Chapter 5: Membranes Main topics --- Membrane structure. Cell adhesion and junctions – Not in lecture but STUDY pp. 82-85. Membrane transport

Membranes are selectively Membranes are selectively* permeable – some substances can pass through and other cannot. (*= differentially permeable, semipermeable)

Passive transport across membranes Simple diffusion – movement of solutes from high concentration to low Osmosis – movement of solvent (water). Like diffusion, water moves from where it is most abundant to where it is least abundant. it moves from the dilute side to the concentrated one i.e. from the hypotonic side to the hypertonic side Facilitated diffusion – movement through channel proteins and carrier proteins.

Active transport Energy is used to pump substances in a direction opposite to that in which they would normally diffuse. Primary – ATP provides energy to pump cations (+) across the membrane Secondary – Re-diffusion of pumped cations drives pumps for other molecules.

Cell Structure and Function - emphasizing eukaryotic cells

Cell size

Prokaryotic Cells Bacteria and Archaea Small (1 µm diameter) Surrounded by plasma membrane (May be walls or other structures outside it.) Contain ribosomes, nucleoid region (DNA) No distinct, membrane-bound organelles (May be internal membrane systems)

Eukaryotic cells Protists, Fungi, Plants, and Animals Larger (10-100 µm) and more complex cells Surrounded by plasma membrane (p.m.) Plant cells have a cellulose wall outside p.m. Internal cytoskeleton Elaborate endomembrane system Distinctive membrane-bound organelles

Cellular Organelles Ribosomes Nucleus Endoplasmic reticulum (ER) and Golgi Vacuoles and lysosomes Mitochondria Chloroplasts and other plastids

Ribosomes Found in both prokaryotes and eukaryotes NOT surrounded by a membrane Made of RNA and protein Some are free, some attached to ER Site where amino acids are assembled into polypeptides. Most cells contain thousands of them.

Nucleus Nuclear envelope is made of two membranes (two phospholipid bilayers) Large pores connect nucleoplasm to cytoplasm. Large dark area called nucleolus Where ribosomal RNA is made DNA can be in various forms Tightly wound-up into distinct chromosomes Loosely distributed as formless chromatin

Endomembrane System The chemical factory of the cell Rough endoplasmic reticulum covered in ribosomes, for protein synthesis. Smooth endoplasmic reticulum for lipid synthesis and modification of proteins Golgi apparatus for further modification and packaging for secretion. Membranes are very fluid and dynamic.

Vacuoles and lysosomes Various membrane-bound compartments May be considered part of endomembrane system Lysosomes contain digestive enzymes Most plant cells contain a very large central vacuole.

Mitochondrion (plural = mitochondria) Where sugars are broken down to yield energy. All eukaryotes have them (plants too!!!). Smooth outer membrane. Separate inner membrane deeply in-folded Folds are called cristae Innner membrane creates two compartments: Inner “matrix” Inter-membrane space btw. inner & outer membranes

Plastids Chloroplasts in plant cells are where photosynthesis occurs and sugars are made. Like mitochondria, they have inner and outer membranes. Will discuss internal structure later. Other plastids in plants include - chromoplasts: pigments other than chlorophyll leucoplasts: colorless, contain starch

Cytoskeleton Protein fibers involved in support and movement of cellular structures. Microfilaments (actin) Intermediate filaments (keratin-like proteins) Microtubules (tubulin) Among other roles, microtubules support and create movement in cilia and flagella.