Cells

• The cell is a basic unit of structure and function. It is the simplest unit with all characteristics of life. Cells can exist as unicellular organisms or as part of multi-cellular organisms. They have great variety in shape, size and structures, due to adaptations to their varied functions
• Cell membrane

  Phospholipids are a major component of biological membranes. Due to the nature of phospholipids, the cell membrane is made up of two layers of phospholipids - phospholipid bilayer

  Besides the phospholipid bilayer, the cell membrane also contains various proteins, carbohydrate chains, and cholesterol attached to or embedded within the phospholipid bilayer.
• Cell membrane

  The fluid mosaic model is used to describe the structure of the cell membrane

  • The cell membrane is fluid in that phospholipids and proteins are not stationary but can move rapidly across the surface of the membrane
  • The cell membrane is mosaic in that many different protein molecules are randomly scattered and embedded throughout the phospholipid bilayer
• Cell membrane

  Due to the nature of phospholipids, only small hydrophobic molecules can pass through the phospholipid bilayer. As such, for hydrophilic or water soluble molecules are unable to pass through the cell membrane, transport proteins are required

  Hence, the cell membrane is partially permeable - allows only certain substances to pass though it. It functions to separates and protects a cell from its surrounding environment and controls how substances move in and out of the cell
• Cytoplasm

  Contains:

  • Cytosol - Aqueous solution of essential ions, soluble proteins (e.g. enzymes) and soluble organic compounds such as sugars and amino acids
  • Cytoskeleton - network of protein fibres that give support, motility and regulations to the cell
  • Organelles - structures with specialised functions, suspended in the cytosol
• Cytoplasm

  Cytoplasmic streaming takes place in the cytoplasm of cells. It is the movement of the fluid substance (cytoplasm) within a plant or animal cell. Cytoplasmic streaming aids in the transport of materials and organelles around the cell (e.g. cytoplasmic streaming of chloroplasts within a plant cell to regions with more light
• Nucleus

  Structure

  • Largest organelle within the eukaryotic cell, usually spherical
  • Consists of the nuclear envelope which separates nucleus from the cytoplasm
  • Contains nucleolus which synthesises ribosomes
  • Contains DNA
  • Found in almost all eukaryotic cells except matured red blood cells

  Functions

  • Contains the hereditary material (DNA) and controls the activities of the cell
• Rough Endoplasmic Reticulum

  Structure

  • A system of flattened membrane-bound sacs
  • Appears "rough" under electron microscope because of the presence of ribosomes on its surface

  Function

  • Protein synthesis
  • The polypeptide chain synthesised by the ribosomes will be folded in the RER
• Smooth Endoplasmic Reticulum

  Structure

  • Consists of fine tubules

  Function

  • Lipid synthesis (e.g. phospholipids, steroid, hormones, cholesterol)
  • Detoxification of drugs and poisons (especially in liver)
  • Storage and release of calcium ions
• Golgi apparatus

  Structure

  • Consist of stacks of flattened membrane-bound sacs

  Functions

  • Chemically modifies products receive from the ER (e.g. proteins and lipids) and packages them, before sending them to destinations inside or out of the cell using vesicles
• Lysosomes

  Structure

  • Small, spherical vesicles surrounded by a single membrane
  • Contain hydrolytic enzymes such as lipase, proteases and nucleases

  Functions

  • To digest materials made in the cell or take in from outside by phagocytosis, e.g. food vacuoles in Amoeba
  • To digest worn-out organelles in the cell, a process known as autophagy
• Animal vacuoles

  Structure

  • Fluid-filled sac bound by a single membrane
  • Formed either by the pinching-off part of the cell membrane, or by enlargement of a vesicle from the Golgi apparatus
  • Usually relatively smaller and exist temporarily

  Function

  • Food vacuoles: formed by phagocytosis

  e.g. in the case of intracellular digestion by macrophages
• Plant vacuoles

  Structure

  • In mature plant cells, the large central vacuole is permanent and occupy over 80% of the cell volume
  • Enclosed by single, partially permeable membrane called the tonoplast
  • Filled with cell sap, a solution of dissolved such as sugars, ions, waste products and pigments

  Functions

  • Storage of nutrients such as protein storage in seeds and inorganic ions e.g. K+ and Cl-
  • Disposal sites for metabolic by-products that would endanger the cell if they accumulated in the cytosol
  • May contain pigments that colour the cells e.g. red and blue pigments of petals that attract pollinators
• Mitochondria

  Structure

  • Rod-shaped or cylindrical
  • Bounded by double membrane

  Function

  • Involved in aerobic respiration
  • ATP is the "energy molecule" in living organisms
  • abundant in metabolically active cells e.g. muscles and liver cells
• Chloroplasts

  Structure

  • Bounded by a double membrane
  • Contain chlorophyll
  • Contain thylakoids and stroma

  Function

  • Site of photosynthesis
• Ribosomes (Non-membranous organelles)

  Structure

  • Small, round structures found in all cells
  • May occur as free ribosomes suspended in the cytosol or bound to rough ER

  Function

  • Sites of polypeptide synthesis
  • Free ribosomes generally make proteins that will function within the cytosol
  • Bound ribosomes generally make proteins that are destined for insertion into membranes, for packaging within certain organelles such as lysosomes, or for export from the cell
• Centrioles (Non-membranous organelles)

  Structure

  • Small, hollow cylinders that occur in pairs
  • Found in animal cells but absent in plant cells

  Function

  • Centrioles play a role in cell division in animal cells
• Comparison between animal and plant cells

  Similarities

  Both cells contain cell membrane, nucleus, ER, Golgi apparatus, mitochondria, ribosomes and cytoplasm
• Comparison between animal and plant cells

  Differences

  • Plant cell is surrounded by the cell wall in addition to the cell membrane whereas in animal cell, cell wall is absent, only surrounded by the cell membrane
  • In plant cell, chloroplasts are present in large numbers in photosynthetic cells whereas in animal cell, chloroplasts are absent
  • In plant cell, centrioles are absent whereas in animal cell, centrioles are present
  • In plant cell, vacuole is present as a single, large and central vacuole whereas in animal cell, vacuoles are small and numerous