Each part of a cell plays an essential role in its functioning.
Not always clear what each part's role is based on appearance alone.
Example: Understanding how a clock works by viewing its individual components.
Biologists face a similar challenge in determining how cell parts work together.
It is more beneficial to study living cells compared to nonliving ones to understand cellular processes.
Energy Use in Cells:
Cells utilize energy released from complex molecules (notably, sugar) through mitochondrial processes.
Energy release occurs in small steps and facilitates various chemical reactions necessary for cellular functions.
Metabolism:
Defined as the sum of all chemical reactions occurring in a cell or organism.
Involves processing important molecules like sugars and transferring energy to adenosine triphosphate (ATP).
Waste materials, including excess carbon (as carbon dioxide, CO2) and nitrogen (as ammonia, NH3), are produced and need to be expelled due to toxicity.
Energy Storage:
Not all energy is used immediately;
For instance, plant cells with chloroplasts can store energy in complex molecules such as sugars.
These sugars can be converted into various organic compounds like amino acids, lipids, or nucleotides, which are vital for cellular maintenance, growth, and the synthesis of new substances.
5.7 Substances Enter and Leave Cells by Diffusion
Constant Motion of Particles:
Atoms, molecules, and small particles are perpetually in motion.
General tendency of molecules: Move from areas of higher concentration to lower concentration until equalized.
Definition of Diffusion:
The process of molecules moving from high to low concentration areas is called diffusion.
Concentration gradients illustrate the variation in molecule concentration across a space.
Molecules move down the concentration gradient during diffusion.
Importance of Diffusion in Cells:
Many substances can enter and exit cells through diffusion, aiding in material distribution within cells.
Water also diffuses through concentrations and can pass through the cell membranes via osmosis.
Osmosis Explained:
The movement of water through membranes is specifically termed osmosis.
Examples:
In a saline solution with lower water concentration than a cell, water exits the cell causing shrinkage (Figure 5.10a).
If in pure water where molecules are more concentrated outside, water moves into the cell, causing it to swell (Figure 5.10b).
Animal cells may burst due to excessive inward osmosis, unlike plant cells which swell without bursting due to their rigid cell walls.
Well-hydrated plant cells support the upright posture, while wilted cells indicate water loss.
5.8 Cells Move Substances in a Variety of Ways
Diffusion through Cell Walls and Membranes:
The cell wall typically permits most substances to diffuse through.
The selective permeability of the cell membrane restricts certain molecules from passing freely (e.g., small glucose vs. large protein molecules).
Types of Transport Mechanisms:
Some substances either exceed size limitations or bear an electrical charge, hindering diffusion.
Two significant methods for how cells transport materials across membranes involve specialized transport proteins:
Passive Transport:
No cellular energy expenditure.
Proteins assist in moving substances down their concentration gradient (from high to low concentration).
Specificity: Each protein is exclusive to a particular substance, crucial for the transport of essential ions.
Active Transport:
This process requires ATP energy.
Enables movement against the concentration gradient (from low to high concentration).
Example: Plants acquire necessary nutrients from soil with low concentrations using active transport techniques.
Another Mechanism: Endocytosis and Exocytosis:
The cell membrane may fold inward to create a pocket for fluid and particles, forming vesicles inside the cell (Figure 5.13a).
This mechanism enables large particles to enter cells and can reverse to expel materials (Figure 5.13b).