Cells and Homeostasis

• All matter in the universe is composed of one or more substances called elements
• hydrogen, oxygen, carbon, nitrogen, calcium, iron are all examples of elements
• compound = two or more different elements chemically joined together
• the smallest unit of any element is an atom
• an atom is made up of subatomic particles such as the proton, electron and neutron
• 2 or more atoms combine to form a molecule
• molecules are the chemical building blocks of all body structures
• a cell is the smallest functioning unit of a living organism
• almost all functions of human physiology are performed or initiated by cells
• a human cell typically consists of flexible membranes that enclose cytoplasm and organelles
• cytoplasm is cellular fluid, which is the cellular material outside the nucleus and inside the plasma/cell membrane
• organelles are specialised structures that carry out specific functions within the cell
• cells carry out the chemical activities needed to sustain life and they divide to form or repair tissue
• a myocyte is a muscle cell which contains multiple nuclei
• an osteocyte is a cell of the bone
• a hepatocyte is a liver cell
• a red blood cell is also known as an erythrocyte, which has no nucleus
• the activity of an organism depends on the collective activities of its cells
• cells usually have 3 main components which are nucleus, plasma membrane and cytoplasm
• a plasma or cell membrane is a selectively permeable barrier that controls the movement of substances into and out of the cell
• the nucleus contains genetic material (DNA) and directs all cellular processes
• the nucleus is surrounded by a double membrane containing pores called the nuclear envelope
• between the two membranes of the nuclear envelope is a fluid filled space, the membranes fuse at various points generating openings called nuclear pores
• the nuclear envelope allows some but not all substances to pass through, but substances pass through it much more freely than elsewhere because of its relatively large pores
• nuclear envelope encloses a jelly-like fluid called nucleoplasm in which other nuclear elements are suspended
• the nucleus contains one or more nucleoli which are small dense bodies where ribosomes are assembled
• most ribosomes eventually migrate into the cytoplasm where the serve as the actual sites of protein synthesis
• when cells divide chromatin condenses forming visible structures known as chromosomes
• chromatin consists of DNA molecules coiled around proteins (histones)
• plasma membrane is made of four components which are: a double layer of phospholipids, proteins, cholesterol and carbohydrates
• the plasma membrane is selectively permeable, so it regulates the entry and exit of substances
• in the plasma membrane, phospholipids water loving end (hydroliphic) is outwards and water hating end (hydrophobic) is inwards creating an oily film
• diffusion is the movement of particles from an area of high concentration to an area of low concentration.
• diffusion is passive transport
• osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration
• osmosis is passive transport
• active transport is the movement of molecules from a low concentration to a high concentration across a cell membrane against a concentration gradient
• active transport requires energy from respiration and ATP to move substances across the membrane
• proteins in the plasma membrane do most of the specialised functions and act as receptors, channels and carriers
• channel proteins allow certain molecules to pass through them directly without being bound by them