BUNDLE 2: CELLS, TISSUES & ORGANIZATION OF THE BODY
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CELL STRUCTURE AND FUNCTIONS
A cell consists of plasma membrane enclosing a number of organelles suspended in a
watery medium called cytosol.
• **Cytoplasm**: Cell content excluding the nucleus
PLASMA MEMBRANE
• Consists of two layers of phospholipids with membrane proteins and sugar
embedded
• Cholesterol is also present
• Phospholipids have:
• **Head**: Electrically charged → hydrophilic (water-loving)
• **Tail**: No charge → hydrophobic (water-hating)
Membrane protein functions:
• Act as receptors for hormones and chemical messengers
• Branched carbohydrates attached outside give the cell its immunological identity
• Some are enzymes
• **Transmembrane proteins** form channels for small water-soluble ions to cross
• Some are involved in pumps that transport substances across the membrane
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ORGANELLES
NUCLEUS
• Found in all cells except mature erythrocytes
• Largest organelle in the body
• Contained within **nuclear envelope** (smaller pores than plasma membrane)
• Contains genetic material (**DNA**) directing all metabolic activities
• Contains **RNA** involved in protein synthesis
• Has **nucleolus** involved in synthesis and assembly of ribosome components
MITOCHONDRIA
• **Powerhouse of the cell**
• Great number found in most active cells (liver, muscle, spermatozoa)
RIBOSOMES
• Tiny granules composed of RNA and protein
, • Synthesize protein from amino acids using RNA as a template
ENDOPLASMIC RETICULUM (ER)
• Extensive series of interconnecting membranous canals
• **Smooth ER**: Synthesizes lipids and steroid hormones; associated with
detoxification of some drugs
• **Rough ER**: Studded with ribosomes; site of protein synthesis
GOLGI APPARATUS
• Stacks of closely folded flattened membranous sacs
• Packaging and transportation of proteins
LYSOSOMES
• Small membranous vesicles pinched off from Golgi apparatus
• Break down fragments of organelles and large molecules (RNA, DNA)
CYTOSKELETON
• Extensive network of tiny protein fibers
• **Microfilaments**: Smallest fibers; provide structural support; maintain cell shape;
permit contraction (e.g., actin in muscle cells)
• **Microtubules**: Largest contractile protein fibers; help movement of organelles,
chromosomes in cell division, and cell extensions
CENTROSOMES
• Direct organization of microtubules within the cell
• Consist of a pair of **centrioles**
• Important role in cell division
CELL EXTENSIONS
• **Microvilli**: Tiny projections containing microfilaments; cover exposed surface of
some cells
• **Cilia**: Microscopic hair-like projections containing microtubules; beat in unison
moving substances along surface (e.g., mucus upward respiratory tract)
• **Flagella**: Single long whip-like projection containing microtubules; forms tail of
spermatozoa propelling them through female reproductive tract
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CELL CYCLE
At the end of cell's natural lifespan, aging cells are programmed to self-destruct;
components removed by phagocytosis in apoptosis.
• All cells divide by **mitosis** except sex cells which divide by **meiosis**
• **Cell cycle**: Period between two cell divisions
__________________________________________________
CELL STRUCTURE AND FUNCTIONS
A cell consists of plasma membrane enclosing a number of organelles suspended in a
watery medium called cytosol.
• **Cytoplasm**: Cell content excluding the nucleus
PLASMA MEMBRANE
• Consists of two layers of phospholipids with membrane proteins and sugar
embedded
• Cholesterol is also present
• Phospholipids have:
• **Head**: Electrically charged → hydrophilic (water-loving)
• **Tail**: No charge → hydrophobic (water-hating)
Membrane protein functions:
• Act as receptors for hormones and chemical messengers
• Branched carbohydrates attached outside give the cell its immunological identity
• Some are enzymes
• **Transmembrane proteins** form channels for small water-soluble ions to cross
• Some are involved in pumps that transport substances across the membrane
__________________________________________________
ORGANELLES
NUCLEUS
• Found in all cells except mature erythrocytes
• Largest organelle in the body
• Contained within **nuclear envelope** (smaller pores than plasma membrane)
• Contains genetic material (**DNA**) directing all metabolic activities
• Contains **RNA** involved in protein synthesis
• Has **nucleolus** involved in synthesis and assembly of ribosome components
MITOCHONDRIA
• **Powerhouse of the cell**
• Great number found in most active cells (liver, muscle, spermatozoa)
RIBOSOMES
• Tiny granules composed of RNA and protein
, • Synthesize protein from amino acids using RNA as a template
ENDOPLASMIC RETICULUM (ER)
• Extensive series of interconnecting membranous canals
• **Smooth ER**: Synthesizes lipids and steroid hormones; associated with
detoxification of some drugs
• **Rough ER**: Studded with ribosomes; site of protein synthesis
GOLGI APPARATUS
• Stacks of closely folded flattened membranous sacs
• Packaging and transportation of proteins
LYSOSOMES
• Small membranous vesicles pinched off from Golgi apparatus
• Break down fragments of organelles and large molecules (RNA, DNA)
CYTOSKELETON
• Extensive network of tiny protein fibers
• **Microfilaments**: Smallest fibers; provide structural support; maintain cell shape;
permit contraction (e.g., actin in muscle cells)
• **Microtubules**: Largest contractile protein fibers; help movement of organelles,
chromosomes in cell division, and cell extensions
CENTROSOMES
• Direct organization of microtubules within the cell
• Consist of a pair of **centrioles**
• Important role in cell division
CELL EXTENSIONS
• **Microvilli**: Tiny projections containing microfilaments; cover exposed surface of
some cells
• **Cilia**: Microscopic hair-like projections containing microtubules; beat in unison
moving substances along surface (e.g., mucus upward respiratory tract)
• **Flagella**: Single long whip-like projection containing microtubules; forms tail of
spermatozoa propelling them through female reproductive tract
__________________________________________________
CELL CYCLE
At the end of cell's natural lifespan, aging cells are programmed to self-destruct;
components removed by phagocytosis in apoptosis.
• All cells divide by **mitosis** except sex cells which divide by **meiosis**
• **Cell cycle**: Period between two cell divisions
