B is the Cytoplasm. This is a jelly-like substance where most of the cell's chemical reactions take place.

  3. 3

    Part C is the Cell Membrane. Its function is to control what substances enter and leave the cell.

Worked example 23 marks

Explain the role of mitochondria in an animal cell.

  1. 1

    Mitochondria are the site of aerobic respiration.

  2. 2

    During respiration, glucose and oxygen react to release energy.

  3. 3

    This energy is used to power all other cellular processes, such as muscle contraction or protein synthesis.

Recap

  • Animal cells have a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes.
  • The nucleus controls the cell and contains the genetic information.
  • The cell membrane is selectively permeable, controlling what enters and leaves.
  • The cytoplasm is where most chemical reactions occur.
  • Mitochondria are the site of aerobic respiration, which releases energy for the cell.
  • Ribosomes are responsible for synthesising proteins.

Quick check

  1. Which organelle controls the activities of the cell?1 mark
  2. Where does aerobic respiration mainly occur?1 mark

2. Plant Cell Structure and Function

Plant cells share many features with animal cells, such as a nucleus, cytoplasm, and cell membrane. However, they also have three key additional structures that allow them to function differently. A rigid cell wall provides support, chloroplasts enable photosynthesis, and a large permanent vacuole helps maintain the cell's shape and store substances.

Key term

Cell Wall: A rigid outer layer made of cellulose that surrounds the cell membrane of plant cells, providing structural support.

Common pitfall

Confusing the cell wall and cell membrane. The cell wall is a fully permeable, strong outer layer for support, while the cell membrane is inside the cell wall and is selectively permeable, controlling what enters the cytoplasm.

Fun fact

The green colour of plants comes from the pigment chlorophyll, which is packed inside the chloroplasts to absorb sunlight for photosynthesis.

Worked example 15 marks

Describe two similarities and three differences in the structure of a typical plant cell and a typical animal cell.

  1. 1

    Similarity 1: Both have a nucleus which contains the genetic material.

  2. 2

    Similarity 2: Both have a cell membrane to control the passage of substances.

  3. 3

    Difference 1: Plant cells have a cell wall made of cellulose for support; animal cells do not.

  4. 4

    Difference 2: Plant cells have chloroplasts for photosynthesis; animal cells do not.

  5. 5

    Difference 3: Plant cells have a large, permanent sap vacuole to maintain turgor pressure; animal cells have small, temporary vacuoles, if any.

Recap

  • Plant cells have all the organelles of an animal cell plus three more: a cell wall, chloroplasts, and a permanent vacuole.
  • The cell wall is made of cellulose and provides structural support.
  • Chloroplasts contain chlorophyll and are the site of photosynthesis.
  • The permanent vacuole contains cell sap and helps keep the cell turgid (firm).
  • Both plant and animal cells are eukaryotic.

Quick check

  1. What is the function of chloroplasts?1 mark
  2. Name the carbohydrate that makes up the plant cell wall.1 mark

3. Specialised Cells and Differentiation

In multicellular organisms, not all cells are the same. They undergo a process called differentiation to become specialised for a particular job. This means they develop specific features or adaptations that allow them to carry out their function efficiently. For example, a nerve cell is long to transmit signals over distances, while a red blood cell has no nucleus to maximise space for carrying oxygen.

Key term

Differentiation: The process by which a cell changes from one cell type to another, more specialized type, to perform a specific function.

Examiner insight

To gain full marks, you must always link a structural feature to its function. Do not just list features. Use connecting phrases like '...which allows it to...' or '...so that it can...'.

Fun fact

Your body contains over 200 different types of specialised cells, from brain cells that can be over a metre long to tiny red blood cells.

Worked example 13 marks

Explain how a root hair cell is adapted for its function.

  1. 1

    Function: The function of a root hair cell is to absorb water and mineral ions from the soil.

  2. 2

    Adaptation 1: It has a long, thin extension, the 'root hair', which massively increases its surface area. This allows for a faster rate of absorption.

  3. 3

    Adaptation 2: It has a large number of mitochondria to provide energy (from respiration) for the active transport of mineral ions from the soil into the root.

Worked example 23 marks

Describe how the structure of a sperm cell relates to its function.

  1. 1

    Function: The function of a sperm cell is to fertilise an egg cell.

  2. 2

    Adaptation 1: It has a long tail (flagellum) that allows it to swim towards the egg.

  3. 3

    Adaptation 2: It has many mitochondria, located in the middle section, to release energy for swimming.

  4. 4

    Adaptation 3: It has an acrosome in its head containing enzymes to digest the outer layer of the egg cell.

Recap

  • Cell differentiation is the process of cells becoming specialised.
  • Specialised cells have adaptations that help them perform their function.
  • Nerve cells are long and branched to transmit electrical impulses.
  • Muscle cells can contract and relax to cause movement.
  • Root hair cells have a large surface area for efficient absorption.
  • Sperm cells have a tail for swimming and mitochondria for energy.

Quick check

  1. State one adaptation of a red blood cell.1 mark

4. Levels of Organisation

In complex multicellular organisms, cells are organised into a clear hierarchy to carry out life processes effectively. The simplest level is the cell. A group of similar cells working together forms a tissue. Different tissues combine to form an organ, which performs a specific, complex function. Several organs then work together in an organ system. Finally, all the organ systems make up the complete organism.

Key term

Organ: A group of different tissues that work together to perform a specific function or set of functions.

Examiner insight

When asked for an example of the levels of organisation, students who use a consistent example throughout all levels (e.g., following the digestive system from an epithelial cell to the whole organism) often score more highly than those who use disconnected examples.

Worked example 13 marks

Place the following in order of increasing complexity, starting with the simplest: Organism, Tissue, Organ System, Organ, Cell.

  1. 1
    1. Cell (e.g., muscle cell)
  2. 2
    1. Tissue (e.g., muscle tissue)
  3. 3
    1. Organ (e.g., the heart)
  4. 4
    1. Organ System (e.g., the circulatory system)
  5. 5
    1. Organism (e.g., a human)

Worked example 23 marks

The stomach is an organ. Explain what this means in terms of its structure.

  1. 1

    As an organ, the stomach is made up of several different tissues working together.

  2. 2

    For example, it has muscle tissue to churn food, glandular tissue to produce digestive juices (like acid and enzymes), and epithelial tissue to line and protect the stomach wall.

Recap

  • The levels of organisation are: Cells → Tissues → Organs → Organ Systems → Organism.
  • A tissue is a group of similar cells performing a specific function.
  • An organ is a group of different tissues working together.
  • An organ system is a group of organs working together for a major function.
  • An organism is a complete living being composed of multiple organ systems.

Quick check

  1. What is a group of similar cells called?1 mark
  2. Name one organ system in the human body.1 mark

5. Viruses: Living or Non-living?

Viruses are fascinating biological entities that exist on the very edge of life. They are not made of cells and are much smaller. They possess some characteristics of living things, such as having genetic material (DNA or RNA) and the ability to evolve. However, they lack many other key features of life. They have no metabolism, cannot grow, and, crucially, cannot reproduce on their own. They must infect a living host cell and hijack its machinery to make more copies of themselves.

Key term

Virus: A non-cellular infectious agent consisting of genetic material (DNA or RNA) inside a protein coat, which can only replicate inside a living host cell.

Common pitfall

Stating definitively that viruses are 'alive' or 'not alive'. Exam questions on this topic often require you to present the arguments for both sides, so a balanced answer is needed.

Fun fact

The word 'virus' comes from the Latin word for 'poison' or 'slimy liquid', reflecting early ideas about the cause of infectious diseases.

Worked example 14 marks

Give two reasons why a virus could be considered living, and two reasons why it could be considered non-living.

  1. 1

    Reason for being living 1: They contain genetic material, either DNA or RNA.

  2. 2

    Reason for being living 2: They can evolve through natural selection.

  3. 3

    Reason for being non-living 1: They are not made of cells (they are acellular).

  4. 4

    Reason for being non-living 2: They cannot reproduce independently; they require a host cell's machinery to replicate.

Recap

  • Viruses are not cells; they consist of genetic material surrounded by a protein coat.
  • Arguments for viruses being 'living' include having genetic material and the ability to evolve.
  • Arguments for viruses being 'non-living' include not being made of cells and being unable to reproduce without a host.
  • Viruses are parasites that must infect host cells to replicate.
  • Because they have features of both, they are often described as being on the 'borderline of life'.

Quick check

  1. State one characteristic of life that viruses do not show.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. List three organelles found in both animal and plant cells, and two organelles found only in plant cells.5 marks
  2. Define the term 'organ system'.1 mark
  3. Describe the function of the mitochondria and the cell membrane in an animal cell.4 marks
  4. Explain how a nerve cell is adapted for its function of transmitting information.3 marks
  5. A student's drawing of a plant cell is 120 mm long. The actual length of the cell is 60 µm. Calculate the magnification of the diagram. Show your working. (1 mm = 1000 µm)3 marks
  6. Explain why viruses are often described as being on the border between living and non-living.4 marks
  7. Describe the hierarchy of organisation in a multicellular organism, starting from the cell. Use the respiratory system as an example for the organ and organ system levels.5 marks
  8. Compare the role and structure of the cell wall with that of the cell membrane in a plant cell.4 marks
  9. Some muscle cells in an athlete's leg can contain over 5,000 mitochondria, whereas a skin cell may only have a few hundred. Explain this difference in biological terms.3 marks
  10. A scientist observes a cell under a microscope. It has a cell wall and cytoplasm, but no visible nucleus, chloroplasts or permanent vacuole. Suggest what type of organism this cell might be from and justify your answer.3 marks

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