Cell Structure (OCR Gateway GCSE Biology: Combined Science)

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Cell Structure

  • Organisms are made of cells which are the smallest unit with the basic properties of life
  • Organisms can be classified as eukaryotes or prokaryotes 
    • Eukaryotes can be unicellular (consist of one cell) or multicellular (consist of many cells) such as animals and plants
    • Prokaryotes (e.g. bacteria) are much smaller than eukaryotes and are always unicellular
  • Both types of cells contain sub-cellular structures (parts of the cell) that each have specific functions
  • But the internal cell structure of eukaryotic and prokaryotic cells are very different

Prokaryote Cell Structure

  • Prokaryotic cells are much smaller in comparison to eukaryotic cells, often measuring ~ 1 µm in size
    • Their small size allows them to have a much simpler structure than eukaryotic cells
  • The inside of the cell contains a gel-like substance called the cytoplasm
    •  Most of the chemical reactions take place here
  • The cell membrane encloses the cytoplasm
    • It is permeable to some substances and provides a selective barrier, controlling what goes in and out of the cell e.g. nutrients, waste
    • Receptors in the membrane help prokaryotes sense their environment  e.g detect nutrients
  • The outside of the cell is surrounded by a cell wall
    • Helps provide strength and maintain the shape of the cell
  • A defining feature of prokaryotic cells is that their chromosomal DNA (genetic material) consists of one long circular chromosome
    • It floats free in the cytoplasm (not enclosed within a nucleus)
    • It controls the cells activities and replication
  • Extra small loops of extra DNA called plasmids may also be present
    • These can be exchanged between bacteria and give additional benefits to the bacteria
      • e.g. antibiotic resistance or ability to use a different energy source

A typical prokaryotic cell, IGCSE & GCSE Biology revision notes

Prokaryotic cells do not have a nucleus, and are much smaller than eukaryotic cells

Eukaryote Cell Structure

  • Eukaryotic cells are larger and more complex than prokaryotic cells
  • They have many more sub-cellular structures to help efficiently carry out specific functions within the cell

Animal Cells

  • Like prokaryotic cells, animal cells also contain a cytoplasm and cell-membrane
  • The cytoplasm is the gel like substance where most of the chemical reactions take place
  • The cell membrane helps to hold the cell together
    • Provides a selective barrier, controlling what goes in and out 
    • Contains receptor molecules used for cell communication e.g. by hormones
  • Unlike prokaryotic cells, the DNA (genetic material) which controls the cell's activities is found in the nucleus
    • DNA is packaged into multiple linear chromosomes 
  • Animal cells contain specialised structures called mitochondria where cellular aerobic respiration occurs
    • Mitochondria contain all the enzymes required for the reactions involved
    • This allows cells to derive energy from food in the presence of oxygen

A typical animal cell

A typical animal cell showing some key sub-cellular structures

Table of animal cell structures

Cell structures table

Plant Cells

  • Plant cells contain the same sub-cellular structures as animal cells and additional structures to help its function:
  • A rigid cell wall made of cellulose to give support for the cell
  • Plant cells found in the leaf and stem contain chloroplasts
    • Photosynthesis takes place in chloroplasts
    • They contain a green substance called chlorophyll to help absorb energy from sunlight

A typical plant cellA typical plant cell showing some key sub-cellular structures (you don't need to know about ribosomes, endoplasmic reticulum or permanent vacuole)

Table of plant cell structures and functions

Cell Structures Table 1Cell Structures Table 2

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Author: Phil

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.