The Cellular Level of Organization
Living things are constructed of cells.
Living things may be unicellular or multicellular.
Cell structure is diverse but all cells share
common characteristics.
The cell theory states:
All organisms are composed of one or more cells.
Cells are the basic unit of structure and function in
organisms.
All cells come only from other cells.
Cells are small so they can exchange materials
with their surroundings.
Surface area relative to the volume decreases as
size of cell increases.
- limits the size of cells
Sizes of living things
Eukaryotic Cells
All cells are surrounded by a plasma membrane
made of phospholipids and proteins.
Eukaryotic cells have membrane bound intracellular
organelles.
The most prominent is the nucleus that controls the
workings of the cell.
The plasma membrane regulates what enters and
exits the cell.
Inside the plasma membrane, the nucleus is
surrounded by cytoplasm.
Plant cells have a cell wall in addition to the
plasma membrane.
Animal and plant cells have organelles.
Organelles compartmentalize functions within
the cell.
The organelles of animal and plant cells are
similar to each other except that centrioles are
present only in animal cells, and chloroplasts
are present only in plant cells.
Animal cell anatomy
Plant cell anatomy
Structure of the Nucleus
Chromatin: DNA and proteins
Nucleolus: Chromatin and ribosomal subunits
Nuclear envelope: Double membrane with pores
Nucleoplasm: semifluid medium inside the nucleus.
The nucleus and the nuclear envelope
Ribosomes
Protein synthesis occurs at tiny organelles called
ribosomes.
Ribosomes are composed of a large subunit and a
small subunit.
Ribosomes can be found alone in the cytoplasm,
in groups called polyribosomes, or attached to the
endoplasmic reticulum.
The endomembrane system consists of:
Nuclear envelope
Endoplasmic reticulum
Golgi apparatus
Vesicles
The endoplasmic reticulum
The endoplasmic reticulum (ER) is a system of
membranous channels and saccules.
Rough ER is studded with ribosomes and is the
site of protein synthesis and processing.
Smooth ER lacks ribosomes and is the site of
synthesis of phospholipids and the packaging of
proteins into vesicles, among other functions.
The Golgi apparatus
The Golgi apparatus consists of a stack of curved
saccules.
The Golgi apparatus receives protein and also
lipid-filled vesicles from the ER, packages,
processes, and distributes them within the cell.
This organelle may also be involved in secretion.
Lysosomes and vacuoles
Lysosomes are vesicles produced by the Golgi
apparatus.
Lysosomes contain hydrolytic enzymes and are
involved in intracellular digestion.
Vacuoles (large) and vesicles (small) are
membranous sacs in the cell that store
substances.
Peroxisomes
Peroxisomes are vesicles than contain enzymes.
The enzymes in these organelles use up oxygen
and produce hydrogen peroxide.
Peroxisomes are abundant in the liver where they
produce bile salts and cholesterol and break down
fats.
Energy-Related Organelles
The two energy-related organelles of eukaryotes
are chloroplasts and mitochondria.
Both organelles house energy in the form of ATP.
Chloroplasts
A chloroplast is bounded by two membranes
enclosing a fluid-filled stroma that contains enzymes.
Membranes inside the stroma are organized into
thylakoids that house chlorophyll.
Chlorophyll absorbs solar energy and carbohydrates
are made in the stroma.
Mitochondria
Mitochondria are found in plant and animal cells.
Mitochondria are bounded by a double membrane
surrounding fluid-filled matrix.
The inner membranes of mitochondria are cristae.
The matrix contains enzymes that break down
carbohydrates and the cristae house protein
complexes that produce ATP.
The Cytoskeleton
The eukaryotic cytoskeleton is a network of
filaments and tubules that extends from the
nucleus to the plasma membrane.
The cytoskeleton contains three types of elements
responsible for cell shape, movement within the
cell, and movement of the cell:
Actin filaments
Microtubules
Intermediate filaments
Actin filaments occur in bundles or mesh-like networks.
Actin filaments play a structural role in intestinal
microvilli and also interact with motor molecules, such
as myosin.
Microtubles are small hollow cylinders made of the
globular protein tubulin.
Microtubules help maintain the shape of the cell and
act as tracks along which organelles can move.
Centrioles
Centrioles are short
cylinders with a 9 + 0
pattern of microtubule
triplets.
Centrioles may be involved
in microtubule formation
and disassembly during
cell division and in the
organization of cilia and
flagella.
Cilia and flagella
Cilia (small and numerous) and flagella (large and
single) have a 9 + 2 pattern of microtubules and
are involved in cell movement.
Cilia and flagella move when the microtubule
doublets slide past one another.
Each cilium and flagellum has a basal body at its
base.
Prokaryotic Cells
Prokaryotic cells include the bacteria and archaea.
Bacterial cells have these constant features:
Outer Boundary: Cell wall
Plasma membrane
Cytoplasm:
Ribosomes
Thylakoids (Cyanobacteria)
Innumerable enzymes
Nucleoid:
Chromosome (DNA only)
Bacterial cells may have plasmids, small accessory
rings of DNA.
Some bacteria have a capsule or a slime layer.
Most bacteria have flagella.
Some also have fimbriae that help cells attach to
surfaces.
Bacteria have a great metabolic diversity.
Evolution of the Eukaryotic Cell
Endosymbiotic hypothesis.
Eukaryotes arose from a symbiotic relationship
between various prokaryotes.
Heterotrophic bacteria became mitochondria.
Cyanobacteria became chloroplasts.
Host cell was a large eukaryotic cell.
Evolution of the eukaryotic cell