Carrier proteins and channels are both transport proteins involved in facilitated diffusion, the passive transport of solutes across a membrane down their concentration or electrochemical gradient. As integral membrane proteins, both carriers and channels protect polar or charged solutes from coming into contact with the hydrophobic interior of the lipid bilayer. Furthermore, all transport proteins are specific for the solutes they transport, owing to the specificity of the interactions between the solute and the transport protein.Channels are protein-lined pores across the membrane. A channel may be open at all times (non-gated), or may be gated such that the channel opens and closes under specific conditions. Channels transport inorganic ions or water.

In contrast, carrier proteins do not have a pore. Binding of the transported solute to the carrier protein on one side of the membrane induces a conformational change in the protein that exposes the solute binding site to the opposite side of the membrane, where the solute is released. Carriers transport small polar solutes such as sugars and amino acids.

Though cells can transport some solutes across the plasma membrane by diffusion alone, many other substances must be transported across the plasma membrane against the existing concentration or electrochemical gradient. These transport processes require an investment of cellular energy, typically in the form of ATP hydrolysis, and are referred to as active transport.Active transport processes move the cell away from chemical or electrical equilibrium. In contrast, passive diffusion moves the cell toward equilibrium. The expenditure of cellular energy to maintain disequilibrium is essential to life.

The concentration gradient of Na+ions across the membrane (higher Na+concentration outside) facilitates the diffusion of Na+into the cell. At the same time, the electrical gradient across the membrane (excess positive charge outside) drives Na+into the cell.The concentration gradient of K+ions across the membrane (higher K+concentration inside) facilitates the diffusion of K+out of the cell. However, the electrical gradient across the membrane (excess positive charge outside) impedes the diffusion of K+out of the cell.The electrochemical gradient for an ion is the sum of the concentration (chemical) gradient and the electrical gradient (charge difference) across the membrane. For Na+ions, diffusion through the Na+channel is driven by both the concentration gradient and the electrical gradient. But for K+ions, the electrical gradient opposes the concentration gradient. Therefore, the electrochemical gradient for Na+is greater than the electrochemical gradient for K+.

In cotransport, the energy required to move one solute against its concentration or electrochemical gradient is provided by an ion moving into the cell down its electrochemical gradient. The ion that moves into the cell down its gradient is usually the same ion that is pumped out of the cell by an active transport pump: for example, Na+in animal cells using the sodium-potassium pump, or H+in plants and prokaryotes using the proton pump.In the case of the glucose-sodium cotransporter in animals, Na+moves back into the cell down its electrochemical gradient, providing the energy for glucose to move into the cell against its concentration gradient. The energy for glucose transport into the cell is supplied indirectly by the sodium-potassium pumps hydrolysis of ATP, and directly by the Na+electrochemical gradient created by the pump.

In exocytosis, substances are transported to the plasma membrane in vesicles derived from the endomembrane system. These vesicles fuse with the plasma membrane, releasing the enclosed substances outside the cell.In endocytosis, substances are taken into the cell by folding in of the plasma membrane and pinching off of the membrane to form a vesicle. Notice that in both exocytosis and endocytosis, the transported substances never actually cross the plasma membrane as they leave or enter the cell.

Most proteins that function in the cytosol (such as actin) or in the nucleus (such as DNA polymerase) are synthesized by free ribosomes. Proteins that function within the endomembrane system (such as lysosomal enzymes) or those that are destined for secretion from the cell (such as insulin) are synthesized by bound ribosomes.As a protein destined for the endomembrane system is being synthesized by a ribosome, the first amino acids in the growing polypeptide chain act as a signal sequence. That signal sequence ensures that the ribosome binds to the outer membrane of the ER and that the protein enters the ER lumen.

As they are being synthesized, secretory proteins enter the lumen of the endoplasmic reticulum. From the ER, vesicles transport these proteins to the Golgi, where they are sequentially modified and concentrated in acis-to-transdirection. Secretory vesicles bud from the Golgi and move along cytoskeletal filaments to eventually fuse with the plasma membrane, secreting their protein cargo. Each of these transport steps requires specialized proteins to ensure that the cargo is sent to the proper location and is able to fuse with the target membrane.

Plant cells and animal cells have more in common than they have differences. Nevertheless, plant cells unique structures play important roles. Photosynthesis occurs in plant cells chloroplasts. The plant cells central vacuole takes up most of the space within the cell and serves a variety of functions, including storage and hydrolysis of organic compounds. Plant cell walls, which are composed primarily of cellulose, protect the cells from damage and maintain their shape. Cellulose is the most abundant organic compound on the planet.

The chloroplast and the chemical reactions associated with it are extremely important for all living things. The photosynthetic reactions are responsible for converting solar energy into chemical energy that is used not only by the cells in which photosynthesis occurs but also by other organisms. For example, all of the foods you eat can be traced back to a plant or other photosynthetic organism. Furthermore, the chloroplast and its reactions have had a profound influence on the constitution of Earths atmosphere due to the fact that oxygen is a byproduct of photosynthesis.

Flagella (singular, flagellum) and cilia (singular, cilium) are microtubular structures that extend outside some cells and that assist in moving the cell or the cell's surroundings. Flagella are much longer than cilia. When a cell has cilia, their number is large, whereas a cell will have few flagella, or a single flagellum. Many protists have cilia, and the sperm of many plants and animals have flagella. Flagella and cilia are assembled from tubulin subunits organized to form a circular arrangement of nine pairs of microtubules attached to a central pair. Flagella and cilia bend to cause movement of either the cell or the surroundings. The bending uses energy derived from the hydrolysis of ATP.

A primary distinction between plant cells and animal cells is the fact that plant cells are surrounded by cell walls. If it were not for plasmodesmata, the cell walls would essentially imprison the plants cells, making intercellular communication and the distribution of nutrients nearly impossible.Animal cells have a variety of intercellular junctions, including tight junctions, desmosomes, and gap junctions. The gap junctions of animal cells are most similar to the plasmodesmata of plants. Gap junctions tend to be limited to tissues that require rapid and coordinated signal relay systems, such as heart muscle; they play no role in nutrient distribution.

Cortisol is a small, hydrophobic steroid hormone that can pass through the plasma membrane of cells. In target cells, cortisol binds to the intracellular receptor protein in the cytoplasm, forming a hormone-receptor complex. The hormone-receptor complex then moves into the nucleus and acts as a transcription factor, binding to specific genes and activating their transcription into mRNA. The mRNA that is produced is eventually translated into specific proteins.Proteins produced in response to the cortisol signal function in the stress response. For example, some of these proteins aid in elevating glucose levels in the blood, helping an animal to meet the demands of starvation or intense physical activity.

Receptor proteins (located in the plasma membrane or inside the cell) bind signaling molecules. The reception of the signal causes a shape change in the receptor molecule, to which other molecules inside the cell respond. The message is then relayed through signal transduction, which may involve a phosphorylation cascade or second messengers such as cAMP, Ca2+, or IP3. Possible responses to the signal may include synthesis of a particular protein or regulation of a particular enzyme.

A mutually dependent relationship exists between chloroplasts and mitochondria in the plant cell. Photosynthesis, which occurs in chloroplasts, generates the sugars and oxygen gas that are used in mitochondria for cellular respiration. Cellular respiration generates carbon dioxide, which in turn is used as a carbon source for the synthesis of sugars during photosynthesis. Cellular respiration also generates ATP and water, which are used in various chemical reactions in the plant cell.

Photosynthesisconverts light energy to thechemical energyof sugars and other organic compounds. This process consists of a series of chemical reactions that require carbon dioxide (CO2) and water (H2O) and store chemical energy in the form of sugar. Light energy drives the reactions. Oxygen (O2) is a by-product of photosynthesis and is released into the atmosphere. The following equation summarizes photosynthesis:

6CO2+ 6H2O --> C6H12O6+ 6O2

Photosynthesis transfers electrons from water to energy-poor CO2molecules, forming energy-rich sugar molecules (C6H12O6). This electron transfer is an example of anoxidation-reductionprocess: the water is oxidized (loses electrons) and the CO2is reduced (gains electrons). Photosynthesis uses light energy to drive the electrons from water to their more energetic states in the sugar products, thus converting solar energy to chemical energy.

In plants,photosynthesisoccurs inchloroplasts, mainly in leaf cells. Thelight reactionsoccur along thethylakoidmembranes within the chloroplasts, where pigments capture light energy. The sugar-making reactions of theCalvin cycleoccur in thestroma, the fluid between the inner membrane of the chloroplast and the thylakoids.

In thelight reactions, light is absorbed bychlorophyllmolecules, exciting their electrons. The energy of excited electrons is then used to join ADP and phosphate to formATP. NADP+joins with excited electrons to form NADPH, which temporarily stores the energized electrons. In the process, water is split and oxygen is released.

In theCalvin cycle, energy from ATP, electrons from NADPH, and carbon from carbon dioxide are combined to produce sugar molecules.

Counting carbonskeeping track of where the carbon atoms go in each reactionis a simple way to help understand what is happening in the Calvin cycle.

To produce 1 molecule of G3P (which contains 3 carbons), the Calvin cycle must take up 3 molecules of CO2(1 carbon atom each). The 3 CO2molecules are added to 3 RuBP molecules (which contain 15 total carbon atoms), next producing 6 molecules of 3-PGA (18 total carbon atoms). In reducing 3-PGA to G3P (Phase 2), there is no addition or removal of carbon atoms. At the end of Phase 2, 1 of the 6 G3P molecules is output from the cycle, removing 3 of the 18 carbons. The remaining 5 G3P molecules (15 total carbon atoms) enter Phase 3, where they are converted to 3 molecules of R5P. Finally, the R5P is converted to RuBP without the addition or loss of carbon atoms.

The Calvin cycle requires a total of 9 ATP and 6 NADPH molecules per G3P output from the cycle (per 3 CO2fixed). In Phase 2, six of the ATP and all of the NADPH are used in Phase 2 to convert 6 molecules of PGA to 6 molecules of G3P. Six phosphate groups are also released in Phase 2 (derived from the 6 ATP used). In the first part of Phase 3, 5 molecules of G3P (1 phosphate group each) are converted to 3 molecules of R5P (also 1 phosphate group each). Thus there is a net release of 2 Pi. In the second part of Phase 3, 3 ATP molecules are used to convert the 3 R5P into 3 RuBP.Note that in the entire cycle, 9 ATP are hydrolyzed to ADP; 8 of the 9 phosphate groups are released as Pi, and the ninth phosphate appears in the G3P output from the cycle.

A reaction or process is dependent on another if the output of the second is an input to the first. For example, the light reactions are dependent on the Calvin cycle because the NADP+and ADP produced by the Calvin cycle are inputs to the light reactions.Thus, if the Calvin cycle slows (because of a decrease in the amount of available CO2), the light reactions will also slow because the supply of NADP+and ADP from the Calvin cycle would be reduced.