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Cells, organelles and membrane
transport
Cells: Living structural and functional units enclosed by a membrane
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Cytology: Study of cellular structure and function
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Eukaryotic cells: Complex cells with a nucleus and subcellular structures (organelles)
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Parts of a cell

For ease of study, we divide the cell into 3 main parts; the plasma membrane, cytoplasm and
the nucleus
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It is a selective barrier which regulated the flow of materials into and out of the cell
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Cytoplasm: Consists of all the cellular contents between the plasma membrane and the
nucleus
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Nucleus: Large organelle which houses most of the cells DNA
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The plasma membrane

It is a flexibly yet sturdy barrier which surrounds and contains the cytoplasm of the cell
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This is because the molecular arrangement of
the membrane resembles a continually moving sea of fluid lipids which contains a mosaic of
many different proteins
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But act as a
barrier to the entry or exit of charged or polar substances
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Other proteins act as signal receptors (telling other cells what type of cells that they are) or
as molecules which link the plasma membrane to intracellular or extracellular proteins
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75% of the membrane is phospholipid (which contain phosphorus)
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5% of the membrane are various glycolipids (with attached carbohydrate groups)
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Polar: Phosphate head (hydrophilic)
Non-Polar: Two long fatty acid tails (hydrophobic)

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Membrane proteins
Membrane proteins are integral or peripheral according to if they are firmly embedded on the
membrane
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The majority of these proteins are transmembrane
proteins as they protrude in both the cytosol and extracellular fluid
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Many integral proteins are glycoproteins
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They are attached to the polar
heads of the lipids or to integral proteins at the inner of outer surface of the membrane
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Impermeable: The structure does not permit the passage of substances through it
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The lipid bilayer portion of the membrane is permeable to molecules which are:


Non polar



Uncharged

Examples: oxygen, carbon dioxide and steroids
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Transport across the plasma membrane

Passive processes – No cellular energy is required
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Non-polar, hydrophobic molecules move through this way
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Water, urea
and small alcohols also pass via simple diffusion
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Facilitated diffusion: An integral membrane protein assists a specific substance across the
membrane (these proteins can be membrane channel or carrier)
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Most membrane channels are ion channels
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The most numerous are K+
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The solute binds to a specific carrier
on one side of the membrane and is releases at the other side after the carrier undergoes a
change in shape
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Example of carrier mediated diffusion: GluT1 – Glucose transporter

1
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2
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3
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This process is concentration-gradient dependent
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Osmosis is a passive process as it moves from high concentration to low concentration
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Water moves through the plasma membrane in 2 ways;
1
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Via aquaporins (integral membrane proteins which act as water channels)
Osmosis is opposed by hydrostatic pressure

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Tonicity
A solutions tonicity is the measurement of the solutions ability the change the volume of cells
by altering their water content
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e
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When a RBC is placed in a hypotonic solution (solution that has a lower concentration of
solutes that the cytosol in the RBC) hemolysis occurs
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Active processes – Cellular energy is required
This is where molecules move against their concentration gradient
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Energy from the hydrolysis of ATP (primary active transport)
2
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This changes the shape of the carrier protein
which pumps the substance across the cell membrane against its concentration gradient
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This is because part of the pump acts
as an ATPase (which hydrolyses ATP)
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40% of the ATP produced within a cell is used on primary active transport
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Secondary active transport

Uses energy stored in Na+ or H+ concentration gradients in order to drive other substances
across the membrane against their own concentration gradients
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How it works – Sodium-potassium pumps maintain a steep concentration gradient of Na+, as a
results the Na+ have stored or potential energy, just like water behind a dam
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Symporters: Carry 2 substances across the membrane in the same direction
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Receptor-mediated endocytosis – Highly selective
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A
receptor protein recognizes and binds to a specific particle
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g
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Binding: LDL binds to specific receptor, forming
a receptor-LDL complex
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Many clathrin molecule come
together forming a basketlike structure around
the receptor-LDL complex
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Uncoating: The clathrin coated vesicle loses its
clathrin
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Fusion with the endosome
Recycling of receptors: Vesicles bud off the
endosome to form a transport vesicle
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Phagocytosis – The cell engulfs large solid particles such as worn out cells, viruses or
whole bacteria
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Learn stages from diagram
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Bulk-phase endocytosis (pinocytosis) – Tiny droplets of extracellular fluid are taken up
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The plasma membrane folds inwards to form the vesicle containing a droplet of
extracellular fluid
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All cells carry out exocytosis, but there are 2 important types of cells which do this;
1
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2
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During exocytosis secretory vesicles form inside the cell, fuse to the plasma membrane and
release contents into the extracellular fluid
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This occurs most often across endothelial cells which line the blood vessels allowing materials
to move between blood plasma and interstitial fluid
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It makes up around 55% of the cell volume
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The cytosol is the site of many chemical reactions required for the cells existence
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g
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Other chemical reactions which occur are vital for the cells growth, maintenance and repair
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Organelles

Cilia and Flagella
Cilia: Numerous, short, hair like projections that extend from the surface of the cell
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Flagella: Are similar in structure to cilia, but they are much longer
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The only example of flagellum in the human body is sperm
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They contain a large amount of ribosomal RNA (rRNA)
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The large and small subunit are made separately in the nucleolus
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Endoplasmic reticulum
Is a network of membranes in the form of flattened sac or tubules
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Rough ER: Is continuous with the nuclear membrane and it usually folded into a series of
flattened sacs
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Rough ER produces
secretory proteins, membrane proteins and many organellar proteins
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Smooth ER: Extends from the rough ER to form a network of membrane tubules
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Function: Synthesis fatty acids and steroids (e
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estrogens and testosterone)
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Golgi complex



Consists of 3–20 flattened, membranous sacs called cisternae



Modify, sort, and package proteins for transport to different destinations



Proteins are transported by various vesicles (secretory, membrane and transport)

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Processing and packaging in golgi

1
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2
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3
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4
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5
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6
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7
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Others are stored in membrane vesicles which deliver contents to the plasma
membrane
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Some proteins will leave the exit face in transport vesicles which carry the proteins to
another cellular destination
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Contain as many as 60 kinds of powerful digestive and hydrolytic enzymes which break down a
wide variety of molecules via endocytosis
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This pH is maintained by active transport pumps which import
H+

Peroxisomes
Similar in structure to lysosomes, but smaller
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Proteasomes
Continuously destroy unneeded, damaged, or faulty proteins
Found in the cytosol and the nucleus
Contain a multitude of protease enzymes which cut proteins into small enzymes
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Generate ATP by aerobic respiration

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Prevalent in active cells: muscles, liver and kidneys

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Self-replicate during times of increased cellular demand or before cell
division

•
•

Contain own DNA - Inherited only from your mother
Plays an important role in apoptosis

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Nucleus

Is a spherical or oval shaped structure
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Both
membranes are lipid bilayers
Nuclear pores - numerous openings in the nuclear envelope, control movement of substances
between nucleus and cytoplasm
Nucleolus - spherical body that produces ribosomes
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It
is not enclosed by a membrane

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