Cell Structure Notes

Cambridge IGCSE Biology: Revision notes

Key facts

  • Animal cells have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes; plant cells also have a cell wall, a large vacuole and chloroplasts.
  • Bacterial cells have no nucleus: their DNA is circular and free in the cytoplasm, and they often have plasmids.
  • Specialised cells have structures adapted to their function.
  • Organisation: cell, tissue, organ, organ system, organism.
  • New cells are produced by division of existing cells: mitosis for growth and repair, meiosis for gametes.

Animal cells

Each structure in a cell has a job: the nucleus controls, mitochondria release energy and ribosomes make proteins.

Describe the function of each structure, not just its name.

  • Nucleus: contains DNA and controls the cell's activities.
  • Cytoplasm: where metabolic reactions happen.
  • Cell membrane: partially permeable barrier controlling what enters and leaves.
  • Mitochondria: site of aerobic respiration.
  • Ribosomes: site of protein synthesis.
RibosomeCytoplasmCell membraneNucleusMitochondrion
An animal cell

Which structure is the site of protein synthesis?

Plant cells

Plant cells have a cell wall, a large permanent vacuole and chloroplasts that animal cells lack.

The cell wall is made of cellulose, outside the membrane, and supports the cell. The large vacuole stores water and keeps the cell firm. Chloroplasts contain chlorophyll for photosynthesis. When comparing, state what is present and what is absent: 'plant cells have a cell wall, animal cells do not'.

Cell wallChloroplastCytoplasmCell membraneRibosomeNucleusPermanent vacuoleMitochondrion
A plant cell

Animal cell

Cell wall:
Absent
Vacuole:
Small and temporary
Chloroplasts:
Absent

Plant cell

Cell wall:
Present (cellulose)
Vacuole:
Large and permanent
Chloroplasts:
Present

Which structure is present in plant cells but not animal cells?

Bacterial cells

A bacterial cell has no nucleus: its DNA is free in the cytoplasm.

Bacteria are prokaryotes. Their circular DNA lies free in the cytoplasm, not enclosed by a membrane, and they often carry small rings of DNA called plasmids. They have a cell wall (peptidoglycan), cell membrane and ribosomes but no chloroplasts or mitochondria. The defining difference from plant and animal cells is the lack of a membrane-bound nucleus.

Cell wallCell membranePlasmidCytoplasmChromosomal DNAFlagellumRibosome
A bacterial cell

Bacterial cell

DNA:
Circular, free in cytoplasm
Cell wall:
Peptidoglycan
Chloroplasts:
Absent
Plasmids:
Often present

Plant cell

DNA:
Inside a nucleus
Cell wall:
Cellulose
Chloroplasts:
Present
Plasmids:
Absent

What is the defining difference between a bacterial cell and a plant cell?

Specialised cells

Specialised cells have structures that suit their function, and good answers link each adaptation to its purpose.

Link every adaptation to its function using 'because' or 'so that'. For example, ciliated cells have many mitochondria because movement needs energy, and root hair cells are long to increase surface area for faster water absorption. Listing features without a reason earns fewer marks.

  1. 1

    Feature

    For example, many mitochondria

  2. 2

    Reason

    because movement needs energy

  3. 3

    Function

    so cilia can move mucus along

Link feature, reason and function

Ciliated cell

Adaptation:
Cilia; many mitochondria
Function:
Move mucus and bacteria out of the lungs

Root hair cell

Adaptation:
Long extension; large vacuole
Function:
Absorbs water and ions from the soil

Palisade mesophyll

Adaptation:
Packed with chloroplasts
Function:
Photosynthesis

Neurone

Adaptation:
Long axon and dendrites
Function:
Transmits impulses over long distances

Red blood cell

Adaptation:
No nucleus; biconcave; haemoglobin
Function:
Transports oxygen

Sperm cell

Adaptation:
Flagellum; many mitochondria
Function:
Swims to and fertilises the egg

Egg cell

Adaptation:
Large; food stores; haploid nucleus
Function:
Provides nutrients and genetic material for a new organism

Why do ciliated cells have many mitochondria?

Organisation

Similar cells form tissues, tissues form organs, organs form organ systems, and the systems make up the organism.

Always state the hierarchy with examples. The heart is an organ made of cardiac muscle tissue, nervous tissue and epithelial tissue. It is part of the circulatory system. Never say 'system of cells' when you mean tissue.

  1. 1

    Cell

    Smallest living unit

  2. 2

    Tissue

    Similar cells

  3. 3

    Organ

    Different tissues

  4. 4

    Organ system

    Organs together

  5. 5

    Organism

    Whole individual

Levels of organisation

Plant example

Cell:
Palisade mesophyll cell
Tissue:
Palisade tissue
Organ:
Leaf
Organ system:
Shoot system
Organism:
Plant

Human example

Cell:
Ciliated cell
Tissue:
Ciliated epithelial tissue
Organ:
Trachea
Organ system:
Respiratory system
Organism:
Human

Which is an organ?

Cell division

New cells are produced by the division of existing cells.

Use the specification phrase 'new cells are produced by division of existing cells'. Mitosis lets an organism grow from a zygote and replaces damaged or worn-out cells, such as skin. Meiosis makes gametes. Without cell division, organisms could not grow or repair themselves.

The stages of mitosis
Mitosis: a cell divides to give two identical new cells

Mitosis

Where:
Growth and repair in body cells
Result:
Two identical daughter cells

Meiosis

Where:
Making gametes
Result:
Cells with half the chromosome number

Which type of cell division produces gametes?

Try an exam question

Describe two ways in which a root hair cell is adapted to its function.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.