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 envelopeEndoplasmic reticulumGolgi apparatusVesicles

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 filamentsMicrotubulesIntermediate 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