Plant cell structure

• Plant cells share similarities with animal cells, but have unique structures: a large vacuole, a cell wall, and chloroplasts.

Cell Wall

• Provides structural support and protection to the cell.
• Composed of cellulose, consisting of a middle lamella, primary cell wall, and secondary cell wall.
• Pits in the cell wall allow cytoplasm to extend between adjacent cells via cytoplasmic threads called plasmodesmata.

Plasmodesmata

• Interconnecting strands of cytoplasm connect the cytoplasm of adjacent cells across the cell wall.
• Enable cell-cell communication.

Vacuole

• Usually large in plant cells, containing cell sap (water, dissolved ions, and pigments).
• Provides support to the cell and helps maintain cell shape.

Tonoplast

• Membrane surrounding the vacuole.
• Similar in structure and function to the cell membrane.

Plastids

• Double-membrane-bound organelles with diverse functions, including making and storing substances.

Chloroplasts

• Occur only in the green parts of plants.
• Contain the green pigment chlorophyll, enabling photosynthesis.

Chromoplasts

• Plastids responsible for making and storing pigments, excluding chlorophyll (mainly carotenoids).
• Present in highly colored plant parts like leaves, flowers, and fruit (yellow, orange, and red).
• Develop from green chloroplasts as unripe fruit ripens, providing the color to ripened fruit.

Leucoplasts

• Smaller and non-pigmented plastids found in roots and non-photosynthetic tissues.
• Often occur in cells specialized for bulk storage of starch, lipids, or proteins.
• Can play a role in the synthesis of fatty acids and amino acids.

Structure of Cellular Membranes

• The fluid mosaic model is the accepted theory describing cell membrane structure.
• The cell membrane is composed of a phospholipid bilayer.

Phospholipids

• Each has a hydrophilic head (attracts water) pointing outwards and hydrophobic tails (repel water) pointing inwards.

Membrane Proteins

• Occur on either surface or are embedded, some extend across the bilayer.
• Act as carrier molecules for water-soluble substances, such as glucose, amino acids, ions, and nucleic acids.
• Bind to molecules on one side and release them on the other.
• Some form small canals for ions to pass through.
• Function as receptor sites for substances like hormones, enabling interaction or entry into cells with the specific receptors.
• Proteins can move within the membrane plane, making it behave more like a fluid than a solid.
• Phospholipids and proteins combine with variations to create a mosaic structure.

Movement of Substances in Organisms

• Diffusion is the movement of substances from high to low concentration until dynamic equilibrium is achieved.
• Diffusion does not require a membrane, but can occur across one.
• Diffusion occurs because all particles possess kinetic energy.
• Diffusion gradient is the concentration difference of a substance between two areas.
• Passive process because no energy is required, occurring down the concentration gradient.

Factors Affecting Diffusion Rate

• Concentration gradient: steeper gradient = faster diffusion.
• Particle size: smaller particles = faster diffusion.
• Distance: shorter distance = faster diffusion.
• Pressure: higher pressure = faster diffusion.

Examples of Diffusion

• Gas exchange, cellular respiration, and photosynthesis.
• Movement of nutrients into cells.
• Movement of metabolic waste out of cells.

Osmosis

• Spontaneous movement of water from high to low water concentration/potential across a differentially permeable membrane until dynamic equilibrium.
• Passive process because it requires no energy and follows the water concentration gradient.
• Influenced by the same factors as diffusion.

Cell Cycle

• Series of activities in a cell that leads to its division and duplication.
• In eukaryotes, the cell cycle includes interphase, karyokinesis, and cytokinesis.

Interphase

• Time between two consecutive cell divisions, the longest stage.
• Cells grow and perform normal metabolic functions.
• DNA replication occurs at the end, forming two chromatids joined by a centromere for each chromosome.
• Chromosomes are visible as a chromatin network in the nucleus.

Karyokinesis

• Nucleus divides.
• Occurs as mitosis in somatic cells.

Cytokinesis

• Separate process from mitosis.
• Starts during the final phase of mitosis.
• Division of the cytoplasm.
• In animal cells: cytoplasm pinches at the equator, halving the cell with a new nucleus in each half.
• In plant cells: cell plate forms at the equator, developing outwards to become the new cell wall.

Chromosomes

• Long, coiled, thread-like structures in the cell nucleus.
• Composed of histones (bead-shaped proteins) with DNA wound around them.
• Genes are the functional units of DNA that contain hereditary information and instructions for making specific proteins.
• Genes determine traits or characteristics.

Mitosis in Plant Cells

• Occurs only in the meristematic tissue.
• Similar process to animal cells, except:
• Centrosome is absent, therefore no centrioles are formed.
• Spindle fibers form from the nucleoplasm.
• A cell plate develops in the equatorial plane region for cytokinesis.